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.

Covalent magnetism and magnetic impurities.

We use the model of covalent magnetism and its application to magnetic insulators applied to the case of insulating carbon doped BaTiO3. Since the usual Stoner mechanism is not applicable we study the possibility of the formation of magnetic order based on a mechanism favoring singly occupied orbitals. On the basis of our model parameters we formulate a criterion similar to the Stoner criterion but also valid for insulators. We describe the model of covalent magnetism using a molecular orbital picture and determine the occupation numbers for spin-up and spin-down states. Our model allows a simulation of the results of our ab initio calculations for E(ℳ) which are found to be in very good agreement.

p-electron magnetism in CdS doped with main group elements.

On the basis of ab initio supercell calculations employing density functional theory (DFT) and post-DFT methods, we investigate the behavior of main group element impurities (B, C, N, Al, Si, P, Ga, Ge) in wurtzite (w) and zincblende (zb) CdS lattices. It is found that the impurities prefer the sulfur position and most of them, depending on the concentration, exhibit magnetic order. We find that for small concentrations (64zb and 72w supercells) a half-metallic behavior is found. For a 16-atom supercell for both the zb- and w-structure partly also unsaturated magnetic moments occur. The field dependence of the magnetic moments in these materials may lead to new technological applications of these magnetic semiconductors as tunable spin injection materials.