Abstract
1. Introduction
Magnetic dynamics of ferromagnet has been an active research field in the past few decades, aiming for the innovative design of low-power, high-speed, nonvolatile, and scalable spintronic devices[
It is known that the magnetic damping governs the magnetization switching time of the magnetic recording device and determines the critical current density in the spin-torque-transfer devices[
In this work, we investigated the ultrafast magnetization dynamics of Co2FeAl thin film under an out-of-plane external magnetic field by using the time-resolved magneto-optical Kerr effect (TR-MOKE) technique. An abnormal huge increase in the intrinsic damping constant of a thin Co2FeAl film was observed at the specific magnetic precession frequencies (or magnetic fields). And the respective precession frequencies were found to be resonant with the coherent acoustic phonons that were simultaneously generated in Co2FeAl/GaAs heterostructure via magnetoelastic effect, with the help of further coherent acoustic phonon detection by the time-resolved pump-probe reflection measurements. The experimental findings demonstrate a strong coherent energy transfer from spins to acoustic phonons via magnetoelastic interaction under a resonant coupling condition, which is of great significance for future information technologies.
2. Experimental details
The Co2FeAl thin films with different thickness were grown on the semi-insulating GaAs (001) substrate using molecular beam epitaxy (MBE) technique. A 150 nm-thick GaAs buffer layer was first grown before deposition of Co2FeAl films, and a 2 nm-thick Al capping layer was deposited to prevent Co2FeAl from oxidation. To study the coherent phonon excitation, a gold film with thickness less than 8 nm was evaporated on the chosen samples’ surface by electron beam evaporation technique, since the gold film can serve as good heat sink to promote coherent acoustic phonon generation under the elastic distortion stress waves with intense optical excitation[
Figure 1.TR-MOKE response for 3 nm-thick Co2FeAl film with an external field of 1.15 T and its best fitting (red solid line). The inset shows the schematic diagram of application of external magnetic field.
Figure 2.For 3 nm-thick Co2FeAl film, (a) TR-MOKE responses measured at different magnetic fields and their fittings (red solid lines), (b) the extracted oscillatory frequency as a function of external magnetic fields (the black squares represent the magnetization precession frequency and the red solid line is its best fitting; the colored dots represent the phonon modes whose frequencies do not depend on the magnetic fields). The inset in panel (b) shows the FFT spectrum of the TR-MOKE data measured at
3. Results and discussions
A thin Co2FeAl film with a thickness of 3 nm was specifically employed for the magnetic dynamics study with TR-MOKE measurements. A typical TR-MOKE response measured at external field of 1.15 T for 3 nm-thick Co2FeAl film is displayed in Fig. 1. It is noted that the dynamic response shows higher-frequency oscillations accompanied by a slowly oscillating envelope, indicating that there is more than one mode in the dynamic response, each with different frequency. With the help of fast Fourier transform (FFT) of the TR-MOKE data, the observed Kerr signal can be further fitted with
where
The transient Kerr signals for 3 nm-thick Co2FeAl film probed at different external magnetic fields are shown in Fig. 2(a). The extracted oscillatory frequency by Eq. (1) as a function of the external magnetic field is summarized in Fig. 2(b), a FFT spectrum of the TR-MOKE data measured at 1.6 T is also displayed in the inset as an example. One can immediately notice that, there is only one mode whose frequency increases almost linearly with the increased magnetic fields, while the frequencies of the other three modes remain constant, being 42.6, 32.3, and 19.6 GHz, respectively. The linearly-varying oscillatory frequency as a function of external field implies its magnetic excitation nature which can be described by the uniform magnetization precession with Landau–Lifshitz–Gilbert (LLG) phenomenological equation[
Figure 3.The typical time-resolved reflection responses and their best fits (red solid line) for Co2FeAl films with different thickness: (a) 3 nm-Co2FeAl measured at probe wavelength of 800 nm, (b) 45 nm-Co2FeAl measured at different probe wavelengths, (c) the extracted coherent acoustic phonon frequencies as a function of probe wavelength. The inset in panel (a) shows the schematic diagram of the experiment.
where
While for the observed other three modes, it is noted that their frequencies did not vary with external magnetic fields, indicating that they are not associated with magnon excitation. But one can readily notice that the frequency of 42.6 GHz is consistent with that the well-known longitudinal acoustic (LA) phonon of GaAs[
The transient reflection first investigated for 3 nm-Co2FeAl/GaAs heterostructure is shown in Fig. 3(a), it consists of two processes: one is the ultrafast electron dynamics reflecting the electron-phonon interaction at the initial time delay and the subsequent thermal relaxation with an exponential decay process; the other is the decaying periodic oscillation signal which is known to result from the interference between the probe pulse reflected from the surface and the probe pulse scattered at the propagating phonon pulses in the sample[
where the parameters
As our previous work could detect the other two low-frequency modes in the thicker Co2MnAl films[
By solving LLG equation, the Gilbert damping factor α under the out-of-plane magnetic field can be determined from the following formula[
where H1 and H2 are defined the same as described in Eq. (2), and the magnetization relaxation time τM can be obtained by fitting TR-MOKE response using Eq. (1). The extracted effective damping factor as a function of external magnetic fields by numerical fitting with Eq. (4) for the 3 nm-thick Co2FeAl film is presented in Fig. 4(a). The remarkable feature in Fig. 4(a) is the abnormal huge increase of magnetic damping factor at the three specific fields (or precession frequencies) in the high-field regime where the damping factor is close to the intrinsic one, which should be independent of the external fields or precession frequencies as previously studied[
Figure 4.For 3 nm-thick Co2FeAl film: (a) the effective damping factors as a function of external magnetic fields, (b) the extracted magnetization relaxation time
Under an external out-of-plane magnetic field as in our experiment, it has been proved that the magnetization precession can be triggered by the longitudinal phonons in (Ga,Mn)As[
Recently, by utilizing ferromagnetic resonance (FMR) technique, An et al. have experimentally observed magnetic dynamics coupling between two YIG films over a macroscopic distance with a non-magnetic GGG spacer sandwiched in between[
4. Conclusion
In conclusion, the magnetic damping of a thin Co2FeAl film is found to exhibit an abnormal huge increase at the precession frequencies that are resonant with that of the acoustic phonon excitations in the Co2FeAl/GaAs heterostructure. This finding is suggested to result from the enhanced coherent energy transfer from spin system to the propagating longitudinal acoustic phonons under a resonant coupling condition via magnetoelastic effect. This acoustic phonon-driven huge energy dissipation of spins leads to a greatly enhanced magnetic damping in Co2FeAl. Our work provides experimental evidence for potential long-range spin manipulation via propagating acoustic phonons in ferromagnetic heterostructures, which is of great significance for developing novel spintronic devices.
Acknowledgements
This work was supported by the National Key R&D Program of China (No. 2017YFB0405700), and National Natural Science Foundation of China (No. 12074370).
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