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ASPHERICAL SPIN DENSITY IN S-STATE CATIONS,
MASSACHUSETTS INST OF TECH LEXINGTON LINCOLN LAB
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A calculation of the first-order effect of spin-orbit coupling on a 3d5 unperturbed S-state ion in a cubic crystal field has led to a new contribution sigma-1 r to the ionic spin density. The integral over space of sigma-1 r is zero and sigma-1 r is perpendicular to the unperturbed spin density sigma-0 r while sigma-0 r is spherically symmetric about the nucleus, sigma-1 r is highly aspherical. In alpha-Fe2O3 at room temperature, the spin density consists of the large antiferromagnetic component sigma-0, plus the new term sigma-1, plus the weak ferromagnetic or Dzialoshinsky term sigma-D, which is spherical. In neutron scattering, it is found that sigma-1 contributes to the same ferromagnetic Bragg peaks as does sigmaD, and in the same order of magnitude. Hence the new term is probably important for understanding the surprising, highly aspherical ferromagnetic spin-density distribution recently observed in alpha-Fe2O3 by Pickart, Nathans, and Halperin. In general, sigma-1 r will be nonzero under much less restrictive symmetry requirements than those needed for the nonvanishing of sigma-D. In the course of discussion, it is pointed out that the Dzialoshinsky-Moriya theory of the weak ferromagnetism in alpha-Fe2O3, for example, implies that the application of a spatially uniform magnetic field should influence the distribution of domains of antiferromagnetic spin components. This effect was observed by Pickart et al. Author
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