Large Magnetic Susceptibility Research Articles

A quantum ruler for orbital magnetism in moiré quantum matter.

For almost a century, magnetic oscillations have been a powerful "quantum ruler" for measuring Fermi surface topology. In this study, we used Landau-level spectroscopy to unravel the energy-resolved valley-contrasting orbital magnetism and large orbital magnetic susceptibility that contribute to the energies of Landau levels of twisted double-bilayer graphene. These orbital magnetism effects led to substantial deviations from the standard Onsager relation, which manifested as a breakdown in scaling of Landau-level orbits. These substantial magnetic responses emerged from the nontrivial quantum geometry of the electronic structure and the large length scale of the moiré lattice potential. Going beyond traditional measurements, Landau-level spectroscopy performed with a scanning tunneling microscope offers a complete quantum ruler that resolves the full energy dependence of orbital magnetic properties in moiré quantum matter.

Preparation of magneto-electro-optically active polymer using Fe ionic liquid as electro-magnetic dopant

In this research, we report the synthesis of conjugated polymers with magnetism. Electrochemical polymerization in liquid crystal was conducted with 1-butyl-3-methylimidazolium tetrachloroferrate (BMITF), which is a paramagnetic character having a large magnetic susceptibility, as a supporting salt. The polymer films thus obtained had magnetism because BMITF remains in the films as a dopant. Poly(2,2′-bithiophene) and poly(bis-(3,4-ethylenedioxythiophene)) were prepared in nematic or cholesteric liquid crystal with BMITF by electrochemical polymerization. Transcription of the liquid crystal structure was succeeded with a small amount of BMITF. Blush or fingerprint structures were confirmed by polarizing optical microscopy observation. The addition of the excess amount of BMITF broke the structure of the liquid crystal. Poly(2,2′-bithiophene) transcribed the cholesteric liquid crystal structure showed a negative Cotton effect, indicating with left-handed helical structure. Poly(bis-(3,4-ethylenedioxythiophene)) transcribed the cholesteric liquid crystal structure showed a positive Cotton effect, indicating the polymer forms a right-handed helical structure. The X-ray photoelectron spectroscopy measurements, confirming the existence of Fe element in the polymers thus obtained. From the electron spin resonance measurements, the polymers thus obtained showed magnetic activity. This method affords facile synthesis of conjugated polymers with magnetism.

In vitro and Bioimaging Studies of Mesoporous Silica Nanocomposites Encapsulated Iron-oxide and Loaded Doxorubicin Drug (DOX/IO@Silica) as Magnetically Guided Drug Delivery System.

In recent years, the delivery of drugs by nanocomposites has emerged as an exciting field of research for bio-imaging tools and targeted cancer treatment. The large surface area and porous volume of mesoporous silica nanocomposites (MSN's) have gained a lot of interest for their application in the delivery of drugs and the magnetic properties of iron oxide (IO) nanocomposites play a key role in the targeted delivery system. In this study, mesoporous silica encapsulated IO nanocomposites loaded with doxorubicin (DOX) were synthesized for the magnetically guided delivery of anticancer drugs. The synthesis of IO nanocomposites was done through the precipitation method, and then silica encapsulation and drug loading were done by the StÖber method. The magnetically driven delivery of the drug is produced by the encapsulation of magnetically active IO in the mesoporous silica shell. The controlled release of DOX is possible because of the MSN's. TEM images show that the nanocomposites have a spherical morphology and average diameter in the range of 120 nm. Power-XRD data confirm the crystalline nature of nanocomposites. The strong absorption peak was observed in UV-Visible spectroscopy at 490 nm and quenching in fluorescence spectra confirms the encapsulation of DOX in the mesoporous silica shell. VSM data showed the magnetic nature of nanocomposites, with large magnetic susceptibility (74.88 emu/g). The use of DOX/IO@Silica nanocomposites as a sustainable drug release and targeted drug delivery vehicle has been reported here. The pH dependent release of DOX was studied and significant release was observed at lower pH. In-vitro cell viability assay and fluorescence imaging assay have demonstrated that these nanocomposites show significant dose-dependent toxicity to cancer cells in the presence of a magnetic field. In-vitro studies via the MTT assay showed that these synthesized nanocomposites in culture are non-toxic to healthy cells compared to DOX-induced cytotoxicity due its controlled release and can be further strengthened by magnetic guidance. Therefore, due to its optical properties and potential for guided delivery of drug to the targeted site, these nanocomposites are ideal as an anticancer agent and bio-imaging prob.

Effects of Hydrothermal Alteration and Mineralization on the Paleomagnetic Properties of Rocks from IODP Expedition 376 at Brothers Volcano

AbstractThe 3-D subseafloor architecture of submarine hydrothermal systems is largely unknown, particularly at arc volcanoes. The alteration of volcanic rocks in these systems produces dramatic changes in their magnetic properties. Here, we present the first comprehensive study of paleomagnetic measurements from oriented samples of hydrothermally altered dacites from Brothers volcano (Kermadec arc), drilled during International Ocean Discovery Program (IODP) Expedition 376. These data have enabled insight into the progressive evolution of magnetic minerals in subseafloor volcanic rocks affected by variable types and degrees of hydrothermal alteration in response to varying fluid temperatures, chemistry, and associated mineralization; from initial chloritization typical of relatively low-temperature interaction with seawater to extremely altered rocks affected by higher-temperature, very acidic magmatic fluids.Hydrothermally altered samples show a significant reduction in natural remanent magnetization intensity (10–4 to 10–2 A/m) compared with unaltered samples (1–10 A/m), suggesting that primary titanomagnetite grains are destroyed during the hydrothermal alteration process. Except for a small region in proximity to the mineralized stockwork zone, no chemical remanent magnetization is observed in association with hydrothermal alteration, consistent with the widespread formation of diamagnetic and/or paramagnetic minerals such as pyrite, rutile, and leucoxene, which do not carry any natural remanent magnetization.Demagnetization experiments show that most of the oriented samples possess a stable characteristic remanent magnetization induced by the residual primary magnetic minerals formed at the time the rocks cooled on the sea floor. Partially chloritized dacites, however, are characterized by large magnetic susceptibilities, low Koenigsberger ratios, and very low magnetic coercivities, consistent with initial dissolution of smaller, singledomain magnetic grains, indicating that intensely hydrothermally altered rocks are better paleomagnetic indicators than initially chloritized samples at the periphery of the hydrothermal systems.The significant magnetic contrast between fresh and hydrothermally altered rocks, in addition to a thick layer (>300 m) of demagnetized rocks observed at Brothers volcano, confirms the empirical results that magnetic anomalies are important geophysical tools to determine the geometry of hydrothermal systems at submarine arc volcanoes.

Magnetic properties of chalcopyrite and arsenopyrite for high-gradient magnetic separation with Crystal-Field Theory

Separating chalcopyrite from arsenopyrite using flotation has been a big challenge due to their similar floatability. However, this challenging condition may be partially solved by employing their difference in magnetic susceptibility. As a matter of fact, pulsating high-gradient magnetic separation (PHGMS), as an environment-friendly and economic strategy, was recently attempted to remove arsenopyrite from chalcopyrite with considerable performance. In this study, the magnetic difference of chalcopyrite and arsenopyrite was confirmed using the Density Functional Theory (DFT) calculation and the Vibrating Sample Magnetometer (VSM) measurement. The results indicated that the two minerals possess significant magnetic property difference, and the chalcopyrite has a much larger magnetic susceptibility than that of arsenopyrite. The theoretical analysis based on the Crystal-Field Theory revealed that the difference was resulted from their spin state of d electrons in Fe ions. This has been further demonstrated by the separation performance of a cyclic PHGMS separator in separating a pure chlacopyrite-arsenopyrite mixture, which assays 15.69% Cu and 22.66% As. Under the optimized conditions, the separator produced a chalcopyrite concentrate assaying 20.25% Cu and 9.92% As, with 79.07% Cu and 26.82% As recovered respectively; in the non-magnetic product, the Cu grade was reduced to 8.48% and the As grade was improved to 42.83%. This investigation has demonstrated a potential perspective for the environmental-friendly and economic removal of arsenopyrite from chalcopyrite using PHGMS method.

Field induced spin freezing and low temperature heat capacity of disordered pyrochlore oxide Ho2Zr2O7

Spin ice materials are the model systems that have a zero-point entropy as T → 0 K, owing to the frozen disordered states. Here, we chemically alter the well-known spin ice Ho2Ti2O7 by replacing Ti sites with isovalent but larger Zr ion. Unlike the Ho2Ti2O7 which is a pyrochlore material, Ho2Zr2O7 crystallizes in disordered pyrochlore structure. We have performed detailed structural, ac magnetic susceptibility and heat capacity studies on Ho2Zr2O7 to investigate the interplay of structural disorder and frustrated interactions. The zero-field ground state exhibits large magnetic susceptibility and remains dynamic down to 300 mK without showing Pauling’s residual entropy. The dynamic state is suppressed continuously with the magnetic field and freezing transition evolves (∼10 K) at a field of ∼10 kOe. These results suggest that the alteration of chemical order and local strain in Ho2Ti2O7 prevents the development of spin ice state and provides a new material to study the geometrical frustration based on the structure.

Anisotropic large diamagnetism in Dirac semimetals ZrTe5 and HfTe5

Dirac semimetals, e.g., ZrTe5 and HfTe5, have been widely investigated and have exhibited various exotic physical properties. Nevertheless, several properties of these compounds, including diamagnetism, are still unclear. In this study, we measured the temperature- and field-dependent diamagnetism of ZrTe5 and HfTe5 along all three crystallographic axes (a-, b-, and c-axis). The temperature-dependent magnetization shows an anomaly, which is a characteristic of Dirac crossing. Diamagnetic signal reaches the highest value of 17.3 × 10−4 emu mol−1 Oe−1 along the van der Waals layers, i.e., the b-axis. However, the diamagnetism remains temperature-independent along the other two axes. The field-dependent diamagnetic signal grows linearly without any sign of saturation and maintains a large value along the b-axis. Interestingly, the observed diamagnetism is anisotropic like other physical properties of these compounds and is strongly related to the effective mass, indicating the dominating contribution of orbital diamagnetism in Dirac semimetals induced by interband effects. ZrTe5 and HfTe5 show one of the largest diamagnetic value among previously reported state-of-the-art topological semimetals. Our present study adds another important experimental aspect to characterize nodal crossing and search for other topological materials with large magnetic susceptibility.

Intracerebral electrical stimulation to understand the neural basis of human face identity recognition.

Recognizing people's identity by their faces is a key function in the human species, supported by regions of the ventral occipito-temporal cortex (VOTC). In the last decade, there have been several reports of perceptual face distortion during direct electrical stimulation (DES) with subdural electrodes positioned over a well-known face-selective VOTC region of the right lateral middle fusiform gyrus (LatMidFG; i.e., the "Fusiform Face Area", FFA). However, transient impairments of face identity recognition (FIR) have been extremely rare and only behaviorally quantified during DES with intracerebral (i.e., depth) electrodes in stereo-electroencephalography (SEEG). The three detailed cases reported so far, summarized here, were specifically impaired at FIR during DES inside different anatomical VOTC regions of the right hemisphere: the inferior occipital gyrus (IOG) and the LatMidFG, as well as a region that lies at the heart of a large magnetic susceptibility artifact in functional magnetic resonance imaging (fMRI): the anterior fusiform gyrus (AntFG). In the first two regions, the eloquent electrode contacts were systematically associated with the highest face-selective and (unfamiliar) face individuation responses as measured with intracerebral electrophysiology. Stimulation in the right AntFG did not lead to perceptual changes but also caused an inability to remember having been presented face pictures, as if the episode was never recorded in memory. These observations support the view of an extensive network of face-selective VOTC regions subtending human FIR, with at least three critical nodes in the right hemisphere associated with differential intrinsic and extrinsic patterns of reentrant connectivity.

Penetrative Brinkman ferroconvection via internal heating in high porosity anisotropic porous layer: influence of boundaries

The penetrative ferrothermal convection (FTC) in a ferrofluid (FF) saturated high porosity anisotropic porous layer via uniform internal heating is investigated. The Brinkman-extended Darcy equation is applied to describe the flow in the porous medium. The permeability in the vertical direction is taken to be twice that of the permeability in the horizontal direction while the ratio of horizontal to vertical effective thermal diffusivity is allowed to vary. The Galerkin method is applied to solve numerically the stability eigenvalue problem for different boundary combinations namely, (i) rigid-paramagnetic (R–P) with large and low magnetic susceptibility, (ii) rigid-ferromagnetic (R–F), and (iii) free-ferromagnetic (F–F). The R–P boundaries with large magnetic susceptibility offer most, while F–F boundaries offer least stabilizing effect against FTC. Besides, the effect of increasing the magnetic number, non-linearity of fluid magnetization parameter, Darcy number and internal heat source strength is to speed up FTC, while the thermal anisotropy and magnetic susceptibility parameter indict a contradictory effect on FTC.

Effect of Dusty Particles on Darcy-Brinkman Gravity-Driven Ferro-Thermal-Convection in a Ferrofluid Saturated Porous Layer with Internal Heat Source: Influence of Boundaries

In the present work, penetrative ferro-thermal-convection (FTC) via internal heating in a ferrofluid-saturated porous layer for different types of velocity, temperature and magnetic potential bounding surfaces, is investigated theoretically. A Galerkin-type based on weighted residual method has been used to obtain the eigenvalue problem. The stability of the system is notably influenced on porous medium via internal heating. It is demonstrated that paramagnetic surface with large magnetic susceptibility $$ (\chi > 0) $$ delay the FTC as compared to very low magnetic susceptibility $$ (\chi = 0) $$ as well as ferromagnetic ones. Increase in heat source strength, non-linearity of magnetization, dust particles and magnetic number hasten, while increase in viscosity ratio, concentration parameter and inverse Darcy number delay the onset. The rigid paramagnetic surfaces induct more stabilizing effect on FTC compared to very low magnetic susceptibility as well as ferromagnetic surfaces. In limiting cases, some results published previously are recovered from the present results.

An equal-TE ultrafast 3D gradient-echo imaging method with high tolerance to magnetic susceptibility artifacts: Application to BOLD functional MRI.

To develop an ultrafast 3D gradient echo-based MRI method with constant TE and high tolerance to B0 inhomogeneity, dubbed ERASE (equal-TE rapid acquisition with sequential excitation), and to introduce its use in BOLD functional MRI (fMRI). Essential features of ERASE, including spin behavior, were characterized, and a comparison study was conducted with conventional EPI. To demonstrate high tolerance to B0 inhomogeneity, in vivo imaging of the mouse brain with a fiber-optic implant was performed at 9.4 T, and human brain imaging (including the orbitofrontal cortex) was performed at 3 T and 7 T. To evaluate the performance of ERASE in BOLD-fMRI, the characteristics of SNR and temporal SNR were analyzed for in vivo rat brains at 9.4 T in comparison with multislice gradient-echo EPI. Percent signal changes and t-scores are also presented. For both mouse brain and human brain imaging, ERASE exhibited a high tolerance to magnetic susceptibility artifacts, showing much lower distortion and signal dropout, especially in the regions involving large magnetic susceptibility effects. For BOLD-fMRI, ERASE provided higher temporal SNR and t-scores than EPI, but exhibited similar percent signal changes in in vivo rat brains at 9.4 T. When compared with conventional EPI, ERASE is much less sensitive, not only to EPI-related artifacts such as Nyquist ghosting, but also to B0 inhomogeneity including magnetic susceptibility effects. It is promising for use in BOLD-fMRI, providing higher temporal SNR and t-scores with constant TE when compared with EPI, although further optimization is needed for human fMRI.

Lanthanide Tagging of Oligonucleotides to Nucleobase for Paramagnetic NMR.

Although lanthanide tags, which have large anisotropic magnetic susceptibilities, have already been introduced to enrich NMR parameters by long‐range pseudoconact shifts (PCSs) and residual dipolar couplings (RDCs) of proteins, their application to nucleotides has so far been limited to one previous report, due to the high affinities of lanthanides for the phosphodiester backbone of nucleotides and difficult organic synthesis. Herein, we report successful attachment of a lanthanide tag to a chemically synthesized oligonucleotide via a disulfide bond. NMR experiments reveal PCSs of up to 1 ppm and H−H RDCs of up to 8 Hz at 950 MHz. Although weaker magnetic alignment was achieved than with proteins, the paramagnetic data could be fitted to the known structure of the DNA, taking the mobility of the tag into account. While further rigidification of the tag is desirable, this tag could also be used to measure heteronuclear RDCs of 13C,15N‐labeled chemically synthesized DNA and RNA.

Modeling of active shimming of metallic needles for interventional MRI.

Artifacts caused by large magnetic susceptibility differences between metallic needles and tissue are a persistent problem in many interventional MRI applications. The signal void caused by the needle can hide procedure targets and prevent accurate image-based monitoring. In this paper, a solution to this problemis presented in the form of an active shim insert inspired from degaussing coils used in naval vessels, that isdesignedto correct the field disturbance (ΔB0 ) caused by the needle. The ΔB0 induced by a 10 gauge hollow single-beveled titanium needle at 3T is modeled in different orientations. A set of 63 orthogonal coil pairs with unique tip paths are evaluated for shimming performance, and an optimal coil pair is chosen. Shimming performance and current demands are evaluated over a range of needle orientations. Robust correction of the titanium needle induced ΔB0 is predicted using a flat no-loop coil combined with an orthogonal 1½ turn loop coil angled at the bevel angle for most orientations, with currents well below 1 amp per coil. Reductions in ΔB0 standard deviations with shimming ranged from ~49% to ~10% depending on needle orientation, with performance worsening as the needle is aligned more along B0 . Simulations predict that it is possible to minimize metallic probe induced ΔB0 and signal losses using externally supplied direct current shim coil inserts in arbitrary orientations for potential benefits in many interventional MRI applications.

Solvent-Induced Encapsulation of Cobalt(II) Ion by a Boron-Capped tris-Pyrazoloximate.

Boron-cross-linked cobalt(II) pseudoclathrochelate was obtained by the template reaction of 2-acetylpyrazoloxime, phenylboronic acid, and a new DMF cobalt(II) solvato complex with a decachloro-closo-decaborate dianion. As confirmed by single-crystal X-ray diffraction, this complex crystallizes with two symmetry-independent cobalt(II) pseudoclathrochelate cations, one decachloro-closo-decaborate dianion, one benzene, one dichloromethane solvent molecule, and two molecules of DMF. The latter act as pseudocapping fragments to the monocapped tris-pyrazoloximate ligands by forming N-H···O hydrogen bonds with their pyrazole groups. The CoIIN6-coordination polyhedra adopt a nearly ideal TP geometry with distortion angles φ equal to 1.22(16) and 2.58(17)° for two symmetry-independent pseudoclathrochelate cations, both containing the encapsulated cobalt(II) ion in its high-spin state (Co-N 2.115(4)-2.198(3) Å). Magnetic properties of this complex were studied both by dc-magnetometry and by solution-state NMR spectroscopy to reveal a high magnetic anisotropy, thus suggesting a large magnetic susceptibility tensor anisotropy (25.8 × 10-32 m3 at 298 K) and a large negative zero-field splitting energy (-85 cm-1). The results of magnetometry studies in the ac magnetic field suggest a single molecule magnet behavior of this TP complex with an effective magnetization reversal barrier of approximately 130 cm-1. Its pseudocapping DMF molecules that form H-bonds with tris-pyrazoloximate fragments are easy to substitute by strong H-bond acceptors, such as chloride ions and di- and tetramethylureas, thus affecting the magnetic properties of a whole pseudomacrobicyclic paramagnetic system.

Yolk-shell structured Fe@void@mesoporous silica with high magnetization for activating peroxymonosulfate

Sulfate radical anion (SO4⿢⿿) based Fenton-like reaction have recently received a large quantity of attention owing to their strong oxidative capacity and high selectivity toward organic pollutants. However, the development of a high-efficient catalyst for activation of peroxymonosulfate (PMS) with a fast separation is still challengeable. Herein, magnetic mesoporous silica composites with a yolk-shell structure (Fe@void@mSiO2) have been prepared via a successive coating strategy, followed by a high-temperature in-situ treatment and demonstrated as a high-efficient and fast magnetic separable catalyst for the activation of PMS. The resultant material possesses a well-defined yolk⿿shell structure with high specific surface area (⿼495.0⿿m2/g), uniform pore size (⿼6.9⿿nm) and super large magnetic susceptibility (⿼105⿿emu/g). Owing to the unique properties, the material possesses an excellent degradation activity for tetracyclines (TC), which is much higher than the commercialized Zero Valent Iron (ZVI) nanoparticles. Additionally, the catalyst is able to work over a broad pH range and be quickly recycled by using an external magnetic field. This research provides a promising strategy for the synthesis and design of multifunctional catalyst for the Fenton-like process.

Structure and Magnetic Properties of Amorphous Fe-Zr-Nb-Cu-B-(Y) Ribons

In this paper the results of the structural and magnetic properties for amorphous alloys ribbons Fe86Zr7Nb1Cu1B5 and Fe86Zr4Y3Nb1Cu1B5 were presented. From X-ray diffraction and M�ssbauer investigations we have stated that amorphous ribbons were fully amorphous. The investigated alloys, exhibit poor soft magnetic properties at room temperature. However, at low temperatures they show relatively large magnetic susceptibility and good its time stability.

Reversible Switching of the Magnetic Orientation of Titanate Nanosheets by Photochemical Reduction and Autoxidation.

Optical properties of aqueous colloidal dispersions of 2D electrolytes, if their aspect ratios are extra-large, can be determined by their orientation preferences. Recently, we reported that a colloidal dispersion of diamagnetic titanate(IV) nanosheets (TiIVNSs), when placed in a magnetic field, is highly anisotropic because TiIVNS anomalously orients its 2D plane orthogonal to the magnetic flux lines due to its large anisotropic magnetic susceptibility. Herein, we report a serendipitous finding that TiIVNSs can be in situ photochemically reduced into a paramagnetic species (TiIV/IIINSs), so that their preference of magnetic orientation changes from orthogonal to parallel. This transition distinctly alters the structural anisotropy and therefore optical appearance of the colloidal dispersion in a magnetic field. We also found that TiIV/IIINSs is autoxidized back to TiIVNSs under non-deaerated conditions. By using an elaborate setup, the dispersion of TiIVNSs serves as an optical switch remotely operable by magnet and light.

Double magnetic proximity in Fe/Fe0.32V0.68 superlattices

The conventional magnetic proximity effect and double-proximity effects were studied in a set of fully coherent high-quality Fe/${\mathrm{Fe}}_{0.32}{\mathrm{V}}_{0.68}$ superlattices. Applying a simple model to the saturation magnetization, it is seen that the magnetic proximity effect is gigantic in magnitude in the alloy---the magnetization is enhanced by 20--450 % and the ordering temperature is enhanced by a factor of 2. The magnitude of the effect can be explained by the large susceptibility of the alloy above its intrinsic ordering temperature. Additionally, a strong dependence of the ordering temperature of single monolayers of Fe on the interlayer distance is observed. The results give insight into new ways of using alloying and large magnetic susceptibility combined with magnetic proximity effects to enhance the functionality of materials that are of interest for spintronic devices.

First-order reversal curve analysis of a Nd-Fe-B sintered magnet with soft X-ray magnetic circular dichroism microscopy

First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets.

Quantifying MRI frequency shifts due to structures with anisotropic magnetic susceptibility using pyrolytic graphite sheet

Magnetic susceptibility is an important source of contrast in magnetic resonance imaging (MRI), with spatial variations in the susceptibility of tissue affecting both the magnitude and phase of the measured signals. This contrast has generally been interpreted by assuming that tissues have isotropic magnetic susceptibility, but recent work has shown that the anisotropic magnetic susceptibility of ordered biological tissues, such as myelinated nerves and cardiac muscle fibers, gives rise to unexpected image contrast. This behavior occurs because the pattern of field variation generated by microstructural elements formed from material of anisotropic susceptibility can be very different from that predicted by modelling the effects in terms of isotropic susceptibility. In MR images of tissue, such elements are manifested at a sub-voxel length-scale, so the patterns of field variation that they generate cannot be directly visualized. Here, we used pyrolytic graphite sheet which has a large magnetic susceptibility anisotropy to form structures of known geometry with sizes large enough that the pattern of field variation could be mapped directly using MRI. This allowed direct validation of theoretical expressions describing the pattern of field variation from anisotropic structures with biologically relevant shapes (slabs, spherical shells and cylindrical shells).