Subnanosecond magnetization dynamics driven by strain waves
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troduction Nanoscale magnetism is used for constructing memory, computing, and communication devices. Static magnetic states serve as basic elements in nonvolatile memories,1 whereas dynamic excitations such as spin waves could be used to transmit signals and encode information in future electronic devices.2 The control of collective magnetization states at the micro- and nanoscale is done traditionally through magnetic fields created with electrical currents, resulting in heat dissipation and stray fields. Magnetostrictive effects connect variations of the magnetic moment to variations of the mechanical strength. The inverse effect is known as the Villari effect or the magnetoelastic effect, and can be used to handle magnetic moment variations through elastic deformations of a material. Electric fields applied to piezoelectric materials can be used to induce elastic deformations (strain) in a nanoscale magnetic material that results in changes in magnetic properties, mostly shown by static experiments.3–10 The magnetoelastic effect is a promising approach to achieve high-speed magnetic moment variations at the nanoscale together with low-power dissipation. In order to achieve high-speed magnetization manipulation through the magnetoelastic effect, the applied strain must change quickly. Surface acoustic waves (SAWs) are propagating strain waves in the MHz–GHz frequency range
that can be generated through radio-frequency (RF) electric fields at the surface of piezoelectric materials. SAWs offer a propagating medium together with direct coupling to the fundamental time scales of the magnetization for ferromagnetic materials. It has been shown that SAWs can couple to magnetic oscillations and can be used to achieve assisted reversal of the magnetic moment.11–16 Hernàndez et al.11 measured an enhanced macroscopic magnetic moment of a Mn12 acetate single crystal upon SAW excitation at low temperatures. Davis et al.12 observed a time-integrated magnetic moment along the hard axis of thin-film microstructures by magneto-optical methods due to a SAW driven magnetoelastic effect. Weiler et al. used SAW to excite ferromagnetic resonance (FMR)13 and perform spin pumping.14 Thevenard et al.15 demonstrated magnetic switching by SAW pulses with an applied magnetic field, and Labanowski et al.16 characterized the SAW–FMR interaction from SAW damping. However, SAW-induced magnetization dynamics have been treated as an effective variation in the magnetic energy, but experiments have provided little information on the phase between the phononic and magnetization modes. Our experimental approach provides a simultaneous direct observation of both strain waves and magnetization modes that unequivocally resolve the dynamic coupling between SAWs and magnetization at the intrinsic time and space scales, which are picosecond and
Michael Foerster, ALBA Synchrotron, Spain; [email protected] Lucia Aballe, ALBA Synchrotron, Spain; [email protected] Joan Manel Hernàndez, Department of Condensed Matter Physics, University of Barcelona, Spain; j
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