Itinerant-electron metamagnetism and giant magnetocaloric effect

Abstract

An isothermal magnetic entropy change in itinerant-electron systems is discussed on the theory of itinerant-electron metamagnetism. The effect of spin fluctuations is taken into account based on the phenomenological Ginzburg-Landau theory. By the Clausius-Clapeyron relation, the magnetic entropy change depends not only on the magnetization jump at the Curie temperature ${T}_{\mathrm{C}},$ but also on the temperature dependence of the critical field of the metamagnetic transition. It is shown that the magnetic entropy change at low fields becomes maximum when ${T}_{\mathrm{C}}$ is about half of the temperature ${T}_{\mathrm{max}}$ where the susceptibility reaches a maximum. The isothermal magnetic entropy changes for $3d$ compounds $\mathrm{Co}(\mathrm{S},\mathrm{S}\mathrm{e}{)}_{2},$ $\mathrm{Lu}(\mathrm{C}\mathrm{o},\mathrm{A}\mathrm{l}{)}_{2},$ and $\mathrm{Lu}(\mathrm{C}\mathrm{o},\mathrm{G}\mathrm{a}{)}_{2}$ and also for itinerant $5f$ compound U(Co,Fe)Al are estimated and compared with those observed for MnFe(P,As) and $\mathrm{La}(\mathrm{F}\mathrm{e},\mathrm{S}\mathrm{i}{)}_{13}.$ It has been found generally that the giant magnetocaloric effect in itinerant-electron metamagnets is expected when the coefficient ${b}_{0}$ of ${M}^{4}$ in the Landau energy expansion with respect to the magnetization M is negative and large.