High-field study of UCo₂Si₂ : Magnetostriction at metamagnetic transition and influence of Fe substitution

UCo₂Si₂ (tetragonal crystal structure) is antiferromagnet below T_N = 83 K with ferromagnetic basal-plane layers of U magnetic moments oriented parallel to the c axis. The layers are coupled in +-+- sequence along this axis. In fields of 45 T applied along the c axis, UCo₂Si₂ exhibits very sharp metamagnetic transition to ++- uncompensated antiferromagnetic state. The transition is accompanied by pronounced magnetostriction effects. The crystal expands along the c axis by 1 * 10^-4 and shrinks in the basal plane by 0.5 * 10^-4 (at 1.5 K) resulting in negligible volume effect. Between 20 K and 40 K the transition changes from the first- to the second-order type. The Fe doping in UCo₂Si₂ reduces T_N from 83 K to 80 K at x = 0.2 in U(Co_1-xFe)₂Si₂. Metamagnetic transition shifts to higher fields (from 45 T at x = 0 - 56 T for x = 0.2). Magnetization jump over the transition remains practically the same which is in agreement with uranium magnetic moment determined by neutron diffraction on crystal with x = 0.1 as 1.29 μ_B, i.e. only slightly lower than that in UCo₂Si₂.