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We present an investigation of the multiferroic lacunar spinel compound GeV 4 S 8 using timedomain terahertz spectroscopy. We find three absorptions which either appear or shift at the antiferromagnetic transition temperature, TN = 17 K, as S=1 magnetic moments develop on vanadium tetrahedra. Two of these absorptions are coupled to the magnetic state and one only appears below the Néel temperature, and is interpreted as a magnon. We also observe isosbestic points in the dielectric constant in both the temperature and frequency domains. Further, we perform analysis on the isosbestic features to reveal an interesting collapse into a single curve as a function of both frequency and temperature, behavior which exists throughout the phase transitions. This analysis suggests the importance of spectral changes in the terahertz range which are linear in frequency and temperature.

Five double perovskites, each containing a transition-metal ion with a 5d1 configuration, have been studied to better understand the surprising diversity of magnetic ground states seen in these isoelectronic compounds. Ba2ZnReO6 adopts the cubic double perovskite structure and magnetically orders below 16 K, with a canted ferromagnetic structure and a saturated magnetization of ∼0.24 μB/Re. X-ray magnetic circular dichroism indicates a substantial orbital moment of approximately 0.4 μB/Re that opposes the spin moment. The structures of Ba2NaOsO6 (canted ferromagnet, T C = 7 K) and Ba2LiOsO6 (antiferromagnet, T N = 8 K) are reinvestigated using time-of-flight neutron powder diffraction and found to crystallize with the cubic double perovskite structure. No evidence for a structural distortion can be found in either compound down to 10 K. Ba2CdReO6 is also cubic at room temperature but undergoes a structural transition upon cooling below ∼180 K to a tetragonal structure with I4/m symmetry that involves compression of the Re–O bonds that are parallel to the c-axis. Sr2LiOsO6 shows a similar tetragonal distortion at room temperature and maintains that structure down to 10 K. Surprisingly, the Os-centered octahedron in Sr2LiOsO6 is distorted in the opposite direction, exhibiting an elongation of the Os–O bonds along the c-axis. Differences in the distortions of the octahedra lead to different magnetic ground states, antiferromagnetic (T N = 4 K) for Ba2CdReO6 and spin glass (T g = 30 K) for Sr2LiOsO6. Theoretical modeling shows that the varied magnetic behaviors of double perovskites containing 5d1 ions are closely tied to crystallographic distortions. These distortions remove the degeneracy of the 5d t2g orbitals, leading to changes in orbital occupation that ultimately determine which of the several competing magnetic ground states is favored.

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