Quantitative Phase Retrieval and Characterization of Magnetic Nanostructures via Lorentz (Scanning) Transmission Electron Microscopy

TL;DR

This paper compares and demonstrates advanced phase retrieval methods for magnetic nanostructures using Lorentz TEM, achieving high-resolution magnetic characterization and revealing proximity effects in nanoscale magnetic materials.

Contribution

It evaluates and applies multiple phase retrieval techniques, including RMAD and ePIE, for high-fidelity magnetic phase reconstruction in both simulations and experiments.

Findings

Successful phase reconstruction of magnetic nanostructures using multiple methods.

Demonstrated high-resolution total phase imaging of NiFe nanowires.

Characterized proximity effects and magnetic interactions at the nanoscale.

Abstract

Magnetic materials phase reconstruction from Lorentz transmission electron microscopy (LTEM) measurements has traditionally been achieved using longstanding methods such as off-axis holography (OAH) and the transport-of-intensity equation (TIE). Amidst the increase in access to processing power and the development of advanced algorithms, phase retrieval of nanoscale magnetic materials with higher fidelity and resolution, potentially down to the few nanometer limit, becomes possible. Specifically, reverse-mode automatic differentiation (RMAD) and the extended electron ptychography iterative engine (ePIE) are two methods that have been utilized for high confidence phase reconstructions using LTEM through-focal series imaging and Lorentz scanning TEM (Ltz-4D-STEM), respectively. This work evaluates phase retrieval using TIE, RMAD, and ePIE in simulations consisting of an array of Permalloy (Ni80Fe20) nanoscale islands. Extending beyond simulations, we demonstrate total phase reconstructions of a NiFe nanowire using OAH and RMAD in LTEM and ePIE in Ltz-4D-STEM experiments and determine the magnetization saturation through corroborations with micromagnetic simulations. Finally, we show how the total phase shift gradient can be utilized to observe and characterize the proximity effects emanating from neighboring magnetic island interactions and an isolated NiFe nanowire.