Successive field-induced transitions in a frustrated antiferromagnet HgCr2O4

A new geometrically frustrated chromium spinel HgCr2O4 was synthesized and characterized. HgCr2O4 is an antiferromagnet whose exchange constant J is significantly smaller than that of other chromium spinels MCr2O4 (M=Mg, Zn, and Cd). We found evidence for antiferromagnetic ordering accompanied by orthorhombic lattice distortions at 5.8 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 T to 27 T at one-half the full moment of Cr3+, which is very similar to the behavior of CdCr2O4 above 28 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-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.