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Abstract: We have studied the thermal stability of L10 FePt thin films and Fe–FePt exchange-spring (ES) bilayers grown on (100) MgO by RF sputtering. The viscosity curves showed both for FePt films and bilayers a clear logarithmic decay of magnetization. Moreover, it was possible to evaluate the viscosity coefficient S for different applied reverse fields and the activation volumes at the coercivity. The latter values were then related to structural, magnetic and morphological measurements performed on the samples. [Copyright &y& Elsevier]

Presents a systematic study of the effects of cobalt substitution on the crystal structure and magnetic anisotropy of Y[sub2]Fe[sub14-x]Co[subx]B and Nd[sub2] Fe[sub14-x] Co[subx]B to clarify the different roles and contributions to the anisotropy of both rare earth and transition metals. Preparation of the polycrystalline samples; Measurement of the effects of cobalt substitution on the tetragonal unit cell parameters.

A study of temperature and composition dependence of the magnetic anisotropy in the tetragonal (Ho[SUBx]Er[SUB1-x])Fe[SUB10]V[SUB2] polycrystalline compounds was made by means of the singular point detection technique. At 293 K the easy magnetization direction lies along the tetragonal c axis [001] in all the samples and the anisotropy field increases linearly with increasing Ho content. However, with decreasing temperature the competition among the anisotropy of the different magnetic sublattices gives rise to a spin reorientation transition axis to cone in Er containing samples. The T[SUBSR] value increases with increasing Er content. A significant in-plane anisotropy was found in all the compounds. This planar anisotropy is larger in HoFe[SUB10]V[SUB2], where it favors the [100] direction of the basal plane, while in Er Fe[SUB10]V[SUB2] the [110] is the preferred direction. The difference between the anisotropy field along [100] and [110] directions was found to decrease with increasing Er content. A first-order magnetization process of type 1 was observed below 120 K in all the compounds when the magnetic field is applied along the [100] direction. [ABSTRACT FROM AUTHOR]

A systematic study of the magnetocaloric effect in La0.7Ca0.15Sr0.15MnO3 perovskite has been performed by both direct and indirect measurement techniques. In particular, the adiabatic temperature change has been measured by a purpose-built probe suitable for laboratory-scale samples, while magnetic measurements were carried out by standard magnetometric techniques. The observation of the effective behavior of the studied systems in adiabatic conditions and the significant value of the isothermal entropy variation indicate that La0.7Ca0.15Sr0.15MnO3 could be considered as a potential candidate for application in magnetic refrigeration around room temperature. Moreover, the critical exponents (, and ) determined using the Kouvel-Fisher method, the modified Arrott plot and the critical isotherm analysis are in good agreement with the theoretical prediction of the 3D-Ising model, indicating that the magnetic interactions are of short-range nature. [ABSTRACT FROM AUTHOR]

Cobalt/iron multilayers with different layer thickness were electron beam evaporated in ultrahigh vacuum, keeping constant both the number of layers and the Co/Fe thickness ratio. Structural and magnetic properties are thoroughly investigated with different techniques. The multilayers have clean and sharp interfaces. All samples show single-phase magnetic behavior in the temperature range of 5–300 K, due to the strong exchange coupling between the layers. The room temperature hysteresis loops present a sharp switching at a field of ∼8 kA/m, followed by a residual hysteresis extending up to saturation, at fields of several tens of kA/m. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigated the time dependence of the magnetic configuration at the mixed magnetic magnetostructural transition in Mn3SnC. The nonergodic nature of the transition involves the stabilization of a final magnetic configuration that involves additional AF ordering which is not present when the transition is initiated and develops only in time. We show the presence of the nonergodicity over a time scale of about 1 hour by field and time-dependent magnetization studies. Two characteristic times related to the transition are observed. We also study the equilibrium thermodynamics under ergodic conditions by heat capacity studies and determine the entropy-change and the adiabatic temperature change around the transition. We find agreement between the indirect and direct methods in determining the adiabatic temperature change and discuss the influence of nonergodic properties on the magnetocaloric effects. [ABSTRACT FROM AUTHOR]

La 0.7 (Ca,Sr) 0.3 Mn 1−x Gd x O 3 (with x = 0; x = 2% and x = 6%) manganites samples have been studied by several techniques including X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), 55 Mn and 139 La nuclear magnetic resonance (NMR) spectroscopy and magnetometric measurements. The X-ray powder diffraction shows that all the prepared samples are single phase. La 0.7 Ca 0.15 Sr 0.15 MnO 3 manganite is crystallized in the rhombohedral structure, whereas a structural transition towards orthorhombic system is observed for x ≥ 2%. Magnetization as a function of temperature shows that all samples exhibit a ferromagnetic (FM)-paramagnetic (PM) phase transition. Gadolinium doping in Manganese site was found to induce a strong initial decrease of the Curie temperature T C . The single double-exchange-narowed 55 Mn NMR line, though broadened by Gd, demonstrates a metallic state in all samples. The current manganites exhibit a second-order magnetic phase transition at T C , which is also confirmed by the criterion of Banerjee. In addition, the normalized entropy change curves of all the compounds collapse onto a universal master curve, a further signature of a second-order transition. The Gd-doped compounds undergo a large magnetocaloric effect, with an enhanced width T FWHM of the entropy change peak as compared to the Gd-free materials, and have consequently potential applications in magnetic refrigeration. [ABSTRACT FROM AUTHOR]

We report a complete structural and magnetothermodynamic characterization of four samples of the Heusler alloy Ni-Co-Mn-Ga-In, characterized by similar compositions, critical temperatures and high inverse magnetocaloric effect across their metamagnetic transformation, but different transition widths. The object of this study is precisely the sharpness of the martensitic transformation, which plays a key role in the effective use of materials and which has its origin in both intrinsic and extrinsic effects. The influence of the transition width on the magnetocaloric properties has been evaluated by exploiting a phenomenological model of the transformation built through geometrical considerations on the entropy versus temperature curves. A clear result is that a large temperature span of the transformation is unfavourable to the magnetocaloric performance of a material, reducing both isothermal entropy change and adiabatic temperature change obtainable in a given magnetic field and increasing the value of the maximum field needed to fully induce the transformation. The model, which is based on standard magnetometric and conventional calorimetric measurements, turns out to be a convenient tool for the determination of the optimum values of transformation temperature span in a trade-off between sheer performance and amplitude of the operating range of a material. [ABSTRACT FROM AUTHOR]

We compare the magnetocaloric effect of samples prepared with different thermal conductivities to investigate the potential of composite materials. By applying the magnetic field under operating conditions we test the material's response and compare this to heat transfer simulations in order to check the reliability of the adiabatic temperature change probe used. As a result of this study we highlight how the material's thermal conductivity influences τ , the time constant of temperature change. This parameter ultimately limits the maximum frequency of a refrigerant cycle and offers fundamental information about the correlation between thermal conductivity and the magnetocaloric effect. [ABSTRACT FROM AUTHOR]

We report on the synthesis and characterization of BiFe0.5Mn0.5O3, a potential type-I multiferroic compound displaying temperature-induced magnetization reversal. Bulk samples were obtained by means of solid-state reaction carried out under the application of hydrostatic pressure of 6 GPa at 1100 °C. The crystal structure is a highly distorted perovskite with no cation order on the B site, where, besides a complex scheme of tilt and rotations of the TM-O6 octahedra, large off-centering of the bismuth ions is detected. Below T1 = 420 K the compound undergoes a first weak ferromagnetic transition related to the ordering of iron-rich clusters. At lower temperatures (just below RT) a complex thermally activated mechanism induces at first an enhancement of the magnetization at T2 = 288 K, then a spontaneous reversal giving rise to a negative response. The complementary use of powder neutron diffraction, superconducting quantum interference device magnetometry, and Mössbauer spectroscopy allowed us to propose as a possible interpretation of the overall magnetic behavior the presence of an uncompensated competitive coupling between nonequivalent clusters of weakly ferromagnetic interactions characterized by different critical temperatures and resultant magnetizations. [ABSTRACT FROM AUTHOR]

Abstract: We report on the optimisation of the growth conditions of manganite La0.7Sr0.3MnO3 thin films prepared by Channel Spark Ablation (CSA). CSA belongs to pulsed electron deposition methods and its energetic and deposition parameters are quite similar to those of pulsed laser deposition. The method has been already proven to provide manganite films with good magnetic properties, but the films were generally relatively rough (a few nm coarseness). Here we show that increasing the oxygen deposition pressure with respect to previously used regimes, reduces the surface roughness down to unit cell size while maintaining a robust magnetism. We analyse in detail the effect of other deposition parameters, like accelerating voltage, discharging energy, chamber pressure and substrate temperature and provide on this basis a set of optimal conditions for the growth of atomically flat films. The thicknesses for which atomically flat surface was achieved is as high as about 10–20nm, corresponding to films with room temperature magnetism. We believe such magnetic layers represent appealing and suitable electrodes for various spintronic devices. [Copyright &y& Elsevier]

Abstract: We report the magnetic and magnetocaloric properties in Co-doped Ni–Mn–Ga Heusler alloys around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the magnetocaloric effect, e.g. the saturation magnetization jump at the transformation (ΔM) and the field dependence of the transformation temperature (dT/dH) are reported, together with high positive values of the isothermal magnetic entropy change (ΔS). The conditions for enhancing the magnetocaloric properties and triggering the sign reversal of the magnetocaloric effect are discussed. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic properties and the magnetization process of electrodeposited thick films of Co-rich CoPt alloys are studied with particular emphasis on the effects of growth rate, controlled by varying the plating current density, and of lateral confinement, analyzing patterned micro-cylinders. We find that varying the plating current density has virtually no effect on the composition of the samples, and hence on the intrinsic magnetic properties, a substantial increase of both coercivity and squareness is obtained when the current is raised. The films are fine-grained, oriented polycrystals with typical grain sizes in the range 50–150nm, depending on the growth rate. The complex magnetization process is studied in detail by Magnetic Force Microscopy and shown to be governed by interaction domains. It is shown that further improvement of the squareness can be obtained by exploiting the lateral confinement in patterned samples. [Copyright &y& Elsevier]

Abstract: A thorough micromagnetic analysis of the exchange-spring problem is reported with special emphasis on multilayers constituted by hard–soft exchange-coupled phases. The developed one—dimensional micromagnetic model leads to a complete magnetic phase diagram in terms of layer thicknesses. Both perpendicular and parallel configurations are considered. The phase diagram provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place, depending on the intrinsic properties of the chosen soft and hard materials. The model has been applied to a variety of hard magnetic phases (e.g., FePt, CoPt, SmCo and NdFeB), coupled to different soft materials (e.g., Fe, FeCo, FeRh or permalloy) both in the form of bilayers and multilayers. The most significant results will be highlighted. [Copyright &y& Elsevier]

Abstract: La0.7Sr0.3MnO3 polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiO x amorphous native oxide. Curie temperatures of about 325K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiO x and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed. [Copyright &y& Elsevier]

Abstract: Single element Co and Fe thin films and Co/Fe/Co trilayers with constant Co-layer thickness were e-beam evaporated in UHV. The samples show a nano-crystalline nature. The trilayers display a rigid composite magnet behavior, i.e., a single-phase-like hysteresis loop. The Co single film and the Co/Fe/Co trilayers show a well-defined uniaxial in-plane anisotropy. In the trilayers, anisotropy is enhanced on increasing Fe-layer thickness. This phenomenon is understood in terms of competing surface and volume contributions from Co/Fe interface and Co magnetocrystalline anisotropy. [Copyright &y& Elsevier]

Abstract: Electroluminescence in organic semiconductors strongly depends on the relative population of singlet and triplet excitonic states, i.e. on the carrier spin statistics. In conventional organic light-emitting diodes (OLED) the optical emission is usually based on fluorescence from excited singlet states, while triplet states provide phosphorescent radiation. Radiative emission from singlet excitons has a very large transition probability providing the main radiative pathway. Quantum constraints determine the statistics of singlet–triplet exciton formation from charge recombination to a 1:3 partition. Controlling the spin statistics by injecting carriers with the desired spin polarisation would open the way to enhance a chosen electronic transition and therefore increase the device efficiency. We show that spin polarised materials can successfully replace conventional electrodes in OLEDs. Electrical and optical characterisations of Alq3/TPD based OLEDs for both normal and spin polarised electrodes are presented. Epitaxial thin films of the manganite La0.7Sr0.3MnO3 were used as spin polarised hole injectors, while iron and cobalt films were used as spin polarised electron injectors. The results are a first step towards the fabrication of devices where the light emission can be tuned by controlling the spin injection. [Copyright &y& Elsevier]

The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, superconducting quantum interference device magnetometry, and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. 59Co NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with 'defective' cobalt favoring growth of 'bulk' cobalt with good magnetic properties. [ABSTRACT FROM AUTHOR]

The magnetisation process has been investigated in thin Ni sheets by means of fluxmetric and alternating force techniques. As-rolled samples exhibit a rotation-dominated reversible magnetisation process, related to stress-induced and magnetocrystalline anisotropies. After annealing the rotations combine with substantial reversible displacements of the domain walls. It is always found that the transverse reversible susceptibility passes through a maximum at a field of the order of the anisotropy field. [Copyright &y& Elsevier]

Co/Fe multilayers with constant Co layer thickness (tCo=5 nm) and Fe layer thickness (tFe) in the range 5–10 nm were electron beam evaporated in ultra-high vacuum. For tFe <10 nm the films showed an in-plane uniaxial magnetic anisotropy. For tFe=10 nm the uniaxial anisotropy lowered and stripe domains formed. The role of temperature and Fe layer thickness was investigated. The observed phenomena can be ascribed to surface anisotropy. [Copyright &y& Elsevier]

Abstract: Magnetic and morphological properties of Cobalt thin films grown by RF sputtering on organic Alq3 layers were investigated by magneto-optical Kerr effect (MOKE) technique and atomic force microscopy (AFM). The AFM images indicate a template growth of Co layers on top of Alq3, the magnetic film “decorates” the surface of organic material. This peculiar morphology induces a strong uniaxial magnetic anisotropy in the Co films, as detected by MOKE measurements. Results are important for the operation of a new class of devices-vertical organic spin valves. [Copyright &y& Elsevier]

Abstract: The switching process of electrodeposited Co-rich Co–Pt thin films with perpendicular magnetic anisotropy is investigated by out-of-plane angle-dependent hysteresis loop measurements. The switching field angular dependence is discussed in terms of basic reversal mechanisms. A model is proposed, based on a two-step switching process, to evaluate the variations of the intensity and orientation of the internal field as the modulus of external magnetic field is varied at each angle φ. Several experimentally observed salient features are well-understood, indicating that switching is due to inverse domain propagation. [Copyright &y& Elsevier]

Abstract: Ultrathin ferromagnetic films of La0.7Sr0.3MnO3 were deposited by electron-beam ablation on standard SrTiO3(100) and NdGaO3(110) substrates. Even 4nm films are metallic and develop spontaneous magnetization at room temperature: similar hysteresis cycles were measured by magneto optical Kerr effect (MOKE) in the longitudinal configuaration and by SQUID magnetometry. To the best of our knowledge the 4nm thickness corresponds so far to the thinnest manganite film showing room-temperature ferromagnetism. [Copyright &y& Elsevier]

Multiferroics, showing the coexistence of two or more ferroic orderings at room temperature, could harness a revolution in multifunctional devices. However, most of the multiferroic compounds known to date are not magnetically and electrically ordered at ambient conditions, so the discovery of new materials is pivotal to allow the development of the field. In this work, we show that BaFe 2 O 4 is a previously unrecognized room temperature multiferroic. X-ray and neutron diffraction allowed to reveal the polar crystal structure of the compound as well as its antiferromagnetic behavior, confirmed by bulk magnetometry characterizations. Piezo force microscopy and electrical measurements show the polarization to be switchable by the application of an external field, while symmetry analysis and calculations based on density functional theory reveal the improper nature of the ferroelectric component. Considering the present findings, we propose BaFe 2 O 4 as a Bi- and Pb-free model for the search of new advanced multiferroic materials., (© 2022. The Author(s).)

Environmental monitoring in Northern Italy, one of the most polluted areas in Europe, is of paramount importance. Leaf monitoring throughout magnetic and scanning electron microscopy (SEM-EDS) analysis could be considered a good complementary analysis to sampling stations, but the lack of evergreen plants in the northern Italy towns may hinder magnetic leaf analysis in the winter season. Therefore, we tested three species of urban vegetation, which are evergreen and commonly found in urban environment, namely Hedera helix L., Parietaria officinalis L. and Rubus caesius L. Magnetic susceptibility, chosen as a simple parameter suitable for monitoring, was measured in seven stations, during the period 25 January 2019 to 8 March 2019 at a weekly step, in the cities of Torino and Parma in the same days. P. officinalis and R. caesius showed the best response, but also H. helix was suitable to detect highly polluted areas. In Torino, the magnetic susceptibility decreased in the last sampling, together with PM10, whereas in Parma it increased, likely for the beginning of the academic period in the University Campus. SEM-EDS analysis was done comparing leaves from the same plant sampled in February 2019, in highly polluted conditions, and in May 2020, after 2 months of very limited traffic, due to national lockdown. Silicate grains of natural minerals, sized between 10 and 20 µm, are present in both samples, whereas Fe oxides, about one micron size, possibly coming from car brake consumption, are prominent in the February 2019 sample. Magnetic susceptibility of leaves form the examined species looks promising to spot urban sites with high metal pollution., (© 2022. The Author(s).)

The stoichiometric Ni 50 Mn 25 In 25 Heusler alloy transforms from a stable ferromagnetic austenitic ground state to an incommensurate modulated martensitic ground state with a progressive replacement of In with Mn without any pre-transition phases. The absence of pre-transition phases like strain glass in Ni 50 Mn 25+ x In 25- x alloys is explained to be the ability of the ferromagnetic cubic structure to accommodate the lattice strain caused by atomic size differences of In and Mn atoms. Beyond the critical value of x = 8.75, the alloys undergo martensitic transformation despite the formation of ferromagnetic and antiferromagnetic clusters and the appearance of a super spin glass state., (© 2020 IOP Publishing Ltd.)

The heterobimetallic metallacrown (MC), (TMA) 2 {Mn(OAc) 2 [12-MC Mn(III)Cu(II)N(shi) -4](CH 3 OH)}·2.90CH 3 OH, 1 , where TMA + is tetramethylammonium, - OAc is acetate, and shi 3- is salicylhydroximate, consists of a Mn II ion captured in the central cavity and alternating unambiguous and ordered manganese(III) and copper(II) sites about the MC ring, a first for the archetypal MC structure design. DC-magnetometry characterization and subsequent simulation with the Spin Hamiltonian H = - J 1 ( s 1 + s 3 )· s 5 - J 2 ( s 2 + s 4 )· s 5 - J 3 Σ i=1 4 s i · s i +1 + d ( s z ,1 2 + s z ,3 2 ) + μ B Σ j=1 5 g j s j · B indicates an S = 5/2 ground state and a sizable axial zero-field splitting on Mn III . AC-susceptibility measurements reveal that 1 displays slow magnetization relaxation akin to single-molecule magnet (SMM) behavior.

Here we report on the magneto-optical Kerr effect employing a nematic liquid crystal (LC) device as an optical modulator. This device allows performing intensity, phase, and polarization modulated measurements with a huge signal-to-noise ratio when compared to those obtained by means of an opto-mechanical chopper and a photo-elastic modulator. The results demonstrate that the optimal performance is achieved modulating the polarization state of the incident light by means of the LCs.

The physical and operating principle of a stress sensor, based on two crossing carbon fibers functionalized with ZnO nanorod-shaped nanostructures, was recently demonstrated. The functionalization process has been here extended to tows made of one thousand fibers, like those commonly used in industrial processing, to prove the idea that the same working principle can be exploited in the creation of smart sensing carbon fiber composites. A stress-sensing device made of two functionalized tows, fixed with epoxy resin and crossing like in a typical carbon fiber texture, was successfully tested. Piezoelectric properties of single nanorods, as well as those of the test device, were measured and discussed.

In this work, both PM 10 filters and leaves have been collected, on a daily basis, over a period of five months and compared systematically. Filters were taken from an air-quality monitoring station and leaves from two Tilia cordata trees, both located near the railway station of Parma. SEM-EDS analysis on the surface and across the leaves shows that magnetic particles are almost entirely made of magnetite, and that they are found invariably on the leaves surface. The saturation isothermal magnetic remanence (SIRM) shows that for both filters and leaves the magnetic fraction mainly consists of a low coercivity, magnetite-like phase. The magnetic signals of filter and leaves and atmospheric PM concentrations are compared. The correlation is better for filters, mostly with parameters related to vehicular pollution, and improved for both filters and leaves once data were averaged on a 10 days basis. Filters and leaves equally show an increase in magnetic signal during the fall-winter period together with PM 10 content. The comparison between leaves and filters shows that: 1) leaves give a qualitative picture, and in our case they could be used as environmental proxies after averaging the results over multiple days; 2) the correlation with PM 10 is weaker, indicating that there is a PM 10 contribution from non-magnetic particles, like calcite and clay minerals, pollen and spores; 3) multidomain particles contribution from filters indicates a strong relation with vehicular polluters, suggesting the important role of larger particles; 4) magnetization from leaves and filters are weakly related, due to the different sampling lapse., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

We report a detailed study on the magnetic properties of the pyroxene series M2M1Si 2 O 6 , with M2  =  Ca and M1  =  Mg, where magnesium and then calcium are progressively substituted by cobalt. For cobalt site occupancy larger than 0.7 at the M1 site, a collinear antiferromagnetic phase is detected for T  <  T N1   =  12 K with a monodimensional character (i.e. M1 site intra-chain order parallel to c axis). Moreover the magnetization easy axis has been estimated to lie roughly along the [1 0 1] direction. Cobalt content  ⩾0.5 at the M2 site (overall content 1.5) determines the formation of a new independent antiferromagnetic order with higher Néel temperature, involving only the M2 site intra-chain interactions. The incoming M2 site order is accompanied by a lowering of the space symmetry which yields to a weakly ferromagnetic resultant due to spin canted distribution of the magnetic moments either along the M1 or M2 chains. Furthermore, metamagnetic transitions are observed for both M1 and M2 site intra-chain orders at relatively low critical magnetic fields, around 2 T, suggesting that this series of pyroxenes can be used as a model system for investigating the fundamental aspects of magnetism in the matter.

Here we report a giant, completely reversible magneto-electric coupling of 100 nm polycrystalline Co layer in contact with ZnO nanorods. When the sample is under an applied bias of ±2 V, the Co magnetic coercivity is reduced by a factor 5 from the un-poled case, with additionally a reduction of total magnetic moment in Co. Taking into account the chemical properties of ZnO nanorods measured by X-rays absorption near edge spectroscopy under bias, we conclude that these macroscopic effects on the magnetic response of the Co layer are due to the microstructure and the strong strain-driven magneto-electric coupling induced by the ZnO nanorods, whose nanostructuration maximizes the piezoelectric response under bias.

We present a comprehensive study of the electrical properties of bulk polycrystalline BiFe0.5Mn0.5O3, a double perovskite synthesized in high-pressure and high-temperature conditions. BiFe0.5Mn0.5O3 shows an antiferromagnetic character with TN = 288 K overlapped with an intrinsic antiferroelectricity due to the Bi(3+) stereochemical effect. Beyond this, the observation of a semiconductor-insulator transition at TP ≈ 140 K allows one to define three distinct temperature ranges with completely different electrical properties. For T > TN, electric transport follows an ordinary thermally activated Arrhenius behavior; the system behaves as a paramagnetic semiconductor. At intermediate temperatures (TP < T < TN), electric transport is best described by Mott's variable range hopping model with lowered dimensionality D = 1, stabilized by the magnetic ordering process and driven by the inhomogeneity of the sample on the B site of the perovskite. Finally, for T < TP, the material becomes a dielectric insulator, showing very unusual poling-induced soft ferroelectricity with high saturation polarization, similar to the parent compound BiFeO3. Under external electric poling, the system irreversibly evolves from antiferroelectric to polar arrangement.

The physical characterization and the extended crystallographic study of the double perovskite system Pb2Mn0.6Co0.4WO6 indicate an improper ferroelectric contribution to the polarization induced by the magnetic ordering. In the paramagnetic phase, the compound displays a centrosymmetric orthorhombic double perovskite structure with the Pmcn1' symmetry. The structure is strongly distorted by the lead stereoactivity. Magnetization measurements show two magnetic transitions at 188 and 9 K, but the time-of-flight neutron diffraction data provide evidence for a long-range magnetic ordering only below the second transition. Quantitative structure refinements combined with a comprehensive symmetry analysis indicate the Pm'c21' magnetic space group to be the adequate symmetry to describe the structural distortions and spin ordering in the ground state of the system. The symmetry implies a coexistence of a spontaneous ferromagnetic moment and a ferroelectric polarization along the orthogonal b- and c-axes, respectively, in the long-range ordered structure. Macroscopic measurements confirm the presence of the spontaneous polarization also below the first transition at 188 K, where only short-range magnetic correlations are evidenced by diffuse scattering in neutron diffraction.

In contrast to rare-earth-based materials, cheaper and more environmentally friendly candidates for cooling applications are found within the family of Ni-Mn Heusler alloys. Initial interest in these materials is focused on the first-order magnetostructural transitions. However, large hysteresis makes a magnetocaloric cycle irreversible. Alternatively, here it is shown how the Heusler family can be used to optimize reversible second-order magnetic phase transitions for magnetocaloric applications., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We report a comprehensive study of the spontaneous magnetization reversal (MRV) performed on the disordered polycrystalline perovskite BiFe(0.5)Mn(0.5)O(3), an intriguing compound synthesized in high pressure-high temperature conditions. In disordered systems, the origin of MRV is not completely clarified, yet. In BiFe(0.5)Mn(0.5)O(3), compositional disorder involves the ions on the B-site of the perovskite determining the presence of mesoscopic clusters, characterized by high concentrations of iron or manganese and thus by different resultant magnetization. This leads to the observation of two singular fields H(1) and H(2) dependent on the degree of inhomogeneity, unpredictably changing from sample to sample due to synthesis effects. These fields separate different magnetic responses of the system; for applied fields H < H(1), the Fe and Mn clusters weakly interact in a competitive way, giving rise to MRV, while for an intermediate field regime the energy of this weak interaction becomes comparable to the energy of the system under field application. As a consequence, the zero field cooled magnetization thermal evolution depends on the sample degree of inhomogeneity. In this field regime, applied field Mössbauer spectroscopy indicates that the iron rich clusters are highly polarized by the field, while the largest part of the material, consisting of AFM clusters characterized by axial anisotropy and uncompensated moments, shows soft or hard magnetism depending on T. Above the higher singular field, the M(T) curves show the trend expected for a classical antiferromagnetic material and the competitive character is suppressed. The MRV phenomenon results to be highly sensitive on both the thermal and magnetic measurement conditions; for this reason the present work proposes a characterization strategy that in principle has a large applicability in the study of disordered perovskites showing similar phenomenology.

In this paper we describe the new ferri-electric compound Pb2MnWO6 (PMW), a double perovskite that can be considered as a novel structural prototype showing complex nuclear structure and interesting electric properties. According to single-crystal synchrotron data, PMW crystallizes in the noncentrosymmetric polar group Pmc21, in which the two symmetry-independent lead atoms give rise to a ferrielectric arrangement. The accurate crystallographic characterization indicates the presence of a complex distortion of the perovskite lattice driven by the local instability induced by the 6s(2) lone pair of the lead atoms. These peculiar structural features are confirmed by the complete electrical characterization of the system. Dielectric and transport measurements indicate an insulating character of the sample, while pyroelectric measurements point out a ferrielectric state characterized by different contributions. The magnetic transition at 45 K is accompanied by a magnetostrictive effect indicating a probable spin-lattice coupling. The characterizations carried out on PMW, showing the evidence of a coexistence of antiferromagnetism and ferrielectricity at low temperature, could lead to the definition of a new class of multiferroic materials.

The synthesis and structural and magnetic characterizations of K3Fe6F19, a new iron potassium fluoride with a complex tungsten bronze related structure, are presented. This phase was found during the investigation of relatively low-temperature (600 °C) synthesis conditions of classical tetragonal tungsten bronze (TTB) fluorides and can be considered an intermediate that forms at this temperature owing to faster crystallization kinetics. The K3Fe6F19 compound has an orthorhombic structure (space group Cmcm (63), a = 7.6975(3) Å, b = 18.2843(7) Å, c = 22.0603(9) Å) related to the TTB one, where the perovskite cage is substituted by a large S-shaped channel simultaneously occupied by two potassium atoms. The magnetic structure, characterized by magnetization measurements on an oriented single crystal and powder neutron diffraction, is dominated by the presence of interconnected double stripes of antiferromagnetic triangular exchange interaction patterns alternately rotated in clock- and anticlockwise fashion. The magnetic order takes place in a wide temperature range, by increasing progressively the interaction dimensionality.