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Dynamic strain waves manipulating magnetic domain wall and skyrmions

Thesis Defense

10:00 am–11:30 am
Jorgensen Hall Room: 207
Target Audiences:
Physics Department, (402) 472-2770,
Anil Adhikari will present his PhD Defense topic, “Dynamic strain waves manipulating magnetic domain wall and skyrmions” in person.

ABSTRACT: Magnetic textures, such as domain walls (DWs) and skyrmions in thin film heterostructures, show potential for racetrack memory applications. Reproducible manipulation, creation, and annihilation of these textures are crucial for device reliability and efficiency. Magnetic fields and currents have been shown to control and drive magnetic textures. Using Magneto-Optical Kerr Effect microscopy, we investigate another approach to manipulating these magnetic textures via high-frequency dynamic strain waves. Surface acoustic waves (SAW) are generated by the electrical; excitation of interdigital transducers on a piezoelectric substrate, in our case, 128º Y cut LiNbO3, via the inverse piezoelectric effect. All magnetic heterostructures are grown on this substrate.
High-frequency SAWs were used to investigate DW velocities in a perpendicular magnetic anisotropy multilayer sample of [Co/Pt]. The velocities of magnetic DWs in magnetic strips increased substantially, 3-8 fold, on the application of relatively low applied voltages of SAW. Our findings suggest that the strain wave-derived effective magnetic field is an additional driver for DW motion.
Next, we investigate the SAW-assisted depinning of magnetic DWs. Pinning sites arise from both random defects and geometric patterned notches. The random defect sites are created via ion milling, and the geometrical notches are via lithography on magnetic stripes. In both samples, we first characterize the thermally-assisted depinning of DWs. Our results show that SAW increases depinning probabilities by factors ranging from 2 to 20. Our data are consistent with a model in which the SAW modulates magnetoelastic anisotropies, thereby reducing the energy barriers that pin the DWs suggesting an alternative route to DW depinning.
Finally, we studied the role of SAW in the manipulation of magnetic skyrmions in Cr(2 nm)/Ta(3 nm)/Pt(3 nm)/CoFeB(0.4 nm)/Pt(2 nm). Skyrmions were stabilized with and without an out-of-plane bias field. Applying SAW, both traveling and standing waves, rapidly increases skyrmion density and decreases skyrmion size.
Our results show promise for an alternative method for the generation and manipulation of magnetic textures that is low power, remote, and allows for useful wave properties that include focusing, directionality, and different elastic modes.

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