Noise Studies of Disordered Materials

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

This condensed matter physics research applies low frequency noise and aging techniques to several unsolved problems of disordered condensed matter. The primary foci will be mixed-phase colossal magnetoresistive (CMR) materials and relaxor ferroelectrics. Noise provides a sensitive probe of the mixed-phase states involved in much CMR, allowing direct thermodynamic measurements of small regions and revealing the extent to which the current flow can be inhomogeneous. The roles of random substitutional disorder, of net strain, and of anisotropic strain constraints in stabilizing mixed phases will be investigated in several materials, particularly manganites. Multilayer materials with net doping equal to that of conventional solid-solution materials will provide a particularly direct test of the role of quenched randomness. Relaxor ferroelectrics freeze into an overall disordered collection of locally ferroelectric nanodomains, allowing them to retain useful high dielectric and piezoelectric coefficients over a broader temperature range than do conventional ferroelectrics. However, there is little consensus on the mechanism or mechanisms behind this glassy freezing in various relaxors. Noise and aging allow determination of the forms of unconventional, glassy freezing on scales both larger and smaller than those of the nanodomains. A new model, analogous to a reentrant spinglass freezing, is suggested by initial results on some standard relaxors. Comparative studies of several dissimilar relaxor materials will sort out which models are applicable to which categories. The graduate students working on these projects have been able to use both table-top small-science techniques and sophisticated lithography and characterization techniques. They have typically gone on to work in the materials side of the computer industry or in further basic research. Although there are many well-developed techniques for studying regularly ordered crystalline materials, which have formed the basis of the semiconductor industry, many of the materials now under development for use in sensors and other applications are disordered. The techniques for studying disordered materials, in which every site is a bit different from every other site, are not so well developed. In disordered materials, there are typically many slightly different physical states among which the material spontaneously fluctuates, giving rise to low-frequency noise. This research focuses on using that noise as a probe to study the basic physics of several materials. One of the main types to be studied will be colossal magnetoresistive materials, which can be driven from a poorly conducting non-magnetic state to a conducting metallic state by application of a magnetic field, providing a potentially useful sensor device. Initial studies show that this change typically happens by a patchwork of conducting and non-conducting regions, rather than by a smooth homogeneous change. Noise provides a way of seeing the behavior of the individual patches, and thus allows a detailed look at how changes in materials affect this transition. The other main topic will be relaxor ferroelectrics, which freeze into a patchwork of regions in which the electrical dipoles of many thousands of crystalline cells line up together, but these units then collectively freeze in a random-looking pattern. That random pattern turns out to have useful properties unlike those of regular patterns. Noise studies (and related studies of slow aging vs. time) allow determination of the scale and type of the interactions driving this peculiar freezing. The students working on these projects have been able to use both table-top small-science techniques and sophisticated large-scale facilities for making and characterizing samples. They have typically gone on to work in the materials side of the computer industry or in further basic research.