Strong Crystalline Anisotropy Research Articles

A level-set method for simulating solid-state dewetting in systems with strong crystalline anisotropy

Accurately modeling solid-state dewetting in materials with strong crystalline anisotropy is an open problem in materials science with importance for both the manufacturing and reliability of nano-scale devices. In this work, we propose and demonstrate a level-set method framework for simulating solid-state dewetting which is capable of modeling systems with strongly anisotropic surface energies and diffusivities. Surface energy anisotropy is handled through the use of the Cahn-Hoffman vector construction, and surface self-diffusivity anisotropy is handled through the use of a diffusivity tensor. We benchmark our method against isotropic phenomena with analytical descriptions and go on to demonstrate that our method is capable of reproducing a host of experimentally observed behaviors in strongly anisotropic single-crystal materials.

Microstructure and texture control of Ni-Mn-Ga magnetic shape memory alloys manufactured by laser powder bed fusion

The Laser Powder Bed Fusion (LPBF) process was used to manufacture Ni-Mn-Ga polycrystalline samples. The initial powders with a fine particle size of about 20 µm were firstly prepared by a ball milling from melt-spun ribbons. The microstructure and texture evolution of Ni-Mn-Ga alloy manufactured by LPBF were investigated employing SEM, TEM, and synchrotron radiation diffraction. Using two different scanning strategies, optimization of laser parameters and post-processing heat treatment, a homogeneous microstructure with a strong crystallographic texture was obtained. The localized heating/cooling conditions allow obtaining a layered structure with preferred <100> fiber orientations along the growth direction. The crystal structure and crystallographic texture drastically change when the laser oscillation mode is chosen. The strong crystalline anisotropy and layered-type microstructure have a significant impact on twining flow behavior giving rise to an anisotropic mechanical response. The variant reorientation during mechanical training is realized by the so-called orthogonal shearing. It is also shown that the resulting crystal structure and characteristic transformation temperatures are strongly dependent on chemical composition related to Mn losses and internal stresses along with chemical order creating metastable phases due to the extremely high cooling rate upon the 3D printing. Therefore, all the above parameters are thoroughly monitored during the three-stage fabrication process.

Lattice preferred orientation of graphite determined by the anisotropy of out-of-phase magnetic susceptibility

Single crystals of graphite are diamagnetic with a strong crystalline anisotropy. Their magnetic susceptibility measured in alternating fields exhibits a weak, negative in-phase component and a relatively strong out-of-phase component due to electric eddy currents. Consequently, if graphite crystals are oriented preferentially by their crystal lattices (LPO) in graphite ore, one would expect a strong anisotropy of magnetic susceptibility (AMS) of the bulk rock. However, the standard AMS, which is the anisotropy of the in-phase component of susceptibility (ipAMS), reflects not only the LPO of graphite, but also the preferred orientation of the paramagnetic and ferromagnetic admixtures. On the other hand, the anisotropy of out-of-phase susceptibility (opAMS) in rocks such as these should directly indicate the graphite LPO due to its strong electrical conductivity, and can therefore be considered free from the contributions of non-conductive paramagnetic and ferromagnetic minerals. These competing factors were tested on natural metamorphic graphite ores near the South Bohemian town of Český Krumlov in the Moldanubian Zone. The ipMS of the ore is very low, 10−6 to 10−4 [SI], being positive in some specimens and negative in others, which indicates mixtures with variable proportions of graphite and paramagnetic-ferromagnetic minerals. On the other hand, opMS is exclusively positive, on the order of 10−5 to 10−4, and is strongly indicative of graphite. The degree of opAMS is distinctly high at 200–300%, and the opAMS lineation is closely perpendicular to the metamorphic foliation in ores and wall rocks. All this indicates a conspicuous LPO of graphite in the ore that was probably created during Variscan regional metamorphism and associated ductile deformation.

Impact of crystalline anisotropy on the extrinsic spin Hall effect in ultrathin films

An efficient conversion of a charge current into a spin current is a crucial point for application of the spin Hall effect in practical spintronic devices. Recently, we revealed that this goal can be achieved by using ultrathin fcc (111) and (001) noble metal films doped with Bi impurities, which possess spin Hall angles up to 80%. Here, we show that the effect can be further amplified in monolayer films with a strong crystalline anisotropy. This is demonstrated by considering noble metal films with fcc (110) geometry. Our theoretical study predicts related spin Hall angles exceeding 100% especially when the crystalline anisotropy is increased, which tunes the Fermi surface topology.

Mathematical modelling of Tyndall star initiation

The superheating that usually occurs when a solid is melted by volumetric heating can produce irregular solid–liquid interfaces. Such interfaces can be visualised in ice, where they are sometimes known as Tyndall stars. This paper describes some of the experimental observations of Tyndall stars and a mathematical model for the early stages of their evolution. The modelling is complicated by the strong crystalline anisotropy, which results in an anisotropic kinetic undercooling at the interface; it leads to an interesting class of free boundary problems that treat the melt region as infinitesimally thin.

Nanomagnets with high shape anisotropy and strong crystalline anisotropy: perspectives onmagnetic force microscopy

We report on a new approach for magnetic imaging, highly sensitive even in the presence ofexternal, strong magnetic fields. Based on FIB-assisted fabricated high-aspect-ratiorare-earth nanomagnets, we produce groundbreaking magnetic force tips with hardmagnetic character where we combine a high aspect ratio (shape anisotropy)together with strong crystalline anisotropy (rare-earth-based alloys). Rare-earth hardnanomagnets are then FIB-integrated to silicon microcantilevers as highly sharpenedtips for high-field magnetic imaging applications. Force resolution and domainreversing and recovery capabilities are at least one order of magnitude better than forconventional magnetic tips. This work opens new, pioneering research fields onthe surface magnetization process of nanostructures based either on relativelyhard magnetic materials—used in magnetic storage media—or on materials likesuperparamagnetic particles, ferro/antiferromagnetic structures or paramagnetic materials.

Dual Easy Axes Effect on Nanosized Near-Spherical Particles for Advanced Tape Storage

Conventional needle-shaped tape particles utilize shape anisotropy to enable magnetic storage. Developed with near-spherical shape and strong crystalline anisotropy, the nanosized composite advanced particles (NanoCAP) are one of the potential candidates for future tape data storage. However, because of the imperfect shape in fabrication, significant shape anisotropy still exists in NanoCAP, forming a second easy axis in addition to that of the crystalline anisotropy. In this paper, the effect of the dual easy axes is studied by micromagnetic modeling. The analysis shows that the parasitic shape anisotropy degrades media performance. Schemes for reducing the parasitic shape anisotropy are quantitatively evaluated.

Local magnetic probes of superconductors

Investigations of the magnetic properties of high temperature superconductors (HTSs) have revealed the existence of striking new vortex phenomena due, in part, to their strong crystalline anisotropy, very short coherence lengths and the much larger thermal energies available at high temperatures. Some of these phenomena, for example vortex lattice 'melting', pose serious problems for technological applications of the most anisotropic HTS materials and a fuller understanding of them is of considerable importance. The most direct information regarding vortex structures and dynamics is obtained through local measurement of the magnetic field within or at the surface of a superconducting sample. A detailed review of such local magnetic probes is presented here including Lorentz microscopy, magnetic force microscopy, Bitter decoration, scanning Hall probe microscopy, magneto-optical imaging, and scanning superconducting quantum interference device microscopy. In each case the principles underpinning the technique are described together with the factors that limit the magnetic field and the spatial and temporal resolution. A range of examples will be given, emphasizing applications in the area of HTSs. In addition the ways in which the existing techniques can be expected to develop over the next few years will be discussed and new approaches that seem likely to be successful described.

Magnetic alignment in powder magnet processing

A strong magnetic field is used to align single-crystal powder particles in the process of producing sintered powder permanent magnets, including hard ferrites and rare-earth permanent magnets. The applied magnetic field aligns the easy direction of magnetization of each particle, owing to strong crystalline anisotropy. Shape anisotropy, existence of particles containing multigrains, and physical interlock between particles reduce the degree of alignment. This study provides a quantitative analysis of magnetic alignment in powder magnet processing. We assume (1) the powder particle is a single crystal; (2) it has the shape of an oblate spheroid and its short axis is the easy direction of magnetization; and (3) the applied magnetic field is strong enough to overcome the resistance of alignment. By applying the minimum-energy principle, it was concluded that the necessary and sufficient condition for a complete magnetic alignment is that the magnetocrystalline anisotropy constant K1 of the particles is greater than its shape anisotropy constant Ks, provided the applied magnetic field is strong enough. When Ks≳K1+2K2, the angle between the short axis of the oblate particle and the direction of applied magnetic field is 90°, and when K1≤Ks≤K1+2K2, the angle is arcsin√(Ks−K1)/2K2.

Vibrating reed studies of magnetic vortices in superconductors

Although high-temperature oxide superconductors have been studied since 1986, the static and dynamic behaviors of magnetic vortices generated by applied magnetic fields and electrical currents remain controversial. These phenomena are important for at least two reasons: (1) vortex motion generates dissipation causing degraded performance in practical superconducting devices; and (2) the high transition temperatures (Tc≊100 K), short coherence length (≤10 Å) and strong crystalline anisotropy of high-Tc oxides are predicted to cause novel structural rearrangements or melting transitions of the ‘‘vortex lattice’’ at temperatures T well below the superconducting/normal phase boundary Hc2(T). Recent mechanical oscillator studies of relatively isotropic, low-Tc superconductors reveal dissipation anomalies and frequency shifts similar to those observed in high-Tc materials, challenging current theories for the vortex state of superconductors. [Work supported by NSF and Research Corp.]

Magnetic fabrics of the Bloodgood Canyon and Shelley Peak Tuffs, southwestern New Mexico: implications for emplacement and alteration processes

Anisotropy of magnetic susceptibility (AMS) of the middle Tertiary Bloodgood Canyon and Shelley Peak Tuffs of the Mogollon-Datil volcanic field has been used to (1) evaluate the ability of AMS to constrain flow lineations in low-susceptibility ash-flow tuffs; (2) establish a correlation between magnetic fabric, magnetic mineralogy, tuff facies, and characteristics of the depositional setting; and (3) constrain source locations of the tuffs. The tuffs are associated with the overlapping Bursum caldera and Gila Cliff Dwellings basin. The high-silica Bloodgood Canyon Tuff fills the Gila Cliff Dwellings basin and occurs as thin outcrops outside of the basin. The older Shelley Peak Tuff occurs as thin outcrops both along the boundary between the two structures, and outside of the complex. AMS data were collected from 16 sites of Bloodgood Canyon Tuff basin fill, 19 sites of Bloodgood Canyon Tuff outflow, and 11 sites of Shelley Peak Tuff. Sites were classified on the basis of within-site clustering of orientations of principal susceptibility axes, based on the categories of Knight et al. (1986). Most microscopically visible oxide minerals in the Bloodgood Canyon Tuff outflow and basin fill, and in the Shelley Peak Tuff are members of the hematite-ilmenite solid solution series. However, IRM acquisition data indicate that Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff have magnetic mineralogy dominated by single- or pseudo-single-domain magnetite, and that the magnetic mineralogy of the Bloodgood Canyon Tuff outflow is dominated by hematite. Hematite in Bloodgood Canyon Tuff outflow is likely to be the result of deuteric and/or low-temperature alteration of magnetite and iron silicate minerals. Bulk magnetic susceptibility is higher in magnetite-dominated ash-flow tuff (Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff) than it is in hematite-dominated ash-flow tuff (Bloodgood Canyon Tuff outflow). Bloodgood Canyon Tuff outflow has the highest total anisotropy (H) of the three units, followed by Shelley Peak Tuff and Bloodgood Canyon Tuff basin fill. All three ash-flow tuffs are genearlly characterized by oblate susceptibility ellipsoids, with those of the Bloodgood Canyon Tuff basin fill nearest to spherical. At high values of total anisotropy, Shelley Peak Tuff susceptibility ellipsoids attain a prolate shape; those of Bloodgood Canyon Tuff outflow attain an increasingly oblate shape. Three factors may influence differences in total anisotropy and susceptibility ellipsoid shape: (1) ash which travelled the greatest distance before deposition may show the best development of magnetic fabric, particularly of magnetic lineation; (2) deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures; and (3) replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane. Scatter in AMS axis orientation within sites may result from: (1) greater orientation inaccuracy in block-sampled than in fielddrilled samples; (2) rheomorphism; and (3) low accuracy of AMS measurement in low-susceptibility ashflow tuffs. Evaluation of flow lineation based on AMS of sites with well-clustered K1 axes indicates that (1) Bloodgood Canyon Tuff basin fill flowed along a generally northwest-southeast azimuth; (2) Shelley Peak Tuff located on the boundary of the Bursum caldera and the Gila Cliff Dwellings basin flowed along a nearly east-west azimuth; and (3) Bloodgood Canyon Tuff outflow sites have K1 susceptibility axes generally radial to the Bursum-Gila Cliff Dwellings complex, but within-site scatter of K1 orientations is generally too large to draw conclusions about flow lineation orientation. Limited petrographic work on pilot thin sections adds flow direction information to AMS-derived flow lineation information.

Numerical micromagnetics of small particles

The results of applying the authors' micromagnetic code to a homogeneous spherical particle that is large enough to support inhomogeneous magnetization are given. For smaller particles, magnetization reversal is by coherent rotation. Larger particles initially exhibit curling as the applied magnetic field is reduced from a saturating value. Then one of two new behaviors is observed. For weak crystalline anisotropy, the axis of the curling state rotates and bends, and the magnetization reversal process is reversible, or nearly so. For strong crystalline anisotropy, a sudden discontinuous transition occurs to a vortex state with axis perpendicular to the anisotropy axis, and the vortex moves across the particle as reversal of the applied magnetic field continues. The formation and disappearance of the vortex are irreversible, but all other aspects of the process are reversible.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>