Spin wave generation by surface acoustic waves in ferromagnetic thin films – SPINSAW

More specifically, the goal of this experimental project is to study on a fundamental level the spin waves generated by strain in carefully chosen magnetostrictive materials. For this we will use the inverse magnetostrictive effect whereby magnetization is modified by application of strain. The strain will be applied in the form of surface acoustic waves (SAWs), generated either electrically using interdigitated combs, or optically using a laser to excite coherent phonons. While the former technique produces quasi-monochromatic strain plane waves and is regularly used on ferromagnets, the latter generates a broader frequency range and isotropic waves; it is very much a novel approach for magnetostrictive spin waves excitation.
Magnetostrictive effects have been studied in a large range of materials: transition metals or rare-earth ferromagnets, garnets or DMS (dilute magnetic semiconductors), and are used in a number of everyday devices. We have chosen to focus on two different families of epitaxied thin films: (i) the ferromagnetic semiconductor Ga1-xMnxAs1-yPy, working at low temperatures but with well understood and fully adjustable magnetic properties, and (ii) Fe1-xGax (Galfenol), a highly magnetostrictive room-temperature ferromagnet. INSP (Institut des Nanosciences de Paris) has a long standing expertise on GaMnAs(P), and has recently succeeded in growing Galfenol in thin film form.

We first optimized the growth of our two materials, GaMnAsP and FeGa, and fully characterized their magnetic anisotropy and magnetostriction. We also optimized the electrical generation of acoustic waves on these two materials. Finally, we developed two novel techniques to determine the absolute amplitude of the surface acoustic waves. This was of paramount importance to optimize the generation of the waves, and be in the best conditions to allow magnetization switching.

On in-plane and out-of-plane GaMnAs(P), we evidenced acoustic wave induced ferromagnetc resonance, and surface acoustic wave induced switching in two geometries/mechanisms: (i) with the field along the easy axis, the waves facilitate domain nucleation/propagation, (ii) with the field perpendicular to the easy axis, deterministic precessional switching can ensue; We evidenced the effect of temperature, acoustic power, frequency and wave-vector direction on the efficiency of the switching. We also evidenced the 'printing' of magnetic patterns usnig stationnary acoustic waves.

On FeGa, we evidenced a non-resonant and resonant coupling of the magnetization to the acoustic wave, and developed several theoretical approaches to describe it, We also designed devices capable of exciting spin waves inductively on this material, in order to then couple them to acoustic waves. Finally, we developed an experiment capable of both exciting acoustic waves optically, and detecting the resulting induced magnetization dynamics

The main result of this project is the controllable coupling of surface acoustic waves to magnetization, harnessed to yield an irreversible switching of the magnetization. This is an exciting perspective since the wave properties of SAWs open up the possibility to rely on focusing or interferences to switch magnetization selectively. In this respect, these results could initiate the building of the first random access SAW-based magnetic storage prototype based on a series of combs addressing individual bits forming an array of ferromagnetic microstructures or SAW-based sensors, where the change of velocity in the presence of an induction could be used to detect local magnetic fields, or phonon detectors using inelastic scattering of SWs on acoustic waves.

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The search for the next generation of spintronics devices will require the investigation of magnetization reversal by alternative techniques, on model and versatile materials. In this context, we propose to study how spin waves can be generated through surface acoustic waves with the idea of using them as information vectors, and also of rivalling some of the dynamic reversal effects currently evidenced in spintronics devices.
More specifically, the goal of this experimental project is to study on a fundamental level the spin waves generated by strain in carefully chosen magnetostrictive materials. For this we will use the inverse magnetostrictive effect whereby magnetization is modified by application of strain. The strain will be applied in the form of surface acoustic waves (SAWs), generated either electrically using interdigitated combs, or optically using a laser to excite coherent phonons. While the former technique produces quasi-monochromatic strain plane waves and is regularly used on ferromagnets, the latter generates a broader frequency range and isotropic waves; it is very much a novel approach for magnetostrictive spin waves excitation.
Magnetostrictive effects have been studied in a large range of materials: transition metals or rare-earth ferromagnets, garnets or DMS (dilute magnetic semiconductors), and are used in a number of everyday devices. We have chosen to focus on two different families of epitaxied thin films: (i) the ferromagnetic semiconductor Ga1-xMnxAs1-yPy, working at low temperatures but with well understood and fully adjustable magnetic properties, and (ii) Fe1-xGax (Galfenol), a highly magnetostrictive room-temperature ferromagnet. INSP (Institut des Nanosciences de Paris) has a long standing expertise on GaMnAs(P), and has recently succeeded in growing Galfenol in thin film form.
When implemented dynamically, magnetostrictive effects are most efficient when the strain frequency matches the natural precession frequency of the ferromagnet. For this reason, and unlike most groups working in this field, we have chosen to work at resonance: first on GaMnAsP, whose low precession rates (<GHz) are easily accessible experimentally, and then on FeGa, with higher frequencies but also larger magnetostriction. The first part of the project will therefore consist in the optimization of the growth/fabrication procedures and of the SAW generation set-up for these frequencies ranges. The core of the scientific program will then reside in two parallel projects. In the first one, the aim will be to generate spin waves efficiently by exploiting the strong coupling between magnon and phonon eigenstates at resonance, i.e. to lie at the crossing of their dispersion curves. Spin waves will be characterized optically by coupling time-resolved Kerr detection to SAW emission, an ambitious experimental approach that will provide more complete information than cavity resonance or electrical detection. If successful, this study could lead to the first experimental evidence of a strongly coupled acoustic phonon-magnon mode, or “polariton”. In the second project, we will achieve the first irreversible magnetization switching in perpendicularly magnetized thin films using SAWs. This will require the coupling of Kerr microscopy to SAW emission. If successful, it could ultimately pave the way towards the first random access SAW-based magnetic storage device. <br />This original project is ambitious because it rests equally on the excellent control over sample growth, as on the fabrication of efficient devices, and finally the implementation of state-of-the art electronic and optical set-ups. While several competing research groups are addressing similar subjects, in particular in Germany, England and the United States, we believe a unique convergence of experimental and theoretical skills exists at INSP to drive this project successfully.