Noise and Aging in Disordered Magnetic Materials

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*****NON-TECHNICAL ABSTRACT*****Technological advancement relies on the development of new materials with properties tailored to applications. For example, the devices that read the magnetic disks in computers rely on special types of material whose electrical resistance is very sensitive to magnetic fields. Although there are well-developed techniques for analyzing materials which have simple crystalline order, the broader class of materials which are not so well ordered, or which even look random ('glassy') are harder to analyze. This project focuses on the use of a technique, the study of random fluctuations ('noise'), to look at disordered magnetic systems, especially ones whose electrical resistance depends on magnetism. This technique allows one to obtain a much better picture of what components of a disordered system are responsible for its response to applied fields, and that in turn is useful for improving material design. One of the key purposes of this project is to train a postdoctoral fellow to become an expert in a collection of noise techniques developed over the years in this laboratory. Undergraduate researchers will also have an opportunity to perform small-scale, accessible science.*****TECHNICAL ABSTRACT*****A variety of qualitative characterizations of common disordered magnetic systems remain unsettled. The best known is the question of what sorts of states realistic spinglasses settle into. There are also many important systems (e.g. cobaltites) in which spinglass physics appears to coexist with domains of more conventional order. This project focuses on the use of noise to investigate disordered magnetic systems. Non-Gaussian noise in small samples of these systems provides a probe of the detailed internal states. It should allow discrimination, for example, between nearly independent ferromagnetic domains imbedded in a spinglass matrix and interacting ferromagnetic domains forming a larger-scale cluster glass. Small-scale fluctuation experiments on spinglasses provide fingerprints of individual states, which should help settle the question of how many non-symmetry-related states are available to several types of conducting spinglasses. Several remaining problems in manganites, particularly whether there are regimes best viewed as a strain-glass, are also amenable to these small-scale noise investigations. Aging investigations, also very sensitive to the form of glassy correlations, will be used in conjunction with the noise work. One of the key purposes of this project is to train a postdoctoral fellow to become an expert in a collection of noise techniques developed over the years in this laboratory. Undergraduate researchers will also have an opportunity to perform small-scale, accessible science