Fluctuating band picture of itinerant electron ferromagnetism(invited)

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Abstract
Renormalized band theory gives a good account of the ground state properties of iron and nickel. Standard finite temperature band theory, the Stoner theory, predicts in disagreement with experiment that the exchange splitting is proportional to Mz and that the transition temperature Tc is an order of magnitude greater than observed. We have worked out a generalization of the band theory in which fluctuations in the direction of the spin quantization are taken into account. This direction changes relatively slowly in space and time, so there is short range magnetic order, SRMO. The magnitude of the exchange splitting also fluctuates, but this is secondary, and the splitting is preserved, even well above Tc. A measure of the SRMO is given by a= (1/2) √ (∇M̂2), where ∇M̂ is the gradient of the local magnetization direction. The SRMO can be observed by neutron scattering. Spin waves with wavenumber q≳a can be set up, even in the absence of long range order above Tc. The lineshape is broadened, shifted, and skewed in a way dependent on a, and the theory is in good agreement with experiment. The value of a observed in this way above Tc is about .3Å−1 in both iron and nickel. By means of a functional integral formulation, our theory is compared with other recent approaches to this problem.