The Magnetics Group's program in magnetodynamics and spin electronics develops new measurement techniques to characterize the high frequency properties and performance of nanomagnetic structures and devices. Hard-disk drives in personal computers continue to get smaller, faster, and cheaper. These magnetic devices push the limits of technology, with current data bit densities of 50 billion per square centimeter. The magnetic orientation of thin-films in write heads, read heads, and recording media all switch extremely fast. The Nanomagnetics Program investigates the high frequency behavior of nanoscale magnetic materials and devices. The switching of magnetization at frequencies in the hundreds of megahertz to hundreds of gigahertz will be the foundation for future magnetic data storage systems and microwave integrated circuits. These technologies will depend on newly discovered properties and limitations of magnetic materials and devices that appear only at the nanoscale.
Figure 2. Simulation showing spin-wave
interaction between two spin-transfer oscillators. Focused ion beam cut breaks coupling. Oscillators phase-lock without the cut. The scanning electron micrograph shows actual device.
Future computer hard disk drives are likely to require patterned media composed of uniform, perpendicularly magnetized nanodots instead of present-day continuous magnetic films. EEEL has demonstrated that the magnetic fields required to switch the magnetization of nanodots is critically dependent on the quality of their edges. EEEL is developing new, highly sensitive, magneto-optic instruments to measure the dynamics of magnetic nanodots as a function of frequency, with the goal of evaluating nanodot quality and homogeneity via a rapid spectroscopic analysis of individual nanodots.
Spintronics exploits the interaction between electrons’ spin angular momentum and the magnetization of a film. EEEL developed the first microwave spin-transfer nano-oscillators based on point current contacts to planar films. These oscillators have very sharp resonant frequencies. EEEL showed how signals emitted by multiple nano-oscillators will naturally synchronize, coherently combining their outputs and stabilizing their oscillation frequency. EEEL explained the mechanism responsible for nano-oscillator coupling and solved a potential barrier to widespread application by demonstrating how oscillations can be made to occur in zero applied magnetic field.
In addition to oscillators, electron spin torque may be used to switch future nonvolatile magnetic random-access memory (MRAM) elements. Compared to switching memory bits with magnetic fields, this method would offer higher speed, greater reliability, lower power, and is scalable to smaller device dimensions. EEEL is developing methods to measure the switching behavior of prototype spin-MRAM.
Imperfections in magnetoresistive sensors give rise to random magnetic fluctuations, which limit their sensitivity. EEEL has obtained evidence of these stochastic processes in the form of electron microscope images of magnetic domain fluctuations in active devices. EEEL has fabricated improved magnetic tunnel-junction sensors by annealing them in high fields in a reducing environment and by using magnetic flux concentrators.
Figure 3. Simulation of magnetization for a
176 nanometer by 60 nanometer device. Colors represent the average x-axis magnetization (red positive, blue negative). Magnetic oscillations grow until device switches.
- Proved that magnetic reversal in perpendicularly magnetized nanostructures is highly dependent on the nature and condition of the edges.
- Magnetic reversal in perpendicularly magnetized nanostructures is highly dependent on the nature and condition of the edges.
- Demonstrated that spin-transfer-driven oscillations in nanocontacts made to spin-valve structures can occur in zero field, which will enable applications such as on-chip timing and signal processing.
- Demonstrated mutual phase-locking of microwave spin-torque nano-oscillators. Spin waves, rather than magnetic fields, are the primary interaction mechanism.
- Identified sources of 1/f noise and time dependent nanoscale fluctuations in magnetic films. Imaged magnetic fluctuations in active magnetic sensors.
- T. J. Silva, W. H. Rippard, “Developments in Nano-Oscillators Based Upon Spin-Transfer Point-Contact Devices,” Journal of Magnetism and Magnetic Materials, vol. 320, pp. 1260-1271, April 2008.
- W. H. Rippard, M. R. Pufall, “Microwave Generation in Magnetic Multilayers and Nanostructures,” in Handbook of Magnetism and Advanced Magnetic Materials, John Wiley (Sussex, U.K.), September 2007.
- R. Heindl, S. E. Russek, T. J. Silva, W. H. Rippard, J. A. Katine, M. J. Carey, “Size Dependence of Intrinsic Spin Transfer Switching Current Density in Elliptical Spin Valves,” Applied Physics Letters, vol. 92, 262504, June 2008.
- J. M. Shaw, S. E. Russek, T. Thomson, M. J. Donahue, B. D. Terris, O. Hellwig, E. Dobisz, M. L. Schneider, “Reversal Mechanisms in Perpendicularly Magnetized Nanostructures,” Physical Review B, vol. 78, 024414, July 2008.
Figure 1. Schematic of a model used to include nanodot edge damage in micromagnetic simulations. Bottom graph: Diameter dependence of the switching field from simulations for various values of edge damage.
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