Valence and spin-state transition in cobaltates revisited by x-ray magnetic circular dichroism

The compounds $({\mathrm{Pr}}_{1-y}{\mathrm{Sm}}_y{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ belong to a class of Pr-based cobaltates presenting a unique case of simultaneous valence (charge transfer between Pr and Co ions) and spin-state transition (of the ${\mathrm{Co}}^{3+}$ ions), hereafter referred to as VSST. The present study sheds light on the debated issues of the ${\mathrm{Co}}^{4+}$ and ${\mathrm{Co}}^{3+}$ spin states, by combining x-ray absorption spectroscopy (XAS) at Co and Pr edges and x-ray magnetic circular dichroism (XMCD) at Co $L_{2,3}$ edges. XAS experiments at both $L_3$ and $M_{4,5}$ Pr edges attest to the appearance of ${\mathrm{Pr}}^{4+}$ below the VSST at ${\mathrm{T}}^{*}=106\mathrm{K}$, and allow a precise characterization of the evolution of the ${\mathrm{Co}}^{4+}$ content as a function of the temperature. XMCD at the Co $L_{2,3}$ edges at 5 K, and in magnetic field up to 9 T, directly tackles the issue of the ${\mathrm{Co}}^{4+}$ spin state. It is found that the ${\mathrm{Co}}^{4+}$ ions are most likely in a low spin state, and experience ferromagnetic interactions at $\mathrm{T}\ll {\mathrm{T}}^{*}$. On the basis of temperature dependent XMCD at 9 T, the fingerprint of the VSST on the Co moments is isolated, and found to be consistent with bulk magnetization data when accounting for the rare-earth contributions derived from reference samples. These temperature dependent XMCD data are used to characterize the evolutions of the various valence/spin state of the Co species involved in the VSST. It appears that the ${\mathrm{Co}}^{3+}$ moments above ${\mathrm{T}}^{*}$ are not consistent with a pure intermediate spin state, whereas they can be well reproduced by considering a low/high spin mixture. Finally, these XMCD results are compared to those derived from fitting of the XAS spectra recorded in zero field at various temperatures.

Figure 1

(a) Pr spectra at the $M_{4,5}$ edges for (${\mathrm{Pr}}_{1-y}{\mathrm{Sm}}_y{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ materials at two temperatures, for $y=0$ and 0.36. (b) Pr spectra at the $L_3$ edge for $y=0.36$, at room temperature (dashed curve) and at low temperature (solid curve). The lower set of curves illustrates the decomposition of the experimental spectrum at $\text{T}=2.2$ K (solid curve), and its comparison to the reconstructed spectrum (open symbols). (c) Temperature dependence of the Pr valence derived from XAS measurements at $L$ and $M$ edges, for $y=0.36$. (d) Temperature dependence of the ${\mathrm{Co}}^{4+}$ content inferred from the Pr valence, for $y=0.36$.

Figure 2

(a) XMCD curves at the Co $L_{2,3}$ edges for (${\mathrm{Pr}}_{0.64}{\mathrm{Sm}}_{0.36}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ at $T\approx 5$ K in different magnetic fields. The inset illustrates the background substraction for the sum-rules analysis on a corresponding XAS curve (average between two helicities for a given magnetic field). (b) Magnetic moments derived from XMCD experiments at 5 K by using the sum-rules analysis on two series of independent data (open squares and filled circles). The horizontal lines represent the expected saturation value for ${\mathrm{Co}}^{4+}$ LS ($S=1/2, g=2$) and for ${\mathrm{Co}}^{4+}$ IS ($S=3/2, g=2$). Magnetization curves calculated from a conventional Brillouin function are shown for ${\mathrm{Co}}^{4+}$ LS (dashed line) and ${\mathrm{Co}}^{4+}$ IS (dotted line). The inset is an enlargement showing various modelizations for the field dependence of the magnetization (see text): constant molecular field (${\mathrm{B}}_{\mathrm{mol}}=2\mathrm{T}$, green), Weiss molecular field [$\lambda =15\mathrm{T}/({\mu }_B/\mathrm{f}.\mathrm{u}.$), blue], or clusterization ($p=3$, red).

Figure 3

(a) Co $L_{2,3}$ XMCD curves at different temperatures measured in (${\mathrm{Pr}}_{0.64}{\mathrm{Sm}}_{0.36}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ at $B=9$ T (an offset of 0.1 is used for clarity between the different temperatures). (b) Temperature dependence of the Co magnetic moment derived from XMCD through sum-rules analysis at $B=9$ T (black circles), bulk magnetization of (${\mathrm{Pr}}_{0.64}{\mathrm{Sm}}_{0.36}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ (red line), and combination of the XMCD data plus rare-earth contributions (gray circles). The blue (magenta) line is the calculated contribution of ${\mathrm{Pr}}^{3+}$ (${\mathrm{Sm}}^{3+}$) derived from the magnetization of ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Ti}}_{0.3}{\mathrm{Ga}}_{0.7}{\mathrm{O}}_3$ (${\mathrm{SmAlO}}_3$).

Figure 4

Temperature dependence of the ${\mathrm{Co}}^{3+}$ HS (red squares) and ${\mathrm{Co}}^{3+}$ LS (blue circles) fractions derived from XMCD. The open symbols with shaded stripes correspond to the same fractions (red for ${\mathrm{Co}}^{3+}$ HS and blue for ${\mathrm{Co}}^{3+}$ LS) derived from XAS, including an estimate of the uncertainty ($+/-5\%$) associated to the fitting of the spectra.

Figure 5

(a) Comparison between experimental Co $L_{2,3}$ XMCD spectra at $\text{T}=5$ K and $\text{B}=9$ T, with simulated XMCD spectra by CTM for ${\mathrm{Co}}^{4+}$ LS. (b) Upper set of curves: experimental XAS spectra at selected temperatures ($\text{B}=0$). The stars mark out the two features discussed in text. Intermediate set of curves: comparison between simulated (dotted line) and experimental (solid line) spectra at 300 K. Lower set of curves: calculated XAS reference spectra for ${\mathrm{Co}}^{4+}$ LS, ${\mathrm{Co}}^{3+}$ LS, and HS.