Abstract
Precision magnetometry is fundamental to the development of novel magnetic materials and devices. Recently, the nitrogen-vacancy (NV) center in diamond has emerged as a promising probe for static magnetism in two-dimensional (2D) van der Waals materials, capable of quantitative imaging with nanoscale spatial resolution. However, the dynamic character of magnetism, crucial for understanding the magnetic phase transition and achieving technological applications, has rarely been experimentally accessible in single 2D crystals. Here, we coherently control the NV center’s spin precession to achieve ultrasensitive, quantitative ac susceptometry of a 2D ferromagnet. Combining dc hysteresis with ac susceptibility measurements varying temperature, field, and frequency, we illuminate the formation, mobility, and consolidation of magnetic domain walls in few-layer CrBr3. We show that domain wall mobility is enhanced in ultrathin CrBr3, with minimal decrease for excitation frequencies exceeding hundreds of kilohertz, and is influenced by the domain morphology and local pinning of the flake. Our technique extends NV magnetometry to the multifunctional ac and dc magnetic characterization of wide-ranging spintronic materials at the nanoscale.10 MoreReceived 17 May 2021Accepted 23 August 2021DOI:https://doi.org/10.1103/PRXQuantum.2.030352Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum sensingPhysical Systems2-dimensional systemsNitrogen vacancy centers in diamondTechniquesAC susceptibility measurementsCondensed Matter, Materials & Applied Physics
Highlights
Two-dimensional van der Waals magnetic materials have broached transformative concepts for integrating and controlling spintronic devices, and presented unique challenges to their magnetic characterization [1,2,3]
The magnetic moment of a micron-sized exfoliated monolayer lies below the sensitivity of commercial magnetometers based on the superconducting quantum interference device (SQUID) [4], the benchmark for analyzing bulk magnetic materials
For example, when the ac susceptibility is contributed by paramagnetic spin rotation, which is nearly homogeneous, or by domain walls with separations that are sufficiently small compared to x
Summary
Two-dimensional van der Waals (vdW) magnetic materials have broached transformative concepts for integrating and controlling spintronic devices, and presented unique challenges to their magnetic characterization [1,2,3]. Key insights in vdW magnetism are most frequently provided by magneto-optical [5,6,7,8] and photoluminescence (PL) imaging [9,10], which feature high sensitivity, or by electrical measurements, which directly incorporate layers inside functional devices [11,12,13]. We exfoliate ultrathin flakes of the ferromagnetic insulator CrBr3 onto a diamond magnetometer chip containing a near-surface ensemble of nitrogen-vacancy (NV) centers This versatile platform directly enables sensitive measurement of both the 2D ferromagnet’s dc magnetization through static shifts of the NV center’s spin energies [18,19,20,21] and its ac susceptibility through dynamically coupling the NV center’s spin precession to ac fields using frequency-selective quantum control sequences [27,28]. Our work achieves an ac field resolution of approximately 40 nT, representing the most sensitive NV magnetometry performed to date on exfoliated 2D magnets [18,19,20,21], and introduces a generic platform for understanding subgigahertz dynamical phenomena in 2D magnetism
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