Nitrogen-Vacancy Magnetometry of Individual Fe-Triazole Spin Crossover Nanorods

Room Temperature Magnetic Skyrmions in Gradient-Composition Engineered CoPt Single Layers

Topologically protected magnetic skyrmions in magnetic materials are stabilized by an interfacial or bulk Dzyaloshinskii-Moriya interaction (DMI). Interfacial DMI decays with an increase of the magnetic layer thickness in just a few nanometers, and bulk DMI typically stabilizes magnetic skyrmions at low temperatures. Consequently, more flexibility in the manipulation of DMI is required for utilizing nanoscale skyrmions in energy-efficient memory and logic devices at room temperature (RT). Here, we demonstrate the observation of RT skyrmions stabilized by gradient DMI (g-DMI) in composition gradient-engineered CoPt single-layer films by employing the topological Hall effect, magnetic force microscopy, and nitrogen-vacancy scanning magnetometry. Skyrmions remain stable over a wide range of applied magnetic fields and are confirmed to be nearly Bloch-type from micromagnetic simulation and analytical magnetization reconstruction. Furthermore, we observe skyrmion pairs, which may be explained by skyrmion-antiskyrmion interactions. Our findings expand the family of magnetic materials hosting RT magnetic skyrmions by tuning g-DMI via gradient polarity and a choice of magnetic elements.

Microwave Remote Sensing with Hybrid Quantum Receiver

Detecting a microwave signal that is emitted or reflected by distant targets is a powerful tool in fundamental science and industrial technology. Solid-state spins provide an opportunity to realize quantum-enhanced remote sensing under ambient conditions. However, the weak interaction between the free-space signal and atomic size sensor limits the sensitivity. This hinders the realization of practical quantum remote sensing. Here, we demonstrate active microwave remote sensing with a diamond-based hybrid quantum receiver by combining electromagnetic field localization at nanoscale with quantum spin manipulation. A method of differential spin refocusing (DSR) is developed to overcome the challenge of reducing the impact of inhomogeneities in spin-signal interaction, while the strength of interaction is enhanced by more than 3 orders with nanostructure. It improves the coherent interaction time of quantum receiver by 30-fold, substantially enhancing the sensitivity and stability. By detecting the reflected microwave with picotesla sensitivity, diamond remote sensing monitors the real-time status of a centimeter-sized target at 2 m distance. Our method is general to various solid-state spins. The results will expand the applications of solid-state spin quantum sensors in areas ranging from medical imaging to resource survey.

Columnar excitation fluorescence microscope for accurate evaluation of quantum properties of color centers in bulk materials

Color centers in wide band-gap semiconductors, which have superior quantum properties even at room temperature and atmospheric pressure, have been actively applied to quantum sensing devices. Characterizing the quantum properties of the color centers in the semiconductor materials and ensuring that these properties are uniform over a wide area are key issues for developing quantum sensing devices based on color centers. In this article, we have developed an optics design protocol optimized for evaluating the quantum properties of color centers and have used this design approach to develop a new microscopy system called columnar excitation fluorescence microscope (CEFM). The essence of this system is to maximize the amount of fluorescence detection of polarized color centers, which is achieved by large-volume and uniform laser excitation along the sample thickness with sufficient laser power density. This laser excitation technique prevents undesirable transitions to undesirable charge states and undesirable light, such as unpolarized color center fluorescence, while significantly increasing the color center fluorescence. This feature enables fast measurements with a high signal-to-noise ratio, making it possible to evaluate the spatial distribution of quantum properties across an entire mm-size sample without using a darkroom, which is difficult with typical confocal microscope systems.

Reversible light-induced spin state switching in a dinuclear Fe(II) spin crossover complex

A dinuclear Fe(II) spin crossover (SCO) complex with the formula [Fe2L5(NCS)4]·2DMF·2H2O (1) was synthesised from 1-naphthylimino-1,2,4-triazole (L). Complex 1 exhibits an incomplete thermally induced spin transition with a transition temperature T1/2 of 95 K and a thermally trapped metastable high-spin state at low temperatures. Furthermore, it undergoes a reversible light-induced spin crossover by alternate irradiation with 532 and 808 nm lasers.

Sensing the Spin State of Room-Temperature Switchable Cyanometallate Frameworks with Nitrogen-Vacancy Centers in Nanodiamonds

Room-temperature magnetically switchable materials play a vital role in current and upcoming quantum technologies, such as spintronics, molecular switches, and data storage devices. The increasing miniaturization of device architectures produces a need to develop analytical tools capable of precisely probing spin information at the single-particle level. In this work, we demonstrate a methodology using negatively charged nitrogen vacancies (NV-) in fluorescent nanodiamond (FND) particles to probe the magnetic switching of a spin crossover (SCO) metal-organic framework (MOF), [Fe(1,6-naphthyridine)2(Ag(CN)2)2] material (1), and a single-molecule photomagnet [X(18-crown-6)(H2O)3]Fe(CN)6·2H2O, where X = Eu and Dy (materials 2a and 2b, respectively), in response to heat, light, and electron beam exposure. We employ correlative light-electron microscopy using transmission electron microscopy (TEM) finder grids to accurately image and sense spin-spin interacting particles down to the single-particle level. We used surface-sensitive optically detected magnetic resonance (ODMR) and magnetic modulation (MM) of FND photoluminescence (PL) to sense spins to a distance of ca. 10-30 nm. We show that ODMR and MM sensing was not sensitive to the temperature-induced SCO of FeII in 1 as formation of paramagnetic FeIII through surface oxidation (detected by X-ray photoelectron spectroscopy) on heating obscured the signal of bulk SCO switching. We found that proximal FNDs could effectively sense the chemical transformations induced by the 200 keV electron beam in 1, namely, AgI → Ag0 and FeII → FeIII. However, transformations induced by the electron beam are irreversible as they substantially disrupt the structure of MOF particles. Finally, we demonstrate NV- sensing of reversible photomagnetic switching, FeIII + (18-crown-6) ⇆ FeII + (18-crown-6)+ •, triggered in 2a and 2b by 405 nm light. The photoredox process of 2a and 2b proved to be the best candidate for room-temperature single-particle magnetic switching utilizing FNDs as a sensor, which could have applications into next-generation quantum technologies.

Nitrogen-Vacancy Magnetic Relaxometry of Nanoclustered Cytochrome C Proteins

Nitrogen-vacancy (NV) magnetometry offers an alternative tool to detect paramagnetic centers in cells with a favorable combination of magnetic sensitivity and spatial resolution. Here, we employ NV magnetic relaxometry to detect cytochrome C (Cyt-C) nanoclusters. Cyt-C is a water-soluble protein that plays a vital role in the electron transport chain of mitochondria. Under ambient conditions, the heme group in Cyt-C remains in the Fe3+ state, which is paramagnetic. We vary the concentration of Cyt-C from 6 to 54 μM and observe a reduction of the NV spin-lattice relaxation time (T1) from 1.2 ms to 150 μs, which is attributed to the spin noise originating from the Fe3+ spins. NV T1 imaging of Cyt-C drop-casted on a nanostructured diamond chip allows us to detect the relaxation rates from the adsorbed Fe3+ within Cyt-C.