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Design and characterization of focused high frequency strain waves
Controlled strain can tune materials’ strain dependent properties. Implementation in future straintronic devices requires an investigation of high frequency strain response, to better understand the dynamics of strain dependent material properties. This thesis is motivated by the investigation of dynamic strain driven phase transitions, specifically the paramagnetic to antiferromagnetic phase transition in magnetoelectric Cr2O3 using surface acoustic waves (SAW). SAW are strain waves which propagate on the media surface and thin films deposited on the SAW surface will experience periodic compressive and tensile strain. Conventional SAW devices produce small strains, on the order of 0.001%. Many strain sensitive properties, including the phase transition of Cr2O3, require strains of ~1%, often achieved in thin films by growth on mismatched substrates. These strain levels can alter ordering temperatures or the type of order in materials. Achieving dynamic strains of this magnitude require focusing of the SAW. This thesis describes the fabrication and characterization of annular interdigital transducers (AIDT) on 128° Y-Cut LiNbO3, with a fundamental resonance frequency of 87.95 MHz. We show, using both optical and time resolved x-ray measurements, that the AIDT generates a tightly focused SAW. The x-ray measurements are enabled by matching the resonance frequency of the SAW to the ring frequency of the Advanced Photon Source. The two measurements confirm separate aspects of SAW focusing and both match the simulated focusing of strain. These focused SAW are shown to drive domain walls in ferromagnetic Co/Pt thin films. We have mapped and locally manipulated the exchange bias (EB) in a Cr 2O3-Co/Pd heterostructure. The writing of EB at a microscopic scale was achieved using a focused laser beam, creating regions of opposite exchange bias. This spatial mapping and manipulation is a necessary precursor for the observation of strain driven changes since it sets a limit on the size of the domains. A unique property of Cr2O3, the roughness insensitive boundary magnetization, also enables us to measure the sign of exchange coupling in Cr2O3-Co/Pd heterostructures. Finally, we discuss and simulate the expected outcomes of future experiments on rf strain driven phase transitions in Cr2O3.
Physics|Condensed matter physics|Materials science
Singh, Uday, "Design and characterization of focused high frequency strain waves" (2016). ETD collection for University of Nebraska - Lincoln. AAI10181967.