Sub-GHz resonant magnetoelastic coupling in epitaxial Fe thin films

Abstract

New fascinating phenomena involving coupled spin waves and phonons are observed at the magneto-elastic resonance: magnetic field dependent spin Seebeck effect [1], acoustically-assisted spin pumping [2, 3] and magnetization swiching [4, 5]. In this context, it has been shown that an efficient mean to excite phonons is by surface acoustic waves (SAW). The integration of SAW-FMR (ferromagnetic resonance) mechanism in spintronic and magnonic devices are claimed to operate with very low power. However, most ferromagnetic materials have precession frequencies well above 5 GHz, i.e. above the highest frequency reached by today's SAW technologies. This restricts the research on SAW-FMR to low magnetization and/or low magnetic anisotropy materials, such as (Ga,Mn)As or (GaMn)(As,P), Ni, and YIG. Here, we show that thin films of Fe epitaxially grown on GaAs(001), a spintronic and magnonic compatible magnetoelastic and piezoelectric heterostucture, give the opportunity to obtain SAW-FMR, at room temperature, below 1 GHz despite the high Fe magnetization. This is obtained by exploiting the softening of the magnetic stiffness when the field is applied along a hard axis, at moderate intensity (60 mT). Slight variations of the magnetic field direction, less than 0.1 deg off, increase dramatically magnetization precession frequency leading to off-resonance interaction and suppressing SAW attenuation. The observed phenomena are described by adopting a very simple magnetization dynamics model that catches the physics of the interaction. The observation of resonant, low frequency and highly directional magnetoelastic coupling in Fe suggests that SAW mature technology offers the opportunity to magnonics and spintronics to handle magnetization precession in a remote way, without using any dissipative electron current. [6]