arXiv subjects

P. M. Braganca

Publications and source records attributed to P. M. Braganca.

9 recordsLinked to original sources

arXiv2013

A quasi-linear spin-torque nano-oscillator via enhanced negative feedback of power fluctuations

We report an approach to improving the performance of spin torque nano-oscillators (STNOs) that utilizes power-dependent negative feedback to achieve a significantly enhanced dynamic damping. In combination with a sufficiently slow variation of frequency with power this can result in a quasi-linear STNO, with very weak non-linear coupling of power and phase fluctuations over a range of bias current and field. An implementation of this approach that utilizes a non-uniform spin-torque demonstrates that highly coherent room temperature STNOs can be achieved while retaining a significant tunability.

cond-mat.mes-hall↗

arXiv2009

Ultrafast switching of a nanomagnet by a combined out-of-plane and in-plane polarized spin-current pulse

We report on spin valve devices that incorporate both an out-of-plane polarizer (OPP) to quickly excite spin torque (ST) switching and an in-plane polarizer/analyzer (IPP). For pulses < 200 ps we observe reliable precessional switching due largely to ST from the OPP. Compared to a conventional spin valve, for a given current in the short pulse regime the addition of the OPP can decrease the pulse width necessary for switching by a factor of 10 or more. The influence of the IPP is most obvious at longer, smaller pulses, but also has beneficial ST consequences for short pulse switching.

cond-mat.other↗

arXiv2007

Enhancement in spin-torque efficiency by nonuniform spin current generated within a tapered nanopillar spin valve

We examine the effect a spatially non-uniform spin current with a component polarized partially out of the plane has on a low saturation magnetization nanomagnet free layer. Micromagnetic simulations indicate that the spin torque efficiency acting upon the reversing nanomagnet can be enhanced through this process, resulting in faster switching with smaller currents. In doing so, we determine that micromagnetic structure within the nanomagnets can be beneficial for reversal processes. We verify this enhancement experimentally in devices with a tapered nanopillar geometry that generates a spin current polarized partly out of plane. Finally, to take even better advantage of these effects, we examine micromagnetically the benefits of a tapered three-magnetic-layer structure that further reduces reversal times while maintaining the thermal stability of the free layer.

cond-mat.other↗

arXiv2007

Spin-Torque Ferromagnetic Resonance Measurements of Damping in Nanomagnets

We measure the magnetic damping parameter a in thin film CoFeB and permalloy (Py) nanomagnets at room temperature using ferromagnetic resonance driven by microwave frequency spin-transfer torque. We obtain $α_{CoFeB} = 0.014 \pm 0.003$ and $α_{Py}=0.010 \pm 0.002$, values comparable to measurements for extended thin films, but significantly less than the effective damping determined previously for similar nanomagnets by fits to time-domain studies of large-angle magnetic excitations and magnetic reversal. The greater damping found for the large amplitude nanomagnet dynamics is attributed to the nonlinear excitation of non-uniform magnetic modes.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic vortex oscillator driven by dc spin-polarized current

Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the dynamics of nanomagnets. A peculiar consequence of this spin-torque, the ability to induce persistent oscillations of a nanomagnet by applying a dc current, has previously been reported only for spatially uniform nanomagnets. Here we demonstrate that a quintessentially nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized dc current. Comparison to micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz, making these highly compact spin-torque vortex oscillator devices potential candidates for microwave signal-processing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures.

cond-mat.other↗

arXiv2006

Mechanisms limiting the coherence time of spontaneous magnetic oscillations driven by DC spin-polarized currents

The spin-transfer torque from a DC spin-polarized current can generate highly-coherent magnetic precession in nanoscale magnetic-multilayer devices. By measuring linewidths of spectra from the resulting resistance oscillations, we argue that the coherence time can be limited at low temperature by thermal deflections about the equilibrium magnetic trajectory, and at high temperature by thermally-activated transitions between dynamical modes. Surprisingly, the coherence time can be longer than predicted by simple macrospin simulations.

cond-mat.other↗

arXiv2006

Spin-transfer-driven ferromagnetic resonance of individual nanomagnets

We demonstrate a technique that enables ferromagnetic resonance (FMR) measurements of the normal modes for magnetic excitations in individual nanoscale ferromagnets, smaller in volume by a factor of 1000 than can be probed by other methods. The measured peak shapes indicate two regimes of response: simple FMR and phase locking. Studies of the resonance frequencies, amplitudes, and linewidths as a function of microwave power, DC current, and magnetic field provide detailed new information about the exchange, damping, and spin-transfer torques that govern the dynamics in magnetic nanostructures.

cond-mat.mtrl-sci↗

arXiv2005

Adjustable spin torque in magnetic tunnel junctions with two fixed layers

We have fabricated nanoscale magnetic tunnel junctions (MTJs) with an additional fixed magnetic layer added above the magnetic free layer of a standard MTJ structure. This acts as a second source of spin-polarized electrons that, depending on the relative alignment of the two fixed layers, either augments or diminishes the net spin-torque exerted on the free layer. The compound structure allows a quantitative comparison of spin-torque from tunneling electrons and from electrons passing through metallic spacer layers, as well as analysis of Joule selfheating effects. This has significance for current-switched magnetic random access memory (MRAM), where spin torque is exploited, and for magnetic sensing, where spin torque is detrimental.

cond-mat.mtrl-sci↗

arXiv2004

Spin-Transfer Effects in Nanoscale Magnetic Tunnel Junctions

We report measurements of magnetic switching and steady-state magnetic precession driven by spin-polarized currents in nanoscale magnetic tunnel junctions with low-resistance, < 5 Ohm-micron-squared, barriers. The current densities required for magnetic switching are similar to values for all-metallic spin-valve devices. In the tunnel junctions, spin-transfer-driven switching can occur at voltages that are high enough to quench the tunnel magnetoresistance, demonstrating that the current remains spin-polarized at these voltages.

cond-mat.mtrl-sci↗