Electronic structure and the magneto-caloric effect.

We present a theoretical investigation of the entropy changes upon the application of an external field leading to the magneto-caloric effect (MCE). The case of localized magnetic moments is treated within the Weiss molecular field model, but special emphasis is given to cases of itinerant electron magnetism. These are described within the Landau theory of phase transitions and the temperature dependence is included via spin fluctuations. Since the parameters of the Landau expansion can be calculated from first-principles calculations of the electronic and magnetic structure, an immediate connection to the electronic band structure and its properties becomes possible. We study ordinary ferromagnets, including magneto-volume coupling and itinerant electron metamagnets, where in a small external field range large changes of the magnetic moments occur. We find that such metamagnetic systems are the most promising candidates for a large MCE in itinerant electron systems. We apply our expressions to several transition metals and their alloys, as well as to the metamagnets YCo2 and Fe2P, and find reasonable agreement with available experimental data.