J.-M. L. Beaujour

Ferromagnetic resonance study of sputtered Co|Ni multilayers

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 the 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 ΔH, on frequency, in the field in-plane geometry, increases with decreasing t. This behaviour is attributed to an enhancement of spin-orbit interactions with decreasing Co layer thickness and in thinner films, to a spin-pumping contribution to the damping.

Strong perpendicular magnetic anisotropy in Ni/Co(111) single crystal superlattices

Single crystal Ni/Co(111) superlattices have been grown by molecular beam epitaxy. The Ni thickness is 3 ML whereas the Co thickness varies from 0.2 to 4 ML. The superlattices were studied using magnetometry and ferromagnetic resonance spectroscopy and they all exhibit strong perpendicular to the plane magnetic anisotropy. The maximum magnetocrystalline anisotropy is obtained for one cobalt monolayer. Kerr microscopy measurements show the variation of domain pattern as the Co layer thickness changes.

Spin-torque driven ferromagnetic resonance of Co/Ni synthetic layers in spin valves

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 1to20GHz. 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.

Ferromagnetic resonance study of polycrystalline cobalt ultrathin films

We present room-temperature ferromagnetic resonance (FMR) studies of polycrystalline ∥Pt∕10nmCu∕tCo∕10nmCu∕Pt∥ films as a function of Co layer thickness (1⩽t⩽10nm) grown by evaporation and magnetron sputtering. FMR was studied with a high-frequency broadband coplanar waveguide (up to 25 GHz) using a flip-chip method. The resonance field and the linewidth were measured as functions of the ferromagnetic layer thickness. The evaporated films exhibit a lower magnetization density (Ms=1131emu∕cm3) compared to the sputtered films (Ms=1333emu∕cm3), with practically equal perpendicular surface anisotropy (Ks≃−0.5erg∕cm2). For both series of films, a strong increase of the linewidth was observed for Co layer thickness below 3 nm. For films with a ferromagnetic layer thinner than 4 nm, the damping of the sputtered films is larger than that of the evaporated films. The dependence of the linewidth can be understood in terms of the spin-pumping effect, from which the interface spin-mixing conductance is deduced.

Spin-torque driven ferromagnetic resonance in a nonlinear regime

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 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.

Ferromagnetic resonance study of polycrystalline Fe1−xVx alloy thin films

Ferromagnetic resonance has been used to study the magnetic properties and magnetization dynamics of polycrystalline Fe1−xVx alloy films with 0⩽x<0.7. Films were produced by cosputtering from separate Fe and V targets, leading to a composition gradient across a Si substrate. Ferromagnetic resonance studies were conducted at room temperature with a broadband coplanar waveguide at frequencies up to 50GHz using the flip-chip method. The effective demagnetization field 4πMeff and the Gilbert damping parameter α have been determined as a function of V concentration. The results are compared to those of epitaxial FeV films.

Spin-transfer in nanopillars with a perpendicularly magnetized spin polarizer

Spin-transfer devices that incorporate a polarizer with its magnetization orthogonal to a switchable (free) layer offer the potential for ultra-fast switching, low power consumption and reliable operation. The non-collinear magnetizations lead to large initial spin-transfer torques, eliminating the incubation delay seen in devices with collinear magnetization. Here we present the basic electrical and magnetic characteristics of spin-valve nanopillars that incorporate a perpendicularly magnetized polarizer and demonstrate current-induced switching with short current pulses, down to 100 ps in duration. We have fabricated devices that have a CoNi polarizer with perpendicular magnetization and an in-plane magnetized 3 nm thick Co free layer and a 12 nm thick Co reference layer, each separated by thin (~ 10 nm) Cu layers. The magnetization of the reference layer is collinear with that of free layer to read out the device state. The reference layer also contributes to the spin-accumulation acting on the free layer and leads to a spin-torque that favors the parallel (P) or antiparallel (AP) state depending on the current pulse polarity, reducing the requirement of precise pulse timing in precessional reversal. The anisotropy field of the perpendicular polarizer is 1.3 T, i.e. it is high enough so that in-plane fields (< 0.3 T) applied to switch the magnetizations of the reference and free layers do not reorient the polarizer. Our typical nanopillar device lateral dimensions are between 60 nm and 300 nm and nanopillars are positioned on coplanar waveguides to allow for broadband electrical connections and studies with fast rise time pulses, generated by an arbitrary waveform generator. The switching probability has been determined for variable pulse amplitude and duration, from 0.1 to 10 ns at room temperature.