Simplified Algorithms for Adaptive Experiment Design in Parameter Estimation.

Measurements to estimate parameters of a model are commonplace in the physical sciences, where the traditional approach to automation is to use a sequence of preselected settings followed by least-squares fitting of a model function to the data. This measure-then-fit approach is simple and effective and entirely appropriate for many applications but when measurement resources are limited, efficiency becomes more important. To increase efficiency, Bayesian experiment design allows measurement settings to be chosen adaptively based on accumulated data and utility, the predicted improvement in results as a function of settings. However, the calculation of utility has been judged too impractical for most applications. In this paper, we introduce computational methods and simplified algorithms that accelerate utility calculations by over an order of magnitude, with only slight degradation in measurement efficiency. The methods eliminate utility calculation as a barrier to practical application of efficient adaptive measurement.

Effect of strain-induced anisotropy on magnetization dynamics in Y3Fe5O12 films recrystallized on a lattice-mismatched substrate.

We report on the correlation of structural and magnetic properties of Y3Fe5O12 (YIG) films deposited on Y3Al5O12 substrates using pulsed laser deposition. The recrystallization process leads to an unexpected formation of interfacial tensile strain and consequently strain-induced anisotropy contributing to the perpendicular magnetic anisotropy. The ferromagnetic resonance linewidth of YIG is significantly increased in comparison to a film on a lattice-matched Gd3Ga5O12 substrate. Notably, the linewidth dependency on frequency has a negative slope. The linewidth behavior is explained with the proposed anisotropy dispersion model.

Sequential Bayesian experiment design for adaptive Ramsey sequence measurements.

The Ramsey sequence is a canonical example of a quantum phase measurement for a spin qubit. In Ramsey measurements, the measurement efficiency can be optimized through careful selection of settings for the phase accumulation time setting, τ. This paper implements a sequential Bayesian experiment design protocol in low-fidelity Ramsey measurements, and its performance is compared to a previously reported adaptive heuristic protocol, a quantum phase estimation algorithm, and random setting choices. A workflow allowing measurements and design calculations to run concurrently largely eliminates computation time from measurement overhead. When precession frequency is the lone parameter to estimate, the Bayesian design is faster by factors of roughly 2 and 4 and 5 relative to the adaptive heuristic, random τ choices and the quantum phase estimation algorithm respectively. When four parameters are to be determined, Bayesian experiment design and random τ choices can converge to roughy equivalent sensitivity, but the Bayesian method converges 4 times faster.

Scalable microresonators for room-temperature detection of electron spin resonance from dilute, sub-nanoliter volume solids.

We report a microresonator platform that allows room temperature detection of electron spins in volumes on the order of 100 pl, and demonstrate its utility to study low levels of dopants in perovskite oxides. We exploit the toroidal moment in a planar anapole, using a single unit of an anapole metamaterial architecture to produce a microwave resonance exhibiting a spatially confined magnetic field hotspot and simultaneously high quality-factor (Q-factor). To demonstrate the broad implementability of this design and its scalability to higher frequencies, we deploy the microresonators in a commercial electron paramagnetic resonance (EPR) spectrometer operating at 10 GHz and a NIST-built EPR spectrometer operating at 35 GHz. We report continuous-wave (CW) EPR spectra for various samples, including a dilute Mn2+-doped perovskite oxide, CaTiO3, and a transition metal complex, CuCl22H2O. The anapole microresonator presented here is expected to enable multifrequency EPR characterization of dopants and defects in perovskite oxide microcrystals and other volume-limited materials of technological importance.

Sequential Bayesian experiment design for optically detected magnetic resonance of nitrogen-vacancy centers.

In magnetometry using optically detected magnetic resonance of nitrogen vacancy (NV-) centers, we demonstrate more than one order-of-magnitude speed up with sequential Bayesian experiment design as compared with conventional frequency-swept measurements. The NV- center is an excellent platform for magnetometry with potential spatial resolution down to few nanometers and demonstrated single-defect sensitivity down to nT/Hz1/2. The NV- center is a quantum defect with spin S = 1 and coherence time up to several milliseconds at room temperature. Zeeman splitting of the NV- energy levels allows detection of the magnetic field via photoluminescence. We compare conventional NV- center photoluminescence measurements that use pre-determined sweeps of the microwave frequency with measurements using a Bayesian inference methodology. In sequential Bayesian experiment design, the settings with maximum utility are chosen for each measurement in real time based on the accumulated experimental data. Using this method, we observe an order of magnitude decrease in the NV- magnetometry measurement time necessary to achieve a set precision.

Quasi-Two-Dimensional Magnon Identification in Antiferromagnetic FePS3via Magneto-Raman Spectroscopy.

Recently it was discovered that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer, opening many avenues in fundamental physics and potential applications of these fascinating materials. One such material is FePS3, a large spin (S=2) Mott insulator where the Fe atoms form a honeycomb lattice. In the bulk, FePS3 has been shown to be a quasi-two-dimensional-Ising antiferromagnet, with additional features in the Raman spectra emerging below the Néel temperature (TN) of approximately 120 K. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that one of these Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of ≈3.7 THz (122 cm-1). Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, where we determine the g-factor to be ≈2. In addition, the symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of FePS3.

A differential rate meter for real-time peak tracking in optically detected magnetic resonance at low photon count rates.

We describe a real-time method to obtain normalized differential count rate signals from modulated systems with photon detection at single-photon count rates. The method is demonstrated with a real-time peak-locking and frequency control to track magnetic field using optically detected magnetic resonance of nitrogen-vacancy centers in diamond. This procedure allows us to measure the magnetic field continuously with a sensitivity of 4.1 µT/Hz1/2 and to track magnetic field sweep rates up to 50 µT/s. The differential rate meter automates the processing of voltage pulse outputs from the photon detector and provides noise levels on par with traditional photon counting methods using digital counters.

Spin-Torque Excitation of Perpendicular Standing Spin Waves in Coupled YIG/Co Heterostructures.

We investigate yttrium iron garnet (YIG)/cobalt (Co) heterostructures using broadband ferromagnetic resonance (FMR). We observe an efficient excitation of perpendicular standing spin waves (PSSWs) in the YIG layer when the resonance frequencies of the YIG PSSWs and the Co FMR line coincide. Avoided crossings of YIG PSSWs and the Co FMR line are found and modeled using mutual spin pumping and exchange torques. The excitation of PSSWs is suppressed by a thin aluminum oxide interlayer but persists with a copper interlayer, in agreement with the proposed model.

Spin wave localization in tangentially magnetized films.

We present an analytical description of localized spin wave modes that form in a parabolic field minimum in a thin ferromagnetic film. Mode profiles proportional to Hermite functions are eigenfuctions of the applied field and exchange parts of the equations of motion, and also provide a basis for numerical approximation of magnetostatic interactions. We find that the spin wave modes are roughly equally spaced in frequency and have roughly equal coupling to a uniform driving field. The calculated mode frequencies and corresponding profiles of localized spin wave modes are in good agreement with micromagnetic modeling and previously published experimental results on multiple resonances from a series of localized modes detected by ferromagnetic resonance force microscopy.

Phase-resolved ferromagnetic resonance using heterodyne detection method.

This paper describes a phase-resolved ferromagnetic resonance (FMR) measurement using a heterodyne method. Spin precession is driven by microwave fields and detected by 1550 nm laser light that is modulated at a frequency slightly shifted with respected to the FMR driving frequency. The evolving phase difference between the spin precession and the modulated light produces a slowly oscillating Kerr rotation signal with a phase equal to the precession phase plus a phase due to the path length difference between the excitation microwave signal and the optical signal. We estimate the accuracy of the precession phase measurement to be 0.1 rad. This heterodyne FMR detection method eliminates the need for field modulation and allows a stronger detection signal at higher intermediate frequency where the 1/f noise floor is reduced.

Noncontact conductivity and dielectric measurement for high throughput roll-to-roll nanomanufacturing.

Advances in roll-to-roll processing of graphene and carbon nanotubes have at last led to the continuous production of high-quality coatings and filaments, ushering in a wave of applications for flexible and wearable electronics, woven fabrics, and wires. These applications often require specific electrical properties, and hence precise control over material micro- and nanostructure. While such control can be achieved, in principle, by closed-loop processing methods, there are relatively few noncontact and nondestructive options for quantifying the electrical properties of materials on a moving web at the speed required in modern nanomanufacturing. Here, we demonstrate a noncontact microwave method for measuring the dielectric constant and conductivity (or geometry for samples of known dielectric properties) of materials in a millisecond. Such measurement times are compatible with current and future industrial needs, enabling real-time materials characterization and in-line control of processing variables without disrupting production.

Effects of disorder and internal dynamics on vortex wall propagation.

Experimental measurements of domain wall propagation are typically interpreted by comparison to reduced models that ignore both the effects of disorder and the internal dynamics of the domain wall structure. Using micromagnetic simulations, we study vortex wall propagation in magnetic nanowires induced by fields or currents in the presence of disorder. We show that the disorder leads to increases and decreases in the domain wall velocity depending on the conditions. These results can be understood in terms of an effective damping that increases as disorder increases. As a domain wall moves through disorder, internal degrees of freedom get excited, increasing the energy dissipation rate.