First-principles theory of ferromagnetic and antiferromagnetic order (abstract)

Abstract

We outline a theory that aims at a consistent description of spin order on a first-principles basis. The theory merely exploits the antisymmetry of the N-electron wave function and its property of solving the associated N-electron Schrödinger equation. As follows directly from the antisymmetry of the wave function, the exchange-correlation energy is generally lowered if one subjects the system to a departure from paramagnetic order. Moreover, it becomes evident that a consistent nonrelativistic N-electron theory can only yield collinear magnetism. We exploit the proven method of mapping the original N-electron Schrödinger equation onto N equivalent one-particle equations familiar from density functional theory. The occurrence of spin order is shown to be connected to Hund’s rule. One obtains closely resembling criteria for antiferromagnetic and ferromagnetic instability, the latter being identical to Stoner’s criterion. By using this criterion we have studied the 3d- and 4d-metals and also two rare-earth metals as to where spin order must or cannot occur. Potassium proves to transform from a simple metal into an insulating ferromagnet as one expands the lattice beyond a certain threshold. From our point of view, it has to be expected that ferromagnetic order can also exist in molten metals as has, in fact, been observed in recent experiments. We can also show that a consistent N-electron theory yields spin fluctuations that will, in general, contribute to the average magnetization and influence its temperature dependence.