Mechanism of the Ferrimagnetic to Antiferromagnetic Transition inMn2−xCrxSb

Abstract

The mechanism of the first-order ferrimagnetic to antiferromagnetic transition (exchange inversion) in ${\mathrm{Mn}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{Sb}$, $x<0.41$, was investigated by exchange magnetostriction. A second-order term has been included in the strain energy which gives an effective elastic constant dependent on magnetization and magnetic ordering. It also introduces a discontinuous change in the magnetization at the transition and hence a contribution to the entropy change in addition to that caused by the change of lattice dimension. For ${\mathrm{Mn}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{Sb}$, the effect of this term is small but necessary to obtain the qualitative behavior of the discontinuous change in lattice dimension and the shift in the Curie temperature with composition. The anisotropy behavior of ${\mathrm{Mn}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{Sb}$ was investigated for various values of $x$, and it is shown that an anisotropy in the critical lattice dimension of only 4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ \AA{} accounts for the observed change in magnetic anisotropy with exchange inversion that occurs for transition temperatures between 200 and 350\ifmmode^\circ\else\textdegree\fi{}K. For transition temperatures above 350\ifmmode^\circ\else\textdegree\fi{}K, anisotropy is positive, and spin-flopping measurements can be made on the antiferromagnetic state. An expression for the critical field in the presence of exchange inversion is obtained and compared with experiment.