Magnetic Properties of a UPtAl Single Crystal

UPtAl is a ferromagnet with a spontaneous magnetic moment M_s = 1.38 μ_B/f.u. directed along the hexagonal c axis and Curie temperature T_C = 43.5 K. The uniaxial magnetocrystalline anisotropy is very strong, the estimated anisotropy field B_a exceeds 290 T. In a thermally-demagnetized sample the initial susceptibility along the c axis is very low for a wide field interval below a critical field B_st. Then the magnetization increases abruptly and saturates in a rather narrow field interval. The low-temperature hysteresis loop is almost rectangular with a 100% remanent magnetization. The coercivity (B_c = 0.7T at 1.7K) weakens rapidly with increasing temperature and vanishes at 20 K. A zero-field cooled sample exhibits a large thermal hysteresis. In the vicinity of B_st and B_c, a very pronounced magnetic viscosity is observed. As the main mechanism of the magnetic hysteresis we propose the pinning of domain walls which are very narrow due to the huge anisotropy and a relatively weak exchange interaction. The susceptibility as a function of temperature in paramagnetic range obeys a Curie-Weiss law with an effective magnetic moment μ_eff = 2.85 μ_B/f.u. The paramagnetic Curie temperatures Θ_p observed in fields applied along the a- and the c-axis differ considerably (-375 and 46 K, respectively) reflecting the fact that the huge uniaxial magnetic anisotropy persists in paramagnetic state. The electrical resistivity of UPtAl is rather isotropic. The large magnetic contribution to resistivity is due to a strong interaction of conduction-electron states with the narrow 5f-band states responsible for U magnetic moments in UPtAl. Up to T = 30 K, the temperature dependence of resistivity obeys a quadratic law ρ(T) = ρ₀ + AT² with A = 0.061 μΩ·cm/K². UPtAl has an enhanced electronic specific-heat coefficient γ = 69 mJ/mol·K². The correlation between A and γ fits to the Kadowaki-Woods plot.