Ferromagnetic Resonance of Disordered FePt Thin Films

FePt ferromagnetic polycrystalline thin films of equiatomic composition deposited on oxidized Si (100) substrates have been studied as a function of film thickness by means of dc magnetization and ferromagnetic resonance measurements. Films in the as-deposited state have a moderate texture along the [111] direction as confirmed by X-ray diffraction experiments. In-plane magnetic hysteresis loops show an increasing coercive filed and a decreasing remanent magnetization when the film thickness increases from 10 to 100 nm. Thicker films (t= 60 and 100 nm) reverse magnetization in a two step process, characteristic of samples that present stripe-like magnetic domains. Ferromagnetic resonance in thinner films (which have an almost square hysteresis loop) is characterized by a single resonance line related to the uniform precession of the magnetization, which correlates well with the static magnetic measurements. The two thicker films show an additional resonance line when the magnetic field is applied at, or very close to, the film plane normal. This line is shifted to lower fields from the main resonance by 400-700 Oe and is observed at both X and Q-bands with a field separation that depends on film thickness. These additional resonances, can be originated in the presence of standing spin waves in the thicker films which could not be observed in the thinner ones.