skip to main content

Title: Estimating the Net Magnetic Moment of Geological Samples From Planar Field Maps Using Multipoles

Recent advances in magnetic microscopy have enabled studies of geological samples whose weak and spatially nonuniform magnetizations were previously inaccessible to standard magnetometry techniques. A quantity of central importance is the net magnetic moment, which reflects the mean direction and the intensity of the magnetization states of numerous ferromagnetic crystals within a certain volume. The planar arrangement of typical magnetic microscopy measurements, which originates from measuring the field immediately above the polished surface of a sample to maximize sensitivity and spatial resolution, makes estimating net moments considerably more challenging than with spherically distributed data. In particular, spatially extended and nonuniform magnetization distributions often cannot be adequately approximated by a single magnetic dipole. To address this limitation, we developed a multipole fitting technique that can accurately estimate net moment using spherical harmonic multipole expansions computed from planar data. Given that the optimal location for the origin of such expansions is unknown beforehand and generally unconstrained, regularization of this inverse problem is critical for obtaining accurate moment estimates from noisy experimental magnetic data. We characterized the performance of the technique using synthetic sources under different conditions (noiseless data, data corrupted with simulated white noise, and data corrupted with measured instrument noise). We then validated and demonstrated the technique using superconducting quantum interference device microscopy measurements of impact melt spherules from Lonar crater, India and dusty olivine chondrules from the CO chondrite meteorite Dominion Range 08006.

more » « less
Award ID(s):
Author(s) / Creator(s):
 ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geochemistry, Geophysics, Geosystems
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Interest in magnetic fields on the ancient Earth and other planetary bodies has motivated the paleomagnetic analysis of complex rocks such as meteorites that carry heterogeneous magnetizations at <<1 mm scales. The net magnetic moment of natural remanent magnetization (NRM) in such small samples is often below the detection threshold of common cryogenic magnetometers. The quantum diamond microscope (QDM) is an emerging magnetic imaging technology with ~1 μm resolution and can, in principle, recover magnetizations as weak as 10−17 Am2. However, the typically 1–100 μm sample‐to‐sensor distance of QDM measurements can result in complex (nondipolar) magnetic field maps, from which the net magnetic moment cannot be determined using a simple algorithm. Here we generate synthetic magnetic field maps to quantify the errors introduced by sample nondipolarity and by map processing procedures such as upward continuation. We find that inversions based on least squares dipole fits of upward continued data can recover the net moment of complex samples with <5% to 10% error for maps with signal‐to‐noise ratio (SNR) in the range typical of current generation QDMs. We validate these error estimates experimentally using comparisons between QDM maps and between QDM and SQUID microscope data, concluding that, within the limitations described here, the QDM is a robust technique for recovering the net magnetic moment of weakly magnetized samples. More sophisticated net moment fitting algorithms in the future can be combined with upward continuation methods described here to improve accuracy.

    more » « less
  2. We report direct imaging of boundary magnetization associated with antiferromagnetic domains in magnetoelectric epitaxial Cr 2 O 3 thin films using diamond nitrogen vacancy microscopy. We found a correlation between magnetic domain size and structural grain size which we associate with the domain formation process. We performed field cooling, i.e. , cooling from above to below the Néel temperature in the presence of a magnetic field, which resulted in the selection of one of the two otherwise degenerate 180° domains. Lifting of such a degeneracy is achievable with a magnetic field alone due to the Zeeman energy of a weak parasitic magnetic moment in Cr 2 O 3 films that originates from defects and the imbalance of the boundary magnetization of opposing interfaces. This boundary magnetization couples to the antiferromagnetic order parameter enabling selection of its orientation. Nanostructuring the Cr 2 O 3 film with mesa structures revealed reversible edge magnetic states with the direction of magnetic field during field cooling. 
    more » « less
  3. Abstract

    Despite the rampant discovery of tunable magnetic properties using magneto‐ionic gating, e.g., magnetic anisotropy, exchange bias, and exchange interactions, there are few studies that give a quantitative understanding of the reversible and irreversible effects of ionic infiltration. In this study, in situ vibrating sample magnetometry, superconducting quantum interference device magnetometry, and X‐ray magnetic circular dichroism (XMCD) reveal the reversible and irreversible control of magnetization, anisotropy, proximity‐effects, spin, and orbital angular momenta. Pd/Co/Pd trilayers, loaded using solid‐state hydrogen‐ion gating, show a decrease in the saturation magnetization of Co, and an increase in the proximity‐induced moment of Pd. This results in little to no change in the net effective magnetization, yet, allows for the effective anisotropy to be reversibly controlled by 270 kJ m−3. The reversible control of the effective anisotropy is dominated by a reversible change in surface anisotropy, however, under repeated cycling, irreversible evolution occurs in the heterostructure. XMCD measurements indicate this is partly due to hydrogen‐induced modification of the spin and orbital angular momenta. Together, these measurements indicate that the origin of the reversible and irreversible effects of magneto‐ionic gating is interfacial and provides crucial insight to scale and optimize thin film heterostructures for enhanced longevity and faster response time.

    more » « less
  4. Abstract

    Transition metal alloys are essential for magnetic recording, memory, and new materials-by-design applications. Saturation magnetization in these alloys have previously been measured by conventional techniques, for a limited number of samples with discrete compositions, a laborious and time-consuming effort. Here, we propose a method to construct complete saturation magnetization diagrams for Co–Fe–Ni alloys using scanning Hall probe microscopy (SHPM). A composition gradient was created by the diffusion multiple technique, generating a full combinatorial materials library with an identical thermal history. The composition and crystallographic phases of the alloys were identified by integrated energy dispersive X-ray spectroscopy and electron backscatter diffraction. “Pixel-by-pixel” perpendicular components of the magnetic field were converted into maps of saturation magnetization using the inversion matrix technique. The saturation magnetization dependence for the binary alloys was consistent with the Slater-Pauling behavior. By using a significantly denser data point distribution than previously available, the maximum of the Slater-Pauling curve for the Co–Fe alloys was identified at ~ 32 at% of Co. By mapping the entire ternary diagram of Co–Fe–Ni alloys recorded in a single experiment, we have demonstrated that SHPM—in concert with the combinatorial approach—is a powerful high-throughput characterization tool, providing an effective metrology platform to advance the search for new magnetic materials.

    more » « less
  5. Spatial confinement of electronic topological surface states (TSSs) in topological insulators poses a formidable challenge because TSSs are protected by time-reversal symmetry. In previous works formation of a gap in the electronic spectrum of TSSs has been successfully demonstrated in topological insulator/magnetic material heterostructures, where ferromagnetic exchange interactions locally lift the time-reversal symmetry. Here we report experimental evidence of exchange interaction between a topological insulator Bi2Se3 and a magnetic insulator EuSe. Spin-polarized neutron reflectometry reveals a reduction of the in-plane magnetic susceptibility within a 2 nm interfacial layer of EuSe, and the combination of superconducting quantum interference device (SQUID) magnetometry and Hall measurements points to the formation of an interfacial layer with a suppressed net magnetic moment. This suppressed magnetization survives up to temperatures five times higher than the Néel temperature of EuSe. Its origin is attributed to the formation of an interfacial antiferromagnetic state. Abrupt resistance changes observed in high magnetic fields are consistent with antiferromagnetic domain reconstruction affecting transport in a TSS via exchange coupling. The high-temperature local control of TSSs with zero net magnetization unlocks new opportunities for the design of electronic, spintronic, and quantum computation devices, ranging from quantization of Hall conductance in zero fields to spatial localization of non-Abelian excitations in superconducting topological qubits. 
    more » « less