Impact of intra-grain spin wave reflections on nano-contact spin torque oscillators

Abstract

We investigate the origin of the experimentally observed varying current-frequency nonlinearity of the propagating spin wave mode in nano-contact spin torque oscillators. Nominally identical devices with 100 nm diameter are characterized by electrical microwave measurements and show large variation in the generated frequency as a function of drive current. This quantitative and qualitative device-to-device variation is described in terms of continuous and discontinuous nonlinear transitions between linear current intervals. The thin film grain microstructure in our samples is determined using atomic force and scanning electron microscopy to be on the scale of 30 nm. Micromagnetic simulations show that the reflection of spin waves against the grain boundaries results in standing wave resonance configurations. For a simulated device with a single artificial grain, the frequency increases linearly with the drive current until the decreased wavelength eventually forces another spin wave anti-node to be formed. This transition results in a discontinuous step in the frequency versus current relation. Simulations of complete, randomly generated grain microstructures additionally shows continuous nonlinearity and a resulting device-to-device variation in frequency that is similar to the experimental levels. The impact of temperature from 4 K to 300 K on the resonance mode-transition nonlinearity and frequency noise is investigated using simulations and it is found that the peak levels of the spectral linewidth as a function of drive current agrees quantitatively with typical levels found in experiments at room temperature.

References in corpus (13)

• The design and verification of Mumax3
• Spin Transfer Torques
• Current-driven vortex oscillations in metallic nanocontacts
• Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven nanocontacts
• Theory for a dissipative droplet soliton excited by a spin torque nanocontact
• Adaptively time stepping the stochastic Landau-Lifshitz-Gilbert equation at nonzero temperature: implementation and validation in MuMax3
• Direct observation and imaging of a spin-wave soliton with $p-$like symmetry
• Faster, farther, stronger: spin transfer torque driven high order propagating spin waves in nano-contact magnetic tunnel junctions
• Model for a collimated spin wave beam generated by a single layer, spin torque nanocontact
• Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators
• Temperature dependence of linewidth in nano-contact based spin torque oscillators: effect of multiple oscillatory modes
• Non-Adlerian phase slip and non stationary synchronization of spin-torque oscillators to a microwave source
• Effects of rf Current on Spin Transfer Torque Induced Dynamics

Cited by in corpus (2)

• Field-free high-frequency exchange-spring spin-torque nano-oscillators
• High frequency oscillations in spin-torque nano oscillator due to bilinear coupling