MAGNETIZATION AND SPIN CORRELATION IN A SINGLE-CRYSTAL OF YFEMNO4

Magnetization and neutron-diffraction measurements were carried out on a single crystal of hexagonal YFeMnO4, a two-dimensional Heisenberg triangular antiferromagnet. The Fe3+ and Mn2+ are randomly distributed on equivalent triangular sites, producing the system with both frustration and randomness. The thermomagnetization curve in 50 Oe shows that a field-cooling effect is observed below 60 K, where the susceptibility along the c axis has a cusplike peak. Neutron-diffraction measurements reveal that a two-dimensional (1/3,1/3,l) magnetic rod is observed in the whole temperature range instead of three-dimensional Bragg points, and the magnetic correlation in the c plane does not become long range even at the lowest temperature. It also shows that spins fluctuate largely at room temperature, and this fluctuation still remains at low temperatures. This magnetic property is similar to that of spin-glass system. Some type of spin freezing occurs below 60 K, and a slight increase of the correlation length down to the lowest temperature suggests that this freezing is imperfect even at the lowest temperature. The temperature and cooling field dependence of thermoremanent magnetization can be explained by assuming a domain structure which is divided into microdomains by random fields induced by the disorder.