Using Co-L2,3 and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La1.5Sr0.5CoO4 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 La2−xSrxCoO4 series. The associated e 2 g and e 0 g 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 La1.5Sr0.5CoO4 is a narrow band material.PACS numbers: 71.28.+d, 78.70.Dm Considerable research effort has been put in cobaltate materials during the last decade in search for new phenomena and extraordinary properties. A key aspect of cobaltates that distinguish them from e.g. the manganates and cuprates [1], is the spin state degree of freedom of the Co 3+/III ions: it can be low spin (LS, S=0), high spin (HS, S=2) and even intermediate spin (IS, S=1) [2,3]. This aspect comes on top of the orbital, spin (up/down) and charge degrees of freedom that already make the manganates and cuprates so exciting. Indeed, numerous cobaltate systems have been discovered with properties that include giant magneto resistance [4,5], superconductivity [6] and ferro-ferri-antiferro-magnetic transitions with various forms of charge, orbital and spin ordering [7,8,9,10,11,12,13,14]. A new and exciting aspect in here is the recognition that the so-called spin blockade mechanism could be present and responsible for several of those unusual properties [15]. If true, this would open up new research opportunities since one could envision exploiting explicitly this mechanism in materials design.Here we focus on the La 2−x Sr x CoO 4 system, which shows quite peculiar transport and magnetic properties [16,17,18,19,20,21,22,23,24,25]. This material is extremely insulating for a very wide range of x values with anomalously high activation energies for conductivity, very much unlike the Mn, Ni, or Cu compounds [1,18,26]. The commensurate antiferromagnetic (AF) state remains stable up to a surprisingly high value of x=0.3 [24,25]. Charge ordering (CO) and spin ordering (SO) at half doping involve quite different transition temperatures, namely T CO ∼ 750 K and T SO ≤ 30 K, respectively. This constitutes a ratio of 25, which is extraordinary since it is an order of magnitude larger than in the Mn and Ni materials [1,21,27].It was already reported that the SO in the La 1.5 Sr 0.5 CoO 4 composition involves non-magnetic Co 3+ ions with the claim that these Co 3+ ions are in the IS state and become non-magnetic due to strong planar anisotropy driven quenching of the spin angular momentum below the T SO [21,22]. Here we go one step further. Using soft x-ray absorption spectroscopy (XAS) we are able to show unambiguously that the Co 3+ ions are in the LS (S=0) state, both below and above T SO . Together with the verification...
As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible2, 3, 4, 5, 6, 7, 8. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics10
The material class of rare earth nickelates with high Ni3+ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO3 thin films has also triggered intensive research efforts. LaNiO3 seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO3 single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni2+ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.
We present measurements of the spin and orbital magnetic moments of Fe3O4 by using SQUID and magnetic circular dichroism in soft x-ray absorption. The measurements show that Fe3O4 has a noninteger spin moment, in contrast to its predicted half-metallic feature. Fe3O4 also exhibits a large unquenched orbital moment. Calculations using the local density approximation including the Hubbard U method and the configuration interaction cluster-model suggest that strong correlations and spin-orbit interaction of the 3d electrons result in the noninteger spin and large orbital moments of Fe3O4.
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