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Register a new userSpin blockade, orbital occupation and charge ordering in La_(1.5)Sr_(0.5)CoO4
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Using Co-L_(2,3) and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La_(1.5)Sr_(0.5)CoO4 involves high spin (S=3/2) Co^2+ and low spin (S=0) Co^3+ ions. This provides evidence for the spin blockade phenomenon as a source for the extremely insulating nature of the La_(2-x)Sr_(x)CoO4 series. The associated e_g^2 and e_g^0 orbital occupation accounts for the large contrast in the Co-O bond lengths, and in turn, the high charge ordering temperature. Yet, the low magnetic ordering temperature is naturally explained by the presence of the non-magnetic (S=0) Co^3+ ions. From the identification of the bands we infer that La_(1.5)Sr_(0.5)CoO4 is a narrow band material.
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A lightly doped perovskite mangantite La_{0.88}Sr_{0.12}MnO_3 exhibits a phase transition at T_{OO}=145 K from a ferromagnetic metal (T_C=172 K) to a novel ferromagnetic insulator.We identify that the key parameter in the transition is the orbital degree of freedom in e_g electrons. By utilizing the resonant x-ray scattering technique, orbital ordering is directly detected below T_{OO}, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The new experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present experimental and theoretical studies uncover a crucial role of the orbital degree in the metal-insulator transition in lightly doped manganites.
We have explored spin, charge and orbitally ordered states in La1-xSrxMnO3 (0 < x < 1/2) using model Hartree-Fock calculations on d-p-type lattice models. At x=1/8, several charge and orbitally modulated states are found to be stable and almost degenerate in energy with a homogeneous ferromagnetic state. The present calculation indicates that a ferromagnetic state with a charge modulation along the c-axis which is consistent with the experiment by Yamada et al. might be responsible for the anomalous behavior around x = 1/8.
We have analyzed the experimental evidence of charge and orbital ordering in La0.5Sr1.5MnO4 using first principles band structure calculations. Our results suggest the presence of two types of Mn sites in the system. One of the Mn sites behaves like an Mn(3+) ion, favoring a Jahn-Teller distortion of the surrounding oxygen atoms, while the distortion around the other is not a simple breathing mode kind. Band structure effects are found to dominate the experimental spectrum for orbital and charge ordering, providing an alternate explanation for the experimentally observed results.
We have performed x-ray linear and circular magnetic dichroism experiments at the Mn L2,3-edge of the La0.7Sr0.3MnO3 ultra thin films. Our measurements show that the antiferromagnetic (AF) insulating phase is stabilized by the interfacial rearrangement of the Mn 3d orbitals, despite the relevant magnetostriction anisotropic effect on the double-exchange ferromagnetic (FM) metallic phase. As a consequence, the Mn atomic magnetic moment orientation and how it reacts to strain differ in the FM and AF phases. In some cases a FM insulating (FMI) phase adds to the AF and FM. Its peculiar magnetic properties include in-plane magnetic anisotropy and partial release of the orbital moment quenching. Nevertheless the FMI phase appears little coupled to the other ones.
We have studied the critical behaviour in $La_{0.5}Sr_{0.5}CoO_{3}$ near the paramagnetic-ferromagnetic transition temperature. We have analysed our dc magnetisation data near the transition temperature with the help of modified Arrott plots, Kouvel-Fisher method. We have determined the critical temperature $T_c$ and the critical exponents, $beta$ and $gamma$. With these values of $T_c$, $beta$ and $gamma$, we plot $M/(1-T/T_c)^{beta}$ vs $H/(1-T/T_c)^{gamma}$. All the data collapse on one of the two curves. This suggests that the data below and above $T_c$ obeys scaling, following a single equation of state.
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