Abstract
The field exponent for the magnetic entropy change n=dlnΔS/dlnH in a magnetic field was evaluated using the Landau model to provide a classification for the nature of the ferromagnetic-to-paramagnetic transitions at the Curie temperature TC in magnetocaloric materials. The magnetic phase transition can be classified as a first-order magnetic transition (FOMT), a second-order magnetic transition (SOMT) or a critical point (CP) at the border between the first and second order transitions. The value of n is mapped as a function of the reduced temperature and the reduced magnetic field for the FOMT, SOMT and CP. The influence of the magnetic field on the transition temperature was found to be relatively weak for the SOMT and corresponds to a power law for the CP and FOMT.
Highlights
Magnetocaloric materials provide promising candidate materials for efficient magnetic cooling, magnetic heat pumping and magnetic energy conversion near room temperature [1,2,3]
As a large magnetocaloric effect (MCE), which is strongest for a discontinuous first-order magnetic transition (FOMT) with latent heat, is often in conflict with a low hysteresis, which is mainly observed for a continuous secondorder magnetic transitions (SOMT) without latent heat, a good compromise is often found near the cross-over point where the latent heat vanishes
For the FOMT we explicitly distinguish the cases for the reversible transition without hysteresis and the irreversible transition with hysteresis, relevant for
Summary
Magnetocaloric materials provide promising candidate materials for efficient magnetic cooling, magnetic heat pumping and magnetic energy conversion near room temperature [1,2,3]. We will apply the Landau model to expand these predictions and provide a detailed model analysis of the influence of the magnetic field on the entropy change and the transition temperature for the firstorder and second-order magnetic transitions, as well as the critical point (CP) at the transition between the FOMT and the SOMT. For magnetocaloric materials the Landau model has been widely applied to characterise the experimental data for a wide range of systems [14,15,16,17,18,19,20] and the related Arrott plots [21] are often used to define the nature of the magnetic transition [22]. It is assumed that the Landau parameters correspond to effective values, where this renormalisation has been included
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