Magnetic Field Resolution Research Articles

Magnetic field amplification and X-ray emission in galaxy minor mergers

We investigate the magnetic field evolution in a series of galaxy minor mergers using the N-body/smoothed particle hydrodynamics (SPH) code gadget. The simulations include the effects of radiative cooling, star formation and supernova feedback. Magnetohydrodynamics is implemented using the SPH method. We present 32 simulations of binary mergers of disc galaxies with mass ratios of 2:1 up to 100:1, whereby we have additionally varied the initial magnetic field strengths, disc orientations and resolutions. We investigate the amplification of a given initial magnetic field within the galaxies and an ambient intergalactic medium (IGM) during the interaction. We find that the magnetic field strengths of merger remnants with mass ratios up to 10:1 saturate at a common value of several μG. For higher mass ratios, the field strength saturates at lower values. The saturation values correspond to the equipartition value of magnetic and turbulent energy density. The initial magnetization, disc orientation and numerical resolution show only minor effects on the saturation value of the magnetic field. We demonstrate that a higher impact energy of the progenitor galaxies leads to a more efficient magnetic field amplification. The magnetic and turbulent energy densities are higher for larger companion galaxies, consistent with the higher impact energy supplied to the system. We present a detailed study of the evolution of the temperature and the bolometric X-ray luminosity within the merging systems. Thereby we find that magnetic fields cause a more efficient increase of the IGM temperature and the corresponding IGM X-ray luminosity after the first encounter. However, the presence of magnetic fields does not enhance the total X-ray luminosity. Generally, the final value of the X-ray luminosity is even clearly lower for higher initial magnetic fields.

Magnetic Imaging on the Nanometer Scale Using Low-Temperature Scanning Probe Techniques

Multiple techniques now exist for the investigation of nanoscale magnetic properties, extending from Lorentz microscopy and magneto-optical imaging (MOKE) to scanning probe microscopy approaches (see Figure 1 for an overview). Among the latter, the most widely used techniques offering both high spatial and high magnetic-field resolution are magnetic force microscopy (MFM) and scanning Hall probe microscopy (SHPM). Both techniques are well known for their versatility and ease of use and can be further adapted for operation in cryogenic conditions. This property is crucial for all areas of research where high magnetic fields are required and where the influence of thermal energy/broadening needs to be suppressed. For example, much of today's fundamental research on superconductivity, spintronics, and magnetic data storage is taking place at low temperatures.

An anisotropic magneto resistive sensor with set/reset field

Using a set/reset magnetic field, an anisotropic magneto resistive (AMR) magnetic field sensing system was developed to reduce the low frequency noise of an AMR sensor. The magnetic field resolution of the AMR sensor was improved by about three times at the frequencies below 30 Hz and a magnetic field resolution of about 150 pT/√Hz was obtained at 1 Hz. For magnetic particle detection using an AMR sensor with set/reset method, the thermal disturbance effect was canceled well and the signal-to-noise ratio was improved by about three times.

HTS Rf-SQUID Microscope for Metallic Contaminant Detection

We have developed metallic contaminant detection systems using HTS-SQUIDs. In this study, we designed and fabricated a single-chip HTS rf-SQUID using SQUID microscope techniques to reduce both the lift-off and cooling temperature as compared with the substrate resonator HTS rf-SQUID. The HTS rf-SQUID was a washer-type rf-SQUID without a flux focuser. By lengthening and narrowing the SQUID hole, we showed that the inductance of the SQUID ring was reduced, and the effective area and the field noise were improved consequently. By introducing a single-chip HTS rf-SQUID with hole size of 800 μm × 50 μm into the detection system, we obtained a field sensitivity of 150 fT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> and a lift-off of 1.5 mm. Since this SQUID has advantages in the reduction of lift-off and cooling temperature, its detectability and magnetic field resolution are superior to that of the substrate resonator SQUID. Finally, we showed the single-chip SQUID can detect 20 μm in diameter iron balls.

50-nm Local Anodic Oxidation Technology of Semiconductor Heterostructures

A novel approach to local anodic oxidation technique, which leads to approximately equal 50 nm wide line patterns, is described. The technique is utilized to prepare quantum point contact on a low-mobility semiconductor heterostructure. Transport measurements show quantized conductance in zero magnetic field at 4.2 K thanks to very short one-dimensional constriction. The technique is also used for the definition of low-to-room temperature sub-micrometer Hall probes to show its applicability for the room temperature applications. The magnetic-field resolution and the sensitivity of the probes are evaluated in dependence of the probe dimensions, bias current, and temperature. The 200-nm probe shows magnetic-field resolution of 47 microT/(Hz)(1/2) at 140 Hz and at 4.2 K, when it is driven by 5 microA bias current. The novel approach is promising for the development of the future nano-devices operated both at low and room temperatures. To our knowledge, local anodic oxidation technique applied directly to shallow semiconductor heterostructure has been successfully used for the room temperature application for the first time.

Comparison of different detectors in low field NMR measurements

Low-field (LF) liquid sample NMR measurements using different detectors are performed at Larmor frequencies fL ranging from 300 Hz to 35 kHz. Five different sensors cooled with liquid nitrogen are compared: a wire-wound coil, a printed planar copper coil, both resonant with capacitance C and read out by a room-temperature amplifier, a high critical temperature Tc thin film rf (bare) SQUID and two tuned SQUID circuits, one with a wire-wound input coil and another with a high-Tc superconducting tape coil. The resonant frequency of each LC circuit is adjusted to fL. The sensitivity of any LC circuit is determined by its inductance, quality factor, and fL. The magnetic field resolution of the sensors was evaluated with a homogenous magnetic field source at different frequencies and with a small source at different positions. We discuss the signal-to-noise ratio of the LF NMR signals recorded by these detectors and find the high-Tc tape coil to be unsuitable for LF NMR measurement. The homogeneity of the magnetic measurement field is deteriorated by this bulk superconducting object. That effect leads to a dramatic reduction of the decay time of the free induction decay (FID) signal. In contrast, the bare SQUID oriented parallel to the measurement field does not influence the FID significantly.

Noise reduction in optically pumped magnetometer assemblies

In most magnetic field measurement configurations the resolution of optically pumped magnetometers is limited by the shot noise of the pump light. However, in practice this noise limit is overwhelmed by other sources. One of them is the conversion of the pump laser’s frequency modulation (FM) noise to amplitude modulation (AM) noise due to the absorption in the magnetometer’s alkali vapour cell. This extra noise can be nearly completely cancelled by the illumination of an additional cell with the same laser light and the subtraction of its photo current from that of the measurement cell. The correlation of the photo signals of different cells is just slightly decreased by the applied measurement and rf fields B 0 and B 1, respectively. As a result, in real magnetic field measurements using the photo-current subtraction, a noise-limited magnetic field resolution of just twice the shot-noise limit can be achieved. This is experimentally shown for the most thrifty setup with two cells; one time using the second cell just for the photo-current subtraction, the other time also serving for magnetic field measurements, forming a gradiometer with the first cell. Yet, the photo-current subtraction method is most appealing for magnetometer arrays, where the photo signal of just one additional vapour cell can be used for the noise reduction of the complete array.

A voltage biased superconducting quantum interference device bootstrap circuit

We present a dc superconducting quantum interference device (SQUID) readoutcircuit operating in the voltage bias mode and called a SQUID bootstrap circuit(SBC). The SBC is an alternative implementation of two existing methods forsuppression of room-temperature amplifier noise: additional voltage feedback andcurrent feedback. Two circuit branches are connected in parallel. In the dc SQUIDbranch, an inductively coupled coil connected in series provides the bias currentfeedback for enhancing the flux-to-current coefficient. The circuit branch parallelto the dc SQUID branch contains an inductively coupled voltage feedback coilwith a shunt resistor in series for suppressing the preamplifier noise current byincreasing the dynamic resistance. We show that the SBC effectively reduces thepreamplifier noise to below the SQUID intrinsic noise. For a helium-cooled planarSQUID magnetometer with a SQUID inductance of 350 pH, a flux noise of about 3 μΦ0 Hz − 1/2 and a magnetic field resolution of less than 3 fT Hz − 1/2 were obtained. The SBC leads to a convenient direct readout electronics for a dc SQUIDwith a wider adjustment tolerance than other feedback schemes.

Granular Co–C nano-Hall sensors by focused-beam-induced deposition

We investigated the performance of Hall sensors with differentCo–C ratios, deposited directly in nanostructured form, usingCo2(CO)8 gas molecules, by focused-electron or ion-beam-induced deposition. Due to the enhancedintergrain scattering in these granular wires, the extraordinary Hall effect can be increasedby two orders of magnitude with respect to pure Co, up to a magnetic field sensitivity of1 Ω T − 1. We show that the best magnetic field resolution at room temperatureis obtained for Co ratios between 60% and 70% and is better than1 µT Hz − 1/2. For an active areaof the sensor of 200 × 200 nm2, the room temperature magnetic flux resolution isϕmin = 2 × 10 − 5ϕ0 in the thermal noise frequency range, i.e. above 100 kHz.

DC SQUID Modulation Electronics for Operation With HTS DC SQUID Magnetometers in the Unshielded Environment

The new variant of DC SQUID modulation electronics for functioning with HTS DC SQUID magnetometers in the unshielded environment, was designed, manufactured and tested. The electronics was optimized for operation with new ultrasensitive HTS DC SQUID magnetometer providing magnetic field resolution of about 15 fT/radicHz at frequencies above 10 Hz and 30 fT/radicHz at 1 Hz. The central commutation core of electronics was based on a complex programmable logic device (CPLD). It has allowed to include into the SQUID-electronics the flexible system of a bias reversal which provides a stable magnetometer operation for various configurations of wiring in the cryostat and various HTS DC SQUID topologies. The electronics was manufactured in one compact box with size 110 mm times60 mm times 15 mm. The channel has standard bandwidth of about 100 kHz with greatly increased dynamic range due to high coupling (8 mV/Phi0) of feedback signal with main SQUID's loop.

Highly sensitive anisotropic magnetoresistance magnetometer for Eddy-current nondestructive evaluation

Using a commercially available anisotropic magnetoresistance (AMR) sensor of HMC1001, we developed a sensitive magnetometer. It could operate in amplifier mode or feedback mode. The magnetic field sensitivity of the AMR sensor was about 3.2 mV/V G. When the AMR sensor was biased by a voltage of 24 V, the magnetic field resolutions of the AMR magnetometer were about 12 pT/square root(Hz) at 1 kHz and 20 pT/square root(Hz) at 100 Hz. We used the AMR magnetometer for Eddy-current nondestructive evaluation in unshielded environment.

High-resolution distributed-feedback fiber laser dc magnetometer based on the Lorentzian force

A low-frequency magnetic field sensor, based on a current-carrying beam driven by the Lorentzian force, is described. The amplitude of the oscillation is measured by a distributed-feedback fiber laser strain sensor attached to the beam. The transduction mechanism of the sensor is derived analytically using conventional beam theory, which is shown to accurately predict the responsivity of a prototype sensor. Excellent linearity and negligible hysteresis are demonstrated. Noise sources in the fiber laser strain sensor are described and thermo-mechanical noise in the transducer is estimated. The prototype sensor achieves a magnetic field resolution of 5 nT Hz for 25 mA of current, which is shown to be close to the predicted thermo-mechanical noise limit of the sensor. The current is supplied optically through a separate optical fiber yielding an electrically passive sensor head.

A Measurement of Magnetocardiogram (MCG) by Planar Type Sensor using CoNbZr Film

We developed a very sensitive high-frequency carrier-type magnetic-field sensor with a sub-pT resolution using a transmission-line-type sensor. Meander-type sensor elements using amorphous CoNbZr films (4μm in thickness), Cu conductor with a meander pattern (30 mm in length, 1.0 mm in width, 18 μm in thickness), and a ground plane were fabricated. We obtained a magnetic field resolution of 9.1×10-13 T/Hz1/2 at 1 Hz. The detectable magnetic field was limited by the 1/f noise. We detected a magnetocardiogram (MCG) signal of heartbeats with the sensor.

A thin film magnetic field sensor of sub-pT resolution and magnetocardiogram (MCG) measurement at room temperature

We developed a very sensitive high-frequency carrier-type thin film sensor with a sub-pT resolution using a transmission line. The sensor element consists of Cu conductor with a meander pattern (20 mm in length, 0.8 mm in width, and 18 μm in thickness), a ground plane, and amorphous CoNbZr film (4 μm in thickness). The amplitude modulation technique was employed to enhance the magnetic field resolution for measurement of the high-frequency field (499 kHz), a resolution of 7.10×10 −13 T/Hz 1/2 being achieved, when we applied an AC magnetic field at 499 kHz. The phase detection technique was applied for measurement of the low frequency field (around 1 Hz). A small phase change was detected using a dual mixer time difference method. A high phase change of 130°/Oe was observed. A magnetic field resolution of 1.35×10 −12 T/Hz 1/2 was obtained when a small AC field at 1 Hz was applied. We applied the sensor for magnetocardiogram (MCG) measurement using the phase detection technique. We succeeded in measuring the MCG signal including typical QRS and T waves, and compared the MCG with a simultaneously obtained conventional electrocardiogram (ECG) signal.

Higher order terms in the geometric resonance of open orbits in unidirectional lateral superlattices

The geometric resonance of open orbits in unidirectional lateral superlattices has been examined with high magnetic-field resolution. Magnetoresistance oscillations periodic in 1 / B , analogous to the well-known commensurability oscillations but orders of magnitude smaller both in magnitude and in the magnetic-field scale, have been observed superposed on the low-field positive magnetoresistance. The periodicity in 1 / B can be interpreted in terms of higher order resonances.

Experimental study of a hybrid single flux quantum digital superconducting quantum interference device magnetometer

Digital superconducting quantum interference devices (SQUIDs) have been proposed to extend the dynamic range of analog SQUIDs whenever magnetic fields varying in a wide range need to be measured. We developed a digital SQUID concept, based on the single flux quantum technique, using the reliable 1 kA/cm2 niobium trilayer technology of the FLUXONICS Foundry. We present an extensive experimental analysis to evaluate the digital SQUID magnetometer operation in shielded and unshielded conditions. Bit error rate measurements have shown reliable operation of the digital superconducting circuitry with operation margins of ±11%, confirmed by stable operation in unshielded magnetometer mode for more than 10 h. The best intrinsic magnetic field resolution, corresponding to one magnetic single flux quantum (Φ0), was found to be about 4.2 nT. Peak-to-peak amplitudes of 14 800 and 2810 flux quanta Φ0 could be measured, respectively, for inductively coupled current and magnetic field inputs. A flux noise level of 220 mΦ0/Hz at 0.1 Hz has been measured, corresponding to a dynamic range of 76 dB (e.g., above 12 bits). Furthermore, preliminary experiments have shown that the integration of a digital SQUID with an analog dc SQUID in a hybrid magnetometer is a solution to reach simultaneously a high dynamic range and an increased field sensitivity. Such a device is of utmost importance to detect magnetic fields with high dynamic range to monitor earthquakes in the presence of high amplitude external magnetic perturbations.

Development of Micro-size Search Coil Magnetometer for Magnetic Field Distribution Measurement

For the measurement of the magnetic field distribution with high spatial resolution and high accuracy, the magnetic field sensing probe must be non-magnetic, but the MFM probe and sub-millimeter-meter size Hall probe use a ferromagnetic tip and block, respectively, to increase the sensitivity. To overcome this drawback, we developed a micro-size search coil magnetometer which consists of a single turn search coil, Terfenol-D actuator, scanning system, and control software. To reduce the noise generated by the stray ac magnetic field of the actuator driving coil, we employed an even function λ-H magnetostriction curve and lock-in technique. Using the developed magnetometer, we were able to measure the magnetic field distribution with a magnetic field resolution of 1 mT and spatial resolution of 0.1 ㎜ × 0.2 ㎜ at a coil vibration frequency of 1.8 ㎑.

Scanning Hall Probe Microscopy (SHPM) Using Quartz Crystal AFM Feedback

Scanning Hall Probe Microscopy (SHPM) is a quantitative and non-invasive technique for imaging localized surface magnetic field fluctuations such as ferromagnetic domains with high spatial and magnetic field resolution of approximately 50 nm and 7 mG/Hz(1/2) at room temperature. In the SHPM technique, scanning tunneling microscope (STM) or atomic force microscope (AFM) feedback is used to keep the Hall sensor in close proximity of the sample surface. However, STM tracking SHPM requires conductive samples; therefore the insulating substrates have to be coated with a thin layer of gold. This constraint can be eliminated with the AFM feedback using sophisticated Hall probes that are integrated with AFM cantilevers. However it is very difficult to micro fabricate these sensors. In this work, we have eliminated the difficulty in the cantilever-Hall probe integration process, just by gluing a Hall Probe chip to a quartz crystal tuning fork force sensor. The Hall sensor chip is simply glued at the end of a 32.768 kHz or 100 kHz Quartz crystal, which is used as force sensor. An LT-SHPM system is used to scan the samples. The sensor assembly is dithered at the resonance frequency using a digital Phase Locked Loop circuit and frequency shifts are used for AFM tracking. SHPM electronics is modified to detect AFM topography and the frequency shift, along with the magnetic field image. Magnetic domains and topography of an Iron Garnet thin film crystal, NdFeB demagnetised magnet and hard disk samples are presented at room temperature. The performance is found to be comparable with the SHPM using STM feedback.

高周波キャリア型薄膜磁界センサを用いた心磁界測定

We developed a very sensitive high-frequency carrier-type magnetic field sensor with a pT resolution using a transmission-line-type sensor. Meander-type sensor elements using amorphous CoNbZr films were fabricated. We obtained a magnetic field resolution of 3.0 x 10-12 T/Hz1/2 at 1 Hz. The detectable magnetic field was limited by the 1/f noise. We detected the magnetocardiogram (MCG) signal by using the sensor.

Application of a new TEM data acquisition system based on a HTS SQUID magnetometer (SQUITEM) to metal exploration in Broken Hill area

JOGMEC completed construction of the SQUITEM, a three-channel TEM data acquisition system based on the HTS SQUID magnetometer, in 2006. A HTS SQUID is a high-sensitivity magnetic sensor that has wide bandwidth, and offers high field sensitivity, even at low frequencies. The SQUID magnetometer offers advantages over an induction coil receiver in depth of investigation and detecting conductive targets especially in the presence of conductive overburden. The magnetic field resolution of the SQUITEM is 300 fT/vHz at 1 kHz as measured in the field. A Slew rate (6.8 mT/sec in maximum) is sufficient to respond to square-wave primary-field variation in metal exploration. Frequency bandwidth from DC to 100 kHz provides minimal distortion of the recorded signal. JOGMEC applied the SQUITEM to metal exploration at the White Roo Prospect in the Border Project, west of Broken Hill, a JOGMEC/Minotaur Joint Venture. The joint venture was targeting Broken Hill Type Pb-Zn-Ag mineralisation. Initial drilling had intersected pyrrhotite/chalcopyrite mineralization hosted within a mafic gneiss. A SQUITEM survey was conducted in October 2006 to investigate bedrock conductor highlighted by a conventional TEM survey. The SQUITEM data was superior to the conventional TEM system with respect to depth of investigation. The SQUITEM data profiles and inverted sections highlighted a conductive zone at depth that was not apparent in the conventional TEM data. Drilling of this bedrock conductor resulted in the intersection of pyrrhotite-chalcopyrite mineralisation.