Direct observation of double exchange in ferromagnetic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{CoO}}_3$ by broadband ellipsometry

We present results of our broadband ellipsometry measurements of the optical response of ferromagnetic ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{CoO}}_3$. Our data show that the ferromagnetic transition is accompanied by a transfer of optical spectral weight from an absorption band centered at 1.5 eV to a narrow component of the Drude-like peak. The associated reduction of the intraband kinetic energy is significantly larger than $k_{\mathrm{B}}{T}_{\mathrm{c}}$, confirming that the double exchange plays a major role in the ferromagnetism of doped cobaltites. In conjunction with results of recent theoretical studies, the temperature dependence of the Drude-like peak suggests that the double exchange is mediated by $t_{2g}$ orbitals.

Figure 1

The real part of the optical conductivity ${\sigma }_1\left(\omega \right)$ of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{CoO}}_3$ deposited on LSAT. (a) ${\sigma }_1\left(\omega \right)$ at $T=200\mathrm{K}\approx T_c$ and at 7 K. (b) and (c) ${\sigma }_1\left(\omega \right)$ for all measured temperatures on a magnified scale. The inset of (b) shows the $T$ dependencies of $\rho \left(0\right)=1/{\sigma }_1\left(0\right)$ and of the dc resistivity ${\rho }_{\mathrm{dc}}$. (d) and (e) $\mathrm{\Delta }{\sigma }_1(\omega ,T)\equiv {\sigma }_1(\omega ,T)-{\sigma }_1(\omega ,7\mathrm{K})$.

Figure 2

(a) Energy dependence of the quantity $N{}_{\mathrm{eff}}(0,\omega ,7\mathrm{K})-N{}_{\mathrm{eff}}(0,\omega ,T)$ introduced in the text. (b) Temperature dependence of the spectral weight $N_{\mathrm{eff}}(0,5\mathrm{eV},T)$. (c) $T$ dependencies of the low and high energy spectral weights $N_{\mathrm{eff}}^{\mathrm{LE}}=N_{\mathrm{eff}}(0,0.3\mathrm{eV})$ and $N_{\mathrm{eff}}^{\mathrm{HE}}=N_{\mathrm{eff}}(0.3,5\mathrm{eV})$. Estimates for the background $T$ dependencies discussed in the text are shown as the dashed lines. (d) Absolute values of the differences $\mathrm{\Delta }{N}_{\mathrm{eff}}^{\mathrm{LE}}$ and $\mathrm{\Delta }{N}_{\mathrm{eff}}^{\mathrm{HE}}$ between the spectral weights and the corresponding background $T$ dependencies together with the square of the magnetization measured at $B=20$ mT. All quantities are normalized to their lowest temperature values.

Figure 3

Measured ${\sigma }_1\left(\omega \right)$ (dotted line), model spectrum (solid green line), and contributions of individual terms in Eq. (2) for $T=7\mathrm{K}$ (a) and for $T=220\mathrm{K}$ (b).

Figure 4

Examples of wrong spin transitions, in the ionic limit, between a ${\mathrm{Co}}^{3+}$ site and ${\mathrm{Co}}^{4+}$ site for various Co spin states. Transition energy values $E^{\mathrm{WST}}$ and the average magnetic moments $\mu$ per site computed for 30% hole doping. The lowest row shows the corresponding situation in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$.