A General Mechanism Underlying Ferromagnetism in Transition Metal Compounds

Among various magnetic orderings exhibited by transition-metal compounds, ferromagnetism is the most important particularly in technological aspects. Recent intensive studies on the colossal magnetoresistance (CMR) have given even another reason for the importance of ferromagnetism. It would be very useful if one could elucidate mechanisms for the stability of ferromagnetism. Zener’s mechanism based on the s-d model and the double exchange mechanism originally proposed also by Zener are such examples. In the present article, we point out that a new general mechanism underlies robust ferromagnetism in a group of transition metal compounds which include MnAs, ordered double perovskites such as A2FeMO6 with A=Ca,Sr and Ba and M=Mo and Re, and organic compound V(TCNE)2· 12CH2Cl2. For some of these materials, the Curie temperature Tc is quite high despite the fact that the magnetic ions are far separated by nonmagnetic ions. The stability of ferromagnetism for these materials can be predicted by the band-structure calculations 9) based on the density functional theory, which automatically take into account several mechanisms for exchange interactions, such as RKKY (including Zener’s mechanism), double exchange and even superexchange. This fact, however, does not mean that the mechanism for the stability of ferromagnetism is clarified. Below we present a physical picture for a new mechanism which may lead to the stability of ferromagnetism. We start with a simple case where the mechanism may be most clearly understood. We assume that the typical elements with the p states as the valence states intervene between the transition-metal elements and that the d states coming from the latter elements split energetically into fully occupied majority-spin state and empty minority-spin one due to the strong intraatomic exchange interaction irrespective of the magnetic ordering. This assumption is reasonable even in the metallic substances of present interest where the d states make bands, because the large inter-atomic distance between transition-metal atoms in these systems may make the d band width smaller than the exchange Fig. 1. A schematic illustration of the situation where ferromagnetism is strongly stabilized.The solid lines denote the density of states when the p-d mixing is switched off. The broken lines denote the valence p density of states with the p-d mixing. The electrons in the shaded area in the majority spin state are transferred to the shaded area in the minority spin state, leading to negative spin polarization in the valence p state and stabilization of ferromagnetic state with respect to antiferromagnetic state.