The impact of stress-driven structural transitions and of film strain on the magnetic properties of nm ferromagnetic films is discussed. The stress-induced bending of film-substrate composites is analysed to derive information on film stress due to lattice mismatch or due to surface-stress effects. The magneto-elastic coupling in epitaxial films is determined directly from the magnetostrictive bending of the substrate. The combination of stress measurements with magnetic investigations by the magneto-optical Kerr effect (MOKE) reveals the modification of the magnetic anisotropy by film stress. Stress-strain relations are derived for various epitaxial orientations to facilitate the analysis of the substrate curvature. Biaxial film stress and magneto-elastic coupling coefficients are measured in epitaxial Fe films in situ on W single-crystal substrates. Tremendous film stress of more than 10 GPa is measured in pseudomorphic Fe layers, and the important role of film stress as a driving force for the formation of misfit distortions and for inducing changes of the growth mode in monolayer thin films is presented. The direct measurement of the magneto-elastic coupling in epitaxial films proves that the magnitude and sign of the magneto-elastic coupling deviate from the respective bulk value. Even a small film strain of order 0.1% is found to induce a significant change of the effective magneto-elastic coupling coefficient. This peculiar behaviour is ascribed to a second-order strain dependence of the magneto-elastic energy density, in contrast to the linear strain dependence that is valid for bulk samples.

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The correlation between mechanical stress and magnetic anisotropy in ultrathin films

The most important milestone in the field of magnetic sensors was when AMR sensors started to replace Hall sensors in many applications where the greater sensitivity of AMRs was an advantage. GMR and SDT sensors finally found applications. We also review the development of miniaturization of fluxgate sensors and refer briefly to SQUIDs, resonant sensors, GMIs, and magnetomechanical sensors.

Advances in Magnetic Field Sensors

Since the 1930s, fluxgate sensors have been used for measuring d.c. magnetic fields up to 1 mT with a maximum resolution of 10 pT. In the sensor core the flux is gated by the excitation field. The preferable sensor geometry is a ring-core; both crystalline and amorphous ferromagnetic materials can be used for the core. Although a lot of fluxgate magnetometer types have appeared, the classical type with detection of the second harmonics by a phase-sensitive detector is the most popular. Fluxgate sensors are reliable and rugged and their applications range from space research to submarine detection.

Review of fluxgate sensors

This paper reviews recent achievements in the technology and design of fluxgate sensors and magnetometers. The major recent trends were decreasing of the sensor size, power consumption and price, and, on the other hand, increasing of the precision in the large range of the measured fields. The potential frequency range was increased up to units of kHz. Present fluxgate sensors have a resolution comparable with high-temperature superconducting quantum interference devices (SQUIDs), while their precision is the best of vectorial field sensors.

Advances in fluxgate sensors

The Magnetometer (MAG) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission is a low-noise, tri-axial, fluxgate instrument with its sensor mounted on a 3.6-m-long boom. The boom was deployed on March 8, 2005. The primary MAG science objectives are to determine the structure of Mercury’s intrinsic magnetic field and infer its origin. Mariner 10 observations indicate a planetary moment in the range 170 to 350 nT R3 M (where RM is Mercury’s mean radius). The uncertainties in the dipole moment are associated with the Mariner 10 trajectory and variability of the measured field. By orbiting Mercury, MESSENGER will significantly improve the determination of dipole and higher-order moments. The latter are essential to understanding the thermal history of the planet. MAG has a coarse range, ±51,300 nT full scale (1.6-nT resolution), for pre-flight testing, and a fine range, ±1,530 nT full scale (0.047-nT resolution), for Mercury operation. A magnetic cleanliness program was followed to minimize variable and static spacecraft-generated fields at the sensor. Observations during and after boom deployment indicate that the fixed residual field is less than a few nT at the location of the sensor, and initial observations indicate that the variable field is below 0.05 nT at least above about 3 Hz. Analog signals from the three axes are low-pass filtered (10-Hz cut- off) and sampled simultaneously by three 20-bit analog-to-digital converters every 50 ms. To accommodate variable telemetry rates, MAG provides 11 output rates from 0.01 s“1 to 20 s-1. Continuous measurement of fluctuations is provided with a digital 1-10 Hz bandpass filter. This fluctuation level is used to trigger high-time-resolution sampling in eight-minute segments to record events of interest when continuous high-rate sampling is not possible. The MAG instrument will provide accurate characterization of the intrinsic planetary field, magnetospheric structure, and dynamics of Mercury’s solar wind interaction.

The magnetometer instrument on MESSENGER

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 Earth's 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.

https://iopscience.iop.org/article/10.1088/0957-0233/11/2/304/pdf

Scalar Calibration of Vector Magnetometers

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Temperature, stress, and structural-relaxation dependence of the magnetostriction in (Co0.94/BFe0.06)75/BSi15B10 glasses.

The experiments and theoretical considerations leading to the construction of a high-performance three-axis fluxgate magnetometer are described. The magnetometer will be used (1996) in the Earth's field mapping satellite named 'OErsted'. The fluxgate sensors are based on stress-annealed metallic glass ribbons as core materials. It is shown that very simple physical models can be used to explain the fluxgate mode of operation, thereby making it easy to calculate the overall sensor performance from first principles. Special attention is drawn to the core excitation current which is analysed on the basis of nonlinear electrical circuitry. It is furthermore shown that the ring-core demagnetizing field obeys a simple cosine law which permits the calculation of the sensor sensitivity with high accuracy. The sensitivity, that is the signal-to-noise ratio, is ultimately determined by the sensor noise which is about 15 pT RMS (0.06-10 Hz), corresponding to a noise power density (1/f noise) of 6.2 pT Hz-1/2 at 1 Hz. The actual magnetometer operating range and sensitivity is determined by the 1 bit resolution of the Earth's field represented by the output from the 18 bit AD converted used in the instrument (+or-65536 nT with 0.5 nT resolution). The maximum attainable bandwidth is half the sensor excitation frequency (1/2*15 kHz) but the OErsted magnetometer bandwidth is limited to 250 Hz. The thermal stability of the sensor has been measured to be better than 1 nT in the temperature range -20 to +60 degrees C.

https://iopscience.iop.org/article/10.1088/0957-0233/6/8/004/pdf

Development, construction and analysis of the 'OErsted' fluxgate magnetometer

The vector fluxgate magnetometer of the Orsted 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 033 nT rms (corresponding to better than ± 1 nT for 98% of the data, and better than ± 2 nT for 9994% 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

https://link.springer.com/content/pdf/10.1186%2FBF03352458.pdf

O.V. Nielsen

Papers

Scalar Calibration of Vector Magnetometers

Temperature, stress, and structural-relaxation dependence of the magnetostriction in (Co0.94/BFe0.06)75/BSi15B10 glasses.

Development, construction and analysis of the 'OErsted' fluxgate magnetometer

Calibration of the Orsted vector magnetometer

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