Crystalline Anisotropy Research Articles

Structural Confinement Assisted a Robust Superparamagnetic State in MgNi2O3 and MgNi1.5Co0.5O3 Nanoparticles at Room Temperature

In this study, ideal spherical-shaped cubic metal oxide nanoparticles of < 30 nm in size were prepared by a citric acid sol-gel combustion method. The detailed structural analysis and microscopic techniques clearly support the existing grade of structural confinement effect in MgNi2O3 and MgNi1.5Co0.5O3. Temperature-dependent magnetisation studies at various applied fields revealed a moderate change of TB near room temperature by distinct particle size. Magnetic crystalline anisotropy and energy barrier density theoretical model results suggested the importance of innermost magnetic particle stability-aided spin orientation with a fascinating magnetic moment in superparamagnetic region exerted by Ni and Co remarkably postured as a significant part of these nanoparticles.

Strength and deformation of shocked diamond single crystals: Orientation dependence

Understanding and quantifying the strength or elastic limit of diamond single crystals is of considerable scientific and technological importance, and has been a subject of long standing theoretical and experimental interest. To examine the effect of crystalline anisotropy on strength and deformation of shocked diamond single crystals, plate impact experiments were conducted to measure wave profiles at various elastic impact stresses up to \ensuremath{\sim}120 GPa along [110] and [111] crystal orientations. Using laser interferometry, particle velocity histories and shock velocities in the diamond samples were measured and were compared with similar measurements published previously for shock compression along the [100] direction. Wave profiles for all three orientations showed large elastic wave amplitudes followed by time-dependent inelastic deformation. From the measured wave profiles, the elastic limits were determined under well characterized uniaxial strain loading conditions. The measured elastic wave amplitudes for the [110] and [111] orientations were lower for higher elastic impact stress (stress attained for an elastic diamond response), consistent with the result reported previously for [100] diamond. The maximum resolved shear stress (MRSS) on the {111}⟨110⟩ slip systems was determined for each orientation, revealing significant orientation dependence. The MRSS values for the [100] and [110] orientations (\ensuremath{\sim}33 GPa) are 25%--30% of theoretical estimates; the MRSS value for the [111] orientation is significantly lower (\ensuremath{\sim}23 GPa). Our results demonstrate that the MRSS depends strongly on the stress component normal to the {111} planes or the resolved normal stress (RNS), suggesting that the RNS plays a key role in inhibiting the onset of inelastic deformation. Lower elastic wave amplitudes at higher peak stress and the effect of the RNS are inconsistent with typical dislocation slip mechanisms of inelastic deformation, suggesting instead an inelastic response characteristic of shocked brittle solids. The present results show that the elastic limit (or material strength) of diamond single crystals cannot be described using traditional isotropic approaches, and typical plasticity models cannot be used to describe the inelastic deformation of diamond. Analysis of the measured wave profiles beyond the elastic limit, including characterization of the peak state, requires numerical simulations that incorporate a time-dependent, anisotropic, inelastic deformation response. Development of such a material description for diamond is an important need.

Thermo-solutal growth of an anisotropic dendrite with six-fold symmetry

A stable growth of dendritic crystal with the six-fold crystalline anisotropy is analyzed in a binary nonisothermal mixture. A selection criterion representing a relationship between the dendrite tip velocity and its tip diameter is derived on the basis of morphological stability analysis and solvability theory. A complete set of nonlinear equations, consisting of the selection criterion and undercooling balance condition, which determines implicit dependencies of the dendrite tip velocity and tip diameter as functions of the total undercooling, is formulated. Exact analytical solutions of these nonlinear equations are found in a parametric form. Asymptotic solutions describing the crystal growth at small Péclet numbers are determined. Theoretical predictions are compared with experimental data obtained for ice dendrites growing in binary water-ethylenglycol solutions as well as in pure water.

Magnetization of Extraterrestrial Allende material may relate to terrestrial descend

The origin of magnetization in Allende may have significant implications for our understanding of core formation/differentiation/dynamo processes in chondrite parent bodies. The magnetic Allende data may contain information that could constrain the magnetic history of Allende. The measurements on Allende chondrules reveal an existence of magnetization component that was likely acquired during the meteorite transit to terrestrial conditions. Both the pyrrhotite carrying magnetic remanence intensity and direction of the chondrules change erratically when subjecting the Allende meteorite's chondrules to temperatures near 77 K and back to room temperature. Chondrules with more intense original magnetization are denser and contain larger inverse thermoremanent magnetization (ITRM). Temperature dependent monitoring of ITRM revealed that magnetization was acquired at temperature near 270 K. Such temperature is consistent with the condition when, in addition to temperature increase, the atmospheric uniaxial pressure applied during the meteorite entry on the porous material was responsible for meteorite break up in the atmosphere. During this process, collapse of the pore space in the matrix and some chondrules would generate crystalline anisotropy energy accumulation within pyrrhotite grains in form of parasitic magnetic transition.

Tuning the ferromagnetic resonance by doping strontium hexa-ferrite nanopowders

Mg–Ti substituted strontium hexa-ferrites nanopowders (SrFe12−x(MgTi)x/2O19, x = 0–3) were prepared by the sol–gel method. The morphology, structure and composition of the nanostructures were examined by field emission scanning electron microscopy (FESEM) and X-ray diffraction. The effect of Mg–Ti doping on the magnetic properties of the powders was investigated by vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) at ambient temperature. Experimental results showed that the materials exhibit hexagonal structures with tunable magnetic properties. The saturation magnetization and the coercive field (Hc) decreased through the Mg and Ti substitution. FMR proved that by incorporation of Mg and Ti in strontium ferrite lattice, crystalline anisotropy, and microwave absorption can be tuned. SrFe12−x(MgTi)x/2O19 ferrites are good candidate for applications at X-band microwave frequencies. A low field absorption signal was observed with the same phase as the FMR absorption in all doped ferrites.

Tailoring magnetic properties of self-biased hexaferrites using an alternative copolymer of isobutylene and maleic anhydride

Discussed is a novel self-biased hexaferrite gelling system based on a nontoxic and water-soluble copolymer of isobutylene and maleic anhydride. This copolymer simultaneously acts as a dispersant and gelling agent, and recently received much attention from the ceramics community. Herein its effects on the rheological conditions throughout magnetic-field pressing, and consequently, orientation, density and magnetic properties of textured hexaferrites were investigated. Ka-band FMR linewidths were measured, and the crystalline anisotropy and porosity induced linewidth broadening were estimated according to Schlömann’s theory. The copolymer allowed to reduce the friction between micron-sized magnetic particulates, resulting in higher density and degree of crystalline orientation, and lower FMR linewidth.

Thermo-solutal and kinetic modes of stable dendritic growth with different symmetries of crystalline anisotropy in the presence of convection.

Motivated by important applications in materials science and geophysics, we consider the steady-state growth of anisotropic needle-like dendrites in undercooled binary mixtures with a forced convective flow. We analyse the stable mode of dendritic evolution in the case of small anisotropies of growth kinetics and surface energy for arbitrary Péclet numbers and n-fold symmetry of dendritic crystals. On the basis of solvability and stability theories, we formulate a selection criterion giving a stable combination between dendrite tip diameter and tip velocity. A set of nonlinear equations consisting of the solvability criterion and undercooling balance is solved analytically for the tip velocity V and tip diameter ρ of dendrites with n-fold symmetry in the absence of convective flow. The case of convective heat and mass transfer mechanisms in a binary mixture occurring as a result of intensive flows in the liquid phase is detailed. A selection criterion that describes such solidification conditions is derived. The theory under consideration comprises previously considered theoretical approaches and results as limiting cases. This article is part of the theme issue 'From atomistic interfaces to dendritic patterns'.This article is part of the theme issue 'From atomistic interfaces to dendritic patterns'.

Geometrically defined spin structures in ultrathin Fe3O4 with bulk like magnetic properties.

We have grown high quality magnetite microcrystals free from antiphase boundaries on Ru(0001) by reactive molecular beam epitaxy, conserving bulk magnetic properties below 20 nm thickness. Magnetization vector maps are obtained by X-ray spectromicroscopy and compared with micromagnetic simulations. The observed domain configurations are dictated purely by shape anisotropy, overcoming the possible influences of (magneto)crystalline anisotropy and defects, thus demonstrating the possibility of designing spin structures in ultrathin, magnetically soft magnetite at will.

Magnetic charges suppress effects of anisotropy in polycrystalline soft ferromagnetic materials

Micromagnetic simulations of polycrystalline iron washers show that grain boundary charges, ρ = -div M, suppress bad effects of magnetocrystalline anisotropy. A single domain wall divides the washer into two domains with opposite magnetization; M is almost = ± Ms ϕ, where ϕ circulates about the hole in the washer. There is a ripple structure. M tilts back and forth toward the inner and outer surfaces. Magnetic charge densities, σm = n·M, on the surfaces keep M at the surfaces very close to lying in the surfaces. The exchange εx and magnetostatic εd energy densities try to keep M parallel to the surfaces throughout the washer, except at the domain wall. An anisotropy energy in each grain is reduced linearly in the angle of rotation away from the circulating pattern towards the nearest anisotropy axis. Both εx and εd near grain boundaries increase as the square of these angles. Anisotropy wins for small rotations. However, the coefficients of the positive quadratic terms are so much larger than the coefficients of the negative linear terms that the rotations are quite small. If the height of the washer is sufficiently greater than 300 nm, M in the washer no longer acts as it would in a thin film. If 300 nm washers are stacked with a spacing of 4 nm, the ripple structure is not lost. The stacked washers can then be used as the core of a transformer. The most remarkable effect is that starting with M = Ms ϕ everywhere, the reversal of M by the field from a current along the z-axis produces a single domain wall. It is stable even in zero field because the wall has Néel caps that act as springs against the surfaces. The suppression of crystalline anisotropy in polycrystalline iron also occurs for geometries other than the toroid; some might be better for creating transformers.

Multiple magnetic states within the A phase determined by field-orientation dependence of Mn0.9Fe0.1Si

We report three distinct regions within the A-phase in Fe-doped MnSi, based on the evolution of magnetoresistance and the Hall effect as a function of orientation of applied field. Fe impurities as pinning centers and crystalline anisotropy are found non-negligible only at the boundary of the A-phase. Electrical transport characteristics unique to the A-phase not only remain robust, but also indicate a freely rotating skyrmion lattice, decoupled from underlying crystal structure or impurity pinning.

First principles examination of electronic structure and optical features of 4H-GaN1−xPx polytype alloys

By using first-principles calculations, we compute the electronic band structures and typical aspects of the optical spectra of hexagonally structured GaN1−xPx alloys. Although a type III–V semiconductor, GaP commonly possesses a zinc-blende structure with an indirect band gap; as such, it may additionally form hexagonal polytypes under specific growth conditions. The electronic structures and optical properties are calculated by combining a non-nitride III–V semiconductor and a nitride III–V semiconductor, as GaP and GaN crystallizing in a 4H polytype, with the N composition ranging between x = 0–1. For all studied materials, the energy gap is found to be direct. The optical properties of the hexagonal materials may illustrate the strong polarization dependence owing to the crystalline anisotropy. This investigation for GaN1−xPx alloys is anticipated to supply paramount information for applications in the visible/ultraviolet spectral regions. At a specific concentration, x, these alloys would be exclusively appealing candidates for solar-cell applications.

Atomistic modelling of magnetic nano-granular thin films

In this work, a complete model for studying the magnetic behaviour of polycrystalline thin films at nanoscale was processed. This model includes terms as exchange interaction, dipolar interaction and various types of anisotropies. For the first term, exchange interaction dependence of the distance n was used with purpose of quantify the interaction, mainly in grain boundaries. The third term includes crystalline, surface and boundary anisotropies. Special attention was paid to the disorder vector that determines the loss of cubic symmetry in the crystalline structure. For the case of the dipolar interaction, a similar implementation of the fast multiple method (FMM) was performed. Using these tools, modelling and simulations were developed varying the number of grains, and the results obtained presented a great dependence of the magnetic properties on this parameter. Comparisons between critical temperature and magnetization of saturation depending on the number of grains were performed for samples with and without factors as the surface and boundary anisotropies, and the dipolar interaction. It was observed that the inclusion of these parameters produced a decrease in the critical temperature and the magnetization of saturation; furthermore, in both cases, including and not including the disorder parameters, not only the critical temperature, but also the magnetization of saturation exhibited a range of values that also depend on the number of grains. This presence of a critical interval is due to each grain can transit toward the ferromagnetic state at different values of critical temperature. The processes of Zero field cooling (ZFC), Field cooling (FCC) and field cooling in warming mode (FCW) were necessary for understanding the mono-domain regime around of transition temperature, due to the high probabilities of a Super-paramagnetic (SPM) state.

Effects of Glycine in DES-Based Plating Baths on Structural and Magnetic Properties of Fe–Ni Films

We have already reported that a deep eutectic solvent is one of hopeful solvents to obtain an electroplated Fe-based soft magnetic film since the plating process shows high current efficiency. In this paper, we employed glycine as an additive to improve surface roughness and soft magnetic properties of electroplated Fe-Ni films. The coercivity of the Fe-rich films (>70 at.%) prepared from the glycine-used bath showed lower values compared with those for the no-glycine baths. From the evaluations for the surface roughness and the crystal structure of the as-plated Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">75</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">25</sub> films, we considered that the reductions in the coercivity of the Fe-rich films for the glycine-used baths are attributed to the smooth surface and the reduction in the effective crystalline anisotropy by increase in the volume fraction of an amorphous magnetic phase in the film.

Precessional switching of sub-micrometer magnetic elements with a perpendicular applied field

Magnetic switching in sub-micrometer permalloy-like thin film elements with shape anisotropy but no crystalline anisotropy is demonstrated by simulation using a tailored magnetic field pulse perpendicular to the sample. This switching mode is novel because no magnetic field is applied in the direction where the magnetic switch occurs. This switching mode is precessional and is significantly faster than switching modes dependent upon domain wall motion.

Tribological behavior anisotropy in sliding interaction of asperities on single-crystal α-iron: A quasi-continuum study

Abstract Fundamental understanding of interfacial phenomena in asperity interactions at the nanoscale is prompted by advances in tribology. In this work, we established a multiscale model of asperity–asperity sliding interaction to study the effects of both crystalline anisotropy and crystallographic orientation matching degree on the tribological behavior. Asperity pairs in cases of both mismatched and matched crystallographic orientations were considered. The results revealed that the crystallographic orientation and its matching degree had a tremendous influence on tribological characteristics. The combination effects of internal defect generation and interfacial adhesion anisotropies were found to be responsible for the anisotropic tribological properties. Wear mechanism for the matched case behaved as adhesion wear, whereas wear mechanism for the mismatched case was identified as abrasive wear.

Reduction in crystalline quality anisotropy and strain for non-polar a-plane GaN epi-layers with nano-scale island-like SiNx interlayer

The reduction in the crystalline quality anisotropy and the in-plane strain for the non-polar a-plane GaN epi-layer with nano-scale island-like SiNx interlayer was studied extensively with scanning electron microscopy, high-resolution X-ray diffraction measurement, and Raman spectroscopy. It was demonstrated that the SiNx interlayer was powerful to suppress the crystalline quality anisotropy in the a-plane GaN epi-layer since the full width at half maximum values of the X-ray rocking curves for both c-direction and m-direction were reduced from 1108 to 780 arcsec and from 2077 to 806 arcsec, respectively. The Raman measurement results also reveal that the SiNx interlayer plays a crucial role in compensating the in-plane strains. In fact, the in-plane compressive strain along m-direction and the tensile strain along c-direction were found to be decreased by approximately 62% and 78%, respectively due to the insertion of the SiNx interlayer.

Structural and electromagnetic evaluations of YIG rare earth doped (Gd, Pr, Ho,Yb) nanoferrites for high frequency applications

Multiple rare earth metal cations (Gd, Yb, Ho and Pr) were doped in yttrium iron garnet (YIG) nanocrystalline ferrites. Following five samples i.e. YIG (Y3Fe5O12), Gd doped YIG (Y2.8Gd0.2Fe5O12), Pr doped YIG (Y2.8Pr0.2Fe5O12), Ho doped YIG (Y2.8Ho0.2Fe5O12) and Yb doped YIG (Y2.8Yb0.2Fe5O12) were prepared using sol-gel auto-combustion synthesis route. The samples were characterized via XRD, FTIR, SEM and VSM whereas VNA was also used to evaluate these samples for high frequency applications. XRD analysis confirms the peaks of garnet ferrites which show cubic phase structure for all YIG doped samples. The lattice constant in all the substituted garnet nanoferrites initially increased and then consequently decreased as the doping increased except for Pr-substituted YIG nanoferrites. This is due to the greater ionic radii of Pr as compared to other doped YIG nanoferrites. FTIR was used to find out the garnet phase in all the substituted YIG nanoferrites. The morphological features of the garnet nanoferrites were observed using the SEM images. High frequency measurements such as permittivity, permeability, dielectric losses, ac conductivity, electric modulus and Q factor of YIG and doped YIG nanoferrites were measured using VNA. Saturation magnetization, remanence, coercivity, squareness ratio, Bohr magneton and magneto crystalline anisotropy were measured from the magnetic hysteresis loops recorded by VSM. Pr-doped YIG show better dielectric and magnetic properties with lower losses at higher frequency suggest the use of these garnet nanoferrites for microwave high frequency devices and applications.

Shape of magnetic domain walls formed by coupling to mobile charges

Magnetic domain walls, which are crucially important in both fundamental physics and technical applications, often have a preference in their form due to many different origins, such as the crystalline shape, lattice symmetry, and magnetic anisotropy. We theoretically investigate yet another origin stemming from the coupling to mobile charges in itinerant magnets. Performing a large-scale numerical simulation in a minimal model for itinerant magnets, i.e., the Kondo lattice model with classical localized spins, we show that the shape of magnetic domain walls depends on the electronic band structure and electron filling. While Neel and 120 antiferromagnetic states do not show a strong preference in the shape of domain walls, noncoplanar spin states with scalar chiral ordering have distinct directional preferences of the domain walls depending on the electron filling. We find that the directional preference is rationalized by the wave-number dependence of the effective magnetic interactions induced by the mobile charges, which are set by the band structure and electron filling. We also observe that, in the noncoplanar chiral states, an electric current is induced along the domain walls owing to the spin Berry phase mechanism, with very different spatial distributions depending on whether the bulk state is metallic or insulating.

Electron spin resonance studies of Bi1-xScxFeO3 nanoparticulates: Observation of an enhanced spin canting over a large temperature range

Bi1-xScxFeO3 (x = 0.0, 0.1, 0.15, 0.25) nano particles were synthesized by sol gel method. We then probed the spin system in these nano particles using electron spin resonance technique. Our ESR results strongly suggest the scenario of modified spin canted structures. Spin canting parameter Δg/g as a function of temperature for Scandium doped BFO is qualitatively different from undoped BFO. A broad peak is observed for all the Scandium doped BFO samples and an enhanced spin canting over a large temperature range (75–210K) in the case of x = 0.15 doping. We also showed that the asymmetry parameter and thereby the magneto-crystalline anisotropy in these BSFO nanoparticles show peaks around 230K for (x = 0.10 and 0.15) and beyond 300K for x = 0.25 system. Thus, we established that the Sc doping significantly modifies the spin canting and magneto crystalline anisotropy in the BFO system.

Critical Behavior and Macroscopic Phase Diagram of the Monoaxial Chiral Helimagnet Cr1/3NbS2

Cr1/3NbS2 is a unique example of a hexagonal chiral helimagnet with high crystalline anisotropy, and has generated growing interest for a possible magnetic field control of the incommensurate spin spiral. Here, we construct a comprehensive phase diagram based on detailed magnetization measurements of a high quality single crystal of Cr1/3NbS2 over three magnetic field regions. An analysis of the critical properties in the forced ferromagnetic region yields 3D Heisenberg exponents β = 0.3460 ± 0.040, γ = 1.344 ± 0.002, and TC = 130.78 K ± 0.044, which are consistent with the localized nature the of Cr3+ moments and suggest short-range ferromagnetic interactions. We exploit the temperature and magnetic field dependence of magnetic entropy change (ΔSM) to accurately map the nonlinear crossover to the chiral soliton lattice regime from the chiral helimagnetic phase. Our observations in the low field region are consistent with the existence of chiral ordering in a temperature range above the Curie temperature, TC < T < T*, where a first-order transition has been previously predicted. An analysis of the universal behavior of ΔSM(T,H) experimentally demonstrates for the first time the first-order nature of the onset of chiral ordering.