The magnetic structure of Ca2MnReO6 double perovskite is investigated by neutron powder diffraction and bulk magnetization, showing dominant non-collinear Mn magnetic moments [4.35(7) µB] that are orthogonally aligned with the small Re moments [0.22(4) µB]. Ab-initio electronic structure calculations show that the strong spin-orbit coupling for Re 5d electrons combined with a relatively modest on-site Coulomb repulsion (U Re ef f 0.6 eV) is sufficient to render this material insulating. This is a rare example of spin-orbit assisted Mott insulator outside the realm of iridates, with remarkable magnetic properties. PACS numbers:The identification of a spin-orbit entangled Mott insulating state in Sr 2 IrO 4 triggered a quest for novel quantum phases in 5d-based materials [1][2][3][4][5]. In such systems with strong spin-orbit coupling (SOC), a Heisenberg Hamiltonian is not sufficient to describe the magnetic ground states and excitations, therefore alternative treatments may be necessary to describe the effect of anisotropic exchange interactions [5]. Also, the electronic correlations in 5d systems are relatively weak, and the atomic moments are normally small compared to the 3d counterparts. Alternating 3d and 5d ions in an ordered double perovskite structure offers a possible pathway to investigate ground states arising from the combination of strong SOC in 5d ions and strong electronic correlation in 3d ions. An example is Ca 2 MnReO 6 (CMRO), which was previously described as a ferromagnetic insulator with a small saturation magnetization of 0.9 µ B /f.u. and coercive field of 4 T [6], also showing Mn valence close to +2 [7,8]. Here, an unusual magnetic structure is demonstrated, with dominating Mn moments forming a largely canted sublattice, while weak Re moments are orthogonally aligned with Mn moments. Electronic structure calculations reveal that a combination of Re SOC and electronic correlations renders this material insulating, revealing a determinant role of the Re moments to the overall electronic and magnetic behavior of this material.The polycrystalline CMRO sample was synthesized by solid state reaction [8]. dc-magnetization and magnetic susceptibility measurements were performed in a commercial platform. Cold neutron powder diffraction (c-NPD) measurements were taken at the G4-1 instrument of Laboratorie Léon Brillouin (LLB), using a highly oriented pyrolytic graphite (HOPG) monochromator with vertical focusing and λ = 2.43Å, a two-axis diffractometer and a BF 3 multicell detector with 80 • aperture and 0.1 • resolution. High resolution thermal neutron powder diffraction (t-NPD) measurements were also taken, and details are given in the Supplemental Material (SM) [9]. The sample was kept sealed under He atmosphere in a vanadium can and mounted into the cold finger of a LHe cryostat. The powder diffraction data were independently analysed using the Fullprof [10] and GSAS [11] suites, with similar results. The refined degree of Mn/Re antisite disorder is 0.6(7) %. The Re 6+ magnetic form factor was...
The crystal and local atomic structure of monoclinic ReO2 (alpha-ReO2) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X-ray absorption spectroscopy and high-resolution synchrotron X-ray powder diffraction and a home-built B4C anvil pressure cell developed for this purpose. Extended X-ray absorption fine-structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re-Re distances and a distorted ReO6 octahedron in the alpha-ReO2 structure. X-ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the alpha-phase, demonstrating a modulation of the Re-Re distances. The relatively small portion of the diffraction pattern accessed in the pressure-dependent measurements does not allow for a detailed study of the crystal structure of alpha-ReO2 under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re-Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that alpha-ReO2 is a possible inner pressure gauge for future experiments up to 1.2 GPa.
We have studied the properties of Ce doping in Ca-site of Ca 2 MnReO 6 double perovskite up to 10%. These compounds have presented a monoclinic cell with P2 1 /n space group. A small increase of lattice parameters in doped sample were observed through Rietveld refinement of X-ray diffraction pattern. The spectra at the Mn K-edge and Re L 3 -edge showed a mixed valence for Mn and Re in both samples. We do not observed any variation on Mn K-edge valence in XANES measurements under pressure. However a change was observed for Re L 3 -edge as a function of applied pressure. The Ce-doped sample showed a magnetic geometric frustration and a small decrease of the temperature frustration as compared with Ca 2 MnReO 6 sample. The decrease of temperature frustration can be related to the MnO 6 and ReO 6 octahedral distortion.
Mercury superconductor samples doped with rhenium, Hg0.82Re0.18Ba2Ca2Cu3O8+d, were produced in a very narrow range of oxygen content (d = 0.05, 0.10 and 0.15). The lattice parameters indicated a slight change of the unitary cell as a function of the oxygen content. Furthermore, the reduction of the volume cell was associated with the slight increment of TC. The measurements of χac versus temperature, under external hydrostatic pressure, have shown sensitive results as a function of distinct oxygen content. In our opinion, this kind of measurement may be used as a tool to determine the oxygen content (underdoped, optimal or overdoped) of samples. For dTC/dP > 1.9 K GPa−1 the sample presents an underdoped oxygen content. One can understand that dTC/dP = (1.9 ± 0.2) K GPa−1 is associated with an intrinsic term value for the optimal oxygen doped sample, Hg0.82Re0.18Ba2Ca2Cu3O8.10.
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