Magnetic Phase Equilibrium in Chromium-Substituted Calcium Ferrite

Abstract

The structure of Ca${\mathrm{Fe}}_2$ ${\mathrm{O}}_4$ has orthorhombic symmetry (space group ${D_{2h}}^{16}-Pnam$) and is characterized by Fe-O-Fe-O chains within which the Fe-O-Fe bond angle is ∼130°. These chains are cross-linked by means of Fe-O-Fe bonds in which the angle is ∼99°. The compounds $\mathrm{Ca}{\left({\mathrm{Cr}}_x{\mathrm{Fe}}_{1-x}\right)}_2{\mathrm{O}}_4$, with $0\le x\le 0.5$, have been found, by means of neutron measurements, to exhibit two magnetic structures in each of which the chains are internally antiferromagnetic, with spins alternating +-+- along a chain. In one structure the chains are linked ferromagnetically through the 99° bonds, with magnetic symmetry $p{n}^{\prime }{a}^{\prime }m$, and in the second they are antiferromagnetically coupled, corresponding to the space group $\mathrm{Pn}{a}^{\prime }m$. A phase diagram has been constructed showing both single-phase regions and a region in which the two phases exist in equilibrium. Transition temperatures as well as the temperature dependence of magnetic moments and phase concentration have been determined as a function of chromium content. The problem posed by the co-existence, at equilibrium, of two magnetic phases belonging to a single crystal structure is discussed.