Spatial mapping of focused surface acoustic waves in the investigation of high frequency strain induced changes.

ABSTRACT

The field of straintronics, in which strain is used to drive phase transitions, ordering and structural changes, has conventionally been limited to dc or low frequency strain. High frequency large strains, which have the potential to serve as a high frequency trigger of strain sensitive physical phenomena, can be generated using focused surface acoustic waves, which produce two dimensional standing strain waves with very high strain at the elliptical focus. Here, the strain standing wave pattern generated by a focused surface acoustic wave is mapped and quantified as a function of voltage and frequency with high spatial resolution. A knife-edge optical reflection method is used to map the strain standing wave pattern generated by a 87.95 MHz annular interdigital transducer on 128° Y-Cut LiNbO3. Subsequent to strain mapping, ferromagnetic Co/Pt multilayers nanostructures are lithographically patterned within the high strain region for preliminary measurements of magnetization changes arising from high frequency fast strain. The knife edge technique is simple, results in excellent spatial resolution and is fully compatible with other optical measurements, such as focused magneto-optic Kerr measurements, while maintaining spatial information. This ability to accurately and reproducibly determine the position of maximum strain and to lock onto a specific strain region is an important step in the investigation of the effects of high frequency strain on thin film materials, which range from magnetic reorientations to strain induced phase transitions.