Spin-transfer Nano-oscillators Research Articles

Tailoring magnetic vortices in nanostructures

Tailoring the properties of magnetic vortices through the preparation of structured multilayers is discussed. The dependence of the vortex core radius r on the effective anisotropy is derived within a simple model, which agrees with our simulations. As the perpendicular anisotropy increases, r also increases until a perpendicular magnetization appears in the disk rim. Co/Pt multilayer disks were studied; x-ray microscopy confirms qualitatively the predicted behavior. This is a favorable system for implementing vortex-based spin-transfer nano-oscillator devices, with enhanced rf power resulting both from the increase in the core size and synchronization afforded by the coupling of the Co layers.

Enhancement of Perpendicular Anisotropy of Co/Cu Multilayer Nanowires by Phase Doping

A multisegment structure of Co/Cu nanowires with tunable perpendicular anisotropy, associated with phase transformation, was fabricated. The mixed crystal structure, formed by doping nano grains of the hcp phase into the fcc phase in the cobalt structure, markedly improves the perpendicular anisotropy. Cobalt nano grains with the hcp phase with a preferred orientation, which were transformed from (100) to (002), by doping fcc nano grain with high preferred (111) orientation and additionally the effective magnetocrystalline energy density of mixed structure was increased, the perpendicular magnetic anisotropy can be enhanced and tunable. The magnetoresistance ratios along the parallel and perpendicular axes of nanowires are 23.4% and 7.2% respectively. This novel approach promises to be of strong interest for subsequent fabrication of phase-locked arrays of spin transfer nano-oscillators with increased output power for microwave applications.

Theory of Injection Locking for Large Magnetization Motion in Spin-Transfer Nano-Oscillators

We study magnetization dynamics in spin-transfer devices subject to DC and microwave injected currents. When the frequency of the injected current is sufficiently close to the self-oscillation frequency of the device, phase-locking occurs. This phenomenon is theoretically studied by using Landau-Lifshitz equation with Slonczewski spin-torque term. By exploiting separation of time scales and using averaging technique, we derive equations which are applicable to the study of phase-locking for arbitrary large magnetization motion. The stability diagram in the (detuning, ac current)-plane is determined and it is shown that phase locking is hysteretic at sufficiently large ac currents.

Advanced Metrology for Nanoelectronics at the National Institute of Standards and Technology

A broad range of programs at the National Institute of Standards and Technology address critical metrology and characterization challenges facing the nanoelectronics industry. A brief history of the program will be included. From these programs we will describe some exemplary projects developing new measurement and characterization techniques for nanodevices. These will include, among others, the use of helium ion microscope, spin-transfer microwave nano-oscillators, characterization of graphene, advanced atomic force microscope, and development of a microcalorimeter-based high resolution X-ray analyzer.

Coupling Efficiency for Phase Locking of a Spin Transfer Nano-Oscillator to a Microwave Current

The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The predicted trends on the coupling strength for phase locking as a function of intrinsic features of the oscillators, i.e., power, linewidth, agility in current, are central to optimize the emitted power in arrays of mutually coupled STNOs.

Impact of the electrical connection of spin transfer nano-oscillators on their synchronization: an analytical study

We analytically study the impact of an electrical connection of spin transfer nano-oscillators (STNOs) on their synchronization. We demonstrate that the phase dynamics of coupled STNO arrays can be described in the framework of the Kuramoto model. The conditions for successful synchronization of an assembly of STNOs are formulated. Synchronizing an assembly of STNOs appears to be the only solution to make the breakthrough on the emitted output power toward frequency synthesizers. In these potential devices, a large number of STNOs will have to be electrically connected, whatever the coupling mechanisms between oscillators.

Current-dependent linewidth of a spin-transfer nano-oscillator

Power spectra of a spin-transfer nano-oscillator, in which a reflection layer is included, are measured for current densities up to 1.2×10 7 A/cm 2. A clear oscillation peak, which is slightly asymmetric with respect to the peak frequency, is observed at 5 GHz under the external field of 700 Oe. The large red shift of the peak frequency on increasing the current, which has been reported in nano-contact devices, is not observed. The relatively low oscillation frequencies as well as the absence of the large red shift indicate the reduction in the demagnetizing field in the free layer. The linewidth decreases with the current and is inversely proportional to the square of the precession amplitude of magnetization in accordance with the recent linear theories.

Template-Grown NiFe/Cu/NiFe Nanowires for Spin Transfer Devices

We have developed a new reliable method combining template synthesis and nanolithography-based contacting technique to elaborate current perpendicular-to-plane giant magnetoresistance spin valve nanowires, which are very promising for the exploration of electrical spin transfer phenomena. The method allows the electrical connection of one single nanowire in a large assembly of wires embedded in anodic porous alumina supported on Si substrate with diameters and periodicities to be controllable to a large extent. Both magnetic excitations and switching phenomena driven by a spin-polarized current were clearly demonstrated in our electrodeposited NiFe/Cu/ NiFe trilayer nanowires. This novel approach promises to be of strong interest for subsequent fabrication of phase-locked arrays of spin transfer nano-oscillators with increased output power for microwave applications.

Phase and amplitude noise spectra in spin-transfer nano-oscillators

Analytical expressions of the phase and the amplitude noise spectra under the thermal fluctuation fields (i=x, y, z) are derived for a simplified model of spin-transfer oscillators within a small amplitude approximation. The line broadening caused by the transverse fluctuations hxT and hyT is calculated by using the linear phase-noise theories developed for electronic oscillators and that caused by the longitudinal fluctuation hzT is treated as a random walk problem well known in the theory of magnetic resonance. It is found that the linewidth caused by hxT and hyT is proportional to (αT∕MV)(1∕A2), where T is the temperature, α, M, and V are the Gilbert damping parameter, the saturation magnetization, and the volume of the free layer, respectively, and A is the amplitude of oscillation. On the other hand, the width caused by hzT is independent of the amplitude and is simply proportional to αT∕MV.

Analytical expression of output power spectra of spin-transfer nano-oscillators

Analytical expressions of the output power spectra are derived for spin-transfer nano-oscillators, by using the phase-noise theories, which have been developed for electronic oscillators. The output spectra, which are proportional to the power spectra of transverse component of magnetization, are calculated under the thermal fluctuation with a small amplitude approximation.

Electrical Measurement of Spin-Wave Interactions of Proximate Spin Transfer Nanooscillators

We have investigated the interaction mechanism between two nanocontact spin transfer oscillators made on the same magnetic spin valve multilayer. The oscillators phase lock when their precession frequencies are made similar, and a giant magnetoresistance signal is detectable at one contact due to precession at the other. Cutting the magnetic mesa between the contacts with a focused-ion beam modifies the contact outputs, eliminates the phase locking, and strongly attenuates the magnetoresistance coupling, which indicates that spin waves rather than magnetic fields are the primary interaction mechanism.

Injection Locking and Phase Control of Spin Transfer Nano-oscillators

We have directly measured phase locking of spin transfer oscillators to an injected ac current. The oscillators lock to signals up to several hundred megahertz away from their natural oscillation frequencies, depending on the relative strength of the input. As the dc current varies over the locking range, time-domain measurements show that the phase of the spin transfer oscillations varies over a range of approximately +/-90 degrees relative to the input. This is in good agreement with general theoretical analysis of injection locking of nonlinear oscillators.