Itinerant-electron metamagnetism and giant magnetocaloric effect

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 TC, 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 TC is about half of the temperature Tₘₐₓ where the susceptibility reaches a maximum. The isothermal magnetic entropy changes for $3d$ compounds Co(S,Se)₂, Lu(Co,Al)₂, and Lu(Co,Ga)₂ and also for itinerant $5f$ compound U(Co,Fe)Al are estimated and compared with those observed for MnFe(P,As) and La(Fe,Si)₁₃. It has been found generally that the giant magnetocaloric effect in itinerant-electron metamagnets is expected when the coefficient b₀ of M⁴ in the Landau energy expansion with respect to the magnetization M is negative and large.