Estudo sistemático do efeito magnetocalórico convencional e girante: aplicação na serie RAl2 (R= Ho, Nd, Tm)

Abstract

The conventional magnetocaloric effect is the heating or cooling of a magnetic material when subjected to a variation of magnetic field applied in a fixed direction. This effect is characterized by the variation of entropy in an isothermal process ([...]Siso) and by the variation of temperature in an adiabatic process ([...]Tad). On the other hand, the rotating magnetocaloric effect is defined as the heating or cooling of a magnetic material, when the magnetic field of constant magnitude, rotates from one direction to another. This effect is characterized by the variation of entropy in an isothermal process and by the variation of temperature in an adiabatic process, in which the applied magnetic field changes direction. In this thesis we have made a systematic study of the magnetic, thermodynamic and of conventional and rotating magnetocaloric effects. An application was made for the compounds of the series RAl2 where R = Ho, Nd and Tm. In this study, we observed the spin reorientation process in the three compounds, characterized by a critical magnetic field (BSR) when we change the temperature, and by a critical temperature (TSR), when we vary the magnitude of the magnetic field. The spin reorientation phenomenon gives rise to the anomalous magnetocaloric effect, i.e., a sign change in the magnetocaloric quantities. In HoAl2 this effect appears when the magnetic field is applied in the directions {100} and {111}. The existence of the anomaly in the conventional quantities, has a reflection on the rotating magnetocaloric quantities. In the NdAl2 the anomalous effect is observed when the magnetic field is applied in the directions {110} and {111}. In the TmAl2 the anomalous magnetocaloric effect is only observed when the magnetic field is applied in the direction {111}. However, the anomaly in the rotating quantities is observed for all rotations of the magnetic field. In this thesis we also propose two alternative ways to calculate the rotating magnetocaloric effect. In the first method, we used the difference of the conventional quantities for each direction studied. In the second method, we use the sum (or difference) of the rotating quantities in two different rotations, to calculate these quantities in another rotation.