Anisotropic Contributions Research Articles

Annealing temperature and thickness dependencies of structural and magnetic properties ofCo2FeAlthin films

${\mathrm{Co}}_{2}\mathrm{FeAl}$ (CFA) thin films, of various thicknesses ($3\phantom{\rule{0.28em}{0ex}}\text{nm}\ensuremath{\le}t\ensuremath{\le}50\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$), have been grown by sputtering on (001) MgO single-crystal substrates and annealed at different temperatures ($\text{RT}\ensuremath{\le}{T}_{a}\ensuremath{\le}600\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$, where RT is the room temperature). The influence of the CFA thickness ($t$), as well as ex situ annealing temperature (${T}_{a}$), on the magnetic and structural properties has been investigated by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed an epitaxial growth of the films with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the $B2$ phase to the $A2$ phase when decreasing $t$ or ${T}_{a}$. The deduced lattice parameters showed an in-plane tetragonal distortion and in-plane and out-plane strains that increase with ${T}_{a}$ and $1/t$. For all ${T}_{a}$ values, the variation of the effective magnetization, deduced from the fit of MS-FMR measurements, shows two different regimes separated by a critical thickness, which is ${T}_{a}$ dependent. It decreases (increases) linearly with the inverse thickness ($1/t$) in the first (second) regime due to the contribution of the magnetoelastic anisotropy to surface (to volume) anisotropy. The observed behavior has been analyzed through a model allowing for the separation of the magnetocrystalline, magnetoelastic, and N\'eel-type interface anisotropy constants to the surface and the volume anisotropies. Similar behavior has been observed for the effective fourfold anisotropy field which governs the in-plane anisotropy present in all the samples. Finally, the MS-FMR data also allow one to conclude that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with ${T}_{a}$.

Vibrational Raman optical activity of bicyclic terpenes: comparison between experimental and calculated vibrational Raman, Raman optical activity, and dimensionless circular intensity difference spectra and their similarity analysis

Experimental vibrational Raman, vibrational Raman optical activity (VROA), and dimensionless circular intensity difference (CID) spectra are analyzed with quantum chemical (QC) predictions of the corresponding spectra using the B3LYP functional and the aug-cc-pVDZ basis set, for five bicyclic terpenes as test cases. The experimentally measured vibrational Raman and VROA spectra have arbitrary y-axis intensities, because of their dependence on instrumental parameters. As a result, the absolute magnitudes of intensities in experimental and calculated vibrational Raman or vibrational ROA spectra are not normally analyzed. However, the experimental dimensionless CID spectrum is independent of the instrumental parameters and can be evaluated quantitatively with corresponding QC predicted CID spectrum. The similarity overlaps between experimental and quantum chemical predictions of vibrational Raman, VROA, and dimensionless CID spectra are evaluated. The quantum chemical predictions of dimensionless CID spectral magnitudes at B3LYP/aug-ccpVDZ level match well with the corresponding experimental magnitudes. The individual anisotropic contributions to VROA and CID spectra are also presented. The anisotropic contribution involving electric dipole-electric quadrupole polarizability derivative tensor is relatively smaller than that involving electric dipole-magnetic dipole polarizability derivative tensor. A new format, which contains all of the information needed for the spectral analysis, has been advanced for presenting the vibrational Raman, VROA, and CID spectra and their similarity analysis with corresponding QC-predicted spectra. Copyright © 2016 John Wiley & Sons, Ltd.

Magnetic properties and magnetoimpedance of FeCoNi/CuBe electroplated tubes with different features of field-annealing induced magnetic anisotropy

The effect of field annealing (in direct (DC) or alternating current (AC) field) on the structure, magnetic properties and giant magnetoimpedance (GMI) of CuBe/ Fe19Co17Ni64 electroplated tubes was studied. The field and frequency dependences of total impedance and its real part were comparatively analyzed together with magnetization processes features. The GMI sensitivity with respect to an applied field was the subject of special attention in view of possible applications of these materials in small magnetic field sensors. The maximum GMI ratio depends strongly on the heat treatments. The DC field annealing leads to the highest total impedance GMI ratio (ΔZ/Z=250%) and real part of the total impedance GMI ratio (ΔR/R=640%), compared to as-cast and AC field annealed samples. The external field response of DC annealed samples presented a single peak GMI response as a consequence of a strong contribution of the longitudinal effective anisotropy. At the same time, the maximum obtained sensitivity of 13.5%/Oe for DC case is much lower compared to the highest sensitivity values obtained for as-prepared (28.6%/Oe) and DC field annealed (22.0%/Oe) tubes for the low frequency of 2.5MHz. The weak dependence of ΔZ/Z ratio in the case of AC field annealed samples in the high frequency range is an important advantage for particular sensor applications.

Size and thickness effect on magnetic structures of maghemite hollow magnetic nanoparticles

The effect of surface anisotropy on the magnetic ground state of hollow maghemite nanoparticles is investigated using atomistic Monte Carlo simulation. The computer modeling is carried on hollow nanostructures as a function of size and shell thickness. It is found that the large contribution of the surface anisotropy imposes a “throttled” spin structure where the moments located at the outer surface tend to orient normal to the surface while those located at the inner surface appear to be more aligned. For increasing values of surface anisotropy in the frame of a radial model, the magnetic moments become radially oriented either inward or outward giving rise to a “hedgehog” configuration with nearly zero net magnetization. We also show the effect of the size of hollow nanoparticle on the spin behavior where the spin non-collinearity increases (for fixed value of surface anisotropy) as the diameter of the hollow nanoparticle increases due to the significant increase in surface-to-volume ratio, the thickness being constant. Moreover, the thickness of the hollow nanoparticle shell influences the spin configuration and thus the relation between surface anisotropy and the size or the thickness of the hollow nanoparticle is established.

Seven 3d-4f coordination polymers of macrocyclic oxamide with polycarboxylates: Syntheses, crystal structures and magnetic properties

Seven new 3d–4f heterometallic coordination polymers, [Ln(CuL)2(Hbtca)(btca)(H2O)]·2H2O (Ln = TbIII1, PrIII2, SmIII3, EuIII4, YbIII5), [Nd(NiL)(nip)(Rnip)]·0·25H2O·0.25CH3OH (R= 0.6CH3, 0.4H) 6 and [Nd2(NiL)(nip)3(H2O)]·2H2O 7(CuL or NiL, H2L = 2, 3-dioxo-5, 6, 14, 15-dibenzo-1, 4, 8, 12-tetraazacyclo-pentadeca-7, 13-dien; H2btca = benzotriazole-5-carboxylic acid; H2nip = 5-nitroisophthalic acid) have been synthesized by a solvothermal method and characterized by single-crystal X-ray diffraction. Complexes 1–5 exhibit a double-strand meso-helical chain structures formed by [LnIIICuII2] units via the oxamide and benzotriazole-5-carboxylate bridges, while complex 6 exhibits a four-strand meso-helical chain formed by NdNi unit via the oxamide and 5-nitroisophthalate bridges. Complex 7 consists of a 2D layer framework formed by four-strand meso-helical chain via the nip2− bridges. Moreover, the magnetic properties of them were investigated, and the best-fit analysis of χMT versus T show that the anisotropic contribution of Ln(III) ions (arising from the spin-orbit coupling or the crystal field perturbation) dominates (weak exchange limit) in these complexes(for 3, λ = 214.6cm−1, zj’ = −0.33cm−1, gav = 1.94; for 5, Δ = 6.98cm−1, zj’ = 1.53cm−1, gav = 1.85).

Competitive and cooperative anisotropy in magnetic nanocrystal chains of magnetotactic bacteria

The formation of cellular magnetic dipoles by chain assemblies of stable single-domain magnetite nanocrystals is a characteristic feature in magnetotactic bacteria (MTB). The dipole strength depends on the competition or cooperation between the various anisotropic energy contributions, mainly between the magnetocrystalline and the interaction-induced shape anisotropy. Ferromagnetic resonance spectroscopy and numerical simulations of intracellular magnetite assemblies in the MTB Desulfovibrio magneticus strain RS-1 show that the alignment of elongated nanocrystallites leads to a predominant uniaxial anisotropy, which is enhanced when the magnetocrystalline symmetry is collinear to the chain, i.e., the anisotropies are cooperative vs. being competitive. This direct insight into the anisotropy variations in chain assemblies provides a physical framework to tailor magnetic nanocomposites, where the collective magnetic properties result from the interactions between the individual nanocrystalline constituents.

Magnetic anisotropy peculiarities of high-temperature ferromagnetic MnxSi1−x (x ≈ 0.5) alloy films

Thin films of MnxSi1−x alloys with different Mn concentration grown by the pulsed-laser deposition (PLD) method onto the Al2O3 (0001) substrate were investigated in the temperature range 4–300 K using ferromagnetic resonance (FMR) measurements in the wide range of frequencies and magnetic fields . For samples with , FMR data show clear evidence of ferromagnetism (FM) with high Curie temperatures . These samples demonstrate the complex and unusual character of magnetic anisotropy described in the frame of phenomenological model as a combination of the essential second-order easy-plane anisotropy contribution and the additional fourth-order anisotropy contribution with the easy direction normal to the film plane. We explain the obtained results by a polycrystalline (mosaic) structure of the films caused by the film-substrate lattice mismatch.

Study of lattice strain evolution during biaxial deformation of stainless steel using a finite element and fast Fourier transform based multi-scale approach

A multi-scale elastic-plastic finite element and fast Fourier transform based approach is proposed to study lattice strain evolution during uniaxial and biaxial loading of stainless steel cruciform shaped samples. At the macroscale, finite element simulations capture the complex coupling between applied forces in the arms and gauge stresses induced by the cruciform geometry. The predicted gauge stresses are used as macroscopic boundary conditions to drive a mesoscale elasto-viscoplastic fast Fourier transform model, from which lattice strains are calculated for particular grain families. The calculated lattice strain evolution matches well with experimental values from in-situ neutron diffraction measurements and demonstrates that the spread in lattice strain evolution between different grain families decreases with increasing biaxial stress ratio. During equibiaxial loading, the model reveals that the lattice strain evolution in all grain families, and not just the 311 grain family, is representative of the polycrystalline response. A detailed quantitative analysis of the 200 and 220 grain family reveals that the contribution of elastic and plastic anisotropy to the lattice strain evolution significantly depends on the applied stress ratio.

Intrinsic magnetic properties inR(Fe1−xCox)11TiZ(R=Yand Ce;Z=H,C,and N)

To guide improved properties coincident with reduction of critical materials in permanent magnets, we investigate via density functional theory (DFT) the intrinsic magnetic properties of a promising system, $R$(Fe$_{1-x}$Co$_{x}$)$_{11}$Ti$Z$ with $R$=Y, Ce and interstitial doping ($Z$=H, C, N). The magnetization $M$, Curie temperature $T_\text{C}$, and magnetocrystalline anisotropy energy $K$ calculated in local density approximation to DFT agree well with measurements. Site-resolved contributions to $K$ reveal that all three Fe sublattices promote uniaxial anisotropy in YFe$_{11}$Ti, while competing anisotropy contributions exist in YCo$_{11}$Ti. As observed in experiments on $R$(Fe$_{1-x}$Co$_{x}$)$_{11}$Ti, we find a complex nonmonotonic dependence of $K$ on Co content, and show that anisotropy variations are a collective effect of MAE contributions from all sites and cannot be solely explained by preferential site occupancy. With interstitial doping, calculated $T_\text{C}$ enhancements are in the sequence of N$>$C$>$H, with volume and chemical effects contributing to the enhancement. The uniaxial anisotropy of $R$(Fe$_{1-x}$Co$_{x}$)$_{11}$Ti$Z$ generally decreases with C and N; although, for $R$=Ce, C doping is found to greatly enhance it for a small range of 0.7$<$$x$$<$0.9.

Detecting Chirality in Molecules by Linearly Polarized Laser Fields.

A new scheme for enantiomer differentiation of chiral molecules using a pair of linearly polarized intense ultrashort laser pulses with skewed mutual polarization is presented. The technique relies on the fact that the off-diagonal anisotropic contributions to the electric polarizability tensor for two enantiomers have different signs. Exploiting this property, we are able to excite a coherent unidirectional rotation of two enantiomers with a π phase difference in the molecular electric dipole moment. The approach is robust and suitable for relatively high temperatures of molecular samples, making it applicable for selective chiral analysis of mixtures, and to chiral molecules with low barriers between enantiomers. As an illustration, we present nanosecond laser-driven dynamics of a tetratomic nonrigid chiral molecule with short-lived chirality. The ultrafast time scale of the proposed technique is well suited to study parity violation in molecular systems in short-lived chiral states.

Diamondoid Structure in a Metal–Organic Framework of Fe4 Single‐Molecule Magnets

A 3D metal-organic framework (MOF) having single-molecule magnet (SMM) linkers was prepared in crystalline form by using a tetrairon(III) complex functionalised with two divergent pyridyl groups, namely [Fe4 (pPy)2 (dpm)6 ] (1; H3 pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane). Reaction of 1 with silver(I) perchlorate afforded {[Fe4 (pPy)2 (dpm)6 ]2 Ag}ClO4 (2), which crystallises in a cubic face-centred lattice and exhibits two interlocked diamondoid networks. In 2, the SMMs act as linear ditopic synthons, and silver(I) ions as tetrahedral nodes coordinated by four pyridyl nitrogen atoms. The magnetic properties of 1 (S=5 and D≈-0.4 cm(-1) in the ground spin state) are largely preserved in 2, which shows slow magnetic relaxation with an anisotropy barrier of Ueff /kB =11.46(10) K in zero field and 14.25(8) K in an applied field of 1 kOe. However, crystal symmetry triggers highly noncollinear magnetic anisotropy contributions oriented at 109.47° from each other along the threefold axes of AgN4 tetrahedra, a unique scenario fully confirmed by a single-crystal cantilever torque magnetometry investigation. Magnetisation curves down to 0.03 K demonstrated the occurrence of a wide hysteresis loop when the magnetic field was swept along one of the four Ag-N bonds. By symmetry, the crystalline compound can then be persistently magnetised parallel or antiparallel to the four main diagonals of the unit cell, although the crystals have no overall second-order anisotropy.

Anisotropy of in-plane holegfactor in CdTe/ZnTe quantum dots

Optical studies of a bright exciton provide only limited information about the hole anisotropy in a quantum dot. In this work we present a universal method to study heavy hole anisotropy using a dark exciton in a moderate in-plane magnetic field. By analysis of the linear polarization of the dark exciton photoluminescence we identify both isotropic and anisotropic contributions to the hole g-factor. We employ this method for a number of individual self-assembled CdTe/ZnTe quantum dots, demonstrating a variety of behaviors of in-plane hole g-factor: from almost fully anisotropic to almost isotropic. We conclude that, in general, both contributions play an important role and neither contribution can be neglected.

Modelling the influence of strain localisation and viscosity on the behaviour of underground drifts drilled in claystone

A benchmark is conducted to characterise the Callovo-Oxfordian claystone behaviour for nuclear waste repository. The objective is to model gallery drilling and Excavation Damaged Zone (EDZ) around galleries. The proposed model is a cross-anisotropic elasto-viscoplastic model whose parameters are calibrated from laboratory tests. Gallery drilling is modelled numerically and shear fractures are reproduced with shear bands. The modelling highlights the contributions of shear banding, viscosity, and anisotropy for the EDZ behaviour. The EDZ and the gallery convergence can be correctly represented only if fractures are reproduced. Modelling shear banding is crucial and leads to a better description of the EDZ.

2D Fourier series representation of gravitational functionals in spherical coordinates

2D Fourier series representation of a scalar field like gravitational potential is conventionally derived by making use of the Fourier series of the Legendre functions in the spherical harmonic representation. This representation has been employed so far only in the case of a scalar field or the functionals that are related to it through a radial derivative. This paper provides a unified scheme to represent any gravitational functional in terms of spherical coordinates using a 2D Fourier series representation. The 2D Fourier series representation for each individual point is derived by transforming the spherical harmonics from the geocentric Earth-fixed frame to a rotated frame so that its equator coincides with the local meridian plane of that point. In the obtained formulation, each functional is linked to the potential in the spectral domain using a spectral transfer. We provide the spectral transfers of the first-, second- and third-order gradients of the gravitational potential in the local north-oriented reference frame and also those of some functionals of frequent use in the physical geodesy. The obtained representation is verified numerically. Moreover, spherical harmonic analysis of anisotropic functionals and contribution analysis of the third-order gradient tensor are provided as two numerical examples to show the power of the formulation. In conclusion, the 2D Fourier series representation on the sphere is generalized to functionals of the potential. In addition, the set of the spectral transfers can be considered as a pocket guide that provides the spectral characteristics of the functionals. Therefore, it extends the so-called Meissl scheme.

Magnetization processes and transitions between two antiferromagnetic spin configurations in single-crystalline MnSn2

The magnetic structure of MnSn2 and magnetic phase transitions in this compound have been investigated by magnetic measurements on single crystals. The results show that two antiferromagnetic (AFM) states exist below 325K and that a transition between these two phases occurs at 74K. Applying a magnetic field (H) has great influence on the transition temperature. An anomalous magnetization process at low fields occurs when the magnetic field applied along the [110] direction, which is ascribed to the contribution of the basal anisotropy. Based on the data for the magnetization processes and the phase transition of the present single crystal, the H–T phase diagram has been established.

Monte Carlo simulations of magnetization state of ellipsoidal CoCu particles in disordered self-assembled arrays

Abstract

On the radial force balance in the quiet time magnetotail current sheet

AbstractUsing Time History of Events and Macroscale Interactions spacecraft observations of the quite magnetotail current sheet within the r∈[9,35]RE region (r is the radial distance from Earth and RE is Earth's radius), we investigate the thermal plasma pressure distribution along the magnetotail. Taking advantage of flapping motions of an ensemble of current sheets at various distances, we estimate the current density magnitude jy (in GSM coordinates). Comparing the tension force jyBz (Bz is the magnetic field component) with the radial gradient of the plasma pressure demonstrates that this gradient is only a small fraction, ∼10–15%, of the Ampere force exerted on the cross‐tail current, in the r &gt; 15RE region. We also estimate the contribution of the electron temperature anisotropy to the pressure balance: in the r &gt; 15RE region the corresponding force can balance only 10–15% of the observed tension force jyBz. Thus, we conclude that about 70% of the tension force is not balanced by the combination of isotropic radial pressure gradient or the electron anisotropy. We discuss mechanisms that could be responsible for balancing the magnetotail current sheet.

Hybridization between microstructure and magnetization improvement in lead and RE co-doped BiFeO3

The crystal structure, magnetic and electrical properties of Bi0.96Pb0.04FeO3 and Bi0.92Pb0.04 RE0.04FeO3 (RE=La, Sm, Dy and Yb) polycrystalline samples were prepared by the flash autocombustion technique. X-ray diffraction (XRD) measurements show that the rare-earth doped compositions crystallized in rhombohedral symmetry of space group R3c. The undoped sample consisted needle shape particles while rare earth substitution preferred platelet like particles as clarified from high resolution transmission electron microscopy (HRTEM). Morphological features were examined using field emission scanning electron microscopy (FESEM). Magnetization measurements showed that Yb3+ samples possessed the highest room temperature saturation magnetization while when Bi3+ ions were substituted by La3+ ions, a smaller MS (0.28 emu/g) was obtained. The coexistence of ferroelectric and magnetic transitions was detected using DSC and χM, indicating the multiferroic characteristics of Bi0.92Pb0.04RE0.04FeO3 crystallites. The Néel temperature shifted upwards with decreasing the ionic radius of rare earth ion. Nice correlation was established between microstructure, morphology and magnetic properties in view of the contribution of magnetocrystalline and shape anisotropy in the magnetic parameters values.

Interactions of Hydrogen Molecules with Halogen-Containing Diatomics from Ab Initio Calculations: Spherical-Harmonics Representation and Characterization of the Intermolecular Potentials.

For the prototypical diatomic-molecule-diatomic-molecule interactions H2-HX and H2-X2, where X = F, Cl, Br, quantum-chemical ab initio calculations are carried out on grids of the configuration space, which permit a spherical-harmonics representation of the potential energy surfaces (PESs). Dimer geometries are considered for sets of representative leading configurations, and the PESs are analyzed in terms of isotropic and anisotropic contributions. The leading configurations are individuated by selecting a minimal set of mutual orientations of molecules needed to build the spherical-harmonic expansion on geometrical and symmetry grounds. The terms of the PESs corresponding to repulsive and bonding dimer geometries and the averaged isotropic term, for each pair of interacting molecules, are compared with representations in terms of a potential function proposed by Pirani et al. (see Chem. Phys. Lett. 2004, 394, 37-44 and references therein). Connections of the involved parameters with molecular properties provide insight into the nature of the interactions.

Ferroelectric polarization in antiferromagnetically coupled ferromagnetic film

We report the influence of interface antiferromagnetic coupling on magnetoelectric properties of ferromagnetic bi-layers. Electric polarization arising at magnetic ingomogeneity in bi-layered ferromagnetic structure with antiferromagnetic coupling at interface in applied magnetic field has been explored. Diagrams representing dependences of electric polarization on magnetic field P(H) have been constructed for two magnetic field geometries (in-plane and out-of plane fields). It has been found out that P(H) dependences demonstrate non-monotonic behavior. Peculiarities of polarization in an in-plane-oriented magnetic field have been explained by magnetization processes. It has been shown that a variety of magnetic configurations of Bloch, Neel and mixed Bloch–Neel types can be realized in antiferromagnetically coupled film due to cubic anisotropy contribution. In the area of Bloch magnetic configuration electric polarization vanishes. The critical values of magnetic fields suppressing polarization have been estimated.