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We present a study of the low-temperature, high-field magnetotransport behavior of epitaxially grown ${\mathrm{CrO}}_2$ thin films on (100) ${\mathrm{TiO}}_2$ substrates. Electron transport measurements confirm high sample quality, with a residual resistivity ratio as large as 45. A logarithmic fit of the low-temperature regime yields a $T^{4.5}$ power law, indicative of the elusive double-magnon scattering in half-metallic ferromagnets. Angle-dependent anisotropic magnetoresistance (AMR) measurements reveal a large projected density of states along [001] with a maximum AMR value of 2.4%. Pronounced, axis-dependent Shubnikov–de Haas (SdH) oscillations are observed in the longitudinal magnetoresistance (MR), with discrete Landau levels mapped at various tilt angles of the magnetic field. Their angular dependence follows a $1/cos\theta$ relation pointing to a highly anisotropic Fermi surface, and amplitude scaling adheres closely to the thermodynamic Lifshitz-Kosevich formalism. The out-of-plane MR reaches 6.58%, the largest reported for ${\mathrm{CrO}}_2$ thin films. Notably, this study presents an observation of SdH oscillations in a half-metallic oxide with near-perfect spin polarization, a significant advancement in the field of spintronics. 

The nitrogen-vacancy (NV) center in diamond is an increasingly popular quantum sensor for microscopy of electrical current, magnetization, and spins. However, efficient NV–sample integration with a robust, high-quality interface remains an outstanding challenge to realize scalable, high-throughput microscopy. In this work, we characterize a diamond micro-chip (DMC) containing a (111)-oriented NV ensemble and demonstrate its utility for high-resolution quantum microscopy. We perform strain imaging of the DMC and find minimal detrimental strain variation across a field of view of tens of micrometer. We find good ensemble NV spin coherence and optical properties in the DMC, suitable for sensitive magnetometry. We then use the DMC to demonstrate wide-field microscopy of electrical current and show that diffraction-limited quantum microscopy can be achieved. We also demonstrate the deterministic transfer of DMCs with multiple materials of interest for next-generation electronics and spintronics. Lastly, we develop a polymer-based technique for DMC placement. This work establishes the DMC's potential to expand the application of NV quantum microscopy in materials, device, geological, biomedical, and chemical sciences. 

Atomic layer etching (ALE) is an emerging technology to etch thin films with atomic level precision for microelectronics industry applications. This approach has been previously demonstrated to work on a number of materials; however, in most cases, only electronic properties of these materials following ALE are investigated. Since ALE of complex magnetic materials is extremely important for use in magnetic tunnel junctins (MTJs), it is imperative to understand how this etching approach affects the magnetic properties of the corresponding films. In this work, we studied the surface morphology, elemental composition, and most importantly, the magnetic properties of the technologically relevant magnetic alloy CoFeB before and after ALE treatment, and compared with the traditional ion milling etching technique. Through ferromagnetic resonance measurements, we find while the change in the saturation magnetization from ALE is small, the Gilbert damping of CoFeB is reduced by 11–35%, possibly due to the suppressed two-magnon scattering processes on the sample surface. Our results show that ALE can be used to etch CoFeB nondestructively and may even improve its magnetization dynamics properties. 

Abstract Second-harmonic Hall voltage (SHV) measurement method has been widely used to characterize the strengths of spin–orbit torques (SOTs) in heavy metal/ferromagnet thin films saturated in the single-domain regime. Here, we show that the magnetic anisotropy of a W/Pt/Co trilayer can be robustly tuned from in-plane to out-of-plane by varying W, Pt, or Co thicknesses. Moreover, in samples with easy-cone anisotropy, SHV measurements exhibit anomalous ‘humps’ in the multidomain regime accessed by applying a nearly out-of-plane external magnetic field. These hump features can only be explained as a result of the formation of Néel-type domain walls, efficiently driven by nevertheless small SOTs in this double heavy metal heterostructure with canceling spin Hall angles. 

Abstract We demonstrate direct probing of strong magnon–photon coupling using Brillouin light scattering (BLS) spectroscopy in a planar geometry. The magnonic hybrid system comprises a split-ring resonator loaded with epitaxial yttrium iron garnet thin films of 200 nm and 2.46  μ m thickness. The BLS measurements are combined with microwave spectroscopy measurements where both biasing magnetic field and microwave excitation frequency are varied. The cooperativity for the 200 nm-thick YIG films is 1.1, and larger cooperativity of 29.1 is found for the 2.46 μ m-thick YIG film. We show that BLS is advantageous for probing the magnonic character of magnon–photon polaritons, while microwave absorption is more sensitive to the photonic character of the hybrid excitation. A miniaturized, planar device design is imperative for the potential integration of magnonic hybrid systems in future coherent information technologies, and our results are a first stepping stone in this regard. Furthermore, successfully detecting the magnonic hybrid excitation by BLS is an essential step for the up-conversion of quantum signals from the microwave to the optical regime in hybrid quantum systems. 

Breaking the time-reversal symmetry on the surface of a topological insulator can open a gap for the linear dispersion and make the Dirac fermions massive. This can be achieved by either doping a topological insulator with magnetic elements or proximity-coupling it to magnetic insulators. While the exchange gap can be directly imaged in the former case, measuring it at the buried magnetic insulator/topological insulator interface remains to be challenging. Here, we report the observation of a large nonlinear Hall effect in iron garnet/Bi2Se3 heterostructures. Besides illuminating its magnetic origin, we also show that this nonlinear Hall effect can be utilized to measure the size of the exchange gap and the magnetic-proximity onset temperature. Our results demonstrate the nonlinear Hall effect as a spectroscopic tool to probe the modified band structure at magnetic insulator/topological insulator interfaces.