Spin waves in exchange spring nanoheterostructured amorphous/nanocrystalline films

We investigate the structural, static, and dynamic magnetic properties of exchange spring (ES) nanohetero-structured, stress-free, optically smooth films of amorphous/nanocrystalline Co-rich cobalt phosphorous (CoP) prepared by electrodeposition technique at room temperature. Static magnetic measurement reveals different hysteresis loop structures of the thin films - evolution from the low coercivity non-ES loop to the staircase-ES loop, giving rise to multiple coercivities for relatively higher thickness films. The first-order reversal curve (FORC) distributions demonstrate the different reversal mechanisms present in the samples and confirm the non-ES and ES natures of the films. The field-dependent Brillouin light scattering (BLS) spectra of the ES thin film unveil two well-resolved spin wave peaks associated with the bulk modes (B) and the so-called Damon-Eshbach (DE) surface spin wave modes (S) while that for non-ES low coercivity sample show a doublet of modes below a certain value of the applied field. Observation of the S mode only on one side of the measured spectra depends on the direction of the external magnetic field due to the nonreciprocal character of the S wave in the micrometer thickness of the investigated films. The external magnetic field-dependent BLS spectra yield an almost linear dependence of the mode frequencies versus the magnetic field intensity. Studied BLS measurements demonstrate the evolution of the ES structure in 5.7-μm-thick nanohetero-structured CoP film compared to the 1.4-μm-thick non-ES sample. Additionally, the increase in the interfacial exchange energy value (JIES-10.12erg/cm2) in the 5.7μm film compared to that (JInon-ES-3.68erg/cm2) of 1.4-μm film (calculated from corresponding asymmetric peak fittings to the measured BLS spectra) reduces the interfacial exchange energy ratio (JInon-ES/JIES) below unity, confirming the enhanced strength of the exchange coupling in the developed ES film.