U 2 Ni 2 Sn and the origin of magnetic anisotropy in uranium compounds
Here, U 2 Ni 2 Sn is a member of a large family of intermetallic compounds with the tetragonal Mo 2 FeB 2 crystal structure. It orders antiferromagnetically at 25 K with propagation vector $q = (0, 0, \frac{1}{2})$. Magnetization, magnetoacoustic, and neutron-diffraction experiments on a single crystal provide evidence that the uranium moments align parallel to the $c$ axis with the anisotropy energy of ≈170 K, indicating that U 2 Ni 2 Sn can be classified as an Ising system. The results are at variance with previous studies on polycrystals, which indicated different magnetic structure, and which were incompatible with the $5f - 5f$ two-ion anisotropy model dominant in most U band systems. High-field magnetization studies exhibit a weak linear response for fields along the basal plane up to the highest field applied (60 T), while the $c$-axis magnetization curve exhibits three metamagnetic transitions at approximately 30, 39, and 50 T. The U magnetic moments of $0.87 μ_B$, the low magnetic entropy, and the enhanced Sommerfeld coefficient $γ$ = 187 mJ/mol f.u.K 2 suggest that U 2 Ni 2 Sn can be classified as an itinerant antiferromagnet with strong electron-electron correlations.