Colorado State University Nano Ferromagnetic Materials for Advanced Battlefield Communications
If one could pole a ferromagnet with an electric field, it would transform the logic and memory technologies used in today’s computer technology. Much for that reason, materials in which a magnetic property can be tuned with an electric field have been sought for many years. This ‘magneto-electric’ coupling is weak, however, and a material or an approach that yields adequate coupling is yet to be discovered. A related magneto-electric effect involves coupling between the precession rather than the flipping of spins in a ferromagnet and an applied electric field. This latter effect is the subject of an accomplishment by Drs. Zbigniew Celinski, Carl Patton and Mingzhong Wu, Colorado State University
In iron and other ferromagnetic materials, the magnetic moments (spin) of the electrons resonate, precessing around the easy axis, when they are exposed to electromagnetic radiation. This resonance, called the ferromagnetic resonance (FMR), can range from 1 MHz to nearly 100 GHz, depending on the material and the device geometry. The FMR of a thin strip of magnetic material can be tuned by changing its physical dimensions or by applying an external magnetic field. Because of the impracticality of applying variable magnetic fields, tuning this resonance with an electric field is tantalizing.
A common solution for electrically-tunable magnetic devices is the juxtaposition of an (electrically-tunable) non-magnetic ferroelectric material with a ferromagnetic material. In such a case, the electric field modifies the dielectric environment of the ferromagnet, affecting the FMR. A difficulty that hindered this approach for many years is that the two materials must be placed close enough together to allow the necessary coupling, with an intervening electrode to provide the electric field. Previous attempts sought to mechanically bond these two components, which limited the necessary coupling.
Under the U.S. Army Research Office Multi-disciplinary University Research Initiative (MURI) project titled, “Giga-Hertz Electromagnetic Wave Science and Devices for Advanced Battlefield Communications,” and led by Professor Zbigniew Celinski, Co-PIs Drs. Carl Patton and Mingzhong Wu at the Colorado State University developed improvements to this approach. The MURI-sponsored researchers recently published their novel approach, which involved the direct deposition of the ferromagnetic and ferroelectric components in a “stack.” As illustrated below, this material is a “sandwich” of ferrimagnetic yttrium iron garnet (YIG), ferroelectric barium strontium titanate (BST), and platinum (Pt) electrode layers, formed by pulsed laser deposition with very high quality layers and interfaces. (See figure above)
Prof. Pamir Alpay at the University of Connecticut, in collaboration with Dr. Cliff Hubbard at the Army Research Laboratory, have developed highly tunable, temperature insensitive polar dielectric films for use in the Joint Tactical Radio System (JTRS) Manpack and Handheld Radio.
By performing a combination of theoretical simulations and experimental studies they were able to determine the optimum parameters for preparing tunable active ferroelectric films that would have low loss and a temperature insensitive tunability from -10 to 90oC. A trilayer heterostructure consisting of three distinct layers of ~220 nm nominal thickness and compositions corresponding to BST 60/40, BST 75/25, and BST 90/10 was identified as the best solution. At room temperature, this heterostructure has a dielectric permittivity of 360, a dissipation factor of 0.012, a dielectric tunability of 65% at 444 kV/cm, and minimal dispersion as a function of temperature ranging from 90 to -10oC, fully satisfying the specifications required for the Joint Tactical Radio System.
Both research accomplishments are noted in 2009 ARO in Review, a U.S. Army Research Office publication of on-going research for military and civilian applications.
