Local Interaction Fields Research Articles

Enhancement of exciton valley polarization in monolayer MoS2 induced by scattering

We report on scattering induced valley polarization enhancement in monolayer molybdenum disulfide. With thermally activated and charge doping introduced scattering, our sample exhibits seven? and twelve-folds of improvements respectively. This counter-intuitive effect is attributed to disruptions to valley pseudospin precession caused by rapid modulation of exciton momentum and concomitant local exchange interaction field, at time scales much shorter than the precession period. In contrast, the valley coherence is improved by thermally activated scattering, but not by charge doping induced scattering. We propose that this is due to anisotropic pseudospin scattering and generalize the Maialle-Silva-Sham model to quantitatively explain our experimental results. Our work illustrates that cleaner samples with minimal scattering, such as those carefully suspended or protected by hexagonal boron nitride, do not necessarily lead to good valley polarization. Well-controlled scattering can in fact provide an interesting approach for improving valleytronic devices.

Monitoring the evolution of Fe3O4 mesocrystal morphology using first-order reversal curves

We report the results of systematic measurements of first-order reversal curves (FORCs) for magnetite (Fe3O4) mesocrystals during the reaction process, where small nanoparticles, all with the same crystallographic orientation, form a cluster, and as the reaction time increases, grow in size. The FORC distribution exhibits a single peak in the FORC diagram, whose position gradually shifts toward a higher coercivity as the reaction time increases, which is associated with the broadening of the peak along both the coercivity and local interaction field axes. In addition, the FORC distribution peak is slightly displaced toward a negative local interaction field. These observations suggest the magnetic hardening and the presence of a positive (magnetizing) mean field acting on an individual nanoparticle from the surrounding nanoparticles within a mesocrystal. As the reaction time is further increased, nanoparticles constituting a mesocrystal split into porous nanoparticles, which is associated with a further shift of the FORC peak toward a higher coercivity. The present study demonstrates that FORCs can be a powerful tool for monitoring the morphology evolution of magnetic mesocrystals composed of small nanoparticles.

Crystal structure and magnetic properties of wafer-size Co nanostrip arrays self-organized on a step-bunched surface of Si(111)-5.55 × 5.55-Cu/(Cu, Cu2Si, W-O) buffers

Structure and magnetization reversal of Co nanostrips self-organized on a Si(111)-5.55 × 5.55-Cu step-bunched surface covered by Cu, Cu2Si and W-O buffer layers are systematically investigated and characterized in details. Optimal conditions for growth of well-ordered arrays of mm-long straight nanostrips with hexagonal close-packed and face-centered cubic crystal phases are defined. We find that the Co nanostrips with epitaxial and multiepitaxial structure grows on monocrystalline buffer layers of Cu and Cu2Si, respectively. The amorphous W-O buffer promotes the growth of polycrystalline Co nanostrips. We reveal the origin and compute the magnitude of magnetic anisotropy in Co nanostrips with different crystal phases. Structure-dependent magnetization reversal modes of nanostrip arrays are determined analytically using angular dependences of the coercive force. Our analysis of distributions of the switching and interaction fields by the first-order reversal curve diagram method specifies contributions of global and local interaction fields. Micromagnetic simulations considering the magnetostatic interaction between nanostrips demonstrate the crystal structure-dependent magnetization reversal mechanisms based on propagation of domain walls, whose type (symmetric or asymmetric transverse domain wall, vortex domain wall) and velocity are determined by the magnetic parameters, interaction fields and driving magnetic fields.

First Order Reversal Curve Study of SmFe₂ Melt-Spun Ribbons.

First-order reversal curves (FORC) and the FORC distribution provide a detailed characterization of the relative proportions of reversible and irreversible components of the magnetization of a material, revealing the dominant interactions in the system. Alloys with the nominal composition SmFe2 were obtained by melt-spinning with a cooper wheel velocity of 30 m/s. X-ray powder diffraction analysis showed a greater part consisting of an amorphous phase and a very small amount of SmFe2 crystalline phase with an average crystallite size of 8 nm. A constant acceleration Mössbauer spectrum, measured at room temperature in transmission mode, was fitted to a continuous distribution of effective fields at the nucleus of the amorphous phase (about 84% of the total area), plus two sextets for the non-equivalent sites of Fe in the SmFe2 crystalline phase. 91 first-order reversal curves were collected in a Quantum Design PPMS-VSM with reversal fields from –800 mT to +800 mT and using a calibration field of 850 mT. The obtained FORC diagrams showed a combined effect of a local interaction field and a mean interaction field, and showed that the reversible magnetization is a function of both, the applied magnetic field and the irreversible magnetization.

Investigation of the Potts model of a diluted magnet by local field averaging technique

Averaging of the local interatomic interaction fields has been applied to the Potts model of a diluted magnet. A self-consistent equation for the magnetization and an equation for the phase transition temperature have been derived. The temperature and magnetic atom density dependences of the spontaneous magnetization have been found for the lattices with the coordination numbers 3 and 4 and various numbers of spin states.

Imperfection of the clustered perovskite structure, phase transitions, and magnetoresistive properties of ceramic La0.6Sr0.2Mn1.2 − x Ni x O3 ± δ (x = 0–0.3)

Ceramic samples of lanthanum strontium manganite perovskites La0.6Sr0.2Mn1.2 − xNixO3 ± δ (0 ≤ x ≤ 0.3) have been investigated using the X-ray diffraction, magnetic (χac), 55Mn NMR, resistive, and magnetoresistive methods. The specific features of the influence of the composition on the structure and properties of nonstoichiometric manganite perovskites have been established. It has been found that the rhombohedrally (R\(\bar 3\)c) distorted perovskite structure contains cation and anion vacancies, as well as nanostructured clusters with Mn2+ ions in the A-positions. The substitution of Ni3+ ions (r = 0.74 A) for Mn3+ ions (r = 0.785 A) leads to a decrease in the lattice parameter a, the ferromagnetic-paramagnetic phase transition temperature TC, and the metal-semiconductor phase transition temperature Tms due to the disturbance of the superexchange interactions between heterovalent manganese ions Mn3+ and Mn4+. The observed anomalous magnetic hysteresis at 77 K has been explained by the antiferromagnetic effect of the unidirectional exchange anisotropy of the ferromagnetic matrix structure on the magnetic moments of the superstoichiometric manganese Mn2+ ions located in nanostructured planar clusters. An analysis of the asymmetrically broadened 55Mn NMR spectra of the compounds has revealed a high-frequency electronic superexchange of the ions Mn3+ ⟷ O2− ⟷ Mn4+; a local heterogeneity of their surrounding by other ions, vacancies, and clusters; and a partial localization of Mn4+ ions. The local hyperfine interaction fields on 55Mn nuclei have been determined. The concentration dependences of the activation energy and charge hopping frequency have confirmed that the Ni ions decrease the electrical conductivity due to the weakening of the electronic superexchange Mn3+ ⟷ O2− ⟷ Mn4+. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures TC and Tms, is caused by scattering at intracrystalline nanostructured heterogeneities, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between magnetic and electrical properties in rare-earth manganites.

Interactions and reversal-field memory in complex magnetic nanowire arrays

Interactions and magnetization reversal of Ni nanowire arrays have been investigated by the first-order reversal curve (FORC) method. Several series of samples with controlled spatial distribution were considered including simple wires of different lengths and diameters (70 and 110 nm) and complex wires with a single modulated diameter along their length. Subtle features of magnetic interactions are revealed through a quantitative analysis of the local interaction field profile distributions obtained from the FORC method. In addition, the FORC analysis indicates that the nanowire systems with a mean diameter of 70 nm appear to be organized in symmetric clusters indicative of a reversal-field memory effect.

Effective permeability tensor and double resonance of interacting bistable ferromagnetic nanowires

We present a model for the effective permeability tensor of nonsaturated arrays of axially magnetized bistable ferromagnetic wires based on a Maxwell-Garnett formalism generalized to include the case of two oppositely magnetized wire populations. Explicit expressions for the complex diagonal and off-diagonal components of the effective permeability tensor are derived to describe the magnetic response of the array and its dependence upon microwave frequency and external magnetic field. The model accounts for the geometrical parameters and the static magnetic configuration of the array, as well as the shape and intrinsic properties of the wires. We incorporate the effect of the static and dynamic interwire dipolar interactions and obtain explicit expressions for the position and relative amplitude of the two ferromagnetic resonance peaks associated with the effective permeability of nonsaturated arrays. The two absorption peaks arise due to the presence of the up and down wire populations, which are predominantly excited by the right-handed and left-handed circularly polarized magnetic field components of the electromagnetic wave propagating inside the array. Excellent agreement is found between the model and experimental results obtained from $S$-parameter broadband microstrip line measurements. Small discrepancies when the array is magnetically unsaturated or when the peaks are close to each other are discussed and attributed to spatial variations of the local interaction field not accounted for by the present model.

Magnetic hysteretic characterization of the irradiation-induced embrittlement of Fe, Fe–Cu model alloys, and reactor pressure vessel steel

Neutron irradiation is known to cause embrittlement of iron-based materials; in the nuclear industry, this effect can be detrimental for reactor pressure vessel steels. In this paper, we investigate the variations of the magnetic hysteretic behavior due to neutron irradiation, for four materials, i.e. nominally pure Fe, Fe-0.1 wt% Cu and Fe-0.3 wt%Cu model alloys, and a reactor pressure vessel steel, JRQ A533-B. Two parameters related to the magnetization loop shape, i.e. maximum relative differential permeability and peak intensity of local interaction field distribution, are measured as a function of neutron fluence. For all materials both parameters decrease with increasing fluence, due to the irradiation-induced formation of nano-size defects. This decreasing trend in magnetic parameters during embrittlement is noticeable regardless the origin of the embrittlement, which can be only Cu-precipitation (thermal aging of Fe–Cu), only matrix damage (irradiation of pure Fe), or both mechanisms (irradiation of Fe–Cu or steel). The magnetic parameters relatively change up to 40%, which indicates the potential of magnetic characterization to assess irradiation-induced material hardening and embrittlement.

Reversible and irreversible magnetization components behavior in Ni nanowires

Polycrystalline Ni nanowires were electrodeposited in nanoporous anodized alumina membranes with mean diameter of approximately 42 nm. Their magnetic properties were studied at 300 K, by measurements of recoil curves from demagnetized state and also from saturated state. M rev and M irr components were obtained and M rev( M irr) H curves were constructed from the experimental data. These curves showed a behavior that suggests a non-uniform reversal mode influenced by the presence of dipolar interactions in the system. A qualitative approach to this behavior is obtained using a Stoner–Wohlfarth model modified by a mean field term and local interaction fields.

Magnetostatic interactions in deep‐sea sediments inferred from first‐order reversal curve diagrams: Implications for relative paleointensity normalization

Understanding magnetostatic interactions in sediments is important for paleomagnetism and rock magnetism. Magnetostatic interactions are known to affect significantly the efficiency of anhysteretic remanent magnetization (ARM) acquisition, and hence the ratio of ARM susceptibility (χARM) to saturation isothermal remanent magnetization (SIRM), a magnetic grain‐size proxy widely used for environmental applications, can be controlled by magnetostatic interactions. Relative paleointensity estimations may also be influenced by magnetostatic interactions because ARM is often used as a normalizer to correct for the difference in magnetizability of sediments. In this study, characterization of magnetic grains in deep‐sea sediments from the North Pacific Ocean was conducted using first‐order reversal curve (FORC) diagrams and IRM acquisition curves. The FORC diagrams indicate that the magnetic grains consist mainly of a noninteracting single‐domain (SD) component and an interacting SD component, and additionally of a multidomain (MD) component. The relative abundances of these components were semiquantitatively estimated by curve fitting of cross sections along a line parallel to the axis of local interaction fields (Hu) and through a peak in the coercivity (Hc). Three components were fitted assuming a Gaussian distribution of interaction fields (Hu). The IRM acquisition curves can be described by two dominant components assuming a log‐Gaussian distribution. The mean coercivities of the two components are ∼40 and ∼100 mT, respectively. It is estimated that the former component is carried by biogenic magnetite, and roughly corresponds to the noninteracting SD component derived from the FORC diagrams, and that the latter component is carried by eolian maghemite, corresponding to the interacting SD component. The χARM/SIRM ratio decreases with increasing concentration of the interacting SD component. This implies that the χARM/SIRM ratio is significantly affected by magnetostatic interactions and that it does not necessarily reflect magnetic grain size. The effect of magnetostatic interactions on natural remanent magnetization acquisition in sediments is not well understood, but if it is similarly sensitive to the SIRM, normalization by ARM in relative paleointensity estimations would overcompensate for the concentration of magnetic grains and cause the significant coherence between the normalized intensity and the normalizer that has been reported in the literature.

Wer regiert Kolumbien?

Who governs Colombia? Socio-economic and social-structural consequences of the economies of violence. An enormous number of publications about the different types of organized violence in Colombia already exists (drug-mafia, guerilla, paramilitaries). This article focuses on the macro- and microeconomic consequences of these criminal organizations in rural and urban areas as well as in the corresponding spatial-structural components, especially in the regional and local interaction fields/“controlled” areas. For a long time the aims of the guerilla-organizations have been oriented by Marxist ideologies. Meanwhile they can be interpreted in the sense of the “new” war economies.

Homogenized Strain Model for Ni—Mn—Ga Driven with Collinear Magnetic Field and Stress

Prior experimental measurements by the authors demonstrated large reversible strains of ·0.41% along the [001] crystal direction of a cylindrical Ni 50Mn28.7Ga21.3 rod driven with a magnetic field along the same direction and no external restoring force. The origin of the reversibility is attributed to internal bias stresses generated by impurities absorbed during manufacture of the alloy. This article presents a macroscopic constitutive model for Ni—Mn—Ga strain in collinear field and stress configuration. The switching between two variant orientations in the presence of Zeeman energy and the pinning energy of the impurities is formulated through a Gibbs energy function for the crystal lattice. Inhomogeneous local interaction fields and impurity distributions are addressed through stochastic homogenization techniques. Attributes of the model are illustrated through comparison of model results with strain—field measurements collected at various compressive loads. Constrained optimization is used to determine the necessary parameters and an error analysis is performed to assess the accuracy of the model for various loading conditions. The collinear field and stress configuration can lead to solenoid transducers with enhanced energy density and bandwidth relative to standard electromagnet devices.

Magnetic evaluation of the hardening and softening of thermally aged iron–copper alloys

We evaluated the variations of the magnetic hysteretic behaviour of Fe–1wt%Cu model alloy samples due to thermal aging and over-aging. In these alloys, the formation and growth of Cu-precipitates during the thermal aging process results in mechanical hardening, whereas for aging times higher than a certain critical value, mechanical softening occurs. Magnetic hysteretic properties such as permeability, remanence, peak value and width of the local interaction field distribution, which is related to the Preisach model, are measured as a function of various Cu-precipitation stages obtained by time dependent heat treatments at 773 K (500 °C). We found that all magnetic parameters exhibit an extremum value for the peak hardening sample. Furthermore, the variation of permeability, remanence, and the peak value of the local interaction field distribution mimics the behaviour of the reciprocal value of the yield stress as a function of aging time. These results suggest that the magnetic domain wall movement is hindered mainly by Cu-precipitates upon thermal aging. The peak hardening values of the investigated magnetic parameters change by approximately 50% when compared with the initial values. This pronounced sensitivity indicates the potential of magnetic non-destructive evaluation for the assessment of the hardening and softening phenomena induced by Cu-precipitation.

Theoretical aspects of dipolar interactions and their appearance in first‐order reversal curves of thermally activated single‐domain particles

Recently, the study of interacting particles was driven by the modeling effort undertaken to interpret first‐order reversal curve (FORC) diagrams of natural rocks and sediments. Understanding the effect of magnetostatic interactions is of primary importance in rock magnetism and paleomagnetism, and FORCs can provide useful information for this purpose. However, fully quantitative theories of FORC measurements have not been formulated yet. The goal of this paper is to fill this gap for the case of interacting single‐domain (SD) particles. First, a general analytical expression is obtained for the local interaction field produced by a random set of magnetic dipoles. This expression is then used to formulate a general theory that explains the effect of the local interaction field on FORCs. Exact solutions are obtained for the case of weakly interacting single‐domain particles, whereby the effect of thermal activations is considered as well. These solutions show that a rigorous analysis of the FORC function and its relationship with the distributions of coercivities and interaction fields is possible.

First-Order Reversal Curves Diagrams of Ferromagnetic Soft Nanowire Arrays

First-order reversal curves (FORC) diagrams map the statistical distributions of magnetic hysterons, based on their critical fields (H c) and local interaction fields (Hu), throughout the magnetization reversal. This technique has been adapted to soft nanowire arrays deposited into alumina templates, with axes perpendicular to the template plane (diameter=50--175 nm, length=5--60 mum). From the topology obtained, the intrinsic physical properties of the array can be determined, notably the coercivities of individual nanowires and the interaction field at saturation. However, the FORC diagram may also yield information on other properties, such as the existence of a distribution of wire lengths

A homogenized energy model for the direct magnetomechanical effect

This paper focuses on the development of a homogenized energy model which quantifies certain facets of the direct magnetomechanical effect. In the first step of the development, Gibbs energy analysis at the lattice level is combined with Boltzmann principles to quantify the local average magnetization as a function of input fields and stresses. A macroscopic magnetization model, which incorporates the effects of polycrystallinity, material nonhomogeneities, stress-dependent interaction fields, and stress-dependent coercive behavior, is constructed through stochastic homogenization techniques based on the tenet that local coercive and interaction fields are manifestations of underlying distributions rather than constants. The resulting framework incorporates previous ferromagnetic hysteresis theory as a special case in the absence of applied stresses. Attributes of the framework are illustrated through comparison with previously published steel and iron data.

A homogenized energy framework for ferromagnetic hysteresis

This paper focuses on the development of a homogenized energy model which quantifies certain facets of the direct magnetomechanical effect. In the first step of the development, Gibbs energy analysis at the lattice level is combined with Boltzmann principles to quantify the local average magnetization as a function of input fields and stresses. A macroscopic magnetization model, which incorporates the effects of polycrystallinity, material nonhomogeneities, stress-dependent interaction fields, and stress-dependent coercive behavior, is constructed through stochastic homogenization techniques based on the tenet that local coercive and interaction fields are manifestations of underlying distributions rather than constants. The resulting framework incorporates previous ferromagnetic hysteresis theory as a special case in the absence of applied stresses. Attributes of the framework are illustrated through comparison with previously published steel and iron data

A homogenized energy framework for ferromagnetic hysteresis

This paper focuses on the development of a homogenized energy model which quantifies certain facets of the direct magnetomechanical effect. In the first step of the development, Gibbs energy analysis at the lattice level is combined with Boltzmann principles to quantify the local average magnetization as a function of input fields and stresses. A macroscopic magnetization model, which incorporates the effects of polycrystallinity, material nonhomogeneities, stress-dependent interaction fields, and stress-dependent coercive behavior, is constructed through stochastic homogenization techniques based on the tenet that local coercive and interaction fields are manifestations of underlying distributions rather than constants. The resulting framework incorporates previous ferromagnetic hysteresis theory as a special case in the absence of applied stresses. Attributes of the framework are illustrated through comparison with previously published steel and iron data

First‐order reversal curve (FORC) diagrams of elongated single‐domain grains at high and low temperatures

First‐order reversal curve (FORC) diagrams are a new technique for identifying magnetic minerals and characterizing their domain structures and interactions. FORC diagrams have potentially important applications for determining magnetic mineral compositions and grain sizes in paleomagnetism and environmental magnetism. However, the interpretation of the FORC parameters Hc and Hu as being equivalent to microcoercivity and particle interaction field, respectively, needs to be thoroughly tested in order to validate the FORC method. In order to test this interpretation, we measured FORC diagrams on elongated single‐domain particles at temperatures ranging from 20 to 853 K (580°C). The FORC distribution contracts significantly along both axes above 430°C, and the bulk coercive force decreases rapidly. The main causes of these features are decreasing spontaneous magnetization Ms(T) and increasing thermal activation of single‐domain moments pinned by shape anisotropy. A profile through the FORC peak along the Hc axis can be integrated to give a synthetic alternating field demagnetization curve or isothermal remanence curve. The synthesized curves match measured curves quite well at a variety of temperatures, and the FORC median Hc varies approximately in proportion to the bulk coercive force. A profile through the FORC peak parallel to the Hu axis contracts with heating approximately as Ms(T), and the half width of the distribution is similar to independent measures of local interaction fields Hi. These properties support the interpretation of the FORC variables Hc and Hu as being equivalent to microcoercivity Hc and interaction field Hi, respectively.