Chaotic Spin Waves and Applications
Recently chaotic signals have been proposed as broadband information carriers with the potential of providing a high level of robustness and privacy in data transmission. We have demonstrated the self-generation of chaotic spin waves in magnetic yttrium iron garnet (YIG) thin film-based active feedback rings. The chaotic excitation was realized through two different nonlinear processes, three-wave interactions and four-wave interactions. The chaotic signals manifested themselves in non-periodic waveforms, broadband power-frequency spectra, positive Lyapunov components and finite correlation dimensions. The dimension of the signals was controllable through the ring gain coefficient. This feature is essential from a practical point of view. Left figure shows the correlation plots for one chaotic signal, where one sees a clear saturation of the slope of the plot as the embedding dimension m is increased from 2 to 20. Such a saturation response is a key signature of deterministic chaos. Right figure shows a picture of a prototype YIG film-based chaotic oscillator which we fabricated in June, 2009.
Spin Wave Envelope Solitons and Applications
Solitons are localized large-amplitude pulse excitations in nonlinear dispersive systems that can travel without a change in shape and survive collisions. Spin waves in magnetic film systems provide a powerful and versatile 'test bed' for the study of fundamental soliton dynamics. Taking the advantage of the spin waves, we discovered many intriguing phenomena involving solitons, including soliton fractals in feedback rings, exact and periodic Fermi-Pasta-Ulam (FPU) recurrence, random formation of coherent solitons from incoherent waves, soliton formation through coupled modulational instability, and the formation of dark solitons through spontaneous modulational instability.
Magnetic Millimeter Wave Devices
Microwave magnetic devices have had a major impact on the development of microwave technology. At present, there is a critical need for the extension of current microwave magnetic device physics and technology into the millimeter (mm) wave range. This need is critical for two reasons. (1) Millimeter waves are recognized as a broadband frequency resource for wireless links. (2) Electromagnetic radiation at mm-wave frequencies can penetrate clouds, fog, and many kinds of smoke, all of which are generally opaque to infrared or visible light.
One strategy to address this need is to use low-loss hexagonal ferrites. The hexagonal ferrites have built-in high anisotropy fields and, therefore, can provide a self-biasing for mm-wave applications in the 30-100 GHz range. Recently we have demonstrated a BaFe12O19 (BaM) slab-based, stripline-type, mm-wave band-stop filter. The band-stop filtering response originates from the ferromagnetic resonance-induced power absorption in the BaM slab. The BaM slab has an in-plane uniaxial anisotropy field of 17 kOe. This built-in high anisotropy field facilitates the operation of the filter beyond 50 GHz without a need of high external fields. Left figure shows the device structure and field configuration. Right figure shows representative transmission profiles for different bias fields, as indicated.
Microwave-Assisted Magnetization Reversal
In the presence of microwaves, magnetization reversal or switching in magnetic materials can take place at significantly reduced switching fields. This effect is called microwave-assisted magnetization reversal (MAMR). The physical mechanism of this effect is that a microwave magnetic field can excite large-angle magnetization precessions which can serve to reduce the magnetic field needed for the magnetization to switch. The MAMR effect has potential applications in high-density hard disks, magnetic random access memory, and microwave devices. We have demonstrated the MAMR responses in a number of different magnetic elements, including Permalloy nano dots and large-damping FeCo thin films.