Interactions Curie temperature

The calculated energy of interaction of an atomic moment and the Weiss field (0.26 uncoupled conduction electrons per atom) for magnetic saturation is 0.135 ev, or 3070 cal. mole-1. According to the Weiss theory the Curie temperature is equal to this energy of interaction divided by 3k, where k is Boltzmann s constant. The effect of spatial quantization of the atomic moment, with spin quantum number S, is to introduce the factor (S + 1)/S that is, the Curie temperature is equal to nt S + l)/3Sk. For iron, with 5 = 1, the predicted value for the Curie constant is 1350°K, in rough agreement with the experimental value, 1043°K. 

An obvious refinement of the simple theory for cobalt and nickel and their alloys can be made which leads to a significant increase in the calculated value of the Curie temperature. The foregoing calculation for nickel, for example, is based upon the assumption that the uncoupled valence electrons spend equal amounts of time on the nickel atoms with / = 1 and the nickel atoms with J = 0. However, the stabilizing interaction of the spins of the valence electrons and the parallel atomic moments would cause an increase in the wave function for the valence electrons in the neighborhood of the atoms with / = 1 and the parallel orientation. This effect also produces a change in the shape of the curve of saturation magnetization as a function of temperature. The details of this refined theory will be published later. 

Above a temperature called the Curie temperature, Tc, all ferromagnetic materials become paramagnetic. The transition to a paramagnetic state comes about when thermal energy is greater than the magnetic interactions and causes the dipoles... 

Unfortunately it appears difficult to prepare the ideal mixed system. Miyahara and SaP observed large saturation magnetizations and high ferrimagnetic Curie temperatures indicative of the presence of FeA-O-Fee interactions. They used X-ray and magnetization data to estimate that 30 percent of the A-sites are Fe " in Zno.5Geo.5Fe204. A study by Robbins has also indicated that Fe ions will tend to displace Ge" " ions on A-sites. 

Fig. 25. Curie temperature as a function of the hole concentration for Gao.95Mno.os As computed from the 6 x 6 Luttinger model (solid line). Straight dashed lines represent results obtained assuming large and small values of the spin-orbit splitting A0, respectively. The dotted line is calculated neglecting the effect of the spin-orbit interaction on the hole spin susceptibility (Dietl et al. 2001c).

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