Low Magnetic Field Sensors Research Articles

Impact of stress induced anisotropic modifications on magneto-impedance behaviour of rapidly quenched CoFe based microwires

The investigation addresses the role of Al in devitrification stage of a series of (Co94Fe6)72.5Si12.5−xAlxB13Cr2 (X = 0, 2, 4, 6 at%) rapidly quenched microwires. The incorporation of Al displayed reduction in primary onset and activation energy (A.E) while it facilitated widening of span between primary and secondary crystallization onsets. The Giant Magneto-Impedance (GMI) behaviour was influenced by Al concentration as well as applied uniaxial stress. Optimal Al containing alloy (x ∼ 2 at%) displayed signature of asymmetric dual peak at the operating frequency of 1 MHz and its prospective application as a very low magnetic field sensor in pre-stressed condition. However, with increasing stress, the asymmetric character transformed to symmetric dual peak which is attributed to the transition of transverse anisotropy to circular anisotropy via helical anisotropy. High stress sensitivity has been attributed to either dominant transverse or circular anisotropy. The transition of stress induced anisotropies in the microwires has been portrayed schematically to elucidate the transitions.

Alternating current magnetic susceptibility of a ferronematic.

We report on experimental studies focusing on the dynamic ac magnetic susceptibility of a ferronematic. It has been shown recently, that in the isotropic phase of a ferronematic, a weak dc bias magnetic field of a few oersteds increases the ac magnetic susceptibility. This increment vanishes irreversibly if the substance is cooled down to the nematic phase, but can be reinduced by applying the dc bias field again in the isotropic phase [Tomašovičová, N. et al. Soft Matter 2016, 12, 5780–5786]. The effect has no analogue in the neat host liquid crystal. Here, we demonstrate that by doubling the concentration of the magnetic nanoparticles, the range of the dc bias magnetic field to which the ferronematic is sensitive without saturation can be increased by about two orders of magnitude. This finding paves a way to application possibilities, such as low magnetic field sensors, or basic logical elements for information storage.

Biasing a ferronematic - a new way to detect weak magnetic field.

The magnetic properties of a ferronematic, i.e., a nematic liquid crystal doped with magnetic nanoparticles in low volume concentration are studied, with the focus on the ac magnetic susceptibility. A weak dc bias magnetic field (a few Oe) applied to the ferronematic in its isotropic phase increases the ac magnetic susceptibility considerably. Passage of the isotropic-to-nematic phase transition resets this enhancement irreversibly (unless the dc bias field is applied again in the isotropic phase).

Low-field microwave absorption in pulse laser deposited FeSi thin film

Low field microwave absorption (LFMA) measurements at 9.4GHz (X-band), were carried out on pulse laser deposited (PLD) polycrystalline B20 cubic structure FeSi thin film grown on Si (111) substrate. The LFMA properties of the films were investigated as a function of DC field, temperature, microwave power and the orientation of DC field with respect to the film surface. The LFMA signal is very strong when the DC field is parallel to the film surface and vanishes at higher angles. The LFMA signal strength increases as the microwave power is increased. The LFMA signal disappears around 340K, which can be attributed to the disappearance of ferromagnetic state well above room temperature in these films. We believe that domain structure evolution in low fields, which in turn modifies the low field permeability as well as the anisotropy, could be the origin of the LFMA observed in these films. The observation of LFMA opens the possibility of the FeSi films to be used as low magnetic field sensors in the microwave and rf frequency regions.

Giant planar Hall effect in pulsed laser deposited permalloy films

Ni80Fe20 thin films with high orientation were grown on Si(1 0 0) using pulsed laser ablation. The anisotropic magnetoresistance (AMR) and the planar Hall measurements show a 2.5% resistance anisotropy and a 45% planar Hall voltage change for magnetic field sweep of 10 Oe. The planar Hall sensitivity dR/dH was found to be 900 Ω T−1 compared with a previously reported maximum of 340 Ω T−1 in the same system. Also these films are found to withstand repeated thermal cycling up to 110 °C and the Hall sensitivity remains constant within this temperature range. This combination of properties makes the system highly suitable for low magnetic field sensors, particularly in geomagnetic and biosensor applications. To elucidate this, we have demonstrated that these sensors are sensitive to Earth's magnetic field. These results are compared with the sputter deposited films which have a very low AMR and planar Hall voltage change as compared with the films grown by PLD. The possible reasons for these contrasting characteristics are also discussed.

Magnetically tunable rf wave absorption in polycrystalline La0.67Ba0.33MnO3

We investigated temperature and magnetic field dependent radio-frequency electromagnetic absorption in La0.67Ba0.33MnO3 by monitoring changes in resonance frequency (fr) and current (I) through a LC resonant circuit powered by an integrated chip oscillator. The ferromagnetic to paramagnetic transition at TC in zero magnetic field is accompanied by an abrupt increase in fr and I and they are tunable by small external magnetic field. We observed fractional changes as much as 46% in Δfr/fr and 23% in ΔI/I around TC under μ0H=0.1 T that can be exploited for low-magnetic field sensors and other applications.

CPT Magnetometer with Atomic Energy Level Modulation

We propose and experimentally investigate a coherent population trapping state based magnetometer prototype with 87Rb atoms. Through modulating Zeeman sublevels with an ac magnetic field, not only a phase sensitive detection scheme suitable for miniature magnetometer is realized, but also the detection resolution of magnetic field intensity could be improved by a factor of two. Our study result indicates that it is a promising low power consumption miniature sensitive low magnetic field sensor offering spatially resolved measurement at the sub-millimetre level.

Spintronics in metal–insulator nanogranular magnetic thin films

Magnetic and electrical properties of metal–insulator nanogranular thin films are overviewed, from the spintronics point of view, by presenting our recent results. (1) The metal-rich ferromagnetic ones possess ultrahigh-frequency (MHz–GHz) permeabilities μ′, μ″, mainly owing to large induced magnetic anisotropy field H k . They are useful for various magnetic devices, such as thin-film inductors and noise suppressors. (2) The insulator-rich superpramagnetic ones exhibit tunnel-type magnetoresistance, being large in (Mg or Al)-fluoride base films. The Granular-In-Gap (GIG) devices with improved field sensitivity are useful for very low magnetic-field sensors.

Development of YBaCuO bulk superconductor for use as low magnetic field sensor

Polycrystalline samples of nonoriented bulk YBaCuO superconducting specimens display high degree of disorder, semiconducting behavior in the normal state, and broad transition temperature. A regime explaining electrical and magnetic behaviors of high critical temperature superconductors called thermally activated flux flow (TAFF) presents a state in which vortex dynamic exhibits an ohmic behavior. The resistivity in this regime is proportional to the applied magnetic field for small current densities. In this work is presented a systematic study of the magnetoresistance as a function of the applied magnetic field in the range 5 to 100 Oe for polycrystalline samples with various thickness. Results of X-ray diffraction, resistivity as a function of the temperature, I-V characteristic curves and magnetoresistance are shown. The later result was fitted through an ad-hoe model showing good agreement in the 5 to 100 Oe magnetic field range.

Low-field microwave absorption in Y1Ba2Cu3Oy-a possible device application

The absorption of microwave radiation by the high-temperature superconducting ceramic Y1Ba2Cu3Oy in nearly zero magnetic field is extremely sensitive to the applied magnetic field. The spectra reported in the literature are very noisy. The authors show that this noise is not inherent to the superconducting system but is due to environmental magnetic noise. This noise can easily be removed by proper magnetic shielding which allows the ceramic to be used, without any further materials processing, as a very low magnetic field sensor. They also report the first measurements of this absorption in a single-crystal sample and show why certain explanations about the nature of this absorption can be ruled out.

Appliations of amorphous magnetic hall devices

Amorphous magnetic materials present a new area of research and provide us with new material for devices. Rare-earth transition metal(RE-TM) amorphous thin films prepared by r.f. sputtering or thermal evaporation have a large extraordinary Hall coefficient ( R 1). We report some characteristics and experimental results of the use of amorphous RE-TM magnetic films as Hall devices. Amorphous magnetic films show various M- H loops, and can be obtained by varying sputtering conditions. All films were used as magnetic heads, but a film with in-plane magnetization was used as a low magnetic field sensor. In hall experiments, 2.5 kBPI signal can be read out using these amorphous films. In Hall sensor experiments, the detectable magnetic field (signal-to-noise ratio, S/N = 1) was about 5 X 10 −3 Oe. Better results in comparison with other Hall sensors were obtained for the temperature dependence of sensitivity and Hall offset voltage.