Spin State Transition inLaCoO3Studied Using Soft X-ray Absorption Spectroscopy and Magnetic Circular Dichroism

Abstract

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L2,3 edge we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply-degenerate high-spin first excited state. From the temperature dependence of the spectral lineshapes we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spinstate system. Crucial is that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low-/intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance and inelastic neutron data.PACS numbers: 71.28.+d, 71.70.Ch, 78.70.Dm LaCoO 3 shows a gradual non-magnetic to magnetic transition with temperature, which has been interpreted originally four decades ago as a gradual population of high spin (HS, t 4 2g e 2 g , S = 2) excited states starting from a low spin (LS, t 6 2g , S = 0) ground state [1,2,3,4,5,6,7,8]. This interpretation continued to be the starting point for experiments carried out up to roughly the first half of the 1990's [9,10,11,12]. All this changed with the theoretical work in 1996 by Korotin et al., who proposed on the basis of local density approximation + Hubbard U (LDA+U) band structure calculations, that the excited states are of the intermediate spin (IS, t 5 2g e 1 g , S = 1) type [13]. Since then many more studies have been carried out on LaCoO 3 with the majority of them [14,15,16,17,18,19,20,21,22,23,24,25,26,27] claiming to have proven the presence of this IS mechanism. In fact, this LDA+U work is so influential [28] that it forms the basis of most explanations for the fascinating properties of the recently synthesized layered cobaltate materials, which show giant magneto resistance as well as metal-insulator and ferroferri-antiferro-magnetic transitions with various forms of charge, orbital and spin ordering [29,30].In this paper we critically re-examine the spin state issue in LaCoO 3 . There has been several attempts made since 1996 in order to revive the LS-HS scenario [31,32,33,34,35], but these were overwhelmed by the above mentioned flurry of studies claiming the IS mechanism [14,15,16,17,18,19,20,21,22,23,24,25,26,27]. Moreover, a new investigation using inelastic neutron scattering (INS) has recently appeared in Phys. Rev. Lett.[36] making again the claim that the spin state transition involves the IS states. Here we used soft xray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the Co-L 2,3 edge and we revealed that the spin state transition in LaCoO 3 can be well described by a LS ground state and a triply degenerate HS excited state, and that an inhomogeneous mixed-spinstate system is formed. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility and electron spin resonance (ESR) data, and provide support for an alternative interpretation of the INS [37]. C...

Strain-driven spin-state transition and superexchange interaction in LaCoO3:Abinitiostudy

“…In Fig. 5 (a), we see that the energy gap becomes less than 58 meV above a strain level of 2.5%, thus allowing for the spin state transition [22]. Note, however, that the band gap also narrows for compressively strained LCO, but this does not result in a magnetic solution as shown in Fig.…”

“…However, the spin structure at different temperatures has been highly debated. For example, the LS-HS [17,18], LS-IS [19,20], and LS-HS/LS crossover scenarios have been discussed in the literature [21][22][23][24][25].…”

Microwave-Assisted Synthesis, Microstructure, and Magnetic Properties of Rare-Earth Cobaltites

“…The Co 3+ spin states may be (a) low-spin (LS) (t2g 6 eg 0 ), (b) intermediate-spin (IS) (t2g 5 eg 1 ), or (c) high-spin (HS) (t2g 4 eg 2 ) [10][11][12]. The competition between different spin states has caused large controversy in the literature previously [10,[12][13][14][15][16][17][18][19], but in most studies only the case of LaCoO3 has been considered. Different studies favor either a LS to IS or a LS to HS state transition by heating across Ts1 ≈ 75 K [20][21][22][23][24].…”

Influence of Al doping in LaCoO3 on structural, electrical and magnetic properties

“…Lanthanum Cobaltate (LaCoO 3 ) has been an intriguing subject of research primarily because of the spin state equilibria and the anomalous magnetic behaviour accompanying this crossover [1][2][3][4][5][6][7][8][9][10][11]. The oxygen octahedra coordinating the cobalt ion in the prototypical perovskite structure controls the delicate balance between the Hund's coupling and the crystal field splitting energy for the d electrons of Co 3?…”