We have analyzed the unusual electronic structure of Sr2FeMoO6 combining ab initio and model Hamiltonian approaches. Our results indicate that there are strong enhancements of the intra-atomic exchange strength at the Mo site as well as the antiferromagnetic coupling strength between Fe and Mo sites. We discuss the possibility of a negative effective Coulomb correlation strength ( U(eff)) at the Mo site due to these renormalized interaction strengths.

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We present magnetization (M) and magnetoresistance (MR) data for a series of Sr2FeMoO6 samples with independent control on antisite defect and grain-boundary densities, which reveal several unexpected features, including a novel switching-like behavior of MR with M. These, in conjunction with model calculations, establish that the MR in Sr2FeMoO6 is dominantly controlled by a new mechanism, derived from the magnetic polarization of grain-boundary regions acting like spin valves, leading to behavior qualitatively different from that usually encountered in tunneling MR. We show that a simple and useful experimental signature for the presence of this spin-valve-type MR (SVMR) is a wider hysteresis in MR compared to that in M.

The degree of B/B;{'} alternate cation order is known to heavily influence the magnetic properties of A_{2}BB;{'}O_{6} double perovskites although the nature of such disorder has never been critically studied. Our detailed x-ray absorption fine structure studies in conjunction with synchrotron radiation x-ray diffraction experiments on polycrystalline Sr_{2}FeMoO_{6} samples with various degrees of disorder reveal that a very high degree of short range order is preserved even in samples with highly reduced long range chemical order. Based on these experimental results and with the help of detailed structural simulations, we are able to model the nature of the disorder in this important class of materials and discuss the consequent implications on its physical properties.

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba 3 ZnIr 2 O 9 is a realization of a novel spin-orbital liquid state. Our results reveal that Ba 3 ZnIr 2 O 9 with Ir 5þ (5d 4 ) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J ¼ 0 state but each Ir ion still possesses a weak moment. Ab initio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir 2 O 9 dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100 mK. DOI: 10.1103/PhysRevLett.116.097205 5d transition metal compounds often exhibit unusual electronic and magnetic properties due to the presence of strong spin-orbit coupling (SOC), comparable to their onsite Coulomb (U) and crystal field (Δ CFE ) interactions [1,2]. In the strong spin-orbit coupling regime, M J ( P m j ) becomes the only valid quantum number instead of m l (orbital) and m s (spin), and the multiplets and their degeneracies are solely determined by the total angular momentum J. The electronic and magnetic responses of a system in such limits are not yet well understood and have generated significant curiosity in recent times. For example, the curious insulating state of the layered tetravalent iridates (Ir 4þ ; 5d 5 ) has recently been explained within single particle theories assuming splitting of t 2g bands into a set of fully filled quartet bands separated from another set of half-filled narrow doublet bands due to finite SOC. The half-filled doublet bands further split into fully occupied lower and empty upper Hubbard bands in the presence of relatively small Hubbard U [3-5].The pentavalent Iridates (Ir 5þ ; 5d 4 ) are more intriguing, where in the strong SOC limit all the spin-orbit entangled electrons will be confined to singlet J ¼ 0 (M J ¼ 0) ground state, with no net moment. The evolution of ground and excited states of a low spin 5d t 4 2g Ir 5þ ion as a function of SOC parameter λ 0 is illustrated in Fig. 1(a) and a J ¼ 0 ground state is realized in the strong SOC limit [6]. A possibility of excitonic magnetism has been predicted for these systems where the energy scale of the singlet-triplet splitting determined by SOC is comparable to superexchange interaction promoted by hopping [10]. The breakdown of the J ¼ 0 nonmagnetic state in d 4 systems can also be realized within a single electron picture primarily driven by band-structure effect that allows the hybridization between the quartet and the doublet redistributed orbitals (eigenstates of the spin-orbit coupled Hamiltonian). Overall, d 4 Ir compounds in the strong SOC limit may host weak magnetic moment unless the λ 0 becomes so large that any excitonic or hopping-assisted magnetism become...

We report the observation of room temperature ferromagnetism in high quality, single crystalline dilute Fe-doped BaTiO 3. The large equilibrium solubility of Fe ions in the matrix refutes uncertainties about secondary phase magnetism, which has often eclipsed this interesting field of research. While room temperature ferromagnetism is observed at and above 5% Fe concentrations, one finds a highly concave temperature dependence of the susceptibility. Using detailed ab initio calculation, this has been related to intrinsic magnetic inhomogeneities arising from positional disorder. Apart from providing a mechanism for the observed high temperature ferromagnetism, our results point out that intrinsic disorder is a generic and essential component of dilute magnetism.

We investigate the electronic structure of Ca1−xSrxVO3 using careful state-of-the-art experiments and calculations. Photoemission spectra using synchrotron radiation reveal a hitherto unnoticed polarization dependence of the photoemission matrix elements for the surface component leading to a substantial suppression of its intensity. Bulk spectra extracted with the help of experimentally determined electron escape depth and estimated suppression of surface contributions resolve outstanding puzzles concerning the electronic structure in Ca1−xSrxVO3.

Single crystalline Fe-doped hexagonal BaTiO 3 samples with varying oxygen content are created by specifically intended post-growth annealing treatments, in order to check the influence of defects on the unusual high temperature ferromagnetism observed in this system. The various defects have been shown to play a crucial role in dilute magnetic systems and therefore, it is important to carry out this check for the Fe-doped BaTiO 3 system also, in which unusual ferromagnetism was reported even in its bulk single crystalline form. The x-ray diffraction and dielectric studies carried out here have confirmed that the Fe doping of Ti is intrinsic, while the high resolution transmission electron microscopy (HRTEM) and x-ray photoemission spectroscopy (XPS) studies proved the absence of unwanted magnetic metal clusters in the sample. The transport studies show that the oxygen concentrations could be varied substantially by the thermal treatments. Finally, magnetization measurements on the samples demonstrated that ferromagnetism is stronger in samples with higher oxygen deficiency, which could interestingly be retreated under high oxygen atmosphere and reversibly be taken back to a lower magnetic state. The vacancy-induced ferromagnetism is further confirmed by EPR measurements, which is consistent with earlier studies and, consequently, put the doped BaTiO 3 in the list of true dilute magnetic oxide (DMO) systems.