Lattice Distortion in Antiferromagnetic CoO under High Magnetic Fields

Cobaltous monoxide, CoO, is an archetypal antiferromagnet, with a Neel temperature TN 1⁄4 290K, which has been studied for more than half a century. Tombs and Rooksby found that CoO exhibits a change from the high temperature NaCl type structure to a tetragonal one below room temperature. Greenwald and Smart pointed out that this structural change is related to the magnetic order. A detailed measurement showed that the lattice constant c contracts with decreasing temperature, T , up to about 1%. Later, the low temperature crystal structure was found to be monoclinic, rather than tetragonal. A lattice distortion associated with a magnetic ordering is called the magnetostriction. Magnetostriction depends on the direction of the magnetic moments and should be distinguished from an exchange striction which does not depend on the moment direction. The magnetic structure of CoO was determined by Shull et al. The structure is such that Co moments in a ð111Þ plane point in the same direction with an antiparallel arrangement of moments in the neighboring ð111Þ planes. Roth determined the direction of the magnetic moments from a detailed neutron diffraction measurement on a powder sample of CoO. The spin direction is parallel to 1⁄2 1 17 which makes an angle 11 300 with the 1⁄2001 axis. Kanamori studied theoretically the magnetic anisotropy and the magnetostriction in CoO. The essential point in the Kanamori’s theory is that the orbital degrees of freedom of Co2þ ion are not quenched in a cubic crystal field and magnetic ordering induces a lattice distortion along the spin direction through the spin–orbit interaction. From a microscopic calculation, Kanamori was successful in explaining the lattice contraction along the spin direction. Despite these extensive studies, the magnetism of CoO, in particular under applied magnetic fields, remains enigmatic. One reason for this is that the exchange interaction is very strong, judging from the Neel temperature, and a conventional magnetic field is insufficient strength to study the field induced phenomena in CoO. In this paper, we report the results of synchrotron X-ray diffraction measurements on a powder sample of CoO under strong magnetic fields. Using an X-ray diffractometer in conjunction with a pulsed magnetic field, we measured the magnetic field dependence of the lattice constants of CoO in the magnetically ordered phase. We find the lattice constants expand with magnetic field, H. Because magnetic ordering in CoO strongly influences the lattice, so we expect that an applied magnetic field will induce a lattice distortion. Nakamichi measured macroscopic deformation of a single crystal of CoO up to 1.1 T using a strain gauge and interpreted the results as due to a movement of the antiferromagnetic domain walls. The present X-ray diffraction measurement gives a direct evidence that the phenomenon takes place at an atomic level. The X-ray diffraction experiment was performed at the beamline BL19LXU at SPring-8. The experimental setup and the method are described in ref. 9. The pulsed magnetic field was applied perpendicular to the scattering plane. The magnetic field dependence of the d spacing at the ð002Þ and ð110Þ reflections obtained from the analysis of the diffraction data is shown in Figs. 1 and 2, respectively. Since the difference between the tetragonal and the monoclinic structures of CoO below TN is small, 4) we use the tetragonal structure to discuss our results. In the tetragonal structure, dð002Þ 1⁄4 c=2 and dð110Þ 1⁄4 a= ffiffiffi