Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing

Metastable face-centered-cubic Fe78Ni22 thin films grown on Cu(001) substrates are excellent candidates for focused ion beam direct writing of magnonic structures due to their favorable magnetic properties after ion-beam-induced transformation. The focused ion beam transforms the originally nonmagnetic fcc phase into the ferromagnetic bcc phase with additional control over the direction of uniaxial magnetic in-plane anisotropy. The magnetocrystalline anisotropy in transformed areas is strong enough to stabilize the magnetization in transverse direction to the long axis of narrow waveguides. Therefore, it is possible to propagate spin waves in these waveguides without the presence of an external magnetic field in the favorable Demon-Eshbach geometry. Phase-resolved micro-focused Brillouin light scattering yields the dispersion relation of these waveguides in zero as well as in nonzero external magnetic fields.