The magnetic and magnetotransport properties of thin Heusler alloy Ni 49.7 Fe 17.4 Co 4.2 Ga 28.7 films deposited onto MgO(100) substrates are studied over a wide temperature range, which includes a martensitic transition (MT). For this composition, the MT is not accompanied by a magnetic phase transition, since the martensitic and austenitic phases are ferromagnets with similar magnetizations. The electrical resistivity does not undergo sharp changes during the MT. The magnetoresistance is negative, decreases in magnitude with increasing temperature in the range 100–250 K corresponding to the MT, and then increases to –1%. The field dependences of the Hall effect resistivity have the shape that is characteristic of homogeneous ferromagnetic alloys. The coefficients of the normal and anomalous Hall effects are determined. The anomalous Hall effect coefficient is shown to be described by the relation R s = αρ + βρ 2 , where ρ is the electrical resistivity and the second term is lower than the first, which indicates an important role of the interference impurity–phonon scattering mechanism.

Magnetoelastic transitions (METs) in bulk in nearly equiatomic Fe-Rh alloys produced by arc melting may show poor reproducibility related to insufficient chemical homogeneity and presence of impurity phases in variable concentrations. To better understand the synthesis conditions that reliably yield bulk FeRh materials with reproducible MET characteristics, Fe100-xRhx alloys with x = 50, 50.5 and 51 at. % were prepared by induction melting and thermal annealing under identical conditions. The fabricated samples were cut into several slices, followed by characterization of METs in each of the slices using isothermal and isofield magnetization measurements, differential scanning calorimetry, and direct measurements of the magnetocaloric effect. All of the slices exhibit METs between the AFM and FM states, but the transitions are abrupt with nearly the same change of magnetization, Delta M, when x = 50.5 and 51, whereas for the x = 50 alloy the transition spreads over a wide temperature interval and Delta M may fluctuate by as much as 10 % from one specimen to another. A comparison of the magnetocaloric responses of x = 50 and 51 materials is presented. The clearly different effect of the magnetic field on the transition in both directions leads to significant differences in the reversibility and maximum values of the magnetic field-induced entropy and adiabatic temperature changes, as well as average hysteresis losses. In terms of reproducibility, our results suggest that induction melting is a more appropriate technique to prepare these binary alloys. (C) 2021 Elsevier B.V. All rights reserved.

Three temperature-induced phase transitions at T=T1, TM/TA, and TC, related to the ferromagnetic order of the martensitic phase (FMMP), martensitic (structural) transitions (MT), and the ferromagnetic order of the austenitic phase (FMAP), respectively, have been observed in the off-stoichiometric Heusler alloys, Ni50-xCoxMn35In15 (x=1, 2.5) and Ni45Co5Mn37In13. The phase transitions temperatures are found to be depended on alloy composition. A kinetic arrest of the AP was observed for Ni47.5Co2.5Mn35In15 in the magnetization measurements during field-cooling cycle (FCC) at 50 kOe. Depending upon the cooling protocols, ZFC and FCC (at H = 50 kOe), two different ground states of the alloys can be found in Ni47.5Co2.5Mn35In15 and Ni45Co5Mn37In13 alloys. The ground states (T=4.2 K and external field H=0) of the alloys was found to be characterized by three main line: two, partially overlapping, at higher frequencies (300-450 MHz), most likely corresponding of manganese resonance lines and one at lower frequency at about 200 MHz. A significant shift in the spectrum of Ni45Co5Mn37In13 by about 100 MHz to higher frequencies was observed. The correlation of magnetizations obtained from magnetic moment and NMR studies is discussed.

We investigate the temperature dependences of the magnetic properties, electrical resistivity, magnetoresistance, and Hall effect resistivity, rho(H)(H), in thin films of the Heusler-type Ni47.3Mn30.6Ga22.1 (at.%) magnetic shape memory alloys epitaxially grown onto a MgO(001) substrate. The results reveal martensitic transformation at about 230 K, premartensitic transition around 285 K, and the Curie temperature of austenite around 380 K. We obtained the coefficients of normal Hall effect (NHE), R-0, and anomalous Hall effect (AHE), R-s by fitting the total Hall resistivity curves rho(H) = R0Bz + 4(pi)R(s)M(z) in several magnetic field ranges (0.1-1, 0-5, 8-16, and 0-16 kOe), using experimental magnetization data. Both coefficients R-0 and R-s strongly depend on the magnetic field. We also fit the Hall effect resistivity with the expression rho(H) = R0Bs + 4 pi RsMz + Delta rho(H) using the coefficients R-0 and R-z obtained from the high-field interval (8-16 kOe), where the last term, Delta rho(H), was considered to correspond either to the topological Hall effect or to the antiferromagnetic Hall effect. The obtained temperature dependence and magnitude of Delta rho(H) discard the presence of the skyrmions or antiskyrmions. We conclude that unconventional field dependences of the NHE and AHE coefficients are produced by the antiferromagnetic correlations and the influence of the magnetic field on the electronic structure.

Magnetization, electrical resistivity, magnetoresistance, and Hall resistivity of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5 Heusler alloys were studied in a temperature range T = 80-400 K in magnetic fields up to 20 kOe. Both alloys exhibit a martensitic transformation from a high-temperature ferromagnetic austenite phase to a low-temperature, low-magnetization martensitic phase. The electrical resistivity nearly doubles as a result of the martensitic transformation, reaching 180 and 100 mu Omega cm in the martensitic states of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5, respectively. The temperature dependence of the electrical resistivity does not corresponded with the Mooij correlation. The magnetoresistance is negative with a narrow negative peak at the martensitic transition. Normal and anomalous Hall effect coefficients were determined by fitting the field dependences of the Hall resistivity using magnetization data. The coefficients of the normal Hall effect for both compositions were found to decrease with temperature from positive values in the austenite to negative values in the martensite phase. None of the known correlations between the anomalous Hall effect coefficient and resistivity were satisfied. Significant changes in the values of the anomalous Hall coefficients during the martensitic transformation are explained by the difference in spin-up and spin-down state occupations in the martensite and austenite phases. First-principles calculations of the electronic structures confirm this explanation.

The magnetic and magneto-optical properties of (Со40Fe40B20)x(SiO2)100x nanocomposites with x = 30–72 at % are studied. The results reveal the inhomogeneous structure of the nanocomposites, which exhibit both large granules and small particles that make independent contributions of different nature to the magnetic properties of the materials. Specific features of the coercive force near the percolation threshold indicate superferromagnetic ordering in the composites at low temperatures.

We present results of experimental studies of magnetic properties, resistivity and magnetoresistance (MR) focusing on MR in high magnetic fields of (Co40Fe40B20)(x)(SiO2)(100-x) and (Co84Nb14Ta2)(x)(Al2O3)(100-x) nanocomposites near the percolation threshold. The nano-composite films with x = 47-58 at.% were deposited onto a glass-ceramic substrate by the ion-beam sputtering. The samples consist of metallic nanogranules embedded into the nonstoichiometric matrix. According to the structural and magnetic data a large amount of metallic atoms are dispersed between magnetic nanogranules. With decreasing metal volume fraction the temperature dependence of conductivity changes from InT behavior, matches a strong tunnel coupling between nanogranules, to the T-1/2 dependence below the metal-insulator transition. MR was studied in pulsed magnetic fields up to 20 T at T = 65-300 K. The pulse duration was 11 ms. Negative MR almost saturates with increasing magnetic field but slowly increases or decreases in high magnetic fields. There is an evidence of linear positive contribution to MR above saturation at 65-300 K with the slope (3-9) 10(-3)% T-1. It was shown that possible reason for positive MR is influence of high magnetic field on the tunnel barrier height and as a result on the tunnel transparency.

Using (Co40Fe40B20)(x)(LiNbO3)(100-x) (x = 6-55 at.%) nanocomposite films as an example, we have performed comparative investigations of granular systems properties with a high (similar to 10(22) cm(-3)) and low (<= 10(21) cm(-3)) content of dispersed atoms (Fe and Co) in an insulating nonstoichiometric matrix. The nanocomposite films were produced using ion-beam sputtering of the composite targets onto glass-ceramic substrates at different growth temperatures (similar to 40 and similar to 80 degrees C). We show that magnetic ions dispersed in a matrix play a crucial role both in the ferromagnetic exchange between granules and in magnetic, transport, and memristive properties below the percolation threshold x(p) approximate to 50-55 at.%. The presence of a high content of dispersed magnetic ions induces intergranular interaction, shifts the critical concentration of the metal-insulator transition x(c) from 48 to 43 at.%, leads to high-field positive magnetoresistance near x(p) and superferromagnetic ordering and magnetic hysteresis below x(c) (down to 33 at.%), and makes the stable resistive switching for capacitorlike structures with resistance ratio R-OFF/R-ON more than 10(2) at x approximate to 10 at.%.

Two metamagnetic Heusler alloys based on Ni45Cr5Mn37In13 with differing Mn concentrations (by 0.2 at. %) were studied using magnetization, resistivity, and Hall resistivity measurements. It is shown that this small variation in composition leads to fundamental changes in magnetic properties. Near the martensitic phase transitions, the anomalous Hall coefficient (R-s) was found to increase with temperature while the resistivity (rho) decreases. Thus, in the case of Ni45Cr5Mn37In13, the general correlation between R-s and rho, considered as applicable for all crystalline and amorphous ferromagnetic alloys, breaks down completely. We explain this behavior in the framework of an s-d model where it is assumed that the resistivity and anomalous Hall effect are determined by different groups of current carriers.

We present results of experimental studies of magnetic properties, resistivity and magnetoresistance (MR) of (Co84Nb14Ta2)x(Al2O3)100-x films deposited onto a glass-ceramic substrate by the ion-beam sputtering, focusing on MR in high magnetic fields for compositions close to the percolation threshold (x=47-57 at.%). The samples consist on Co-Nb-Ta metallic nanogranules size of 2-5 nm which are embedded into the non-stoichiometric Al-O matrix. Magnetization was measured by SQUID magnetometer at T=4.2-350 K. MR was studied in the pulsed magnetic fields μ0H up to 20 T at T=70-300 K in three geometries: magnetic field in plane parallel and perpendicular to current, magnetic field perpendicular to plane. The pulse duration was 11-12 ms. For the sample with x=57 at.% the temperature dependence of conductivity follows the lnT behavior that matches a strong tunnel coupling between nanogranules. With decreasing metal volume fraction lnT behavior gradually changes to the T1/2 dependence at 47 at.%. For all samples MR is small (<1%) and negative. For x<57 at.% it is slightly anisotropic at μ0H<1.0 T and almost saturates with increasing magnetic field. There is an evidence of small positive contribution to MR at μ0H=20T. Accordingly to structural and magnetic data a large amount of metallic atoms are located between magnetic nanogranules that diminish the tunnel barrier height and make tunnel MR small and weakly dependent on temperature.

The structural, magnetic, and magnetotransport properties of Ni50-xCrxMn37In13 Heusler alloys have been synthesized and investigated by x-ray diffraction (XRD), field and pressure dependent magnetization, and electrical resistivity measurements. The partial substitution of Ni by Cr in Ni50Mn37In13 significantly improves the magnetocaloric effect in the vicinity of the martensitic transition (TM). This system also shows a large negative entropy change at the Curie temperature (TC), making it a candidate material for application in a refrigeration cycle that exploits both positive and negative magnetic entropy changes. The refrigeration capacity (RC) values at TM and TC increase significantly by more than 20 % with Cr substitution. The application of hydrostatic pressure increases the temperature stability of the martensitic phase in Ni45Cr5Mn37In13. The influence of Cr substitution on the transport properties of Ni48Cr2Mn37In13 is discussed. An asymmetric magnetoresistance, i.e., a spin-valve-like behavior, has been observed near TM for Ni48Cr2Mn37In13.

Direct measurements of the adiabatic temperature change (ΔTAD) of Ni50Mn35In14.5B0.5 have been done using an adiabatic magnetocalorimeter in a temperature range of 250–350 K, and with magnetic field changes up to ΔH = 1.8 T. The initial susceptibility in the low magnetic field region drastically increases with temperature starting at about 300 K. Magnetocaloric effects parameters, adiabatic temperature changes, and magnetic entropy changes were found to be a linear function of H2/3 in the vicinity of the second order transitions (SOT), whereas the first order transitions do not obey the H2/3 law due to the discontinuity of the transition. The relative cooling power based on the adiabatic temperature change for a magnetic field change of 1.8 T has been estimated. Maximum values of ΔTAD = −2.6 K and 1.7 K were observed at the magnetostructural transition (MST) and SOT for ΔH = 1.8 T, respectively. The observed ΔTAD at the MST exceeds the ΔTAD for Ni50Mn35In14X with X = In, Al, and Ge by more than 20% and is larger than the Gd based Heusler alloys.

The magnetic and magnetocaloric characteristics of Ni50Mn35In15 Heusler alloy are studied in low and high applied magnetic field. At a magnetic field of 14 T, the adiabatic temperature change ΔT ad measured by the sample extraction technique near the martensitic transformation (≈315 K) is as large as 11 K. This value is an order of magnitude larger than the corresponding change measured at 1.6 T. The observed giant values of the magnetocaloric effect could be related to the suppression of antiferromagnetic correlations.