Microwave Excitation Research Articles

Dramatically Enhanced Spin Dynamo with Plasmonic Diabolo Cavity

The applications of spin dynamos, which could potentially power complex nanoscopic devices, have so far been limited owing to their extremely low energy conversion efficiencies. Here, we present a unique plasmonic diabolo cavity (PDC) that dramatically improves the spin rectification signal (enhancement of more than three orders of magnitude) under microwave excitation; further, it enables an energy conversion efficiency of up to ~0.69 mV/mW, compared with ~0.27 μV/mW without a PDC. This remarkable improvement arises from the simultaneous enhancement of the microwave electric field (~13-fold) and the magnetic field (~195-fold), which cooperate in the spin precession process generates photovoltage (PV) efficiently under ferromagnetic resonance (FMR) conditions. The interplay of the microwave electromagnetic resonance and the ferromagnetic resonance originates from a hybridized mode based on the plasmonic resonance of the diabolo structure and Fabry-Perot-like modes in the PDC. Our work sheds light on how more efficient spin dynamo devices for practical applications could be realized and paves the way for future studies utilizing both artificial and natural magnetism for applications in many disciplines, such as for the design of future efficient wireless energy conversion devices, high frequent resonant spintronic devices, and magnonic metamaterials.

Ensemble-averaged Rabi oscillations in a ferromagnetic CoFeB film

Rabi oscillations describe the process whereby electromagnetic radiation interacts coherently with spin states in a non-equilibrium interaction. To date, Rabi oscillations have not been studied in one of the most common spin ensembles in nature: spins in ferromagnets. Here, using a combination of femtosecond laser pulses and microwave excitations, we report the classical analogue of Rabi oscillations in ensemble-averaged spins of a ferromagnet. The microwave stimuli are shown to extend the coherence-time resulting in resonant spin amplification. The results we present in a dense magnetic system are qualitatively similar to those reported previously in semiconductors which have five orders of magnitude fewer spins and which require resonant optical excitations to spin-polarize the ensemble. Our study is a step towards connecting concepts used in quantum processing with spin-transport effects in ferromagnets. For example, coherent control may become possible without the complications of driving an electromagnetic field but rather by using spin-polarized currents.

Paramagnetic Ce3+ optical emitters in garnets: Optically detected magnetic resonance study and evidence of Gd-Ce cross-relaxation effects

Paramagnetic ${\mathrm{Ce}}^{3+}$optical emitters have been studied by means of optically detected magnetic resonance (ODMR) via ${\mathrm{Ce}}^{3+}$ spin-dependent emission in cerium-doped garnet crystals which were both gadolinium free and contain gadolinium in a concentration from the lowest (0.1%) to 100%, i.e., to the superparamagnetic state. It has been shown that the intensity of photoluminescence excited by circularly polarized light into ${\mathrm{Ce}}^{3+}$ absorption bands can be used for selective monitoring the population of the ${\mathrm{Ce}}^{3+}$ ground-state spin sublevels. Direct evidence of the cross-relaxation effects in garnet crystals containing two electron spin systems, i.e., the simplest one of ${\mathrm{Ce}}^{3+}$ ions with the effective spin $S=\frac{1}{2}$ and the system of ${\mathrm{Gd}}^{3+}$ ions with the maximum spin $S=\frac{7}{2}$, has been demonstrated. Magnetic resonance of ${\mathrm{Gd}}^{3+}$ has been found by monitoring ${\mathrm{Ce}}^{3+}$ emission in cerium-doped garnet crystals with gadolinium concentrations of 0.1 at. %, 4%--8%, and 100%, which implies the impact of the ${\mathrm{Gd}}^{3+}$ spin polarization on the optical properties of ${\mathrm{Ce}}^{3+}$. Strong internal magnetic fields in superparamagnetic crystals were shown to modify the processes of recombination between UV-radiation-induced electron and hole centers that lead to the recombination-induced ${\mathrm{Ce}}^{3+}$ emission. Observation of spikes and subsequent decay in the cross-relaxation-induced ODMR signals under pulsed microwave excitation is suggested to be an informative method to investigate transient processes in the many-spin system of ${\mathrm{Ce}}^{3+}, {\mathrm{Gd}}^{3+}$, and electron and hole radiation-induced centers.

Microwave-excited ultrasound and thermoacoustic dual imaging

We designed a microwave-excited ultrasound (MUI) and thermoacoustic dual imaging system. Under the pulsed microwave excitation, the piezoelectric transducer used for thermoacoustic signal detection will also emit a highly directional ultrasonic beam based on the inverse piezoelectric effect. With this beam, the ultrasonic transmitter circuitry of the traditional ultrasound imaging (TUI) system can be replaced by a microwave source. In other words, TUI can be fully integrated into the thermoacoustic imaging system by sharing the microwave excitation source and the transducer. Moreover, the signals of the two imaging modalities do not interfere with each other due to the existence of the sound path difference, so that MUI can be performed simultaneously with microwave-induced thermoacoustic imaging. In the study, the performance characteristics and imaging capabilities of this hybrid system are demonstrated. The results indicate that our design provides one easy method for low-cost platform integration and has the potential to offer a clinically useful dual-modality tool for the detection of accurate diseases.

Abstracted Random Mediums for Electromagnetic Hotspot Observation in Finite-Difference Time-Domain Simulation

Microwave excitation of energetic materials can remotely incite ignition and deflagration. While this has been experimentally observed, the underlying principles are not well understood due to the complexity of analytical solutions. Further complicating understanding is the sensitivity of the effect to geometry and the difficulty of creating simulation environments representative of realistic materials. Manageable analysis requires abstractions, and here abstractions ranging from regularly arranged spheres and cubes to randomly arranged spheres, cubes, and arbitrarily shaped crystals are examined. The randomly arranged cube model provides acceptable prediction of thermal and peak electric field hotspots while having manageable computational complexity. The hotspots resulting from multiple subwavelength scattering occur inside the body of the energetic material and are localized in time and space having spans that are a few percent or less of the period and wavelength of pulsed electromagnetic excitation.

Continuous-flow DNP polarizer for MRI applications at 1.5\u2009T

Here we describe a new hyperpolarization approach for magnetic resonance imaging applications at 1.5 T. Proton signal enhancements of more than 20 were achieved with a newly designed multimode microwave resonator situated inside the bore of the imager and used for Overhauser dynamic nuclear polarization of the water proton signal. Different from other approaches in our setup the hyperpolarization is achieved continuously by liquid water flowing through the polarizer under continuous microwave excitation. With an available flow rate of up to 1.5 ml/min, which should be high enough for DNP MR angiography applications in small animals like mice and rats. The hyperpolarized liquid cooled to physiological temperature can be routed by a mechanical switch to a quartz capillary for injection into the blood vessels of the target object. This new approach allows hyperpolarization of protons without the need of an additional magnet and avoids the losses arising from the transfer of the hyperpolarized solution between magnets. The signal-to-noise improvement of this method is demonstrated on two- and three-dimensional phantoms of blood vessels.

Spatial mapping of spin pumping from magnetic insulator

We demonstrate the spatial mapping of the spin pumping from a magnetic insulator, yttrium iron garnet (YIG), using the inverse spin Hall effect. The inverse spin Hall effect in thin Pt wires placed on a YIG film enables local detection of the spin-current emission from the insulator, revealing spatially nonuniform spin pumping at various microwave excitation frequencies. We found that the spatial variation of the spin pumping is sensitive to the wavelength of the magnons responsible for the spin pumping as well as the thickness of the magnetic insulator. These findings provide an important information for studying spintronic phenomena in metal/insulator bilayers.

CONTROL OF A MOS INVERTER BY OUT-OF-BAND PULSED MICROWAVE EXCITATION

An intentional focusing of High-Power Microwave (HPM) energy on microelectronic systems can produce effects that will potentially upset or damage the target. However, the physical mechanisms at work within the device are not often well understood. We provide a detailed understanding of the physical mechanisms involved in a common-source Metal Oxide Semiconductor (MOS) transistor inverter when Pulsed Microwave Excitation (PME) in a frequency range from 10 MHz to 1 GHz is applied on the gate terminal. Our study is based on the measurements of the current waveforms on all transistor access and explains the MOS response with and without the Radio-Frequency (RF) interference.

Control of Magnon-Photon Coupling Strength in a Planar Resonator/Yttrium-Iron-Garnet Thin-Film Configuration

A systematic study of the coupling at room temperature between ferromagnetic resonance and a planar resonator is presented. The chosen magnetic material is a ferrimagnetic insulator (yttrium iron garnet, YIG) which is positioned on top of a stop band (notch) filter based on a stub line capacitively coupled to a 50 Ω microstrip line resonating at 4.731 GHz. Control of the magnon-photon coupling strength is discussed in terms of the microwave excitation configuration and the YIG thickness from 0.2 to 41 μm. From the latter dependence, we extract a single spin-photon coupling of g 0 /2π = 162 ± 6 mHz and a maximum of an effective coupling of 290 MHz.

Construction of 0.15 Tesla Overhauser Enhanced MRI.

Overhauser enhanced MRI (OMRI) is one of the free radical imaging technologies and has been used in biomedical research such as for partial oxygen measurements in tumor, and redox status in acute oxidative diseases. The external magnetic field of OMRI is frequently in the range of 5-10 mTesla to ensure microwave penetration into small animals, and the S/N ratio is limited. In this study, a 0.15 Tesla OMRI was constructed and tested to improve the S/N ratio for a small sample, or skin measurement. Specification of the main magnet was as follows: 0.15 Tesla permanent magnet; gap size 160mm; homogenous spherical volume of 80 mm in diameter. The OMRI resonator was designed based on TE101 cavity mode and machined from a phosphorus deoxidized copper block for electron spin resonance (ESR) excitation and a solenoid transmission/receive resonator for NMR detection. The resonant frequencies and Q values were 6.38MHz/150 and 4.31-4.41GHz/120 for NMR and ESR, respectively. The Q values were comparable to those of conventional low field OMRI resonators at 15 mTesla. As expected, the MRI S/N ratio was improved by a factor of 30. Triplet dynamic nuclear polarization spectra were observed for 14N carboxy-PROXYL, along the excitation microwave sweep. In the current setup, the enhancement factor was ca. 0.5. In conclusion, the results of this preliminary evaluation indicate that the 0.15 Tesla OMRI could be useful for free radical measurement for small samples.

Influence of static and dynamic dipolar fields in bulk YIG/thin film NiFe systems probed via spin rectification effect

The characteristics of the static and dynamic components of the dipolar fields originating from a bulk polycrystalline yttrium iron garnet (YIG) substrate are probed by depositing a NiFe (Permalloy) layer on it, which acts as a detector. By measuring dc voltages generated via spin rectification effect (SRE) within the NiFe layer under microwave excitation, we characterize the influence of dipolar fields from bulk YIG on the NiFe layer. It is found that the dynamic YIG dipolar fields modify the self-SRE of NiFe, driving its own rectification voltages within the NiFe layer, an effect we term as non-local SRE. This non-local SRE only occurs near the simultaneous resonance of both YIG and NiFe. On the other hand, the static dipolar field from YIG manifests itself as a negative anisotropy in the NiFe layer which shifts the latter’s ferromagnetic resonance frequency.

Adaptive microwave impedance memory effect in a ferromagnetic insulator.

Adaptive electronics, which are often referred to as memristive systems as they often rely on a memristor (memory resistor), are an emerging technology inspired by adaptive biological systems. Dissipative systems may provide a proper platform to implement an adaptive system due to its inherent adaptive property that parameters describing the system are optimized to maximize the entropy production for a given environment. Here, we report that a non-volatile and reversible adaptive microwave impedance memory device can be realized through the adaptive property of the dissipative structure of the driven ferromagnetic system. Like the memristive device, the microwave impedance of the device is modulated as a function of excitation microwave passing through the device. This kind of new device may not only helpful to implement adaptive information processing technologies, but also may be useful to investigate and understand the underlying mechanism of spontaneous formation of complex and ordered structures.

Scanning Microwave Microscopy for Electronic Device Analysis on Nanometre Scale

Probing electrical properties of state-of-the-art electronic devices is one of the key features of Scanning Microwave Microscopy. While providing valuable information on charge carrier properties, the combination of an atomic force microscope cantilever with a microwave signal raises the question on the actual spatial resolution of the system. On the example of the highly confined two-dimensional electron gas of an AlGaN/GaN structure, the effective tip radius is demonstrated to be in the range of the theoretical tip radius for sharp tips, while both values differ for unevenly shaped cantilever tips. The presented method demonstrates the role of the microwave excitation region for the spatial resolution of the system as well as the potential of this method to characterise the effective tip radius.

Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla

Abstract Dynamic nuclear polarization (DNP) is a methodology to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. It relies on the transfer of the electron spin polarization from a radical to coupled nuclear spins, driven by microwave excitation resonant with the electron spin transitions. In this work we explore the potential of pulsed multi-frequency microwave excitation in liquids. Here, the relevant DNP mechanism is the Overhauser effect. The experiments were performed with TEMPOL radicals in aqueous solution at room temperature using a Q-band frequency (1.2 T) electron paramagnetic resonance (EPR) spectrometer combined with a Minispec NMR spectrometer. A fast arbitrary waveform generator (AWG) enabled the generation of multi-frequency pulses used to either sequentially or simultaneously excite all three 14N-hyperfine lines of the nitroxide radical. The multi-frequency excitation resulted in a doubling of the observed DNP enhancements compared to single-frequency microwave excitation. Q-band free induction decay (FID) signals of TEMPOL were measured as a function of the excitation pulse length allowing the efficiency of the electron spin manipulation by the microwave pulses to be extracted. Based on this knowledge we could quantitatively model our pulsed DNP enhancements at 1.2 T by numerical solution of the Bloch equations, including electron spin relaxation and experimental parameters. Our results are in good agreement with theoretical predictions. Whereas for a narrow and homogeneous single EPR line continuous wave excitation leads to more efficient DNP enhancements compared to pulsed excitation for the same amount of averaged microwave power. The situation is different for radicals with several hyperfine lines or in the presence of inhomogeneous line broadening. In such cases pulsed single/multi-frequency excitation can lead to larger DNP enhancements.

Combined Brillouin light scattering and microwave absorption study of magnon-photon coupling in a split-ring resonator/YIG film system

Microfocused Brillouin light scattering (BLS) and microwave absorption (MA) are used to study magnon-photon coupling in a system consisting of a split-ring microwave resonator and an yttrium iron garnet (YIG) film. The split-ring resonator is defined by optical lithography and loaded with a 1 μm-thick YIG film grown by liquid phase epitaxy. BLS and MA spectra of the hybrid system are simultaneously recorded as a function of the applied magnetic field magnitude and microwave excitation frequency. Strong coupling of the magnon and microwave resonator modes is found with a coupling strength of geff /2π = 63 MHz. The combined BLS and MA data allow us to study the continuous transition of the hybridized modes from a purely magnonic to a purely photonic mode by varying the applied magnetic field and microwave frequency. Furthermore, the BLS data represent an up-conversion of the microwave frequency coupling to optical frequencies.

Signatures of topological Josephson junctions

Quasiparticle poisoning and diabatic transitions may significantly narrow the window for the experimental observation of the $4\pi$-periodic $dc$ Josephson effect predicted for topological Josephson junctions. Here, we show that switching current measurements provide accessible and robust signatures for topological superconductivity which persist in the presence of quasiparticle poisoning processes. Such measurements provide access to the phase-dependent subgap spectrum and Josephson currents of the topological junction when incorporating it into an asymmetric SQUID together with a conventional Josephson junction with large critical current. We also argue that pump-probe experiments with multiple current pulses can be used to measure the quasiparticle poisoning rates of the topological junction. The proposed signatures are particularly robust, even in the presence of Zeeman fields and spin-orbit coupling, when focusing on short Josephson junctions. Finally, we also consider microwave excitations of short topological Josephson junctions which may complement switching current measurements.

Catalytic degradation of PNP and stabilization/solidification of Cd simultaneously in soil using microwave-assisted Fe-bearing attapulgite

A novel Fe-bearing attapulgite (Fe/ATP) as microwave (MW) absorber, cadmium (Cd) stabilizer and active catalyst to simultaneously stabilize/solidify Cd and oxidize p-nitrophenol (PNP) in soil was reported. The toxicity characteristic leaching procedure tests showed 80% of Cd was successfully stabilized in soil with Fe/ATP addition, whereas Cd stabilization efficiency of 99.3% was successfully achieved in MW-assisted Fe/ATP system. In addition, the long-term stabilization efficiency of Cd (180-day of curing) increased about 2-fold in MW-assisted Fe/ATP system compared with Fe/ATP system. XRD, FTIR, and XPS studies showed that Cd was more mineral-integrated than surface-bound with Fe/ATP, and a part of the stabilized Cd in Fe/ATP transformed into ferrite or encapsulated in vitrification with MW irradiation. Fe/ATP also showed extremely high catalytic performance for PNP oxidation, achieving complete PNP removal from soil within 20min of MW irradiation. The enhancements of Cd stabilization and PNP removal were attributed to the “hot spots” effect on the Fe/ATP surface with MW excitation. In addition, active species (holes, OH, and O2−) generated from the redox of Fe2+–Fe3+ species were responsible for the PNP degradation. The use of Fe/ATP to catalytic oxidize PNP and stabilize/solidify Cd under MW irradiation was demonstrated to be an effective method for rapid remediation of organic-heavy metal co-contaminated soil.

Magnetoelectric control of spin currents

The ability to control the spin current injection has been explored on a hybrid magnetoelectric system consisting of a (011)-cut ferroelectric lead magnesium niobate-lead titanate (PMNT) single crystal, a ferromagnetic FePt alloy, and a metallic Pt. With this PMNT/FePt/Pt structure we have been able to control the magnetic field position or the microwave excitation frequency at which the spin pumping phenomenon between FePt and Pt occurs. We demonstrate that the magnetoelectric heterostructure operating in the L-T (longitudinal magnetized-transverse polarized) mode couples the PMNT crystal to the magnetostrictive FePt/Pt bilayer, displaying a strong magnetoelectric coefficient of ∼140 Oe cm kV−1. Our results show that this mechanism can be effectively exploited as a tunable spin current intensity emitter and open the possibility to create an oscillating or a bistable switch to effectively manipulate spin currents.

Interplay between electron overheating and ac Josephson effect

We study the response of high-critical current proximity Josephson junctions to a microwave excitation. Electron over-heating in such devices is known to create hysteretic dc voltage-current characteristics. Here we demonstrate that it also strongly influences the ac response. The interplay of electron over-heating and ac Josephson dynamics is revealed by the evolution of the Shapiro steps with the microwave drive amplitude. Extending the resistively shunted Josephson junction model by including a thermal balance for the electronic bath coupled to phonons, a strong electron over-heating is obtained.

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.