Istituto Nazionale di Ricerca Metrologica, Torino, Italy.
J Phys Condens Matter. 2011 Jun 8;23(22):226004. doi: 10.1088/0953-8984/23/22/226004. Epub 2011 May 17.
This paper presents a study of the magnetocaloric effect at the first-order magneto-elastic phase transition. The entropy change Δs at the transition temperature is given by the sum of the magnetic and the structural contributions. By using a thermodynamic model, it is shown that the sign and amplitude of the structural contribution to Δs are determined by the dimensionless parameter ζ (zeta) which depends on β, the steepness of the change of exchange forces with volume, and on α(p), the thermal expansion coefficient of the structural lattice. For ζ < 0 the change Δs can be larger than the magnetic contribution alone, because the structural entropy change and the magnetic entropy change have the same sign, giving rise to the giant magnetocaloric effect. For 0 < ζ < 1 the two contributions partially cancel each other and the total entropy change is lower than the magnetic entropy change. For ζ > 1 the structural entropy dominates and a transition occurs upon heating from a low temperature paramagnet to a high temperature ferromagnet.
本文研究了一级磁弹相变中的磁热效应。在相变温度下的熵变 Δs 由磁贡献和结构贡献之和给出。通过热力学模型表明，结构贡献对 Δs 的符号和幅度由无量纲参数 ζ（zeta）决定，ζ 取决于β，即交换力随体积变化的陡峭程度，以及α(p)，结构晶格的热膨胀系数。对于 ζ < 0，变化 Δs 可以大于磁贡献，因为结构熵变化和磁熵变化具有相同的符号，导致巨大的磁热效应。对于 0 < ζ < 1，两个贡献部分抵消，总熵变低于磁熵变。对于 ζ > 1，结构熵起主导作用，从低温顺磁体加热到高温铁磁体时会发生相变。
