O.V. Nielsen

Scalar Calibration of Vector Magnetometers

The calibration parameters of a vector magnetometer are estimated only by the use of a scalar reference magnetometer. The method presented in this paper differs from those previously reported in its linearized parametrization. This allows the determination of three offsets or signals in the absence of a magnetic field, three scale factors for normalization of the axes and three non-orthogonality angles which build up an orthogonal system intrinsically in the sensor. The advantage of this method compared with others lies in its linear least squares estimator, which finds independently and uniquely the parameters for a given data set. Therefore, a magnetometer may be characterized inexpensively in the Earths magnetic-field environment. This procedure has been used successfully in the pre-flight calibration of the state-of-the-art magnetometers on board the magnetic mapping satellites Orsted, Astrid-2, CHAMP and SAC-C. By using this method, full-Earth-field-range magnetometers (± 65536.0 nT) can be characterized down to an absolute precision of 0.5 nT, non-orthogonality of only 2 arcsec and a resolution of 0.2 nT.

Effects of longitudinal and torsional stress annealing on the magnetic anisotropy in amorphous ribbon materials

The influence of tensile stress annealing on the magnetic properties of amorphous alloys is reported for ribbons belonging to the pseudo ternary system (FeCoNi) 75 Si 15 B 10 . Ribbons of compositions (Co 1-x Fe x ) 75 Si 15 B 10 are susceptible to stress annealing in the whole composition range 0 \leq x \leq 1 . A very large easy ribbon axis anisotromy can be produced in the range 0.2 \leq x \leq 0.7 whereas the largest hard ribbon axis anisotropy is producable in the composition range around x=0.15. Ribbons of compositions [CO 1-x (Fe 0.5 Ni 0.5 ) x ] 75 Si 15 B 10 are found susceptible to stress annealing only in the range 0 \leq x \leq 0.7 . In all the compositions the induced easy ribbon axis anisotropies are irreversible whereas the hard ribbon axis anisotropies are recoverable with activation energies ≥ 1.55 eV.

Magnetic anisotropy in Co73Mo2Si15B10 and (Co0.89Fe0.11) 72Mo3Si15B10 metallic glasses, induced by stress-annealing

Abstract Annealing temperature dependence of the stress-induced magnetic anisotropy in Co 73 Mo 2 Si 15 B 10 (λ s 0.89 Fe 0.11 ) 72 Mo 3 Si 15 B 10 (λ > 0) metallic glass ribbons has been studied experimentally. The room temperature values of the induced anisotropy constants K depend strongly on the annealing conditions, and reveal a change of sign at annealing temperatures well below T cryst . It is concluded that transient creep and steady-state creep at the elevated temperature give rise to compressive and tensile stresses, respectively, in the expression for the magnetoelastic coupling energy at room temperature.

Calibration of the Ørsted vector magnetometer

The vector fluxgate magnetometer of the Ørsted satellite is routinely calibrated by comparing its output with measurements of the absolute magnetic intensity from the Overhauser instrument, which is the second magnetometer of the satellite. We describe the method used for and the result obtained in that calibration. Using three years of data the agreement between the two magnetometers after calibration is 0.33 nT rms (corresponding to better than ± 1 nT for 98% of the data, and better than ± 2 nT for 99.94% of the data). We also report on the determination of the transformation between the magnetometer coordinate system and the reference system of the star imager. This is done by comparing the magnetic and attitude measurements with a model of Earth’s magnetic field. The Euler angles describing this rotation are determined in this way with an accuracy of better than 4 arcsec.

Experiments concerning the origin of stress anneal induced magnetic anisotropy in metallic glass ribbons

Abstract The stress anneal induced anisotropy has been studied in metallic glass ribbons of the compositions (Co 1- x Fe x ) 75 Si 15 B 10 , 0≤ x ≤0.12. The induced anisotropies are found to consist of two contributions, an irreversable one and a recoverable one. From their thermal variations and from their stress dependence the anisotropy constants have tentatively been related to the constituents of the saturation magnetostriction coefficients. The kinetics reveal for the recoverable anisotropy a continuous spectrum of activation energies in the range 1.55–1.8 eV.

Magnetic anisotropy in (Fe, Co)75Si15B10 and (Fe0.11Co0.89)72Mo3Si15B10 metallic glass ribbons, induced by constant stress and constant strain annealing

Abstract The magnetic anistropy induced by constant tensile stress and constant torsional strain annealing has been studied for liquid quenched alloys of the compositions (FexCo1−x)75Si15B10 (x = 0.1−1.0) and (Fe0.11Co0.89)72Mo3Si15B10. Within a certain annealing temperature range (depending on composition, pre-annealing time, applied stress and stress-annealing duration) the induced easy axis is perpendicular to the ribbon axis. After subsequent stress relief the easy axis turns parallel to the ribbon axis. The easy ribbon axis anisotropy produced in this way reveals a similar composition dependence as the anisotropy obtained by magnetic field annealing, but the anisotropy constant may be up to one order of magnitude larger, depending on the stress annealing conditions. In contrast to the field anneal induced anisotropy, the stress anneal induced anisotropy can be made permanent in the sense that it still remains after subsequent stress relief. It is furthermore shown that constant strain annealing induced anisotropy can be deduced from the properties observed after constant stress annealing, and a method for selecting proper stress relaxation conditions, with the purpose of obtaining maximum magnetic softness (isotropy) is proposed.

Co-Si-B and Fe-Co-B amorphous alloys: Induced anisotropy and various magnetic properties

Abstract The dependence on the metalloid content of some magnetic properties of Co 100− x (Si 0.6 B 0.4 ) x (22.5 ⩽ x ⩽ 30) and Co 75 Si 25− x B x (10 ⩽ x ⩽ 25) amorphous alloys has been studied. Ribbons were subjected to different kinds of heating treatments: field annealing, stress annealing and stress-field annealing (tensile stress and longitudinal magnetic field applied simultaneously). While the anisotropies induced by simple field annealings are of the order of magnitude of 0.1 kJm -3 , the anisotropy induced by stress-field annealing can reach values up to 0.5 kJm -3 . The preferred axis is longitudinal for most of the annealing conditions. The temperature and composition dependence of the magnetostriction have been studied too. Stress, field and stress-field induced anisotropies have also been measured in Co 66 Fe 9 B 25 samples (λ s > 0). In this case the preferred axis is transverse to the ribbon axis.

Digital detection and feedback fluxgate magnetometer

A new full Earths field dynamic feedback fluxgate magnetometer is described. It is based entirely on digital signal processing and digital feedback control, thereby replacing the classical second harmonic tuned analogue electronics by processor algorithms. Discrete mathematical cross-correlation routines and substantial oversampling reduce the noise to 71 pT root-mean-square in a 0.25 - 10 Hz bandwidth for a full Earths field range instrument.

Digital fluxgate magnetometer for the Astrid-2 satellite

The design and performance of the Astrid-2 magnetometer are described. The magnetometer uses mathematical routines, implemented by software for commercially available digital signal processors, to determine the magnetic field from the fluxgate sensor. The sensor is from the latest generation of amorphous metal sensors developed by the Department of Automation at the Technical University of Denmark.

Short range order in (Fe, Co, Ni)75Si15B10 amorphous alloys determined from magnetic anisotropy

Abstract Some previously reported experimental results dealing with magnetostriction and induced magnetic anisotropy are analyzed with respect to the alloy composition. It is found that the properties which are affected by directional order vary with the composition as the probability for finding Bernal tetrahedral holes surround by 3Co1Fe or 3Fe1Co and not as the probability for finding metal—metal pairs as predicted by diatomic ordering theory for ideal solutions.