Quantum Imaging of Magnetic Phase Transitions and Spin Fluctuations in Intrinsic Magnetic Topological Nanoflakes

Abstract

Topological materials featuring exotic band structures, unconventional current flow patterns, and emergent organizing principles offer attractive platforms for the development of next-generation transformative quantum electronic technologies. The family of MnBi2Te4 (Bi2Te3) n materials is naturally relevant in this context due to their nontrivial band topology, tunable magnetism, and recently discovered extraordinary quantum transport behaviors. Despite numerous pioneering studies to date, the local magnetic properties of MnBi2Te4 (Bi2Te3) n remain an open question, hindering a comprehensive understanding of their fundamental material properties. Exploiting nitrogen-vacancy (NV) centers in diamond, we report nanoscale quantum imaging of the magnetic phase transitions and spin fluctuations in exfoliated MnBi4Te7 flakes, revealing the underlying spin transport physics and magnetic domains at the nanoscale. Our results highlight the unique advantage of NV centers in exploring the magnetic properties of emergent quantum materials, opening new opportunities for investigating the interplay between topology and magnetism.

Wide field imaging of van der Waals ferromagnet Fe3GeTe2 by spin defects in hexagonal boron nitride

“…5a-g show a series of V À B spin relaxation rate maps measured with temperatures between 165 K and 223 K. Note that the background of the intrinsic relaxation rate of V À B has been subtracted to highlight the contribution Γ M from the fluctuating magnetic fields generated by FGT (see Supplementary Information Note 4 for details). Due to the strong perpendicular magnetic anisotropy, the minimum magnon energy of FGT is larger than the ESR frequencies of V À B spin defects in our experimentally accessible magnetic field range, hence, the measured spin relaxation is driven by the longitudinal spin fluctuations of FGT, which is further related to the static longitudinal magnetic susceptibility χ 0 and the diffusive spin transport constant D 46,60,61 . When temperature is away from the quantum critical point, fluctuations in FGT are largely suppressed due to its vanishingly small magnetic susceptibility, leading to negligible spin relaxation rate Γ M of V À B defects (Fig.…”

“…By performing spatially dependent ODMR measurements over the spin ensembles, we are able to obtain a 2D stray field map as shown in Fig. 2b , which is measured at a temperature T = 6 K and an external magnetic field B ext = 142 G. Through well-established reverse-propagation protocols 46 , 57 , 58 (see Supplementary Information Note 3 for details), we can reconstruct the corresponding magnetization 4 πM map of the FGT flake, as shown in Fig. 2c .…”

“…In this work, we report nanoscale quantum sensing and imaging of exfoliated 2D ferromagnet Fe 3 GeTe 2 (FGT) flakes [33][34][35][36][37][38][39][40][41] by boron vacancy V À B spin defects in an adjacent hBN capping layer. Exploiting a wide-field magnetometry method [42][43][44][45][46] , we directly image the local magnetic texture of the FGT flake and its characteristic temperature and field dependent magnetization evolution behavior. Taking advantage of spin relaxometry techniques [47][48][49][50][51] , we have observed spatially varying spin fluctuations in the FGT flake, whose magnitude reaches a peak value around the Curie temperature, consistent with the expected ferromagnetic phase transition.…”

Ferromagnetic Resonance in Two-dimensional van der Waals Magnets: A Probe for Spin Dynamics

“…Over the past years, NV magnetometry techniques have been demonstrated as a transformative tool in exploring the local static and dynamic spin behaviors in 2D magnets with competitive field sensitivity and spatial resolution. Examples include nanoscale imaging layer-dependent 2D magnetization [195], 2D magnetic domains [196], magnetization reversal processes [197], moiré magnetism [198], room-temperature 2D ferromagnetism [199,200], and others [188,201]. Here, we briefly discuss the opportunity to use NV relaxometry method to probe intrinsic spin fluctuations in 2D magnetic materials, which is challenging to access by conventional magnetometry techniques.…”

“…Figure 13a shows a schematic of NV-based wide-field imaging platform. An MnBi 4 Te 7 nanoflake is transferred onto the diamond surface with shallowly implanted NV ensembles [188]. Figure 13b show an example of the 2D NV relaxation maps.…”