Abstract. We report on room temperature ferromagnetic resonance (FMR) studies of [t Co|2t Ni]×N sputtered films, where 0.1 ≤ t ≤ 0.6 nm. Two series of films were investigated: films with same number of Co|Ni bilayer repeats (N=12), and samples in which the overall magnetic layer thickness is kept constant at 3.6 nm (N=1.2/t). The FMR measurements were conducted with a high frequency broadband coplanar waveguide up to 50 GHz using a flip-chip method. The resonance field and the full width at half maximum were measured as a function of frequency for the field in-plane and field normal to the plane, and as a function of angle to the plane for several frequencies. For both sets of films, we find evidence for the presence of first and second order anisotropy constants, K1 and K2. The anisotropy constants are strongly dependent on the thickness t, and to a lesser extent on the total thickness of the magnetic multilayer. The Landé g-factor increases with decreasing t and is practically independent of the multilayer thickness. The magnetic damping parameter α, estimated from the linear dependence of the linewidth, △H, on frequency, in the field in-plane geometry, increases with decreasing t. This behaviour is attributed to an enhancement of spin-orbit interactions with t decreasing and in thinner films, to a spin-pumping contribution to the damping.

Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study magnetic excitations in Co/Ni synthetic layers confined in nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude excitation in a linear response regime. The resulting resonance fields were compared with those obtained using conventional rf field driven FMR on extended films with the same Co/Ni layer structure. A lower resonance field is found in confined structures. The effect of both dipolar fields acting on the Co/Ni layer emanating from other magnetic layers in the device and finite size effects on the spin wave spectrum are discussed.One approach to study ferromagnetic resonance (FMR) of a magnetic layer in a confined structure is to use the spin transfer interaction [1, 2] in a currentperpendicular (CPP) nanojunction. An rf current is applied to a magnetic tunnel junction [3] or spin valve [4], to drive FMR, in a method known as the spin-torque-driven ferromagnetic resonance (ST-FMR). This new technique enables quantitative studies of magnetic properties of materials in nanopillars, such as their magnetic excitations, anisotropy and damping.Spin-transfer devices that incorporate materials with perpendicular magnetic anisotropy are of great interest. This is because of their potential to lead to faster STdevices, with lower power dissipation [5] and critical current [6]. Recently, Mangin et al. studied perpendicular spin valves with a Co/Ni multilayer free layer, where a large magnetoresistance value and a high spin torque efficiency were observed [7].In this work, we present ST-FMR studies on bilayer spin valves, where the thin (free) layer is composed of a Co/Ni synthetic layer and the thick (fixed) layer is pure Co. By comparing the ST-FMR resonance fields with those of conventional rf field driven FMR of extended Co/Ni films with the same layer structure, we illustrate interactions important in ST-FMR of nanojunctions. Specifically, we discuss both dipolar interactions between the Co/Ni layer and other magnetic layers in the device, and finite size effects on the magnetic excitation spectrum.Pillar junctions with submicron lateral dimensions and rectangular shape, shown in Fig. 1a, were patterned on a silicon wafer using a nanostencil process [8]. Junctions were deposited by evaporation, and have the layer structure 1.5 nm Cr| 100 nm Cu| 20 nm Pt| 10 nm Cu| [0.4 nm Co| 0.8 nm Ni]×3 | 10 nm Cu| 12 nm Co| 200 nm Cu . The ST-FMR measurement setup is shown in Fig. 1(a). An rf current generated by a high frequency source is coupled with a dc current through a bias-T (the dashed-line box in Fig. 1(a)) into the spin valve. At resonance, the rf current and spin valve resistance oscillate at the same frequency. This results in a dc voltage (V =< I(t)R(t) >) [3,4]. Assuming a small angle circular precession of the free layer on resonance,where β is the angle between the free and fixed layers (before applying the rf curre...