[en] Gd₅Si₄, Gd₅Ge₄, and Gd_5.09Ge_2.03Si_1.88 compounds were studied by electron spin resonance. The arc-melted samples were initially characterized by optical metallography, x-ray diffraction, and static magnetization measurements. The electron spin resonance results show a negative paramagnetic g shift for Gd₅Si₄ and Gd_5.09Ge_2.03Si_1.88, and a smaller positive one for Gd₅Ge₄. The values of the exchange parameter (j) between the localized Gd-4f spins and the conduction electrons are obtained from the g shifts. These values are positive and of the same order of magnitude for Gd₅Si₄ and Gd_5.09Ge_2.03Si_1.88, and negative one order of magnitude smaller for Gd₅Ge₄. The electron spin resonance data were interpreted considering the strongly bottlenecked solution of the coupled Bloch-Hasegawa equations

[en] We report on pressure effects on the magnetic and magnetocaloric properties of the compound UGa₂. Using a mean field approximation, we were able to calculate the isothermal entropy change and the adiabatic temperature change. Neither the applied pressure nor the chemical substitution experiments within the ranges studied revealed a remarkable improvement on the magnetocaloric effect (MCE) except for the Al substitutions. Nevertheless, we found that mechanical pressure and chemical pressure are equivalent in terms of the Curie temperature shift when Al, Ge and Si are substituted for Ga, but a different behavior is found when Ni, Fe and Co are used. Our results also show that a composite to operate between 80 and 120 K can be obtained using different concentrations of U(Ga,Ni)₂.

[en] The structural and magnetic properties of Fe₂MnSi_1-x_Ga_x ( x = 0, 0.02, 0.04, 0.08, 0.09, 0.12, 0.16, 0.20, 0.30 and 0.50) Heusler alloys were studied by diffraction and energy dispersive spectroscopy (EDS), X-ray diffraction and magnetic measurements, in order to investigate potential magnetocaloric properties and to optimize the possible use of these alloys for magnetic refrigeration. We found a monotonic behaviour in the structural properties with Ga content (x), but with increasing x, T_C decrease, which at first is not expected for these compounds. (author)

[en] Effects of Al, Mn and Sb dopings in CeFe₂ and the effect of applied pressure have been investigated. Al doping gives rise to the FM-AFM transition and a reduction in the magnetic moment and T_C values, clearly indicating the growth of the AFM component. Mn and Sb dopings only cause a reduction in the T_C value. It is found that, in general, external pressure enhances the antiferromagnetism in both the pure and the doped alloys. Enhancement of the Ce 4f-Fe 3d hybridization as a result of doping and with the external pressure may be the reason for the stabilization of antiferromagnetism in these alloys.

[en] This paper presents a method for detecting the magnetocaloric effect (MCE), based on the acoustic detection. Small temperature oscillations, due to the application of a modulated magnetic field, are detected by a microphone in a closed cell. The continuous scanning of a superimposed dc magnetic field allows, by numerical calculation, the determination of large temperature variations caused by magnetic field steps from zero to tens of kOe. Measurements were performed in Gd and Gd₅(Si_xGe_1-x)₄ compounds. The obtained results show the efficiency of the technique, which is suitable for the investigation of materials undergoing both purely magnetic phase transitions and magnetic-crystallographic first order ones.

[en] RMn₂Si₂ (R = Tm, Dy, and Tb) and HoCoSi alloys, presenting multiple magnetic phase transitions (except for TmMn₂Si₂ and HoCoSi) and large magnetocaloric effect (MCE), were physically mixed to produce a composite magnetic refrigerant. The used concentration of each alloy was 0.1(TmMn₂Si₂) + 0.35(DyMn₂Si₂) + 0.4(TbMn₂Si₂) + 0.15(HoCoSi). X-ray diffraction and magnetization measurements were used to investigate the crystallographic and magnetic properties of the samples. The results indicate the maximum values of magnetic entropy change ($-<!--\; ???\; -->{\mathrm{\Delta <!--\; ??\; -->}S}_M^{max}$) of ~ 4.6 J/kg K under magnetic field change (ΔH) of 50 kOe. A large table-like MCE was observed for the composite sample. The full width at half maximum of ΔS_M peak (δT_FWHM) values reaches ~ 60.4 K (ΔH = 20 kOe) and ~ 67.2 K (ΔH = 50 kOe), which correspond to a respective ΔS_M peak broadening of 230% and 180% for the composite sample compared with each compound studied individually. Such property is important for a material to operate in an ideal Ericsson cycle.

[en] The effect of external pressure on the magnetic properties and magnetocaloric effect of polycrystalline compounds DyCo₂ and Er(Co_1-xSi_x)₂ (x = 0,0.025 and 0.05) has been studied. The ordering temperatures of both the parent and the Si-substituted compounds are found to decrease with pressure. In all the compounds, the critical field for metamagnetic transition increases with pressure. It is seen that the magnetocaloric effect in the parent compounds is almost insensitive to pressure, while there is considerable enhancement in the case of Si-substituted compounds. Spin fluctuations arising from the magnetovolume effect play a crucial role in determining the pressure dependence of the magnetocaloric effect in these compounds. The variation of the magnetocaloric effect is explained on the basis of the Landau theory of magnetic phase transitions

[en] The so-called half-metallic magnets have been proposed as good candidates for spintronic applications due to the feature of exhibiting a hundred percent spin polarization at the Fermi level. Such materials follow the Slater-Pauling rule, which relates the magnetic moment with the valence electrons in the system. In this paper, we study the bulk polycrystalline half-metallic Fe₂MnSi Heusler compound replacing Si by Ga to determine how the Ga addition changes the magnetic, the structural, and the half-metal properties of this compound. The material does not follow the Slater-Pauling rule, probably due to a minor structural disorder degree in the system, but a linear dependence on the magnetic transition temperature with the valence electron number points to the half-metallic behavior of this compound

[en] The magnetic and magnetocaloric properties of the intermetallic compounds Tb_1-xTm_xCo₂ (with x = 0, 0.2 and 0.5) have been studied. It is found that partial replacement of Tb by Tm in TbCo₂ leads to a reduction in the ordering temperature, which is attributed to the decrease in the exchange strength due to the lower spin value of Tm³⁺ as compared with that of Tb³⁺. The analysis of the zero-field heat capacity data at low temperature shows that the coefficient of electronic heat capacity increases with increase in Tm content and is attributed to the presence of local spin fluctuations. The variation of the magnetocaloric effect (MCE) has been explained on the basis of the magnetic properties. Temperature dependence of the MCE shows that this system may be useful for magnetic refrigeration applications in a sub-room temperature regime

[en] The ferromagnetically coupled cobalt ion is observed to create a magnetocrystalline anisotropy in the PrNi_5-xCo_x structure above a critical composition of x = 2. The competition of the anisotropy energies between Co and Pr sublattices gives rise to a spin reorientation (SR) phenomenon in PrNi_5-xCo_x compounds at a low temperature (∼150 K) which is then followed by a magnetic transition at a higher temperature. Co-doping has a strong influence on the Curie temperature, changing it from ∼60 K (x = 1.95) to ∼537 K (x = 3). The magnetic entropy change is associated with SR as well as a magnetic transition, and correspondingly a large full width at half maximum (δT_FWHM) is obtained for this series of compounds. For example, the PrNi_2.85Co_2.15 compound presents δT_FWHM = 166 K at a 1 T field. This series therefore has an appreciable relative cooling power, which makes this material a suitable magnetic refrigerant over a large temperature span.