Abstract
Research on van der Waals materials is an important topic in condensed matter physics. Since the first exfoliations of graphene in 2004, the family of van der Waals materials has grown extremely rapidly, covering a wide range of different physical properties, from insulating materials to semi-metals, including semiconductors. These materials also host phenomena such as superconductivity, or the strong spin-valley coupling in transition metal dichalcogenides. In parallel, a second field has also developed rapidly: van der Waals heterostructures. It consists of stacking two-dimensional materials, layer by layer, in a very specific order. These structures offer the possibility of combining the properties of several different materials, in a single artificial material, in addition to allowing the appearance of proximity effects at the interfaces of the different layers. A promising prospect would be to bring spintronics towards van der Waals materials, thus taking advantage of their great diversity, their atomic thickness, as well as their mechanical robustness.However, a member remained missing in this large family of van der Waals materials: two-dimensional magnetic materials. Obtaining a two-dimensional magnetic order would open the way to many opportunities, whether at the fundamental level with the study of two-dimensional phase transitions or for new spintronic devices. It is only very recently, in 2017, that a magnetic order was observed in a CrI3 monolayer as well as in a Cr2Ge2Te6 bilayer. There followed a considerable international research effort, quickly adding many new members to the family of magnetic van der Waals materials.The emergence of these two-dimensional magnets has also been accompanied by new issues. First of all, in an instrumental point of view, because the quantitative measurement of the magnetic properties of these materials is a complex undertaking, due to their small size. It requires the use of non-invasive magnetometry techniques, with high sensitivity and nanometric spatial resolution. A second issue, this time at the material level, is that due to their lamellar nature, the Curie temperature Tc of these van der Waals magnets is low. Almost all of these two-dimensional magnetic materials have a Curie temperature well below room temperature, which limits the development of new applications.I carried out my thesis work in such context, on the study of magnetic van der Waals materials at room temperature, at the nanometer scale. This thesis work revolves around two axes, with first of all the use of the nitrogen-vacancy center (NV) in diamond as a quantum sensor at an atomic scale. I used a scanning NV magnetometer to measure the magnetization of exfoliated flakes of CrTe2, a ferromagnetic van der Waals magnet at room temperature. I also studied the effects of hBN encapsulation on their magnetic properties.The second part focuses on the in-situ study of van der Waals heterostructures by a quantum sensor hosted in a two-dimensional material. The VB- center exhibits magneto-optical properties similar to the NV center in diamond and is present in hBN. I developed a new magnetic microscope based on this defect and showed a proof of principle on magnetic imaging of exfoliated CrTe2 flakes.