Ultra-low Magnetic Field Research Articles

Size and Compositional Effects on Contrast Efficiency of Functionalized Superparamagnetic Nanoparticles at Ultralow and Ultrahigh Magnetic Fields

Magnetic resonance imaging (MRI) systems at ultralow and ultrahigh field have been developed for biomedical imaging. We synthesized and functionalized superparamagnetic (SPM) nanoparticles (NPs) and studied the spin–lattice (T1) and spin–spin (T2) relaxation time of protons in water surrounding these NPs at ultralow and at ultrahigh magnetic fields. In these fields, size and compositional effects of SPM NPs were observed contributing to the spin–lattice (r1) and spin–spin (r2) relaxivities as well as r2/r1 ratios. These results reveal the relationship between magnetic characteristics of SPM NPs and relaxation behavior of water proton at ultralow or ultrahigh field.

Two-component Bose-Hubbard model with higher-angular-momentum states

We study a Bose-Hubbard Hamiltonian of ultracold two component gas of spinor Chromium atoms. Dipolar interactions of magnetic moments while tuned resonantly by ultralow magnetic field can lead to spin flipping. Due to approximate axial symmetry of individual lattice site, total angular momentum is conserved. Therefore, all changes of the spin are accompanied by the appearance of the angular orbital momentum. This way excited Wannier states with non vanishing angular orbital momentum can be created. Resonant dipolar coupling of the two component Bose gas introduces additional degree of control of the system, and leads to a variety of different stable phases. The phase diagram for small number of particles is discussed.

Searching for non-Abelian phases in the Bose-Einstein condensate of dysprosium

The recently realized Bose-Einstein condensate of dysprosium will become a spin-8 spinor condensate at low magnetic fields. In such a high-spin condensate, many phases with different symmetries can exist. Among them the most interesting ones are those with non-Abelian point group symmetry. In this Rapid Communication we discuss the variety of symmetry phases in a spin-8 condensate resulting from numerical solutions of the Hamiltonian. We show that these symmetries can be determined uniquely from the measurements of density population on each spin component in an ultralow magnetic field, together with the measurements of the collective modes in the zero-field limit. This method can also be applied to Bose-Einstein condensates of other magnetic atoms, such as Cr and Er.

SQUID-based systems for co-registration of ultra-low field nuclear magnetic resonance images and magnetoencephalography

The ability to perform magnetic resonance imaging (MRI) in ultra-low magnetic fields (ULF) of ∼100μT, using superconducting quantum interference device (SQUID) detection, has enabled a new class of magnetoencephalography (MEG) instrumentation capable of recording both anatomical (via the ULF MRI) and functional (biomagnetic) information about the brain. The combined ULF MRI/MEG instrument allows both structural and functional information to be co-registered to a single coordinate system and acquired in a single device. In this paper we discuss the considerations and challenges required to develop a combined ULF MRI/MEG device, including pulse sequence development, magnetic field generation, SQUID operation in an environment of pulsed pre-polarization, and optimization of pick-up coil geometries for MRI in different noise environments. We also discuss the design of a “hybrid” ULF MRI/MEG system under development in our laboratory that uses SQUID pick-up coils separately optimized for MEG and ULF MRI.

Anomaly disturbances of the magnetic fields before the strong earthquake in Japan on March 11, 2011

One of the strongest earthquakes, with magnitude M 8.9, occurred at the sea bottom near to the east coast of Japan on March 11, 2011. This study is devoted to the investigation of anomaly disturbances in the main magnetic field of the Earth and in ultra-low frequency magnetic variations (F <10 Hz) observed before this earthquake. Secular variations of the main geomagnetic field were investigated using three-component 1-h data from three magnetic observatories over the 11-year period of January 1, 2000, to January 31, 2011. The Esashi and Mizusawa magnetic stations are situated northwest of the earthquake epicenter, at distances of around 170 km to 200 km, and the Kakioka observatory is situated southwest of the earthquake epicenter, at a distance of about 300 km. During this period, there were four local anomalies in the secular variations. The last anomaly was the biggest, which began around 3 years prior to the earthquake moment. All of the anomalies can be most distinctly recognized, in the form of differences in the corresponding magnetic components at these remote magnetic stations. For investigations of the ultra-low frequency magnetic field disturbances, three-component 1-s data at two magnetic stations (Kakioka and Uchiura) were used. The Uchiura station is situated 119 km south of Kakioka, at a distance of about 420 km from the earthquake epicenter. Data from the time interval of February 18, 2011 to March 10, 2011 (only at night-time: 01:00 to 04:00 local time) were investigated in a wide frequency range. In the frequency range of 0.033 Hz to 0.01 Hz, there was the clearest anomaly, seen as a decrease in the correlation coefficients of the corresponding magnetic components at these two stations, from February 22, 2011. Differences in the Z components showed an increase, and became positive after this date. This might suggest that the ultra-low frequency lithospheric source appeared north of the Kakioka station. Outside this specified frequency range, the anomalies were not well defined.

Change in the microhardness of nonmagnetic crystals after their exposure to the Earth’s magnetic field and AC pump field in the EPR scheme

Changes in the microhardness of ZnO, triglycine sulfate (TGS), and potassium acid phthalate (KAP) crystals after their exposure to crossed ultralow magnetic fields, i.e., the Earth’s field B Earth ≈ 50 μT and the alternating-current field \(\tilde B \approx 3 \mu {\rm T}\) orthogonal to it, have been revealed. In ZnO crystals, the microhardness increases, whereas in TGS and KAP, it decreases. A maximum change (10–15%) is reached within 1–3 h after magnetic treatment; then, the microhardness gradually recovers to its initial value for the first day. After a sufficient pause, the effect is completely reproduced under the same conditions. The resonant frequency of the pump field \(\tilde B\) corresponds to the EPR condition with a g-factor close to two. The magnetic memory exhibits a strong anisotropy: for each of the crystals, a direction is found, which, being coincident with the Earth’s magnetic field vector B Earth, causes complete or partial suppression of the effect. In ZnO and TGS crystals, these are symmetry axes 6 and 2, respectively. In the KAP crystal, it is the direction in the cleavage plane orthogonal the 2 axis. Possible physical mechanisms of the observed phenomena have been discussed.

Relaxation Behavior Study of Ultrasmall Superparamagnetic Iron Oxide Nanoparticles at Ultralow and Ultrahigh Magnetic Fields

Ultrasmall superparamagnetic iron oxide nanoparticles (USPIOs) have attracted attention because of their current and potential usefulness as contrast agents for magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR). USPIOs are usually used for their significant capacity to produce predominant proton relaxation effects, which result in signal reduction. However, most previous studies that utilized USPIOs have been focused on the relaxation behavior at commonly used magnetic fields of clinical MRI systems (typically 1-3 T). In this paper, magnetic relaxation processes of protons in water surrounding the USPIOs are studied at ultralow (≤10 mT) and ultrahigh magnetic fields (14.1 T). USPIOs used in our experiments were synthesized with a core size of 6 nm, and transferred from organic to water by ligand exchange. The proton spin-lattice relaxation time (T(1)) and spin-spin relaxation time (T(2)) were investigated at ultralow (212 μT for T(2) and 10 mT for T(1)) and at 14.1 T with different iron concentrations. At all of the fields, there is a linear relationship between the inverse of relaxation times and the iron concentration. The spin-spin relaxivity (r(2)) at 14.1 T is much larger than that value of the ultralow field. At ultralow field, however, the spin-lattice relaxivity (r(1)) is larger than the r(1) at ultrahigh field. The results provide a perspective on potential in vivo and in vitro applications of USPIOs in ultralow and ultrahigh field NMR and MRI.

Water's response to ultralow magnetic fields comes as a surprise

<xhtml:span xmlns:xhtml="http://www.w3.org/1999/xhtml" xml:lang="en"> A record-sensitive nuclear magnetic resonance experiment proves we still don't know all there is to know about the most abundant liquid on the planet. </xhtml:span>

Review of Noise Sources in Magnetic Tunnel Junction Sensors

Noise problem limits the sensitivity of magnetic tunnel junction (MTJ) sensors for ultra-low magnetic field applications. Noise analysis not only helps in finding ways to eliminate noise disturbances but also essential for understanding the electronic and magnetic properties of MTJs. These approaches provide insight for optimizing the design of MTJ sensors before fabrication. This paper reviews the noise sources in MTJ sensors reported in recent years. Both the origins and mathematical derivations of the noise sources are presented, illustrating how different factors affecting the performance of MTJ sensors. A brief outlook of challenges in the future is also given.

Stray magnetic field compensation with a scalar atomic magnetometer

We describe a system for the compensation of time-dependent stray magnetic fields using a dual channel scalar magnetometer based on nonlinear Faraday rotation in synchronously optically pumped Cs vapor. We detail the active control strategy, with an emphasis on the electronic circuitry, based on a simple phase-locked-loop integrated circuit. The performance and limits of the system developed are tested and discussed. The system was applied to significantly improve the detection of free induction decay signals from protons of remotely magnetized water precessing in an ultralow magnetic field.

Charge transfer and in‐cloud structure of large‐charge‐moment positive lightning strokes in a mesoscale convective system

Lightning observations in the very high frequency band and measurements of ultra low frequency magnetic fields are analyzed to investigate the charge transfer and in‐cloud structure of eight positive cloud‐to‐ground (+CG) strokes in a mesoscale convective system. Although no high altitude images were recorded, these strokes contained large charge moment changes (1500–3200 C·km) capable of producing nighttime sprites. Even though the convective region of the storm was where the flashes originated and where the CG strokes could occur, the charge transferred to ground was mainly from the stratiform region. The post‐stroke long continuing currents were connected to highly branched negative leader extension into the stratiform region. While the storm dissipated, the altitude of negative leader propagation in the stratiform area dropped gradually from 8 to 5 km, indicating that in some and perhaps all of these strokes, it was the upper positive charge in the stratiform region that was transferred.

SQUIDs for Magnetic Resonance Imaging at Ultra-low Magnetic Field

Nuclear magnetic resonance methods are widely used in medicine, chemistry and industry. One application area is magnetic resonance imaging or MRI. It is among the most effective diagnostic tools in medicine. Modern medical MRI scanners use strong magnetic fields. Recently it has become possible to perform NMR and MRI in ultra-low field regime that requires measurement field strengths only of the order of 1 gauss. These ultra-low field techniques exploit the advantages offered by superconducting quantum interference devices or SQUIDs. We describe the world’s first multichannel SQUID-based instruments that are capable of performing ULF MRI for different applications.

Nuclear magnetic resonance on room temperature samples in nanotesla fields using a two-stage dc superconducting quantum interference device sensor

We describe a compact system for pulsed nuclear magnetic resonance at ultralow magnetic fields on small liquid samples (∼0.14ml) at room temperature. The broadband spectrometer employs an integrated two-stage superconducting quantum interference device current sensor with a coupled energy sensitivity of 50h, in the white noise limit. Environmental noise is screened using a compact arrangement of mu-metal and a superconducting shield. Proton signals in water have been observed down to 93nT (a Larmor frequency of 4.0Hz), with a minimum linewidth of 0.16Hz measured at ∼40Hz. Two-component free induction decays were observed from oil/water mixtures between 275 and 300K.

Magnetic Resonance at or below the Earth's Magnetic Field

NMR resonances of two nuclei can be obtained in one spectrum at ultralow magnetic fields. At such low fields, chemical shifts become negligible and heteronuclear couplings dominate the NMR spectra. The Highlight gives an overview of “pure J-coupling spectroscopy” and (functional) imaging applications at magnetic-field strengths in the range of the Earth's magnetic field.

Analysis of molecular structures by homo- and hetero-nuclear J-coupled NMR in ultra-low field

The drawback of nuclear magnetic resonance (NMR) spectroscopy at ultra-low magnetic fields appears to be the lack of access to chemical information in terms of chemical shifts and homo-nuclear J-couplings. Here we report that a chemical group can be identified by the multiplet structure of the NMR spectrum in ultra-low fields if the condition of the strong hetero-nuclear J-coupling is fulfilled. Moreover we found that high-resolution ultra-low field proton-NMR spectra of liquids indeed reveal all hetero- and homo-nuclear J-couplings in terms of pairs of multiplets. This opens the door for the study of molecular structures at ultra-low magnetic fields.

Free precession and transverse relaxation of hyperpolarized 129Xe gas detected by SQUIDs in ultra-low magnetic fields

We studied the free precession of the nuclear magnetization of hyperpolarized 129Xe gas in external magnetic fields as low as B0 = 4.5 nT, using SQUIDs as magnetic flux detectors. The transverse relaxation was mainly caused by the restricted diffusion of 129Xe in the presence of ambient magnetic field gradients. Its pressure dependence was measured in the range from 30 mbar to 850 mbar and compared quantitatively to theory. Motional narrowing was observed at low pressure, yielding transverse relaxation times of up to 8000 s.

Ultra-low field NMR measurements of liquids and gases with short relaxation times

Interest in nuclear magnetic resonance measurements at ultra-low magnetic fields (ULF, ∼μT fields) has been motivated by various benefits and novel applications including narrow NMR peak-width, negligible susceptibility artifacts, imaging of samples inside metal containers, and possibility of directly imaging neuronal currents. ULF NMR/MRI is also compatible with simultaneous measurements of biomagnetic signals. However the most widely used technique in ULF NMR—prepolarization at high field and measurement at lower field—results in large transient signals which distort the free induction decay signal. This is especially severe for the measurement of signals from samples and materials with short T 1 time. We have devised an approach that largely cancels the transient signals. The technique was successfully used to measure NMR signals from liquids and gases with T 1 in the range 1–4 ms.

Magnetodielectric effect of Bi6Fe2Ti3O18 film under an ultra-low magnetic field

Good-quality and fine-grain Bi6Fe2Ti3O18 magnetic ferroelectric films with single-phase layered perovskite structure have beenprepared successfully via the metal organic decomposition (MOD) method. The results oflow-temperature magnetocapacitance measurements reveal that an ultra-low magnetic field of10 Oe can produce a non-trivial magnetodielectric response in zero-field cooling conditions,and the relative variation of dielectric constants in a magnetic field is positive, i.e.[εr(H)−εr(0)]/εr(0) = 0.05, whenT<55 K, but negativewith a maximum of [εr(H)−εr(0)]/εr(0) = −0.14 when 55 K<T<190 K. The magnetodielectric effect shows a sign change at 55 K, which is due to a transitionfrom an antiferromagnetic to a weak ferromagnetic, and vanishes abruptly at around 190 K,which is thought to be associated with an order–disorder transition of iron ions at the Bsite of perovskite structures. Our results allow the expectation of low-cost applications ofdetectors and switches for extremely weak magnetic fields over a wide temperature range of55–190 K.

J-Coupling Nuclear Magnetic Resonance Spectroscopy of Liquids in nT Fields

In ultralow magnetic fields, liquid state nuclear magnetic resonance (NMR) spectra of homonuclear spin systems exhibit line widths dominated by their natural lifetime. Chemical shifts become negligible, and heteronuclear NMR spectra show predominantly the electron-mediated field-independent J-coupling. However, weak polarization and Larmor frequencies down to a few hertz require special detectors, such as Superconducting Quantum Interference Devices (SQUID), that also enable the simultaneous detection of broad band spectra of heteronuclear spin systems. We acquired spectra of 2,2,2-trifluoroethanol and trimethyl phosphate at detection fields varying from 444 nT to 3.34 muT after prepolarizing the sample in a field of 250 muT. Down to a 1H Larmor frequency of 40 Hz, the spectra of trifluoroethanol exhibited four clearly resolvable peaks. The numerical simulation agreed well with the measured spectra. Trimethyl phosphate exhibited two major groups of nonresolved proton lines. At 1H Larmor frequencies below 150 Hz, the separation of the two groups decreased, reflecting the transition from weakly to strongly coupled spin systems. Direct determination of 3J(H,P) from the peak separation is possible only at Larmor frequencies above 150 Hz. The experimental setup allowed an easy adjustment of the detection field over several octaves. This enabled us to adapt the detection field to the best-suited measurement window providing the maximum spectral information. Low-field NMR may open new applications, such as monitoring heteronuclear reactions, low-field imaging, simultaneous NMR/magnetoencephalography measurements, or quantum computing.

SQUID-Based Simultaneous Detection of NMR and Biomagnetic Signals at Ultra-Low Magnetic Fields

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) at ultra-low magnetic fields (ULF, fields of /spl sim//spl mu/T) have several advantages over their counterparts at higher magnetic fields. These include narrow line widths, the possibility of novel imaging schemes such as T/sub 1/ weighted images, and reduced system cost and complexity. In addition, ULF NMR/MRI with superconducting quantum interference devices (SQUIDs) is compatible with simultaneous measurements of biomagnetic signals, a capability conventional systems cannot offer. SQUID-based ULF MRI has already been demonstrated, as have measurements of simultaneous MEG and NMR at ULF. In this paper we will show simultaneous magnetocardiography (MCG) and magnetomyography (MMG) with NMR are also possible. Another compelling application of NMR/MRI at ULF is the possibility of directly measuring magnetic resonance consequences of neuronal signals. In this paper we explore simultaneous MMG/NMR and MCG/NMR for an effect on the NMR signal, in T/sub 2//sup */, that might be associated with the effects of bioelectric currents.