Ferromagnetic Resonance in Granular Co/Ag and NiFe/Ag Multilayers

Abstract

This paper is concerned with the ways in which ferromagnetic resonance (FMR) may be used to study some properties of ferromagnetic particles in multilayered granular systems. The Co/Ag and NiFe/Ag multilayers have been investigated since the relation between their microstructure and magnetic/magnetotransport properties has already been well documented [1, 2] and, thus, these systems may be regarded as model materials for FMR investigations of fine magnetic particle systems. The samples were prepared by rf sputtering with a deposition rate of about 0.04 nm/s for each component. Two series of samples were made with variable Co or NiFe thickness 0.2 nm <d Co(NiFe) <1.5 nm and a constant Ag thickness of about 2 nm with the number of repetitions between 40 and 200. The structural characterisation were performed with the high- and low-angle X-ray diffraction and X-ray fluorescence. The high-angle X-ray diffraction shows that the multilayer films have coherent interfaces and a highly textured structure with [111] texture and a random orientation of crystallographic directions in the film plane. In low-angle XRD, the Bragg reflections up to the second order were observed. Ferromagnetic resonance spectra were taken at 9.4 GHz at the inplane and perpendicular to plane geometry in fields up to 1.2 T and at temperature range 300 - 600 K. The magnetic properties of our multilayers with ultrathin Co (NiFe) layers are very similar to those already reported in literature [1, 2]. For a nominal thickness of Co or NiFe below 0.6 - 0.7 nm, these multilayers consist of quasi two-dimensional arrays of flat particles embedded in a Ag matrix. With increasing the nominal thickness of Co or NiFe, the granular layers approach the behaviour of quasi-continuous magnetic films. However, a threshold between granular and quasi-continuous structure seems not to be very sharp and the transition region extends over 0.4 - 0.6 nm for the Co/Ag multilayers and even more (0.4 - 0.8 nm) for the NiFe/Ag system. For a very small nominal thickness (0.2 - 0.3 nm for Co/Ag and 0.2 - 0.6 nm for NiFe/Ag) the Co (NiFe) particles exhibit almost pure superparamagnetic behaviour since thermally activated magnetisation fluctuations diminish the apparent anisotropy field nearly to zero [4]. Important factors that determine the positions and shape of FMR spectra are anisotropies, particles size, their distribution [3, 4], a spatial variation of particles orientation around the film normal [5] and, for very fine particles, the influence of thermally activated magnetization reversal [3]. Due to limited space, we confine our discussion to the main points related to the anisotropy, the size effects, and thermally activated fluctuations in our granular Co/Ag multilayers.