ENERGY AND ENTROPY IN ITINERANT MAGNETS

Abstract

This paper describes a method for calculating short-range magnetic order in itinerant magnets. An energy and an entropy are assigned to configurations of the magnetization. The energy comes from the electronic structure and the entropy from the number of configurations satisfying constraints on the short-range order. In mean field theory (MFT) one takes the energy E to be a function of an order parameter X, and calcu- lates the free energy F (X) = E (X) - TS (X) by as- sociating an entropy S (X) to each value of X. This is given by the logarithm of the number of configurations satisfying a constraint given by the order parameter. The free energy and order parameter are then found by minimizing F (X) . The approximation is exact if the energy is a function of the order parameter only. It is unsatisfactory for the present Purpose, where we are interested in the SRO. We therefore take an energy that depends on both the long-range order parameter and a short-range order parameter, such as the nearest neighbour correlation function. By minimizing the free energy with respect to both of these parameters one can determine the SRO. Similar approaches have been used elsewhere with a different approximation for the entropy