Manipulating spin-waves by strain in BiFeO₃ thin films

Since the first realization of BiFeO₃ film 20 years ago [1], this material form has been extensively studied as it is promising for spintronic and magnonic applications. Due to the strong magneto-electric and magneto-elastic couplings of BiFeO₃, the magnetic properties of thin films (e.g., the magnetic ground states, the spin-wave energies at the zone center) can be effectively manipulated by strain [2-3]. However, the lack of suitable techniques sensitive to the spin-wave dispersion in thin films prevented so far to understand the relation between the magnetic exchange interaction and the strain.

Here, we employ Fe L_3,2-edge resonant inelastic X-ray scattering (RIXS) to probe the spin-wave dispersion of BiFeO₃ thin films versus strain, from compressive (Δa/a=-2.3 %) to tensile (Δa/a=0.4 %). The momentum-dependent RIXS spectra reveal a clear change in the spin-wave dispersion: a linear decrease of the magnon bandwidth is observed as compressive strain increases. Using a minimal model for unstrained and strained cases, we reveal an increased in-plane exchange interaction by 8 % and a decreased out-of-plane exchange interaction by 27 % for the mostly compressed film (-2.3 %), with respect to the unstrained case. Our results elucidate the microscopic mechanisms at play behind the manipulation of the spin-wave in BiFeO₃ thin films.

[1] J. Wang, et al., Science 299, 1719 (2003)

[2] D. Sando, et al., Nat. Mater. 12, 641 (2013).

[3] S. R. Burns, et al., Adv. Mater. 32, 2003711 (2020)