# Quantum Imaging of Ferromagnetic van der Waals Magnetic Domain Structures at Ambient Conditions

## TL;DR

This paper uses quantum imaging to study the magnetic properties of a 2D material, revealing thickness-independent behavior and new structural features.

## Contribution

The study introduces quantum magnetic microscopy for noninvasive imaging of 2D magnets under ambient conditions.

## Key findings

- Magnetic phase transition in Fe5GeTe2 is largely thickness-independent down to 15 nm.
- Stripe features in images are attributed to element modulations during crystal synthesis and oxidation.
- Magnetic anisotropy does not significantly influence the material's magnetic properties.

## Abstract

Recently discovered 2D van der Waals magnetic materials,
and specifically
iron–germanium–telluride (Fe5GeTe2), have attracted significant attention both from a fundamental perspective
and for potential applications. Key open questions concern their domain
structure and magnetic phase transition temperature as a function
of sample thickness and external field, as well as implications for
integration into devices such as magnetic memories and logic. Here
we address key questions using a nitrogen-vacancy center based quantum
magnetic microscope, enabling direct imaging of the magnetization
of Fe5GeTe2 at submicrometer spatial resolution
as a function of temperature, magnetic field, and thickness. This
quantum imaging technique provides noninvasive, high-sensitivity measurements
with high spatial resolution under ambient conditions, making it particularly
well suited for probing 2D magnets. We employ spatially resolved measures,
including magnetization variance and cross-correlation, and find a
significant spread in transition temperature yet with no clear dependence
on thickness down to 15 nm. We also identify previously unknown stripe
features in the optical as well as magnetic images, which we attribute
to modulations of the constituting elements during crystal synthesis
and subsequent oxidation. Our results suggest that the magnetic anisotropy
in this material does not play a crucial role in their magnetic properties,
leading to a magnetic phase transition of Fe5GeTe2 which is largely thickness-independent down to 15 nm. Our findings
could be significant in designing future spintronic devices, magnetic
memories, and logic with 2D van der Waals magnetic materials.