Nonlinear magnetization dynamics are of great interest, being, for instance, leveraged for neuromorphic computing in spin-transfer torque nano-oscillators. Here, we demonstrate how to implement magnetoacoustics to reach this regime, using monochromatic (f saw = 450 MHz) surface acoustic waves traveling on a thin layer of (Ga,Mn)As. By careful tuning of the precession frequency to both f saw and 2 f saw using the magnetic field and temperature, we evidence clear signatures of a nonlinear magnetoacoustic response of the magnetic dynamics using the time-and space-resolved Kerr effect: (i) frequency and wave-vector doubling in time and space, respectively, (ii) quadratic (sublinear) evolution of the precession amplitude at 2 f saw (f saw) with acoustic amplitude, and (iii) resonance field shift. While (i) can be well reproduced by a parametric resonance model where nonlinearities arise solely from the SAW, we show that features (ii) and (iii) also involve intrinsic magnetic nonlinearities. Understanding the conditions leading to these nonlinearities will mean better control of the acoustic-wave-driven magnetization dynamics, in order to implement optimally the wave properties enabled by this approach.
We present an experimental and k⋅p theoretical study on the origin of the strong in-plane uniaxial magnetic anisotropy in (Ga,Mn)As layers, unexpected from the cubic crystalline structure. The symmetry lowering can be accounted for by structural or effective shear strains. We find theoretically out-of-plane and in-plane magnetic anisotropy constants being linear with the shear strain. Searching for a real shear strain arising from lattice relaxation, we perform two types of measurements: anomalous x-ray diffraction and strain-induced optical birefringence, at room temperature. Working on a strongly anisotropic (Ga,Mn)As layer, the estimated ϵxy=10−4 was not found although it lied an order of magnitude above the detection threshold. This ensemble of results indicates as unlikely a relaxation-driven uniaxial anisotropy. As previously suggested theoretically, the magnetic symmetry-lowering could instead originate from the anisotropic incorporation of Mn atoms during growth. This would yield a perfectly in-plane matched lattice, with an anisotropy that could nevertheless be modeled as an effective shear strain and modified by an external shear stress, in agreement with the existing experimental literature.
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