U2Ni2Sn and the origin of magnetic anisotropy in uranium compounds

${\mathrm{U}}₂\mathrm{N}{\mathrm{i}}₂\mathrm{Sn}$ is a member of a large family of intermetallic compounds with the tetragonal $\mathrm{M}{\mathrm{o}}₂\mathrm{Fe}{\mathrm{B}}₂$ 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 \ensuremath{\approx}170 K, indicating that ${\mathrm{U}}₂\mathrm{N}{\mathrm{i}}₂\mathrm{Sn}$ 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 $5f\text{\ensuremath{-}}5f$ 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 $0.87\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, the low magnetic entropy, and the enhanced Sommerfeld coefficient $\ensuremath{\gamma}=187\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/\mathrm{mol}\phantom{\rule{0.16em}{0ex}}\mathrm{f}.\mathrm{u}.\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}²$ suggest that ${\mathrm{U}}₂\mathrm{N}{\mathrm{i}}₂\mathrm{Sn}$ can be classified as an itinerant antiferromagnet with strong electron-electron correlations.