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We present the design, simulation, and characterization of a magnetic shape-memory alloy (MSMA) film actuator that transitions from bistable switching to resonant self-actuation when subjected to a stationary heat source. The actuator design comprises two Ni-Mn-Ga films of 10 µm thickness integrated at the front on either side of an elastic cantilever that moves freely between two heatable miniature permanent magnets and, thus, forms a bistable microswitch. Switching between the two states is induced by selectively heating the MSMA films above their Curie temperature Tc. When continuously heating the permanent magnets above Tc, the MSMA film actuator exhibits an oscillatory motion in between the magnets with large oscillation stroke in the frequency range of 50–60 Hz due to resonant self-actuation. A lumped-element model (LEM) is introduced to describe the coupled thermo-magnetic and magneto-mechanical performance of the actuator. We demonstrate that this performance can be used for the thermomagnetic energy generation of low-grade waste heat (T < 150 °C) with a high power output per footprint in the order of 2.3 µW/cm2.

In the current study we have obtained Co2FeSi glass-coated microwires with different geometrical aspect ratios, ρ = d/Dtot (diameter of metallic nucleus, d and total diameter, Dtot). The structure and magnetic properties are investigated at a wide range of temperatures. XRD analysis illustrates a notable change in the microstructure by increasing the aspect ratio of Co2FeSi-glass-coated microwires. The amorphous structure is detected for the sample with the lowest aspect ratio (ρ = 0.23), whereas a growth of crystalline structure is observed in the other samples (aspect ratio ρ = 0.30 and 0.43). This change in the microstructure properties correlates with dramatic changing in magnetic properties. For the sample with the lowest ρ-ratio, non-perfect square loops are obtained with low normalized remanent magnetization. A notable enhancement in the squareness and coercivity are obtained by increasing ρ-ratio. Changing the internal stresses strongly affects the microstructure, resulting in a complex magnetic reversal process. The thermomagnetic curves show large irreversibility for the Co2FeSi with low ρ-ratio. Meanwhile, if we increase the ρ-ratio, the sample shows perfect ferromagnetic behavior without irreversibility. The current result illustrates the ability to control the microstructure and magnetic properties of Co2FeSi glass-coated microwires by changing only their geometric properties without performing any additional heat treatment. The modification of geometric parameters of Co2FeSi glass-coated microwires allows to obtain microwires that exhibit an unusual magnetization behavior that offers opportunities to understand the phenomena of various types of magnetic domain structures, which is essentially helpful for designing sensing devices based on thermal magnetization switching. This research was funded by the Spanish MICIN, under PID2022-141373NBI00, by EU under “INFINITE” (Horizon Europe) project and by the Government of the Basque Country, under PUE_2021_1_0009 and Elkartek (MINERVA, ZE-KONP and MAGAF) projects and by under the scheme of “Ayuda a Grupos Consolidados” (Ref.: IT1670-22). MS wish to acknowledge the funding within the Maria Zambrano contract by the Spanish Ministerio de Universidades and European Union –Next Generation EU (“Financiado por la Unión Europea-Next Generation EU”).

Funding: Engineering and Physical Sciences Research Council EP/J000884/1, EP/W037300/1 (RAD, BFK). Materials with the TiNiSi structure have recently been highlighted as potential thermoelectric materials. Here we report the thermoelectric properties of TiNiX (X = Si and Ge). Both materials behave as defective metals or heavily doped degenerate semiconductors. Room temperature Seebeck coefficients are -45 μV K-1 (Si) and -20 μV K-1 (Ge) with electrical resistivities of 0.5-1 mΩ cm. The lattice thermal conductivities are 8 W m-1 K-1 (Si) and 6 W m-1 K-1 (Ge) at 360 K, which is promising in the absence of alloying. The calculated power factors and figures of merit remain small, with the largest S2/ρ = 0.17 mW m-1 K-2 and peak zT = 5 × 10-3 seen in TiNiSi near 300 K. Both compositions show Kondo behaviour at low-temperatures, linked to the emergence of local moment magnetism, and have substantial magnetoresistance effects at 2 K. This work provides property characterisation for two members of this large class of intermetallic materials. Publisher PDF

The ability of Heusler alloys to accommodate broad variations of composition, doping and ordering provides multiple options for tailoring their ferromagnetic and ferroelastic properties. Moreover, existing coupling between these ferroic properties ranging from coupled ferroic transitions to a coupling of their ferromagnetic and ferroelastic microstructure allows for manifold multifunctionalities. Here we focus on ferromagnetic, metamagnetic and reentrant shape memory alloys explaining the principles and sketch effects’ rich susceptibility to temperature, magnetic field and stress. We illustrate how these can provide a path to a multitude of emerging applications for actuation, sensing, and energy use. As an outlook, we discuss time dependency, fatigue, and finite size effects, which are not yet fully explored.

In the current study, we concentrated on the influence of annealing on the magnetic behavior of Co2MnSi-based Heusler microwires. We set the annealing temperature at 1023 K for 2 h, as the sample did not show any significant changes in the magnetic properties at lower temperatures, while annealing at temperatures above 1023 K damages the glass coating. Strong in-plane magnetocrystalline anisotropy parallel to the microwire axis was evident in the magnetic behavior at room temperature for as-prepared and annealed samples. The coercivity of the annealed sample was four times higher than that of the as-prepared sample across a wide range of measuring temperatures. Both annealed and as-prepared samples exhibit quite stable coercivity behavior with temperature, which may have interesting applications. The an nealed sample did not exhibit magnetic saturation for M-H loops measured below 50 K. Sharp irreversible magnetic behavior has been detected for annealed samples at a blocking temperature of 220 K; at the same time, the blocking temperature for the as-prepared sample was 150 K. The strong internal mechanical stress induced during the fabrication of Co2MnSi microwires in addition to the internal stress relaxation caused by the annealing induced the onset of magnetic phases resulting in unusual and irreversible magnetic behavior. This research was funded by the Spanish MICIN, under PID2022-141373NB-I00 project, by EU under “INFINITE” (HORIZON-CL5-2021-D5-01-06) project and by the Government of the Basque Country, under PUE_2021_1_0009 and Elkartek (MINERVA and ZE-KONP) projects and by under the scheme of “Ayuda a Grupos Consolidados” (Ref.: IT1670-22). In addition, “Financiado por la Unión Europea-Next Generation EU”. We also wish to thank the administration of the University of the Basque Country, which not only provides very limited funding, but even expropriates the resources received by the research group from private companies for the research activities of the group. Such interference helps keep us on our toes.

В работе представлены результаты исследований микро- и наноструктуры поверхности быстрозакаленных лент сплавов Гейслера (NiMnAl,NiMn AlSi, NiCoMn Al) методами сканирующей электронной и атомно-силовой микроскопии. Рассмотрено влияние химического состава на размер, геометрию границ и структуру зерен. Показано, что все исследуемые образцы обладают наноразмерным мартенситным рельефом, определены его параметры. Установлено, что поперечное сечение лент представлено кристаллическими зернами разной формы и размера, что обусловлено отличием скоростей охлаждения по краям и в центре ленты. Проведено элементное картирование поверхности поперечного сечения лент с помощью рентгеновского энергодисперсионного спектрометра, установлено равномерное распределение химических элементов в образцах. Показано, что легирование лент состава NiMnAl кобальтом вызывает изменение микроморфологии поверхности и оказывает значительное влияние на ход полевых зависимостей намагниченности и доменную структуру образцов. The paper describes the results of the scanning electronic and atomic force microscopy research of the surface structure of the NiMn Al, NiMn AlSi, NiCoMnAl rapidly quenched ribbons. The influence of the chemical composition on the size, boundary geometry and structure of the grains is considered. It is shown that all the test samples have a nano-sized martensitic relief, and its parameters are determined. It has been established that the cross section of the ribbon is represented by crystalline grains of different shapes which are due to differences in the cooling rates along the edges and in the center of the samples. Elemental mapping of the cross-sectional surface of the ribbons was carried out using an X-ray energy dispersive spectrometer, and a uniform distribution of chemical elements in the samples was established. It is shown that the cobalt doping of NiMn Al ribbon causes changes in the micromorphology of the surface and has a significant effect on the magnetic properties of rapidly quenched ribbons: field dependences of themagnetization and domain structure.

We study the effect of annealing and the applied magnetic field from 50 Oe to 20 kOe on the magneto-structural behavior of Ni2FeSi-based Heusler microwires fabricated by using Taylor-Ulitovsky technique. Using the XRD analysis, a strong effect of annealing, manifested as the development of the crystallization process, was observed. The average grain size and crystalline phase content of annealed sample increase from 21.3 nm and 34% to 32.8 nm and 79%, respectively, as-compared to the as-prepared one. In addition, upon annealing, phase transforms into a monoclinic martensitic structure with a modulation of 10 M, which cannot be found in the as-prepared sample. Concerning the magnetic properties, both samples show ferromagnetic behavior below and above the room temperature, where the Curie temperature of Ni2FeSi is higher than the room temperature. The induced secondary phases have a noticeable effect on the magnetic behavior of the annealed sample, where a high normalized saturation magnetization (NMs) and low normalized reduced remenance (Mr = M/M5K), compared to the as-prepared have been detected. Additionally, the coercivity of annealed sample shows one flipping point at 155 K where its behavior changes with temperature. Meanwhile, the as-prepared sample show two flipped point at 205 K and 55 K. A mismatch between field cooling (FC) and field heating (FH) magnetization curves with temperature has been detected for annealed sample at low applied magnetic field. The difference in magnetic and structure behavior of Ni2FeSi microwires sample is discussed considering the effect of induced internal stresses by the presence of a glass coating and the recrystallization and stresses relaxation upon annealing. This research was funded by the Spanish MCIU, under PGC2018-099530-B-C31 (MCIU/AEI/FEDER, UE), by EU under “INFINITE”(Horizon 2020) project, by the Government of unding have been chethe Basque Country, under PUE_2021_1_0009 and Elkartek (MINERVA and ZE-KONP) projects and by under the scheme of “Ayuda a Grupos Consolidados” (Ref.: IT1670-22), by the University of Basque Country under the COLAB20/15 project and by the Diputación Foral de Gipuzkoa in the frame of Programa “Red guipuzcoana de Ciencia, Tecnología e Innovación 2021” under 2021-CIEN-000007-01 project. We also wish to thank the administration of the University of the Basque Country, which not only provides very limited funding, but even expropriates the resources received by the research group from private companies for the research activities of the group. Such interference helps keep us on our toes.

In the current study, we have succeeded to fabricate Co2FeSi Heusler alloy glass-covered microwires with magnetic core nucleus diameter d = 4.36 mu m and total diameter D = 17.55 mu m, with high Curie temperature (T-e > 1100 K) and well-defined magnetic anisotropy for high temperature spintronic devices application. The magnetic properties of as-prepared and annealed at different temperature (873 K, 973 K and 1073 K for 1 h) of Co2FeSi Heusler alloy glass-covered microwires have been investigated. Strong dependence of the magnetic properties on the annealing conditions has been indicated. Anomalous magnetic behavior for annealed samples at 873 K and 973 K has been found and structural properties of such samples have been analyzed. Critical temperatures 155 K and 250 K have been detected for annealed samples at 873 K and 973 K, respectively, where the behavior of M-H loops and coercivity changed. Below the critical point the M -H curve shows "kink or wasp-waisted" magnetic behavior and complex magnetic reversal mechanism is supposed. The anomalous magnetic behavior is due to the martensitic phases induced by annealing con-ditions below the room temperature. This unusual magnetization behavior provides opportunities to un-derstand the phenomena of different types of magnetic domain structures in the preferred crystallographically oriented Co2FeSi Heusler alloy glass-coated micro-wires, essentially helpful for de-signing the devices based on magnetization switching. This work was supported by the Spanish MCIU, under PGC2018-099530-B-C31 (MCIU/AEI/FEDER, UE), by EU under “INFINITE” (HORIZON-CL5-2021-D5-01-06) project, by the Government of the Basque Country, under PUE_2021_1_0009 and Elkartek (Composens, Minerva and ZE-KONP) projects, by the University of Basque Country, under the scheme of “Ayuda a Grupos Consolidados” (Ref.: GIU18/192) and under the COLAB20/15 project and by the Diputación Foral de Gipuzkoa in the frame of Programa “Red guipuzcoana de Ciencia, Tecnología e Innovación 2021” under 2021-CIEN-000007-01 project. The authors thank for technical and human support provided by SGIker (Medidas Magneticas Gipuzkoa) (UPV/EHU/ ERDF, EU). We wish to thank the administration of the University of the Basque Country, which not only provides very limited funding, but even expropriates the resources received by the research group from private companies for the research activities of the group. Such interference helps keep us on our toes.

Using zone calculations in the FLAPW (the full-potential linearized augmented-plane-waves) model, the information on the energy, charge, and spin characteristics of Pt1-xNixMnSb alloys (x = 0.0÷1.0) is obtained. It is established that, with an increase in the concentration of nickel atoms in Pt1-xNixMnSb alloys, the interatomic space density of electrons decreases, covalent bonds weaken, and the cohesive energies of the alloys decrease. The dominant contributions to the formation of magnetic moments in Pt1-xNixMnSb alloys are made by 3d electrons of manganese atoms. In alloys with x ≥ 0.50, the complete polarization of Fermi electrons is registered, which converts these alloys to a half-metallic state., За допомогою зонних розрахункiв у моделi FLAPW (the full-potential linearized augmented-plane-waves) отримано iнформацiю про енергетичнi, зарядовi та спiновi характеристики сплавiв Pt1-xNixMnSb (x = 0,0–1,0). Встановлено, що зi збiльшенням концентрацiї атомiв нiкелю в сплавах Pt1-xNixMnSb зменшується мiжатомна просторова густина електронiв, послаблюються ковалентнi зв’язки i знижуються когезiйнi енергiї сплавiв. Домiнуючий внесок у формування магнiтних моментiв у сплавах Pt1-xNixMnSb роблять 3d-електрони атомiв марганцю. У сплавах з x ≥ 0,50 зареєстрована повна поляризацiя фермiєвських електронiв, що переводить цi сплави в half-metallic стан.

Funding: The LEAP 5000XR at Oxford is supported by EPSRC grant EP/M022803/1. The P-FIB UXe DualBeam FIB/SEM at Glasgow is supported by EPSRC grant EP/P001483/1, and the EPSRC is also acknowledged for funding the work on nanostructured half-Heuslers for thermoelectric waste heat recovery (grants EP/N01717X/1 and EP/N017218/1) and a studentship (grant EP/N509668/1). The performance of thermoelectric materials depends on both their atomic-scale chemistry and the nature of microstructural details such as grain boundaries and inclusions. Here, the elemental distribution throughout a TiNiCu0.1Sn thermoelectric material has been examined in a correlative study deploying atom-probe tomography (APT) and electron microscopies and spectroscopies. Elemental mapping and electron diffraction reveal two distinct types of grain boundary that are either topologically rough and meandering in profile or more regular and geometric. Transmission electron microscopy studies indicate that the Cu dopant segregates at both grain boundary types, attributed to extrusion from the bulk during hot-pressing. The geometric boundaries are found to have a degree of crystallographic coherence between neighboring grains; the rough boundaries are decorated with oxide impurity precipitates. APT was used to study the three-dimensional character of rough grain boundaries and reveals that Cu is present as discrete, elongated nanoprecipitates cosegregating alongside larger substoichiometric titanium oxide precipitates. Away from the grain boundary, the alloy microstructure is relatively homogeneous, and the atom-probe results suggest a statistical and uniform distribution of Cu with no evidence for segregation within grains. The extrusion suggests a solubility limit for Cu in the bulk material, with the potential to influence carrier and phonon transport properties across grain boundaries. These results underline the importance of fully understanding localized variations in chemistry that influence the functionality of materials, particularly at grain boundaries. Publisher PDF

We used the Taylor–Ulitovsky technique to prepare nanocrystalline Co2MnGe Heusler alloy glass-coated microwires with a metallic nucleus diameter of 18 ± 0.1 µm and a total diameter of 27.2 ± 0.1 µm. Magnetic and structural studies were carried out to determine the fundamental magneto-structural characteristics of Co2MnGe glass-coated microwires. XRD revealed a well-defined nanocrystalline structure with an average grain size of about 63 nm, lattice parameter a = 5.62 and a unique mixture of L21 and B2 phases. The hysteresis loops measured at different temperatures indicated a well-known ferromagnetic behavior for the reduced remanent, where a monotonic increasing in the reduced remanent and saturation magnetization occurs. The coercivity shows anomalous behavior compared to the Co2Mn-based glass-coated microwires. The magnetization curves for field cooling and field heating (FC–FH) demonstrate a considerable dependence on the applied magnetic field, ranging from 50 Oe to 20 kOe. Internal stresses, originated by the production process, resulted in various magnetic phases, which were responsible for the notable difference of FC and FH curves on magnetization dependence versus temperature. Furthermore, the ferromagnetic behavior and expected high Curie temperature, together with high degree of the L21 order, make it a promising candidate for many applications. This research was funded by the Spanish MICIN, under PID2022-141373NB-I00 project, by EU under “INFINITE” (Horizon Europe) project and by the Government of the Basque Country, under PUE_2021_1_0009 and Elkartek (MINERVA and ZE-KONP) projects and by under the scheme of “Ayuda a Grupos Consolidados” (Ref.: IT1670-22). In addition, MS wishes to acknowledge the funding within the Maria Zambrano contract by the Spanish Ministerio de Universidades and European Union–Next Generation EU (“Financiado por la Unión Europea-Next Generation EU”).

Several studies have confirmed that current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) technology is appropriate for next-generation read sensors for ultrahigh areal densities (ADs) of data storage applications. Since the physical dimension of the read sensor is a crucial factor for developing the reader to overcome its limitations, this paper proposes an optimal sizing prediction of the CPP-GMR read heads for ADs of 1-4 Tb/in2. Micromagnetic modelling was performed in the simulations. The appropriate length of the stripe height (SH) and the read width (RW) of the readers was estimated based on a consideration of sensor outputs including the readback signal, asymmetry parameter, dibit response and power spectral density (PSD) profile. It was found that a variation of SH and RW lengths had an influential impact on the readback signal waveform. Those affectations were further characterized through the echoes of dibit response showing that shortening the SH length or increasing the RW length could improve the resolution and reduce the distortion occurring in the readback signal. Moreover, the PSD profile indicated that the reader operation became more stable at shorter SH lengths or longer RW lengths. The head response spectrum was also examined. In addition, the magnitude of the bias current was studied in relation to the head response. Lastly, the optimal physical dimension (SH $\times $ RW) of the CPP-GMR readers for ADs of 1-4 Tb/in2 was predicted to be ( $40 \times 48$ ) nm, ( $28 \times 29$ ) nm, ( $25 \times 26$ ) nm and ( $19 \times 20$ ) nm, respectively. The results can be utilized to design the CPP-GMR sensors at ultrahigh magnetic recording capacities.

This study presents the concentration-dependent structural, morphological, and magnetic properties of Fe2V1-xMnxGe Heusler alloys (x = 0.0, 0.25, and 0.50) have not been studied previously in the literature, and are investigated by using X-ray Diffractometer, Scanning Electron Microscope (SEM), and Physical Properties Measurement System (PPMS). We observed from the X-ray diffraction measurements that (except x = 0) Fe2V1-xMnxGe samples have had L21-type crystal structure, matching well with the literature. In all SEM images, some holes and crack-type formations are observed. Increasing Mn content in Fe2V1-xMnxGe Heusler alloys can effectively inhibit grain boundary migration. Except for Fe2VGe, the other alloys in the series are investigated to be soft ferromagnets at 5 K with saturation magnetic moment. The (M-H) graphs especially for Fe2V0.75Mn0.25Ge and Fe2V0.5Mn0.5Ge measured at 5 K reveal almost no coercive fields and easy magnetization. Namely, the system easily saturates without needing any high fields. The obtained results show that these new samples may be suitable candidates for technological applications.

Shape memory alloys (SMA) are now widely studied in academia and have increasing usage in industry, as they allow efficient performance with reduced dimensions and the possibility of sensing through the material itself. A subclass of the SMAs are the alloys with magnetic shape memory, still little explored as intelligent materials in Brazil. They combine the best properties of SMAs and common magnetostrictive materials. These alloys not only have a relevant memory effect, but can also be used in totally new applications, given their complex mechanical behavior. The magnetic shape memory alloys (MSMA) can be activated not only by the presence of a thermal stimulus, but also by the presence of a magnetic field. In this research, the polycrystalline magnetic shape memory alloys Ni51,3Mn24Ga24,7 and Ni54Mn21Ga25 were manufactured and characterized, a new characterization method was also developed through the measurement of the material’s magnetic permeability when the material is exposed to a magnetic field. Differential scanning calorimetry, X-ray diffraction, X-ray spectroscopy and scanning electron microscopy analyzes were performed to thermoanalytically and crystallographically characterize the alloys. Magnetic characterizations were also performed through magnetic saturation analysis and atomic force microscopy. From the results obtained with the developed permeability device, it was possible to establish the alloy’s permeability and also to demonstrate that there was a decrease in magnetic permeability when exposing the alloys to a constant magnetic field. Additional characterizations evidenced that the Ni54Mn21Ga25 alloy exhibited larger magnetic force than the Ni51,3Mn24Ga24,7 alloy, as well as identified that there is a hysteresis in the force behavior of these alloys in relation to the distancing and approximation from a constant magnetic field source. Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq As ligas de memória de forma (LMF) são, hoje, amplamente estudadas na academia e têm crescente utilização na indústria, pois permitem uma atuação eficiente com reduzidas dimensões e possibilidade de sensoriamento através do próprio material. Uma subclasse das LMFs são as ligas com memória de forma magnética (LMFM), ainda pouco exploradas en quanto materiais inteligentes no Brasil, que combinam as melhores propriedades dos LMFs e dos materiais magnetoestrictivos comuns. Estas ligas não só apresentam um efeito de memória de forma relevante como também podem ser utilizadas em aplicações totalmente novas, dado o seu comportamento mecânico complexo. Estas ligas podem ser ativadas não apenas na presença de um estímulo térmico, mas também pela presença de um campo magnético. Neste trabalho de pesquisa foram desenvolvidas e caracterizadas as ligas policristalinas Ni51,3Mn24Ga24,7 e Ni54Mn21Ga25, com efeito de memória de forma magnética, assim como foi desenvolvido um novo método de caracterização através da mensuração da permeabilidade magnética do material ao expor a ligas a um campo magnético. Foram realizadas análises de calorimetria exploratória diferencial, difração de raios X, espectroscopia de raios X e microscopia eletrônica de varredura para caracterizar termoanalítica e cristalograficamente as ligas produzidas. Ca racterizações magnéticas também foram realizadas através de ensaios de saturação magnética e microscopia de força atômica. Com os resultados obtidos com o dispositivo desenvolvido foi pos sível determinar a permeabilidade das ligas e também demonstrar que houve uma diminuição da permeabilidade magnética ao expor as ligas a um campo magnético constante. Caracterizações adicionais demonstraram que a liga Ni54Mn21Ga25 apresentou força magnética maior que a liga Ni51,3Mn24Ga24,7 , assim como identificaram que há uma histerese no comportamento de força destas ligas com relação ao afastamento e aproximação de uma fonte de campo magnético constante.

Fully compensated ferrimagnets do not create any magnetic stray field and allow for a completely polarized current of charges. As a result, these alloys show promising prospects for applications as spintronic devices. In this paper, we investigated the phase stability, the site preference, the tetragonal distortion and the influence of symmetry from the crystal structure and chemical environments of magnetic ions on the magnetic properties of Cr2YZ and Mn2YZ (Y = void, Ni, Cu, and Zn; Z = Ga, Ge, and As) full Heusler alloys by first-principles calculations. We found that the selected Cr2-based alloys, except for Cr2NiGa and Cr2NiGe, prefer to crystallize in the centrosymmetric L21-type structure, while the selected Mn2-based alloys, except for Mn2CuAs, Mn2ZnGe and Mn2ZnAs, tend to crystallize in the non-centrosymmetric XA-type structure. Due to the symmetry, the antiferromagnetism of the selected L21-type alloys is very stable, and no spin-polarized density of states could be generated. In contrast, the magnetic moment of the selected XA-type alloys depends heavily on the number of valence electrons and tetragonal distortion, and spin-polarized density of states is generated. Therefore, the selected alloys with L21-type structures and their tetragonal-distorted structure are potential candidates for conventional antiferromagnets, while those with XA-type structure and their tetragonal-distorted structure are promising candidates for (fully) compensated ferrimagnets.

A key part of optimising thermoelectric materials is understanding the electronic scattering mechanism. For half-Heusler (HH) thermoelectrics, the dominant mechanisms are acoustic phonon scattering in pure systems and alloy scattering in highly alloyed systems. In this report, the significance of the residual resistivity ρ 0 is highlighted. Large ρ 0 values can lead to misidentification of the dominant scattering mechanism when only high-temperature ρ(T) data is available. A straightforward approach to analyse ρ(T) is proposed and applied to a range of HH systems. This reveals large levels of structural disorder in XIVNiSn, whilst XVFeSb has the strongest coupling with acoustic phonons. The electronic scattering mechanism depends sensitively on composition, with acoustic (ρ sim T 1.5), metallic (simT 1) and alloy (simT 0.5) scattering observed within the main HH families. With the aid of velocity of sound, band mass and carrier concentration data, the deformation potential can be obtained, enabling quantification of the interaction between phonons and carriers, from fits to resistivity data. This work provides a route for the analysis of experimental ρ(T) data that can be applied to a range of thermoelectric materials.

In the purpose of exploring new Heusler alloys with different magnetic applications, we have employed first principles calculations method within density functional theory. After checking the structural stability of X2YZ Heusler alloys (X = Fe, Co; Y =Zr, Mo and Z = Ge, Sb), we found that Cu2MnAl type structure is more favorable for most compounds except for X2MoGe and Co2MoSb, were the Hg2CuTi structure is energetically more stable. The trends in magnetic and electronic structures can be predicted by the structure types as well as the different kinds of hybridizations between the constituents. Among the two series only two compounds were identified to be true half metals with potential applications in spintronic devices. While one compound was classified as a nonmagnetic semiconductor with a small band gap. For the rest of materials, we found that the metallic behavior is dominant. These materials show possible interesting features in technical applications as well. The effect of distortion on the magnetic properties of Co2ZrGe and Fe2ZrSb showed that the half metallic character was preserved within a moderate range of volume changes, which makes it possible to grow these materials as thin films with modern techniques.

The magnetocaloric Ni50Mn25Sn25 Heusler compound was prepared via ultra-fast and sustainable microwave synthesis, by using the two ways hybrid microwave heating method. The structural, microstructural, magnetic and magnetocaloric properties of the prepared specimen are presented. The functional features of the synthetized sample result comparable with compounds obtained with other high-temperature preparation techniques that require larger amount of energy. The proposed time and energy-efficient preparation method could promote the development of large mass production routes of multifunctional huge-potential Heusler compounds, exploitable as active materials in thermomagnetic energy conversion machines and in other technological applications.

Ferromagnetic resonance experiment was performed to study the magnonic modes of an antidot lattice nanopatterned in a sputtered Co2MnSi Heusler alloy thin film. The magnonic crystal was prepared with a Ga+ focused ion beam, and micromagnetic simulations were used to explain qualitatively and quantitatively the complex experimental spin waves spectrum. We demonstrate the necessity to consider the geometrical imperfections and the modification of the Co2MnSi magnetic parameters induced by the nanofabrication process to describe the evolution of the frequencies and spatial profiles of the principal experimental spin waves modes in the 0–300 mT magnetic field range. In particular, our model suggests that Ga+ milling induces a drastic decrease (between 80% and 90%) in the bulk Co2MnSi magnetic parameters. In addition, simulations reveal the presence of a diversity of localized and extended spin waves modes whose spatial profiles are closely related to the evolution of the magnetic state at equilibrium from a very non-collinear configuration up to a quasi-saturated state.

The reduction of the thermal hysteresis in first order magnetostructural transition is a determining factor to decrease energy losses and to improve the efficiency of magnetocaloric cooling based systems. In this work, a Cu doped NiMnInCo metamagnetic shape memory alloy (MMSMA) exhibiting a narrow thermal hysteresis (around 5 K) at room temperature has been designed. In this alloy, the induced L21 ordering process affects the phase stability in an unusual way compared to that observed in NiMnInCo and other NiMn based alloys. This ordering produces an increase in the Curie temperature of the austenite but hardly affects the mar tensitic transformation temperatures. As a consequence, the ordering increases the magnetization of the austenite without changing the transformation temperatures, doubles the sensitivity of the transformation to magnetic fields (the Claussius-Clapeyron slope goes from 2.1 to 3.9 K/T), improves the magnetocaloric effect, the reversibility and finally, enhances the refrigeration capacity. In addition, the magnetic hysteresis losses are among the lowest reported in the literature and the effective cooling capacity coefficient RCeff reaches 86 J/Kg for 2 T (15 % higher than those found in Ni-Mn based alloys) and 314 J/Kg for 6 T fields. Therefore, the ordered alloy possesses an excellent combination of low thermal hysteresis and high RCeff, not achieved previously in metamagnetic shape memory alloys near room temperature. This work has been carried out with the financial support of the Spanish “Agencia Estatal de Investigación (AEI), Ministerio de Ciencia, Innovación y Universidades” (Projects number RTI2018–094683-B-C54 (MCIU/AEI/FEDER, UE)), Navarra Government (Project number PC017–018 AMELEC). P. La Roca has received funding from “la Caixa” and "Caja Navarra" Foundations, under agreement LCF/PR/PR13/51080004.

Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties.

The review considers various groups of Heusler compounds, which can have the properties of a semiconductor, a half-metallic ferromagnet, a spin gapless semiconductor, a topological semimetal, and a non-collinear antiferromagnet. In these Heusler compounds, “conventional” from the point of view of the crystal structure, unusual kinetic and magnetic properties can be observed, which are caused by the features of their electronic structure (e.g., presence of an energy gap for one spin projection) and magnetic state (e.g., strong ferromagnetism, compensated ferrimagnetism). Their magnetic and kinetic characteristics are very sensitive to external influences. Depending on the alloy composition and external parameters, transitions between the considered states can be realized. All this opens up further prospects for controlling the electronic and magnetic characteristics of such compounds and their practical application. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature. Ministry of Education and Science of the Russian Federation, Minobrnauka: 122021000036–3, 122021000038–7; Russian Science Foundation, RSF: 22–22-00935 The research was carried out within the state assignment of Ministry of Science and Higher Education of the Russian Federation (themes “Spin,” No. 122021000036–3 and “Quantum,” No. 122021000038–7). Research funding was provided by the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority-2030 Program). The Sect. was prepared with the financial support of the Russian Science Foundation within the framework of research project No. 22–22-00935.

Agradecimentos: MSR thanks the Brazilian agencies CNPq, FAPERJ, and PROPPI-UFF for financial support. BMP thanks the CNPq for financial support. VGP thanks the CNPq for the financial support under grant no. 160744/2019-9. VMA thanks IFIMUP and FCT for the contract under the project POCI-01-0145-FEDER-028676 and the financial support through the UIDB/04968/2020. MSR belongs to the INCT of Refrigeração e Termofísica, funding by CNPq by grant number 465448/2014-3. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 and project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES. This work was also financed by the Fundação de Amparo a Pesquisa do Estado de São Paulo, Brasil (FAPESP) - Project No. 2017/10581-1. JSA acknowledges FCT for grant IF/01089/2015. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory Abstract: The competition between the stability of the cubic and hexagonal full Heusler alloys and the implications concerning their magnetic properties were systematically studied through the detailed structural and magnetic characterization of the Fe2Mn(Si1-xGex) system. This system was specifically chosen as the parent compositions are cubic (x = 0) and hexagonal (x = 1). It is found that the formation of hexagonal phases occurs for the x >= 0.6 samples, whereas its phase fraction monotonically increases with x until the pure hexagonal Fe2MnGe is formed. The change in structure results in high sensitiveness of both the saturation of magnetization (MS) and Curie temperature (TC) with x values, related to a strong magnetocrystalline anisotropy of the hexagonal phase. Both cubic and hexagonal magnetic features were qualitatively reproduced by Density Functional Theory (DFT) calculations. This work provides an experimental and theoretical foundation for further design of Heusler systems with controlled structures and magnetic properties CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ FUNDAÇÃO CARLOS CHAGAS FILHO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO - FAPERJ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP Fechado

The classical Bardeen-Cooper-Schrieffer and Eliashberg theories of the electron-phonon-mediated superconductivity are based on the Migdal theorem, which is an assumption that the energy of charge carriers, $k{_B}T{_F}$, significantly exceeds the phononic energy, $\hbar{\omega{_D}} $, of the crystalline lattice. This assumption, which is also known as adiabatic approximation, implies that the superconductor exhibits fast charge carriers and slow phonons. This picture is valid for pure metals and metallic alloys because these superconductors exhibit $\hbar{\omega{_D}}$/$k{_B}T{_F}, Comment: 22 pages, 3 figures, 1 table

In this work, scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry were used to investigate the solidification structure, thermal behavior, and kinetics of the martensitic transformations of the (Mn,Ni)Sn as-spun and annealed ribbons synthesized by melt-spinning. At room temperature, the as-spun and annealed (Mn,Ni)Sn ribbons exhibited a cubic single-phase Heusler L21 structure. The kinetics of the martensitic transformation (MT) was studied, together with their microstructure evolution and cooling rate dependence. The mechanism was also investigated. Additionally, a high dependence between the cooling rates and energy activation (Ea) was detected. A more detailed characterization of MT and account of thermodynamic parameters were examined after annealing.

[EN] In this work, we were able to produce Co2FeSi Heusler alloy glass-covered microwires with a metallic nucleus diameter of about 4.4 µm and total sample diameter of about 17.6 μm by the Taylor–Ulitovsky Technique. This low cost and single step fabrication process allowed the preparation of up to kilometers long glass-coated microwires starting from a few grams of high purity inexpensive elements (Co, Fe and Si), for a wide range of applications. From the X-ray diffraction, XRD, analysis of the metallic nucleus, it was shown that the structure consists of a mixture of crystalline and amorphous phases. The single and wide crystalline peak was attributed to a L21 crystalline structure (5.640 Å), with a possible B2 disorder. In addition, nanocrystalline structure with an average grain size, Dg = 17.8 nm, and crystalline phase content of about 52% was obtained. The magnetic measurements indicated a well-defined magnetic anisotropy for all ranges of temperature. Moreover, soft magnetic behavior was observed for the temperature measuring range of 5–1000 K. Strong dependence of the magnetic properties on the applied magnetic field and temperature was observed. Zero field cooling and field cooling magnetization curves showed large irreversibility magnetic behavior with a blocking temperature (TB = 205 K). The in-plane magnetization remanence and coercivity showed quite different behavior with temperature, due to the existence of different magnetic phases induced from the internal stress created by the glass-coated layer. Moreover, a high Curie temperature was reported (Tc ≈ 1059 K), which predisposes this material to being a suitable candidate for high temperature spintronic applications. This research was funded by the Spanish MCIU, under PGC2018-099530-B-C31 (MCIU/ AEI/FEDER, UE), by EU under “INFINITE” (HORIZON-CL5-2021-D5-01-06) project, by the Gov- ernment of the Basque Country under PUE_2021_1_0009 and Elkartek (MINERVA, ZE-KONP and COMPONENS) projects, by the University of the Basque Country, under the scheme of “Ayuda a Grupos Consolidados” (Ref.: GIU18/192) and under the COLAB20/15 project and by the Diputación Foral de Gipuzkoa in the frame of Programa “Red guipuzcoana de Ciencia, Tecnología e Innovación 2021” under 2021-CIEN-000007-01 project

To explain the observed features of k-space photoelectron images taken on off-stoichiometric Heusler Ni49.7Mn29.1Ga21.2 single-crystals in the cubic austenitic and pseudotetragonal martensitic phases, the images were simulated theoretically. Despite the moderate structural difference of both phases, there is large difference in photoemission spectra. Analysis of the final states’ structure, matrix elements, and interface barrier scattering was performed to interpret discrepancies between the external photoemission of the austenite and martensite. The missing signal at the surface-normal emission of the martensitic phase is, ultimately, explained by repeated scatterings of escaping electrons on the interfaces between nanotwins.

As part of this diploma thesis a measurement setup to study the Seebeck- and Nernst effect at high temperatures and high magnetic fields was designed and created.The measurement setup allows for simultaneous measurements of both effects for temperatures up to 560 Kelvin and magnetic fields up to 10 Tesla. Apart from the experimental work, the underlying theory of both effects is presented. Based on the theoretical concepts, measurement equations are derived, that counteract constant spurious offset voltages and systematic errors of the measurement setup.For automation of the measurement process, a measurement software and a graphical user interface were developed. The measurement setup was tested and optimized using a wide variety of polycrystalline materials for which reliable reference data were published. For all reference samples the published results could be reproduced for both the Seebeck- and Nernst effect, allowing the measurement setup to be a reliable system for future research at the institute.

Были исследованы структуры и фазовые превращения сплавов Гейслера системы Ni–Mn–Ga. Для исследования использовались сплавы Ni2,16Mn0,84Ga, Ni2,19Mn0,81Ga и Ni2,36Mn0,64Ga. Сплавы данной системы обладают рядом привлекательных свойств, которые связаны с термоупругим мартенситным превращением и магнитными свойствами, присущими этим сплавам. The structures and phase transformations of Heusler alloys of the Ni–Mn–Ga system were investigated. For the study, we used the alloys Ni2,16Mn0,84Ga, Ni2,19Mn0,81Ga, and Ni2,36Mn0,64Ga. Alloys of this system have a number of attractive properties that are associated with thermoelastic martensitic transformation and magnetic properties inherent in these alloys. Работа выполнена при финансированной поддержке Рроссийского научного фонда по гранту № 19-72-20080. Автор признателен профессору В.В. Ховайло за образцы для исследований и проф. А.М. Балагурову за возможность их исследования на фурье-дифрактометре ibr2 в Объединенном Институте Ядерных Исследований (ОИЯИ) в г. Дубна. Автор выражает благодарность научному руководителю — доктору физико-математических наук, профессору И.С. Головину. The work was carried out with the funded support of the Russian Science Foundation under grant No. 19-72-20080. The author is grateful to prof. V.V. Khovailo for the samples for research and prof. A.M. Balagurov for the opportunity to study them on the ibr2 Fourier diffractometer at the Joint Institute for Nuclear Research (JINR) in Dubna. The author expresses gratitude to the scientific advisor — doctor of physical and mathematical sciences, professor I.S. Golovin.

Structural, electronic, and magnetic properties of Mn2Co1-xVxZ (Z = Ga, Al, x = 0, 0.25, 0.5, 0.75, 1) Heusler alloys were theoretically investigated for the case of L21 (space group Fm3¯m), L21b (L21 structure with partial disordering between Co and Mn atoms) and XA (space group F4¯3m) structures. It was found that the XA structure is more stable at low V concentrations, while the L21 structure is energetically favorable at high V concentrations. A transition from L21 to XA ordering occurs near x = 0.5, which qualitatively agrees with the experimental results. Comparison of the energies of the L21b and XA structures leads to the fact that the phase transition between these structures occurs at x = 0.25, which is in excellent agreement with the experimental data. The lattice parameters linearly change as x grows. For the L21 structure, a slight decrease in the lattice constant a was observed, while for the XA structure, an increase in a was found. The experimentally observed nonlinear behavior of the lattice parameters with a change in the V content is most likely a manifestation of the presence of a mixture of phases. Almost complete compensation of the magnetic moment was achieved for the Mn2Co1-xVxZ alloy (Z = Ga, Al) at x = 0.5 for XA ordering. In the case of the L21 ordering, it is necessary to consider a partial disorder of atoms in the Mn and Co sublattices in order to achieve compensation of the magnetic moment.

International audience; The family of Co-based Heusler compounds contains promising candidates for spintronic applications regarding their predicted Half-Metal-Magnetic nature, ultra-low magnetic damping coefficients, high curie temperatures and tunable electronic properties. Here we focused on the transport properties of Co2MnSi thin films with thickness in the range of 4-44 nm exhibiting magnetic damping in the 10-4 range. The goals of this study are to examine the impact of the peculiar electronic band structure on the transport properties, to identify the temperature-dependent scattering process, and to extract robust conduction parameters to exploit this material in magnetoelectric devices. In order to undoubtedly correlate all results, the full study has been performed on the same series of samples. Scanning transmission electron microscopy experiments were performed to check the chemically-ordered L21 phase in our films, and also allowed us to identify misfit dislocations generated at the interface with the substrate. The variation of the resistivity with film thickness was measured at different temperatures. The results are examined under the Fuchs and Sondheimer model which allowed us to extract the electron mean free path in Co2MnSi in the temperature range 5-300 K. Values for the residual resistivity, Debye temperature, and distance between the Fermi energy and the conduction band for minority spins were obtained from the fit of the resistivity versus temperatures curves. A negative AMR ratio was measured for all the samples which confirmed the Half-metallic nature of the Co2MnSi films. The determination of the ordinary Hall coefficient alloys allowed us to extract the carrier concentration and carrier mobility and their dependency on the temperature. Finally, scaling of the anomalous Hall coefficient with the longitudinal resistivity was performed indicating that skew scattering is the dominant temperature-dependent scattering mechanism in our films.

The electronic structure and metallicity of Si-doped Mn2VGe Heusler alloys within the first-principles density-functional theory framework are discussed. Mn2VGe is found to assume two stable structures at (cubic) lattice constants of 5.7 and 6.05 Å, which are well-separated in energy and correspond to half-metallic (low-spin) and metallic (high-spin) phases, respectively. Substitution of Ge by Si reduces the energy difference between these two phases, which become nearly degenerate at an Si concentration of ≈3.125 at% at zero temperature. The analysis shows that the switching between the high- and low-spin phases for this composition can be triggered via a pressure of 2.8 GPa at 300 K. Si-doped Mn2VGe can thus provide an ultrafast, low power, cost-effective materials platform for spintronics applications. Post-print / Final draft

The martensitic transition featuring the ternary Heusler alloy Ni2.09Mn0.91Ga was investigated by neutron diffraction. Differential scanning calorimetry indicated that structural transition starts at 230 K on cooling with a significant increase in the martensitic transformation onset compared to the classical Ni2MnGa. The low-temperature martensite presents the 5M type of modulated structure, and the structural analysis was performed by the application of the superspace approach. As already observed in Mn-rich modulated martensites, the periodical distortion corresponds to an incommensurate wave-like shift of the atomic layers. The symmetry of the modulated martensite at 220 K is orthorhombic with unit cell constants a = 4.2172(3) Å, b = 5.5482(2) Å, and c = 4.1899(2) Å; space group Immm(00γ)s00; and modulation vector q = γc* = 0.4226(5)c*. Considering the different neutron scattering lengths of the elements involved in this alloy, it was possible to ascertain that the chemical composition was Ni2.07Mn0.93Ga, close to the nominal formula. In order to characterize the martensitic transformation upon increasing the temperature, a series of neutron diffraction patterns was collected at different temperatures. The structural analysis indicated that the progressive change of the martensitic lattice is characterized by the exponential change of the c/a parameter approaching the limit value c/a = 1 of the cubic austenite.

We present a study of the annealing effect and its influence on magnetic and structural properties for a series of Heusler alloys Ni50−xCoxMn35.5In14.5 (x=0,3,5) prepared in ribbon form. We studied the morphology and composition using scanning electron microscopy (SEM) equipped with an X-ray microanalyzer (EDX). The magnetic properties were determined by two methods: electron magnetic resonance (EMR) and vibrating sample magetometer (VSM). We found that cobalt content in the annealed samples reveals an additional magnetic phase transition at lower temperatures.

Current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read sensors based on Heusler alloys are promising candidates for ultrahigh areal densities of magnetic data storage technology. In particular, the thickness of reader structures is one of the key factors for the development of practical CPP-GMR sensors. In this research, we studied the dependence of the free layer thickness on the stability of the Co2(Mn0.6Fe0.4)Ge Heusler-based CPP-GMR read head for an areal density of 1 Tb/in2, aiming to determine the appropriate layer thickness. The evaluations were done through simulations based on micromagnetic modelling. The reader stability indicators, including the magnetoresistance (MR) ratio, readback signal, dibit response asymmetry parameter, and power spectral density profile, were characterized and discussed. Our analysis demonstrates that the reader with a free layer thickness of 3 nm indicates the best stability performance for this particular head. A reasonably large MR ratio of 26% was obtained by the reader having this suitable layer thickness. The findings can be utilized to improve the design of the CPP-GMR reader for use in ultrahigh magnetic recording densities.

Current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) read sensors based on Heusler alloys are promising candidates for ultrahigh areal densities of magnetic data storage technology. In particular, the thickness of reader structures is one of the key factors for the development of practical CPP-GMR sensors. In this research, we studied the dependence of the free layer thickness on the stability of the Co2(Mn0.6Fe0.4)Ge Heusler-based CPP-GMR read head for an areal density of 1 Tb/in2, aiming to determine the appropriate layer thickness. The evaluations were done through simulations based on micromagnetic modelling. The reader stability indicators, including the magnetoresistance (MR) ratio, readback signal, dibit response asymmetry parameter, and power spectral density profile, were characterized and discussed. Our analysis demonstrates that the reader with a free layer thickness of 3 nm indicates the best stability performance for this particular head. A reasonably large MR ratio of 26% was obtained by the reader having this suitable layer thickness. The findings can be utilized to improve the design of the CPP-GMR reader for use in ultrahigh magnetic recording densities.

First-principles calculations suggest that C1b-ordered FeMnZ (Z = Al, Ga, In and Al0.5Ga0.5) alloys are half-metallic fully compensated ferrimagnets with a spin-gapless semiconducting nature. In such materials, full spin polarization, spin-gapless properties and absence of external stray fields, thus exhibiting minimal energy losses, are expected., Materials with full spin polarization that exhibit zero net magnetization attract great scientific interest because of their potential applications in spintronics. Here, the structural, magnetic and electronic properties of a C1b-ordered FeMnGa alloy are reported using first-principles calculations. The results indicate that the corresponding band structure exhibits a considerable gap in one of the spin channels and a zero gap in the other thus allowing for high mobility of fully spin-polarized carriers. The localized magnetic moments of Fe and Mn atoms have an antiparallel arrangement leading to fully compensated ferrimagnetism, which possesses broken magnetic inversion symmetry. Such magnetic systems do not produce dipole fields and are extremely stable against external magnetic fields. Therefore, this will improve the performance of spintronic devices. Using this principle, similar band dispersion and compensated magnetic moments were predicted in a C1b-ordered FeMnAl0.5In0.5 Heusler alloy.

In this study, the Co2FeX (X=Ge, Si) Heusler compounds with 30 valence electrons, which are made by using mechanical alloying and arc melting methods were studied. The crystallization of samples was confirmed by XRD data in both manufacturing methods. The results showed that the presence of Si than Ge in the compound played a more effective role to creation a large scale atomic ordering, and the maximum saturation magnetization is related to one of the Co2FeSi samples equal to 5.24 . However, the amount obtained is still less than the predicted value by the Slater-Pauling due to this fact that the sample isn’t single crystalline. The microstructure analysis showed that Ge has been working more effective than Si on the enhancement of crystalline nucleation in the milling process. Although the crystallite growth in the annealing process is more rapid in the presence of Ge due to the lower melting point of Ge than Si. The great coercivity observed in the milled sample of Co2FeSi compound was related to the significant anisotropy caused by the atomic ordering and also the large volume of crystallite boundary as barriers into wall movement of domains.

In this study, the improvement of the magnetic properties of Co2FeSi Heusler compound was followed by the utilization of different experimental synthesizing procedures. Comparing the crystal structure showed that the milled samples had a higher crystalline order than the arc-melted ones. Annealing of the milled sample improved the crystalline order, resulting in the highest saturation magnetization (5/24 μB/F.u.). The difference in the saturation magnetization of the other samples was explained by the core-shell model. Comparison of the various coercivity mechanisms showed that the decrease in the size of crystallites played a key role in the higher value of the milled samples coercivity.

This paper contains a detailed study of low-field microwave absorption, which is observed in EMR spectra registered for a series of Ni50−xCoxMn35.5In14.5 (x=0,3,5) Heusler alloys polycrystalline in situ and annealed at 1173 K ribbons. The LFMA spectra for all ribbons were performed at X-band (∼9.5 GHz), at temperatures below Curie temperature. Additionally, for annealed Ni45Co5Mn35.5In14.5 ribbons, the LFMA signal dependencies of the external magnetic field modulation amplitude, modulation frequency, microwave power and microwave magnetic field phase were registered. These results confirm the resonant character of LFMA. To determine the basic EMR parameters, such as linewidth and resonance field, the experimental data were fitted by the Dyson function. The LFMA signal is satisfactorily matched by the two lines, and the variability of the component lines with temperature is remarkable.

In the search for new magnetic functional materials, non-stoichiometric compounds remain a relatively unexplored territory. While experimentalists create new compositions looking for improved functional properties, their work is not guided by systematic theoretical predictions. Being designed for perfect periodic crystals, the majority of first-principles approaches struggle with the concept of a non-stoichiometric system. In this work, we attempt a systematic computational study of magnetic and structural properties of Ni–Mn–Ga, mapped onto ternary composition diagrams. Compositional stability was examined using the convex hull analysis. We show that the cubic austenite has its stability region close to the stoichiometric Ni2MnGa, in agreement with experimental data, while the tetragonal martensite spreads its stability over a wider range of Mn and Ni contents. The unstable compositions in both austenite and martensite states are located in the Ga-rich corner of the ternary diagram. We note that simultaneous stability of the austenite and martensite should be considered for potentially stable compounds suitable for synthesis. The majority of compounds are predicted to be ferrimagnetically ordered in both austenitic and martensitic states. The methodology used in this work is computationally tractable, yet it delivers some predictive power. For experimentalists who plan to synthesize stable Ni–Mn–Ga compounds with ferromagnetic order, we narrow the target compositional range substantially.

The aim of the present work is to study the influence of a partial substitution of Mn by Zr in MnCoGe alloys. The X-ray diffraction (XRD) studies revealed a coexistence of the orthorhombic TiNiSi-type and hexagonal Ni2In- type phases. The Rietveld analysis showed that the changes in lattice constants and content of recognized phases depended on the Zr addition. The occurrence of structural transformation was detected. This transformation was confirmed by analysis of the temperature dependence of exponent n given in the relation ΔSM = C·(BMAX)n. A decrease of the Curie temperature with an increase of the Zr content in the alloy composition was detected. The magnetic entropy changes were 6.93, 13.42, 3.96, and 2.94 J/(kg K) for Mn0.97Zr0.03CoGe, Mn0.95Zr0.05CoGe, Mn0.93Zr0.07CoGe, and Mn0.9Zr0.1CoGe, respectively. A significant rise in the magnetic entropy change for samples doped by Zr (x = 0.05) was caused by structural transformation.

This article belongs to the Special Issue Magnetic and Magnetoelectric Nanomaterials: Synthesis, Characterization and Applications., Thin polycrystalline Co2FeGe films with composition close to stoichiometry have been fabricated using magnetron co-sputtering technique. Effects of substrate temperature (TS) and post-deposition annealing (Ta) on structure, static and dynamic magnetic properties were systematically studied. It is shown that elevated TS (Ta) promote formation of ordered L21 crystal structure. Variation of TS (Ta) allow modification of magnetic properties in a broad range. Saturation magnetization ~920 emu/cm3 and low magnetization damping parameter α ~ 0.004 were achieved for TS = 573 K. This in combination with soft ferromagnetic properties (coercivity below 6 Oe) makes the films attractive candidates for spin-transfer torque and magnonic devices., Portuguese team acknowledges Network of Extreme Conditions Laboratories-NECL and Portuguese Foundation of Science and Technology (FCT) support through the projects PTDC/FIS-MAC/31302/2017, NORTE-01-0145-FEDER-022096, EXPL/IF/00541/2015 (S.A.B.), SFRH/BPD/84948/2012 (A.V.) and SFRH/BPD/87430/2012 (A.A.). This work was partially supported by Spanish Ministerio de Economía y Competitividad through project MAT2017-82970-C2 and from regional Gobierno de Aragón through project E28 20R including FEDER funding.

Diese Arbeit befasst sich mit dem Wachstum von Dünnschichtstrukturen, die zur Herstellung eines Spin-selektiven Schottky-Barrier-Tunneltransistors (SS-SBTT) erforderlich sind. Das Bauelement basiert auf dem Transport von Ladungsträgern durch eine dünne halbleitende (SC) Schicht, die zwei ferromagnetische (FM) Kontakte trennt. Daher müssen hochqualitative und gitterangepasste vertikale FM/SC/FM-Trilayer gezüchtet werden, was aufgrund der inkompatiblen Kristallisationsenergien zwischen SC und Metallen eine experimentelle Herausforderung darstellt. Das Problem wurde mit einem Festphasenepitaxie-Ansatz gelöst, bei dem eine dünne amorphe Ge-Schicht (4-8 nm) durch Ausglühen über Fe3Si auf GaAs(001)-Substraten kristallisiert wird. Langsame Glühgeschwindigkeiten bis zu einer Temperatur von 260°C konnten ein neues gitterangepasstes Polymorph von FeGe2 erzeugen, über das ein zweites Fe3Si mittels Molekularstrahlepitaxie gezüchtet werden könnte. SQUID-Magnetometermessungen zeigen, dass die dreischichtigen Proben in antiparallele Magnetisierungszustände versetzt werden können. Vertikale Spin-Ventil-Bauelemente, die mit verschiedenen Trilayern hergestellt wurden, wurden verwendet, um zu demonstrieren, dass der Ladungstransport über die Heteroübergänge spinselektiv ist und bei Raumtemperatur einen Magnetowiderstand von höchstens 0,3% aufweist. Der Effekt nimmt bei niedrigen Temperaturen ab, was mit einem ferromagnetischen Übergang in der FeGe2-Schicht korreliert. Durch TEM- und XRD-Experimente konnte festgestellt werden, dass das neue FeGe2-Polymorph die Raumgruppe P4mm aufweist und bis zu 17% Si-Atome als Ersatz für Ge-Stellen enthält. Die Isolierung von FeGe2 war möglich, indem das Verhältnis von Fe-, Si- und Ge-Atomen so eingestellt wurde, dass die richtige Stöchiometrie bei vollständiger Durchmischung erreicht wurde. Anhand von FeGe2-Dünnschichten wurde ein zunehmender spezifischer Widerstand bei niedriger Temperatur und ein semi-metallischer Charakter beobachtet., This thesis discusses the growth of thin film structures required to fabricate a Spin-Selective Schottky Barrier Tunnel transistor (SS-SBTT). The device relies on charge carriers being transported through a thin semiconducting (SC) layer separating two ferromagnetic (FM) contacts. Thus, high quality and lattice-matched FM/SC/FM vertical trilayers must be grown, which is experimentally challenging due to incompatible crystallization energies between SC and metals. The problem was solved using a solid-phase epitaxy approach, whereby a thin amorphous layer of Ge (4-8 nm) is crystallized by annealing over Fe3Si on GaAs(001) substrates. Slow annealing rates up to a temperature of 260°C could produce a lattice-matched Ge-rich compound, over which a second Fe3Si could be grown my molecular-beam epitaxy. The compound obtained during annealing is a new layered polymorph of FeGe2. SQUID magnetometry measurements indicate that the trilayer samples can be placed in states of antiparallel magnetization. Vertical spin valve devices created using various trilayers were used to demonstrate that charge transport is spin-selective across the heterojunctions, showing a magnetoresistance of at most 0.3% at room temperature. The effect decreases at low temperature, correlating with a ferromagnetic transition in the FeGe2 layer. TEM and XRD experiments could determine that the new FeGe2 polymorph has a space group P4mm, containing up to 17% Si atoms substituting Ge sites. Isolating FeGe2 was possible by tuning the proportion Fe, Si and Ge atoms required to obtain the right stoichiometry upon full intermixing. Hall bars fabricated on FeGe2 thin films were used to observe an increasing resistivity at low temperature and semimetallic character.

Isothermal annealing of a eutectic dual phase Ni–Mn–Sn–Fe alloy was carried out to encourage grain growth and investigate the effects of grain size of the γ phase on the martensitic transformation behaviour and mechanical properties of the alloy. It is found that with the increase of the annealing time, the grain size and volume fraction of the γ phase both increased with the annealing time predominantly by the inter-diffusion of Fe and Sn elements between the γ phase and the Heusler matrix. The isothermal anneals resulted in the decrease of the e/a ratio and suppression of the martensitic transformation of the matrix phase. The fine γ phase microstructure with an average grain size of 0.31 μm showed higher fracture strength and ductility values by 28% and 77% compared to the coarse-grained counterpart with an average grain size of 3.31 μm. The fine dual phase microstructure shows a quasi-linear superelasticity of 4.2% and very small stress hysteresis during cyclic loading, while the coarse dual phase counterpart presents degraded superelasticity of 2.6% and large stress hysteresis. These findings indicate that grain size refinement of the γ phase is an effective approach in improving the mechanical and transformation properties of dual phase Heusler alloys.

Determining and acting on thermo-physical properties at the nanoscale is essential for understanding/managing heat distribution in micro/nanostructured materials and miniaturized devices. Adequate thermal nano-characterization techniques are required to control device thermal behavior and address thermal issues deterrent to their desired performance. In this context, Scanning Thermal Microscopy (SThM) is a high spatial resolution probing and acting technique based on Atomic Force Microscopy (AFM) using a nano-probe as thermometer and resistive heater. Enabling direct observation and mapping of thermal properties, it is becoming a powerful tool in several fields, from material science to device thermal management. The mechanisms driving heat transfer at the nano/microscopic-scale are essential for device development and optimization, particularly when using first-order phase transition materials exhibiting coupled structural and ferrocaloric effects, as Ni-Mn-Ga, BaTiO3 and Ni-Ti (magnetocaloric, electrocaloric, elastocaloric), researched for multifunctional actuating, sensing and solid-state heat-management technologies (cooling) applications. This work aims to raise the understanding of the interplay of these effects by local probing of thermal properties, combined with magnetic, structural, and electric studies: - local thermomagnetic, thermoelectric and thermoelastic phenomena by scanning thermal microscopy (SThM) and microscopic-infrared-thermography - local actuation by heating performed by SThM-tip to evaluate thermal conductivity, phase change contrast and map sample dynamic inhomogeneities. - time-dependence of local thermal phenomena correlated with thermal diffusion processes across phase transformations for Ni-Mn-Ga, BaTiO3 and Ni-Ti. - local probing of “thermal return-point-memory” and avalanche effects on Ni-Mn- Ga films produced by temperature cycling. One of our most significant results consisted in finding that locally inducing the structural transformation in Ni-Mn-Ga thin films results in much wider martensitic transformation temperatures (TM) intervals than for overall structural transformation inducing, within a same micrometric area of the material. We also observed large differences in the behavior of the magnetocaloric effect in bulk Co-doped Ni-Mn-Ga on heating and cooling, at the microscopic scale, in the vicinity of the structural transformation. Mapping of the contribution of each microscopic area of the sample to the effect was achieved. Determinar e atuar nas propriedades termofísicas à nanoescala é essencial para compreender/gerir a distribuição térmica de calor em materiais e dispositivos miniaturizados. Técnicas de nanocaracterização térmica são necessárias para analisar e controlar o comportamento térmico de dispositivos, evitando prejuízos à performance desejada. Neste contexto, a microscopia de varrimento térmico (SThM) é uma técnica de sondagem e atuação de elevada resolução espacial baseada na microscopia de força atómica (AFM), utilizando uma nano-sonda como termómetro e elemento resistivo de aquecimento. Permitindo a observação direta e o mapeamento de propriedades térmicas, esta é uma ferramenta poderosa em várias áreas, desde a ciência de materiais até à gestão térmica em dispositivos. Os mecanismos responsáveis pela transferência de calor à nano/microescala são essenciais para desenvolver e otimizar dispositivos, particularmente aquando da utilização de materiais que sofrem transições de fase de primeira ordem, que exibem efeitos ferrocalóricos (magnetocalórico, eletrocalórico e elastocalórico) e estruturais acoplados, tais como o Ni-Mn-Ga, o BaTiO3 e o Ni-Ti, investigados para aplicações de atuação, deteção e tecnologias de gestão térmica em estado sólido (arrefecimento). Este trabalho pretende aumentar a compreensão da interação destes efeitos através da sondagem local das propriedades térmicas, combinada com estudos magnéticos, eletricos e estruturais: - fenómenos termomagnéticos, termoeletricos e termoelasticos locais por microscópio de varrimento térmico (SThM) e termografia de infravermelhos. - atuação local por aquecimento levada a cabo pela sonda SThM para avaliar condutividade térmica, contraste de mudança de fase e mapeamento de inomogeneidades dinâmicas de amostras. - dependência temporal de fenómenos térmicos locais correlacionados com processos de difusão térmica através de transformações de fase para o Ni-Mn- Ga, o BaTiO3 e o Ni-Ti. - sondagem local de “memória de ponto de retorno térmica” e efeitos de avalanche em filmes de Ni-Mn-Ga provocados por ciclagens térmicas. Um dos resultados mais significativos consistiu na constatação de que a indução local da transformação estrutural em filmes finos de Ni-Mn-Ga resulta em intervalos muito mais alargados de temperaturas de transformação martensítica do que no caso de indução global da transformação estrutural, numa mesma área microscópica do material. Observámos ainda grandes diferenças no comportamento do efeito magnetocalórico no Ni-Mn-Ga dopado com Co, em aquecimento e arrefecimento, à escala microscópica, na vizinhança da transformação estrutural. Foi realizado com sucesso o mapeamento da contribuição para o efeito de cada área microscópica da amostra. Programa Doutoral em Engenharia Física

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Parameters of the energy gap and, consequently, electronic, magnetic and galvanomagnetic properties in different X$_2$YZ Heusler alloys can vary quite strongly. In particular, half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) with almost 100% spin polarization of charge carriers are promising materials for spintronics. The changes in the electrical, magnetic and galvanomagnetic properties of the Co$_2$YSi (Y = Ti, V, Cr, Mn, Fe) and Co$_2$MnZ Heusler alloys (Z = Al, Si, Ga, Ge) in possible HMF and/or SGS states were followed and their interconnection was established. Significant changes in the values of the magnetization and residual resistivity were found. At the same time, the correlations between the changes in these electronic and magnetic characteristics depending on the number of valence electrons and spin polarization are observed., 10 pages

Shape memory alloys witch exhibit magnetically induced phase transformations at room temperatures are the most interesting for magnetic cooling applications. Heusler type alloys (both stoichiometric and non-stoichiometric) are among the most studied systems, as they permit the change of the transition temperatures due to compositional variations. The first Heusler alloys that have been studied in depth are Ni- Mn-Ga alloys. However, to overcome the high cost of Gallium and low martensitic transformation temperature, the search for Ga-free alloys has been recently endeavoured, principally, by introducing In, Sn or Sb. The purpose of this work is the production and characterization of non-stoichiometric Heusler alloys based on a Ni-Mn-Sn composition, with copper or Palladium doping. The effect of doping element will be determined, on the crystalline structure, the phase transition temperatures, the thermodynamic and magnetocaloric parameters. On the other hand, a heat treatment will be exerted on Ni-Mn-Sn-Cu alloys then characterize them in the same way. Shape memory alloys will be produced in the first step, by arc melting technique, to produce the bulk and then by melt spinning, to obtain shape memory ribbons. Produced samples are characterized by scanning electron microscopy (SEM) technique, to check the morphological structure of the alloys, microanalysis technique (EDX) to have the final composition experimentally, X-ray diffraction technique (XRD) to extract information from the crystal structure, differential scanning calorimetry (DSC) to study the thermal and thermodynamic variations induced by structural phase transformations and finally magnetometry techniques (Physical property measurement system, PPMS: VSM, resistivity, hysteresis cycles) to characterize the phase transformations behaviour and magnetocaloric effect under applied external magnetic fields. Among the results obtained in the present work it can be concluded that both Cu-doped and Pd-doped alloys have similar morphology. The reversible austenite-martensite transformation was detected in all ribbons. Cu or Pd doping tends to increase the structural transition temperatures. These changes in the magnetic and martensitic transformation temperatures are confirmed report directly to the ratio (e/a) of the alloy. Moreover, it’s emphasized the fact that Pd doped alloys show a good magnetocaloric effect (the maximum variation of ΔS is approximately 4.5 J / (Kg.K) for an applied magnetic field of 50 kOe and the refrigeration capacity is 28 J / kg). This dependence must allow the selection of the appropriate composition for the production of alloys with transformation into a desired temperature range. On the other hand, measurements of DRX, DSC and magnometry carried out on Ni-Mn-Sn-Cu alloys after application of thermal cycles of heating-cooling of 100 times, allowed us to certify the great stability of the alloys and the shape memory effect (minor change in phase transformation temperatures and curie temperatures) Els aliatges tipus Heusler (tant estequiomètrics com no estequiomètrics) es troben entre els sistemes més estudiats, ja que permeten el canvi de les temperatures de transició a causa de les variacions de la composició. Els primers aliatges de Heusler que s’han estudiat en profunditat són els aliatges de Ni-Mn-Ga. No obstant això, per superar l’elevat cost del gal i la baixa temperatura de transformació martensítica, recentment s’ha intentat buscar aliatges sense Ga, principalment, introduint In, Sn o Sb. L’objectiu d’aquest treball és la producció i caracterització d’aliatges de Heusler no estequiomètrics a partir d’una composició de Ni-Mn-Sn, amb dopatge de cobalt o pal·ladi. L'efecte de l'element dopant es determinarà, sobre l'estructura cristal·lina, les temperatures de transició de fase, els paràmetres termodinàmics i magnetocalòrics. D’altra banda, s’exercirà un tractament tèrmic sobre els aliatges de Ni-Mn-Sn-Cu per després caracteritzar-los de la mateixa manera. Els aliatges de memòria de forma es produiran en el primer pas, mitjançant la tècnica de la fusió de l’arc, per produir l’aliatge massiu i després per la fusió per obtenir cintes de memòria de forma. Les mostres produïdes es caracteritzen per la tècnica de microscòpia electrònica d’escombratge (SEM), per comprovar l’estructura morfològica dels aliatges, la tècnica de microanàlisi (EDX) per tenir la composició final de manera experimental, la tècnica de difracció de raigs X (XRD) per extreure informació de l’estructura cristal·lina, calorimetria d’escaneig diferencial (DSC) per estudiar les variacions tèrmiques i termodinàmiques induïdes per transformacions de fase estructurals i finalment tècniques de magnetometria (PPMS: VSM, resistivitat, cicles d’histèresi) per caracteritzar el comportament de les transformacions de fase i l’efecte magnetocalòric sota camps magnètics externs aplicats. Entre els resultats obtinguts en el present treball es pot concloure que tant els aliatges dopats amb Cu com els aliatges dopats amb Pd tenen una morfologia similar. La transformació reversible d'austenita-martensita es va detectar a totes les cintes. El dopatge Cu o Pd tendeix a augmentar les temperatures de transició estructural. Aquests canvis en les temperatures de transformació magnètica i martensítica es confirmen directament a la relació (e / a) de l'aliatge. A més, s’ha destacat el fet que els aliatges dopats amb Pd mostren un bon efecte magnetocalòric. Aquesta dependència ha de permetre seleccionar la composició adequada per a la producció d'aliatges amb transformació en un rang de temperatura desitjat. D’altra banda, les mesures de DRX, DSC i magnometria realitzades sobre aliatges Ni-Mn-Sn-Cu després de l’aplicació de cicles tèrmics de calefacció-refrigeració de 100 vegades, ens van permetre certificar la gran estabilitat dels aliatges i la efecte memòria de forma Programa de Doctorat en Tecnologia