Nanoscale magnetic field imaging for 2D materials

Abstract

Nanometer-scale imaging of magnetization and current density is the key to deciphering the mechanisms behind a variety of new and poorly understood condensed matter phenomena. The recently discovered correlated states hosted in atomically layered materials such as twisted bilayer graphene or van der Waals heterostructures are noteworthy examples. Manifestations of these states range from superconductivity, to highly insulating states, to magnetism. Their fragility and susceptibility to spatial inhomogeneities limits their macroscopic manifestation and complicates conventional transport or magnetization measurements, which integrate over an entire sample. In contrast, techniques for imaging weak magnetic field patterns with high spatial resolution overcome inhomogeneity by measuring the local fields produced by magnetization and current density. Already, such imaging techniques have shown the vulnerability of correlated states in twisted bilayer graphene to twist-angle disorder and revealed the complex current flows in quantum Hall edge states. Here, we review the state-of-the-art techniques most amenable to the investigation of such systems, because they combine the highest magnetic field sensitivity with the highest spatial resolution and are minimally invasive: magnetic force microscopy, scanning superconducting quantum interference device microscopy, and scanning nitrogen-vacancy center microscopy. We compare the capabilities of these techniques, their required operating conditions, and assess their suitability to different types of source contrast, in particular magnetization and current density. Finally, we focus on the prospects for improving each technique and speculate on its potential impact, especially in the rapidly growing field of two-dimensional materials.

Magnetic, Thermal, and Topographic Imaging with a Nanometer-Scale SQUID-On-Lever Scanning Probe

“…2(f). Imaging such spatial magnetic field derivatives also increases the maximum sensitivity towards smaller features sizes, compared with imaging magnetic fields [1]. In fact, detection of magnetic spatial derivatives is 034002-4 the standard mode for magnetic force microscopy and has also been implemented using a tuning fork resonator in scanning SQUID-on-tip microscopy [11] to increase sensitivity to small dc features in B z or local T.…”

“…They provide local information about length scales, inhomogeneity, and interactions that is inaccessible in bulk measurements of transport, magnetization, susceptibility, or heat capacity. Prominent among these techniques, for their combination of high sensitivity and high spatial resolution, are scanning probe microscopies such as magnetic force microscopy, scanning nitrogenvacancy center microscopy, and scanning superconducting quantum interference (SQUID) device microscopy [1]. Exploiting its intrinsic sensitivity to magnetic flux and its minimal interaction with the sample, researchers have used scanning SQUID microscopy to image a wide variety of nanometer-scale phenomena, including superconducting vortices [2][3][4][5], persistent currents in normal metal rings [6], magnetism and superconductivity at oxide interfaces [7,8], and magnetic reversal in nanomagnets [9,10].…”

Observation of dimension-crossover of a tunable 1D Dirac fermion in topological semimetal NbSixTe2

“…Low dimensional electronic states bear fascinating emergent physics and have attracted great research interests in the past decade. For example, some of the two-dimensional (2D) electronic states under intensive investigation include electronic structure of van der Waals (vdW) materials in the atomic limit [1][2][3][4][5][6][7] , the quantum well states confined on the surface/interfaces of semiconductors 1,8,9 and topological surface states of strong topological insulators [10][11][12][13] . They host profound physics including Ising superconductivity [14][15][16] , charge density wave [17][18][19][20] , spin-momentum locking 21,22 , and magnetism [23][24][25] .…”

Scanning nitrogen-vacancy magnetometry down to 350mK

“…Imaging magnetic fields at the micro-to nanoscale is a powerful method for materials characterization and provides insights into a plethora of physical phenomena, ranging from magnetic ordering to the flow of electric currents [1]. Over the past decade, nitrogen-vacancy (NV) centers in diamond scanning probes [2,3] have been introduced as versatile magnetic field sensors that are applicable over a broad temperature range.…”