Abstract
The spinel HgCr2S4 has a spiral antiferromagnetic spin structure which can be ferromagnetically aligned by application of a moderate magnetic field.1 The presence of the nonmagnetic Hg and S ions affords an excellent opportunity to corroborate and investigate the details of the magnetic structure by study of the hyperfine interaction of these nuclei. We have observed the NMR of the 199Hg, 201Hg, and 53Cr nuclei in polycrystalline samples at 1.4°K. The Hg nuclei, which are situated in layers between aligned sheets of Cr spins, exhibit a magnetic anisotropy which disappears with ferromagnetic alignment. The spectral position and shape depend strongly on magnetic field. In zero magnetic field, the center of the Hg spectra is at 507 kOe, and that of the Cr spectrum is at 190 kOe. The intensities of the spectra are field dependent, and are observed to initially increase up to 5 kOe and then decrease in higher magnetic fields. An analysis of the Hg spectra has been made in which the magnetic anisotropy is shown to be a consequence of the spiral spin structure. The strong magnetic-field dependence of the spectra is found to be a result of the changing spin structure. A fuller account of this work will be presented elsewhere.
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