Successive field-induced transitions in a frustrated antiferromagnetHgCr2O4

Abstract

A new geometrically frustrated chromium spinel $\mathrm{Hg}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ was synthesized and characterized. $\mathrm{Hg}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ is an antiferromagnet whose exchange constant $J$ is significantly smaller than that of other chromium spinels $M{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ ($M=\mathrm{Mg}$, Zn, and Cd). We found evidence for antiferromagnetic ordering accompanied by orthorhombic lattice distortions at $5.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. By applying a magnetic field, we observed successive transitions from an antiferromagnetic to an intermediate phase and, finally, to a ferromagnetic state. The intermediate state appears as a very wide magnetization plateau from $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}27\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at one-half the full moment of ${\mathrm{Cr}}^{3+}$, which is very similar to the behavior of $\mathrm{Cd}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ above $28\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The magnetic field required to realize the magnetization plateau phase is considerably small, reflecting the small value of $J$, which makes it possible to make a more complete study of the physical properties of the system. The facts that the nearest-neighbor interaction is antiferromagnetic and that thermal fluctuation stabilizes the magnetization plateau phase indicate that a collinear three-up and one-down state is realized in this phase. From the obtained $H\text{\ensuremath{-}}T$ phase diagram, this phase is stable even at $T=0$. We conclude that lattice distortions, in addition to thermal fluctuations, are the most probable cause of the plateau phase.