Agility of vortex-based nanocontact spin torque oscillators (original) (raw)
2009, Applied Physics Letters
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.
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