Influence of spin reorientation on magnetocaloric effect in NdAl2: A microscopic model

We report a theoretical investigation about the influence of the spin reorientation from easy magnetic direction to the applied magnetic field direction on the magnetocaloric properties of NdAl2. This compound was fully investigated using a model Hamiltonian which includes the Zeeman-exchange interactions and the crystalline electrical field, which are responsible for the magnetic anisotropy. All theoretical results were obtained using the proper model parameters for NdAl2, found in the literature. The existence of a minimum in magnetic entropy change below the phase transition was predicted and ascribed to the strong jump on the spin reorientation.74

The giant anisotropic magnetocaloric effect in DyAl(2)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)We report on calculations of the anisotropic magnetocaloric effect in DyAl(2) using a model Hamiltonian including crystalline electrical field effects. The anisotropic effect is produced by the rotation of a constant magnetic field from the easy to a hard magnetic direction in the crystal and is enhanced by the first order nature of the field induced spin reorientation transition. The calculated results indicate that for a field with modulus of 2 T rotating from a hard to the easy direction, the isothermal magnetic entropy (Delta S(iso)) and adiabatic temperature (Delta T(ad)) changes present peak values higher than 60% the ones observed in the usual process, in which the field direction is kept constant and the modulus of the field is varied. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.3009974]1049Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Magnetocaloric effect in the RNi5 (R=Pr, Nd, Gd, Tb, Dy, Ho, Er) series

In this paper, the magnetocaloric effect in the hexagonal intermetallic compounds belonging to the RNi5 series was calculated using a Hamiltonian including the crystalline electrical field, exchange interaction, and the Zeeman effect. Experimental work was performed and the two thermodynamics quantities, namely, isothermal entropy change and adiabatic temperature change were obtained for polycrystalline samples, using heat capacity measurements, and compared to the theoretical predictions.701

Influence of the strong magnetocrystalline anisotropy on the magnetocaloric properties of MnP single crystal

Manganese monophosphate MnP single crystal deserves attention due to its rich magnetic phase diagram, which is quite different depending on the direction of the applied magnetic field. Generally speaking, it has a Curie temperature around 291 K and several other magnetic arrangements at low temperatures (cone-, screw-, fan-, and ferromagnetic-type structures). This richness is due to the strong magnetocrystalline anisotropy. In this sense, the present paper makes a thorough description of the influence of this anisotropy on the magnetocaloric properties of this material. From a fundamental view we could point out, among those several magnetic arrangements, the most stable one. On the other hand, from an applied view, we could show that the magnetic entropy change around room temperature ranges from -4.7 to -3.2 J/kg K, when the magnetic field (5T) is applied along the easy and hard magnetization directions, respectively. In addition, we have shown that it is also possible to take advantage of the magnetic anisotropy for magnetocaloric applications, i.e., we have found a quite flat magnetic entropy change (with a huge relative cooling power), at a fixed value of magnetic field, only rotating the crystal by 90 degrees.771

Theoretical investigations on giant magnetocaloric effect in MnAs1-xSbx

In this work we apply a model to describe the magnetocaloric effect for the MnAs1-x Sb-x series of compounds, 0 less than or equal to x less than or equal to 0.4. The behavior of the material under first order phase transitions is well described, and we are able to obtain the magnetocaloric potential for the series of compounds presenting first order magnetic phase transitions. Based on these results we predict the performance of a composite comprising a combination of compositions of this compound to work as active element in a magnetic refrigerator using an Erickson cycle spanning a great temperature range down from room temperature. (C) 2003 Elsevier B.V. All rights reserved.320430230

The influence of spontaneous and field-induced spin reorientation transitions on the magnetocaloric properties of HoZn and ErZn

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We report a theoretical investigation on the magnetocaloric properties of the cubic CsCl-type HoZn and ErZn compounds. Several anomalies in the magnetocaloric quantities, Delta S(T) and Delta T(S), are observed due to spontaneous and/or field-induced spin reorientation transitions in these compounds. In HoZn, a discontinuity in the isothermal entropy change and in the adiabatic temperature change around T(1) = 23 K is ascribed to the spontaneous reorientation transition. Under a magnetic field variation from 0 up to 2 T in the < 110 > and < 100 > directions, an almost table-like behavior in Delta S(T) is predicted between T(1) and T(SR1). The peak around the ferromagnetic-paramagnetic transition temperature in the magnetocaloric quantities shows a dependence on the direction of the applied field. For mu(0)Delta H = 2 T, it reaches 11.9 J/kg K (magnetic field along the < 111 > direction) and 7.9 J/kg K (magnetic field in the < 100 > direction). In ErZn there is also a dependence of Delta S(T) and Delta T(S) on field direction. From the analysis of the spin reorientations in both compounds we have built spin reorientation diagrams that summarize their temperature and field dependence. Our theoretical approach is based on a model Hamiltonian that includes exchange, crystal field, and quadrupolar interactions. (C) 2011 American Institute of Physics. [doi:10.1063/1.3554725]1096Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Isothermal variation of the entropy (Delta S(T)) for the compound Gd(5)Ge(4) under hydrostatic pressure

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)In the present work, the isothermal variation of the entropy (Delta S(T)) for the compound Gd(5)Ge(4) was studied at different applied hydrostatic pressures (from 0 up to 0.58 GPa). In all pressure ranges, we observe the giant magnetocaloric effect. The AST data for the compound Gd(5)Ge(4) at zero applied pressure present two peaks: the lowest temperature peak is due to irreversible processes and the highest temperature peak is due to magneto structural transitions. Increasing the pressure, the lowest temperature peak displaces to lower temperatures and disappears. The magnitude of the other peak has a nonlinear behavior with pressure. Different protocols were used to obtain Delta S(T) at zero applied pressure and the results indicate that Delta S(T) strongly depends on the initial and final states of Gd(5)Ge(4) compound. We also present a T-P magnetic phase diagram built from the available magnetic data. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2980040]1046Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Experimental and theoretical analyses of PrAl2 and NdAl2 composite for use as an active magnetic regenerator

We report the theoretical and experimental investigations on the magnetocaloric effect in the PrAl2 and NdAl2 compounds and a composite of these compounds for use as an active magnetic regenerator. The theoretical calculations were performed considering the crystalline electrical field anisotropy and the magnetocaloric potentials were calculated in the three main crystallographic directions. The experimental data, obtained for the polycrystalline samples, are in good agreement with the theoretical results. Also, an optimum molar fraction of the PrAl2 and NdAl2 composite was determined theoretically and experimentally and discussed in the framework of the optimum regeneration Ericsson cycle. (C) 2005 American Institute of Physics.97

Investigation of the first-order metamagnetic transitions and the colossal magnetocaloric effect using a Landau expansion applied to MnAs compound

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We have explored a simple Landau model to calculate magnetization isotherms considering magnetic hysteresis. The model parameters have been chosen to fit the magnetic and magnetocaloric data of MnAs compound. Experimental data show that there is a great difference between the isothermal variation of the entropy (S(T)) obtained from isotherms measured increasing and decreasing magnetic field. This great difference is reproduced theoretically. From the experimental and phenomenological isotherms, we calculated the S(T). From the theoretical entropy, we also obtained S(T), which does not present the colossal peak.6816772Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq