Intricacies of the Co3+ spin state in Sr2Co0.5Ir0.5<

Abstract

We report on a combined soft x-ray absorption and magnetic circular dichroism (XMCD) study at the Co-L3,2 on the hybrid 3d/5d solid state oxide Sr2Co0.5Ir0.5O4 with the K2NiF4 structure. Our data indicate unambiguously a pure high spin state (S = 2) for the Co 3+ (3d 6 ) ions with a significant unquenched orbital moment Lz/2Sz = 0.25 despite the sizeable elongation of the CoO6 octahedra. Using quantitative model calculations based on parameters consistent with our spectra, we have investigated the stability of this high spin state with respect to the competing low spin and intermediate spin states.PACS numbers: 71.70. Ch, 75.47.Lx, 78.70.Dm, 72.80.Ga Cobalt compounds have aroused a great deal of attention in the scientific community because of the complex and large diversity of physical phenomena displayed, including metal-insulating transitions 1-3 , superconductivity 4 , large magneto-resistance 5 and high thermoelectric power 6 . This richness of electronic and magnetic properties is closely related not only to the possibility of stabilizing cobalt in different valence states but also to its ability to present different spin states, the so-called spinstate degree of freedom 7,8 . For example, in an octahedral coordination, Co 3+ ions, which have the d 6 configuration, can exist in three possible spin states: a high spin (HS) state (S = 2, t 4 2g e 2 g ), a low spin (LS) state (S = 0, t 6 2g e 0 g ) and even an intermediate spin (IS) state (S = 1, t 5 2g e 1 g ) 7,9,10 . This spin state degree of freedom is evident in LaCoO 3 where the Co 3+ ions have a non-magnetic LS ground state and undergo a gradual transition with increasing temperature to a magnetic spin state [11][12][13] . The nature of the magnetic spin state (IS or HS) was heavily disputed in literature for over four decades, till an XMCD study clearly demonstrated it to be HS 10 . Calculations of the Co 3+ energy level diagram show that the LS (HS) state can be stabilized by a large (small) value of the crystal field 10Dq. The IS is always higher in energy for CoO 6 octahedra close to regular, like in LaCoO 3 10 .However, the IS state, with one electron in the e g states, is Jahn-Teller active and, hence, can gain energy and become the ground state in the presence of a sufficiently large distortion of the local structure 14 . For this reason the spin state of the Co 3+ ions in layered cobaltates, where the elongated distortion of the CoO 6 octahedra favors and may stabilize the IS state, have been subject of intense debate. In the case of layered La 2−x Sr x CoO 4 , contradicting scenarios for the Co 3+ ions were considered to interpret the complex structural, magnetic and transport properties of the system as a function of Sr doping: LS Co 3+ , IS Co 3+ 15-17 , HS-to-IS transition 18 , and mixing of HS/IS 19,20 . Only recently the spin state of Co 3+ in layered La 2−x Sr x CoO 4 was demonstrated by X-ray absorption spectroscopy (XAS) studies to be LS 21,22 for x = 0.5 and a mixture of LS/HS for x ≥ 1 22-24 . Band formation has been proposed to p...

Strain-induced changes of the electronic properties of B -site ordered double-perovskite Sr2CoIrO6 thin films

“…The experimental isotropic XAS spectra before subtracting Co 2+ contributions can be found in Figure S8 of the SM 27 . Further, the spectral features indicate a high spin (HS) state of the Co 3+ ions 38,39,42 in the SCIO thin films which is independent of substrates underneath, i.e., a strain independent HS state. On the other hand, the orbital state in these SCIO thin films is quite different as indicated by the opposite sign of the polarization-dependent difference of spectra.…”

“…The line shape strongly depends on the multiplet structure given by the Co 3d-3d and 2p-3d Coulomb and exchange interactions, as well as by the local crystal fields and the hybridization with the O 2p ligands. Unique to soft XAS is that the dipole selection rules are very sensitive in determining which of the 2p 5 3d n+1 final states can be reached and with what intensity, starting from a particular 2p 6 3d n initial state (n = 6 for Co 3+ ) 38,39,42 . This makes the technique extremely sensitive to the symmetry of the initial state, i.e., the spin, orbital and valence states of the ions.…”

“…We infer that the change of the anisotropy in the crystal field parameters in going from room temperature to low temperatures due to thermal contraction of the substrates is negligible, and that we therefore can use these XAS derived parameters also for the analysis of the low temperature magnetic properties (see the Supplemental Material for details). Accordingly, we can calculate the magnetic anisotropy energy as a function of the spin direction for each case as shown in Figure 7 (d) (tensile, SCIO/GSO) and (e) (compressive, SCIO/NGO).The magnitude of the exchange field was set to 4 meV in accordance with the the magnetic ordering temperature of 43 K. 42 This magnetic anisotropy energy is expressed as E = K 0 + K 1 sin 2 (θ ) + K 2 sin 4 (θ ) + K 3 sin 4 (θ ) sin 2 (ϕ) cos 2 (ϕ), where θ is the angle between the exchange field and the [001] pc , and ϕ is the azimuthal angle which is set to 45 • . We find for the SCIO/GSO (tensile) K 1 = −1.42 meV, K 2 = 0.14 meV, and K 3 = 0.56 meV, while for the SCIO/NGO (compressive) we obtain K 1 = 0.48 meV, K 2 = 0.08 meV, and K 3 = 0.33 meV.…”

Comparative Study of Potentially Jeff = 0 Ground State Iridium(V) in SrLaNiIrO6, SrLaMgIrO6, and SrLaZnIrO6

“…To analyze the results quantitatively, we need to make a theoretical estimate for the effective magnetic moments for the Ni and Ir ions. To this end we resort to configuration interaction cluster model calculations, an approach which is very successful to explain quantitatively the basic local electronic and magnetic properties of a wide range of insulating transition metal oxides . This model includes the full atomic multiplet theory and takes into account the intra‐atomic transition metal d–d Coulomb interactions, the atomic d spin‐orbit coupling, the O2p to transition metal d hybridization, and the local ionic crystal field parameters .…”

High-pressure synthesis, crystal structure, and properties of iron-based spin-chain compound Ba9Fe3Se15

“…where J = 3.4 μ B is the sum of L and S. The obtained μ eff from the XMCD experiments agrees well with that from magnetic susceptibility measurement, which confirms the unquenched orbital moment in the system and the assumption of the HS form with S=2. The observation of sizeable orbital moment is quite common in cobalt based compounds with a high spin 3d 6 ionic configuration in octahedral coordination [28][29][30]. Also several iron based compounds such as FeBr 2 [31,32] and Ba 9 Fe 3 Te 15 [15] have been reported to possess unquenched orbital moment.…”