Shehzaad Kaka

Mutual phase-locking of microwave spin torque nano-oscillators

The spin torque effect that occurs in nanometre-scale magnetic multilayer devices can be used to generate steady-state microwave signals in response to a d.c. electrical current. This establishes a new functionality for magneto-electronic structures that are more commonly used as magnetic field sensors and magnetic memory elements. The microwave power emitted from a single spin torque nano-oscillator (STNO) is at present typically less than 1 nW. To achieve a more useful power level (on the order of microwatts), a device could consist of an array of phase coherent STNOs, in a manner analogous to arrays of Josephson junctions and larger semiconductor oscillators. Here we show that two STNOs in close proximity mutually phase-lock—that is, they synchronize, which is a general tendency of interacting nonlinear oscillator systems. The phase-locked state is distinct, characterized by a sudden narrowing of signal linewidth and an increase in power due to the coherence of the individual oscillators. Arrays of phase-locked STNOs could be used as nanometre-scale reference oscillators. Furthermore, phase control of array elements (phased array) could lead to nanometre-scale directional transmitters and receivers for wireless communications.

Direct-current induced dynamics in Co90Fe10/Ni80Fe20 point contacts

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precession frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz. This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800. We compare our results with those from single-domain simulations of current-induced dynamics.

Direct-Current Induced Dynamics inCo90Fe10/Ni80Fe20Point Contacts

We have directly measured coherent high-frequency magnetization dynamics in ferromagnetic films induced by a spin-polarized dc current. The precession frequency can be tuned over a range of several gigahertz by varying the applied current. The frequencies of excitation also vary with applied field, resulting in a microwave oscillator that can be tuned from below 5 to above 40 GHz. This novel method of inducing high-frequency dynamics yields oscillations having quality factors from 200 to 800. We compare our results with those from single-domain simulations of current-induced dynamics.

Frequency modulation of spin-transfer oscillators

Spin-polarized dc electric current flowing into a magnetic layer can induce precession of the magnetization at a frequency that depends on current. We show that addition of an ac current to this dc bias current results in a frequency modulated (FM) spectral output, generating sidebands spaced at the modulation frequency. The sideband amplitudes and shift of the center frequency with drive amplitude are in good agreement with a nonlinear FM model that takes into account the nonlinear frequency-current relation generally induced by spin transfer. Single-domain simulations show that ac current modulates the cone angle of the magnetization precession, in turn modulating the frequency via the demagnetizing field. These results are promising for communications and signal processing applications of spin-transfer oscillators.

Precessional switching of submicrometer spin valves

Small spin valves are switched using a subnanosecond field pulse applied along the magnetization hard axis. The measured probability for switching due to pulsed hard-axis fields increases as the duration is decreased from 325 to 230 ps. This indicates a large-angle precessional motion in response to the transverse applied field. The behavior is modeled with a single-domain, Landau–Lifshitz simulation. Switching in this manner requires only single-polarity transverse pulses that toggle the state of magnetic devices. This process consumes less energy than traditional quasi-static switching using long-duration field pulses along both directions.

High-speed dynamics, damping, and relaxation times in submicrometer spin-valve devices

The dynamical response of spin-valve devices with linewidths of 0.8 μm has been measured after excitation with 160 ps magnetic impulses. The devices show resonant frequencies of 2–4 GHz which determine the upper limit of their operation frequency. The dynamical response can be fit with Landau–Lifshitz models to extract an effective uniform-mode damping constant, αum. The measured values of αum were between 0.04 and 0.01 depending on the magnitude of the longitudinal bias field. The appropriate damping coefficient for use in micromagnetic modeling, αmm, was extracted from the dynamical response with large longitudinal bias field. This value was used to model the switching of a 0.1 μm×1.0 μm magnetoresistive random access memory cell. The micromagnetic model included shape disorder that is expected to be found in real devices. The simulations showed that, while the magnetization reverses rapidly (<0.5 ns), it took several nanoseconds for the energy to be removed from the magnetic system. The switching energ...

High-Speed Characterization of Submicrometer Giant Magnetoresistive Devices

A microwave test structure has been designed to measure the high-speed response of giant magnetoresistive (GMR) devices. The test structure uses microwave transmission lines for both writing and sensing the devices. Pseudo-spin-valve devices, with line widths between 0.4 and 0.8 μm, were successfully switched with pulses whose full width at half-maximum was 0.5 ns. For small pulse widths τpw the switching fields are observed to increase linearly with 1/τpw. The increase in switching fields at short pulse widths is characterized by a slope which, for the current devices, varies between 4 and 16 μA s/m (50–200 Oe ns). The magnetoresistive response during rotation and switching was observed. For small rotations (∼45° between layer magnetizations) the GMR response pulses had widths of 0.46 ns, which is at the bandwidth limit of our electronics. For larger rotations (∼90°) the response pulses broadened considerably as the magnetic layers were rotated near the unstable equilibrium point perpendicular to the dev...

Coherent control of nanomagnet dynamics via ultrafast spin torque pulses

We demonstrate reliable manipulation of the magnetization dynamics of a precessing nanomagnet by precisely controlling the spin transfer torque on the subnanosecond time scale. Using a simple pulse shaping scheme consisting of two ultrafast spin torque pulses with variable amplitudes and delay, we demonstrate coherent control over the precessional orbits and the ability to tune the switching probability of a nanomagnet at room temperature and 77 K. Our measurements suggest that appropriately shaped spin transfer can be used to efficiently manipulate the orientation of a free layer nanomagnet, thus providing an alternative for spin torque driven spintronic devices.

Time and frequency domain measurements of ferromagnetic resonance in small spin-valve

Time and frequency domain magnetoresistance measurements of ferromagnetic resonance (FMR) in small spin-valve devices are presented along with comparisons to single-domain simulations. The measurements and simulations give consistent results for rotational motion with angular deviations of less than 30/spl deg/ from the easy axis. While the time and frequency domain measurements produce similar results fur ideal devices in the linear regime, each technique provides different information as the devices become nonlinear and less ideal. Both time and frequency domain magnetoresistance measurements allow the study of FMR in considerably smaller magnetic structures than can be done with conventional techniques.

High Speed Switching and Rotational Dynamics in Small Magnetic Thin Film Devices

The intent of this chapter is to review high-frequency magnetic device measurements and modeling work at NIST which is being conducted to support the development of high-speed read sensors, magnetic random access memory, and magnetoelectronic applications (such as isolators and microwaves components). The chapter will concentrate on magnetoresistive devices, those devices whose resistance is a function of the magnetic state of the device, which can in turn be controlled by a magnetic field. The low-frequency characteristics of magnetoresistive devices will be reviewed. Simulated high-frequency device dynamics, using single-domain and micromagnetic models, will be discussed. Next, high-speed measurements of magnetization rotation and switching in micrometer-size devices will be presented. The effects of thermal fluctuations and disorder on device dynamics will be examined, and high-frequency magnetic noise data will be presented. Finally, the need to understand and control high-frequency magnetic damping will be discussed, and a method for engineering high-frequency magnetization damping using rare-earth doping will be presented.