Pressure effect study on uranium ferromagnet U4Ru7Ge6 with negligible magnetocrystalline anisotropy

Although the first U4Ru7Ge6 single crystals were grown in the late 1980’s, only a vague report on ferromagnetism (TC ∼ 7 K, 0.2 μB/U ion in 5 T at 4.3 K) [1, 2, 3] and no information on anisotropy can be found in literature. Therefore we have grown a high-quality single crystal of this compound and measured its magnetization, ac susceptibility, thermal expansion, specific heat, and electrical resistivity with respect to temperature, magnetic field and pressure.
Strong magnetocrystalline anisotropy is a well-known property of uranium compounds. U4Ru7Ge6 behaves as an itinerant 5f -electron ferromagnet (TC = 10.7 K, μS = 0.85 μB/f.u. at 1.9 K) [4]. The ground-state easy magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field μ0Ha along the [001] direction is only about 0.3 T, which is at least three orders of magnitude smaller than for other U ferromagnets. At Tr = 5.9 K the easy magnetization direction changes to [001], and remains [001] up to TC. This transition is due to a change in magnetic symmetry, and is quite apparent in the low-field magnetization, ac susceptibility and thermal expansion data, whereas only weak anomalies are observed at Tr in the temperature dependence of the specific heat and electrical resistivity. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in the case of the [111] ([001]) easy magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites.
We studied the evolution of magnetism and its anisotropy of the U4Ru7Ge6 single crystal under applied hydrostatic pressures up to 3 GPa in piston cylindric pressure cell. We will present results for magnetization, AC susceptibility and electrical resistivity measurements performed on a U4Ru7Ge6 single crystal at various temperatures and magnetic fieldscans present the magnetic phase diagram with respect to pressure covering the evolution of both, Curie and reorientation temperature.

[1] A. A. Menovsky, J. Magn. Magn. Mater. 76-77, 631 (1988).
[2] Nascimento, et al., Physical Review B, 98(17) doi:10.1103/PhysRevB.98.174431 (2018)
[3] Hiroyuki Hidaka et al., Journal of the Physical Society of Japan, DOI: 10.1143/JPSJS.80SA.SA102 (2011)
[4] M. Valiska et al., Phys. Rev. B 95, DOI: 10.1103/PhysRevB.95.085142 (2017)