Orientation Of Moments Research Articles

The structural, magnetic and magnetocaloric properties of MnCrNiGeSi high-entropy alloy

The structural, magnetic and magnetocaloric properties of Mn20Cr14Ni33Ge25Si8 and Mn24Cr10Ni33Ge25Si8 high entropy alloys (HEAs) were investigated. The HEAs were prepared by arc melting method. The structural analysis indicates that the structure of HEAs is orthorhombic with Pnma space group. In magnetic measurements, the maximum saturation magnetization was found to be 56.7 A m2kg−1. The Curie temperature of HEAs is 361 and 387 K, respectively. The Curie temperature shifts to the high temperature, when 4% Mn is added, resulting the long-range orientation of the magnetic moments. The magnetic entropy change of Mn24Cr10Ni33Ge25Si8 is 3.6 Jkg−1K−1 at around 387 K. Consequently, Large magnetic entropy change is achieved with low hysteresis and soft magnetic property at high temperature.

Sprayed Nanometer-Thick Hard-Magnetic Coatings with Strong Perpendicular Anisotropy for Data Storage Applications

The rapid growth of digital information in the world necessitates a big leap in improving the existing technologies for magnetic recording. For the best modern perpendicular recording, the highest coercivity materials with minimal volume are required. We present a study of a facile technology for establishing mono- and multilayer surfaces from various single-domain flat magnetic nanoparticles that exhibit a strong perpendicular-oriented magnetic moment on solid and flexible substrates. Surfactant-free, hard ferromagnetic, and single-domain anisotropic strontium hexaferrite SrFe12O19 nanoparticles with a perpendicular magnetic moment orientation and two different aspect ratios are self-ordered into magnetic thin nanofilms, exploiting the templating effect of cellulose nanofibrils and magnetic fields. Uniform magnetic coatings obtained by the scalable layer-by-layer spray deposition from a monolayer coverage up to thicknesses of a few tens of nanometers show a preferred in-plane orientation of the hard-magnetic nanoparticles. High coercivities of the films of up to 5 kOe and a high perpendicular anisotropy of Mr⊥/Ms > 80% are found. The application of the magnetic field during film deposition ensures additional improvement in perpendicular magnetic anisotropy and the appearance of residual magnetization in the film of up to 0.6Ms. For low-aspect-ratio nanoparticles stacked in periodic planar structures, the signs of the photonic band gap are revealed. The ability to create scalable, thin magnetic structures based on nanosized particles/building blocks opens great opportunities for their application in a wide variety of optoelectronic and magnetic storage devices.

Magnetophotoselection in the Investigation of Excitonically Coupled Chromophores: The Case of the Water-Soluble Chlorophyll Protein.

A magnetophotoselection (MPS) investigation of the photoexcited triplet state of chlorophyll a both in a frozen organic solvent and in a protein environment, provided by the water-soluble chlorophyll protein (WSCP) of Lepidium virginicum, is reported. The MPS experiment combines the photoselection achieved by exciting with linearly polarized light with the magnetic selection of electron paramagnetic resonance (EPR) spectroscopy, allowing the determination of the relative orientation of the optical transition dipole moment and the zero-field splitting tensor axes in both environments. We demonstrate the robustness of the proposed methodology for a quantitative description of the excitonic interactions among pigments. The orientation of the optical transition dipole moments determined by the EPR analysis in WSCP, identified as an appropriate model system, are in excellent agreement with those calculated in the point-dipole approximation. In addition, MPS provides information on the electronic properties of the triplet state, localized on a single chlorophyll a pigment of the protein cluster, in terms of orientation of the zero-field splitting tensor axes in the molecular frame.

Potential high-performance magnet materials: Co- and Al-alloyedSm2Fe17

${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ has long been known as a potential high-performance magnet whose deficiencies---planar anisotropy and lower-than-optimal ${T}_{c}$---can be remedied by nitrogen addition, but which presents synthesis difficulties. Herein we apply first-principles calculations to search for alternative low-cost, high-performance permanent magnets in this family, by exploring simultaneous Fe and Al substitution. Specifically, the goal is to improve properties of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ easy-plane magnet at the stoichiometric composition. Density functional theory calculations were executed for three series of compounds, i.e., ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{14}{\mathrm{Al}}_{3}$, ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{15}{\mathrm{Al}}_{2}$, and ${\mathrm{Sm}}_{2}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{16}\mathrm{Al}$. We find that substitution of Fe with 12--18 of Co in $%\phantom{\rule{4pt}{0ex}}{\mathrm{Sm}}_{2}{\mathrm{Fe}}_{14}{\mathrm{Al}}_{3}$ modifies the magnetic anisotropy type from easy plane to easy axis with a substantial anisotropy of 7.1 MJ/${\mathrm{m}}^{3}$. We also demonstrate that the largest part of magnetic anisotropy is introduced by $4f$ Sm atom electrons. Thus the rotation of magnetic moment orientation from $\ensuremath{\langle}1\overline{1}0\ensuremath{\rangle}$ to $\ensuremath{\langle}111\ensuremath{\rangle}$ is followed by an increase of the occupied $4f$ state number and, as a result, the orbital part of the magnetic moment of one of the Sm atoms. This increase of the occupied $4f$ state number at an energy $\ensuremath{\sim}\ensuremath{-}$4.3 eV results in a significant reduction of band structure energy. The substitution of Fe by Co does not significantly reduce the magnetization of the compound and keeps it slightly above 1 T. This combination of magnetic anisotropy and magnetization makes the compound a promising candidate for a permanent magnet.

Spontaneous formation of metastable orientation with well-organized permanent dipole moment in organic glassy films.

The performance of organic optoelectronic and energy-harvesting devices is largely determined by the molecular orientation and resultant permanent dipole moment, yet this property is difficult to control during film preparation. Here, we demonstrate the active control of dipole direction-that is, vector direction and magnitude-in organic glassy films by physical vapour deposition. An organic glassy film with metastable permanent dipole moment orientation can be obtained by utilizing the small surface free energy of a trifluoromethyl unit and intramolecular permanent dipole moment induced by functional groups. The proposed molecular design rule could pave a way toward the formation of spontaneously polarized organic glassy films, leading to improvement in the performance of organic molecular devices.

Colossal Orbital Edelstein Effect in Noncentrosymmetric Superconductors.

In superconductors that lack inversion symmetry, the flow of supercurrent can induce a nonvanishing magnetization, a phenomenon which is at the heart of nondissipative magnetoelectric effects, also known as Edelstein effects. For electrons carrying spin and orbital moments, a question of fundamental relevance deals with the orbital nature of magnetoelectric effects in conventional spin-singlet superconductors with Rashba coupling. Remarkably, we find that the supercurrent-induced orbital magnetization is more than 1 order of magnitude greater than that due to the spin, giving rise to a colossal magnetoelectric effect. The induced orbital magnetization is shown to be sign tunable, with the sign change occurring for the Fermi level lying in proximity of avoiding crossing points in the Brillouin zone. In the presence of superconducting phase inhomogeneities, a modulation of the Edelstein signal on the scale of the superconducting coherence length appears, leading to domains with opposite orbital moment orientations. These hallmarks are robust to real-space self-consistent treatment of the superconducting order parameter. The orbital-dominated magnetoelectric phenomena, hence, have clear-cut marks for detection both in the bulk and at the edge of the system and are expected to be a general feature of multiorbital superconductors with inversion symmetry breaking.

A study on the relationship of magnetic moments orientation in L10 FePt network nanostructured film by electron energy-loss magnetic chiral dichroism using semi-core excitation spectra

In this study, we applied electron energy-loss magnetic chiral dichroism (EMCD), an electron counterpart of X-ray magnetic circular dichroism (XMCD), to a network nanostructured FePt L10 ordered alloy film to examine the relative orientation of magnetic moments between neighboring Fe and Pt atoms using the Fe-M2,3, Pt-O2,3, and Pt-N6,7 semi-core excitation spectra with transmission electron microscopy and electron energy-loss spectroscopy. EMCD signals were successfully extracted from a large number of spectra using a dedicated data analysis procedure to obtain sufficient noise statistics. Results showed that the relative sign relation of the EMCD signals between the Fe and Pt absorption edges was consistent with that of the theoretical dielectric tensor while assuming that parallel magnetic moments exist between neighboring Fe and Pt. We believe the results of this study can be applied to alloys with different nanostructures to determine whether the spin configuration depends on the size and geometry of the nanostructures.

Kapitza pendulum effects in a Josephson junction coupled to a nanomagnet under external periodic drive

We investigate reorientation effects under external periodic drive in the nanomagnet dynamics coupled to a Josephson junction. The Kapitsa pendulum is introduced as a mechanical analog to this system and we demonstrate the reorientation of the easy axis of the nanomagnet. The magnetic field generated by the Josephson junction and external drive plays the role of the oscillating force of the suspension point in the Kapitsa pendulum. The high frequency oscillations change the orientation of the magnetic moment. The magnetic field of the quasiparticle current determines the frequency dependence of the magnetic moment's orientation. We obtain simple analytical formulas for the stable position of the magnetic moment, both under the external periodic drive and without it. The influence of external periodic drive on the voltage of complete reorientation have been demonstrated.

Janus 2H-VSSe monolayer: two-dimensional valleytronic semiconductor with nonvolatile valley polarization

Valleytronic as a hot topic in recent years focuses on electrons’ valley degree of freedom as a quantum information carrier. Here, by combining two-band k.p model with high-throughput density functional theory (DFT) calculations, the valley states of Janus 2H-VSSe monolayer are studied which have spontaneous polarization. Nonvolatile valley polarization state is mainly arises from intrinsic ferromagnetism contributed by V-3d electronic configuration and not the spontaneous out-of-plane dipole moment of VSSe monolayer. The effective Hamiltonian model and DFT calculations both showed that the valley splitting mainly originates from the smaller spin splitting coming from the spin–orbit coupling effect rather than the spin splitting of magnetic exchange field. By using the effective Dirac Hamiltonian and Kubo formula, we further calculated the longitudinal and transversal conductivities and absorption spectra of VSSe monolayer which exhibits an anomalous valley Hall effect and clear valley-selective circular dichroism. Our calculations indicate that the modification of valley and spin splitting related to Berry curvature by applying an external strain is more noticeable than by the change of the magnetic moment orientation and electric field. We found that carriers accumulation with particular spin and valley label can be manipulated by tuning effective Hamiltonian parameters. The coexistence of robust in-plane magnetic ordering and spontaneous valley polarization of 2H-VSSe monolayer supports the possibility of applications in spintronics, valleytronics and optoelectronics devices.

Target magnetic moment orientation estimation method based on full magnetic gradient orthonormal basis function

Magnetic anomaly detection (MAD) is an effective method to detect the existence and localization of magnetic targets. Magnetic signal processing technology can extract target signals from complex background noise. However, traditional magnetic signal processing methods cannot greatly improve the signal-to-noise ratio (SNR) while also restoring information concerning the target. This is because the main existing method to calculate a target’s magnetic moment requires a pure target signal. Our research regarding the full magnetic gradient orthonormal basis function (FMG-OBF) addresses the problem of low SNRs for the target magnetic anomaly (TMA) signal. However, this method can only detect the presence of the target and cannot obtain the magnetic moment characteristics of the target. Benefiting from the FMG tensor, which contains large amounts of spatial magnetic field information, this paper is devoted to characterizing TMA in different orientations from the signal energy point of view. We analyze the influences of the target magnetic moment’s variation on the energy components of the TMA signal in various orientations, and further propose a target magnetic moment orientation estimation method. Compared with the traditional signal processing method, the proposed method can estimate the magnetic moment orientation of the target while greatly improving the SNR. Therefore, this method has significant application potential for the classification and identification of weak TMA signals in MAD.

MARMOT: magnetism, anisotropy, and more, using the relativistic disordered local moment picture at finite temperature

We present MARMOT, a hybrid Python/FORTRAN implementation of the disordered local moment picture within multiple scattering density-functional theory. MARMOT takes atom-centred, scalar-relativistic potentials and constructs an effective medium (within the coherent potential approximation) to describe the disordered magnetic moment orientations at finite temperature. By solving the single-site scattering problem fully relativistically, spin–orbit effects are included, allowing the magnetocrystalline anisotropy to be calculated. Magnetic transition temperatures, spin and orbital moments, the density-of-states, and analytical parameterizations of the magnetic potential energy surface can also be calculated. Here, we describe the theory and practical implementation of MARMOT, and demonstrate its use by calculating Curie temperatures, magnetizations and anisotropies of bcc Fe, GdFe2 and YCo5.

Second Moment Specification of Complex States of Polymer Chain Orientation in Fabricated Plastic Parts

AbstractA generalized representation of second moment orientation in polymer systems is presented. Orientation is expressed through the asymmetry of the polarizability tensor. The formulation presumes that local symmetry directions differ from macroscopic machine and transverse directions. The formulation leads to a set of biaxial orientation factors (equivalent to those of White and Spruiell) defined relative to symmetry axes I, II, III and a set of anglesϑI1,ϑII2 and ϑIII3${{\vartheta }_{I1}},{{\vartheta }_{II2}}\text{ }\!\!~\!\!\text{ and }\!\!~\!\!\text{ }{{\vartheta }_{III3}}$representing the directions of the axes relative to the laboratory axes (1,2, 3) such as the machine (1), transverse (2) and thickness (normal) directions (3). Any two of these angles eγ,Θ11${{\Theta }_{11}}$and ΘII2are sufficient. The formulation is applied to interpret orientation developed in simple shear.

Effect of symmetry reduction on the magnetic properties of LnIr[formula omitted]Si[formula omitted] polymorphs

Many tetragonal compounds LnIr2Si2 (Ln = lanthanoid) occur in two polymorphous phases and are therefore well suited to study the relationship between crystal structure and magnetic properties. We have grown GdIr2Si2 single crystals of both polymorphs from a high-temperature indium flux and investigated their anisotropic magnetic properties. The higher symmetric form with the ThCr2Si2 structure (space group I4/mmm) orders antiferromagnetically at TN=87K while for the lower symmetric compound in the CaBe2Ge2 structure (space group P4/nmm) we determined a much lower Néel temperature TN=12K. Our magnetic characterization of the single crystals reveals that for both compounds the magnetic moments are aligned in the a−a plane of the tetragonal lattice, but that the change of the symmetry strongly effects the inplane alignment of the moment orientation. For Ln=Er and Ho, we confirmed the existence of LnIr2Si2 in the space group P4/nmm. The magnetic properties of these lower symmetric compounds are in remarkable difference to their related compounds in the space group I4/mmm.

Influence of Hydrogen Bonds and π–π Interactions on the Fluorescence of Crystalline (N-Alkylpyridyl)enamino-pyrrolo[2,3-b]quinoxalin-2-one Derivatives

This paper presents the relationship between the fluorescent properties of (E/Z)-(N-alkylpyridyl)enamino-pyrrolo[2,3-b]quinoxalin-2-one derivatives in the crystalline state and the molecular packing governed by the conformation of the molecules, hydrogen bonding, and π–π interactions. In particular, the type of 2-pyridyl alkyl chain (2-Py(CH2)n with n = 0,1,2) is responsible for the molecular packing and influences the fluorescence properties of crystalline pyrrolo[2,3-b]quinoxalines. The molecules studied exhibit permanent dipole moments that, in the crystalline phases, promote either the formation of dimers with an antiparallel orientation or stacks with a parallel dipole moment orientation. In dimers and stacks, the main observed interaction is the π–π type. The conformations of both (E)- and (Z)-enamines are stabilized by intramolecular hydrogen bonds from the NH group of the enamine to either the nitrogen atom N4 of the quinoxaline (E)-diastereoisomer or the oxygen atom of the amide carbonyl group in the (Z)-diastereoisomer. When an additional intramolecular hydrogen bond from the enamine NH group to the pyridyl nitrogen atom forms, it affects the conformation of the molecules, causing the reorientation of the molecular permanent dipole moment. Density functional theory (DFT) calculations performed for two neighboring molecules in dimers or stacks indicate two charge transfer mechanisms: intra- and intermolecular mechanisms. For centrosymmetric dimers, charge transfer occurs within each component molecule (intramolecular charge transfer). For stacks with molecules arranged by translation and noncentrosymmetric dimers, charge transfer is of an intermolecular nature. Higher absolute fluorescence quantum yields (Φf = 12.06–13.77%) are exhibited by the (E-diastereoisomers, which contain methylene or ethylene chains and form either translational stacks of the molecules or noncentrosymmetric dimers. The lowest absolute fluorescence quantum yields (Φf = 3.80–4.00%) are typical for the centrosymmetric dimers present in crystalline (Z)-(N-pyridyl)enaminopyrrolo[2,3-b]quinoxaline and (E)-(N-ethylpyridyl)enamino-pyrrolo[2,3-b]quinoxaline. The fluorescence liftimes τ were determined for single crystals of the studied phases (from 13.3 to 15.6 ns) and revealed one single process of radiative energy transfer.

In-plane magnetization and electronic structures in BiFeO3/graphene superlattice

We predict the presence of gapped electronic structures in an artificial superlattice of graphene embedded in (111)-oriented BiFeO3 layers based on first-principles calculations. Due to the electron transfer and the proximity effect at the BiFeO3/graphene interface, we find that magnetic moments of Fe atoms near a graphene layer were slightly less than that of bulk Fe atoms. Regarding the ferromagnetic moment orientation of Fe atoms in perovskite BiFeO3, we reveal that the in-plane magnetization gives the ground state. The bandgap depends on the magnetization direction and the separation between the graphene layer and the perovskite BiFeO3 slab, which might be adjusted by applying external uniaxial stress in an experiment. Our results provide a route for designing hybrid 2D materials with emerging properties that are not available in single materials alone.

Giant electromagnetic proximity effect in superconductor/ferromagnet superlattices

We show that in superlattices with alternating superconducting (S) and ferromagnetic (F) layers the spontaneous magnetic field induced in the superconducting layers due to the electromagnetic proximity effect becomes dramatically enhanced compared to the previously studied S/F bilayers. The effect reveals itself for the in-plane orientation of the magnetic moments both for ferromagnetic and anti-ferromagnetic ordering of the moments in the F layers. In the finite size samples the magnetic field decays from the sample surface towards the bulk of the structure, and the decay length strongly depends on the relative orientation of the sample surface, the layers planes and magnetic moments in the F layers. The obtained results provide additional insights into experimental data on the neutron scattering in Nb/Gd superlattices.

Ab initiocalculation of the magnetic Gibbs free energy of materials using magnetically constrained supercells

We present a first-principles approach for the computation of the magnetic Gibbs free energy of materials using magnetically constrained supercell calculations. Our approach is based on an adiabatic approximation of slowly varying local moment orientations, the so-called finite-temperature disordered local moment picture. It describes magnetic phase transitions and how electronic and/or magnetostructural mechanisms generate a discontinuous (first-order) character. We demonstrate that the statistical mechanics of the local moment orientations can be described by an affordable number of supercell calculations containing noncollinear magnetic configurations. The applicability of our approach is illustrated by firstly studying the ferromagnetic state in bcc Fe. We then investigate the temperature-dependent properties of a triangular antiferromagnetic state stabilizing in two antiperovskite systems Mn$_3$AN (A = Ga, Ni). Our calculations provide the negative volume expansion of these materials as well as the ab initio origin of the discontinuous character of the phase transitions, electronic and/or magnetostructural, in good agreement with experiment.

Dynamics of transition dipole orientation in representative fluorescent proteins

Molecules of fluorescent proteins behave like antennas: they exhibit distinct directionality of light absorption and emission. In our previous work we determined the mean orientations of the ‘molecular antennas’ (transition dipole moments) within the atomic framework of fluorescent protein molecules. In the presented work we use molecular dynamics simulations, as well as time resolved fluorescence anisotropy measurements, in order to investigate the dynamics of transition dipole moment orientation in representative fluorescent proteins.

Transitions of a two-atom system in an entangled state and the Fulling–Davies–Unruh effect: The electromagnetic field case

We explore, respectively, from the viewpoints of a locally inertial observer and a coaccelerated observer, the transition rates of a two-atom system in the symmetric or antisymmetric entangled state, [Formula: see text] or [Formula: see text], and coupled to the electromagnetic fields. The interatomic separation is assumed to be constant and perpendicular to the acceleration; the fields to which the atoms are coupled are assumed to be in the vacuum state in the locally inertial frame and in a thermal state in the coaccelerated frame. We find that, in the viewpoints of the two observers, both the downward transition [Formula: see text] and the upward transition [Formula: see text] can take place, where [Formula: see text] and [Formula: see text] denote two eigenstates of the two-atom system with both atoms in their ground states and excited states, respectively. With an appropriate choice of the orientations of the atomic dipole moments, coherent radiation can happen for a system in acceleration but not for one in inertial motion. By comparing the transition rates viewed by the two observers, we show that the transition rates of the accelerated two-atom system viewed by a locally inertial observer can only be recovered in the coaccelerated frame by assuming a thermal bath at the Fulling–Davies–Unruh temperature. The effects of these collective transitions on the evolution of energy of the two-atom system are also analyzed.

Enhanced performance of solution-processed OLEDs by altering the molecular transition dipole moment orientation of emission layers

Transition dipole moment orientation is one of the crucial factors, which can enhance the performance of organic light-emitting diodes (OLEDs). In this work, the transition dipole moment orientation of the host-guest emission layers (EMLs) prepared by solution method annealed at different temperatures were systematically studied by analyzing the angle-dependent PL spectrum. When the EMLs of 2DPAc-MPM (20 wt%): DPEPO was annealed at the temperature of 115 °C, the photoluminescence quantum yield (PLQY) was enhanced to 78%±3.5%, and the vertical dipole ratio was reduced to 0.34. The lower vertical orientation of transition dipoles is conducted to improve the electron and hole mobility, which was testified by the hole and electron only devices. The lower vertical dipole ratio, higher PLQY and carrier mobility together enhanced the performance of solution processed OLEDs.