Magnetic phase diagram of UNi 2Si 2 in high fields

Abstract

The tetragonal UNi 2Si 2 exhibits three different AF phases in zero magnetic field below T N=125 K. In all of them the U moment is parallel to the c-axis, forming ferromagnetic basal planes. Between T N and 108 K the incommensurate AF1 phase with q = ( 0, 0, q z ) is established ( q z ∼0.74). The phase AF2, between 108 and 40 K, is of the AF-I type with q=(0, 0, 1) and the ground-state phase AF3 is the uncompensated squared-up AF structure with the (++−) stacking of basal planes along the c-axis ( q=(0, 0, 2 3 ) ). Both 108 and 40 K transitions are of the first order. The neutron scattering study of UNi 2Si 2 single crystal in magnetic fields up to 15 T shows a complex magnetic phase diagram. The 108 K phase boundary splits above 3 T and the AF2 phase disappears above 3.8 T. The T N remains unchanged up to 15 T and the AF1 phase is getting narrower in high magnetic field, vanishing above 14 T. Only the AF3 phase exists up to T N in higher fields. Strong field and temperature dependence of the q z were observed in the AF1 phase, it shows steep decrease with applied field at the AF1–AF3 boundary.