Jacek Cwik

The magnetotransport properties of polycrystalline silicon layers in silicon-on-insulator (SOI) structures were studied at low temperatures down to 4.2 K and in magnetic fields up to 14 T. Experimental results reveal both negative and anomalous positive magnetoresistance in nonuniform polycrystalline silicon films, depending on the level of p-type doping near the metal–insulator transition. Low-temperature behavior of the magnetoresistance is caused to the trapping of free charge carriers at grain boundary defects. The estimated Curie temperatures for highly defective polycrystalline layers in SOI structures show both positive and negative values, indicating the coexistence of ferromagnetic and s–d antiferromagnetic interactions. In lightly doped samples with NB = 2.4⋅1018 cm–3, the negative magnetoresistance is associated with the ordering of localized spins, in samples with NB = 3.9⋅1019 cm–3, it is dominated by the polarization of conduction carriers and their additional scattering by localized magnetic moments near grain boundaries. Interaction between charge carriers within the framework of hopping conductivity in doubly occupied localized states (in the temperature range of 4.2–20 K), due to spin-dependent scattering in polycrystalline silicon films with NB = 3.9⋅1019 cm–3, can serve as a model analogy for understanding related effects in high-temperature superconductors, in particular pseudogap behavior and unconventional superconducting carrier pairing.

Cryogenic magnetocaloric in a disordered double-perovskite Gd ₂ MnZnO ₆ .

The critical behavior around ferromagnetic–paramagnetic phase transition of polycrystalline Tb1−xHoxNi2 solid solutions (x = 0.25, 0.5, 0.75) was investigated using magnetization measurements in a magnetic field range of 0–5 T. The critical exponents obtained using the Kouvel–Fisher modified Arrott plot methods and the Widom scaling relation are consistent. The tricritical and 3D-Ising models adequately describe the samples, likely due to Ho substitution affecting the critical parameters. The spin interactions indicate long-range character in all studied compositions. Critical parameter analysis shows that the magnetic transition temperature decreases with the increasing Ho content, from 29.3 K for Tb0.75Ho0.25Ni2 to 17.1 K for Tb0.25Ho0.75Ni2. For the Tb0.5Ho0.5Ni2, direct measurements of adiabatic temperature change near the Curie temperature were performed in magnetic fields up to 14 T. The maximum ΔTad reaches ∼11 K near 26.3 K for μ0ΔH = 14 T. These results are compared with TbNi2 and HoNi2 and analyzed within the Landau theory of second-order phase transitions. To demonstrate application potential, composite materials based on Tb1−xHoxNi2 (x = 0.25–0.75) were proposed. Optimal molar ratios of Tb0.75Ho0.25Ni2, Tb0.5Ho0.5Ni2, and Tb0.25Ho0.75Ni2 were theoretically determined. The composites exhibit a nearly constant calculated magnetic entropy changes, ∼3.4 J/kg K for μ0ΔH = 1 T and ∼ 6.3 J/kg K for μ0ΔH = 2 T over a wide temperature range of 18–30 K. These results indicate that the proposed composites are promising candidates for use as refrigerants in low-temperature magnetic refrigerators.

The ΔT_ad values are high and reach their maximum within a narrow temperature range below 20 K. This indicates that Dy_1−xEr_xNi₂ solid solutions may be a viable option for refrigerants in magnetic cryocoolers suitable for hydrogen liquefaction.

Direct measurements of the magnetocaloric effect were performed in a Heusler Ni_44.4Mn_36.2Sn_14.9Cu_4.5 alloy at cryogenic temperatures in magnetic fields up to 10 T. The maximum value of the inverse magnetocaloric effect in a 10 T field was ∆T_ad = –2.7 K in the vicinity of the first-order magnetostructural phase transition at T₀ = 117 K. Ab initio and Monte Carlo calculations were performed to discuss the effect of Cu doping into a Ni-Mn-Sn compound on the ground-state structural and magnetic properties. It is shown that with increasing Cu content the martensitic transition temperature decreases and the Curie temperature of austenite slightly increases. In general, the calculated transition temperatures and magnetization values correlated well with the experimental ones.