Theory of the magnetic phase diagram and magnetostriction of FeGe2.

Abstract

A Landau model based on the competition between quadratic and quartic exchange among moments in the four-sublattice structure of ${\mathrm{FeGe}}_{2}$ has been used to derive an expression for the free energy, and hence to determine the boundaries of the homogeneous antiferromagnetic (AF) phases in the phase diagram. Two types of AF spiral structure of different symmetry are found. The transverse AF spiral phase exists in comparatively small magnetic fields, H${H}_{p}$\ensuremath{\approxeq}${H}_{e}$(${a}_{o}$${q}_{c}$${)}^{2}$ (${H}_{e}$ is the exchange field, ${q}_{c}$ the spiral wave vector, and ${a}_{o}$ the lattice constant), while the longitudinal AF spiral phase exists in stronger magnetic fields up to ${H}_{t}$\ensuremath{\approxeq}${H}_{e}$${a}_{o}$${q}_{c}$. The phase transition between these two AF spiral structures in the field-temperature phase diagram is first order. The temperature dependence of the wave vector of the spiral structure in the absence of magnetic field is also determined, and the results of magnetostriction experiments are explained.