RF Magnetic Field Research Articles

Hairpin RF resonators for MR imaging transceiver arrays with high inter-channel isolation and B1 efficiency at ultrahigh field 7 T

Electromagnetic decoupling among a close-fitting or high-density transceiver RF array elements is required to maintain the integrity of the magnetic flux density from individual channel for enhanced performance in detection sensitivity and parallel imaging. High-impedance RF coils have demonstrated to be a prominent design method to circumvent these coupling issues. Yet, inherent characteristics of these coils have ramification on the B1 field efficiency and SNR. In this work, we propose a hairpin high impedance RF resonator design for highly decoupled multichannel transceiver arrays at ultrahigh magnetic fields. Due to the high impedance property of the hairpin resonators, the proposed transceiver array can provide high decoupling performance without using any dedicated decoupling circuit among the resonant elements. Because of elimination of lumped inductors in the resonator circuit, higher B1 field efficiency in imaging subjects can be expected. In order to validate the feasibility of the proposed hairpin RF coils, systematical studies on decoupling performance, field distribution, and SNR are performed, and the results are compared with those obtained from existing high-impedance RF coil, e.g., “self-decoupled RF coil”. To further investigate its performance, an 8-channel head coil array using the proposed hairpin resonators loaded with a cylindrical phantom is designed, demonstrating a 19 % increase of the B1+ field intensity compared to the self-decoupled coils at 7 T. Furthermore, the characteristics of the hairpin RF coils are evaluated using a more realistic human head voxel model numerically. The proposed hairpin RF coil provides excellent decoupling performance and superior RF magnetic field efficiency compared to the “self-decoupled” high impedance coils. Bench test of a pair of fabricated hairpin coils prove to be in good accordance with numerical results.

High-Sensitivity rf Detection Using an Optically Pumped Comagnetometer Based on Natural-Abundance Rubidium with Active Ambient-Field Cancellation

To detect a specific radio-frequency (rf) magnetic field, rf optically pumped magnetometers (OPMs) require a static magnetic field to set the Larmor frequency of the atoms equal to the frequency of interest. However, unshielded and variable magnetic field environments (e.g., an rf OPM on a moving platform) pose a problem for rf OPM operation. Here, we demonstrate the use of a natural-abundance rubidium vapor to make a comagnetometer to address this challenge. Our implementation builds upon the simultaneous application of several OPM techniques within the same vapor cell. First, we use a modified implementation of an OPM variometer based on ${}^{87}$$\mathrm{Rb}$ to detect and actively cancel unwanted external fields at frequencies $⪅60\phantom{\rule{0.2em}{0ex}}\mathrm{Hz}$ using active feedback to a set of field control coils. We exploit this stabilized field environment to implement a high-sensitivity rf magnetometer using ${}^{85}$$\mathrm{Rb}$. Using this approach, we demonstrate the ability to measure rf fields with a sensitivity of approximately $9\phantom{\rule{0.2em}{0ex}}\mathrm{fT}\phantom{\rule{0.1em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$ inside a magnetic shield in the presence of an applied field of approximately $20\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{T}$ along three mutually orthogonal directions. This demonstration opens up a path toward completely unshielded operation of a high-sensitivity rf OPM.

Injection locking of spin torque nano oscillators using surface acoustic waves

Conventional methods use RF currents or magnetic fields for injection locking the Spin Torque Nano Oscillators (STNO) devices. In this paper, we demonstrate the injection locking of STNO devices using the Surface Acoustic Waves (SAW). The SAW interacts with the free layer of the STNO devices through magneto elastic coupling. This interaction produces some effective magnetic field, which we show, can be used for injection locking of the STNO device. We have studied the dynamics of the injection locking process and how it depends upon the characteristics of the driving SAW signal in great details. This is done through macrospin as well as micromagnetic simulations. Our results suggest that SAW can be very good candidates for injection locking of the STNO devices with a locking range of 50 MHz and locking time of 10 ns for a strain of 100 ppm. This roughly corresponds to the requirement of RF current of amplitude 0.05 mA or field of amplitude 5 Oe, if we use the conventional methods. To explain our simulation results, we use the universal auto oscillator model. The amplitude of the locking signal depends strongly upon the power of the oscillations in case of SAW. We have extended the universal auto oscillator model to consider this effect and explain our results.

Design and simulation of the high efficiency ion source for Damavand NBI heating system

Neutral beam injection (NBI) is known as one of the important methods of plasma auxiliary heating and current drive in fusion reactors. Optimization of the ion sources, as one of the main components of NBI systems, with high current and minimum divergence, can be very effective in order to reach more optimal dimensions of NBI systems. A high efficiency ion source designed using a helicon plasma source for neutral beam injection system (NBI) of Damavand Tokamak. The extraction system has the ability to extract a positive hydrogen ion beam with the current and voltage of 7A and 4.5 kV, respectively. In this paper, an ion source with optimized helicon plasma and optimized beam extraction system is developed using the COMSOL software. All parameters, including RF power, magnetic field, gas pressure, antenna length, and RF frequency, for the production of helicon plasma and ion beam extraction system with optimal geometry are investigated in the most effective way, in order to reduce the beam divergence and beam current increment. Finally, the helicon plasma with density of 4.12×1018m−3, electron temperature of 2.8eV and hydrogen beam extraction system consists of three Accel-Decel grid, each grid with 280 apertures of 3.5mm diameter, has been designed for neutral beam injection system of Damavand Tokamak. This ion production system has an extractor grid with very large transparency (60%) and the minimum single beamlet divergence of 27 mrad.

Nonlinear magnetoelectric effect in atomic vapor and its application to precision radio-frequency magnetometry

We demonstrate the nonlinear magnetoelectric (ME) effect in atomic vapor achieved through the parametric interaction of optical electric and radio-frequency (rf) magnetic fields leading to the generation of new optical electric fields. Density matrix calculations are performed to validate the experimental results. Moreover, the predicted dependence of the generated optical electric field amplitudes on the rf magnetic field strength is experimentally verified to confirm the ME effect. The system provides a technique for precision rf magnetometry based on this phenomenon. We could experimentally achieve an rf magnetic field sensitivity of 70 $\mathrm{fT}/\sqrt{\text{Hz}}$ at 1 kHz to 7.5 $\mathrm{pT}/\sqrt{\text{Hz}}$ at 3 MHz for zero bias field in an unshielded environment. The appealing features of the proposed rf magnetometer using our system include a high dynamic range up to ${10}^{12}$, 6 dB bandwidth of 450 kHz, and arbitrary frequency resolution, which are intrinsic to the nonlinear ME effect in the medium.

Electrically detected electron nuclear double resonance in amorphous hydrogenated boron thin films

We report on electrically detected electron nuclear double resonance (EDENDOR) observations via spin dependent trap assisted tunneling (SDTAT) in amorphous boron thin films at 250 K. We observe EDENDOR from both 10B and 11B nuclei, likely interacting with carbon impurities. In SDTAT, electron paramagnetic resonance (EPR) is detected through a change in the trap assisted tunneling current through the thin film. As in conventional electron nuclear double resonance (ENDOR) the ENDOR response is detected through a modest change in the EPR response. Since SDTAT detection sensitivity is about seven orders of magnitude greater than that of conventional EPR, it, in principle, can provide an enormous boost in ENDOR sensitivity. This enormous potential sensitivity improvement can be compromised by a coupling of the RF electromagnetic field driving the nuclear spin response and the electrical leads which are utilized to monitor the resonance induced changes in device current. We overcome this difficulty with the introduction of proportional-integral-derivative (PID) control of the RF circuitry which greatly suppresses fluctuations in the amplitude of the RF magnetic field at the thin film structure under investigation.

Suppression of potential roughness in atom-chip ac Zeeman traps

We present several toy models for predicting the character and strength of the roughness of ac Zeeman trapping potentials generated by imperfections in wire traces on an atom chip. An ac Zeeman trapping potential is generated by targeting a microwave or rf magnetic field at hyperfine or Zeeman ground-state transitions, respectively, while a dc Zeeman trap uses a static magnetic field to manipulate an atom via its magnetic moment. We find that an ac Zeeman trap suppresses potential roughness by several orders of magnitude with respect to a comparable dc Zeeman trap. This suppression stems from the inherent differences between an atom's response to dc and ac (namely rf and microwave) magnetic fields, in concert with how the ac skin effect smooths out a current distribution disturbed by a conductivity variation. For chip wires fabricated by evaporation, we find that an ac Zeeman trap for $^{87}\mathrm{Rb}$ atoms, located $100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the chip and operating on a 6.8 GHz hyperfine transition, is expected to suppress potential roughness by a factor of $4\ifmmode\times\else\texttimes\fi{}{10}^{5}$ compared with a similar dc Zeeman trap. For a comparable ac Zeeman trap operating on a 10 MHz Zeeman transition, the suppression is $2\ifmmode\times\else\texttimes\fi{}{10}^{4}$. Conversely, the ac skin effect also slightly exacerbates potential roughness, in ac Zeeman traps, from defects in the sidewall of a wire trace.

Direct current magnetic Hall probe technique for measurement of field penetration in thin film superconductors for superconducting radio frequency resonators.

Superconducting Radio Frequency (SRF) cavities used in particle accelerators are typically formed from or coated with superconducting materials. Currently, high purity niobium is the material of choice for SRF cavities that have been optimized to operate near their theoretical field limits. This brings about the need for significant R & D efforts to develop next generation superconducting materials that could outperform Nb and keep up with the demands of new accelerator facilities. To achieve high quality factors and accelerating gradients, the cavity material should be able to remain in the superconducting Meissner state under a high RF magnetic field without penetration of quantized magnetic vortices through the cavity wall. Therefore, the magnetic field at which vortices penetrate a superconductor is one of the key parameters of merit of SRF cavities. Techniques to measure the onset of magnetic field penetration on thin film samples need to be developed to mitigate the issues with the conventional magnetometry measurements that are strongly influenced by the film orientation and shape and edge effects. In this work, we report the development of an experimental setup to measure the field of full flux penetration through films and multi-layered superconductors. Our system combines a small superconducting solenoid that can generate a magnetic field of up to 500 mT at the sample surface and three Hall probes to detect the full flux penetration through the superconductor. This setup can be used to study alternative materials that could potentially outperform niobium, as well as superconductor-insulator-superconductor (SIS) multilayer coatings on niobium.

About the possibility for using graphene layers 12 C–13 C for quantum computation

We used a graphene 12 C matrix doped with 13 C atoms in a squared and center-triangular configuration, where only the interaction 13 C–13 C with their magnetic moments are considered. This is done in the presence of a constant magnetic field parallel to the layers and a transverse rf-magnetic field in order to simulate the NOT gate (single layer), controlled-not quantum gate (two layers), and controlled-controlled-not quantum gate (three layers). However, as it would be explained in more detail throughout this article, a single qubit rotation defined by a single graphene layer was not possible to construct. We found that frequencies needed to perform this transitions do not depend on the number of 13 C atoms on each layer, which means that we can make transitions between two distant states with big separation of energy levels (m and n states) with lower frequency pulses .

Enhanced radio-frequency performance of niobium films on copper substrates deposited by high power impulse magnetron sputtering

We present a study of radio frequency properties of niobium films deposited on copper by two different approaches based on High Power Impulse Magnetron Sputtering, namely with a DC voltage biased substrate and with bipolar target voltage. Such approaches enable the synthesis of dense superconducting (SC) layers. The SC radio frequency losses of these films are characterized as a function of the applied RF magnetic field using a dedicated calorimetric method. We report on a significant reduction of the Q-slope phenomenon and the residual surface resistance in the characterized films, achieving similar values as those obtained on bulk niobium surfaces qualified with the same technique. Our results pave the way towards the realization of Nb/Cu coated accelerating cavities featuring a surface resistance 2–3 times lower than the state-of-the-art values at working frequencies of 400 and 800 MHz, making this technology even more appealing for future particle accelerators and colliders.

Effect of Ti Atoms on Néel Relaxation Mechanism at Magnetic Heating Performance of Iron Oxide Nanoparticles

The study was based on understanding the relationship between titanium (Ti) doping amount and magnetic heating performance of magnetite (Fe3O4). Superparamagnetic nanosized Ti-doped magnetite ((Fe1−x,Tix)3O4; x = 0.02, 0.03 and 0.05) particles were synthesized by sol-gel technique. In addition to (Fe1−x,Tix)3O4 nanoparticles, SiO2 coated (Fe1−x,Tix)3O4 nanoparticles were produced as core-shell structures to understand the effects of silica coating on the magnetic properties of nanoparticles. Moreover, the magnetic properties were associated with the Néel relaxation mechanism due to the magnetic heating ability of single-domain state nanoparticles. In terms of results, it was observed that the induced RF magnetic field for SiO2 coated (Fe0.97,Ti0.03)3O4 nanoparticles caused an increase in temperature difference (ΔT), which reached up to 22 °C in 10 min. The ΔT values of SiO2 coated (Fe0.97,Ti0.03)3O4 nanoparticles were very close to the values of uncoated Fe3O4 nanoparticles.

Surface-induced reduction of the switching field in nanomagnets

Magnetization reversal in a many-spin nanomagnet subjected to an rf magnetic field, on top of a DC magnetic field, is studied by numerically solving the system of coupled (damped) Landau–Lifshitz equations. It is demonstrated that spin-misalignment induced by surface anisotropy favors switching with a DC magnetic field weaker than the Stoner–Wohlfarth switching field, for optimal intensities and frequencies of the rf field.

Optimization of a traveling wave superconducting rf cavity for upgrading the International Linear Collider

The Standing Wave (SW) TESLA niobium-based superconducting radio frequency structure is limited to an accelerating gradient of about 50 MV/m by the critical RF magnetic field. To break through this barrier, we explore the option of niobium-based traveling wave (TW) structures. Optimization of TW structures was done considering experimentally known limiting electric and magnetic fields. It is shown that a TW structure can have an accelerating gradient above 70 MeV/m that is about 1.5 times higher than contemporary standing wave structures with the same critical magnetic field. The other benefit of TW structures shown is R/Q about 2 times higher than TESLA structure that reduces the dynamic heat load by a factor of 2. A method is proposed how to make TW structures multipactor-free. Some design proposals are offered to facilitate fabrication. Further increase of the real-estate gradient (equivalent to 80 MV/m active gradient) is also possible by increasing the length of the accelerating structure because of higher group velocity and cell-to-cell coupling. Realization of this work opens paths to ILC energy upgrades beyond 1 TeV to 3 TeV in competition with CLIC. The paper will discuss corresponding opportunities and challenges.

Intermediate spin pair relaxation through modulation of isotropic hyperfine interaction in frequency-swept spin-dependent recombination in 4H–SiC

Electrically detected magnetic resonance (EDMR) measurements have been extended to sub-mT magnetic fields through utilization of frequency sweeping of the oscillating magnetic field, where conventional electron paramagnetic resonance-based measurements traditionally utilize magnetic field magnitude ramping. In spin-dependent transport measurements in devices, an oftentimes pervasive near-zero field magnetoresistance effect overwhelms the sub-mT regime. This magnetoresistance effect is independent of the RF drive. Thus, by utilizing a constant DC magnetic field and a frequency sweep of the RF magnetic field, the magnetoresistance effect is not detected, leaving only the EDMR response. Interesting EDMR-based phenomena emerge at sub-mT fields when the oscillating field magnitude approaches the static field, such as multiple-photon transitions caused by the emergence of Floquet spin states and Bloch–Siegert shifts. A spectral-narrowing effect also emerges as the static field is reduced. In this work, we show that the narrowing of the frequency-swept EDMR response with static field can be modeled by changes in intermediate spin-pair relaxation through modulation of hyperfine fields caused by stochastic perturbations from the environment. We utilize recently developed theory to model the relaxation of spin pairs and show that stochastic interactions of the electron spin with the environment yield both Floquet spin states and changes in intermediate spin-pair relaxation.

Physical processes in a low-power inductive plasma source in the presence of a weak magnetic field

Research into the influence of a weak external magnetic field (<150 G) on the efficiency of power coupling to plasma and on the structure of axial RF fields in plasma is presented. Power
 coupling to the discharge plasma as well as the structure of the axial component of RF magnetic fields are shown to depend on the external magnetic field’s magnitude in a nonmonotous manner.

Phase-Modulated Cavity Magnon Polaritons as a Precise Magnetic Field Probe

We describe and operate a spin-magnetometer based on the phase modulation of cavity magnon polaritons. In this scheme a rf magnetic field is detected through the sidebands it induces on a pump, and the experimental configuration allows for a negligible pump noise and a high-frequency readout. The demonstrator setup, based on a copper cavity coupled to an yttrium iron garnet sphere hybrid system, reached a sensitivity of $2.0\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at 220 MHz, evading the pump noise and matching the theoretical predictions. An optimized setup can attain a rf magnetic field sensitivity of about $8\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/\sqrt{\mathrm{Hz}}$ at room temperature. An orders of magnitude improvement is expected at lower temperatures, making this instrument one of the few magnetometers accessing the subfemtotesla limit.

Decoherence-free radio-frequency-dressed subspaces

We study the spectral signatures and coherence properties of radiofrequency dressed hyperfine Zeeman sub-levels of 87Rb. Experimentally, we engineer combinations of static and RF magnetic fields to modify the response of the atomic spin states to environmental magnetic field noise. We demonstrate analytically and experimentally the existence of 'magic' dressing conditions where decoherence due to electromagnetic field noise is strongly suppressed. Building upon this result, we propose a bi-chromatic dressing configuration that reduces the global sensitivity of the atomic ground states to low-frequency noise, and enables the simultaneous protection of multiple transitions between the two ground hyperfine manifolds of atomic alkali species. Our methods produce protected transitions between any pair of hyperfine sub-levels at arbitrary (low) DC-magnetic fields.

Radial spatiotemporal evolution of rf magnetic field and plasma current in a very-high-frequency plasma source over a wide range of plasma density and pressure.

We present detailed spatiotemporal measurements of rf magnetic fields and plasma conduction current as well as the corresponding plasma density profiles when very-high-frequency 100-MHz waves are introduced to a parallel plate plasma. The magnetic probe, optical emission, and plasma absorption probe measurements support a model in which the interaction of high-frequency waves with the moving plasma sheath at specific phases of the very high frequencies (VHF) period leads to local reversals in the vertical rf electric field which can lead to conduction current reversals and circulations. The presence of current reversals and circulation correlate with measured plasma uniformity. In particular, measured electric field and current reversals coincide with the strongly center-peaked plasma density profiles frequently observed in VHF plasma. The magnitude of the center peak increases with the number and magnitude of the current reversal cells. The scale length of the current reversal cells is on the order of what is expected for plasma surface waves. Measurements indicate that plasma conductivity is the common scaling factor for both plasma uniformity and the circulation. The propagation of high-frequency waves and the generation of rf current reversal in one phase of the rf cycle rather than another results in the appearance of harmonics in Fourier decomposed magnetic probe wave forms. These harmonics turn out not to be actual waves or evidence of standing waves at these frequencies but rather consequences of packets of wave propagation that occur periodically.

Study of the Structural and Magnetic Properties of COFE2O4 Nanoparticles for Therapeutic Agent for Cancer Therapy

The cobalt ferrite (CoFe2O4) nanoparticles were synthesized by chemical co-precipitation technique and characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. The CoFe2O4 nanoparticles coated with biocompatible chitosan at different concentrations to produce homogeneous suspensions were also characterized by dynamic light scattering (DLS) and hyperthermia set-up. XRD data of the sample explored the particle size near about 7 nm in as-dried condition. TEM micrograph of bare CoFe2O4 nanoparticles provided the particle size nearly too about 8 nm which is in good agreement with the XRD data. This particle size after coating with chitosan became 14 nm. EDX results of this sample confirmed its nano dimension with spinel structure. VSM results of the sample in as-dried condition showed the ferromagnetic character which has been beyond proved by Mössbauer spectroscopy. The hydrodynamic diameter (Hd) and polydispersity index (PDI) of the chitosan-coated samples also provided promising results. The samples were tested for their induction heating properties with an RF magnetic field of 20 mT and a frequency of 342 kHz. We also studied therapeutic efficiency in-vitro on 9L gliosarcoma cancer cells, which revealed > 98% of mortality through hyperthermia protocol with the same RF field and frequency using chitosan-coated CoFe2O4nanoparticles.
 GUB JOURNAL OF SCIENCE AND ENGINEERING, Vol 7, Dec 2020 P 69-75

Proton MRS on sub-microliter volume in rat brain using implantable NMR microcoils.

The use of miniaturized NMR receiver coils is an effective approach for improving detection sensitivity in studies using MRS and MRI. By optimizing the filling factor (the fraction of the coil occupied by the sample), and by increasing the RF magnetic field produced per unit current, the sensitivity gain offered by NMR microcoils is particularly interesting when small volumes or regions of interest are investigated. For in vivo studies, millimetric or sub-millimetric microcoils can be deployed in tissues to access regions of interest located at a certain depth. In this study, the implementation and application of a tissue-implantable NMR microcoil with a detection volume of 850 nL is described. The RF magnetic field generated by the microcoil was evaluated using a finite element method simulation and experimentally determined by high spatial resolution MRI acquisitions. The performance of the microcoil in terms of spectral resolution and limit of detection was measured at 7 T in vitro and in vivo in rodent brains. These performances were compared with those of a conventional external detection coil. Proton MR spectra were acquired in the cortex of rat brain. The concentrations of main metabolites were quantified and compared with reference values from the literature. In vitro and in vivo results obtained with the implantable microcoil showed a gain in sensitivity greater than 50 compared with detection using an external coil. In vivo proton spectra of diagnostic value were obtained from brain regions of a few hundred nanoliters. The similarities between spectra obtained with the implanted microcoil and those obtained with the external NMR coil highlight the minimally invasive nature of the coil implantation procedure. These implantable microcoils represent new tools for probing tissue metabolism in very small healthy or diseased regions using MRS.