This presentation will describe unexpected magnetic properties observed in artificially nano-structured materials. Three different examples will be described. Two-dimensional arrays of nano-dots of Fe can be fabricated with controlled density and size. They exhibit "superferromagnetic" behavior instead of the expected superparamagnetic ordering. Stacked arrays of these dots show the transition from ferromagnetism to a pseudo spin-glass accompanying the d-dimensional to 3-dimensional cross over. Oscillatory coupling of magnetic dot arrays on both surfaces of a Cu wedge will also be described. Second, wires of transition-metal oxide films reveal discrete jumps in the phase transition order parameter due to dimensional confinement. The final example illustrates the use of "subsurfactant" epitaxy growth to generated a dilute magnetic semiconductor with ~0.25% interstitial Mn in Ge. The ferromagnetic ordering temperature is TC > 440K. All of these examples can’t be explained with conventional pictures of magnetic coupling.
Research at ORNL sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy, and by the National Science Foundation. UT Research supported by NSF and DOE (DMS&E) (NSF-DMR-0451163).
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