Study of Exchange Bias and Interfacial Coupling Effects in a Micromagnetic Thin Film Bilayer

• Author / Creator

  Thibault, John Anthony

• Exchange bias and interfacial coupling effects in thin-film bilayers of magnetic materials were investigated during the period July, 2018, to Sep, 2021. Thin films of permalloy and cobalt oxide were deposited, in the form of micrometer size bilayer disks, on doubly-clamped nanomechanical resonators that were fabricated at the University of Alberta NanoFAB facility. The samples were mounted in an evacuated cryostat and were studied by observing their AC torque response to externally applied DC and RF magnetic fields. The apparatus was a three-axis AC torque magnetometer developed at the University of Alberta by our research group. Preliminary experimental data from ambient and field-cooled linear hysteresis loops showed a shift of the coercive field points thus providing the traditional signature of exchange bias behaviour and demonstrating that the samples and measurement techniques were suitable for further study of exchange bias effects. The original intent of previous researchers in our group was to investigate the effect of exchange bias on magnetic vortex behaviour, but that endeavor evolved toward a more fundamental study of exchange bias and interfacial coupling effects, the findings of which are reported herein. After modification to some of the samples to increase their torque response and improvements to the apparatus to enhance thermal and mechanical stability, the work continued by collecting and examining data from both linear and rotating hysteresis loops at ambient and cryogenic temperatures. The data were compared to predictions obtained from a phenomenological energy equation and macrospin model. Results from micromagnetic simulations were used to augment the investigative process.

  Data from the linear hysteresis loops indicated exchange bias behaviour much in agreement with currently held views, but the findings from the rotating hysteresis loops suggested that exchange bias effects were not the dominant contributor to hysteretic behaviour in the rotating loops and that at least one additional interfacial coupling mechanism was involved. An additional task was to search for an exchange bias signature that did not require collecting data from a complete hysteresis loop. It was hoped that such a signature could be found by examining the torque response as a function of temperature. An experiment to this end was conducted wherein the findings indicated a progressive increase in torque as the temperature was lowered. The results were consistent with the view that exchange bias effects become observable as the sample temperature is decreased below the Néel temperature and that the strength of the effect increases as the temperature is further decreased.

• Subjects / Keywords

  • Exchange Bias
  • AC Torque Magnetometry
  • Nanoscale Resonators

• Graduation date

  Fall 2022

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-v20a-2w74

• License

  This thesis is made available by the University of Alberta Library with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.