Magnetoelastic properties of 5f ferromagnet UCu₂P₂

UCu₂P₂, a Zintl phase with the CaAl₂Si₂ structure type, is a 5f ferromagnet with record-high Curie temperature among U compounds, T_C = 216 K [1]. While the size of the magnetic moment 2.0 μ_B/U is not surprising due to U-U spacing exceeding the Hill limit, the reasons for the high T_C are less understood. Ab-initio calculations reveal only very weak hybridization of the U-5f states with the 6d states as well as with electronic states of Cu and P. It seems that a transfer of U-6d states to the P-3p states is an important ingredient, which was highlighted by a rapid increase of T_C under hydrostatic pressure so that a room-temperature 5f ferromagnetism could be demonstrated. Besides magnetization, transport, and heat capacity studies on single crystals we performed also characterization of a polycrystalline material, which has T_C = 219 K. Thermal expansion study revealed a moderate increase of both lattice parameters just below T_C, so we can exclude that the pressure enhancement of T_C is driven simply by thermodynamics (via the Ehrenfest relation). Hence the reasons have to be attributed to the enhancement of specific U-U couplings upon compression. Indeed, ab initio calculations probing the energy enhancement upon moments reversal gave a semi-quantitative account of the observed tendency of T_C.

[1] D. Kaczorowski, R. Troc, Magnetic and transport properties of a strongly anisotropic ferromagnet, UCu2P2, J. Phys. Condens. Matter. 2, 4185 (1990).