Magnetogyration

Abstract

This paper discusses the theory of magnetoelastic coupling between localized magnetic moments and the rotation of molecular groups. The interaction can produce crystallographic transitions involving the discontinuous rotation of such molecular groups through large angles (a phenomenon termed magnetogyration). Such transitions do not necessarily involve the simultaneous onset of long-range magnetic order, although magnetic ordering will normally occur eventually as the temperature is lowered in the magnetogyric phase. The magnetically ordered magnetogyric phase can be destroyed discontinuously by application of an applied magnetic field of sufficient magnitude. A transition of magnetogyric origin appears to be present in potassium hyperoxide K${\mathrm{O}}_2$ and involves a rotation of the oxygen molecules at $T\sim 1.5T_N$, where $T_N$ is the Néel temperature. The magnetic electrons are in molecular orbitals on the ${\mathrm{O}}_2^{-}$ ions and the magnetoelastic coupling occurs as the result of a modulation of exchange interactions (i.e., of magnetic-orbital overlaps) by a change in orientation of the molecular axes.