The objective of this research is to design, fabricate and test microwave circulators that operate in the band between 5 and 50 GHz utilizing magnetostatic waves as the dominant energy transport mechanism. This will allow total device sizes of 1 µm or less for competitive integration with standard microelectronic processes. The approach is threefold 1) a simplified numerical model (i.e., Green's function or integral equation) will be developed 2) new materials processing techniques will be created for low conductivity dilute magnetic semiconductor materials and 3) dc bias circuitry will be fabricated. Intellectual Merit The proposed activity aims to dramatically change microwave ferrite circulator design approaches as well as communication/RADAR system functionality. Magnetostatic Wave phenomena will significantly change perspectives on nonreciprocal device operation and promises the extraordinary outcome of expanded bandwidths in smaller structures than are currently employed. In addition to these explicit outcomes, the proposed research is expected to provide glimpses of new applications and phenomena associated with magnetoelectric semiconducting effects of dilute magnetic semiconductor materials; new theories, models and devices are expected. Broader Impacts The proposed activities integrate magnetostatic wave phenomena and dilute magnetic semiconductor material fabrication discoveries with significant teaching, training and learning activities. In particular, underrepresented undergraduate and graduate students will participate in the proposed research wherever possible. Also, K-12 students will be included via presentations, workshops and laboratory tours. Results are planned for dissemination in top-tier peer-reviewed journals; intermediate results will be presented to the international community through conferences and symposia.
The overall goal of this work was to develop microwave circulators that employ a unique phenomena called magnetostatic waves (or spin waves). These waves operate based on interaction between a radio frequency magnetic field and the direction of magnetic dipoles within a crystal lattice structure. Such waves were demonstrated in the 1950s, and have been experimentally verified extensively since. The key feature that we wish to exploit is the very small wavelength that is seen in these waves, as compared to that of electromagnetic waves. Specifically, the wavelength of magnetostatic waves depends on the atomic spacing and can be as small as nanometers, even at microwave frequencies. The contributions of this research may be separated into the broad areas of analytical modeling, numerical simulation, and materials fabrication. To pursue the realization of a magnetostatic wave based circulator, or any other device, it is necessary to fully understand the behavior of these waves in the context of specific boundaries. To this end, we developed the theory associated with wave propagation within magnetic materials in guides having both rectangular, and circular, symmetry. When waves propagate within a rectangular waveguide, with conducting boundaries, and magnetic bias transverse to the direction of propagation, the magnetic field of the wave tends to accumulate to one side of the waveguide. However, the electric field is unchanged. Thus a hybrid mode is developed that consists of a TE mode in the electric field, but an approximately TM field in the magnetic field. Such behavior is unique to this type of guiding structure. We have also investigated wave behavior in axially magnetized ferrite rods. We have developed theory that incorporates the boundary conditions with vacuum, and developed the cutoff frequencies associated with waves in this type of guide. In the broad area of numerical modeling, we have developed a Green's function that describes a novel circulator topology, a fully second order, 3D time domain simulation scheme that incorporates dominant mode magnetic materials, as well as a 2D time domain scheme that incorporates electromagnetic as well as magnetostatic wave phenomena. Traditional circulators have circular symmetry, with ports equally spaced, and emanating from a circular disk. It has long been supposed that the center of the ferrite disk plays little role in circulation, but has never been verified. We have developed a Green's function that describes the behavior of a circulator made of an annular ring of ferrite material, and with the option of ports on the inside, as well as the outside of the ferrite material (See Figure 1). This Green's function has led to the understanding that circulation can occur between ports on the outside and inside of the ferrite, but only if the ring is either very thin (Rout is only a little largen than Rin), or the magnetic response of the material is fairly weak (i.e., 4piMs is small). In addition to this computationally efficient trans-impedance Green's function, we have developed a full-wave solver that incorporates dominant-mode magnetic behavior (See Figure 2); this scheme also includes a full error analysis that verifies second order accuracy. Finally, we have developed a 2D simulation scheme that incorporates both electromagnetic and magnetostatic wave behavior. We have successfully reproduced measured results from the open literature using this scheme, and have simulated a first pass at a circulator topology based on magnetostatic wave phenomena. Because of the significant differences in wavelength associated with these two phenomena, and the time-domain nature of the simulation scheme, this routine requires a long simulation time to produce meaningful results. Thus, we have investigated the use of parallel processing (such as multi-core processors as well as graphics processing units) to accelerate simulation time with very promising results. It is a goal of this project to investigate materials fabrication to support the proposed devices. It has been shown that when GaN is heavily doped with Mn, a strong ferromagnetic response is obtained, with very low conductivity. When considering spintronic devices, the low conductivity is problematic, however for circulator design this is a very important benefit. Thus, we have successfully fabricated, and characterized, numerous samples of GaN materials, as well as GaN doped with Mn. These materials have demonstrated very consistent crystal structure with very few impurities. The magnetic response appears to be appropriate for the devices proposed.
