Brillouin Light Scattering Study of Spin Waves in CoP Exchange Spring Thin Films

Abstract

Brillouin Light Scattering (BLS) [1, 2] experiment has been used to determine the spin waves in Co-rich cobalt phosphorous (CoP) exchange spring [3, 4] thin films prepared by the electrodeposition techniques [5, 6] at room temperature. The static magnetic measurement using hysteresis loop tracer (ShB Instruments, USA) shown in FIG. 1 (a)-(b) reveals the hysteresis loop structures for the electrodeposited thin films – the staircase like exchange spring nature with the presence of two coercivities in the hysteresis loops – composed by the soft and hard magnetic phases in a single material. Contrasted with the conventional single-phase magnetic material having a comparatively low coercivity – such kind of exchange spring nanomagnetic frameworks are promising for cutting edge, forefront magnetic applications due to their relatively higher energy product – the blend of magnetization and permanent magnetic field [7]. The differential of the magnetization shown in FIG. 1 (c)-(d) confirms the presence of two coercivities in the 2.8 μm, 5.7 μm thick thin films, respectively.Utilizing a (3+3)-pass tandem Fabry-Perot interferometer, and considering the backscattering geometry, the BLS spectra of the thermally excited SWs were measured. The BLS Spectra of the 2.8 µm thick CoP thin film shows that the peaks are very broad. The field dependent BLS spectra of 5.7 µm thick CoP thin film electrodeposited for 60 min is shown in FIG. 2(a). The BLS study yields two main modes present in the BLS spectrum: the lowest frequency one is a band of bulk modes (B) while the one at the higher frequency (sharp peak) is the so-called Damon-Eshbach surface spin wave (S) [8]. On the Stokes-side of the spectra, two well resolved peaks (S and B, as mentioned in the figure) are observed while the anti-Stokes side of the spectra shows only one peak (B). The S mode is observed only on one side of the spectrum, depending on the direction of the external magnetic field. This is the consequence of the nonreciprocal character of the S wave and of the micrometer thickness of the investigated films. The external magnetic field dependent BLS spectra shows an almost linear dependence of the mode frequencies vs the magnetic field intensity. The BLS spectra shown in FIG. 2(b) reveals the effect of magnetization reversal at constant polarization which unveils the predicted side change of Damon Eshbach (DE) surface mode. The change in Stokes/anti-Stokes ratio has also been affirmed because of the associated transfer of the S and B mode interaction with this magnetization reversal [8]. **