Anisotropic Contributions Research Articles

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.

Band inversion amplifies 31 P-31 P nuclear overhauser effects: Relaxation mechanism and dynamic behavior of ATP in the human brain by 31 P MRS at 7 T.

To develop an improved method to measure the 31 P nuclear Overhauser effect (NOE) for evaluation of adenosine triphosphate (ATP) dynamics in terms of correlation time (τc ), and contribution of dipole-dipole (DD) and chemical shift anisotropy (CSA) mechanisms to T1 relaxation of ATP in human brain. The NOE of ATP in human brain was evaluated by monitoring changes in magnetization in the β-ATP signal following a band inversion of all downfield 31 P resonances. The magnetization changes observed were analyzed using the Bloch-McConnell-Solomon formulation to evaluate the relaxation and motion dynamic parameters that describe interactions of ATP with cellular solids in human brain tissue. The maximal transient NOE, observed as a reduction in the β-ATP signal, was 24 ± 2% upon band inversion of γ- and α-ATP, which is 2-3-fold higher than achievable by frequency-selective inversion of either γ- or α-ATP. The rate of 31 P-31 P cross relaxation (0.21 ± 0.02 s-1 ) led to a τc value of (9.1 ± 0.8) × 10-8 s for ATP in human brain. The T1 relaxation of β-ATP is dominated by CSA over the DD mechanism (60%: 40%). The band inversion method proved effective in amplifying 31 P NOE, and thus facilitating ATP τc and relaxation measurements. This technique renders ATP a potentially useful reporter molecule for cellular environments. Magn Reson Med 77:1409-1418, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear Mn(II)2Gd(III)2 and Co(II)2Eu(III)2 Complexes.

In view of a wide perspective of 3d-4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalix[4]arene tetranuclear heterometallic complexes Mn(II)2Gd(III)2 and Co(II)2Eu(III)2. The energy pattern of the Mn(II)2Gd(III)2 complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Landé intervals are affected by the biquadratic spin-spin interactions. The nonmonotonic temperature dependence of the χT product observed for the Mn(II)2Gd(III)2 complex is attributed to the competitive influence of the ferromagnetic Mn-Gd and antiferromagnetic Mn-Mn exchange interactions, the latter being stronger (J(Mn, Mn) = -1.6 cm(-1), Js(Mn, Gd) = 0.8 cm(-1), g = 1.97). The model for the Co(II)2Eu(III)2 complex includes uniaxial anisotropy of the seven-coordinate Co(II) ions and an isotropic exchange interaction in the Co(II)2 pair, while the Eu(III) ions are diamagnetic in their ground states. Best-fit analysis of χT versus T showed that the anisotropic contribution (arising from a large zero-field splitting in Co(II) ions) dominates (weak-exchange limit) in the Co(II)2Eu(III)2 complex (D = 20.5 cm(-1), J = -0.4 cm(-1), gCo = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the Co(II) pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-(1)/2 Hamiltonian that is deduced for the Co(II)2 pair in the weak-exchange limit.

Statistic regularities of magnetization jumps in K0.4[Cr(CN)6][Mn(R/S)‐pn](R/S)‐pnH0.6 ferrimagnet

A series of two types of stochastic magnetization jumps in K0.4[Cr(CN)6][Mn(R/S)‐pn](R/S)‐pnH0.6 single crystals (pn = 1,2‐diaminopropane) are reported. Large jumps appear on the demagnetization branch of the hysteresis loop at the critical value of the decreasing magnetic field. The correlation of the temperature dependence of jump magnetic field and the temperature dependence of magnetic anisotropy field indicates a contribution of magnetic anisotropy to nonlinear spin‐soliton structure. Small magnetization jumps occur in both magnetization and demagnetization regimes.

Critical thickness for stripe domain formation in FePt thin films: Dependence on residual stress

Magnetically soft FePt thin films of varying thickness (20 nm ≤ d ≤ 100 nm) were sputter-deposited at different Ar pressures in order to systematically modify the residual stress and hence the magnetic anisotropy. The magnetic domain structure of FePt thin films showed a transition from planar to nearly parallel stripes above a critical thickness, dcr, which was found to depend on an anisotropy contribution perpendicular to the film plane, originated essentially in magnetoelastic effects. A careful structural characterization was made in order to obtain the strain and the stress induced magnetic anisotropy in the samples. Vibrating sample magnetometry and magnetic force microscopy were used to investigate the changes occurring in the magnetic domain structure and the critical thickness of each set of films. Joining together structural and magnetic results, we have been able to construct a phase diagram that divided regions of different domain structures, either by changing the film thickness or the perpendicular magnetic anisotropy. The experimental results could be satisfactorily explained by using a model developed by Murayama. The observed dependence of the magnetic properties of soft FePt thin films on the fabrication conditions opens the possibility to tune the magnetic domain configuration from planar to stripe-like domains by changing the argon sputtering pressure used during film deposition.

Large magnetic anisotropy enhancement in size controlled Ni nanowires electrodeposited into nanoporous alumina templates

A large enhancement of the magnetic anisotropy of Ni nanowires (NWs) embedded in anodic aluminium oxide porous membranes is obtained as a result of an induced magnetoelastic (ME) anisotropy contribution. This unusual large anisotropy enhancement depends on the diameter of the NWs and exceeds the magnetostatic (MS) contribution. As a consequence, it leads to effective magnetic anisotropy energies as large as erg cm−3, which are of the same order of magnitude and comparable to the MS energies of harder magnetic materials like Co NWs. Specifically, from ferromagnetic resonance experiments, the magnetic anisotropy of the NWs has been observed to increase as its diameter is decreased, leading to values that are about four times larger than the corresponding value when only the MS anisotropy is present. Our results are consistent with the recently proposed growth mechanism of Ni NWs that proceeds via a poly-crystalline stage at the bottom followed by a single-crystalline stage with texture [110] parallel to the axis of the NWs. A strong correlation between reducing the diameter of the NWs with the decrease of the length of the poly-crystalline segment and the enhancement of the effective magnetic anisotropy has been shown. Magnetization curves obtained from alternating gradient magnetometry experiments show that the average ME anisotropy results from the competition between the magnetic anisotropies of both crystalline segments of the NWs. Understanding the influence of size and confinement effects on the magnetic properties of nanocomposites is of prime interest for the development of novel and agile devices.

Local electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation.

We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV.

Primordial power spectrum of tensor perturbations in Finsler spacetime

We first investigate the gravitational wave in the flat Finsler spacetime. In the Finslerian universe, we derive the perturbed gravitational field equation with tensor perturbations. The Finslerian background spacetime breaks rotational symmetry and induces parity violation. Then we obtain the modified primordial power spectrum of tensor perturbations. The parity violation feature requires that the anisotropic effect contributes to $TT,TE,EE,BB$ angular correlation coefficients with $l'=l+1$ and $TB,EB$ with $l'=l$. The numerical results show that the anisotropic contributions to angular correlation coefficients depend on $m$, and $TE$ and $ET$ angular correlation coefficients are different.

Impact of anisotropy on the interaction of an atom with a one-dimensional nano-grating

We study the interaction of an atom with a one-dimensional nano-grating within the framework of macroscopic QED, with special emphasis on possible anisotropic contributions. To this end, we first derive the scattering Green’s tensor of the grating by means of a Rayleigh expansion and discuss its symmetry properties and asymptotes. We then determine the Casimir–Polder potential of an atom with the grating. In particular, we find that strong anisotropy can lead to a repulsive Casimir–Polder potential in the normal direction.

Magnetic and microwave properties of amorphous FeCoNbBCu thin films

The soft magnetic and microwave properties of amorphous FeCoNbBCu thin films with thicknesses varying from 70 nm to 450 nm have been systematically investigated. Due to the amorphous structure, the coercivity is 1.5 Oe in thicker films. The thickness-dependent microwave characteristics of the films were measured over the range 0.5–6 GHz and analyzed using the Landau–Lifshitz–Gilbert equation. Without applying magnetic field during deposition and measurement, an in-plane uniaxial anisotropy in amorphous thin films was obtained, ranging from 21 to 45 Oe. The interface interaction between substrate and film is confirmed to be the origin of the induced anisotropy, whereas the volume anisotropy contribution is more pronounced with increasing film thickness. For films possessing an in-plane uniaxial anisotropy, the shift of resonance frequency with thickness is observed and verified by the Kittel equation. The demonstration of a controllable and tunable anisotropy suggests that the FeCoNbBCu thin films have potential application as magnetic materials for Spintronics-based microwave devices.

Spatial spin-modulated structure and hyperfine interactions of 57Fe nuclei in multiferroics BiFe1–x T x O3 (T = Sc, Mn; x = 0, 0.05)

The results of the Mossbauer studies on 57Fe nuclei in multiferroics BiFe1–x T x O3 (T = Sc, Mn; x = 0, 0.05) in the temperature range of 5.2–300 K have been presented. The Mossbauer spectra have been analyzed in terms of the model of an incommensurate spatial spin-modulated structure of cycloid type. Information has been obtained about the effect of the substitution of Sc and Mn atoms for Fe atoms on the hyperfine parameters of the spectrum: the shift and the quadrupole shift of the Mossbauer line, the isotropic and anisotropic contributions to the hyperfine magnetic field, and also the parameter of anharmonicity of the spatial spin-modulated structure.

Traveling surface spin-wave resonance spectroscopy using surface acoustic waves

Coherent gigahertz-frequency surface acoustic waves (SAWs) traveling on the surface of a piezoelectric crystal can, via the magnetoelastic interaction, resonantly excite traveling surface spin waves in an adjacent thin-film ferromagnet. These excited surface spin waves, traveling with a definite in-plane wave-vector q∥ enforced by the SAW, can be detected by measuring changes in the electro-acoustical transmission of a SAW delay line. Here, we provide a demonstration that such measurements constitute a precise and quantitative technique for spin-wave spectroscopy, providing a means to determine both isotropic and anisotropic contributions to the spin-wave dispersion and damping. We demonstrate the effectiveness of this spectroscopic technique by measuring the spin-wave properties of a Ni thin film for a large range of wave vectors, |q∥| = 2.5 × 104–8 × 104 cm−1, over which anisotropic dipolar interactions vary from being negligible to quite significant.

Azimuthal anisotropy of charged jet production in [formula omitted] Pb–Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central sNN=2.76 TeV Pb–Pb collisions with respect to the second harmonic event plane, quantified as v2ch jet. Jet finding is performed employing the anti-kT algorithm with a resolution parameter R=0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero v2ch jet is observed in semi-central collisions (30–50% centrality) for 20<pTch jet<90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the v2 of single charged particles at high pT. Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions.

Modeling of hysteresis loops by Monte Carlo simulation

Recent advances in MC simulations of magnetic properties are rather devoted to non-interacting systems or ultrafast phenomena, while the modeling of quasi-static hysteresis loops of an assembly of spins with strong internal exchange interactions remains limited to specific cases. In the case of any assembly of magnetic moments, we propose MC simulations on the basis of a three dimensional classical Heisenberg model applied to an isolated magnetic slab involving first nearest neighbors exchange interactions and uniaxial anisotropy. Three different algorithms were successively implemented in order to simulate hysteresis loops: the classical free algorithm, the cone algorithm and a mixed one consisting of adding some global rotations. We focus particularly our study on the impact of varying the anisotropic constant parameter on the coercive field for different temperatures and algorithms. A study of the angular acceptation move distribution allows the dynamics of our simulations to be characterized. The results reveal that the coercive field is linearly related to the anisotropy providing that the algorithm and the numeric conditions are carefully chosen. In a general tendency, it is found that the efficiency of the simulation can be greatly enhanced by using the mixed algorithm that mimic the physics of collective behavior. Consequently, this study lead as to better quantified coercive fields measurements resulting from physical phenomena of complex magnetic (nano)architectures with different anisotropy contributions.

Estimating conductivity anisotropy of lower mantle from geomagnetic observatory data

Anisotropy of electrical conductivity in the lower mantle, presumably caused by phase change of dielectric magnesiowustite, can be estimated from geomagnetic records of the globally distributed geomagnetic array. The components of the conductivity tensor corresponding to the tangential components of the geomagnetic field are obtained from the magnetic mode alone recorded on the surface. Inversion by the perturbation method is applied to monthly means of geomagnetic variations along the X, Y, and Z components for the period from 1920 to 2009, borrowed from the World Monthly Means Database. The anisotropy contribution is expressed as corrections to the conductivity tensor components. The contribution of anisotropy to induction responses becomes notable for geomagnetic variations with at least decadal periods and increases with the period length.

Fully anisotropic finite strain viscoelasticity based on a reverse multiplicative decomposition and logarithmic strains

In this paper we present a novel formulation for phenomenological anisotropic finite visco-hyperelasticity. The formulation is based on a multiplicative decomposition of the equilibrated deformation gradient into nonequilibrated elastic and viscous contributions. The proposal in this paper is a decomposition reversed respect to that from Sidoroff allowing for anisotropic viscous contributions. Independent anisotropic stored energies are employed for equilibrated and non-equilibrated parts. The formulation uses logarithmic strain measures in order to be teamed with spline-based hyperelasticity. Some examples compare the results with formulations that use the Sidoroff decomposition and also show the enhanced capabilities of the present model.

Anomalous scaling in magnetohydrodynamic turbulence: Effects of anisotropy and compressibility in the kinematic approximation.

The field-theoretic renormalization group and the operator product expansion are applied to the model of passive vector (magnetic) field advected by a random turbulent velocity field. The latter is governed by the Navier-Stokes equation for compressible fluid, subject to external random force with the covariance ∝ δ(t-t')k(4-d-y), where d is the dimension of space and y is an arbitrary exponent. From physics viewpoints, the model describes magnetohydrodynamic turbulence in the so-called kinematic approximation, where the effects of the magnetic field on the dynamics of the fluid are neglected. The original stochastic problem is reformulated as a multiplicatively renormalizable field-theoretic model; the corresponding renormalization group equations possess an infrared attractive fixed point. It is shown that various correlation functions of the magnetic field and its powers demonstrate anomalous scaling behavior in the inertial-convective range already for small values of y. The corresponding anomalous exponents, identified with scaling (critical) dimensions of certain composite fields ("operators" in the quantum-field terminology), can be systematically calculated as series in y. The practical calculation is performed in the leading one-loop approximation, including exponents in anisotropic contributions. It should be emphasized that, in contrast to Gaussian ensembles with finite correlation time, the model and the perturbation theory presented here are manifestly Galilean covariant.

Structural, electrical, and magnetic properties of polypyrrole–Zn0.5Ni0.45Mn0.05Fe2O4 nanocomposites prepared by in situ chemical polymerization

Polypyrrole–Zn0.5Ni0.45Mn0.05Fe2O4 nanocomposites are conducting polymer-ferrites with useful properties and new functions. The ferrite and polypyrrole–ferrite nanocomposites were synthesized by wow sol gel and in situ chemical polymerization respectively. The structures, morphology, electrical and magnetic properties of the samples were characterized by various instruments. The results of X-ray diffraction (XRD) patterns, Fourier transform infrared (FTIR) spectra and field emission scanning electron microscopy (FE-SEM) showed the presence of the two intended phases. FE-SEM results show that phase separation was found to increase with increase of ferrite contents. The crystallite size measured by XRD was found to be in the range 40.9–88.6 nm. Possible bonding effect between metal cations and Polypyrrole (PPy) made the electronic density of the polymer chain reduce and hence resulted in the decrease of the conductivity. The room temperature dielectric measurements were performed in the frequency range 0.1 kHz–1 MHz. The dielectric constant followed the Maxwell–Wagner interfacial polarization. The frequency dependent conductivity obeyed a power law of frequency. Incorporation of ferrites in the conducting polypyrrol matrix leads to higher values of dielectric constant and dielectric loss. The loss tangent was found to have high values for composite materials as compared to pure samples. Under applied magnetic field, the Hysteresis measurements revealed that the saturation magnetization increased with increase in ferrite content. Anisotropy constant K values showed that grains were not single domains and anisotropy contribution is not uniaxial. In general the different variety of the results obtained by just changing the relative percentage of PPy–Zn,Ni,Mn ferrites nanocomposites, made the present studied samples to be useful in radiation absorption and shielding applications.

Model Driven Optimization of Magnetic Anisotropy of Exchange-coupled Core-Shell Ferrite Nanoparticles for Maximal Hysteretic Loss.

This study provides a guide to maximizing hysteretic loss by matching the design and synthesis of superparamagnetic nanoparticles to the desired hyperthermia application. The maximal heat release from magnetic nanoparticles to the environment depends on intrinsic properties of magnetic nanoparticles (e.g. size, magnetization, and magnetic anisotropy), and extrinsic properties of the applied fields (e.g. frequency, field strength). Often, the biomedical hyperthermia application limits flexibility in setting of many parameters (e.g. nanoparticle size and mobility, field strength and frequency). We show that core-shell nanoparticles combining a soft (Mn ferrite) and a hard (Co ferrite) magnetic material form a system in which the effective magnetic anisotropy can be easily tuned independently of the nanoparticle size. A theoretical framework to include the crystal anisotropy contribution of the Co ferrite phase to the nanoparticles total anisotropy is developed. The experimental results confirm that this framework predicts the hysteretic heating loss correctly when including non-linear effects in an effective susceptibility. Hence, we provide a guide on how to characterize the magnetic anisotropy of core-shell magnetic nanoparticles, model the expected heat loss and therefore, synthesize tuned nanoparticles for a particular biomedical application.

Top-down control of dynamic anisotropy in permalloy thin films with stripe domains

Permalloy films with different thickness were prepared for comparison, and the dynamic properties of the stripe domains were investigated by using a vector network analyzer ferromagnetic resonance (VNA-FMR) technique. A top-down approach was used to control the thickness of the films. In the measurement of permeability spectra, an applied magnetic field Happ was employed parallel and perpendicular to the stripe domains, respectively. Dynamic pseudo-uniaxial anisotropy fields are derived. In order to explain the anisotropy variation of permalloy films, a theoretical model has been proposed, and the result shows that the pseudo-uniaxial anisotropy can be attributed to the contributions of saturation magnetization, exchange interaction, perpendicular anisotropy, and film thickness. In addition, with the increase of Happ, the stripe domains gradually rotate towards the direction of the applied field, which can be clearly observed by magnetic force microscopy. Moreover, the dynamic permeability spectra exhibit a rhythmic variation dependent on the rotation of the stripe domains, i.e. the relationship between the rotational stripe domains and resonance frequency.