Xiaoqin Li

Quantitative measurement of nanoscale stray magnetic fields during domain reversal is important for optimizing magnetic memory and logic devices. Yet, achieving high spatial resolution with minimally invasive probes remains challenging. Here we demonstrate nitrogen vacancy centers in nanodiamonds (NDs) as local sensors in a CoFeB strip. During magnetic-field-driven reversal, the ND sensors resolve three distinct local responses depending on their positions relative to the strip: a positive frequency jump at the edge, an unexpected negative jump just outside the strip, and a negligible change near the center. These contrasting signals are quantitatively explained by magnetostatic boundary fields from micromagnetic simulations. We further implement a fully constrained fitting procedure for optically detected magnetic resonance spectra, enabling robust field extraction even when resonances are partially resolved. Together, these results extend ND magnetometry to quantitatively characterize heterogeneous stray fields during magnetic domain reversal in thin films and spintronic devices.

Phase transition between different polymorphs of two-dimensional (2D) transition metal dichalcogenides is of great interest for the development of nanoelectronics and optoelectronics. Here, we report the visualization of laser-driven 2H-to-1T′ phase transition in few-layer molybdenum ditelluride (MoTe2) nanosheets by microwave impedance microscopy. The region after intense illumination exhibits clear Raman signatures of the 1T′ phase, as well as much higher local conductance than the untreated area. Moreover, the semiconducting 2H region displays strong photoresponse, whereas the metallic 1T′ region shows little photoconductivity. The spatially resolved information is important for correlating electrical and structural properties in functional 2D materials.

Ferrimagnetic insulators offer a promising route toward low-loss magnonic devices. Here, we report the first direct observation of magnons in high-quality epitaxial lithium aluminum ferrite (LAFO) thin films using inelastic Brillouin light scattering. We observe remarkably sharp magnon resonances with instrument-limited line widths whose frequencies can be tuned by film thickness and applied magnetic field (<300 mT) across a broad 8-28 GHz range. The film thickness dependence confirms that they are discrete standing-wave magnons confined in the direction perpendicular to the film. The field-dependence of the magnon frequencies allows us to extract the exchange constant and magnetic anisotropy. Electronic structure calculations reveal the microscopic origin of the ferrimagnetic order. The insights into magnons in LAFO establish the low-damping spinel ferrite, with its controllable dynamic magnetic properties, as a versatile and Si-compatible platform for future magnonic technologies.

The study of collective behaviour driven by spontaneous symmetry breaking and topology is crucial for understanding phase transitions in quantum matter. The two-dimensional (2D) XY model, describing spins with continuous in-plane rotational symmetry, hosts the topological Berezinskii–Kosterlitz–Thouless (BKT) transition, where vortex–antivortex binding induces quasi-long-range order. This model was later extended to include anisotropy fields, leading to the six-state clock model, which predicts the instability of the BKT phase toward true long-range order at low temperatures. Here we investigate this physics in the van der Waals antiferromagnet NiPS₃ using nonlinear optical micropolarimetry. As the material is thinned to a monolayer, its magnetic response switches abruptly from the 3D XXZ behaviour of multilayers to a distinct 2D regime consistent with a BKT state. Upon further cooling, the monolayer BKT phase becomes unstable and transforms into a pinned state with long-range order. These results, corroborated by Monte Carlo simulations, open pathways to explore spin vortices and topological dynamics in 2D antiferromagnets.

Magnetic bilayers consisting of an epitaxially grown ferrimagnetic insulator and a heavy metal layer are attractive for spintronic application because of the opportunity for electric control and readout of spin textures via spin–orbit torque. Here, we investigate ultrathin thulium iron garnet (TmIG)/Pt bilayers for TmIG layer thicknesses of 3, 2.7, and 2.3 nm using a sensitive Sagnac magneto-optical Kerr effect technique. We compare the hysteresis loops from out-of-plane and in-plane applied magnetic fields. The preferred magnetization orientation evolves with the TmIG thickness and the presence of the Pt overlayer. We quantify the evolution of the magnetic anisotropy in these ultrathin films and find a significant change even when the TmIG thickness is varied by less than 1 nm. In these ultrathin films, the presence of a Pt overlayer changes the effective anisotropy field by more than a factor of 2, suggesting that the interfacial anisotropy at the Pt/TmIG interface plays a critical role in this regime.

Antiferromagnetic (AFM) insulators exhibit many desirable features for spintronic applications such as fast dynamics in the THz range and robustness to fluctuating external fields. However, large damping typically associated with THz magnons presents a serious challenge for THz magnonic applications. Here, we report long-lived short-wavelength zone boundary magnons in the honeycomb AFM insulator CoTiO₃, recently found to host topological magnons. We find that its zone-boundary THz magnons exhibit longer lifetimes than its zone-center magnons. This unusual momentum-dependent long magnon lifetime originates from several factors including the antiferromagnetic order, exchange anisotropy, a finite magnon gap, and magnon band dispersion. Our work suggests that magnon-magnon interaction may not be detrimental to magnon lifetimes and should be included in future searches for topological magnons.

Spin defects in solid-state materials offer a platform for quantum sensing that combines the properties of atom-like systems with the scalability, versatility, and technological maturity of semiconductor devices. The past decade has seen increasing interest in host materials beyond diamond which can offer additional functionality and more effectively leverage the advantage of the existing semiconductor ecosystem. This review provides a survey and comparison of spin defects in silicon carbide, hexagonal boron nitride, and gallium nitride with an emphasis on their applications to magnetometry, electrometry, thermometry, and strain sensing. A practical overview of quantum sensing protocols and sensitivity enhancement is provided along with a final discussion of the future direction of the field and remaining challenges.