Oxides Exhibiting Half-Metallicity

Schwarz. Moreover, this half-metallicity leads to an integral magnetic moment of 2 gB/f-u- (f.u. = formula unit), the same as predicted by Hund s rule for the spin moment of Cr (3d ) ion. This agrees with the measured saturation moment of 2 

Electronic structure calculations of Fe304 in its high temperature cubic form have been carried out with the self-consistent spin-polarised 

The diverse structural, electronic and magnetic properties of Fe304 have attracted interest to other spinel oxides. LiMn204 is a geometrically frustrated compound with a spinel structure. The magnetisation and 

MR studies near the charge ordering temperature (T=280 K) show a large negative MR of 20% at 7 T. The MR also exhibits field hysteresis, which is not reflected in the magnetisation a behaviour very different from the CMR oxomanganates. 

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Gadolinium, like other related rare earth metals, is silvery white, has a metallic luster, and is malleable and ductile. The element is relatively stable in dry air but in moist air tarnishes with the formation of a loosely adhering oxide film. The metal reacts slowly with water and is soluble in diluted acid. Gadolinium exhibits a trivalent oxidation state and because of the half-filled 4f level the Gd " " ion is especially stable and unique for its high paramagnetic moment. It has the highest thermal neutron capture cross-section of any known element. 

The fluorite-related lanthanide oxides exhibit unusual diffusional properties. The conventional rule-of-thumb is that atomic mobility in a solid does not become significant until one-half of the melting point temperature (the Tammann temperature) is reached. In these oxides this value is about 1200°C. At the Tammann temperature the metal atoms in lanthanide oxides just begin to become mobile as confirmed by the temperatures required for solid-state reactions. The oxygen substructure, to the contrary, is mobile below 300°C. This leads to a situation where equilibration and reaction must be considered for each substructure separately (see Bevan and Summerville 1979). This places the lanthanide oxides with fluorite-related structures in the category of fast-ion conductors along with, e.g., calcia-stabilized zirconia as indicated in table 18. 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

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