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Seminar: Michal Vališka

  • Michal Vališka: Uranium ferromagnet with negligible magnetocrystalline anisotropy - U4Ru7Ge6
  • 30. 11. 2016, 14:30
  • lecture room F2, first floor Ke Karlovu 5
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Abstract:

Strong magnetocrystalline anisotropy is a well-known property of uranium compounds. The almost isotropic ferromagnetism in U4Ru7Ge6 reported in this paper represents a striking exception. We present results of magnetization, AC susceptibility, thermal expansion, specific heat and electrical resistivity measurements performed on a U4Ru7Ge6 single crystal at various temperatures and magnetic fields and discuss them in conjunction with results of first-principles electronic-structure calculations. U4Ru7Ge6 behaves as an itinerant 5f-electron ferromagnet (TC = 10.7 K, µs = 0.85 µB/f.u. at 1.9 K). The ground-state easy-magnetization direction is along the [111] axis of the cubic lattice. The anisotropy field m0Ha along the [001] direction is only of ~ 0.3 T, which is at least 3 orders of magnitude smaller value than in other U ferromagnets. At Tr = 5.9 K the easy magnetization direction changes for [001] which holds at temperatures up to TC. This transition causing a change of magnetic symmetry is significantly projected in low-field magnetization, AC susceptibility and thermal-expansion data whereas only a weak anomaly is observed at Tr is observed in the temperature dependence of specific heat and electrical resistivity, respectively. The magnetoelastic interaction induces a rhombohedral (tetragonal) distortion of the paramagnetic cubic crystal lattice in case of the [111]([001]) easy-magnetization direction. The rhombohedral distortion is connected with two crystallographically inequivalent U sites. The ab initio calculated ground-state magnetic moment of 1.01 µB/f.u. is oriented along [111]. The two crystallographically inequivalent U sites are a consequence of spin-orbit coupling of the U 5f-electrons. In the excited state which is only 0.9 meV above the ground state the moment points to the [001] direction in agreement with experiment. A scenario of the origin of the very weak magnetic anisotropy of U4Ru7Ge6 is discussed considering interactions of the U-ion 5f-electron orbitals with the nearest neighbor ions.

For details see: https://arxiv.org/abs/1611.07335