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[en] Electron spin resonance was applied on samples of Gd_5.09Ge_2.03Si_1.88. The results are discussed under the scope of magnetization measurements, optical metallography, and wavelength dispersive spectroscopy. Polycrystalline arc-melted samples submitted to different heat treatments were investigated. The correlation of the electron spin resonance and magnetization results permitted a characterization of the present phases and their transitions. Two coexisting phases in the temperature range between two phase transitions have been identified and associated to distinct crystallographic phases. Additionally, the magnetic moment at high temperatures has been estimated from the measured effective g factor. A peak value of 21.5 J/kg K for the magnetocaloric effect was obtained for a sample heat treated at 1500 deg. C for 16 h

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[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] Full text: Sometimes, in the production of materials unwanted crystalline phases are formed besides desired crystalline phase. The presence of these secondary phases, even if in low concentrations, can change the overall physical properties of the sample. Nevertheless, it is worth mentioning that production of materials with two crystalline phases can result in new and interesting effects. Therefore, the development of new methods that facilitate the control of the purity of samples and control of the proportion of components in multiphase samples are very important. Recently, in our earlier work, exchange bias like effect in polycrystalline NiO/NiMn₂O₄ composite produced by a simple sol-gel method was observed, which resulted from the exchange coupling between the spins of NiO and NiMn₂O₄ at interfaces. X-ray powder diffraction with synchrotron radiation, Rietveld method, scanning electron microscopy and energy dispersive X-ray (EDX) analysis are used for characterizing structural features that can be affected by this method of synthesis. It includes strong evidences of interfaces between nickel oxide and nickel manganite particles, shading light on the mechanism of spin exchange coupling between these materials. Rietveld refinement results for NiO/NiMn₂O₄ sample provided with high accuracy the percentages of NiO and NiMn₂O₄ crystalline phases. EDX mapping images showed that NiO particle is fixed to NiMn₂O₄ particle, which leads to believe in interface between NiO and NiMn₂O₄ materials. (author)

[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] To present day, the maximum magnetocaloric effect (MCE) at room temperature for a magnetic field change of 5 T is 40 J/(kg K) for MnAs. In this Letter we present colossal MCE measurements on MnAs under pressure, reaching values up to 267 J/(kg K), far greater than the magnetic limit arising from the assumption of magnetic field independence of the lattice and electronic entropy contributions. The origin of the effect is the contribution to the entropy variation coming from the lattice through the magnetoelastic coupling

[en] Magnetic refrigeration is an innovative technology owing to its high-efficiency, low-energy consumption, and environmental friendliness. However, the manufacturing process of the magnetocaloric materials is an obstacle to the effective application of those materials on refrigeration systems. In this work, we present a magnetic characterization of the Gd_5.09Ge_2.03Si_1.88.alloy, analyzing the as-cast bulk and the sintered tablets after some heat treatments. The study of powder metallurgy parameters such as grain size, compacting pressure and sintering process has showed results comparable to the as-cast alloy, which presents ΔS_T=-18 /kg K

[en] We report the strong correlations between the magnetoresistivity and the magnetic entropy change in the cubic antiferromagnetic TbSb compound. The theoretical investigation was performed through a microscopic model which takes into account the crystalline electrical field anisotropy, exchange coupling interactions between the up and down magnetic sublattices, and the Zeeman interaction. The easy magnetization directions changes from 〈001〉 to 〈110〉 and then to 〈111〉 observed experimentally was successfully theoretically described. Also, the calculation of the temperature dependence of electric resistivity showed good agreement with the experimental data. Theoretical predictions were calculated for the temperature dependence of the magnetic entropy and resistivity changes upon magnetic field variation. Besides, the difference in the spin up and down sublattices resistivity was investigated.

No abstract available