Quantification of magnetic interactions in van der Waals heterostructures using Lorentz transmission electron microscopy and electron holography

TL;DR

This study uses advanced electron microscopy techniques to quantify magnetic interactions in layered van der Waals heterostructures, revealing how magnetic domain alignment and surface effects influence device design.

Contribution

It introduces a method to measure local magnetic fields in cross-sectional vdW heterostructures, providing new insights into interlayer magnetic coupling and domain behavior.

Findings

Domain alignment weakens with increased layer separation.

Dipolar coupling length scale is approximately 34 nm.

Surface effects cause magnetic moment canting up to 100 nm from surface.

Abstract

Magnetic van der Waals (vdW) materials are promising for memory and logic applications because of their highly tunable magnetic properties and compatibility with vdW heterostructure devices. However, in conventional plan-view measurements, coupling between magnetic textures in stacked layers is difficult to resolve because the magnetic signal is integrated over the sample thickness. Here, these interactions are quantified in Fe$_3$GeTe$_2$ (FGT)/graphite/FGT heterostructures using cross-sectional Lorentz transmission electron microscopy and electron holography, enabling reconstruction of the local magnetic field within and between the layers. Domain alignment weakens with increasing FGT separation, yielding a dipolar coupling length scale of $\lambda = 34 \pm 4$ nm for the cross-sectional geometry studied here, corresponding to the average separation at which domain misalignment first emerges. This length scale coincides with an approximately 50\% reduction in the interlayer magnetic field relative to bulk FGT. Surface effects result in canting of the magnetic moments away from the easy axis up to $\sim$100 nm from a surface. Finally, the domain walls are narrow ($\sim$9 nm), while micromagnetic simulations reproduce the observed textures without invoking Dzyaloshinskii-Moriya interaction. These results quantify the internal and stray fields in stacked vdW magnets and guide the design of devices that require controllable coupling between magnetic textures.