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 thin Co/Ni synthetic layers with perpendicular anisotropy confined in spin-valve based 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 rf excitation, from 1 to 20 GHz. These results are compared with those obtained using conventional rf field driven FMR on extended films with the same Co/Ni layer structure. The layers confined in spin valves have a lower resonance field, a narrower resonance linewidth and approximately the same linewidth vs frequency slope, implying the same damping parameter. The critical current for magnetic excitations is determined from measurements of the resonance linewidth vs dc current and is in accord with the one determined from I-V measurements.Spin-transfer torque has been theoretically predicted and experimentally demonstrated to drive magnetic excitations in nanostructured spin valves and magnetic tunnel junctions [1,2,3,4,5]. With an rf current, spin transfer can be used to study ferromagnetic resonance [6,7]. This technique, known as spin-torque driven ferromagnetic resonance (ST-FMR), enables quantitative studies of the magnetic properties of thin layers in a spin-transfer device. Specifically, the layer magnetic anisotropy and damping can be determined [8], which are important parameters that need to be optimized in spin-torque-based memory and rf oscillator applications.Spin-transfer memory devices will likely include magnetic layers with perpendicular magnetic anisotropy that counteracts their shape-induced easy-plane anisotropy. This will allow efficient use of spin current for magnetic reversal with a reduced switching threshold [9] and a faster switching process [10]. Recent work by Mangin et al. [11] has demonstrated improvements of spin-torque efficiency in a spin valve that has perpendicularly magnetized Co/Ni synthetic layers. For further optimization of perpendicular anisotropy materials, it is important to have quantitative measurements of their anisotropy field and damping in a nanostructured device, as both of these parameters directly affect the threshold current for spintransfer induced switching.In this Letter, we present ST-FMR studies of bilayer nanopillars, where the thin (free) layer is composed of a Co/Ni synthetic layer and the thick (fixed) layer is pure Co. The magnetic anisotropy and damping of the Co/Ni have been determined by ST-FMR. We compare these results with those obtained from extended films with the same Co/Ni layer stack measured using traditional rf field driven FMR.Pillar junctions with submicron lateral dimensions ( Fig. 1(a)) were patterned on a silicon wafer using a nanostencil process [12]. Junctions were deposited using metal evaporation with the layer structure 1.5 nm (a)

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Spin transfer in asymmetric Co-Cu-Co bilayer magnetic nanopillars junctions has been studied at low temperature as a function of free-layer thickness. The phase diagram for current-induced magnetic excitations has been determined for magnetic fields up to 7.5 T applied perpendicular to the junction surface and freelayers thicknesses from 2 to 5 nm. The junction magnetoresistance is independent of thickness. The critical current for magnetic excitations decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness. The limiting current is in quantitative agreement with that expected due to a spin-pumping contribution to the magnetization damping. It may also be indicative of a decrease in the spin-transfer torque efficiency in ultrathin magnetic layers.Spin transfer in magnetic nanopillar has become a major focus of experimental research 1-3 since Slonczewski and Berger's seminal theoretical work in 1996. 4,5 A spin current has been demonstrated to switch the magnetization direction of a small magnet at a specific current density, as well as to induce microwave excitations. There are applications of this effect to magnetic random access memory ͑MRAM͒ and high-frequency electronics. 1,6,7 It is of importance to determine the factors that control the critical current for magnetization dynamics for both the physics and technology of spin transfer. For instance, it is of interest to reduce the critical current for MRAM applications, and to increase it in magnetic sensor designs.In Slonczewski's theory, spin transfer is an interface effect: spin-angular momentum is transferred to the background magnetization when the spin current enters the ferromagnet-within the first few atomic layers. 8 For one polarity of the current, this generates a torque on the magnetization that is opposed by bulk damping. As a result, there is a threshold current to excite magnetization dynamics that is proportional to the volume of the magnet or, equivalently, the threshold current density is proportional to the thickness of the magnetic layer. There are alternative models in which the spin-transfer interaction occurs on a longer length scale, which predict a decrease in the efficiency of the torque in very thin magnetic layers. 9,10 It is also now widely appreciated that the magnetization damping in thin layers can be dominated by interfaces, in an effect known as "spin pumping." 11 For these reasons it is of importance to study spin transfer in samples in which the layer thicknesses are varied to gain insight into the factors that determine the strength and length scales of the spin-transfer interaction.Albert et al. 12 studied current-induced magnetization switching as a function of free-layer thickness at room temperature. Here thermal fluctuations are important and the intrinsic ͑zero temperature͒ critical current was determined by extrapolating from pulsed current measurements. The switching was between in-plane magnetized states, parallel and antiparallel to the f...

Spin-valve based nanojunctions incorporating Co|Ni multilayers with perpendicular anisotropy were used to study spin-torque driven ferromagnetic resonance (ST-FMR) in a nonlinear regime. Perpendicular field swept resonance lines were measured under a large amplitude microwave current excitation, which produces a large angle precession of the Co|Ni layer magnetization. With increasing rf power the resonance lines broaden and become asymmetric, with their peak shifting to lower applied field. A nonhysteretic step jump in ST-FMR voltage signal was also observed at high powers. The results are analyzed in in terms of the foldover effect of a forced nonlinear oscillator and compared to macrospin simulations. The ST-FMR nonhysteretic step response may have applications in frequency and amplitude tunable nanoscale field sensors.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter