Anisotropic Magnetic Resistance Research Articles

Fragmentation of dense rotation-dominated structures fed by collapsing gravo-magneto-sheetlets and origin of misaligned 100 au-scale binaries and multiple systems

ABSTRACT The majority of stars are in binary/multiple systems. How such systems form in turbulent, magnetized cores of molecular clouds in the presence of non-ideal magnetohydrodynamic (MHD) effects remains relatively underexplored. Through athena++-based non-ideal MHD adaptive mesh refinement simulations with ambipolar diffusion, we show that the collapsing protostellar envelope is dominated by dense gravo-magneto-sheetlets, a turbulence-warped version of the classic pseudodisc produced by anisotropic magnetic resistance to the gravitational collapse, in agreement with previous simulations of turbulent, magnetized single-star formation. The sheetlets feed mass, magnetic fields, and angular momentum to a Dense ROtation-Dominated (DROD) structure, which fragments into binary/multiple systems. This DROD fragmentation scenario is a more dynamic variant of the traditional disc fragmentation scenario for binary/multiple formation, with dense spiral filaments created by inhomogeneous feeding from the highly structured larger-scale sheetlets rather than the need for angular momentum transport, which is dominated by magnetic braking. Provided that the local material is sufficiently demagnetized, with a plasma-$\beta$ of 10 or more, collisions between the dense spiralling filaments play a key role in facilitating gravitational collapse and stellar companion formation by pushing the local magnetic Toomre parameter $Q_\mathrm{m}$ below unity. This mechanism can naturally produce in situ misaligned systems on the 100-au scale, often detected with high-resolution Atacama Large Millimeter Array (ALMA) observations. Our simulations also highlight the importance of non-ideal MHD effects, which affect whether fragmentation occurs and, if so, the masses and orbital parameters of the stellar companions formed.

Protostellar discs fed by dense collapsing gravomagneto sheetlets

ABSTRACT Stars form from the gravitational collapse of turbulent, magnetized molecular cloud cores. Our non-ideal MHD simulations reveal that the intrinsically anisotropic magnetic resistance to gravity during the core collapse naturally generates dense gravomagneto sheetlets within inner protostellar envelopes – disrupted versions of classical sheet-like pseudo-discs. They are embedded in a magnetically dominant background, where less dense materials flow along the local magnetic field lines and accumulate in the dense sheetlets. The sheetlets, which feed the disc predominantly through its upper and lower surfaces, are the primary channels for mass and angular momentum transfer from the envelope to the disc. The protostellar disc inherits a small fraction (up to 10 per cent) of the magnetic flux from the envelope, resulting in a disc-averaged net vertical field strength of 1–10 mG and a somewhat stronger toroidal field, potentially detectable through ALMA Zeeman observations. The inherited magnetic field from the envelope plays a dominant role in disc angular momentum evolution, enabling the formation of gravitationally stable discs in cases where the disc field is relatively well-coupled to the gas. Its influence remains significant even in marginally gravitationally unstable discs formed in the more magnetically diffusive cases, removing angular momentum at a rate comparable to or greater than that caused by spiral arms. The magnetically driven disc evolution is consistent with the apparent scarcity of prominent spirals capable of driving rapid accretion in deeply embedded protostellar discs. The dense gravomagneto sheetlets observed in our simulations may correspond to the ‘accretion streamers’ increasingly detected around protostars.

Defect detection in lithium ion cells by magnetic field imaging and current reconstruction

The detection of production failures in commercial lithium ion cell productions is important to decrease scrap rates and save energy by early recognition. However, there is still a lack of fast and inexpensive methods to test for cell defects, which affect the cell’s current distribution and therefore the cell’s aging.This work presents an improved setup for magnetic field imaging (MFI) and current reconstruction of lithium ion cells that can be used to detect and locate production failures. The method bases on a previous paper using anisotropic magnetic resistance (AMR) sensors to scan the magnetic field in a two-dimensional plane above the cell. Experimental cells with various defects were produced, simulated and measured. The defects include missing welds, cuts and cracks in the active material and current collector as well as blocking elements between the layers of the cell. Our results show that many, although not all, types of defects can be observed using the proposed setup.

Magnetic Detection of Steel Corrosion at a Buried Position Near the Ground Level Using a Magnetic Resistance Sensor

Magnetic Detection of Steel Corrosion at a Buried Position Near the Ground Level Using a Magnetic Resistance Sensor

Detection of Inner Cracks in Thick Steel Plates Using Unsaturated AC Magnetic Flux Leakage Testing With a Magnetic Resistance Gradiometer

In order to ensure the safety of infrastructure, it is important to be able to detect both surface and inner cracks in the thick steel plates used. However, it is difficult to use conventional magnetic measurement techniques for inspecting inner cracks in steel because of its high permeability as well as owing to the variations in it. To solve this problem, we developed a method called unsaturated ac magnetic flux leakage (MFL) testing based on a magnetic resistance gradiometer for analyzing the inner cracks in steel. The proposed method uses a sensor probe consisting of a semicircular yoke with induction coils at each end, and a gradiometer with two anisotropic magnetic resistance sensors for detecting the components perpendicular to the steel surface. The conventional MFL testing method requires a strong power source for observing the MFL in the magnetic saturation region. In contrast, in this paper, a gradiometer-based method for detecting the differential intensity and phase is used to detect low levels of magnetic leakage using a weak power source when the test steel sample is unsaturated. During the measurements, a deep inner crack could be detected by decreasing the frequency. The line-scanned differential signal exhibited a peak just above the crack position and was observed to be depth dependent. Based on these results, the method could be used to determine the position of the crack as well as its depth.

Magnetic Properties of Fe/Ni and Fe/Co Multilayer Thin Films

In this work, the magnetic and transport properties of Fe/SiO2/Ni and Fe/SiO2/Co multilayers grown on Si/SiO2 substrates have been studied. The samples have been prepared by two-stage deposition process. In the first stage, Fe layer and SiO2 interlayer of both samples are grown by ion beam deposition technique at room temperature. Then the samples are taken out to ambient atmosphere and loaded into a pulse laser deposition (PLD) chamber. Prior to the deposition of top layer, the samples are cleaned by annealing at 150 °C. In the second stage, Ni (or Co) layer is prepared by PLD technique at room temperature. The thickness of deposited layers has been measured by Rutherford back scattering (RBS). Magnetic properties of ferromagnetic bilayers have been investigated by room-temperature ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. Standard four-point magneto-transport measurements at various temperatures have been performed. Two-step switching in the in-plane hysteresis loops of Fe/SiO2/Ni and Fe/SiO2/Co samples is observed. A crossing in the middle of hysteresis loops of both samples points to a weak antiferromagnetic interaction between the magnetic layers of the stacks. Saturation magnetization values have been obtained from the VSM measurements of samples with DC magnetic field perpendicular to the films surface. Magneto-transport measurements have shown the predominant contribution of anisotropic magnetic resistance both at room and low temperatures. FMR studies of Fe/SiO2/Ni and Fe/SiO2/Co samples have revealed additional non-uniform (surface and bulk SWR) modes, which behavior has been explained in the framework of the surface inhomogeneity model. An origin of the antiferromagnetic interaction has been discussed.

Detection of Inner Corrosion of Steel Construction Using Magnetic Resistance Sensor and Magnetic Spectroscopy Analysis

The detection of the inner corrosion of steel construction, specifically backside corrosion of a steel plate, is highly demanded to maintain safety. Eddy-current testing (ECT) is widely used as a nondestructive testing and an evaluation method for detecting surface and subsurface flaws. However, the ECT is usually applied to non-ferromagnetic materials, but has difficulties when applied to ferromagnetic materials with high permeability, since the skin depth is smaller, and the signal-to-noise ratio (SNR) decreases due to magnetic flux fluctuation. In this paper, we developed a magnetic probe using an anisotropic magnetic resistance (AMR) sensor with the sensitivity of 1 nT/ $\surd $ Hz that can detect inner corrosion at extremely low frequency. We also developed an analysis method using magnetic spectroscopy to delete the magnetic flux fluctuation. To improve the SNR of the detected signal, a small cancellation coil around the AMR sensor canceled the applied magnetic field directly coupled to the sensor. The frequency of the applied magnetic field ranged from 1 Hz to 1 kHz. The sensor output at each frequency was lock-in detected. The weak ac magnetic field was applied to the initial permeability region of the magnetization curve. The obtained magnetic vector signal consisting of a real part and an imaginary part at each frequency was plotted as magnetic spectroscopy, and the magnetic component of the eddy current and the magnetic component of the magnetization of the steel were separated. By using the magnetic component of the eddy current, the thickness of an iron steel plate thinner than 16 mm could be measured. Moreover, the shape of the back-side corrosion was determined by scanning with the magnetic probe.

Characterizations of Electrodeposited NiCoFe Ternary Alloys: Influence of deposition potential

Microstructural, magnetoresistance and magnetic properties of electrodeposited NiCoFe ternary alloys were investigated in terms of deposition potentials changing from −1.8 to −3.0 V with respect to saturated calomel electrode. The alloys were grown on polycrystalline Titanium substrates. The potentials were obtained from cyclic voltammetry and the current–time transients were also recorded to control the growth of proper alloys. From the structural analysis by X-ray diffraction, all alloys had a face-centred cubic structure. The alloy compositions by energy dispersive X-ray spectroscopy revealed that the Ni content increases whereas the Co and Fe contents decrease as the deposition potential increase. And, the scanning electron microscope images disclosed that the alloy morphologies changed as the deposition potential changed. All alloys showed anisotropic magnetic resistance and their magnitudes were between 4.5 and 8.7 %. The vibrating sample magnetometer measurements showed that the saturation magnetization of the alloys increases from 748 to 786 emu/cm3 and coercivity decreases from 28.7 to 25.9 Oe as the deposition potential increases. The easy axis of magnetization was found to be parallel to the film plane for all alloys. The variations in magnetic and magnetoresistive properties related to the microstructure were attributed to the compositional changes caused by the deposition potential.

Influence of deposition potential on the electrodeposited Ternary CoFeCu films

Ternary CoFeCu films, relating their magnetic and magnetoresistance properties with film composition, surface morphology and the corresponding crystal structure, were investigated in terms of different deposition potentials in electrodeposition. The films were grown on polycrystalline titanium substrates. The potentials were obtained from cyclic voltammetry and the current–time transients were also recorded to control the growth of proper films. From the structural analysis by X-ray diffraction, all films had a face-centred cubic structure and the calculated grain size increased with increasing deposition potential. The film compositions by energy dispersive X-ray spectroscopy revealed that the Co, Fe and Cu contents varied and the scanning electron microscope images disclosed that the film morphologies changed as the deposition potential changed. The saturation magnetization was high and coercivity was low at high deposition potential. The easy axis of magnetization was parallel to the film plane for all films. All films showed anisotropic magnetic resistance and their magnitudes were between 3.2 and 3.8 %. The variations in magnetic and magnetoresistive properties related to the microstructure were attributed to the variation of the film contents caused by deposition potential.

Co–Fe Films: Effect of Fe Content on Their Properties

The characterizations of Co-Fe films electrodeposited on Ti substrates under potentiostatic conditions were investigated as a function of the Fe content in the films. The compositional analysis was carried out by energy dispersive X-ray spectroscopy. Scanning electron microscopy used to analyze the surface morphology of the films revealed that the film surface became rather smooth with the increase of the Fe contents. X-ray diffraction patterns showed that the crystal structure changed depending on the Co:Fe ratio in the films. It was observed that the crystal structure converted from the predominant face-centered-cubic to the body-centered-cubic with increasing Fe content. All films showed anisotropic magnetic resistance, but their magnitudes decrease as the Fe content increases. Magnetic data obtained from by vibrating sample magnetometer revealed that the changes observed in the saturation magnetization and coercivity values may arise from the Fe content of the films. The different magnetic and magnetotransport properties may come from the structural differences caused by the Fe content.

Properties of Co–Fe Films: Dependence of Cathode Potentials

Co-Fe films were electrodeposited on Titanium substrates at the cathode potentials changing from - 1.8 to - 2.7 V with respect to saturated calomel electrode (SCE). The structural analysis by X-ray diffraction revealed that all films have a mixed phase of face centered cubic and body centered cubic structure, but the phase ratios change depending on the cathode potentials. The compositional analysis, which was made using an energy dispersive X-ray spectrometry, demonstrated that the Co content of the films slightly increases as the deposition potential increases. The morphological analysis of the films grown at high deposition potential (-2.7 V versus SCE), studied by scanning electron microscopy, indicate that they have larger grains. All Co-Fe films showed anisotropic magnetic resistance up to 4% and its magnitude was affected by the cathode potentials. Magnetic measurements carried out by the vibrating sample magnetometer indicated that the saturation magnetization varied and the coercivity decreased from 46.98 to 31.74 Oe as the cathode potential increased. The easy axis of magnetization was found to be in the film plane for all films. The variation in magnetoresistance and magnetic properties may be related to the structural changes in the films.