High-frequency magnetoacoustics and magnetic nanomechanics
In cooperation with Dortmund Technical University and the University of Nottingham.
In this research, we focus on dynamical processes driven by interaction between the spin system and the lattice in magnetically ordered nanostructures. Our goal is effective control of the magnetic and vibrational excitations on the nanoscale and at gigahertz frequencies for next generation high-speed nanoscale applications: transducers, microwave and hypersound sources, sensors, etc.
Research directions
1. In the frame of magnetic nanomechanics, we focus on resonance interaction of magnetic excitations and mechanical Eigenmodes of a ferromagnetic nanostructure. Such an interaction may result in intriguing collective magneto-elastic effects (strong magneto-elastic coupling at GHz frequencies, parametric amplification of mechanical modes, anomalouslylong lifetime of magnetic excitations, etc.). The main advantage of magnetic nanomechanical systems is tunability of their resonances by external magnetic field. The structure under study are based on materials with strong magneto-elastic coupling: Galfenol, (Ga,Mn)As etc.
2. We aim to disclose feasibility fornanoscale magnetostrictive films to serve as an effective source of picosecond acoustic pulses of various polarization for further picosecond acoustic research.
Backgound
1. Coherent magnetization precession induced by the picosecond strain pulses.
The pump laser pulse of femtosecond duration/high fluence (up to 50 mJ/cm2) hits a metal film, which plays a role of opto-acoustic transducer. The thermal extension of a film results in injection of a bipolar strain pulse into the substrate (i.e. GaAs). This coherent acoustic wavepacket aduration of ~10 ps, spatial size of ~100 nm and contains frequencies up to 100 GHz. The peak amplitude of the strain pulse may achieve 10-2 and strong nonlinear elastic effects may accompany its propagation.
Injection of the strain pulse into a ferromagnetic layer results in ultrafast modulation of the magneto-crystalline anisotropy, which launches the coherent magnetization precession at ferromagnetic resonance frequency until relaxation processes return magnetization into its equilibrium orientation. We monitor the magnetization kinetics optically by means of time-resolved magneto-optical Kerr rotation. The variable delay between the pump and probe pulses provides femtosecond time resolution. Here we show the typical kinetic signals excited by the longitudinal strain pulses in films of (Ga,Mn)As and Galfenol [1,2]. The grey areasin both panels show the time intervals of strain pulse propagation in a ferromagnetic film.
Importantly, the interaction of picosecond acoustic pulses with magnetic subsystem and the induced magnetization kinetics can be modeled with no fitting parameters.[3]
2. Fineacoustic control of magnetic excitation in a ferromagnetic nanostructure.
Optically detected magnetization kinetics excited by picoseconds acoustic pulse in a thin film is in fact a superposition of standing spin waves (SWs). This is demonstrated in the right panel of the figure where the spatial profiles of standing SW (solid lines) and several spectral components of strain pulse are shown.We have demonstrated [4] that a certain SWmode canbe selectively excited by adjusting its spatial matching with the resonance component of the strain pulse [4]. This can be done by tuning the applied magnetic field. Here the left panel of the figure shows the FFT spectra of magnetization kinetics, where the presence of two or more lines indicates excitation of several standing spin wave modes. At 200-250 mT,however, a single line in the FFT appears indicating the regime where selective excitation of the ground (n=0) SW mode is achieved.
3. High-amplitude modulation of magnetization by the shear strain pulses.
The flexibility of picoseconds acoustic excitation of magnetization kinetics can be tremendously enhanced by utilizing shear strain. Generation of such pulses is possible in low-symmetry substrates Here the left-hand side of the figure shows the magnetization kinetics induced by the longitudinal and shear strain pulses in Galfenol films [2].
Furthermore, the modulation amplitudes under the shear strain pulse may exceed 10% of the saturation magnetization, while the excitation density dependence remains linear. The right hand side of the figure shown the magnetization kinetics excited by the shear strain pulse in a (Ga,Mn)As film [5] under various optical pump fluencies. The inset demonstrates the linear dependence of the modulation amplitude in the wide range of excitation densities.
References:
- A.V. Scherbakov, A.S. Salasyuk, A.V. Akimov, X. Liu, M. Bombeck, C. Bruggemann, D.R. Yakovlev, V.F. Sapega, J.K. Furdyna, and M. Bayer, "Coherent magnetization precession in ferromagnetic (Ga,Mn)As induced by picosecond acoustic pulses", Phys. Rev. Lett. 105, 117204 (2010). http://dx.doi.org/10.1103/PhysRevLett.105.117204
- J. V. Jäger, A. V. Scherbakov, T.L. Linnik, D.R. Yakovlev, M. Wang, P. Wadley, V. Holy, S.A. Cavill, A.V. Akimov, A. W. Rushforth, and M. Bayer "Picosecond inverse magnetostriction in galfenol thin films" Appl. Phys. Lett. 103, 032409 (2013). http://dx.doi.org/10.1063/1.4816014
- T. Linnik, A. V. Scherbakov, D.R. Yakovlev, X. Liu, J. Furdyna, M. Bayer "Theory of magnetization precession induced by a picosecond strain pulse in ferromagnetic semiconductor (Ga,Mn)As" Phys. Rev. B 84, 214432 (2011).
- M. Bombeck, A. S. Salasyuk, B. A. Glavin, A.V. Scherbakov, C. Brueggemann, D.R. Yakovlev, V.F. Sapega, X. Liu, J. K. Furdyna, A. V. Akimov, and M. Bayer "Excitation of spin waves in ferromagnetic (Ga,Mn)As layers by picosecond strain pulses" Phys. Rev. B 85, 195324 (2012). http://dx.doi.org/10.1103/PhysRevB.85.195324
- M. Bombeck, J.V. Jager, A.V. Scherbakov, T. Linnik, D.R. Yakovlev, X. Liu, J. K. Furdyna, A.V . Akimov, and M. Bayer "Magnetization precession induced by quasitransverse picosecond strain pulses in (311) ferromagnetic (Ga,Mn)As" Phys. Rev. B 87, 060302(R) (2013). http://dx.doi.org/10.1103/PhysRevB.87.060302