Superconducting Quantum Interference Device Data Research Articles

A Small Scale Optically Pumped Fetal Magnetocardiography System.

Fetal magnetocardiography (fMCG) is considered the best technique for diagnosis of fetal arrhythmia. It is superior to more widely used methods such as fetal, fetal electrocardiography, and cardiotocography for evaluation of fetal rhythm. The combination of fMCG and fetal echocardiography can provide a more comprehensive evaluation of fetal cardiac rhythm and function than is currently possible. In this study, we demonstrate a practical fMCG system based on optically pumped magnetometers (OPMs). Seven pregnant women with uncomplicated pregnancies underwent fMCG at 26-36 weeks' gestation. The recordings were made using an OPM-based fMCG system and a person-sized magnetic shield. The shield is much smaller than a shielded room and provides easy access with a large opening that allows the pregnant woman to lie comfortably in a prone position. The data show no significant loss of quality compared to data acquired in a shielded room. Measurements of standard cardiac time intervals yielded the following results: PR = 104 ± 6 ms, QRS = 52.6 ± 1.5 ms, and QTc = 387 ± 19 ms. These results are compatible with those from prior studies performed using superconducting quantum interference device (SQUID) fMCG systems. To our knowledge, this is the first European fMCG device with OPM technology commissioned for basic research in a pediatric cardiology unit. We demonstrated a patient-friendly, comfortable, and open fMCG system. The data yielded consistent cardiac intervals, measured from time-averaged waveforms, compatible with published SQUID and OPM data. This is an important step toward making the method widely accessible.

Search for enhanced magnetism at the interface between Bi2Se3 and EuSe

Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that superconducting quantum interference device (SQUID) magnetometry of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$/EuSe heterostructures reveals precisely the magnetic phase diagram known from EuSe, including the ferrimagnetic phase below 5 K, without any apparent changes from the bulk behavior. Choosing a temperature of 10 K to search for magnetic enhancement, we determine an upper limit for a possible magnetic coercive field of 3 mT. Using interface sensitive x-ray absorption spectroscopy we verify the magnetic divalent configuration of the Eu at the interface without contamination by ${\mathrm{Eu}}^{3+}$, and by x-ray magnetic circular dichroism (XMCD) we confirm at the interface the magnetic hysteresis obtained by SQUID. XMCD data obtained at 10 K in a magnetic field of 6 T indicate a magnetic spin moment of ${m}_{z,\mathrm{spin}}=7\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}/{\mathrm{Eu}}^{2+}$, in good agreement with the SQUID data and the expected theoretical moment of ${\mathrm{Eu}}^{2+}$. Subsequent XMCD measurements in zero field show, however, that sizable remanent magnetization is absent at the interface for temperatures down to about 10 K.

Recording and quantifying fetal magnetocardiography signals using a flexible array of optically-pumped magnetometers

Objective: Fetal magnetocardiography (fMCG) is a non-invasive biomagnetic technique that provides detailed beat-to-beat fetal heart rate analysis, both in normal rhythm as well as in fetal arrhythmias. New cryogenic-free sensors called optically pumped magnetometers (OPMs) have emerged as a less expensive and more geometrically flexible alternative to traditional Superconducting Quantum Interference Device (SQUID) technology for performing fMCG. The objective of the study was to show the ability of OPMs to record fMCG using flexible geometry while seeking to preserve signal quality, and to quantify fetal heart rate variability (FHRV). Approach: Biomagnetic measurements were performed with OPMs in 24 healthy pregnant women with uncomplicated singleton pregnancies between 28 and 38 weeks gestation (GA). A total of 96 recordings were analyzed from OPM data that was collected using sensors placed in two different maternal configurations over the abdomen. The fMCG signals were extracted and the quality of the recordings were quantified by peak amplitudes and signal-to-noise ratio (SNR). R peaks were used to perform both time and frequency domain FHRV analysis. FHRV measures obtained from OPMs were compared descriptively to the same measures obtained from GA-matched existing SQUID data. Main results: The fMCG derived from OPMs were observed in 21 of the 24 participants. Higher detection rates (85%) of fMCG signals were observed in the data sets recorded at GA >32 weeks. Peak amplitudes and SNR values were similar between two maternal configurations, but peak amplitudes were significantly higher (p = 0.013) in late GA compared to early GA. FHRV indicators were successfully extracted and their values overlapped substantially with those obtained from SQUID recordings. Significance: Taking advantage of the geometric flexibility of the OPMs, we have demonstrated their ability to record and quantify fMCG in different maternal positions as opposed to rigid SQUID configurations.

Capability of low-temperature SQUID for transient electromagnetics under anthropogenic noise conditions

Transient electromagnetics (TEM) is a well-established method for mineral, groundwater, and geothermal exploration. Superconducting quantum interference device (SQUID)-based magnetic-field receivers used for TEM have quantitative advantages and higher sensitivity compared with commonly used induction coils. Special applications are deep soundings with target depths [Formula: see text] and settings with conductive overburden. However, SQUIDs have rarely been applied for TEM measurements in environments with significant anthropogenic noise. We compared a low-temperature SQUID with a commercially available induction coil in an area affected by anthropogenic noise. We acquired four fixed-loop data sets with totally 61 receiver stations close to Bad Frankenhausen, Germany. The high sensitivity of the SQUID enables low noise levels, which lead to longer high-quality transient data compared with the induction coil. The effect of anthropogenic and natural noise sources is more critical for the coil than for the SQUID data. In the vicinity of the transmitter loop, systematic distortion of the coil signals occurs at early times, most probably caused by sferic interferences. We have developed 1D inversion results of both receivers that matched well in general. However, the SQUID-based models were more consistent and showed greater depths of investigation. This led to a superior resolution of deeper layers and even enabled a potential detection of thin conducting targets at up to a 500 m depth. Moreover, we find that the SQUID data inversion revealed multidimensional effects within the conductive overburden. In this regard, we applied forward modeling to analyze systematic differences between inversion results of SQUID and coil data. We determine that low-temperature SQUIDs have the potential to significantly improve the reliability of subsurface models in suburban environments. Nevertheless, we recommend combined application of both types of receivers.

Synthesis and Characterization of Co3O4-MnxCo3-xO4 Core-Shell Nanoparticles

ABSTRACTThe mixed-valence oxide Co3O4 nanoparticles, having the normal spinel structure, possess large surface area, active-site surface adsorption properties, and fast ion diffusivities. Consequently, they are widely used in lithium-ion batteries, as well as for gas sensing and heterogeneous catalysis applications. In our research, we use a two-step method to synthesize Co3O4–based core-shell nanoparticles (CSNs). Cobalt oxide (Co3O4) nanoparticles were successfully synthesized using a wet synthesis method employing KOH and cobalt acetate. Manganese was incorporated into the Co3O4 structure to synthesize inverted Co3O4@MnxCo3-xO4 CSNs using a hydrothermal method. By adjustment of pH value, we obtained two different morphologies of CSNs, one resulting in pseudo-spherical and octahedron-shaped nanoparticles (PS type) whereas the second type predominantly have a nanoplate (NP type) morphology. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) have been performed in order to determine the morphological and structural properties of our CSNs, whereas the magnetic properties have been characterized using a superconducting quantum interference device (SQUID) magnetometer. XRD and TEM results show that the CSNs have the same spinel crystal structure throughout the core and shell with an average particle size of ∼19.8 nm. Our Co3O4 nanoparticles, as measured prior to CSN formation, are shown to be antiferromagnetic (AFM) in nature as shown by the magnetization data. Our SQUID data indicate that the core-shell nanoparticles have both AFM (due to the Co3O4 core) and ferrimagnetic properties (of the shell) with a coercivity field of 300 Oe and 150 Oe at 5 K for the PS and NP samples, respectively. The magnetization vs temperature data show a spin order-disorder transition at ∼33 K and a superparamagnetic blocking temperature of ∼90 K for both batches.

Effect of crystal morphology on magnetic structure of nano-magnetites

Abstract The nano-magnetites with particle, rod, tube, and ring crystal morphologies were synthesized and the differences between macroscopic and microscopic magnetic properties were studied. The macroscopic magnetic properties of nano-magnetites obtained via a superconducting quantum interference device (SQUID) showed that both coercive magnetic field and saturation magnetization per unit volume followed the orders of ring > particle > tube > rod, respectively. This indicated that the crystal morphology affected macroscopic magnetic properties. The particle nano-magnetite contained a single domain while the others contained multiple domains measured by a magnetic force microscope (MFM). However, the domain structure of nano-magnetites calculated from SQUID data showed that all were pseudo-single domains. This suggested that the MFM may be a precise tool to determine magnetic structures. Moreover, the crystal morphology of nano-magnetites affected magnetic properties owing to different magnetic-domain structures.

Origin of magnetism in undoped TiO2 nanotubes

Magnetic properties of undoped anatase, rutile, and amorphous titanium dioxide (TiO2) nanotubes grown by electrochemical anodization were studied by superconducting quantum interference device (SQUID) and electron spin resonance (ESR) methods in the temperature range 1.8–300 K. All anatase, rutile, and amorphous TiO2 nanotubes were found to exhibit paramagnetic behaviors in the entire temperature range when tested with magnetic center concentrations of 6×1017, 3×1016, and 3 × 1015 cm−3, respectively. The diameter of the TiO2 nanotubes varied from 40–160 nm and has no significant effect on the magnetic properties observed. SQUID data showed strong nonlinear M–H relationships for anatase at low temperatures, and Arrot plot analysis suggested ferromagnetism in the sample with a Curie temperature TC ∼ 6 K. However, ESR studies showed no evidence for long-distance magnetic ordering. ESR studies revealed two magnetic centers with g1 = 1.928 and g2 = 2.028 that were common to all samples. The resonance peak at g1 = 1.922 was ascribed to Ti3+ cations centers resulting from oxygen vacancies, while the peak at g2 = 2.028 was ascribed to surface absorbents. The amorphous sample ESR spectrum contained additional resonance peaks with corresponding g values at 2.228, 1.873, and 1.715 that possibly resulted from the disordered nature of these samples.

Incorporation, valence state, and electronic structure of Mn and Cr in bulk single crystal β–Ga2O3

Single crystals of transition metal (TM) doped β–Ga2O3, a wide gap semiconductor system of interest for transparent conductive oxide and diluted magnetic semiconductor applications, have been studied in the dilute, non-interacting limit (≤0.06 cation %). Based on optical absorption, particle induced x-ray emission, and Rutherford backscattering measurements, Mn does not incorporate as well as Cr, and Mn degrades the crystal quality. Using superconducting quantum interference device (SQuID) magnetometry, a Brillouin type paramagnetic magnetization is observed for Mn or Cr doped crystals with an effective number of Bohr magnetons per TM ion of 5.88 ± 0.1 or 3.95 ± 0.1, respectively. A trace ferromagnetic signal is consistent with a very small concentration of secondary phases in the Mn-doped crystal. The position of the edge in x-ray absorption near edge structure (XANES) measurements suggests that the Cr takes the 3+ valence, while a mixture of Mn2+ and Mn3+ are present; based on the absence of a prominent pre-edge feature in the XANES, both TM predominantly occupy an octahedral site in β–Ga2O3. Density functional theory (DFT) results, optical absorption and SQuID data are consistent with this assignment. While the Cr-doped crystal is conductive, the Mn-doped crystal is insulating, which is consistent with the Mn2+/Mn3+ mixed valence, assuming the Fermi level is pinned mid-gap at the Mn 2+/3+ transition level, which is predicted by DFT to be 1.8 eV above the valence band maximum.

Magnetic and magnetoelectric study of the pyroxeneNaCrSi2O6

We investigated the magnetic, magnetoelectric, and crystal structures of the pyroxene ${\text{NaCrSi}}_{2}{\text{O}}_{6}$ by superconducting quantum interference device (SQUID) magnetometry, electrical polarization measurement, and powder neutron diffraction. Magnetic exchange couplings extracted from magnetization measurements are found to be ${J}_{intra}/{k}_{b}=\ensuremath{-}0.48(4)\text{ }\text{K}$ and ${J}_{inter}/{k}_{b}=0.24(8)\text{ }\text{K}$. This is in perfect agreement with the antiferromagnetic order determined below ${T}_{N}=2.8(2)\text{ }\text{K}$ by neutron diffraction. Corroborating the determined magnetic structure (magnetic symmetry $\text{C-}{1}^{\ensuremath{'}}$), the magnetic field dependence of electrical polarization evidences a clear magnetoelectric effect below ${T}_{N}$. An induced magnetic field transition toward a ferromagnetic state (magnetic symmetry $\text{C}{2}^{\ensuremath{'}}/{\text{c}}^{\ensuremath{'}}$) is observed in the SQUID data and confirmed by neutron diffraction.

Growth and characterization of ferromagnetic cubic GaCrN: Structural and magnetic properties

Structural and magnetic properties of cubic GaCrN (c-GaCrN) grown on MgO (0 0 1) substrate by plasma-assisted molecular beam epitaxy (PAMBE) were studied with X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) analysis and a superconducting quantum interference device (SQUID) magnetometer. The Cr concentration dependence of XRD rocking curve showed that high Cr-doping induced mosaic block effect in the c-GaCrN layer. It was found that the saturation magnetization ( M s) and the coercive field ( H c) obtained from the M– H curves decrease with increase in the Cr concentration. XAFS results showed that the ratios of Cr atoms substituted for Ga sites to Cr atoms incorporated in low Cr-doped samples (Cr∼0.2%) are higher than those in highly Cr-doped samples (Cr∼3%), and that the other Cr atoms are segregated to form a CrN phase with NaCl structure. This XAFS results correspond to the SQUID data that the amounts of magnetically active Cr atoms in c-GaCrN decrease for the highly Cr-doped samples. X-ray absorption near-edge structure spectra also supports these results.

Magnetic properties ofLiBeH3andLi2BeH4

${\mathrm{LiBeH}}_{3}$ and ${\mathrm{Li}}_{2}{\mathrm{BeH}}_{4}$ have previously been proposed as candidates for high-temperature superconductivity because positions of hydrogen atoms in the respective crystal structures may lead to the formation of hydrogen-atom-based conduction bands. In the present work, the magnetization of ${\mathrm{LiBeH}}_{3}$ and ${\mathrm{Li}}_{2}{\mathrm{BeH}}_{4}$ was measured as a function of applied magnetic field and temperature with a superconducting quantum interference device (SQUID) magnetometer. Small magnetic-susceptibility values and the highly reactive nature of the samples led to the use of a background correction procedure involving the raw SQUID data. The magnetization data for ${\mathrm{LiBeH}}_{3}$ and ${\mathrm{Li}}_{2}{\mathrm{BeH}}_{4}$ samples appear to exhibit a superposition of both weak diamagnetic and superparamagnetic behaviors, and show no irreversibility with respect to either temperature or applied magnetic field within experimental uncertainty. At $T=300\mathrm{K},$ diamagnetic components of susceptibilities are observed to be $\ensuremath{-}3.3\ifmmode\pm\else\textpm\fi{}0.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{e}\mathrm{m}\mathrm{u}/\mathrm{m}\mathrm{o}\mathrm{l}$ for ${\mathrm{LiBeH}}_{3}$ and $\ensuremath{-}3.1\ifmmode\pm\else\textpm\fi{}0.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{e}\mathrm{m}\mathrm{u}/\mathrm{m}\mathrm{o}\mathrm{l}$ for ${\mathrm{Li}}_{2}{\mathrm{BeH}}_{4}.$ Furthermore, the possible origin of the observed superparamagnetic contribution is discussed. These measurements are contrasted with previous work, which had suggested that the magnetic behavior of ${\mathrm{Li}}_{2}{\mathrm{BeH}}_{4}$ could be related to type-II superconductivity.