Triggering magnetic properties of epitaxied thin films by Surface Acoustic Waves : the cases of magnetocaloric MnAs and magnetostrictive Fe_(1-x)Ga_x

by M. Marangolo (Institut des NanoSciences de Paris, UPMC-CNRS (France))

Friday 6 May 2016, 14:30 → 15:30 Europe/Rome

Aula 4 (Dip. di Fisica - Edificio E. Fermi)

In our laboratories we are performing experimental studies of the propagation of surface acoustic waves (SAW) in thin films (~50-100 nm) between 100MHz and 2 GHz. We are interested to trigger the magnetic properties (magnetic anisotropies, spin waves dispersion, magnetocaloric effect, magnetization direction) by acoustic means. In this seminar, I will present some recent results obtained on two magnetic and metallic systems: MnAs and Fe1-xGax grown by Molecular Beam Epitaxy (MBE) on GaAs(001). Bulk MnAs is a well-known magnetocaloric material exhibiting a magnetic and structural phase transition at 40 °C between a ferromagnetic and a non‑ferromagnetic phase. MnAs thin fims epitaxied on GaAs are strained by the substrate and the elastic coupling is responsible for a phase coexistence, in a wide temperature range. Our group has shown that coexistence spreads the temperature range where the magnetocaloric effect is significant. We have studied the propagation of surface acoustic waves on MnAs thin film (100‑200 nm) epitaxied on GaAs(001). We show the surface acoustic waves trigger a dynamic magnetocaloric effect in MnAs. Indeed, the elastic strain modulates the Weiss molecular field and is equivalent to an effective dynamic magnetic field. This leads to a magnetic entropy modulation in the material and heat exchange with the substrate. Energy is transferred from the acoustic wave to the magnetic excitations. A theory of the effect is presented. Concerning Fe1-xGax, a highly magnetostrictive material, we are interested to controlling the magnetization direction and spin waves dispersion by an elastically driven ferromagnetic resonance phenomenon. For metallic magnetic alloys, the expected resonance frequencies are in the GHz regime and measureable at room temperature. As a prerequisite to understand resonant phenomena (GHz) regime, we systematically investigated non-resonant coupling between SAW and magnetization (different film thickness, direction of the applied field, working frequency). I will present these recent out-of-resonance results and a phenomenological model to understand SAW propagation.