Phase transitions refer to changes in a material's properties, often driven by temperature changes. The ubiquitous phase transition of water, which transforms from ice to liquid water to steam as the temperature increases, serves as an excellent example. Pressure significantly alters the temperature of water's transitions, as anyone who has baked a cake at high altitude well knows. The research component of this project will investigate the effects of rapidly changing pressure on phase transitions of technologically important magnetic materials by focusing a very fast (ultrasonic) sound wave on the material. By varying the pressure at rates of up to 10 billion times a second, the investigators will probe how the material's properties vary. The fundamental questions being probed are how quickly can the material respond, what will the response be and why does a particular material respond the way it does. The educational component is informed by the PIs' extensive experience in outreach and education at all levels ranging from preschool, K-12 and undergraduate education. New activities on phase transitions, tailored to the needs of a particular age group, will be developed to tie in with this research project.

Technical Abstract

The research activity will investigate the dynamics of strain driven phase transitions in materials that show intricate entanglement between structure and ordering, viz. the magnetoelectric antiferromagnet Cr2O3 and the antiferromagnetic Mott insulator NdNiO3. In both cases the antiferromagnetic ordering is coincident with the appearance of another transition, surface boundary magnetization for Cr2O3 and a metal-insulator transition for NdNiO3. Focused surface acoustic wave transducers will generate strains that are comparable in magnitude to fixed, epitaxial strains and allow for controlled variable strain on a single sample, as well as the ability to drive strain at high frequencies. This approach of precisely controlled lattice excitation is unique in its the ability to drive materials back and forth across the phase transition at GHz frequencies, resulting in large changes in the order parameter, rather than small perturbative changes. This approach can be extended to a wide variety of strain sensitive ordering in thin film materials and will answer two fundamental questions. First, it will quantify the effects of external strain on the phase transition temperature of a single thin film sample, eliminating the uncertainties associated with thin film growth. Second, it will measure the temporal scale over which these strain driven phase transitions occur. The educational objectives will extend the reach of this cutting edge science by expanding the PIs' portfolios to include topics that are relevant to this project. The PIs will share their knowledge, skills and excitement with the already extensive ongoing outreach and education projects that include K-12 students and teachers, science cafes, science clubs, and senior citizen groups.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Susan Dexheimer
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University of Nebraska-Lincoln
United States
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