Acoustics for antiferromagnets – ACAF

The ACAF project is at the crossroads of two very exciting and promising fields that have emerged and gained increasing international visibility in recent years, namely antiferromagnets, for novel spintronic devices, and magneto-acoustics, for novel magnetization manipulation schemes. There is now a keen interest to use colinear antiferromagnets (AFMs) instead of ferromagnets (FM) to code information magnetically, e.g. on two orthogonal directions of the Néel vector. The immunity of AFMs to external magnetic fields because of their vanishing magnetization however makes it challenging to detect and control the antiferromagnetic (AF) state. To bypass the bottlenecks of static and dynamic manipulation of AFMs, the project will implement a well-known effect in an original fashion: strain waves in the hypersound range (acoustic waves) will be used to actuate on the AF dynamics via magnetoelasticity. More specifically we will use surface acoustic waves (SAWs). With their energy–efficient generation, low attenuation, and power-flow confined to the surface, they have proved over the last couple years to be a very relevant tool for non-inductive wave-manipulation of magnetization in ultra-thin ferromagnets, opening a new route toward “straintronics”. Via magneto-elasticity they generate an effective radio-frequency magnetic field, particularly efficient to actuate on magnetization dynamics when their frequency matches spin wave frequencies.
To investigate fundamentally the potential of SAWs to control AF states, we have chosen two magnetostrictive model systems that can function around room temperature in order to be compatible with potential applications. Both systems reintroduce a finite and variable magnetization that will ease the detection of its dynamics and/or the Néel vector manipulation. In GdCo, a ferrimagnet, we will assess the potential of SAWs to drive the dynamics at the temperature of angular momenta compensation, where the dynamics is AF-like but the magnetization does not cancel. In the AFM FeRh, the ambitious goal will be to control the static AF state using SAWs, relying on the first-order magneto-structural AFM-FM transition, very sensitive to strain. At constant temperature the SAW will induce a local transition to the FM state, allowing for magnetization rotation by 90 degrees, leaving the Néel vector rotated after returning to the AFM state.
The project will develop sample design and growth for efficient electrical generation of SAWs on thin optimized FeRh and GdCo layers and microstructures, magnetic and structural characterization of the samples, investigation of the magnon and phonon dispersions by Brillouin light scattering, optical time- and space domain observation of acoustically-driven AF dynamics and AF-FM transition. The project gathers three French laboratories with complementary skills and techniques, and benefits from a well-recognized expertise in magneto-acoustics. It also includes collaboration with two foreign laboratories, for FeRh growth and theory.
In the longer term, this project might open new routes to implement magnetic components with lower Joule dissipation as strain will be excited by electrical fields. It could also allow alternative data storage design in AF materials using the wave properties of SAWs: focusing, interferences, wave-front shaping and waveguiding, or remote accessing of bits thanks to the weak attenuation of SAWs.