Coupled resonance modes in ferromagnetic films with stripe domains

E. BURGOS; E. SALLICA LEVA; F. J. MARTÍNEZ TABARES; M. VÁSQUEZ MANSILLA; A. BUTERA

Bariloche

Encuentro; X Encuentro CNEA “Superficies y Materiales Nanoestructurados”; 2010

Resumen: The intermetallic compound FePt presents a great technological interest because of their unique magnetic properties, particularly the very large coercivities and the high magnetic anisotropy, which can be larger than 7x107 emu/cm3 in the ordered FCT phase. However, when thin films are fabricated using dc magnetron sputtering, they often form in an FCC crystalline-disordered relatively soft magnetic phase, and the high coercivity, high anisotropy properties are observed only after proper annealing at elevated temperatures. The remanent magnetic domain configuration of the FCC disordered phase is strongly dependent on the film thickness. Films thinner than a critical thickness, tcr ~ 30 nm, show an almost square in-plane hysteresis loop, with magnetic domains that are completely oriented parallel to the film plane. For tcr > 30 nm the domain structure develops a stripe-like pattern in which a small uniaxial out of plane component of the magnetic anisotropy favors the misalignment of the domains from the film plane. The dynamic response of the magnetization was investigated in a series of FePt films with thicknesses in the range 9-100 nm using ferromagnetic resonance techniques at both X-band (9.5 GHz) and K-band (24 GHz). From the out of plane angular variation of the uniform resonance mode we have deduced the effective demagnetizing field which is around 9000 Oe for all films. This value correlates very well with the saturation field deduced from magnetization measurements. The obtained average g-value was 2.09 which is coincident with the value of ferromagnetic iron. In the thinner films (t < 28 nm) we have only observed a single resonance absorption that is related to the uniform precession of the magnetization. Thicker films, in which a stripe domain pattern is observed at zero field, show an additional resonance line when the magnetic field is applied at, or very close to, the film plane normal. This line appears at fields below the main resonance and is observed at both X and K-bands with approximately the same field separation from the uniform mode. We have found that the line separation between the two resonances scales both with the film thickness and the stripe period, indicating that the appearance of an additional resonance is probably related to some kind of confinement. We have also analyzed the variation of the linewidth as a function of film thickness and have found that the line broadens above tcr. From these data we have obtained further evidence that the formation of a stripe structure produces considerable changes in the resonance spectra.