Solid State Phenomena Vols. 233-234

Abstract: In this paper, we investigated the magnetocaloric effect (MCE) in one-dimensional magnets with different types of ordering in the Ising model, Heisenberg, XY-model, the standard, planar, and modified Potts models. Exact analytical solutions to MCE as functions of exchange parameters, temperature, values and directions of an external magnetic field are obtained. The temperature and magnetic field dependences of MCE in the presence of frustrations in the system in a magnetic field are numerically computed in detail.

Abstract: High cooling power of magnetocaloric refrigeration can be achieved only at large amounts of heat, which can be transferred in one cycle from cold end to hot end at quasi-isothermal conditions. The simple experimental method for direct measurement of the transferred heat from material with magnetocaloric effect (MCE) to massive nonmagnetic block at quasi-isothermal conditions was proposed. The vacuum calorimeter was designed for the simultaneous measurements of MCE both at adiabatic conditions (∆T) and quasi-isothermal conditions (∆Q) in the magnetic fields of Bitter coil magnet. This calorimeter was tested on samples of pure polycrystalline Gd with direct MCE. The maximal obtained values were ∆T = 17.7 K and ∆Q = 5900 J/kg at initial temperature 20 °C in magnetic field 140 kOe.

Abstract: In this work we report the results of experimental investigation of the magnetocaloric effect in Gd_100-xZr_x (x = 0, 1, 2, 3) cold rolled ribbons. As it is shown the magnetocaloric effect exist within the large temperature interval 279-295 K and comparable with magnetocaloric effect observed on pure Gd. As it shown cold rolling is one of promising techniques for producing thin forms of magnetocaloric materials for heat exchangers of magnetic cooling devices.

Abstract: Transverse Kerr effect (TKE) was used to study magneto-optical (MO) properties of Ni₄₅Mn_36.7In_13.3Co₅ (at.%) single crystals. A single crystalline ingot of such composition was grown by the Czochralski method. One series of samples was quenched into cold water (WQ) and the other series after quenching was heated at 770 K for 20 min and slowly cooled to assure a complete atomic order (SC). Accordingly to differential scanning calorimetry (DSC), magnetic and magneto-optical (MO) measurements, WQ samples exhibit well-defined martensitic transition (MT), but the SC samples do not show MT. It is found that TKE for WQ samples shows the following features (i) MO signal is well pronounced far below the martensitic transition in spite of a weak magnetization of martensitic phase; (ii) the characteristic temperatures of martensitic transition differ from those for the bulk and depend on annealing conditions; (iii) MO spectra profile do not change significantly during the martensitic transition and is similar but not identical with that for Ni₅₀Mn₃₅In₁₅ thin films and Ni_43.7Mn_43.6In_12.7 ribbons measured before; (iv) MO signal is anisotropic.

Abstract: The structural and magnetic properties of Mn-based stoichiometric Heusler alloys have investigated by means of ab initio calculations in framework of the density functional theory. First principles electronic structure calculations have shown that Mn₂NiZ (Z = Ga, In, Sn, Sb) alloys are ferrimagnets with antiparallel alignment between the Mn atoms. The martensitic transition can be realized in Mn₂NiGa and Mn₂NiSn alloys with tetragonal ratio of 1.27 and 1.16, respectively. Calculated properties are in a good agreement with available experimental data.

Abstract: Magnetic, magnetocaloric and transport properties of alloys of system Ni-Mn-In has been investigated. Large magnetocaloric effect have been observed at temperatures below room, making it a potential material system for various applications.

Abstract: In this work we report the results of investigation of the magnetocaloric effect in Gd_100-xY_x (x = 0, 5, 10, 15) cold rolled ribbons. It is shown that the magnetocaloric effect exists within a wide temperature interval 258-295 K and it is comparable with the magnetocaloric effect observed in bulk samples of pure gadolinium. The value of the magnetocaloric effect in the rolled samples is reduced in comparison with the bulk samples and strongly depends on a degree of plastic deformation. High temperature heat treatment can restore a value of the magnetocaloric effect in the cold rolled ribbons up to initial ones. Thus, cold rolling is proposed to be a promising technique for producing thin forms of magnetocaloric materials for heat exchangers of magnetic cooling devices.

Abstract: The paper presents the investigation of GdNiH_3.2 and TbNiH_3.4 hydrides magnetic transitions and magnetocaloric properties. The isothermal magnetization data in the fields up to 5T are obtained for GdNi and TbNi compounds and their hydrides and the values of magnetic entropy change are calculated. The maximum values of magnetic entropy change ΔS_M in GdNiH_3.2 and TbNiH_3.4 are extremely large. It is shown that the hydrogenation shifts ΔS_M(T) maximum to lower temperatures.

Abstract: A study of crystalline structure, magnetic and magnetocaloric properties of Dy(Co_1-хFe_х)₂ (х = 0.10, 0.15) intermetallic compounds was undertaken. Phase composition was controlled by X-ray diffraction analysis. Magnetic properties were measured with a help of SQUID magnetometer in magnetic fields up to 7 Т in the temperature range from 4.2 K to 400 K. Magnetic transition temperatures from paramagnetic to magnetically ordered state were inferred as 288 K and 350 K, respectively. It is shown that at an increase of iron concentration and/or magnetic field intensity, a considerable maximum broadenings on a temperature dependence of magnetic entropy change is observed. The calculated value of the relative cooling power (RCP) of Dy(Co_0.90Fe_0.10)₂, in a magnetic field of 1.7 T is equal to 152 J/kg that is close to that for Gd metal with RCP = 181 J/kg at μ₀Н = 2 T.

Abstract: In this work, we report on the theoretical calculations of the temperature dependence of magnetization in Ni₂MnGa alloy by Monte Carlo simulation. The calculations have been carried out for the single crystal and polycrystalline structures with 16, 32 and 64 magnetic domains. The theoretically calculated results are in good agreement with the experimental data.