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In this project, optical excitation of spin precession is investigated in Fe/CoO exchangecoupled heterostructure with timeresolved magnetooptic Kerr effect (TRMOKE). Photoexcited chargetransfer processes in AFM CoO layer create a strong transient exchangecoupling torque τ_{ex}(t) on FM Fe layer through FM–AFM exchange coupling. The efficiency of spin precession excitation is significantly higher and the recovery is notably faster than the demagnetization procedure. The precession amplitude peaks around room temperature and with external magnetic field competitive to the magnetic anisotropy field, indicating that this efficient excitation mechanism originates from the modulation of the uniaxial magnetic anisotropy K_{u} induced by the FM/AFM exchange coupling. Our results will help promote the development of lowenergy consumption magnetic device concepts for fast spin manipulation at room temperature. The observed ultrafast spin precession excitation is described by a modified Landau–Lifshitz–Gilbert (LLG) equation with an additional torque term: where γ is gyromagnetic ratio, M is the magnetization, Heff is the effective magnetic field, α is the Gilbert damping constant, τ_{ex}(t) = γ(M×△H_{ex}(t)) denotes the instant spin exchangecoupling torque, and △H_{ex}(t) is the change of FM–AFM exchange field He_{ex} by modulation of uniaxial exchange anisotropy K_{u}. The geometry of instant spin exchangecoupling torque τ_{ex}(t) is illustrated in Fig. 1. Figure 1  Illustration of photoexcited spin exchangecoupling torque. When t is less than 0, the magnetization M (purple arrow) in the Fe layer aligns along the effective field direction Heff (black arrow). H denotes the external magnetic field, and H_{ex} is the field established by FM–AFM exchange coupling. At t=0, the 400nm pump pulse (blue arrow) generates photoexcited carriers in the CoO layer, which leads to the reorientation of AFM spins (green arrows). This modifies the exchange coupling, △H_{ex} (brown arrow), causing a change of the effective field direction. Then the exchangecoupling torque τ_{ex}(red arrow) forms, which triggers the precession of M. In Fig. 2c the sudden rise and decay of Kerr signal indicates the demagnetization process. Meanwhile, the magnetization starts to precess around the equilibrium direction in a damped circling way described by LLG equation. The measured Kerr signal can be wellfitted by the following equation: where parameters A, τ, f and φ are the amplitude, magnetic relaxation time, frequency and initial phase of the magnetization precession mainly along the polar direction, respectively. Figure 2  Experimental design and observation of ultrafast exchangecoupling torque. Schematic of TRMOKE measurement geometry, depiction of magnetization precession and longitudinal hysteresis loops (a). Two pump strategies to optically excite the spin precession (b), where the black arrows represent the magnetic moments. Optical chargetransfer transition in CoO is depicted. TRMOKE results from Fe/MgO (black squares) and Fe/CoO (red circles) with pumppulse intensity 3.1 mJ cm^{2}, and Fe/CoO (blue triangles) with pump intensity 0.16 mJ cm^{2} (c). The solid lines represent fits of equation (2). Here the pronounced spin precession is observed with much lower pumppulse energy 0.16 mJ cm^{2} at 400nm wavelength, as shown in Fig. 2c (blue triangles). Moreover, the TRMOKE data reveal the absence of obvious demagnetization and slow recovery of M. Furthermore, the magnetic relaxation time t decreases from 330 ps (red circles) to 100ps (blue triangles) in Fe/CoO with 400nm pump pulses, which is desirable for fast switching. To determine the origin of the optical excitation mechanism, temperature and fielddependent TRMOKE measurements are carried out (Fig. 3). Figure 3  Temperature and fielddependent TRMOKE studies. TRMOKE results from Fe/CoO at different temperatures T (a), where the solid lines are fits of equation (2). Spin precession amplitude A (black squares) and frequency f (red circles) as a function of H (b). The solid lines are simulations. The arrows indicate the proper yaxis for the different plots (b,c). Temperature dependence of f for two different fields (c). The derived K_{u}/M_{S} is also plotted (blue triangles) and linked with spline cubic analysis fitting. Temperature dependence of A for two different fields (d), where solid lines represent simulated results. Error bar of A is estimated by the deviation of fitting with equation (2). For the ultrafast spin exchangecoupling torque, the carrier excitation is instant on photoexcitation, thus the AFM modulation is fast. In addition, the exchange interaction between Fe FM spins and CoO AFM spins are very strong and the modulation of this exchange interaction is fast. Among the magnetic interactions, the spin exchangecoupling interaction has the largest energy and thus the shortest time scale. This process is also repeatable many times without any thermally induced degradation. Larger magnetization precession may be obtained with shorter wavelengths, thereby enhancing (reducing) the absorption in the CoO (Fe) layer, which will also lower the thermal load (larger heat capacity of CoO) allowing the application of higher pumppulse power. References:
Funding: DOE



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