The inverse magnetocaloric effect occurs when a magnetic material cools down under applied magnetic field in an adiabatic process. Although the existence of the inverse magnetocaloric effect was recently reported experimentally, a theoretical microscopic description is almost nonexistent. In this paper we theoretically describe the inverse magnetocaloric effect in antiferro- and ferrimagnetic systems. The inverse magnetocaloric effects were systematically investigated as a function of the model parameters. The influence of the Néel and the compensation temperature on the magnetocaloric effect is also analyzed using a microscopic model.

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 ͑⌬S iso ͒ and adiabatic temperature ͑⌬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.

Frequently, magnetothermal studies of many ferromagnetic materials are focused considering the system such as a paramagnet subject to a molecular field being characterized by an exchange constant. However, magnetostructural transitions can produce abrupt changes in the exchange constant triggering magnetic transitions of first order at transformation temperatures, which can be shifted by applying magnetic field or pressure. On thermodynamic bases we show how an abrupt but continuous variation of the exchange constant at the transition can play an important role in the magnetocaloric and barocaloric isothermal potentials in systems showing structural transformation.

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a b s t r a c tA theoretical and experimental investigation on the magnetocaloric properties of the rare earth pseudobinary compounds Gd 1 À n Pr n Al 2 is presented. The calculated isothermal entropy and adiabatic temperature changes under magnetic field variations from 0 to 2 T and from 0 to 5 T are in good agreement with the experimental data. For the Pr-concentrations n ¼ 0.25, 0.5 and 0.75 the experimental data present an inverse magnetocaloric effect which was theoretically predicted and associated with the competition between the opposite magnetizations of the Gd and Pr sublattices. The two-sublattice Hamiltonian used in the calculations takes into account the crystal field, exchange and Zeeman interactions.

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Conventional and anisotropic magnetocaloric effects were studied in cubic rare earth RNi 2 ͑R =Nd,Gd,Tb͒ 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.