Commercial Electron Paramagnetic Resonance Research Articles

Enhancement of microwave fields in pulse EPR of quantum paraelectrics

The pulse electron paramagnetic resonance (EPR) is widely used in different branches of material and life sciences, including promising applications in quantum information processing and quantum sensing. Here, we study the effect of the high polarizability of KTaO3 and SrTiO3 quantum paraelectrics on local electric and magnetic field components of microwaves (MW) at Fe3+ and Mn2+ paramagnetic ions. The measurements are performed with a commercial EPR spectrometer using dielectric and split-ring resonators. It is found that the power of MW pulses used in coherent spin manipulation at nanoseconds timescale decreases to milliwatts as compared to the tens–hundreds of watts usually used for spins in conventional materials. The amplification of MW fields is related to the very high dielectric permittivity (up to 25 000 in SrTiO3) of quantum paraelectrics at GHz frequencies and temperatures below 20 K. This creates the large induced polarization and, thus, huge displacement current and in turn the secondary MW magnetic field. Numerical simulations support the observation of the enhanced magnetic MW field in the high-permittivity sample. The low MW power for excitation of spin transitions in quantum paraelectrics eliminates the requirement of expensive high-power MW equipment. This approach also allows to globally control spin qubits in tandem with integrated devices based on conventional semiconductor MW circuits working at mW powers. It is suggested that quantum paraelectrics can also be used as substrates for deposition of nanoparticles or films of other materials, which would be manipulated by the low-power MW pulses.

Statistical analysis of ENDOR spectra

Electron-nuclear double resonance (ENDOR) measures the hyperfine interaction of magnetic nuclei with paramagnetic centers and is hence a powerful tool for spectroscopic investigations extending from biophysics to material science. Progress in microwave technology and the recent availability of commercial electron paramagnetic resonance (EPR) spectrometers up to an electron Larmor frequency of 263 GHz now open the opportunity for a more quantitative spectral analysis. Using representative spectra of a prototype amino acid radical in a biologically relevant enzyme, the [Formula: see text] in Escherichia coli ribonucleotide reductase, we developed a statistical model for ENDOR data and conducted statistical inference on the spectra including uncertainty estimation and hypothesis testing. Our approach in conjunction with 1H/2H isotopic labeling of [Formula: see text] in the protein unambiguously established new unexpected spectral contributions. Density functional theory (DFT) calculations and ENDOR spectral simulations indicated that these features result from the beta-methylene hyperfine coupling and are caused by a distribution of molecular conformations, likely important for the biological function of this essential radical. The results demonstrate that model-based statistical analysis in combination with state-of-the-art spectroscopy accesses information hitherto beyond standard approaches.

Extending electron paramagnetic resonance to nanoliter volume protein single crystals using a self-resonant microhelix.

Electron paramagnetic resonance (EPR) spectroscopy on protein single crystals is the ultimate method for determining the electronic structure of paramagnetic intermediates at the active site of an enzyme and relating the magnetic tensor to a molecular structure. However, crystals of dimensions typical for protein crystallography (0.05 to 0.3mm) provide insufficient signal intensity. In this work, we present a microwave self-resonant microhelix for nanoliter samples that can be implemented in a commercial X-band (9.5 GHz) EPR spectrometer. The self-resonant microhelix provides a measured signal-to-noise improvement up to a factor of 28 with respect to commercial EPR resonators. This work opens up the possibility to use advanced EPR techniques for studying protein single crystals of dimensions typical for x-ray crystallography. The technique is demonstrated by EPR experiments on single crystal [FeFe]-hydrogenase (Clostridium pasteurianum; CpI) with dimensions of 0.3 mm by 0.1 mm by 0.1 mm, yielding a proposed g-tensor orientation of the Hox state.

Six Decades of Progress in Magnetic Resonance: The Contributions of James S. Hyde

The development of electron paramagnetic resonance (EPR) and magnetic resonance imaging (MRI) over six decades is sketched with an emphasis on the contributions of James S. Hyde. For twenty years starting three years after the first commercial EPR spectrometer was shipped by Varian, he led commercial EPR developments, and then for more than forty years, he led development of instrumentation and biomedical applications of EPR at the Medical College of Wisconsin. It was there that he also made major contributions to MRI, and especially functional MRI.

Identification of Superoxide O2− during Thermal Decomposition of Molten KNO3-NaNO2-NaNO3 Salt by Electron Paramagnetic Resonance and UV-Vis Absorption Spectroscopy

On account of excellent thermal physical properties, molten nitrates/nitrites salt has been widely employed in heat transfer and thermal storage industry, especially in concentrated solar power system. The thermal stability study of molten nitrate/nitrite salt is of great importance for this system, and the decomposition mechanism is the most complicated part of it. The oxide species O22− and O2− were considered as intermediates in molten KNO3-NaNO3 while hard to been detected in high temperature molten salt due to their trace concentration and low stability. In this work, the homemade in situ high temperature UV-Vis instrument and a commercial electron paramagnetic resonance were utilized to supply evidence for the formation of superoxide during a slow decomposition process of heat transfer salt (HTS, 53 wt% KNO3/40 wt% NaNO2/7 wt% NaNO3). It is found that the superoxide is more easily generated from molten NaNO2 compared to NaNO3, and it has an absorption band at 420–440 nm in HTS which red shifts as temperature increases. The band is assigned to charge-transfer transition in NaO2 or KO2, responsible for the yellow color of the molten nitrate/nitrite salt. Furthermore, the UV absorption bands of molten NaNO2 and NaNO3 are also obtained and compared with that of HTS.

Measuring errors in single-qubit rotations by pulsed electron paramagnetic resonance

The ability to measure and reduce systematic errors in single-qubit logic gates is crucial when evaluating quantum computing implementations. We describe pulsed electron paramagnetic resonance (EPR) sequences that can be used to measure precisely even small systematic errors in rotations of electron-spin-based qubits. Using these sequences we obtain values for errors in the rotation angle and axis for single-qubit rotations using a commercial EPR spectrometer. We conclude that errors in qubit operations by pulsed EPR are not limiting factors in the implementation of electron-spin-based quantum computers.

Adapting a hall probe controller for current control of an air‐core magnet

AbstractAir‐core magnets are preferable to iron‐core magnets for electron paramagnetic resonance (EPR) magnetic fields less than ∼150 G (15 mT). The preferred way to control the field of an air‐core magnet is by direct control of the current. For imaging applications, magnetic fields produced by the gradient coils interfere with the use of the Hall sensor. Direct current control of the field avoids this problem. However, most suppliers of commercial EPR spectrometer systems provide only Hall effect control of the magnetic field. To provide direct current control of an air‐coil magnet interfaced to a commercial console, an adapter was constructed that effectively converts the Hall effect probe into a current sensor by measuring the field produced in coils carrying the same current as the main magnet coils, but positioned away from and shielded from the main magnet. The effectiveness of this approach is demonstrated. © 2002 Wiley Periodicals, Inc. Concepts in Magnetic Resonance (Magn Reson Engineering) 15: 47–50, 2002.

New magnetic material spectroscopy (abstract)

Sensitive commercial electron paramagnetic resonance (EPR) spectrometers are able to measure 10∧13 spins. Using a high Tc weak link, we were able to measure FMR on yttrium ion garnet (YIG) films which were exposed to an effective area of 3 nm by 15 micrometers. Given that this is the area of excitation and the thickness of the YIG was 0.5 micrometers, we could measure less than 10∧9 spins in a YIG film. Our weak link was fabricated by patterning a 0.5 micrometer thick film of YBCO down to a 15 micrometer wide bridge across an artificial grain boundary in an MgO substrate. Our technique uses the ac Josephson effect to generate a microwave field which couples into a YIG film which is placed in intimate contact atop the link. The frequency of this microwave field is proportional to the voltage across the weak link. The absorption of the microwaves by the YIG will affect the voltage versus current behavior across the weak link. For a YIG film of 0.5 micrometers thickness, we measured an in-plane linewidth of 125 Gauss using the weak link technique. The FMR linewidth measured by a conventional EPR technique was 40 Gauss. This implies that the excitation by the weak link may be nonuniform. We present data which illustrate this effect and numerical results for the corresponding circuit model. Our conclusions are that we have developed a viable spectroscopy to characterize extremely local magnetic interactions.

Saturation recovery electron paramagnetic resonance spectrometer

A versatile saturation recovery accessory based on a small, special-purpose timing controller and an efficient mix of coaxial and waveguide microwave components has been added to a commercial electron paramagnetic resonance (EPR) spectrometer. The spectrometer was designed for study of the spin lattice relaxation time of transition metals and free radicals over a wide temperature range, such as are of interest in materials science and metallobiochemistry. A microprocessor-based timing system was designed to provide a wide dynamic range with a simple user interface. Waveguide phase shifters and rotary vane attenuators were used for their precision and resetability, while coaxial components were used where their superior performance could be exploited. Sensitivity is provided by a low-noise GaAs field-effect transistor (FET) microwave preamplifier and a double-balanced mixer (DBM) detector. A biphase modulator (±180° phase shifter) on the lo side of the DBM provided efficient addition/subtraction of successive recovery curves, obviating slow data transfer to a computer prior to the arithmetic operations. Because the system is designed for the study of inherently broad spectra, it does not use the magnetic field step modulation of some prior spectrometers. In addition, although most EPR spectrometers have been designed with a phase shifter in the reference arm, we have found it more convenient to keep the reference arm phase constant and adjust the phases of the pump and observe arms of this three-arm bridge spectrometer. The saturation recovery system described here could be implemented on any commercial EPR spectrometer with only minor modifications.

Ferromagnetic resonance imaging using photothermal deflection

The local variation of the ferromagnetic (FMR) resonance absorption scanning along the surface of ferromagnetic metallic films was studied with the photothermal deflection technique. Using a modified commercial electron paramagnetic resonance spectrometer the FMR of an inhomogeneous Ni film was measured as a function of the external magnetic field and laser beam position. The local resolution which depends on characteristics of the laser probe beam and on the thermal diffusion length of the surrounding medium was investigated at different modulation frequencies between 35 Hz and 1 kHz.

A rapid-scanning electron paramagnetic resonance spectrometer with stopped-flow mixing

Modifications to a commercial electron paramagnetic resonance (e.p.r.) spectrometer are described which allow data to be obtained from 64 e.p.r, spectra (0.1–100 gauss per scan) with scan times of 0.010–0.900 s and with 0.020–900 s delay times between scans. Reagents are delivered into an e.p.r, cell from a stopped-flow mixer that triggers the generation of a microprocessor-controlled waveform to drive a rapid-scan unit. This digitally synthesized waveform is designed to correct inherent imperfections in the Helmholtz-type sweep coil circuit in the rapid-scan unit. The spectra are digitized (250 points per scan) and can be processed to determine rates of reaction. Performance of the system is demonstrated by the determination of the kinetics of rearrangement of a bis(di- peptide)nickelate(III) complex.