P.J. von Ranke

Investigations on magnetic refrigeration: Application to RNi2 (R=Nd, Gd, Tb, Dy, Ho, and Er)

In this article we report the thermodynamic investigations on the Ericsson cycle with application on RNi2 (R=Nd, Gd, Tb, Dy, Ho, and Er) series. Besides the Zeeman and exchange interactions, these compounds present an important contribution from crystalline electrical field interaction. The Ericsson coefficient of performance and refrigerant capacity was investigated under the crystal field influence. An optimum molar composite of Er–Dy–TbNi2 was proposed to work as refrigerant material in the temperature interval from 7 to 22 K.

Magnetocaloric effect in

Abstract In this paper we calculate the magnetocaloric effect in the compound Gd ( Pd 1 - x Rh x ) by using the Heisenberg Hamiltonian where the indirect exchange interaction parameter between localized spins depends on the Rh concentration and the spin–spin interaction is treated in the molecular field approximation. The calculated adiabatic temperature changes upon magnetic field variations are in good agreement with the available experimental data.

The giant anisotropic magnetocaloric effect in DyAl2

We report on calculations of the anisotropic magnetocaloric effect in DyAl2 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 (ΔSiso) and adiabatic temperature (ΔTad) 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.

Theoretical calculations of the magnetocaloric effect in MnFeP0.45As0.55: a model of itinerant electrons

In this paper we calculate the magnetocaloric effect in the compound MnFeP0.45As0.55. We use a microscopical model in the picture of the band theory, including a magnetoelastic interaction. The theoretically calculated isothermal entropy changes upon magnetic field variations are in good agreement with the available experimental data.

Understanding the table-like magnetocaloric effect

Abstract In this work, we explain the table-like magnetocaloric effect as a consequence of multiple magnetic phase transitions for different temperatures. Using a Hamiltonian that takes into account the dipolar and quadrupolar interactions and the crystalline electric field, it was possible to study the magnetocaloric effect of the Γ 3 –Γ 5 reduced magnetic system. We analyze the table-like magnetocaloric effect for different λ and K , dipolar and quadrupolar parameters, and for a suitable energy gap between Γ 3 and Γ 5 that permits successive magnetic orderings at different temperatures.

Anisotropic magnetocaloric effect in antiferromagnetic systems: Application to EuTiO3

In this work, we theoretically predicted an anisotropic magnetocaloric effect of the same order of magnitude of the usual magnetocaloric effect for antiferromagnetic systems. The anisotropic magnetic properties come from the anti-parallel alignment of the magnetic sites and can be optimized depending on the magnetic field change. This result highlights the applicability of antiferromagnetic compounds as refrigerants based on the anisotropic magnetocaloric effect. For this purpose, we considered a Hamiltonian model, including the exchange and Zeeman interactions in a two sublattices framework. It is worth noting that no anisotropy is explicitly included on the Hamiltonian model, although the system presents an anisotropic behavior. The calculations were applied to the G-type antiferromagnetic compound EuTiO3.

The magnetocaloric effect in R5Si4 (R = Gd, Tb): a Monte Carlo calculation

In this work we calculate the magnetocaloric effect in the compounds Gd5Si4 and Tb5Si4. We use a model Hamiltonian of interacting spins, and treat the spin–spin interaction in the Monte Carlo simulation. The theoretically calculated isothermal entropy change and the adiabatic temperature change upon variation of the magnetic field are in good agreement with the available experimental data.

The anomalous magnetocaloric effect in HoNi2

Abstract In this work, we report a theoretical investigation of the magnetocaloric effect in the ferromagnet HoNi 2 . To carry out this investigation, we have used a model Hamiltonian that takes into account the crystalline electric field (CEF) and the exchange interaction. Using the proper experimental CEF and exchange parameter, ascribed for HoNi 2 , a change of the easy magnetization direction, from 〈110〉 to 〈001〉 was predicted at T =1.5 K for the critical magnetic field H ∼2.4 T. The anomalous peak in the isothermal magnetic entropy change and in the adiabatic temperature change with magnetic field was calculated and analysed for the three main crystallographic directions.

A comparative study of the magnetocaloric effect in RNi2 (R=Nd,Gd,Tb) intermetallic compounds

Conventional and anisotropic magnetocaloric effects were studied in cubic rare earth RNi2 (R=Nd,Gd,Tb) ferromagnetic intermetallic compounds. These three compounds are representative of small, null, and large magnetocrystalline anisotropy in the series, respectively. Magnetic measurements were performed in polycrystalline samples in order to obtain the isothermal magnetocaloric data, which were confronted with theoretical results based on mean field calculations. For the R=Tb case, we explore the crystalline electrical-field anisotropy to predict the anisotropic magnetocaloric behavior due to the rotation of an applied magnetic field of constant intensity. Our results suggest the possibility of using both conventional and anisotropic magnetic entropy changes to extend the range of temperatures for use in the magnetocaloric effect.

Theoretical investigation on the magnetocaloric effect in amorphous systems, application to: Gd80Au20 and Gd70Ni30

The temperature dependence of the magnetocaloric effect in Gd80Au20 and Gd70Ni30 amorphous alloys were investigated, using the Handrich-Kobe model with a modified Brillouin function that takes an additional exchange fluctuation term. The exchange fluctuation parameters were determined to give better fits to magnetic entropy changes and adiabatic temperature changes. The magnetic entropy changes of 2.20 Jmol−1K−1 and 1.50 Jmol−1K−1 and the refrigerant capacity values of 135 Jmol−1 (ΔB=5 T) and 146 Jmol−1 (ΔB=7 T) are obtained for Gd80Au20 and Gd70Ni30, respectively. In addition, the influence of phase changes between crystalline and amorphous states on the isothermal entropy change was investigated.