Photoinduced giant magnetic polarons in EuTe

Photoinduced magnetic polarons in EuTe, with a magnetic moment of several hundred Bohr magnetons, were investigated as a function of pump intensity and temperature by pump-probe Faraday rotation. The quantum efficiency for optical generation of magnetic polarons is found to be 0.09. The pump-intensity dependence of the photoinduced Faraday rotation shows a sublinear increase, from which we deduce that the population of photoexcited polarons is limited by a maximum value of $4.5\times {10}^{15}{\mathrm{cm}}^{-3}$. This is four orders of magnitude less than the concentration of polarons that would completely fill the crystal, which suggests that the photoexcited polarons are anchored by defects. In addition to the generation of polarons, at high pump densities the modulated pump light also causes a small alternating heating of the illuminated region. The temperature dependence of the polaron magnetic moment is well described by the Curie–Weiss law. Above 100 K, polarons are thermally quenched with an activation energy of 11 meV.

Figure 1

Faraday rotation by the bulk EuTe sample as a function of the applied magnetic field at $T=5$ K and $T=50$ K. The inset shows the ratio of the internal magnetic field to the applied magnetic field.

Figure 2

Photoinduced Faraday rotation (PFR) signal as a function of the internal magnetic field at $T=5$ K.

Figure 3

(a) Photoinduced Faraday rotation at 5 K and (b) at 50 K. (c) Faraday rotation by the bulk EuTe sample as a function of temperature for various magnetic fields. (Notice that for our sample the observed Néel temperature is slightly larger than the accepted value of 9.6 K for EuTe, in agreement with direct measurements reported in Ref. [13]). The slope of the dependance of $\partial {\theta }_F/\partial T$ on $B$ is shown in the inset. (d) The deduced temperature modulation of the illuminated region is shown as a function of the pump intensity for $T=5$ K and $T=50$ K.

Figure 4

Dots show the photoinduced Faraday rotation angle at saturation as a function of the pump intensity. The full line shows a fit of the dots with Eq. (14), which yields the maximum polaron concentration to be $n_{\text{D}}=4.5\times {10}^{15}{\mathrm{cm}}^{-3}$.

Figure 5

(a) Temperature dependence of the magnetic moment of a polaron. (b) Temperature dependence of the photoinduced Faraday rotation angle at saturation. Above $\mathrm{T}\sim 50$ K, $\mathrm{\Delta }{\theta }_F^{\text{SAT}}$ decreases exponentially with a characteristic activation energy of $E_A=11$ meV. The inset shows schematically the magnetic polaron energy level, the ground state (when the photoexcited electron is absent), and the thermally activated state, which drains the polaron population when the temperature of the sample is increased.