Non-collinear Magnetic States: From Density Functional Theory to Model Hamiltonians

Recent observation of \colossal magnetoresistance" in La 1?x Ca x MnO 3 and related compounds suggest the importance of orientational disordering of Mn magnetic moments, especially near the Curie temperature. Several interesting compounds even display non-collinear moments (NCM) in their ground states. Although self-consistent local density functional based treatments of NCM are already available (within certain simpliications), it will be instructive to have realistic tight binding methods to simulate the more complex spin conngurations. In this paper the process of simplifying the ab initio density functional theory of NCM to tight binding form is discussed. It is suggested that in the La-based manganites, the e g moments on the Mn ion may not be strongly coupled to the t 2g moments, and that these two suborbitals may become non-collinear on the same ion. A tight binding approach can model such a situation.