We investigated the orbital anisotropy of GaFeO3 using the Fe L2,3-edge x-ray magnetic circular dichroism and the polarization dependent O K-edge x-ray absorption spectroscopy. We found that the system shows a considerably large orbital momentum and anisotropic Fe-O bonding, which are unexpected in a half-filled d5 system such as GaFeO3. The orbital and bonding anisotropies, which turn out to be induced by the lattice distortions with exotic off-centering site movements, contribute the large magnetocrystalline energy and magnetoelasticity. These results provide critical clues on the microscopic understanding of the magnetoelectricity.
Spin-orbit coupling results in technologically-crucial phenomena underlying magnetic devices like magnetic memories and energy-efficient motors. In heavy element materials, the strength of spin-orbit coupling becomes large to affect the overall electronic nature and induces novel states such as topological insulators and spin-orbit-integrated Mott states. Here we report an unprecedented charge-ordering cascade in IrTe 2 without the loss of metallicity, which involves localized spin-orbit Mott states with diamagnetic Ir 4 þ -Ir 4 þ dimers. The cascade in cooling, uncompensated in heating, consists of first order-type consecutive transitions from a pure Ir 3 þ phase to Ir 3 þ -Ir 4 þ charge-ordered phases, which originate from Ir 5d to Te 5p charge transfer involving anionic polymeric bond breaking. Considering that the system exhibits superconductivity with suppression of the charge order by doping, analogously to cuprates, these results provide a new electronic paradigm of localized charge-ordered states interacting with itinerant electrons through large spin-orbit coupling.
We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional sensitivity to chemical compositions to investigate high-quality single crystal samples of La(1/4)Pr(3/8)Ca(3/8)MnO3, establishing the formation of distinct insulating domains embedded in the metallic host at low temperatures. These domains are found to be at least an order of magnitude larger in size compared to previous estimates and exhibit memory effects on temperature cycling in the absence of any perceptible chemical inhomogeneity, suggesting long-range strains as the probable origin.
Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice order in highly frustrated ZnCr2O4 where magnetic chromium ions occupy the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of anti-aligning spins surrounding the triangular faces of tetrahedra is resolved by establishing weak interactions on each triangle through an intricate lattice distortion. The resulting spin order is however, not simply a Néel state on strong bonds. A complex co-planar spin structure indicates that antisymmetric and/or further neighbor exchange interactions also play a role as ZnCr2O4 resolves conflicting magnetic interactions. PACS numbers:While tetrahedral atomic clusters are a natural consequence of close packing, they are particularly inconvenient for antiferromagnetically interacting spins. This is because no spin configuration can simultaneously satisfy all six antiferromagnetic interactions amongst spins on the vertices of a tetrahedron [1,2,3,4,5]. The consequence of such "geometrical frustration" is deep suppression of magnetic order and a range of temperatures where spins remain fluctuating despite interactions that far exceed thermal energies [6,7]. Indeed for spins on a lattice of corner-sharing tetrahedra, it appears there is no conventional order in the quantum limit (S = 1/2, T = 0) [5]. Because they entail higher energy spin configurations, geometrically frustrating lattices however typically do not survive in the low temperature limit. Instead a compromise between spin and lattice energy is reached through a first order phase transition that freezes the spin liquid and distorts the lattice [8,9,10,11,12,13]. Such phase transitions challenge conventional theories of magnetism because they involve strongly correlated spins and the collapse of the rigid lattice approximation [14,15,16].A case in point is ZnCr 2 O 4 . At room temperature, it has a cubic F d3m crystal structure where Cr 3+ (S = 3/2) ions form a network of corner-sharing tetrahedra [9]. The Curie-Weiss temperature is -390 K indicating strong antiferromagnetic frustration, yet chromium spins remain in a cooperative paramagnetic phase down to T C = 12.5 K [6,9]. There, a first order phase transition from a cubic paramagnet to a tetragonal antiferromagnet signals the end of distinct spin and lattice degrees of freedom. Tetragonal strain energy alone does not account for the difference between magnetic energy gain and overall latent heat and this was a first indication of a more comprehensive rearrangement of the lattice [9]. Subsequently X-ray superlattice peaks were detected at ( 2 ) c type reflections (see Fig. 1 (a)) [17]. This indicates that below T N the tetragonal lattice has I4m2 symmetry and a √ 2 × √ 2 × 2 chemical unit cell [18]. Theoretical efforts to understand the nature of the phase transition have focused on magneto-elastic couplings that involve symmetric isotropic nearest neighbor (NN) exchange interactions [14,15,16].Here we report a combined synchrotron X-ray and magnetic neutron diffraction study ...
We report x-ray scattering studies of nanoscale structural correlations in the paramagnetic phases of the perovskite manganites La 0.75 (Ca 0.45 Sr 0.55 ) 0.25 MnO 3 , La 0.625 Sr 0.375 MnO 3 , and Nd 0.45 Sr 0.55 MnO 3 . We find that these correlations are present in the orthorhombic O phase in La 0.75 (Ca 0.45 Sr 0.55 ) 0.25 MnO 3 , but they disappear abruptly at the orthorhombic-to-rhombohedral transition in this compound. The orthorhombic phase exhibits increased electrical resistivity and reduced ferromagnetic coupling, in agreement with the association of the nanoscale correlations with insulating regions. In contrast, the correlations were not detected in the two other compounds, which exhibit rhombohedral and tetragonal phases. Based on these results, as well as on previously published work, we propose that the local structure of the paramagnetic phase correlates strongly with the average lattice symmetry, and that the nanoscale correlations are an important factor distinguishing the insulating and the metallic phases in these compounds.The physical mechanism underlying the magnetic-fieldinduced insulator-metal transition in perovskite manganites A 1Ϫx B x MnO 3 has been the subject of intense experimental and theoretical investigation since its rediscovery in 1993. 1,2 One of the motivations for such considerable attention is the unusually large diminution of the electrical resistivity observed at the magnetic-field-induced transition, which is now commonly referred to as the colossal magnetoresistance ͑CMR͒. Several different kinds of the CMR effect are known. 2 The most widely studied variant of this effect is the transition from a paramagnetic insulating ͑PI͒ to a ferromagnetic metallic ͑FM͒ phase. The large difference between the resistivities of these two phases lies at the heart of the CMR effect. The metallic nature of the FM phase has been explained within the framework of the double-exchange mechanism. 2 However, the physical mechanism responsible for the large resistivity of the PI phase remains poorly understood.The situation is complicated significantly by the fact that transport properties of the paramagnetic state in manganites with the same doping level x, but with different cations A and B, are often very different. For example, the electrical resistivity of La 0.7 Sr 0.3 MnO 3 shows a metallic behavior ͑i.e., grows with temperature͒, 3 while the resistivity of La 0.7 Ca 0.3 MnO 3 exhibits the temperature dependence typical of an insulator, decreasing with temperature. 4 As a consequence, the resistivity of the latter compound is significantly larger than that of the former. These differences cannot be explained by steric modification of the electronic bandwidth W due to the evolution of the average structural parameters from one composition to another as a result of the differing size of the divalent dopants. 5,6 In fact, the value of W in La 0.7 Ca 0.3 MnO 3 is expected to differ from that in La 0.7 Sr 0.3 MnO 3 by less than 1% as a result of such effects. 5 The widely accepted solution...
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