No abstract

We present a systematic study of the spin transport properties in two different bilayer systems, Ni 80 Fe 20 /Ru and Ni 80 Fe 20 /Ta, combining ferromagnetic resonance (FMR) and inverse spin Hall effect (ISHE) voltage measurements. We have estimated the effective spin mixing conductance g ↑↓ by analyzing the permalloy (Py) thickness dependence of the FMR linewidth obtaining g ↑↓ = (3.8 ± 0.7) × 10 15 cm −2 and g ↑↓ = (1.3 ± 0.4) × 10 15 cm −2 for Py/Ru and Py/Ta, respectively. Analyzing the Ta thickness dependence of the ISHE voltage, we have been able to extract the spin diffusion length, λ SD = 1.5 ± 0.5 nm, and spin Hall angle, SH = −0.03 ± 0.01, of Ta. From the two series of Py/Ta bilayers-with thickness variation of ferromagnetic and nonmagnetic layers, respectively-we demonstrate a path to estimate the spin diffusion length from the experimental data, independent of the spin Hall angle and the microwave field amplitude.

The dynamical behaviour of FePt thin films in the chemically disordered fcc phase has been studied using ferromagnetic resonance spectroscopy in a broad range of frequencies, from 1 to 100 GHz. A set of samples with thicknesses between 9 and 94 nm was investigated at room temperature in order to analyse the damping of the magnetization as a function of the excitation frequency. As expected, the increase in frequency is accompanied by larger resonance fields and linewidths. These two quantities also display an anisotropic response when the external applied field is rotated from the in-plane to the out-of-plane direction.We have observed that in the thicker samples the linewidth is different for fields applied parallel or perpendicular to the film plane which, in principle, should not occur in homogeneous ferromagnetic films. This behaviour could be understood by the presence of a frequency-independent anisotropic inhomogeneous broadening that depends on film thickness, in addition to the Gilbert damping term (α) which gives a linear frequency dependence. Although this last term is in general isotropic, we have measured different values of α for in-plane (α = 0.025(1)) and out-of-plane (α ⊥ = 0.021(1)) geometries. These values of the damping constant are considerably smaller than those previously reported in chemically ordered alloys, at variance with the prediction of recent developed theoretical models.

Magnetically soft FePt thin films of varying thickness (20 nm ≤ d ≤ 100 nm) were sputter-deposited at different Ar pressures in order to systematically modify the residual stress and hence the magnetic anisotropy. The magnetic domain structure of FePt thin films showed a transition from planar to nearly parallel stripes above a critical thickness, dcr, which was found to depend on an anisotropy contribution perpendicular to the film plane, originated essentially in magnetoelastic effects. A careful structural characterization was made in order to obtain the strain and the stress induced magnetic anisotropy in the samples. Vibrating sample magnetometry and magnetic force microscopy were used to investigate the changes occurring in the magnetic domain structure and the critical thickness of each set of films. Joining together structural and magnetic results, we have been able to construct a phase diagram that divided regions of different domain structures, either by changing the film thickness or the perpendicular magnetic anisotropy. The experimental results could be satisfactorily explained by using a model developed by Murayama. The observed dependence of the magnetic properties of soft FePt thin films on the fabrication conditions opens the possibility to tune the magnetic domain configuration from planar to stripe-like domains by changing the argon sputtering pressure used during film deposition.