Transition metal oxides reveal a variety of unusual phenomena, among which the magneto-electronic properties are of great interest because of the potential applications of these materials in spin electronics devices. In particular, the solid solution of LaMn1-xCoxO3 has attracted a considerable interest due to the occurrence of a ferromagnetic ground state at intermediate x, which is in contrast to the behaviour of the end members of the series which are the antiferromagnetic LaMnO3 and the diamagnetic LaCoO3. The effect was originally attributed to the superexchange interaction between MnIII ions, without any magnetic contribution of a low-spin CoIII (t2g6eg0, S=0). More recently, the strong tendency towards formation of MnIV+CoII clusters coupled via positive superexchange interaction was suggested and later observed in LaMn0.5Co0.5O3[1]. For other intermediate compositions both scenarios of charge segregation were discussed: the solution of CoIII/MnIII and/or the stable pairs of MnIV+CoII coexisting with a surplus of MnIII and CoIII, generating the magnetic moment from a complex set of oxygen mediated interactions. Among them, the ferromagnetic MnIII-O-MnIV, MnIII-O-MnIII, CoII-O-MnIV and antiferromagnetic MnIV-O-MnIV, CoII-O-CoII, MnIII-O-CoII are the most likely.

To determine which of the proposed scenarios dominates the properties of the series, a study of the effective charge and spin state of manganese and cobalt has been carried out at room temperature by means of Kß emission (XES) and high resolution K-edge absorption (XAS) spectroscopy at the ID26 beamline. K-edge XAS probes the density of p-like unoccupied electronic states and exhibits a strong dependence on the effective charge and symmetry of the probed site in transition metal oxides. Kß emission spectra (3p 1s decay) probe the localised spin moment of transition metals revealed in a multiplet structure due to exchange interaction between the 3p hole and the 3d orbitals.

Figure 17 shows examples of the measured spectra for the extreme Mn and Co concentrations. Absorption spectra (insets) reveal a gradual shift of the edge position to higher energy with increasing Co content at both edges, indicating the electron transfer from the Mn to the Co site. The Mn emission spectra reveal the distinct exchange feature (Kß’) over the entire series indicating a high spin state for Mn irrespective of the average Mn valence state. The Co Kß spectra also show a gradual shift of the Kß1,3 line to lower energy with increasing Co content while the Kß’ feature diminishes for the highest Co doping indicating a lowering of spin configuration. This unequivocally proves that the Co spin state changes substantially upon doping.

Fig. 17: Room temperature Mn (upper) and Co (lower) high resolution Kß XES and K-edge XAS (insets) spectra of x = 0.02 (blue, solid symbols) and 0.98 (red, open symbols) compounds. Increasing Co content, x, is indicated by arrows.


Assuming a linear dependence between edge position and effective charge, the average spin states were obtained from XAS spectra. The Mn spin was derived within the high-spin only model, while in the case of Co, a gradual variation between the end member configurations was assumed, i.e. the change from high spin S=3/2 CoII to mixed spin S=1/2 CoIII [2]. These indirect predictions are compared to the values derived from the Kb XES spectra by means of two techniques: integrals of the absolute values of the difference spectra (IAD) method [2,3] and shift of the first moment of the Mn Kß1,3 line (Figure 18). A perfect correlation of the Mn spin predictions confirms consistency of the methods employed. In the case of Co these two dependences do not agree revealing a considerable difference in the 0.4 < x < 0.9 region. The effect is attributed to the average CoIII spin state lower than the assumed S=1/2 due to a larger admixture of the low spin configuration. A variation of the CoIII spin state can plausibly explain anomalies observed in bulk magnetic measurements of the system in the Co rich region. The experimental method presented can be applied to other mixed transition metal systems allowing exploration of the spin and magnetic properties irrespective of the magnetic ordering.

Fig. 18: The evolution of the average spin of Mn and Co ions as determined from XAS (stars and lines) and XES (circles for IAD and squares for first moment).


[1] J.B. Goodenough et al., Phys. Rev. 124, 373 (1961); G.H. Jonker, J. Appl. Phys. 37, 1424 (1966); R.I. Dass and J.B. Goodenough, Phys. Rev. B 67, 014401 (2003).
[2] G. Vankó et al., Phys. Rev. B 73, 024424 (2006).
[3] J.P. Rueff et al., Phys. Rev. B 63, 132409 (2001).

Principal publication and authors

M. Sikora (a,b), K. KníÏek (c), Cz. Kapusta (b), P. Glatzel (a), J. Appl. Phys (2008) - in press.
(a) ESRF
(b) Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow (Poland)
(c) Institute of Physics, Czech Academy of Science, Prague (Czech Republic)