Thermodynamic models of ferromagnetism and the magnetocaloric effect
J. S. Amaral1,2, J. P. Araújo2 and V. S. Amaral1
1Departamento de Física and CICECO, Universidade de Aveiro, Portugal
2IFIMUP-IN and Departamento de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Portugal
The magnetocaloric effect (MCE) is a property common to all magnetic materials. It is associated with the magnetic entropy change that occurs due to a change in applied magnetic field. This effect is the basis of magnetic refrigeration technology, which promises efficient and environmentally friendly refrigeration devices to substitute common vapour-cycle based ones.
In this talk, the basics of the MCE and magnetic refrigeration will be reviewed, together with the typical methods used to estimate the MCE from magnetization data. We highlight the usefulness of theoretical models such as the Landau theory of phase transitions and the Weiss mean-field model together with the Bean-Rodbell magneto-volume coupling, in this context.
The intensity of the MCE directly affects the efficiency of the refrigeration devices. The discovery of the giant MCE [1], which is usually the result of coupled structural and magnetic phase transitions, gave an enormous boost to MCE research worldwide. We present novel methods to interpret the magnetic and magnetocaloric properties of these compounds [2,3].
The first-order nature of the transition in giant MCE materials results in some additional precautions on estimating the MCE effect from magnetization measurements. The controversial Colossal MCE [4] was found to be the result of ignoring the existence of metastable and mixed-phase states [5], typical of first-order phase transition systems.
Finally, we present recent results on deriving an equation of state for a ferromagnetic system, establishing analytic expressions for the MCE effect and critical behavior of a Weiss mean-field system [6]. Some preliminary results on the use of critical phenomena theory and scaling methods in MCE studies will also be presented.
References
[1] V. K. Pecharsky and K. A. Gschneidner, Phys. Rev. Lett. 78, 4494 (1997)
[2] V. S. Amaral and J. S. Amaral, J. Magn. Magn. Mater. 272, 2104 (2004).
[3] J. S. Amaral, N. J. O. Silva and V. S. Amaral, Appl. Phys. Lett. 91, 172503 (2007).
[4] A. De Campos, D. L. Rocco, A. M. G. Carvalho, et al. Nature Mat. 5 ,802 (2006).
[5] J. S. Amaral and V. S. Amaral, Appl. Phys. Lett. 94, 042506 (2009).
[6] J. H. Belo, J. S. Amaral, A. M. Pereira, et al. Appl. Phys. Lett. (accepted for publication, 2012).