Surface Magnetic Measurements Research Articles

Mapping Magnetic Signals of Individual Magnetite Grains to Their Internal Magnetic Configurations Using Micromagnetic Models.

Micromagnetic tomography (MMT) is a technique that combines X‐ray micro computed tomography and scanning magnetometry data to obtain information about the magnetic potential of individual grains embedded in a sample. Recovering magnetic signals of individual grains in natural and synthetic samples provides a new pathway to study the remanent magnetization that carries information about the ancient geomagnetic field and is the basis of all paleomagnetic studies. MMT infers the magnetic potential of individual grains by numerical inversion of surface magnetic measurements using spherical harmonic expansions. The magnetic potential of individual particles in principle is uniquely determined by MMT, not only by the dipole approximation, but also more complex, higher order, multipole moments. Here, we show that such complex magnetic information together with both particle shape and mineral properties severely constrains the internal magnetization structure of an individual grain. To this end, we apply a three dimensional micromagnetic model to predict the multipole signal from magnetization states of different local energy minima. We show that for certain grains it is even possible to uniquely infer the magnetic configuration from the inverted magnetic multipole moments. This result is crucial to discriminate single‐domain particles from grains in more complex configurations such as multi‐domain or vortex states. As a consequence, our investigation proves that by MMT it is feasible to select statistical ensembles of magnetic grains based on their magnetization states, which opens new possibilities to identify and characterize stable paleomagnetic recorders in natural samples.

Monitoring the cooling of the 1959 Kīlauea Iki lava lake using surface magnetic measurements

Lava lakes can be considered as proxies for small magma chambers, offering a unique opportunity to investigate magma evolution and solidification. Repeated magnetic ground surveys over more than 50 years each show a large vertical magnetic intensity anomaly associated with Kīlauea Iki Crater, partly filled with a lava lake during the 1959 eruption of Kīlauea Volcano (Island of Hawai’i). The magnetic field values recorded across the Kīlauea Iki crater floor and the cooling lava lake below result from three simple effects: the static remnant magnetization of the rocks forming the steep crater walls, the solidifying lava lake crust, and the hot, but shrinking, paramagnetic non-magnetic lens (>540 °C). We calculate 2D magnetic models to reconstruct the temporal evolution of the geometry of this non-magnetic body, its depth below the surface, and its thickness. Our results are in good agreement with the theoretical increase in thickness of the solidifying crust with time. Using the 2D magnetic models and the theoretical curve for crustal growth over a lava lake, we estimate that the former lava lake will be totally cooled below the Curie temperature in about 20 years. This study shows the potential of magnetic methods for detecting and monitoring magmatic intrusions at various scales.

Influence of uniaxial plastic deformation on surface magnetic field in steel

Under geomagnetic field, surface magnetization measurements were made on 1045 and a3 steels samples uniaxially deformed to differing magnitudes of plastic strain, to study the dependence of surface magnetic field and its distribution on plastic deformation. The results indicated initial increase in surface magnetic field with increasing plastic strain followed by a decrease at higher plastic deformation. At still higher plastic deformation, the surface magnetic field was found to be almost independent of plastic strain. In the middle part of the sample, the uneven plastic deformation interrupted the linear distribution of surface magnetic field. The behavior of surface magnetic field and its distribution with plastic deformation were largely controlled by the different mechanisms of interaction of domain walls with isolated dislocation, dislocation tangles and dislocation cellular structures.

Changes in the hyperfine interactions in the Fe 80Nb 3Cu 1B 16 metallic glass under tensile loading

The influence of tensile stress on the changes in hyperfine parameters obtained by Mössbauer spectroscopy of the ferromagnetic amorphous NANOPERM-type Fe 80Nb 3Cu 1B 16 alloy has been investigated. The bulk changes are obtained by Mössbauer spectroscopy in a transmission geometry using γ-rays; the surface properties are studied in a scattering geometry using conversion electrons. The Mössbauer spectra are analyzed by two distributions of hyperfine inductions, which result in high-field and low-field components corresponding with a model of clusters formed by predominantly iron atoms and intermediate phase containing mainly the Nb, Cu and B atoms. The obtained results are completed by the bulk and surface magnetic measurements, with observations of surface morphology by AFM and microstructure by SEM. The investigations yield approximately linear increase of hyperfine parameters and slight deterioration of the bulk as well as surface magnetic characteristics with tensile loading.

On induction effects of geomagnetic daily variations from equatorial electrojet and solar quiet sources at low and middle latitudes

We investigate the spatiotemporal behavior of the magnetic vertical component, Z, of the daily ionospheric current systems: the equatorial electrojet (EEJ) and solar quiet (Sq) variations, considering induction in the mantle and oceans. The inducing EEJ and Sq current systems are provided by the comprehensive model of Sabaka et al. (2004). The three‐dimensional (3‐D) conductivity model of the Earth includes oceans of laterally variable conductance and a spherical conductor (1‐D) underneath. Our model studies demonstrate that induction effects in Z due to the EEJ are negligible everywhere inland for all local times. At CHAMP altitude (400 km) the magnetic signal induced by EEJ above the oceans does not exceed 2–5% of the external field during local noon. This, in particular, means that considering the induction effects is not necessary when modeling the EEJ current strength from inland surface magnetic measurements and/or satellite data. As expected, induction in the oceans strongly affects the Sq field. The model studies show that the anomalous induction effect (defined as the difference between results obtained with 1‐D and 3‐D conductivity models) of Sq is substantial at CHAMP altitude, comprising 50% of the total field. It is therefore necessary to consider induction in the oceans when modeling Sq variations for both ground‐based and satellite data. Finally, we demonstrate that the anomalous behavior of the daily variations in Z at south Indian sites, namely, a large positive prenoon peak, can be explained by 3‐D induction of the Sq variations, with no contribution from the EEJ.

Improvement of magnetic interface coupling through a magnetic coupling gel

Surface measurement of magnetic properties is potentially a very advantageous technique for nondestructive evaluation. The effect of a magnetic coupling gel on surface magnetic measurement, is reported in this paper. Test results under three interface conditions are presented and compared. It was found that the magnetic gel reduces the variability of the measurement by improving the magnetic interface coupling between the sensor and test specimen. A simple model of a magnetic circuit is analyzed to show the coupling function of magnetic gel. From these results, it is deduced that the magnetic coupling gel plays an important part in improving reliability of the magnetic measurements.

High precision LEED structure analysis of ultra-thin epitaxial fcc Fe films on Cu(100)

We present structure determinations of ultra-thin epitaxial fcc Fe films on Cu(100) by low energy electron diffraction in the coverage regime of 5–10 monolayers. Special emphasis is on the precision of both the measurement of diffraction intensities and their full dynamical analysis. We demonstrate that it is possible to achieve an almost perfect fit between theory and experiment mirrored by R-factors as low as R P = 0.090, R ZJ= 0.026 and R 2 = 0.028 which to our knowledge are the best reported for non-trivial structures. The structure of the films, which stays practically constant in the coverage regime considered, is characterized by a substantial surface reconstruction, an expanded spacing between the first two iron layers and an unstrained in-plane lattice constant a P(Fe) = 2.52 Å rather than that for the substrate ( a P(Cu) = 2.55 Å). The surface reconstruction, which always moves to the top layer when the film thickness is made to grow, can be interpreted as a precursor for the transition to bcc Fe which takes place at higher coverages. The expansion of the top layer spacing, which is consistent with the reconstruction, is fully in line with published surface magnetic measurements which exhibit ferromagnetic coupling in the first two layers.

Origins of surface induced magnetic structures

Surface anisotropy and the surface-bulk exchange interaction may induce surface magnetic structures (SMS). SMS can reduce coercivity and must be taken into account when interpreting surface magnetic measurements. Fe(100) very likely exhibits SMS. In 3d-4f alloys, SMS occur naturally and can be characterized with few parameters.

Polarized Electron Probes of Magnetic Surfaces

The magnetic properties of surfaces are now being explored with electron spectroscopies that use electron spin polarization techniques. The increased activity in surface magnetic measurements with polarized electron beams is spurred by new scientific and technological challenges and is made feasible by recent advances in the technology of sources and detectors of polarized electrons. The ability to grow thin films and to engineer artificial structures permits new phenomena to be investigated at magnetic surfaces and interfaces. For such investigations, spin-polarized electron techniques-such as polarized electron scattering, polarized photoemission, polarized Auger spectroscopy, and scanning electron microscopy with polarization analysis-have been and will probably continue to be used to great advantage.

Geophysical transitions across the northwest Atlantic magnetic quiet-zone border

Deep and surface magnetic measurements, gravity and subbottom seismic profiling data have been gathered across a part of the northwest Atlantic smooth-rough magnetic border. These data indicate that the transition involves only a change in the magnetic field, without associated gravity or subbottom topographic signature. Model studies suggest that a change in basement magnetization from 0.010 emu/cm 3 to 0.005 emu/cm 3 could account for the magnetic field change across the boundary from the rough to the smooth sectors. Various theories previously proposed to explain this magnetic boundary are discussed with respect to these newer data; reduction of basement magnetization by weathering of the proto-Atlantic Ocean floor as described by Drake et al. (1968) is the preferred explanation.

Cosmic ray evidence for a ring current

Many observations show that low rigidity protons from solar flares are permitted entry at Minneapolis only during the main phase of magnetic storms. The measured energy is much below the normal Stormer cutoff at that time. This paper develops the idea that a ring current responsible for the main storm field reduces the Stormer cosmic-ray cutoffs. The model of the ring chosen is actually an azimuthal current on the surface of a sphere with current intensity proportional to sin θ where θ = colatitude. It is shown that this mathematical form permits great simplicity in the analysis and leads to essentially the same result as a diffuse ring corresponding to the actual trapped radiation. The magnetic moment of the ring required to produce the cutoff change can be provided by reasonable intensities of very low energy trapped radiation. It is shown that the cosmic-ray data permit one to evaluate both the moment (M′) and radius (R) of the ring, whereas the surface magnetic measurements determine only the quantity M′/R3. Since observation shows that the cosmic-ray cutoffs return to normal during the main phase, it must be assumed that the ring shrinks inward so that the surface field is maintained negative. Cosmic-ray evidence concerning the presence of a permanent ring current is discussed. Such a ring is measured directly by satellites during quiet times.