This work describes experimental study by a direct and indirect method of magnetocaloric properties of Ni-based alloys: Ni 93.8 V 6.2 , Ni 89.5 Al 10.5 , Ni 72.5 Cu 27.8 , which are promising materials for self adjusting magnetic hyperthermia. It has been demonstrated that the temperature of the maximum magnetocaloric effect determined by these methods can differ significantly, which is of critical importance for self-adjusting hyperthermia.

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.

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.

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.

The isothermal magnetic entropy changes are studied in Ni80Fe20/Ni67Cu33/Co90Cu10/Mn80Ir20 stacks at temperatures near the Curie point of the Ni67Cu33 spacer by applying magnetic fields in a few tens of Oersted. Such low fields were sufficient for toggling magnetic moments in the soft ferromagnetic (FM) layer (Ni80Fe20). It is found out that this switching provides the magnetic entropy change, which is up to 20 times larger than that achievable in a single Ni67Cu33 film subjected to such low fields. Our finding holds promise to be utilized in the magnetocaloric devices that would be based on FM/PM/FM heterostructures and would operate with moderate magnetic fields.

We present the results of the studies of magnetic and magnetocaloric (MCE) properties of quaternary Heusler alloys Ni48.5Mn35In15Co1.5 and Ni50Mn35In13.5Al1.5 in temperature range between 80 K and 400 K. While doping initial ternary Ni-Mn-In alloy with 1.5 at.% Al does not change TCA as well as MS, doping with 1.5 at.% Co significantly increases both TCA and MS. The direct magnetocaloric measurements around martensitic transition and austenite Curie temperature reveal the competition between the contributions of different signs. The latter is related to the vicinity of transition temperatures of austenite and martensite, resulting in mixed state, where both phases undergo magnetic order-disorder transformation in the same temperature region. Additionally temperature dependencies of adiabatic temperature change ΔTad and magnetic entropy ΔSM of the system exhibit additional peak feature which we attribute to intermartensitic transition occurring in these alloys.

The magnetic properties and reversibility of the magnetocaloric effect of Ni50Mn35In15 have been studied in the vicinity of the phase transition using magnetization and direct adiabatic temperature change (Delta T-ad) measurements in magnetic fields up to 14 T. The magnetostructural phase transitions (MSTs) between a martensitic phase (MP) with low magnetization (paramagnetic or antiferromagnetic) and a nearly ferromagnetic austenitic phase were detected from thermomagnetic curves, M(T,H), at the applied magnetic fields up to 5 T. The MST temperature was found to be nearly independent of magnetic field for H < 5 T, and shifted to lower temperature with the further increase of magnetic field to 14 T. A large and nearly reversible inverse magnetocaloric effect (MCE) with Delta T-ad similar to -11 kappa for a magnetic field change of Delta H = 14 T was observed in the vicinity of the MST. The irreversibility of Delta T-ad was found to be 1 K. A direct Delta T-ad of +7 K for Delta H = 14 T was detected at the second order ferromagnetic-paramagnetic phase transition. The obtained results have been discussed in terms of the suppression of antiferromagnetic correlations with the application of a strong magnetic field, and a reversibility of the initial magnetic state of the MP with applied magnetic field when the MST coincides with T-C. (C) 2016 Elsevier B.V. All rights reserved.

The temperature dependence of the electrical conductivity σ( T ) of (Co 41 Fe 39 B 20 ) x (Al–O) 100– x of nanocomposite films for different concentrations x of amorphous ferromagnetic metal (56 > x > 30) has been studied in the temperature range of 4.2–300 K. It has been shown that, for concentrations in the interval 56 > x > 49, the conductivity obeys the logarithmic law σ( T ) = A (1 + αln T ), where A and α depend on the concentration. According to the theory developed by Efetov et al., this logarithmic dependence is connected with specificities of the Coulomb interaction in nanogranulated alloys on the intergranule tunneling in the transient region of concentrations from metallic conduction to the dielectric regime. The comparison of the theory with the experiment has revealed only qualitative agreement. The reasons of the quantitative disagreement have been discussed. The resistivity of samples with the concentrations lying in the range 49 > x > 30 obeys the 1/2 power law.

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.

We present our last results on anomalous Hall effect (AHE) in (Co 41 Fe 39 B 20 ) x (Al–O) 100-x nanocomposites focusing on the possible correlation between temperature dependence of AHE and resistivity. It is shown that the temperature dependence of conductivity G =1/ R xx , where R xx is resistivity, for compositions with x =49-56% at 10K< T < T k follows the relation, where the parameters A, , T k depend on x . For x =47% this relation changes to the exponential law “1/2” R xx ∝ exp (Т 0 /T) 1/2 . The correlation between AHE resistivity R H (T) and resistivity R xx (T) can be described as R H ∝ ( R xx ) m , where m increases from 0.38 to 0.58 with an increase of x from 49 to 56 %.

The impact of B substitution in Ni50Mn35In15−xBx Heusler alloys on the structural, magnetic, transport, and parameters of the magnetocaloric effect (MCE) has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (H) up to 5 T, and in the temperature interval 5–400 K). Direct adiabatic temperature change (ΔTAD) measurements have been carried out for an applied magnetic field change of 1.8 T. The transition temperatures (T-x) phase diagram has been constructed for H = 0.005 T. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. The maximum absolute value of ΔTAD = 2.5 K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5.