Magnetic order and physical properties of the kagome metal UNb6Sn6

Abstract

The RM6X6 family of materials (R = rare-earth, M = transition metal, X = Ga, Si, Ge, Sn) produces an array of emergent phenomena, such as charge density waves, intrinsic Hall effects, and complex magnetic order, due to its kagome net of transition metal atoms, its local-moment magnetic anisotropies, and its extensive chemical tunability. Here, we report UNb6Sn6, a new “166” material containing both an actinide and a 4d transition metal, along with its nonmagnetic analog ThNb6Sn6, to investigate the properties of a 5f-4d electron 166 system. UNb6Sn6 crystallizes in the hexagonal P6/mmm space group with a small degree of disorder due to shifts in the size of the CoSn-like cages along the c axis. Upon cooling at zero magnetic field, the material undergoes two magnetic phase transitions at T2 = 46K and TN = 43K. The low-temperature, zero-field phase is an antiferromagnet with ordered uranium moments and a k = (0,0,1/2) propagation vector determined by neutron diffraction. Remarkably, with a magnetic field applied along the c axis, five additional magnetic transitions occur, evidenced by magnetization and resistivity data, before the moment saturates at 2.62 μB/U at 2 K and ≥ 13.6T. In two magnetic phase regions, the Hall resistivity of UNb6Sn6 significantly deviates from the magnetization, suggesting that the phases have a large Berry curvature or a change in the Fermi surface. The unknown magnetic ordering of the field-dependent phases of UNb6Sn6 demonstrates the complexity of the 5f-4d 166 system and encourages further study of its properties.