Spin Blockade, Orbital Occupation, and Charge Ordering inLa1.5Sr0.5CoO4

Abstract

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...

Correlation effects in CaCu3Ru4O12

“…27 At the maximum of the L 3 line it is seen that doping with one electron per formula unit (four Ru atoms) results in a shift of about an quarter of an eV, in agreement with the notion that a valence change of one electron results in a shift of the transition metal L 2,3 edge of the order of 1 eV. 28 …”

Electronic and Magnetic Nano Phase Separation in Cobaltates La2−x Sr x CoO4

“…This re-arrangement of charges into stripes at low temperatures seems to be hampered in the cobaltates which also have a much more robust CBCO with distinctly higher CBCO ordering temperatures than in the nickelates. An apparent difference between cobaltates and nickelates is the higher insulating properties of cobaltates according to the spin-blockade mechanism [12] as well as the stability of both Co 2+ and Co 3+ oxidation states (charges are localized at Co-sites and not at the oxygen ions) together with a very large difference in their ionic sizes (the ratio of the Co 2+ and Co 3+ ionic radii amounts to ∼1.367 [17]). All these effects together hamper the hopping of electrons from Co 2+ to Co 3+ sites and, thus, probably the re-arrangement of charges into stripes at low temperatures.…”

“…At half-doping, a robust checkerboard charge order (CBCO) has been reported in these cobaltates (La 1.5 Sr 0.5 CoO 4 ) with a charge ordering temperature of the order of ∼800 K [10,11]. It was shown that the Co 2+ and Co 3+ ions in La 1.5 Sr 0.5 CoO 4 are in the high spin and in the nonmagnetic low-spin state, respectively, [11,12].…”

Femtosecond Reflection Spectroscopy in La1.5Sr0.5CoO4

“…addition, the recent X-ray absorption spectroscopy [7], neutron diffraction [8], and band calculation [9] revealed that the electronic states of Co 2+ and Co 3+ are high-spin (HS, e ) and low spin (LS, t 6 2g ). Upon T CO , it is reasonable to consider some spin state transition in Co 3+ may occur but it has not been clarified.…”