Vol 68, No 4 (2023)

The possible presence of half-metallic properties in Fe2RhZ (Z = Al, Si, Ga, Ge, In, Sn) ferromagnetic Heusler alloys is ab initio studied. The calculation is carried out taking into account the exchange correlation effects using the generalized and meta-generalized gradient approximations. It is shown that the ground state of most investigated alloys is ferromagnetic ordering in the Тр model lattice. An analysis of the phase stability of the alloys showed that, in the considered approximations, most of them are stable in both the regular and inverse lattice, as well as in the three model lattices belonging to inverse-type Heusler structures with different atomic orders. It was predicted using the strongly constrained and appropriately normed (SCAN) functional that the Fe2RhSi alloy can exhibit the properties of half-metals, since it has an integer magnetic moment, a fairly high degree of polarization, and an energy gap at the Fermi level of spinup electrons.

The results of a study of the magnetocaloric effect (MCE) in Ni2.25Mn0.75Ga0.93Si0.07 alloy are presented in the cast state and in the state after multi-axial isothermal forging (MIF) at 700°C and true degree of deformation e = 3.19. It is shown that as a result of MIF, the initial equiaxed microstructure is transformed into a bimodal one in which large grains 100–200 μm in size are surrounded by a layer of fine-grained microstructure.
As a result of MIF, the range of martensitic transformation is slightly shifted to the region of low temperatures by about 5°C. The analysis of phase transformations in the region of room temperatures shows that the intervals of martensitic and magnetic phase transformations are superimposed on each other. The MCE value in a magnetic field of 1.8 T is 0.59ºC in the initial cast state, and as a result of forging it decreases to 0.55°C.

A physical model of a mechanical thermal switch at cryogenic temperatures is studied. In the model, heat is transferred due to contact heat conduction in a detachable contact pair of two copper cylinders. A mechanical thermal switch is developed using a cryomagnetic system with a 10-T superconducting solenoid, and the values of thermal contact conductance are determined in a temperature interval of 10–160 K, including values at a magnetic field of 5 T. In an experimental temperature interval of 60–80 K, close to the phase transition of the DyAl2 and GdNi2 compounds, the thermal contact conductance is 2300–3300 W/(m2 K). The effect of magnetic field of up to 5 T on thermal contact resistance is experimentally determined under vacuum conditions

The magnetic and magnetocaloric characteristics of the Mn1.9Cu0.1Sb alloy were studied. The presence of a relatively sharp decrease in the magnetization in the region of 100 K is established, which, according to ab initio calculations, can be interpreted as antiferromagnetism–ferrimagnetism transitions. The presence of a magnetic phase transition from a ferrimagnetic to an antiferromagnetic state (F ↔ AF) leads to the appearance of an inverse magnetocaloric effect, which is preserved in magnetic fields up to 10 T.

Experimental studies of the magnetic and magnetocaloric properties of the Laves phase of GdNi2 have been carried out in external static up to 3 T and pulsed up to 50 T magnetic fields. It has been found that in a magnetic field of 3 T the change in the magnetic entropy of the alloy reaches its maximum value ΔSm = −8 J/(kg K) in the vicinity of the Curie temperature TC = 73.6 K. The corresponding adiabatic temperature change in this case, calculated by an indirect method, is ΔTad ≈ 3 K. The maximum value of the adiabatic temperature change measured by the direct method in a pulsed magnetic field of 50 T at T0 = 77 K, was equal to ΔTad = 15 K, which agrees well with theoretical predictions.

The heat treatment effect of the magnetostrictive component in magnetoelectric (ME) composites consisting of a piezoelectric and magnetostrictive material has been studied. The dependence of the ME voltage coefficient on frequency was experimentally found without heat treatment and with annealing from 200 to 500°C of the AMAG493 amorphous alloy, which acted as a magnetostrictive component. It is shown that with an increase in the processing temperature of an amorphous alloy, an increase in the ME voltage coefficient is observed: the maximum value of the ME coefficient was observed at a temperature of 350°C and amounted to 29.52 V cm–1 Oe–1 at a resonance frequency of 54 kHz. It has been proven that the increase in the ME voltage coefficient occurs due to the improvement in the characteristics of the amorphous alloy during heat treatment, which leads to partial nanocrystallization of the material.