Dissertation Defense: Christopher Jensen

Candidate: Christopher Jensen

Major: Physics

Advisor: Kai Liu, Ph.D.

Title: Magneto-Ionic Control of Thin Film Heterostructures

The magneto-ionic (MI) effect has shown promise to control magnetic thin film heterostructures through ionic migration, enabling a wide variety of spintronic applications through the manipulation of properties such as magnetic anisotropy, ferromagnetism, antiferromagnetism, and chiral spin textures. The MI effect can be tailored by using electric fields to drive ions, thus avoiding the use of the relatively inefficient electric current actuation and the associated energy dissipation. MI and electric field control of saturation magnetization (Ms) and exchange bias (EB), an interfacial interaction in ferromagnetic/antiferromagnetic (FM/AF) systems, in a variety of magnetic thin films has been explored. The electric field manipulation of EB in Gd/NiCoO was first explored, which shows chemically induced O2- motion through a redox reaction at the Gd/NiCoO interface, producing a thin FM NiCo layer. The magnitude of EB in this system could be enhanced by up to 35% using electric fields, and changes in Ms and FM thickness caused by O2- motion were correlated to changes in EB. Subsequently other ionic species were explored to improve ionic switching speed and reversibility, including the paramagnetic to FM transition in electrolyte-gated α-Co(OH)2, which demonstrated the MI control of Ms . Additionally, the MI control of EB in nitride based CoFe (FM)/MnN (AF) system was explored. Here, N3-motion was shown to be driven by electric fields, causing an EB increase up to 19% at 10K, which is reversible, and is related to the increase in N content in MnN at the CoFe interface. Finally, MI was incorporated into a magnetic tunneling junction (MTJ) to demonstrate the practical applicability to spintronic devices. In this study, GdOx acts as a source of O2-, which can be driven by electric fields to produce large changes in the electrical resistance readout. These studies demonstrate that the MI effect offers a new handle, through ionic migration that can be electrically driven, to toggle the magnetic properties, opening the door to electric manipulation of magnetic tunnel junctions and other devices.