arXiv subjects

R. A. Buhrman

Publications and source records attributed to R. A. Buhrman.

At least 55 records · Page 3Linked to original sources

arXiv2010

Spin-torque driven magnetic vortex self-oscillations in perpendicular magnetic fields

We have employed complete micromagnetic simulations to analyze dc current driven self-oscillations of a vortex core in a spin-valve nanopillar in a perpendicular field by including the coupled effect of the spin-torque and the magnetostatic field computed self-consistently for the entire spin-valve. The vortex in the thicker nanomagnet moves along a quasi-elliptical trajectory that expands with applied current, resulting in blue-shifting of the frequency, while the magnetization of the thinner nanomagnet is non-uniform due to the bias current. The simulations explain the experimental magnetoresistance-field hysteresis loop and yield good agreement with the measured frequency vs. current behavior of this spin-torque vortex oscillator.

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↗

arXiv2009

Single-shot time-domain studies of spin-torque-driven switching in magnetic tunnel junctions

We report single-shot measurements of resistance versus time for thermally assisted spin-torque-driven switching in magnetic tunnel junctions. We achieve sufficient sensitivity to resolve the resistance signals leading up to switching, including the variations between individual switching events. Analyses of pre-switching thermal fluctuations allow detailed measurements of coherence times and variations in magnetization precession amplitude. We find that with a small in-plane hard-axis magnetic field the magnetization dynamics are more spatially coherent than for the case of zero field.

cond-mat.mes-hall↗

arXiv2009

Sensitivity of spin-torque diodes for frequency-tunable resonant microwave detection

We calculate the efficiency with which magnetic tunnel junctions can be used as resonant detectors of incident microwave radiation via the spin-torque diode effect. The expression we derive is in good agreement with the sensitivities we measure for MgO-based magnetic tunnel junctions with an extended (unpatterned) magnetic pinned layer. However, the measured sensitivities are reduced below our estimate for a second set of devices in which the pinned layer is a patterned synthetic antiferromagnet (SAF). We suggest that this reduction may be due to an undesirable coupling between the magnetic free layer and one of the magnetic layers within the etched SAF. Our calculations suggest that optimized tunnel junctions should achieve sensitivities for resonant detection exceeding 10,000 mV/mW.

cond-mat.mes-hall↗

arXiv2009

Bias and angular dependence of spin-transfer torque in magnetic tunnel junctions

We use spin-transfer-driven ferromagnetic resonance (ST-FMR) to measure the spin-transfer torque vector T in MgO-based magnetic tunnel junctions as a function of the offset angle between the magnetic moments of the electrodes and as a function of bias, V. We explain the conflicting conclusions of two previous experiments by accounting for additional terms that contribute to the ST-FMR signal at large |V|. Including the additional terms gives us improved precision in the determination of T(V), allowing us to distinguish among competing predictions. We determine that the in-plane component of has a weak but non-zero dependence on bias, varying by 30-35% over the bias range where the measurements are accurate, and that the perpendicular component can be large enough to be technologically significant. We also make comparisons to other experimental techniques that have been used to try to measure T(V).

cond-mat.other↗

arXiv2008

Spin-Torque-Induced Rotational Dynamics of a Magnetic Vortex Dipole

We study, both experimentally and by numerical modeling, the magnetic dynamics that can be excited in a magnetic thin-film nanopillar device using the spin torque from a spatially localized current injected via a 10s-of-nm-diameter aperture. The current-driven magnetic dynamics can produce large amplitude microwave emission at zero magnetic field, with a frequency well below that of the uniform ferromagnetic resonance mode. Micromagnetic simulations indicate that the physical origin of this efficient microwave nano-oscillator is the nucleation and subsequent steady-state rotational dynamics of a magnetic vortex dipole driven by the localized spin torque. These results show this novel implementation of a spintronic nano-oscillator is a promising candidate for microwave technology applications.

cond-mat.mes-hall↗

arXiv2008

Resonant Spin-Transfer-Driven Switching of Magnetic Devices Assisted by Microwave Current Pulses

The torque generated by the transfer of spin angular momentum from a spin-polarized current to a nanoscale ferromagnet can switch the orientation of the nanomagnet much more efficiently than a current-generated magnetic field, and is therefore in development for use in next-generation magnetic random access memory (MRAM). Up to now, only DC currents and square-wave current pulses have been investigated in spin-torque switching experiments. Here we present measurements showing that spin transfer from a microwave-frequency pulse can produce a resonant excitation of a nanomagnet and lead to improved switching characteristics in combination with a square current pulse. With the assistance of a microwave-frequency pulse, the switching time is reduced and achieves a narrower distribution than when driven by a square current pulse alone, and this can permit significant reductions in the integrated power required for switching. Resonantly excited switching may also enable alternative, more compact MRAM circuit architectures.

cond-mat.other↗

arXiv2008

Strong linewidth variation for spin-torque nano-oscillators as a function of in-plane magnetic field angle

We measure the microwave signals produced by spin-torque-driven magnetization dynamics in patterned magnetic multilayer devices at room temperature, as a function of the angle of a magnetic field applied in the sample plane. We find strong variations in the frequency linewidth of the signals, with a decrease by more than a factor of 20 as the field is rotated from the magnetic easy axis to the in-plane hard axis. Based on micromagnetic simulations, we identify these variations as due to a transition from spatially incoherent to coherent precession.

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

Time-Domain Studies of Very-Large-Angle Magnetization Dynamics Excited by Spin Transfer Torques

We describe time-domain measurements which provide new information about the large-angle nonlinear dynamics of nanomagnets excited by spin-transfer torque from a spin-polarized current. Sampling-oscilloscope measurements, which average over thousands of experimental time traces, show that the mean reversal time for spin-transfer-driven magnetic switching has a step-like dependence on magnetic field, because an integer number of precession cycles is required for reversal. Storage-oscilloscope measurements of individual experimental traces reveal non-periodic large-amplitude resistance variations at values of magnetic field and current in a crossover region between the regimes of spin-transfer-driven switching and steady-state precession. We also observe directly the existence of time-dependent switching, on the nanosecond scale, between different precessional modes and between a precessional mode and a static state, at particular values of magnetic field and current bias.

cond-mat.mtrl-sci↗

arXiv2007

Measurement of the Spin-Transfer-Torque Vector in Magnetic Tunnel Junctions

Spin-polarized currents can transfer spin angular momentum to a ferromagnet, generating a torque that can efficiently reorient its magnetization. Achieving quantitative measurements of the spin-transfer-torque vector in magnetic tunnel junctions (MTJs) is important for understanding fundamental mechanisms affecting spin-dependent tunneling, and for developing magnetic memories and nanoscale microwave oscillators. Here we present direct measurements of both the magnitude and direction of the spin torque in Co60Fe20B20/MgO/Co60Fe20B20 MTJs. At low bias V, the differential torque vector d{tau}/dV lies in the plane defined by the electrode magnetizations, and its magnitude is in excellent agreement with a prediction for highly-spin-polarized tunneling. With increasing bias, the in-plane component d{tau}_{parallel}/dV remains large, in striking contrast to the decreasing magnetoresistance ratio. The differential torque vector also rotates out of the plane under bias; we measure a perpendicular component tau_{perp}(V) with bias dependence proportional to V^2 for low V, that becomes as large as 30% of the in-plane torque.

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

Large-amplitude coherent spin waves exited by spin-polarized current in nanoscale spin valves

We present spectral measurements of spin-wave excitations driven by direct spinpolarized current in the free layer of nanoscale Ir20Mn80/Ni80Fe20/Cu/Ni80Fe20 spin valves. The measurements reveal that large-amplitude coherent spin wave modes are excited over a wide range of bias current. The frequency of these excitations exhibits a series of jumps as a function of current due to transitions between different localized nonlinear spin wave modes of the Ni80Fe20 nanomagnet. We find that micromagnetic simulations employing the Landau-Lifshitz-Gilbert equation of motion augmented by the Slonczewski spin torque term (LLGS) accurately describe the frequency of the current-driven excitations including the mode transition behavior. However LLGS simulations give qualitatively incorrect predictions for the amplitude of excited spin waves as a function of current.

cond-mat.mes-hall↗

arXiv2007

X-ray photoemission study of CoFeB/MgO thin film bi-layers

We present results from an X-ray photoemission spectroscopy (XPS) study of CoFeB/MgO bi-layers where we observe process-dependent formation of B, Fe, and Co oxides at the CoFeB/MgO interface due to oxidation of CoFeB during MgO deposition. Vacuum annealing reduces the Co and Fe oxides but further incorporates B into the MgO forming a composite MgBxOy layer. Inserting an Mg layer between CoFeB and MgO introduces an oxygen sink, providing increased control over B content in the barrier.

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

Disorder, defects and bandgaps in ultra thin (001) MgO tunnel barrier layers

We report scanning tunneling spectroscopy studies of the electronic structure of 1.5 to 3 nm (001) textured MgO layers grown on (001) Fe. Thick MgO layers exhibit a bulk-like band gap, approximately 5-7 eV, and sparse, localized defect states with characteristics attributable to oxygen and, in some cases, Mg vacancies. Thin MgO layers exhibit electronic structure indicative of interacting defect states forming band tails which in the thinnest case extend to approximately 0.5 V of the Fermi level. These vacancy defects are ascribed to compressive strain from the MgO/Fe lattice mismatch, accommodated as the MgO grows.

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↗