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Register a new userElectronic and Magnetic Structures of Sr2FeMoO6
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Sugata Ray
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We have investigated the electronic and magnetic structures of Sr2FeMoO6 employing site-specific direct probes, namely x-ray absorption spectroscopy with linearly and circularly polarized photons. In contrast to some previous suggestions, the results clearly establish that Fe is in the formal trivalent state in this compound. With the help of circularly polarized light, it is unambiguously shown that the moment at the Mo sites is below the limit of detection (< 0.25mu_B), resolving a previous controversy. We also show that the decrease of the observed moment in magnetization measurements from the theoretically expected value is driven by the presence of mis-site disorder between Fe and Mo sites.
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Ordered double perovskite oxides of the general formula, A2BBO6, have been known for several decades to have interesting electronic and magnetic properties. However, a recent report of a spectacular negative magnetoresistance effect in a specific member of this family, namely Sr2FeMoO6, has brought this class of compounds under intense scrutiny. It is now believed that the origin of magnetism in this class of compounds is based on a novel kinetically-driven mechanism. This new mechanism is also likely to be responsible for the unusually high temperature ferromagnetism in several other systems, such as dilute magnetic semiconductors, as well as in various half-metallic ferromagnetic systems, such as Heussler alloys.
The crystal structures and the physical (magnetic, electrical transport and thermodynamic) properties of the ternary compounds CeRhSi2 and Ce2Rh3Si5 (orthorhombic CeNiSi2- and U2Co3Si5-type structures, respectively) were studied in wide ranges of temperature and magnetic field strength. The results revealed that both materials are valence fluctuating systems, in line with previous literature reports. Direct evidence for valence fluctuations was obtained by means of Ce LIII-edge x-ray absorption spectroscopy and Ce 3d core-level x-ray photoelectron spectroscopy. The experimental data were confronted with the results of ab initio calculations of the electronic band structures in both compounds.
The electronic properties of two spinels Fe$_3$O$_4$ and Fe$_2$SiO$_4$ are studied by the density functional theory. The local Coulomb repulsion $U$ and the Hunds exchange $J$ between the $3d$ electrons on iron are included. For $U=0$, both spinels are half-metals, with the minority $t_{2g}$ states at the Fermi level. Magnetite remains a metal in a cubic phase even at large values of $U$. The metal-insulator transition is induced by the $X_3$ phonon, which lowers the total energy and stabilizes the charge-orbital ordering. Fe$_2$SiO$_4$ transforms to a Mott insulating state for $U>2$ eV with a gap $Delta_gsim U$. The antiferromagnetic interactions induce the tetragonal distortion, which releases the geometrical frustration and stabilizes the long-range order. The differences of electronic structures in the high-symmetry cubic phases and the distorted low-symmetry phases of both spinels are discussed.
Besides a drastic reduction in saturation magnetization of disordered Sr2FeMoO6 compared to highly ordered samples, magnetizations as a function of the temperature for different disordered samples may also show qualitatively different behaviors. We investigate the origin of such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish that extensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples is responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with composition fluctuations of the type Sr2Fe1+xMo1-xO6 with Fe-rich (x>0) and Mo-rich (x<0) regions.
We report a combined study for the electronic structures of ferromagnetic CeAgSb$_2$ using soft X-ray absorption (XAS), magnetic circular dichroism (XMCD), and angle-resolved photoemission (ARPES) spectroscopies. The Ce $M_{4, 5}$ XAS spectra show very small satellite structures, reflecting a strongly localized character of the Ce $4f$ electrons. The linear dichroism effects in the Ce $M_{4, 5}$ XAS spectra demonstrate the ground state Ce $4f$ symmetry of $Gamma{_6}$, the spatial distribution of which is directed along the $c$-axis. The XMCD results give support to the picture of local-moment magnetism in CeAgSb$_2$. Moreover it is also found that the theoretical band dispersions for LaAgSb$_2$ provides better description of the ARPES band structures than those for CeAgSb$_2$. Nevertheless, ARPES spectra at the Ce $3d$-$4f$ resonance show the momentum dependence for the intensity ratio between Ce $4f^{1}_{5/2}$ and $4f^{1}_{7/2}$ peaks in a part of the Brillouin zone, suggesting the non-negligible momentum dependent hybridization effect between the Ce $4f$ and the conduction electrons. This is associated with the moderate mass enhancement in CeAgSb$_2$.
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