In situ Manipulation of Magnetization via Direct Mechanical Interaction in Magnetostrictive Thin Films

Abstract

The pursuit of a universal memory- possessing fast write/read times, nonvolatile and unlimited data endurance, low operating power, low manufacture costs, high bit density, as well as being easily integrable with on-trend complementary metal-oxide semiconductor (CMOS) devices- has reenergized research in the field of multiferroic and magnetoelectric materials. Such materials simultaneously exhibit ferroelectricity and ferromagnetism, and allow for the coupling of the two order parameters, known as magnetoelectric coupling. This coupling is enhanced in magnetostrictive/piezoelectric bilayer systems where applied electrical bias can modify magnetic order via strain-mediation, a mechanism that can reduce the power demands in emerging magnetic random access memory (MRAM) technologies. We have previously investigated this relationship in an Fe0.7Ga0.3/BaTiO3 bilayer structure using magnetic contrast imaging techniques with in situ applied electric fields. The goal of this thesis was to explore methods to better control magnetoelectric effects in order to enhance local magnetic response to external stimuli.

Specifically, we investigated magnetoelastic response of freestanding, magnetostrictive Fe0.7Ga0.3 thin films via direct mechanical interaction with an external probe, as the well known strain-mediated mechanism in magnetoelectric devices depends on the lesser known magnetoelastic nature of strain transfer between the distinct material phases. Magnetoelastic effects are directly associated with both external magnetic field and stress via Lorentz-force transmission electron microscopy (LTEM) contrast techniques, and the hysteresis of magnetic order was charted with respect to both stimuli. For relevant application to MRAM devices, we have initiated studying these effects in patterned media as well, where individual, nanoscale magnetic geometries represent bistable bits for memory.

We demonstrate static pure stress effects on the magnetoelastic response in continuous thin films, as well as real-time mechanical "writing" of stable domain states. The external probe is directed into the film, inducing a non-uniform, radially symmetric local strain.

Micromagnetic simulation reveals that the strength of observed magnetoelastic effects is offset by small, undulating variations in magnetization characteristic of polycrystalline thin films, known as magnetization ripple. Imposing a threshold function on the effective anisotropy of the film describes the spontaneous onset of these effects and the differences in magnetic order for films with hysteresis solely due to stress, or with both field and stress. Thus, a method to achieve bistable logic for MRAM applications using direct uniform stress, in lieu of external fields, is proposed.