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Ni-Mn-Ga based alloys are widely studied due to their potential for practical applications, making use of their martensitic transformations. However, hysteresis is a long-standing drawback that reduces the chance of transferring these alloys from the laboratory to industry. In this work, we studied a Cr-doped Ni2.15Mn0.70Cr0.15Ga alloy. We arrived at this composition by integrating data obtained from the previous phase and hysteresis diagrams taken from the literature. The compound presents a magnetostructural transition at room temperature, with a ferromagnetic martensite phase, and moderate thermal hysteresis of approximately 4 K. The magnetocaloric and ferromagnetic shape memory effects were explored, showing a reversible entropy change of approximately 9 Jkg-1K-1 under 0–5 T field change, and a cyclical magnetic-field-induced deformation close to 0.81% under 0–9 T, an advance towards high reversibility for this family alloys.

The magnetically frustrated manganese nitride antiperovskite family displays significant changes of entropy under hydrostatic pressure that can be useful for the emerging field of barocaloric cooling. Here we show that barocaloric properties of metallic antiperovskite Mn nitrides can be tailored for room-temperature application through quaternary alloying. We find an enhanced entropy change of $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{t}}|=37\phantom{\rule{4pt}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}$ at the ${T}_{t}=300\phantom{\rule{4pt}{0ex}}\mathrm{K}$ antiferromagnetic transition of quaternary ${\mathrm{Mn}}_{3}{\mathrm{Zn}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{N}$ relative to the ternary end members. The pressure-driven barocaloric entropy change of ${\mathrm{Mn}}_{3}{\mathrm{Zn}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{N}$ reaches $|\mathrm{\ensuremath{\Delta}}{S}_{\mathrm{BCE}}|=20\phantom{\rule{4pt}{0ex}}\mathrm{J}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{kg}}^{\ensuremath{-}1}$ in 2.9 kbar. Our results open up a large phase space where compounds with improved barocaloric properties may be found.

Second magnetization peak (SMP) in hole-doped cuprates and iron pnictide superconductors has been widely explored. However, similar feature in the family of electron-doped cuprates is not common. Here, we report the vortex dynamics study in the single crystal of an electron-doped cuprate ${\mathrm{Pr}}_{0.87}{\mathrm{LaCe}}_{0.13}{\mathrm{CuO}}_{4}$ superconductor using dc magnetization measurements. A second magnetization peak feature in the isothermal $M(H)$ was observed for $H\ensuremath{\parallel}ab$ planes. On the other hand, no such feature was observed for the $H\ensuremath{\parallel}c$ axis in the crystal. Using magnetic relaxation data, a detailed analysis of activation pinning energy via collective creep theory suggests an elastic to plastic creep crossover across the SMP. Moreover, for the $H\ensuremath{\parallel}ab$, a peak in the temperature dependence of critical current density is also observed near 7 K, which is likely be related to a dimensional crossover [three-dimensional--two dimensional (3D-2D)] associated with the emergence of Josephson vortices at low temperatures. The anisotropy parameter obtained $\ensuremath{\gamma}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}8--11$ indicates the 3D nature of vortex lattice mainly for the $H\ensuremath{\parallel}c$ axis. The $H\ensuremath{-}T$ phase diagrams for $H\ensuremath{\parallel}c$ and $H\ensuremath{\parallel}ab$ are presented.

Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural and magnetic properties of the Ni2Mn1-xCuxGa0.84Al0.16 Heusler alloys. The alloys present a thermal hysteresis reduction of ~60% when the Cu content in the compound varies from x = 0.10 to x = 0.25, with a minimum hysteresis width of 6 K being achieved. We applied the geometric non-linear theory of martensite to address the phase compatibility, quantified by the parameter lambda2, the middle eigenvalue of the transformation stretch tensor, and found that the minimum of hysteresis is associated with a better crystallographic compatibility (lambda2 closer to 1) between the austenite and martensite phases. In addition, we show that the valley-like properties of hysteresis found in the Ni2Mn1-xCuxGa0.84Al0.16 compounds is present in several other alloys in the literature. These results provide new pathways to understand as well as to masters the phase compatibility and ultimately achieve a low thermal hysteresis in multifunctional Heusler alloys., 29 pages and 8 figures

In this work we address the interplay between two phenomena which are signatures of the out-of-equilibrium state in phase separated manganites: irreversibility against thermal cycling and aging/rejuvenation process. The sample investigated is La0.5Ca0.5MnO3, a prototypical manganite exhibiting phase separation. Two regimes for isothermal relaxation were observed according to the temperature range: for T > 100 K, aging/rejuvenation effects are observed, while for T < 100 K an irreversible aging was found. Our results show that thermal cycles act as a tool to unveil the dynamical behavior of the phase separated state in manganites, revealing the close interplay between static and dynamic properties of phase separated manganites., 16 pages, 5 figures, accepted in JMMM (2018)

In this work we investigate the critical behavior of two manganite compounds, Pr0.5Sr0.5MnO3 (PSMO) and Pr0.5Sr0.41Ca0.09MnO3 (PSCMO). These are complex magnetic systems, which undergo a paramagnetic to ferromagnetic phase transition on cooling, followed by a first order magnetic transition to a charge ordered antiferromagnetic states at lower temperatures. Magnetization measurements and magnetocaloric calculations were performed in order to determine their critical behavior. As a main result we show that the scaling of the entropy change - Δ S M with magnetization M , instead of the usual scaling with magnetic field H , is a powerful tool to study the critical behavior through magnetocaloric analysis. We use this property, in conjunction with Modified Arrot Plot technique, to determine the basic critical exponents β and γ for each sample. The exponents values indicate that the samples are near to universality class models based in nearest neighbor interactions. Besides intrinsic differences between the Ca-doped and the undoped sample, we confirm the applicability of the magnetocaloric analysis in order to determine critical behavior in complex magnetic systems.

Polycrystalline Heusler compounds Ni2Mn0.75Cu0.25Ga0.84Al0.16 with a martensitic transition between ferromagnetic phases and Ni2Mn0.70Cu0.30Ga0.84Al0.16 with a magnetostructural transformation were investigated by magnetization and thermal measurements, both as a function of temperature and magnetic field. The compound Ni2Mn0.75Cu0.25Ga0.84Al0.16 presents a large magnetocaloric effect among magnetically aligned structures and its causes are explored. In addition, Ni2Mn0.70Cu0.30Ga0.84Al0.16 shows very high, although irreversible, entropy and adiabatic temperature change at room temperature under a magnetic field change 0-1 T. Improved refrigerant capacity is also a highlight of the 30% Cu material when compared to similar Ni2MnGa-based alloys., Comment: 26 pages and 6 figures

Ferromagnetic Ni2MnGa-based alloys play an important role in technological fields, such as smart actuators, magnetic refrigeration and robotics. The possibility of obtaining large non-contact deformation induced by an external perturbation is one of its key strengths for applications. However, the search for materials with low cost, practical fabrication procedures and large signal output under small perturbing fields still poses challenges. In the present study we demonstrate that by judicial choice of substitution on the Mn site, an abrupt magnetostructural transition from a paramagnetic austenite phase to a ferromagnetic martensite one can be tuned to close to room temperature achieving large and reproducible strains. The required magnetic field to induce the strain varies from small values, as low as 0.25 T for 297.4 K and 1.6% of strain, to 8 T for 305 K and 2.6% of strain. Our findings point to encouraging possibilities for application of shape memory alloys in relatively inexpensive, scalable polycrystalline materials., Comment: 26 pages and 7 figures

The mixed-valent homometallic ludwigite $({\mathrm{Co}}_{2}^{2+}{\mathrm{Co}}^{3+}){\mathrm{O}}_{2}{\mathrm{BO}}_{3}$ is investigated above the ferrimagnetic ordering temperature ${T}_{C}=43\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ through structural, thermal, magnetic, electric, and spectroscopic probes. X-ray absorption at the Co ${L}_{2,3}$ edges is consistent with the coexistence of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ ions, as expected by the sample stoichiometry. Magnetic susceptibility shows a relatively large net paramagnetic moment per Co ion above room temperature, $p=4.87$, indicating that the ${\mathrm{Co}}^{3+}$ ions are not in a pure low-spin configuration at high temperatures, also showing a non-Curie-Weiss behavior below 300 K. Electrical conductivity and differential scanning calorimetry measurements on single crystals indicate two phase transitions at $\ensuremath{\sim}475$ and $\ensuremath{\sim}495\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. X-ray powder diffraction shows substantial lattice parameter anomalies below 500 K. These results indicate phase transitions associated with changes in the Co oxidation state in each of its four crystallographic sites. Such transitions are possibly dictated by a competition between (i) an ordered ground state with all ${\mathrm{Co}}^{3+}$ ions occupying the same crystallographic site and (ii) either partially or totally charge-disordered states that are favored at high temperatures due to their higher entropy.

In this work, we demonstrate that La0.225Pr0.4(Ca1−xSrx)0.375MnO3 manganites with 0 ≤ x ≤ 0.10 , exhibit a discontinuous first order ferromagnetic-paramagnetic transition at temperatures ~ 230–240 K, which evolves to a continuous second order ferromagnetic-paramagnetic transition according to the intensity of the applied magnetic field. Under high magnetic fields and through a magnetization scaling analysis, we have determined the critical exponents of the sample with x = 0.05 - β = 0.15, γ = 0.77, and δ = 6.24, and x = 0.10 - β = 0.165, γ = 0.62, and δ = 4.69. The exponents values obey the scaling laws but do not belong to any existing universality class. This analysis of critical behavior can be used to predict the magnetic entropy change under high magnetic fields.

We study the effect of proton irradiation on Ba(Fe$_{0.96}$Co$_{0.04}$)$_2$As$_2$ superconducting single crystals from combined magnetisation and magnetoresistivity measurements. The study allows the extraction of the values of the apparent pinning energy $U_0$ of the samples prior to and after irradiation, as well as comparison of the values of $U_0$ obtained from the flux-flow reversible region with those from the flux-creep irreversible region. Irradiation reduces $T_c$ modestly, but significantly reduces $U_0$ in both regimes: the critical current density $J_c$ is modified, most strikingly by the disappearance of the second magnetisation peak after irradiation. Analysis of the functional form of the pinning force and of the temperature dependence of $J_c$ for zero field, indicates that proton irradiation in this case has not changed the pinning regime, but has introduced a high density of shallow point-like defects. By consideration of a model that takes into account the effect of disorder on the irreversibility line, the data suggests that irradiation produced a considerable reduction in the average effective disorder overall, consistent with the changes observed in $U_0$ and $J_c$., 5 pages 7 figures

We investigate the nature of ultrasharp metamagnetic transitions in La5/8-yNdyCa3/8MnO3 manganites. These compounds change from a low temperature ferromagnetic metallic state at low Nd doping to a charge-ordered antiferromagnetic insulator for high Nd content. At an intermediate doping a phase-separated state is established. At low temperatures (∼2 K), an avalanche-like field-induced metamagnetic transition is observed, where the entire compound changes abruptly from one phase to the other. The behavior of this ultrasharp transition is investigated using magnetization and specific heat measurements. We found a first order transition in the specific heat associated with discontinuous jumps in the magnetization. A strong increase of the sample temperature is simultaneously observed. The results are interpreted in terms of latent heat released from the field-induced enhancement of the ferromagnetic fraction, triggering the avalanche process.

Using a combination of theoretical (first-principles total-energy and electronic structure calculations) as well as experimental (structural, thermodynamics) techniques, we systematically investigated the influence of B incorporation on the structural, electronic and thermodynamic properties of a series of technologically-important B-containing AlNi matrix. Special attention was paid to calculating the energy cost of placing B at various sites within the cubic unit cell. The most energetically favorable defects were identified to be, depending on initial stoichiometry, substitutional B at Al site (B Al ), Ni vacancy ( V Ni ), or Ni antisite (Ni Al ). We show that the induced variation in the lattice parameters can be correlated with the type and concentration of the involved defects: e.g. the surge of V Ni defects leads to a stronger lattice-parameter reduction, that of Ni Al ones to a relatively weaker reduction while that of B Al defects to a much weaker influence. Both electronic band structure calculations as well as thermodynamics measurements indicate that the 3 d bands of Ni are fully occupied and magnetically unpolarized and that the resulting N ( E F ) is very small: all studied compounds are normal conductors with no trace of superconductivity or magnetic polarization.

A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density ($J_c$) and the vortex-pinning behaviour in these compounds. The observed $J_c$ is higher than the threshold limit (10$^5$ A/cm$^2$) considered for the technological potential and even greater than 1 MA/cm$^2$ for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen $et$ $al$, which suggest the dominant role of $\delta l$-type pinning., Comment: 26 pages, 13 figures, Accepted for publication in ACS Applied Electronic Materials 2019

We present magnetization studies as a function of time, temperature and magnetic field for $H$ $\parallel$ c-axis, in a hole-doped pnictide superconductor, La$_{0.34}$Na$_{0.66}$Fe$_2$As$_2$, with, $T_c$ $\approx$ 27 K. The obtained vortex phase-diagram shows that the magnetic irreversibility line is very close to the mean-field superconducting transition line, similar to the low $T_c$ superconductors, evidencing a strong pinning behavior. The irreversibility line does not follow a power law behavior with ($T_c$-$T$), however, it is well described using an expression developed in the literature considering the effect of disorder in the system. The critical current density estimated using the Bean's critical-state model is found to be of the order of 10$^5$ A/cm$^2$ below 12 K in the limit of zero magnetic field. A plot of the normalized pinning force density as a function of the reduced magnetic field at different temperatures shows a good scaling and the analysis suggests that the vortex pinning is due to normal point like pinning centers. The temperature dependence of the critical current density suggests that the pinning due to the variation in charge carrier mean free path alone is not sufficient to explain the experimental data. Magnetic relaxation rate as a function of temperature and magnetic field is also studied., Comment: 19 pages, 6 Figures, Accepted for Publication in the "Journal of Applied Physics 2019"

Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here, we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite Mn

Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here, we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite Mn3NiN, facilitated by its geometrically frustrated antiferromagnetism opening the possibility of new memory device concepts. Films of Mn3NiN with intrinsic biaxial strains of ±0.25% result in Néel transition shifts up to 60 K and magnetization changes consistent with theory. Films grown on BaTiO3 display a striking magnetization jump in response to uniaxial strain from the intrinsic BaTiO3 structural transition, with an inferred 44% strain coupling efficiency and a magnetoelectric coefficient α (where α = dB/dE) of 0.018 G cm/V. The latter agrees with the 1000-fold increase over Cr2O3 predicted by theory. Overall, our observations pave the way for further research into the broader family of Mn-based antiperovskites where yet larger piezomagnetic effects are predicted to occur at room temperature.

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with $H\ensuremath{\parallel}c$ on an underdoped ${\mathrm{Ba}}_{0.75}{\mathrm{K}}_{0.25}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ pnictide single crystal, with superconducting transition temperature ${T}_{c}=28$ K. The second magnetization peak (SMP) has been observed at temperatures below ${T}_{c}/2$ and vanished at higher temperatures. The observed behavior of the SMP has been studied by measuring the magnetic field dependence of relaxation rate $R(H)$ and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting $H\text{\ensuremath{-}}T$ phase diagram is obtained. The critical current density $({J}_{c})$ was estimated using Bean's model and found to be $\ensuremath{\sim}3.4\ifmmode\times\else\texttimes\fi{}{10}^{9}$ A/${\mathrm{m}}^{2}$ at 10 K in the SMP region, which is comparable to an optimally doped Ba-${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual $\ensuremath{\delta}l$ and $\ensuremath{\delta}{T}_{c}$ pinning models, which suggest the intrinsic nature of pinning in the compound.

We investigate the effects of A-site cation disorder on the magnetic transition in doped Sr2YRuO6 insulating double perovskites. Disorder is induced by partially substituting Sr for Ca, Ba and La. The relative concentration of the doping ions was carefully chosen in order to warrant a constant average ionic radius equal to the one found in Sr2+. The variance, σ2 = − 2, where rA is the ionic radius of each type of ion occupying the A-site, is the parameter used to quantify disorder. Unexpectedly, in spite of the fact that the magnetic ground state in Sr2YRuO6 is the result of a delicate balance between competitive ferro- and antiferromagnetic interactions with bound lengths and angles playing a relevant role, the magnetic transition temperature is virtually insensitive to disorder, in sharp contrast to the behavior observed for metallic perovskites. This result supports the idea that the leading mechanism suppressing the magnetic transition in Sr2YRuO6 is the deviation from planarity of the RuO2 layers.

s This work investigates in the insulating double perovskites Sr 2− x Ca x YRuO 6 . We address the angular dependence of the strength of the magnetic coupling due to the deviation from planarity of the basal layers of the monoclinic structure, characterized by the in-plane angle α α =144° for x =1) while the bond distances vary in a complex way as a consequence of the strong monoclinic distortion. We found that the magnetic transition temperature, T M , shows a linear dependence on cos [( π − α )/2]. This result is discussed in terms of t 2 g ( π )– e g ( σ ) mixing of the magnetic orbitals of the Ru 5+ ions and unbalanced competitive super-exchange interactions. The deleterious effect of Ca doping for the magnetic coupling is confirmed by the reduction in the short-range antiferromagnetic correlations characteristic of the parent compound at T >> T M and the enhancement of magnetic frustration for T T M .

We examine the evidence for multiband superconductivity and non s-wave pairing in the non-centrosymmetric superconductor Nb$_{0.18}$Re$_{0.82}$, using electrical transport, magnetization and specific heat measurements. In the normal state, both the evolution of resistivity with temperature and with magnetic field support a multiband picture. In the superconducting state, the Werthamer, Helfand and Hohenberg (WHH) model cannot adequately describe the temperature dependence of the upper critical field, $H_{c2}(T)$, over the whole temperature range measured. In addition, the observed $H_{c2}(0)$ exceeds the Pauli limit, suggesting non-s-wave pairing. Interestingly, the Kadowaki-Woods ratio and Uemura plot reveal a behavior in Nb$_{0.18}$Re$_{0.82}$ which is similar to that found in unconventional superconductors. The temperature dependence of the lower critical field, $H_{c1}(T)$, follows an anomalous $T^3$ behavior and the derived normalized superfluid density ($\rho_s$) is well explained using a nodeless two-gap description. Phase-fluctuation analysis conducted on the reversible magnetization data, reveals a significant deviation from the mean-field conventional s-wave behavior. This trend is interpreted in terms of a non s-wave spin-triplet component in the pairing symmetry as might be anticipated in a non-centrosymmetric superconductor where anti-symmetric spin-orbit coupling plays a dominant role., Comment: 24 pages, 7 figures

Sr2YRuO6 is a double perovskite with a complex magnetic state, including non-collinear arrangement of the ruthenium moments and spin re-orientation. One open issue is the possibility of magnetization reversal leading to an apparent diamagnetic response. In this work we show that when the oxygen content is reduced, the susceptibility curve develops a sharp minimum, in contrast to the broad maximum observed for a well-oxygenated sample. However, the magnetization remains positive regardless of the thermomagnetic history. We explain this effect in terms of the reversal of a net ferromagnetic moment in opposition to the applied field and by a spatial average of the contribution of the grains in the polycrystalline sample. The reversal is observed in deoxygenated samples because the delicate balance between competitive ferromagnetic and antiferromagnetic interactions is altered. The significant reduction of the antiferromagnetic correlations at high temperatures, as evidenced by the magnetic contribution to the specific heat, is consistent with this scenario. The role of the oxygen content is conclusively confirmed by reversibility after re-oxygenation. Our results suggest that Sr2YRuO6−x might host inhomogeneous superconductivity due to local enhancement of spin fluctuations in the neighborhood of the oxygen vacancies.

In the present work we investigate the alloys (Fe 0.8 Al 0.2 ) 100− x B x , for x > 2.0 up to x = 5.0. A maximum of the total magnetostriction was found for the alloy (Fe 0.80 Al 0.20 ) 97 B 3 . In this sample we measured the boron contents using WDS and we did not find boron in the matrix, while the precipitates formed after annealing of as cast samples are essentially the phase Fe 2 B. By a detailed XRD study of this sample using Mo Kα radiation, the presence of the phases Fe 3 Al (D03) and α (A2) was determined. The coexistence of both phases was determined to be the origin of the increase of the magnetostriction of this material. The value of λ long = 80 × 10 −6 achieved in the as cast sample with x = 3 indicates that the introduction of texturing to the material should provide even higher enhancement of the magnetostriction. In addition to high magnetostriction values, the alloy can be machined, welded and is also a low cost material, therefore the alloy (Fe 0.80 Al 0.20 ) 97 B 3 reveals a great promise as a material to be used on sensors and actuators assembly.

We investigate the magnetostriction of polycrystalline Fe 100− x V x alloys as a function of vanadium content, with values of x varying from 5.2% to 40.7% (at.%V). The lattice parameter curve behavior as a function of vanadium deviates from a linear behavior at x ≈ 10.2 and x ≈ 30.7. Close to x = 10.2 is possible to observe a small lattice contraction and close to x = 30.7 an expansion. The magnetostriction curve as a function of vanadium content shows two maxima at x = 11.8 and 30.7 and has values of 3/2 λ s of 68.8 ppm and 49.3, respectively. The alloys microstructures investigations do not offer enough elements to connect it with magnetostriction. Therefore first-principles calculations were performed and resulted that the lattice contraction at ∼10.2% of V is due to the formation of strengthened Fe–V interactions associated to stronger hybridization between minority spin d subbands, while the lattice expansion observed in the range 25.5–30.7% of V is associated to the weakening of the Fe–V interaction. As for the case of simulating V–V pairs for the alloy Fe 12 V 4 , representative of x = 25.0, we associated the experimental result of the lattice expansion observed in the range among 25.5–30.7% of V to the weakening of the Fe–V interaction. These calculated electronic properties explain the lattice contraction (at x = 10.2) and expansion ( x = 25.5–30.7) and are expected to be the sources of the magnetostrictive behavior of these Fe–V alloys.

It is now well established that the paramagnetic-to-ferromagnetic transition in the magnetocaloric $\mathrm{La}{(\mathrm{FeSi})}_{13}$ is a cooperative effect involving spin, charge, and lattice degrees of freedom. However, the influence of this correlated behavior on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of $\mathrm{LaF}{\mathrm{e}}_{x}\mathrm{M}{\mathrm{n}}_{y}\mathrm{S}{\mathrm{i}}_{z}$ samples, with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature, and the spin-wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced $d$-band electronic interactions at play. The Sommerfeld coefficient is found to be $90--210\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$, unusually high compared to that expected from band-structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin-wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly and provide a basis for future theoretical development.

Here, we study the temporal evolution of the magnetic field-driven paramagnetic to ferromagnetic transition in the La(Fe,Mn,Si) 13 material family. Three compositions are chosen that show varying strengths of the first-order character of the transition, as determined by the relative magnitude of their magnetic hysteresis and temperature separation between the zero-field transition temperature T c and the temperature T crit , where the transition becomes continuous. Systematic variations in the fixed field, isothermal rate of relaxation are observed as a function of temperature and as a function of the degree of first-order character. The relaxation rate is reduced in more weakly first-order compositions and is also reduced as the temperature is increased towards T crit . At temperatures above T crit , the metastability of the transition vanishes along with its associated temporal dynamics. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

The paramagnetic Meissner effect (PME) is related to the appearance of a positive magnetization when a superconducting specimen is field cooled through its critical temperature. In this work we report on the PME and ac magnetization in roll-bonded Cu–Nb (RB/Cu–Nb) layered composites. We present typical DC magnetization loops obtained in the normal magnetic field configuration that show the PME. In addition, we present ac magnetization measurements that reveal a crossover behavior at a characteristic field value. We show evidence that such a crossover behavior, attributed to activation processes of vortices, is probably related to the disappearance of the PME in the RB/Cu–Nb layered composites.

The Fex(Cr2O3)1−x system, with 0.10 ≤ X ≤ 0.80, was mechanically processed for 24 h in a high-energy ball-mill. In order to examine the possible formation of iron–chromium oxides and alloys, the milled samples were, later, thermally annealed in inert (argon) and reducing (hydrogen) atmospheres. The as-milled and annealed products were characterized by X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy and magnetization. The as-milled samples showed the formation of an Fe1+YCr2−YO4−δ nanostructured and disordered spinel phase, the α1-Fe(Cr) and α2-Cr(Fe) solid solutions and the presence of non-exhausted precursors. For the samples annealed in inert atmosphere, the chromite (FeCr2O4) formation and the recrystallization of the precursors were verified. The hydrogen treated samples revealed the reduction of the spinel phase, with the phase separation of the chromia phase and retention of the Fe–Cr solid solutions. All the samples, either as-milled or annealed, presented the magnetization versus applied field curves typical for superparamagnetic systems.

In this work we focus on the microstructural and magnetic characterization of wires with different architectures (design and reinforcement). The microstructural characterization was performed using scanning electron microscopy. AC magnetic susceptibility was measured with field applied both parallel and perpendicular to the wire axis. The heat treatment performed to form the A-15 superconducting phase leads to partial spheroidization followed by coarsening of the Nb filaments in the reinforcement material. The differences concerning the microstructure of the reinforcement material among the investigated wires were reflected in the broadening of the superconducting transition of Nb, more evident for a field applied parallel to the wire axis. From the magnetic data the wires were also compared in terms of the superconducting volume fraction.

We investigate a series of manganese oxides, the La0.225Pr0.4(Ca1−xSrx)0.375MnO3 system. The x=0 sample is a prototype compound for the study of phase separation in manganites, where ferromagnetic and charge ordered antiferromagnetic phases coexist. Replacing Ca2+ by Sr2+ gradually turns the system into a homogeneous ferromagnet. Our results show that the material structure plays a major role in the observed magnetic properties. On cooling, at temperatures below ∼100 K, a strong contraction of the lattice is followed by an increase in the magnetization. This is observed both through thermal expansion and magnetostriction measurements, providing distinct evidence of magneto-elastic coupling in these phase separated compounds. Fil: Eslava, G.G.. Universidade Federal do Rio de Janeiro; Brasil Fil: Parisi, F.. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Bernardo, P.L.. Universidade Federal do Rio de Janeiro; Brasil Fil: Quintero, Mariano Horacio. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comisión Nacional de Energía Atómica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Leyva, G.. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Cohen, L.F.. Imperial College. Theoretical Physics Group. Blackett Laboratory; Reino Unido Fil: Ghivelder, L.. Universidade Federal do Rio de Janeiro; Brasil

The influence of Co substitution on the structural and magnetic properties of La(Fe 1− x Co x ) 11.4 SP 1.6 compounds (SP=Al or Si) was investigated by X-ray diffraction, Mossbauer spectroscopy and magnetization measurements. The studied compounds were crystallized in the NaZn 13 structure, with a fraction of α-Fe(Co,SP) phase. Partial substitution of Fe by Co atoms changes (i) the magnetic ordering temperature (200 K T x = 0 to about 0.5 T for x = 0.04 . For x > 0.04 , the compounds are ferromagnetic as the ground state. On the other hand, the Si compounds have ferromagnetic ground state for all Co compositions studied. The field-induced (5 T) maximum magnetic entropy variation remains almost constant at about −5 J/kg K, for x values up to 0.12, in the Al compound. However, this entropy variation reduces from −18 to −5 J/kg K (∼72%), when Co substitution is increased from 0 to 8 at%. in the Si compounds. This effect may be associated with the disappearing of the first-order transition with the addition of Co. Metamagnetic transition of the Co free Al compound has entropy variation of 13 J/kg K at about 11 K.

We report on the influence of the microstructure on the AC and DC magnetic properties of Cu–3.5% Nb nanofilamentary wires. Samples obtained from a single Cu–3.5% Nb wire were subsequently submitted to different plastic strain levels via drawing so that their microstructure was altered. Noticeable differences are observed in their isothermal DC magnetization curves that present a double-peak structure. The first peak, which occurs at low magnetic fields, is attributed to superconductivity induced in the Cu matrix due to the proximity effect. It is argued that the second peak is related exclusively to niobium. The dependence of these two distinct peaks on the characteristic nanometre length scales of the samples, i.e. dimension of the Nb filaments and interfilamentary spacing, are discussed.

The magnetic properties of the superconducting ruthenate-cuprate RuSr2Eu 1.4Ce0.6Cu2O10-δ (Ru-1222) have been studied by a scaling analysis of the dc magnetization and ac susceptibility measurements. Non-linear M(H) curves reveal the presence of nano-size clusters with a net magnetic moment of ∼102 μB at 180 K, near the deviation from a Curie-Weiss behavior. On cooling, no scaling was observed down to 90 K, discarding the possibility of magnetic phase separation of collinear ferromagnetic particles. We explain this result in terms of a variable number of contributing particles, with a temperature dependent net magnetic moment. For 70 K ≤T≤ 90 K the scaling plots evidence the emergence of a system of non-interacting particles, which couple on further cooling. The observed cluster-glass features are preserved down to the lowest measured temperature (10 K); no signature of long-range order was detected. The frequency shift of the peak in the real part of the ac susceptibility does not follow the Vogel-Fulcher type dependence, as previously reported. The puzzling temperature dependence of the coercive field, HC(T), is correlated with the changes in the scaling factors.

The present work presents the steps for the manufacture of an Nb?Ti artificial pinning centre (APC) multifilament wire for AC applications. The wire production starts from a Ti bar inserted in a Nb tube. The superconductor Nb?Ti alloy was obtained using heat treatments during the wire production steps through the growth of the diffusion layer between the pure Nb and Ti regions. The final wire is composed of many filaments dispersed in a Cu?Ni matrix. The transport critical current density was measured in samples for different conditions of heat treatment, followed by wire diameter reduction. Magnetization hysteresis loops were measured to extract the hysteretic losses in wires with filament diameters of 76 and 92??m and compared to the results in commercial wires with comparable filament diameters. We obtained an Nb?Ti APC wire with hysteretic losses of the same order of magnitude as the commercial wires. However, critical current densities Jc are higher for fields lower than 2?T, where AC applications are most important. The Jc results confirmed a previous result that the decrease of normal region sizes is a less important pinning source in this material than the pinning from variations in the composition of the diffusion layer.

We report on the microstructure evolution in Cu-15% Nb multifilamentary wires upon annealing and the corresponding effects on their electrical and magnetic properties. During annealing at temperatures higher than 800degC thermal instability mechanisms take place in the microstructure of the Cu-15%Nb composite, leading to spheroidization, followed by coarsening of niobium filaments. Such microstructural changes exert considerable influence on the magnitude of the normal electrical resistivity for the Cu-Nb composite. In addition, the annealing process leads to a double-peak structure in the isothermal DC magnetization curves of the composite, which is strongly dependent on the annealing time. This behavior suggests that the spheroidization kinetics can be obtained from M(H) curves

The lower critical field of the grains, $H_{c1}$, and the intragrain critical current density, $J_{c}$, were determined for the superconducting ruthenate-cuprate RuSr$_{2}$Gd$_{1.5}$Ce$_{0.5}$Cu$_{2}$O$_{10-\delta}$ [Ru-1222(Gd)] through a systematic study of the hysteresis in magnetoresistance loops. A reliable method, based on the effects of the magnetization of the grains on the net local field at the intergranular junctions is provided, circumventing the problem of the strong masking of the superconducting diamagnetic signal by the ferromagnetic background. The temperature dependency of $H_{c1}$ and $J_{c}$ both exhibit a smooth increase on cooling without saturation down to $T/T_{SC}$ $\cong $ 0.2. The obtained $H_{c1}$ values vary between 150 and 1500 Oe in the 0.2 $\leq $ $% T/T_{SC}$ $\leq $ 0.4 interval, for samples annealed in an oxygen flow; oxygenation under high pressure (50 atm) leads to a further increase. These values are much larger than the previously reported rough assessments (25-50 Oe), using conventional magnetization measurements. High $J_{c}$ values of $% \sim $ 10$^{7}$ A/cm$^{2}$, comparable to the high-T$_{c}$ cuprates, were obtained. The $H_{c1}(T)$ and $J_{c}(T)$ dependencies are explained in the context of a magnetic phase separation scenario., Comment: 7 pages, 6 figures, accepted for publication in European Physical Journal B

We report on the microstructural evolution in Cu15%Nb multifilamentary wires upon annealing and the corresponding effects on their magnetic properties. During annealing at temperatures higher than 800 °C thermal instability mechanisms take place in the microstructure of the Cu15%Nb composite, leading to the spheroidization of niobium filaments. Another important consequence of annealing is the recrystallization of Nb, that has a straightforward influence on the magnetization data. When compared to the as-drawn composite, in the annealed Cu15%Nb samples the bulk upper critical field Hc 2(T) is reduced significantly, while the lower critical field Hc 1(T) is increased as a function of the annealing temperature and time. This indicates that we may employ this process as an efficient method to modify the basic superconducting parameters of Nb, namely the coherence length ξ(T) and the penetration depth λ(T).

We studied electric and magnetic properties of polycrystalline La 0.325 Pr 0.300 Ca 0.375 MnO 3 , a manganese oxide-based compound exhibiting intrinsic multiphase coexistence of submicrometric ferromagnetic, charge-ordered and charge-disordered regions. Time relaxation in both resistivity and magnetization were observed in a broad temperature range, signaling that coexisting phases are out of equilibrium. Different regimes of instability are presented and discussed.

We have studied a non-volatile memory effect in the mixed valent compound La 0.5 Ca 0.5 MnO 3 induced by magnetic field ( H ). In a previous work (Phys. Rev. B 65 (2002) 104403), it has been shown that the response of this system upon application of H strongly depends on the temperature range, related to three well-differentiated regimes of phase separation occurring below 220 K. In this work we compare memory capabilities of the compound, determined following two different experimental procedures for applying H , namely zero-field cooling and field cooling the sample. These results are analyzed and discussed within the scenario of phase separation.

We report on the annealing effects on the microstructure, electrical, and magnetic properties of jelly-rolled Cu –xvol %Nb (x = 25, 33, 50, 63) composite wires, a promising material to be used in high-field magnets. During annealing at temperatures as high as 850 °C, noticeable changes take place in the microstructure, including recovery and recrystallization of copper and niobium, followed by partial spheroidization and further coarsening of niobium filaments. The influence of these microstructural changes on the electrical and magnetic properties of jelly-rolled Cu–Nb composite wires is discussed.

We have studied the effect of irreversibility induced by repeated thermal cycles on the electric transport and magnetization of polycrystalline samples of La0.5Ca0.5MnO3 and La0.325Pr0.3Ca0.375MnO3. An increase of the resistivity and a decrease of the magnetization at different temperature ranges after cycling is obtained in the temperature range between 300 K and 30 K. Both compounds are known to exhibit intrinsic submicrometric coexistence of phases and undergo a sequence of phase transitions related to structural changes. Changes induced by thermal cycling can be partially inhibited by applying magnetic field and hydrostatic pressure. Our results suggest that the growth and coexistence of phases with different structures gives rise to microstructural tracks and strain accommodation, producing the observed irreversibility. Irrespective of the actual ground state of each compound, the effect of thermal cycling is towards an increase of the amount of the insulating phase in both compounds., to appear in Journal of Alloys and Compounds (2003)

Comparison of the DC resistivity, magnetisation, 55 Mn NMR, AC susceptibility and microwave impedance of single crystalline and bulk polycrystalline samples of La 0.9 Ca 0.1 MnO 3 reveals the complexity of these materials. The observation of a motionally narrowed peak in the NMR data is indicative of double exchange ferromagnetism existing in isolated metallic regions, whilst combined magnetisation and DC resistivity suggest that at macroscopic scales the material is a super-exchange mediated ferromagnetic insulator. High-frequency conductivity measurements are used to search for evidence of non-percolative metallic clusters. The microwave signature associated with the metal insulator transition in micron sized isolated particulates of La 0.67 Ca 0.33 MnO 3 were studied in order to establish how metallic cluster formation might be identified.

In this paper, the annealing effects on the magnetic properties of a multifilamentary Cu–15vol%Nb composite were investigated. During vacuum annealing, noticeable changes take place in the microstructure, mostly the partial spheroidization and further coarsening of the niobium filaments. Results show that spheroidization becomes noticeable at about 700 °C and, even after annealing at severe conditions, e.g. 1050 °C for 32 h, the continuity of the niobium-conducting path is partially preserved. The influence of these microstructural changes on the magnetic properties of the multifilamentary Cu–Nb composite conductor is discussed.

A new binuclear copper(II) complex, namely [Cu2L(pz)(DMSO)], where L = 2,6-bis[(2-phenoxy)iminomethyl]-4-methylthiophenolate(3-) and pz = pyrazolate ligand, has been synthesized by a one-pot synthesis involving copper(II) acetate monohydrate, the S-protected ligand precursor 2-(N,Ndimethylthiocarbamato)-5-methylisophthalaldehyde di-2?-hydroxy anil, (I), and pyrazole, in which a metal-promoted S-deprotection reaction occurs during the formation of the complex. This was characterized by routine physicochemical studies, single crystal X-ray diffraction and electron paramagneticresonance (EPR) techniques. The structure analysis reveals that there are copper centres in two different environments, a slightly distorted square planar and a distorted square-pyramidal, arranged in binuclear units. The EPR study of these binuclear units performed at 9.4 GHz in the temperature range between 4 and 293 K shows an antiferromagnetic interaction between CuII ions, and allows evaluating g factors gx = 2.068(1), gy = 2.091(1) and gz = 2.165(1), with = 2.108(1), an exchange coupling parameter J0 = -26(1) cm−1 (defined as Hex = - J0 S1 S2), and a zero field splitting of the ground triplet state described by D = 86(2) × 10-4 cm-1 and E = -48(3) × 10-4 cm-1. These results are discussed and compared with the existing literature. Fil: Khadir, Narjes. Sharif University of Technology. Department of Chemistry; Irán Fil: Boghaei, Davar M.. Sharif University of Technology. Department of Chemistry; Irán Fil: Assoud, Abdeljalil. University of Waterloo. Department of Chemistry; Canadá Fil: Nascimento, Otaciro R.. Universidade de São Paulo. Instituto de Física de São Carlos. Departamento de Física e Ciência Interdisciplinar. Grupo de Biofísica Molecular Sergio Mascarenhas; Brasil Fil: Nicotina, Amanda. Universidade Federal do Rio de Janeiro. Instituto de Física; Brasil Fil: Ghivelder, Luis. Universidade Federal do Rio de Janeiro. Instituto de Física; Brasil Fil: Calvo, Rafael. Universidad Nacional del Litoral. Facultad de Bioquímica y Ciencias Biológicas. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Física del Litoral; Argentina

Made available in DSpace on 2018-12-11T17:11:37Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-04-10 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) The oxyborate Fe3O2BO3 presents a charge density wave (CDW) transition close to room temperature. As we show here, this is associated with a well defined anomaly in the specific heat. Below this transition, when applying in a single crystal of Fe3O2BO3 a DC voltage above a temperature dependent threshold, a high current is liberated in this material. We study the conduction in single crystals of Fe3O2BO3 with voltage applied parallel and perpendicular to the crystallographic c axis direction. The observed currents are attributed to the depinning of charge ordered domains above a threshold voltage V T2 that gives rise to a collective conduction due to coherent domains. Compliance limited DC data shows that above a lower threshold voltage depinning is smooth and follows a power law scaling. Similar depinning with power law scaling is also revealed in the AC conductivity. Departamento de Física IGCE Universidade Estadual Paulista UNESP - C.P. 178 Instituto de Física Universidade Federal Fluminense Campus da Praia Vermelha Instituto de Física Universidade Federal Do Rio de Janeiro, Caixa Postal 68528 Centro Brasileiro de Pesquisas Físicas Rua Dr. Xavier Sigaud, 150 - Urca Departamento de Física IGCE Universidade Estadual Paulista UNESP - C.P. 178

Cu, Fe, Gd and Nd dopants were added to HNb2O5 in a range of concentrations to investigate the influence on the magnetic properties. The dopants fill the vacant sites in the channels along the b axis of the structure. Magnetic moments tend to remain localized in the dopants. AC susceptibility measurements display low temperature undulations associated with short range magnetic correlations within small clusters (of two or three ions) of dopants. Glauber spin-flip kinetics for Ising spin rings can be applied to these data. Although the magnetic behaviour is mostly Curie–Weiss above 20 K, the susceptibility data display a temperature-independent residual paramagnetic signal. We attribute this signal to Van Vleck orbital paramagnetism associated with bonding–antibonding transitions within Nb4+ or Cu2+ spin-paired dimers.

The exchange interactions between copper ions in (l-tryptophyl-glycinato)copper(II), C13H13CuN3O3 (named Cu(II)Trp-Gly) are determined by ac magnetic susceptibility and dc magnetization measurements in powder samples, and by EPR measurements in single-crystal samples. The copper ions in this compound are arranged in two symmetry-related types of chains along the b axis. Neighbor coppers in a chain at 5.14 A are connected by equatorial syn−anti carboxylate bridges, which transmit the intrachain exchange interactions. Indole rings of neighbor tryptophan residues at apical positions of the copper ions provide cation−π contacts that give rise to pathways supporting exchange interactions between the chains. The susceptibility and magnetization data obtained in the temperature range 1.8 < T < 100 K, with applied dc magnetic fields (B) up to 9 T are explained assuming one-dimensional spin chains with ferromagnetic intrachain coupling 2J/k = 3.9 ± 0.1 K, calculated from the zero-field susceptibility. The magnetiz...

The existence of superconductivity in samples of PrBa 2 Cu 3 O 7−δ (Pr-123) is still a polemic subject. To further study this problem, we have prepared high-quality polycrystalline Pr-123 samples by following a sol-gel method, associated with heat treatment in inert atmosphere. In this paper we report the structural, thermal and magnetic properties of superconducting sintered-polycrystalline Pr-123 and give a descriptive explanation about its unique dependence on the preparation procedure. The list of evidences of superconductivity in Pr-123 is further expanded with new data on a superconducting thin film. The present results confirming the occurrence of superconductivity in Pr-123 revitalizes the hypothesis that superconductivity originates outside the cuprate-plane.