Fred Mancoff

Direct observation of a propagating spin wave induced by spin-transfer torque

Spin torque oscillators with nanoscale electrical contacts are able to produce coherent spin waves in extended magnetic films, and offer an attractive combination of electrical and magnetic field control, broadband operation, fast spin-wave frequency modulation, and the possibility of synchronizing multiple spin-wave injection sites. However, many potential applications rely on propagating (as opposed to localized) spin waves, and direct evidence for propagation has been lacking. Here, we directly observe a propagating spin wave launched from a spin torque oscillator with a nanoscale electrical contact into an extended Permalloy (nickel iron) film through the spin transfer torque effect. The data, obtained by wave-vector-resolved micro-focused Brillouin light scattering, show that spin waves with tunable frequencies can propagate for several micrometres. Micromagnetic simulations provide the theoretical support to quantitatively reproduce the results.

Spin torque oscillator frequency versus magnetic field angle: The prospect of operation beyond 65 GHz

We study the impact of the magnetic field angle on the oscillation frequency of a nanocontact spin torque oscillator (STO) in magnetic fields up to 2.1 T. A model based on a single nonlinear, nonpr ...

Experimental evidence of self-localized and propagating spin wave modes in obliquely magnetized current-driven nanocontacts.

Through detailed experimental studies of the angular dependence of spin wave excitations in nanocontact-based spin-torque oscillators, we demonstrate that two distinct spin wave modes can be excited, with different frequency, threshold currents, and frequency tunability. Using analytical theory and micromagnetic simulations we identify one mode as an exchange-dominated propagating spin wave, and the other as a self-localized nonlinear spin wave bullet. Wavelet-based analysis of the simulations indicates that the apparent simultaneous excitation of both modes results from rapid mode hopping induced by the Oersted field.

Nonlinear frequency and amplitude modulation of a nanocontact-based spin-torque oscillator

We study the current-controlled modulation of a nanocontact spin-torque oscillator. Three principally different cases of frequency nonlinearity (d(2)f/dI(dc)(2) being zero, positive, and negative) ...

Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators

The integrated power and linewidth of a propagating and a self-localized spin-wave mode excited by spin-polarized current in an obliquely magnetized magnetic nanocontact are studied experimentally ...

Frequency modulation of spin torque oscillator pairs

The current controlled modulation of nanocontact based spin torque oscillator (STO) pairs is studied in both the synchronized and nonsynchronized states. The synchronized state shows a well behaved modulation and demonstrates robust mutual locking even under strong modulation. The power distribution of the modulation sidebands can be quantitatively described by assuming a single oscillator model. However, in the nonsynchronized state, the modulation sidebands are not well described by the model, indicating interactions between the two individual nanocontact STOs. These findings are promising for potential applications requiring the modulation of large synchronized STO arrays.

Spin-torque oscillator linewidth narrowing under current modulation

We study the behavior of the linewidth of a nanocontact based spin torque oscillator (STO) under application of a radio frequency (100 MHz) modulating current. We achieve a significant (up to 85%) reduction in the STO linewidth when it is modulated across a region of high nonlinearity. The mechanism responsible for the linewidth reduction is the nonlinear frequency shift under the influence of current modulation, which reduces the nonlinear amplification of the linewidth. The reduction in the linewidth during modulation can be quantitatively calculated from the free-running behavior of the STO.

Spin Torque Oscillators and RF Currents—Modulation, Locking, and Ringing

We study the interaction between a nano-contact spin torque oscillator (STO) and injected radio-frequency and microwave currents. Modulation of the STO signal is observed over a wide frequency range from 100 MHz to 3.2 GHz. The modulation sidebands agree well with macrospin simulations. When the injected microwave frequency approaches that of the STO, we observe injection locking, frequency pulling/pushing, and intermodulation peaks. While the intermodulation peaks are reasonably well reproduced by macrospin simulations, they do not follow the Adlers model. We argue that this discrepancy is due to intrinsic ringing effects stemming from the internal dynamics of the STO.

Modulation of Individual and Mutually Synchronized Nanocontact-Based Spin Torque Oscillators

We study current modulation of spin torque oscillators (STOs) based on single and double nano-contacts. For single nano-contact devices, we show modulation for a carrier frequency of 19-40 GHz and modulation frequencies from 50 MHz to 1 GHz. We show that the behavior of modulation and the asymmetry in the power of the modulation sidebands can be very well modeled, and predicted, using nonlinear frequency and amplitude modulation (NFAM) calculations based on the free-running STO parameters. Modulation of synchronized double nano-contact STOs is equally well modeled by NFAM theory and demonstrates the feasibility of communication applications based on arrays of synchronized STOs.

Modulation of single and double spin torque oscillators

We discuss modulation of metallic spin torque oscillators (STOs) based on single and double nano‐contacts. The modulation behavior of both types of devices is characterized by equally spaced multiple sidebands, which can be ascribed mostly to a frequency modulation phenomenon. However, quantitative examination reveals that amplitude modulation is also present in both types of devices. We also show the feasibility of modulating the synchronized state of a double nano‐contact STO. The synchronized state can be treated as a single oscillator, which is a very promising feature for future applications of STO arrays.z