Ferromagnetic resonance studies of Py bilayers for the system (Permalloy/Al2O3) (abstract)

Abstract

We report a detailed ferromagnetic resonance study, performed at X band and room temperature, of magnetic permalloy (Py) bilayers (Py1/Al2O3/Py2) deposited by rf sputtering on mica substrates. We will present and discuss results relative to two series of Py bilayers characterized by an ultrathin Py layer thickness and corresponding to the two following sets of spacer thickness ts: (40;30;19 Å) and (15;12;9 Å). These films have been deposited at room temperature and subsequently annealed (at T=220 and 190 °C, respectively, for these two sets). The resonance spectrum is studied as a function of the orientation [ΘH=(N,H) with respect to the film normal N] of the applied dc field H in a plane perpendicular to the film. (i) For the three samples of the first series the resonance spectrum is composed of two resolved lines which can be attributed to the resonances of the individual layers (Py1 and Py2) dynamically uncoupled. As for a single Py layer, the angular variation Hres(ΘH) of each line observed is well accounted for in terms of a unique parameter (4ΠM−HA), where HA is a uniaxial perpendicular anisotropy field arising from the interface-induced anisotropy. The anisotropy fields HA1 (for Py1) and HA2 (for Py2) are found slightly different (HA2−HA1=500 G), reflecting some differences in the structural characteristics of the (Py/Al2O3) interfaces in the two Py layers. (ii) In the second series of lower ts values, by contrast, the observed resonance spectrum is more complex revealing for a given orientation ΘH two main modes (with a more intense one) and for specific geometries (around ΘH=0) the presence of other resonance modes of much weaker intensities. For these samples the angular variation Hres(ΘH) of the different modes observed, by comparison to the behavior found in the Py single-layer control sample, provides clear evidence for the presence of a dynamical coupling between the magnetizations of the two Py layers. The main mechanisms, likely responsible for this intermagnetic layer coupling, will be discussed: dipolar coupling and exchange coupling (either direct through pinholes or indirect).