Probing magnetism in 2D materials at the nanoscale with single-spin microscopy

Abstract

The recent discovery of ferromagnetism in 2D van der Waals (vdW) crystals has generated widespread interest, owing to their potential for fundamental and applied research. Advancing the understanding and applications of vdW magnets requires methods to quantitatively probe their magnetic properties on the nanoscale. Here, we report the study of atomically thin crystals of the vdW magnet CrI 3 down to individual monolayers using scanning single-spin magnetometry, and demonstrate quantitative, nanoscale imaging of magnetisation, localised defects and magnetic domains. We determine the magnetisation of CrI 3 monolayers to be ≈ 16 µ B /nm 2 and find comparable values in samples with odd numbers of layers, whereas the magnetisation vanishes when the number of layers is even. We also establish that this inscrutable even-odd effect is intimately connected to the material structure, and that structural modifications can induce switching between ferro-and anti-ferromagnetic interlayer ordering. Besides revealing new aspects of magnetism in atomically thin CrI 3 crystals, these results demonstrate the power of single-spin scanning magnetometry for the study of magnetism in 2D vdW magnets.Magnetism in individual monolayers of vdW crystals has recently been observed in a range of materials, including semiconducting [3,4] and metallic [5][6][7] compounds. The discovery of such two dimensional magnetic order is per se non-trivial [8] and has triggered significant attention owing to emerging exotic phenomena including Kitaev spin liquids [9,10], or novel magneto-electric effects [11][12][13][14]. Remarkable efforts have led to the use of two-dimensional magnets as functional elements in spintronics, such as spin-filters [15, 16], spin-transistors [17], tunnelling magnetoresistance devices [18,19] or magnetoelectric switches [12][13][14]. Further advances hinge on methods for the quantitative study of the magnetic response of these atomically thin crystals at the nanoscale, but despite their central importance, the required experimental methods are still lacking. Indeed, transport ex-A z NV e NV z θ NV 3 µm 3 l a y e r s 2 l a y e r s B C, D 0.35 -0.35 0 B NV -B NV (mT) C 2 µm Magne�c stray-field map bias D 20 -20 0 σ (µ B /nm 2 ) 2 µm Magne�sa�on map FIG. 1.Nanoscale imaging of magnetism in twodimensional van der Waals magnets. A Schematic of the scanning single spin magnetometry technique employed in this work. A single Nitrogen-Vacancy (NV) electronic spin is scanned across few layer flakes of encapsulated CrI3 (encapsulation not shown). Stray magnetic fields from the sample are sensed by optically detected Zeeman shifts of the NV spin states, and imaged with nanoscale resolution (set by the sensor-sample separation zNV) by lateral scanning of the NV. The method detects magnetic fields along the NV spin quantisation axis eNV, at an angle θNV ∼ 54 • from the sample normal. B Optical micrograph of the CrI3 bi-and tri-layer flake of sample D1. C Magnetic field map of BNV across sample D1 recorded in a bias field B bias NV = 172.5...

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