Ultrafast spin exchange-coupling torque via photo-excited charge-transfer processes

Optical control of spin is of central importance in the research of ultrafast spintronic devices utilizing spin dynamics at short time scales. Recently developed optical approaches such as ultrafast demagnetization, spin-transfer and spin-orbit torques open new pathways to manipulate spin through its interaction with photon, orbit, charge or phonon. However, these processes are limited by either the long thermal recovery time or the low-temperature requirement. Here we experimentally demonstrate ultrafast coherent spin precession via optical charge-transfer processes in the exchange-coupled Fe/CoO system at room temperature. The efficiency of spin precession excitation is significantly higher and the recovery time of the exchange-coupling torque is much shorter than for the demagnetization procedure, which is desirable for fast switching. The exchange coupling is a key issue in spin valves and tunnelling junctions, and hence our findings will help promote the development of exchange-coupled device concepts for ultrafast coherent spin manipulation.

(a) in the main text, the hysteresis loops show perfect squareness for the field along the easy axis; however, it is hard to reach saturation magnetization at the largest field, restricted by our electromagnet, along the hard axis perpendicular to the cooling field. These results indicate a well-defined easy axis and homogeneous anisotropy of the sample.
Supplementary Figure 3 shows that the total magnetic moment of Fe layer in Fe/CoO thin film structure remains almost unchanged from 100 to 300 K, which is measured with a superconducting quantum interference device (SQUID). The decrease of M S at higher T is within 4%, indicating that the Curie temperature of the Fe thin film layer is much higher than 300 K.

Supplementary Note 2 | Magnetic anisotropies
We determine the magnetic anisotropies Although the Fe 3d/O 2p hybridization at the interface may result in the perpendicular magnetic anisotropy K  , the Fe-O bond formation at the Fe/CoO interface is not responsible for the enhanced uniaxial magnetic anisotropy K u because we found that K u of the Fe film grown on 1-nm-thick CoO is very similar to that of the Fe film directly grown on MgO. The CoO thickness must be large enough to establish the AFM order and form exchange coupling with the Fe spins. Since the spin precession excitation is correlated with the modulations of the CoO AFM spins and the resultant exchange coupling-induced anisotropy, we may exclude the role of Fe-O bond in the excitation mechanism. Moreover, we did not observe apparent temperature dependence of the spin precession amplitude in the Fe/MgO (001) capped with a 3-nm-thick MgO layer, so we believe that the modulation of perpendicular magnetic anisotropy due to the MgO layer does not play an important role in the spin precession excitation. We also note that the MgO has the optical band gap of ~7.8 eV and its absorption of 400-nm light is extremely small, so the 3-nm-thick capping layer is nearly transparent for 400 nm. We thus may neglect the contribution of 400-nm light absorption by the MgO capping layer to the spin precession excitation.

Supplementary Note 3 | Simulation of precession amplitude
We simulate the dependence of spin precession amplitude A on H and T.  can be expressed as a function of anisotropy fields and H, through minimizing the magnetic free energy in Supplementary Equation 2. If the duration of K u change is comparable to the magnetization precession period, A can be assumed to be proportional to the equilibrium direction change by modulation of K u ( ⊿ K u ) [Supplementary Reference 6-8, Supplementary Equation 3]. The scaling pre-factor s represents the convention from magnetization precession amplitude to detected Kerr signal. First, the H dependence of A is fitted with parameters ⊿K u and s. Then for the temperature dependence of A, ⊿K u is parameterized as K u (T)-K u (T+20K), and s is set as derived from the H dependence simulation.
Since we do not have the K u values above 300K, we extend the values of K u to higher temperature from the fitted dependence of K u on T in Fig. 3(c) of main text. The simulation on temperature dependence of A above 280K is based on those values. The presented A at 300 K is derived from fitting the field dependence of A shown in Fig. 3(b) of main text.

Supplementary Note 4 | Estimation of thermal effect
The conventional heat diffusion from Fe layer to CoO layer can not explain the fast excitation mechanism, because the heat diffusion is too slow and the 400-nm pumpinduced heat of Fe layer ( ) will mainly diffuse to the MgO layer, since the heat conduction is much higher in the Fe and MgO layers than in the CoO layer.
The direct heating of CoO layer by the blue light is also negligible, i.e.
. The temperature raise in CoO layer caused by instant laser pulse heating is estimated through , where E is the light energy absorbed, C the molar heat capacity, the density, t the thickness of layer, r the radius of pumped region and M the molar mass. In the calculation of E, the reflection from Fe surface and Fe/CoO interface and absorption in Fe layer are also considered.

Supplementary Note 5 | Simulation of recovery time
Real-time simulation of magnetization precession is carried out with LLG equations with time interval t=0.2ps in Supplementary Equations 4 and 5.
Supplementary Figure 4 displays the simulation results with other τ r values, where shorter or longer τ r leads to mismatch in the oscillation phase. Generally speaking, the longer τ r postpone the phase of oscillation, and the precession frequency during the τ r is smaller compared with that after K u is recovered.

Supplementary Equation 1 | Field (H) dependence of spin precession frequency (f)
where the θ is the angle between the equilibrium direction of magnetization and Fe [001] direction.
The scaling pre-factor s represents the convention from magnetization precession amplitude to detected Kerr signal. where the first term represents the Gilbert damping effect and the second term denotes the circling of spin precession. As for m y , there is an additional term describing the change of equilibrium direction driven by modulation of K u .

Supplementary Equations 4 and 5 | Real time simulation of spin precession
Supplementary References: