The static, paramagnetic, spin susceptibility of metals at finite temperatures

Abstract

The authors present a theory for the static, paramagnetic, spin susceptibility for metals at finite temperatures. It is based on a 'first principles' mean-field theory of itinerant magnetism and provides an expression which resembles that of a classical Heisenberg model added to an itinerant component. The quantities which occur in this expression all depend on the underlying electronic structure of the disordered local moment (DLM) paramagnetic state and thus demonstrate the subtle combination of the localised and itinerant aspects of this problem. The theory is applied to BCC iron and FCC nickel. A Curie temperature of 1280K and effective Curie constant moment of 1.97 mu B is obtained for iron in reasonable agreement with experiment. From the calculations of the wavevector dependent susceptibility, which are compared with quasi-elastic neutron scattering data, equal time spatial, magnetic correlation functions are inferred which are consistent with the magnetic structure of the initially imposed (DLM) paramagnetic state. The calculations for nickel provide a very different picture. At temperatures at which the DLM paramagnetic state differs from the conventional 'Stoner' state, it is shown that this state is unstable via the sensitivity of the moments to their orientational environment and that the theory for the paramagnetic state is equivalent to that of the Stoner-Wohlfarth picture. It is concluded that an improved theory for nickel must incorporate a mechanism for 'local' moment formation on several sites.