Study of the spin-reordering transition in Cr5S6

It is known that Cr5S6 becomes ferrimagnetic at T-C = 305 degrees K and undergoes a ferrimagnetic-antiferromagnetic transition at 150 < T-t < 160 degrees K. A neutron-diffraction study by van Laar indicates this transition to be of second order, whereas the existence of thermal hysteresis points to a first-order effect. We have investigated the magnetization of Cr5S6 as a function of hydrostatic pressure as well as of temperature and applied field. We find that partial derivative T-C/partial derivative P = 1.83 degrees K/kbar, partial derivative T-t/partial derivative P = 0.04 degrees K/kbar, and sigma(-1)(0)(partial derivative sigma(0)/partial derivative P) = -0.011/kbar. partial derivative T-t/partial derivative H runs from -0.54 degrees K/kOe at low fields to -0.45 degrees K/kOe at 9-12 kOe; these values, together with the shape of the normalized curve of magnetization versus temperature, are independent of pressure. In addition, we have investigated theoretically the classical ground-state spin configuration of the Cr5S6 lattice, using the generalized Luttinger-Tisza method, and have computed the thermal evolution of the ground state in the molecular field approximation. The observed spiral ground state is found to require not only that all the nearest-neighbor interactions be antiferromagnetic, but also that antiferromagnetic next-nearest-neighbor interactions be present. The calculated temperature dependencies of spins on the different sublattices lead directly to a second-order spin transition to a collinear ferrimagnetic configuration at T-t, in good agreement with experiment. We conclude that the transition at T-t is basically of second order, with secondary magnetostrictive forces responsible for the thermal hysteresis. The pressure dependence of sigma(0) indicates that some of the magnetization of Cr5S6 arises from band electrons. The decrease of T-C with pressure is largely due to a decrease in the nearest-neighbor interaction along the c axis, which interaction is also the one most likely to involve band electrons.