Local electronic structure and magnetic properties of $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ studied by x-ray absorption and magnetic circular dichroism spectroscopy

We have studied the local electronic structure of $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ using soft-x-ray absorption spectroscopy at the $\mathrm{Co}\text{−}{L}_{3,2}$ and $\mathrm{Mn}\text{−}{L}_{3,2}$ edges. We found a high-spin ${\mathrm{Co}}^{2+}\text{−}{\mathrm{Mn}}^{4+}$ valence state for samples with the optimal Curie temperature. We discovered that samples with lower Curie temperatures contain low-spin nonmagnetic ${\mathrm{Co}}^{3+}$ ions. Using soft-x-ray magnetic circular dichroism, we established that the ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ ions are ferromagnetically aligned. We also revealed that the ${\mathrm{Co}}^{2+}$ ions have a large orbital moment: $m_{\mathrm{orb}}∕m_{\text{spin}}\approx 0.47$. Together with model calculations, this suggests the presence of a large magnetocrystalline anisotropy in the material and predicts a nontrivial temperature dependence for the magnetic susceptibility.

Figure 1

$\mathrm{Co}\text{−}{L}_{2,3}$ XAS spectra of (a) $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ as a ${\mathrm{Co}}^{3+}$ reference, of the $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ samples with (b) $T_C=150\mathrm{K}$ and (c) $T_C=225\mathrm{K}$, and (d) of CoO as a ${\mathrm{Co}}^{2+}$ reference.

Figure 2

(Color online) $\mathrm{Co}\text{−}{L}_{2,3}$ XAS spectra of (a) the $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ samples with $T_C=225\mathrm{K}$ (solid black curve) and $T_C=150\mathrm{K}$ (dashed red curve), their difference (dotted blue curve), and (b) of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ as ${\mathrm{Co}}^{3+}$ reference.

Figure 3

$\mathrm{Mn}\text{−}{L}_{2,3}$ XAS spectra of the $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ sample with (a) $T_C=150\mathrm{K}$ and (b) $T_C=225\mathrm{K}$ together with (c) $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ (${\mathrm{Mn}}^{4+}$, taken from Ref. 43), (d) $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_3$ $\left({\mathrm{Mn}}^{3+}\right)$, and (e) MnO $\left({\mathrm{Mn}}^{2+}\right)$ for comparison.

Figure 4

(Color online) $\mathrm{Mn}\text{−}{L}_{2,3}$ XAS spectra of (a) the two $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ samples with $T_C=150\mathrm{K}$ (dashed red curve), $T_C=225\mathrm{K}$ (black solid curve) and their difference (dotted blue curve), and (b) $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_3$ $\left({\mathrm{Mn}}^{3+}\right)$ for comparison.

Figure 5

(Color online) $\mathrm{Co}\text{−}{L}_{2,3}$ (a) and $\mathrm{Mn}\text{−}{L}_{2,3}$ (b) spectra of $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{O}}_3$ taken with circularly polarized x rays at $135\mathrm{K}$. The photon spin was aligned parallel (${\mu }^{+}$, black solid) and antiparallel (${\mu }^{-}$, red dashed) to the $1\mathrm{T}$ magnetic field, respectively; the difference spectra is shown in dotted blue. Top: measured spectra. Bottom: simulated spectra.

Figure 6

Energy level diagram of the ${\mathrm{Co}}^{2+}$ ion (left panel) in a cubic field depending on the strength of the exchange field $H_{\mathrm{ex}}$, (middle panel) the effect of lowering the symmetry $[\Delta e_g=4\Delta t_{2g}=4(E_{zx-xy,yz}-E_{zx+xy})]$, and (right panel) the low-symmetry energy splitting depending on $H_{\mathrm{ex}}$.

Figure 7

Calculated inverse susceptibility for a single ${\mathrm{Co}}^{2+}$ ion in a cubic crystal field; (top inset) the (apparent) effective moment ${\mu }_{\mathrm{eff}}$ and (bottom inset) the (apparent) Weiss temperature $\Theta$ as defined in the text.