Experimental study of current-driven vortex oscillations in magnetic nanocontacts (original) (raw)
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Journal of Physics D: Applied Physics, 2009
We present an experimental and theoretical study of the low-field dynamics of current-driven vortex oscillations in nanocontacts based on spin-valve multilayers. These oscillations appear as low-frequency (250-500 MHz) excitations in the electrical power spectrum which arise from to variations in the giant-magnetoresistance. We show that the vortex oscillations, once nucleated at large fields applied perpendicular to the film plane, persist at zero applied magnetic fields. Some training effects on the oscillation frequency and linewidth also observed for small in-plane magnetic fields.
Current-Driven Vortex Oscillations in Metallic Nanocontacts
Physical Review Letters, 2008
We present experimental evidence of sub-GHz spin-transfer oscillations in metallic nano-contacts that are due to the translational motion of a magnetic vortex. The vortex is shown to execute large-amplitude orbital motion outside the contact region. Good agreement with analytical theory and micromagnetics simulations is found.
Applied Physics Letters, 2010
Vortex oscillations induced by dc currents (Idc) through a metallic nanocontact subject to in-plane magnetic fields (Hext) are studied by measuring voltage power spectra. Two oscillations modes exist: at large Idc the oscillation frequency (fosc) is substantially insensitive to Hext, whereas at low Idc, fosc decreases with Hext increasing. At intermediate Idc the two modes coexist. This behavior is ascribed to the magnetic states of the device ferromagnetic layers: in the first mode vortices are formed in both layers while in the second mode one layer is in a vortex state while the other is in a quasiuniform state.
Magnetic vortex driven by non-uniform injection of spin-polarized current in nano-scale spin valves
Journal of Magnetism and Magnetic Materials, 2009
We demonstrate that a current pulse of a non-uniform spin-polarized current density in a nanomagnet can drive, apart from magnetization reversal a static magnetic vortex. This vortex configuration can be achieved in low shape anisotropy spin valves of elliptical cross-sectional area. These non-uniform configurations exist also in presence of either ion mill damages below the nano-aperture or thermal effects at low temperature. We performed a numerical experiment of spin-torque driven ferromagnetic resonance in a magnetic vortex configuration, our results predict a frequency response with a few maxima and minima related to small oscillation of the vortex state around its equilibrium configuration.
Influence of geometry on current-driven vortex oscillations in nanocontact devices
2011
We present a computational study of current-driven vortex dynamics in a particular geometry, a hybrid Co/Au/Py nanocontact, in which the Co layer is not flat. The experimental measurements validate the numerical results. We identify the Py layer as dynamically active. The nonuniform magnetization configuration in the Co layer, which acts as spin polarizer, and the interlayer magnetostatic stray field, both of which are mostly determined by geometry, are shown to have crucial influence on the dynamic properties of the system. The frequency as a function of current at zero field and also as a function of an out-of-plane field for a fixed current are computed. An excellent quantitative agreement with experimental data is obtained, demonstrating a novel approach for tailoring vortex nano-oscillators.
Spin-torque driven magnetic vortex self-oscillations in perpendicular magnetic fields
Applied Physics Letters, 2010
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 "blueshifting" of the frequency, while the magnetization of the thinner nanomagnet is nonuniform 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.
Nanocontact size dependence of the properties of vortex-based spin torque oscillators
physica status solidi (b), 2011
We study the frequency, linewidth, and power of spin torque driven vortex oscillators, based on a nanocontacted spin-valve (SV). The oscillation frequency strongly decreases with the contact size, and increases with the current. The power delivered by the oscillator is not quadratic with the current, in contrast with the behavior expected from the rigid vortex model (RVM). The linewidth is almost independent of the current at low current and does not strongly depend on the nanocontact size. We compare our findings with the outcomes of the RVM.
Vortex nucleation in spin-torque nanocontact oscillators
Applied Physics Letters, 2010
We present an experimental study of the nucleation rate associated with current-driven vortex oscillations in magnetic nanocontacts. We find that the nucleation and subsequent steady-state oscillation of a vortex can be initiated using current pulses as short as a few nanoseconds, yielding instant on oscillator capability. The nucleation rate is governed by an Arrhenius law, with an exponential dependence on the applied current magnitude. The mechanism for the vortex nucleation is discussed and compared to analytical estimates assuming the transient presence of a vortex-antivortex pair.
Agility of vortex-based nanocontact spin torque oscillators
Applied Physics Letters, 2009
We study the agility of current-tunable oscillators based on a magnetic vortex orbiting around a point contact in spin-valves. Theory predicts frequency-tuning by currents occurs at constant orbital radius, so an exceptional agility is anticipated. To test this, we have inserted an oscillator in a microwave interferometer to apply abrupt current variations while time resolving its emission. Using frequency shift keying, we show that the oscillator can switch between two stabilized frequencies differing by 25% in less than ten periods. With a wide frequency tunability and a good agility, such oscillators possess desirable figures of merit for modulation-based rf applications. PACS numbers: 75.75.+a, 72.25.Pn, In the context of spin torque oscillators, metallic nanocontacts on spin-valves represent an important system in which both spin wave radiation and large-amplitude excitations of magnetic vortices are possible. Indeed for certain nanocontact sizes and multilayer configurations, the Oersted field associated with the current can nucleate a magnetic vortex, which is subsequently set into steady-state rotation about the nanocontact by competing damping and spin-torques . A stable oscillation of the point contact resistance is generated at 100-500 MHz as the vortex orbits around the magneto-resistive contact, which opens up possible applications as compact rf oscillators [10] operating at zero field [8] and allowing multi-octave frequency coverage. From an rf application perspective, the frequency tunability of such vortex based spin torque oscillators is outstanding since the free running frequency can be varied by a factor of more than three . However many rf applications require modulation schemes. The oscillator agility -the rate at which the frequency can be effectively tuned -is therefore an essential figure of merit that needs to be measured and understood.
Vortex Nucleation Phase in Spin Torque Oscillators Based on Nanocontacts
IEEE Transactions on Magnetics, 2011
We study the starting up phase of a current-controlled oscillator based on a magnetic vortex orbiting around a nanocontact in a spin-valve. From the idle state, current pulses down to a few nanoseconds can create the vortex, which is detected through the electrical signature of its steady-state gyration. Two ns are needed to reach the in-current equilibrium. The process can then be described by an Arrhenius law, with an activation energy that is consistent with the Oersted-field-induced separation of a vortex-antivortex pair. Requirements for deterministic nucleation are deduced, with prospects for instant-on oscillator capability.