Antiferromagnetic-to-ferromagnetic transition induced by diluted Co in $\mathrm{Sr}{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$: Magnetic circular x-ray dichroism study

The antiferromagnetic-to-ferromagnetic transition in $\mathrm{Sr}{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$ $(x_c\simeq 0.15)$ induced by Co doping has been studied by magnetic circular x-ray dichroism. The orbital and spin magnetic moments of the Fe and Co $3d$ states under the magenetic field of $2\mathrm{T}$ are found to show different $x$ dependences: The spin polarization of Fe $3d$ gradually increases with Co concentration; on the other hand, a large spin polarization of Co $3d$ is induced already in the antiferromagnetic phase, indicating that the Co moment is nearly fully alligned already in the antiferromagnetic phase. This suggests that the alignment of the Fe magnetic moment in $\mathrm{Sr}{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$ is induced by interaction with doped Co magnetic moment. Possible formation of ferromagnetic Co rich region is discussed.

Figure 1

Magnetization of $\mathrm{Sr}{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$ at $30\mathrm{K}$. (a) Magnetization under the magnetic field of $2\mathrm{T}$ plotted against Co concentration $x$. (b) Magnetization plotted against the magnetic field for each Co concentration $x$.

Figure 2

Fe $2p$ XAS spectra ${\mu }^{+}$ and ${\mu }^{-}$ of $\mathrm{Sr}{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$ $(0⩽x⩽0.5)$ (upper panel) and Fe $2p$ MCXD spectra $\Delta \mu ={\mu }^{+}-{\mu }^{-}$ with their energy integrals (lower panel). All the spectra have been normalized to the average Fe $2p_{3∕2}$ XAS $[({\mu }^{+}+{\mu }^{-})∕2]$ peak height. The dotted lines in lower panel show the zero levels of the MCXD spectra.

Figure 3

Co $2p$ XAS spectra ${\mu }^{+}$ and ${\mu }^{-}$ of $\mathrm{Sr}{\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$ $(0.1⩽x⩽0.5)$ (upper panel), Co $2p$ MCXD spectra $\Delta \mu ={\mu }^{+}-{\mu }^{-}$, and their energy integrals (lower panel). All the spectra have been normalized to the average Co $2p_{3∕2}$ XAS $[({\mu }^{+}+{\mu }^{-})∕2]$ peak height. The dotted lines in lower panel show the zero levels of the MCXD spectra.

Figure 4

Orbital $M_{\mathit{orb}}$, spin $M_{\mathit{\text{spin}}}$ and total $M_{\mathit{\text{total}}}$ magnetic-moments of Fe $3d$ (a) and of Co $3d$ (b) as functions of Co concentration $x$ obtained using the MCXD orbital (Ref. 12) and spin sum rules (Ref. 13). Correction factors (Ref. 14) are applied in estimating $M_{\mathit{\text{spin}}}$.

Figure 5

Total magnetic moment obtained from the SQUID measurements and that obtained from the MCXD sum rules. Dotted line shows the total magnetization from the ionic model assuming magnetization of ${\mathrm{Fe}}^{4+}$ with $4{\mu }_B$ and ${\mathrm{Co}}^{4+}$ with $1.8{\mu }_B$.