![](cpb_26_1_016802/cpb_26_1_016802ieqn1.gif)
![](cpb_26_1_016802/cpb_26_1_016802ieqn1.gif)
† Corresponding author. E-mail:
In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co2MnAl films which are epitaxially grown on GaAs substrates are observed and investigated as a function of film thickness, probe wavelength, external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of (3.85 ± 0.1)×103 m/s at 800 nm for the epitaxial Co2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.
Achieving a high degree of spin polarization is critical for the development of ultra-high-density magnetic storage and other spin-dependent devices. Some of Co2-based Heusler alloys exhibit half metallicity with high Curie temperature and spin polarization and have been widely investigated as promising materials in spintronics, such as spin transport-based electronics and spin injection sources.[1–8] For device physics towards future applications in these directions, it is not only crucial to understand the magnetic and spin dynamics, but also vital to understand the properties of phonons that govern the thermal conductivity and electron scattering in the Co2-based Heusler alloys. Moreover, the recent studies have shown that the coupling between magnons and elastic phonons, known as the magnetoelastic coupling, can efficiently trigger the magnetization switching and precession.[9–13] Especially, the acoustically driven magnetization switching on an ultrafast time scale offers great capability for a high-speed control of local magnetization.[10–12, 14] The knowledge of phonon spectrum in the ferromagnetic films allows us to understand the elastic properties of materials and the coupling mechanism between phonons and magnons as well as external stimuli such as light field. Thus it is important to study the properties of coherent acoustic phonons in Co2-based Heusler alloys, which remains to be explored, in order to investigate the effective magnetization modulation through magnetoelastic coupling.
With the fast development of ultrafast lasers, comprehensive information about phonon properties can be obtained experimentally by using picosecond acoustics techniques in which the coherent acoustic phonon can be generated by an optical pump pulse from a femtosecond laser and detected with femtosecond temporal resolution by the probe pulse from the same laser.[15] This technique has been successfully used to study the coherent acoustic phonons in various thin-film structures,[15–18] bulk materials,[15, 19, 20] semiconductor superlattices,[21] and quantum wells.[22, 23] One of the mechanisms of coherent acoustic phonon generation and detection is that the absorption of ultrafast laser pulses would induce the propagation of an ultrafast, impulsive strain excitation through material. A periodic oscillation can be observed due to the self-interference of the reflected probe beam from the crystal surface and the surface defined by the propagating strain pulse.[15, 24] Other generation mechanisms of coherent acoustic phonon include the impulsive stimulated Raman scattering[21, 25] and displacive excitation of coherent phonons,[26] which have been observed in bulk semiconductors and superlattices. In both cases the coherent acoustic oscillations are detected through changes in the reflection or transmission by the modulation of interband transitions coupled to the deformation potential. Large amplitude oscillations in GaN heterostructures have been observed recently and are attributed to the screening of the piezoelectric field by the photoexcited carriers.[22]
In this work, we report the time-resolved reflectivity study of Co2MnAl film epitaxially grown on a GaAs substrate by a two-color femtosecond pump–probe technique. The oscillatory response is observed in the transient reflectivity response and systematically investigated as a function of probe wavelength, temperature, and magnetic field, and film thickness. It is suggested that the laser-induced coherent acoustic phonon generation in Co2MnAl film is responsible for the observed oscillation response. The physical mechanism for the coherent phonon generation in Co2MnAl film can be explained by a propagating strain pulse model.
The two Co2MnAl films with different thicknesses are grown on GaAs (001) substrates with a temperature of 280
The typical time-resolved reflectivity responses of the 60-nm-thickness and 100-nm-thickness Co2MnAl films measured with probe wavelength at 800 nm are shown in Figs.
We then use the following damped harmonic function superimposed onto an exponentially decaying background to fit the measured transient reflection response:[21, 32, 33]
![]() | (1) |
![](cpb_26_1_016802/cpb_26_1_016802ieqn5.gif)
![](cpb_26_1_016802/cpb_26_1_016802ieqn6.gif)
![](cpb_26_1_016802/cpb_26_1_016802ieqn7.gif)
In order to look into the nature of the oscillatory response observed in the transient reflectivity response, we investigate the wavelength dependent response of the time-resolved reflectivity at room temperature by tuning the probe wavelength. The typical time-resolved reflectivity response for 100-nm-thick Co2MnAl film probed at different wavelengths is presented in Fig.
As is well known, in the case of impulsive stimulated Raman scattering, the generated acoustic phonon wave vector q is well defined, i.e.,
![]() | (2) |
![](cpb_26_1_016802/cpb_26_1_016802ieqn12.gif)
![](cpb_26_1_016802/cpb_26_1_016802ieqn13.gif)
![]() | (3) |
Therefore, the observed coherent oscillation frequencies measured at several probe wavelengths are retrieved by fitting the transient reflectance responses through Eq. (
Now, we come to discuss the role of the thickness of the sample related to the strain-pulse penetration depth. The damping of the oscillatory signal arises from two factors in the propagating strain model. First, the absorption loss of the probe which bounces back and forth in the multilayer film must be taken into account. A second contribution comes from the absorption and/or scattering of the acoustic phonons during propagation, with the decay time of acoustic wave corresponding to a time when the strain pulse can propagate into the film within its characteristic length before complete energy transfer takes place from the coherent mode to a distribution of incoherent modes. Fitting the observed oscillations with Eq. (
Another possible origination of the observed periodic oscillations could be the coherent magnons since the detection of coherent magnons via time-resolved reflectivity is also possible and has already been observed in some of the multiferroics.[44–46] Coherent magnon excitation can be viewed as triggering magnetic precession when the sublattice equilibrium position is modified through transient optical heating of the crystalline lattice from energy relaxation of an initial hot electron distribution, leading to a rapid modification of the exchange coupling between the sublattices. Thus the coherent magnon could be generated and show strong dependence on the external magnetic field. To elucidate whether the observed periodic oscillations in our time-resolved reflectivity measurement is associated with the coherent magnon generation in Co2MnAl film, we further perform the magnetic field and temperature dependent measurements. The extracted oscillation frequency as a function of the applied in-plane magnetic field is shown in Fig.
In this work, we perform two-color pump–probe reflectivity measurements on Co2MnAl films epitaxially deposited on GaAs substrates. A periodic oscillation response is observed, and its frequency depends on the film thickness, probe wavelength, external magnetic field, and temperature. It is suggested that the observed periodic oscillation should result from the coherent phonon generation in the Co2MnAl layer and GaAs buffer/substrate and be able to be explained by a propagating strain pulse model. From the measured acoustic phonon frequency, the room-temperature sound velocity of longitudinal acoustic phonon in Co2MnAl film is evaluated to be
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