# Micromagnetometry of two-dimensional ferromagnets

## Abstract

The study of atomically thin ferromagnetic crystals has led to the discovery of unusual magnetic behaviour and provided insight into the magnetic properties of bulk materials. However, the experimental techniques that have been used to explore ferromagnetism in such materials cannot probe the magnetic field directly. Here, we show that ballistic Hall micromagnetometry can be used to measure the magnetization of individual two-dimensional ferromagnets. Our devices are made by van der Waals assembly in such a way that the investigated ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from encapsulated graphene. We use the micromagnetometry technique to study atomically thin chromium tribromide (CrBr3). We find that the material remains ferromagnetic down to monolayer thickness and exhibits strong out-of-plane anisotropy. We also find that the magnetic response of CrBr3 varies little with the number of layers and its temperature dependence cannot be described by the simple Ising model of two-dimensional ferromagnetism.

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## Data availability

The data that support our findings are available from the corresponding authors upon reasonable request.

## Code availability

The computer code used in this study is available from the corresponding authors upon reasonable request.

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## Acknowledgements

This work was supported by the European Research Council, the Graphene Flagship and Lloyd’s Register Foundation. M.K. was partly supported by the National Research Foundation of Korea (grant 2018R1A6A3A03010943). W.K., D.G.H. and A.I.B. were supported by the Graphene NowNANO Doctoral Training Programme. J.H.E. and S.L. acknowledge support from the NSF (grant CMMI 1538127).

## Author information

M.K. and P.K. carried out the project and analysed the experimental data. A.K.G. suggested and supervised the project. P.K., J.B. and S.G.X. fabricated devices. M.K., A.I.B. and W.K. performed electrical and superconducting quantum interference device measurements. D.G.H. and S.J.H. provided transmission electron microscopy analysis. M.K. performed the finite-element simulations. J.K. and P.A.M. provided theoretical support. S.L. and J.H.E. supplied hBN crystals. M.K., P.K., K.S.N., I.V.G., J.B. and A.K.G. wrote the manuscript. All of the authors contributed to discussions.

Correspondence to M. Kim or P. Kumaravadivel or A. K. Geim.

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### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–7 and references.

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