Ultrafast Light Switching of Ferromagnetism in EuSe

We demonstrate that light resonant with the band gap forces the antiferromagnetic semiconductor EuSe to enter ferromagnetic alignment in the picosecond timescale. A photon generates an electron-hole pair, whose electron forms a supergiant spin polaron of magnetic moment of nearly 6000 Bohr magnetons. By increasing the light intensity, the whole of the illuminated region can be fully magnetized. The key to the novel large photoinduced magnetization mechanism is the huge enhancement of the magnetic susceptibility when both antiferromagnetic and ferromagnetic interactions are present in the material and are of nearly equal magnitude, as is the case in EuSe.

Figure 1

Scheme of the setup for measuring the time-resolved photoinduced Faraday rotation angle. The linearly polarized probe pulse arrives at the sample when a time $\mathrm{\Delta }t$ has elapsed after the arrival of the pump pulse. We measured $\mathrm{\Delta }{\theta }_F$, the increment in the Faraday rotation of the probe, induced by the pump illumination.

Figure 2

Photoinduced Faraday rotation associated with spin polarons in EuSe. The maximum PFR is indicated by $\mathrm{\Delta }{\theta }_F^{\mathrm{SAT}}$.

Figure 3

PFR excitation spectrum.

Figure 4

PFR as a function of the pump modulation frequency.

Figure 5

PFR associated with spin polarons as a function of the internal magnetic field. The ratio ${\chi }_{\perp }/{\chi }_{||}$, used to compute the internal magnetic field, is shown in the inset, as a function of the temperature. The arrow in the inset indicates the Néel temperature for the sample studied, $T_N=4.75\mathrm{K}$.

Figure 6

Magnetic moment of a photoinduced polaron as a function of the temperature. Full circles, empty circles, and inverted triangles correspond to probe energies of 1.865, 1.699, and 1.632 eV, respectively. The dashed line shows the paramagnetic approximation, with $J_{\text{Xf}}=0.075\mathrm{eV}$ obtained from the linear fit shown in the inset.

Figure 7

PFR as a function of the pump excitation power.

Figure 8

PFR as a function of the delay, $\mathrm{\Delta }t$, between the pump and probe pulses.