Role of spin fluctuations in the conductivity of ${\mathrm{CrO}}_2$

We present a time-resolved terahertz spectroscopic study of the half-metallic ferromagnet ${\mathrm{CrO}}_2$. The ultrafast conductivity dynamics excited by an optical pump displays very short (several picoseconds) and a very long (several hundred picoseconds) characteristic time scales. We attribute the former to the electron-phonon relaxation and the latter to the spin-lattice relaxation. We use this distinction to quantify the relative contribution of the scattering by spin fluctuations to the resistivity of ${\mathrm{CrO}}_2$: We find that they contribute less than one half of all scattering events below room temperature. This contribution rises to $\sim 70\%$ as the temperature approaches $T_C=390$ K. The small effect of spin fluctuations on the resistivity is unexpected in light of the proposed double-exchange nature of the electronic and magnetic properties of ${\mathrm{CrO}}_2$.

Figure 1

(a) Red line: THz pulse transmitted by the ${\mathrm{CrO}}_2$ film in equilibrium state (no pump excitation) at 9 K. Blue line: Transmitted THz pulse 200 ps after the optical pump at 9 K. (b), (c) Real and imaginary parts of the THz conductivity at 9 and 70 K in equilibrium state. Symbols: Measured conductivity. Solid lines: Drude model fit. (d) THz conductivity at 150 and 330 K. Only the real part is shown for the 330 K data. Solid lines show the Drude model fit for the 150 K data.

Figure 2

The temperature dependence of the dc resistivity and the scattering rate $\gamma$ determined by THz TDS. Drude fit error bars are smaller than the size of the symbols.

Figure 3

(a) TRTS spectra and the time-resolved measurement of the ultrafast change in the peak transmitted THz electric field at various temperatures. (b) TRTS spectra similar to (a), but zoomed in on the first 30–60 ps of the photoinduced response and recorded with a shorter time step. The spectra were shifted horizontally for clarity.

Figure 4

(a) TRTS and TR-MOKE spectra at room temperature. Solid lines are fits to the two-temperature model. (b) TRTS spectra at 150 and 380 K. The solid lines are fits to the two-temperature model.

Figure 5

(a) The computed electron-phonon and spin specific heat. (b) Temperature dependence of the parameter $\alpha$, which quantifies the relative contribution of the spin temperature change to the photoinduced resistivity change. The vertical line indicates the Curie temperature $T_C=390$ K.

Figure 6

Time evolution of the difference in the instantaneous electron-lattice and spin temperature computed in the two-temperature model. Colors correspond to different equilibrium temperatures.