Large Magnetic Field Effects Research Articles

ON α- MARTENSITE NUCLEATION IN STEELS AND IRON ALLOYS

Purpose. To investigate the possibility of martensite nucleation on magnetic (concentration) inhomogeneities of chrome-nickel steels austenite.
 Research methods. Generalization of experimental and theoretical studies of the strong magnetic field effect on the martensitic transformation in steels and iron alloys, taking into account the magnetic state of austenite
 Results. The analysis of the magnetic state of austenite and the abnormally large strong magnetic field effect on the martensite point displacement in 40X2H20 steel indicates that the transformation of austenite into martensite in it is a “magnetic first order phase transition”.
 Scientific novelty. Some features of the martensitic transformation in steels and iron alloys are explained. It is assumed that martensite during the “magnetic γ→α- transition” originates on magnetic inhomogeneities with a disoriented magnetic structure of the spin glass type. Forced magnetostriction during paraprocess and spontaneous magnetostriction below the Curie point lead to the emergence of collinear ferromagnetism with a change in the coordination number from 12 to 8. High all-round pressure leads to the occurrence of collinear antiferromagnetism and the γ-phase stabilization. In paramagnetic austenite the nucleation of α-martensite occurs on dislocation nuclei, the sizes of which have the same order of magnitude (by volume) as the magnetic inhomogeneities in the Fe-Ni system.
 Practical value. The results obtained in the work expand the concept of martensitic transformation in steels and provide grounds for explaining some kinetic features in them.

Enhanced charge density wave with mobile superconducting vortices in La1.885Sr0.115CuO4

Superconductivity in the cuprates is found to be intertwined with charge and spin density waves. Determining the interactions between the different types of order is crucial for understanding these important materials. Here, we elucidate the role of the charge density wave (CDW) in the prototypical cuprate La1.885Sr0.115CuO4, by studying the effects of large magnetic fields (H) up to 24 Tesla. At low temperatures (T), the observed CDW peaks reveal two distinct regions in the material: a majority phase with short-range CDW coexisting with superconductivity, and a minority phase with longer-range CDW coexisting with static spin density wave (SDW). With increasing magnetic field, the CDW first grows smoothly in a manner similar to the SDW. However, at high fields we discover a sudden increase in the CDW amplitude upon entering the vortex-liquid state. Our results signify strong coupling of the CDW to mobile superconducting vortices and link enhanced CDW amplitude with local superconducting pairing across the H − T phase diagram.

INFLUENCE OF TEMPERATURE ON THE BEARING CAPACITY OF PASSIVE MAGNETIC BEARINGS

When magnetic-passive bearings are heated, the magnetic properties of permanent magnets change and, therefore, the force of interaction between them changes. Depending on the temperature and exposure time, irreversible changes in the strength of the interaction of magnets occur, caused by changes in the magnetic structure, phase composition, as well as reversible changes due to the temperature dependence of the magnetization and coercive force of the magnet material. A distinctive feature of the operation of magnets in bearings is the effect of large external magnetic fields on them. For this reason, it is impossible to predict in advance the properties of bearings at elevated temperatures using only the data given in the literature for magnetically rigid materials or individual magnets. The influence of temperature on the interaction of two coaxially arranged magnets modeling a part of a bearing is theoretically and experimentally considered. Numerical values of temperature coefficients of changes in bearing capacity and stiffness of bearings are determined, as well as the theoretical conclusion about the constancy of these coefficients for bearings with optimal magnet sizes is confirmed.

Spinterfaces Manipulate Large Magnetic Field Effects in 3D and Quasi‐2D Organic–Inorganic Hybrid Perovskites

AbstractSpinterfaces are due to the orbital hybridization at an interface of a ferromagnet and a nonmagnetic semiconductor. Interfacial densities of states (i‐DOS) are spin‐dependent, which may offer the spin‐filter effect for enhancing spin‐polarized charge injection efficiency and magnetic field effects (MFEs). In the work, a prototypical perovskite spin valve (PeSV) comprising Ni/MAPbI3−xClx/Ni (MA = CH3NH3) is fabricated, showing a clear magnetic switching behavior in the ambient condition. It is further explored by studying the magnetophotocurrent (MPC) at the short‐circuit current density condition (magneto‐Jsc). Anomalously large magneto‐Jsc can be detected by comparing to devices with nonmagnetic Au electrodes. The effect can be more pronounced for the quasi‐2D perovskite such as (PEA)2(MA)3Pb4I13 (PEA = C6H5(CH2)2NH3). At low fields, the hyperfine field induced spin‐mixing shows remarkable magneto‐Jsc signals for it. In addition, the circularly and linearly polarized laser beam can effectively manipulate the magnitude of magneto‐Jsc for both MAPbI3−xClx and (PEA)2(MA)3Pb4I13. It is postulated that the formation of the spinterfaces may play a dominant role for the large and tunable magneto‐Jsc effect. These studies may pave the way for future experimental and theoretical studies in the field.

Spin‐Dependent Electron–Hole Recombination and Dissociation in Nonfullerene Acceptor ITIC‐Based Organic Photovoltaic Systems

A nonfullerene acceptor (NFA), named 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′] dithiophene (ITIC) with the fused aromatic conjugated backbone structure, is an intriguing small molecular semiconductor in the application of organic bulk heterojunction (BHJ) solar cells. However, the underlying spin‐dependent photo‐physics in the photovoltaic process remains deficient. Here, ITIC‐based thin solid film, binary, and ternary organic blends are designed and fabricated with two polymeric donors, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]‐dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T) and poly[(2,6‐(4,8‐Bis(3‐((2‐ethylhexyl)oxy)‐phenyl)‐benzo[1,2‐b:4,5‐b′]‐dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBPD‐Th). With the spin‐sensitive magneto‐photocurrent and magneto‐electroluminescence measurements, ITIC always exhibits large magnetic field effects with negative signs in the ambient temperature. The effect is attributed to the exciton‐charge reaction. However, both positive and negative signs are detected in the binary and ternary organic blends at small and large fields, respectively. The results elucidate that the mutual combination of the spin‐dependent polaron pair dissociation and exciton‐charge reaction plays a decisive role in the photovoltaic process. Furthermore, with photo‐induced electron paramagnetic resonance (EPR) studies, the full width at half maximum (FWHM) of the line shape for the positive magneto‐photocurrent is firmly associated to the magnitude of the effective g‐factor. The present study may shed a new light on the deep understanding of spin‐dependent photo‐physical process in NFA‐based solar cells for organic opto‐spintronic developments.

Effect of intense magnetic fields on reduced-magnetohydrodynamics evolution in sNN=200 GeV Au + Au collisions

We investigate the effect of large magnetic fields on the $2+1$ dimensional reduced-magnetohydrodynamical expansion of hot and dense nuclear matter produced in $\sqrt{s_{\rm NN}}$ = 200 GeV Au+Au collisions. For the sake of simplicity, we consider the case where the magnetic field points in the direction perpendicular to the reaction plane. We also consider this field to be external, with energy density parametrized as a two-dimensional Gaussian. The width of the Gaussian along the directions orthogonal to the beam axis varies with the centrality of the collision. The dependence of the magnetic field on proper time ($\tau$) for the case of zero electrical conductivity of the QGP is parametrized following [Deng 2012], and for finite electrical conductivity following [Tuchin 2013]. We solve the equations of motion of ideal hydrodynamics for such an external magnetic field. For collisions with non-zero impact parameter we observe considerable changes in the evolution of the momentum eccentricities of the fireball when comparing the case when the magnetic field decays in a conducting QGP medium and when no magnetic field is present. The elliptic-flow coefficient $v_2$ of $\pi^{-}$ is shown to increase in the presence of an external magnetic field and the increment in $v_2$ is found to depend on the evolution and the initial magnitude of the magnetic field.

Measurement of untruncated nuclear spin interactions via zero- to ultralow-field nuclear magnetic resonance

Zero- to ultra-low-field nuclear magnetic resonance (ZULF NMR) provides a new regime for the measurement of nuclear spin-spin interactions free from effects of large magnetic fields, such as truncation of terms that do not commute with the Zeeman Hamiltonian. One such interaction, the magnetic dipole-dipole coupling, is a valuable source of spatial information in NMR, though many terms are unobservable in high-field NMR, and the coupling averages to zero under isotropic molecular tumbling. Under partial alignment, this information is retained in the form of so-called residual dipolar couplings. We report zero- to ultra-low-field NMR measurements of residual dipolar couplings in acetonitrile-2-$^{13}$C aligned in stretched polyvinyl acetate gels. This represents the first investigation of dipolar couplings as a perturbation on the indirect spin-spin $J$-coupling in the absence of an applied magnetic field. As a consequence of working at zero magnetic field, we observe terms of the dipole-dipole coupling Hamiltonian that are invisible in conventional high-field NMR. This technique expands the capabilities of zero- to ultra-low-field NMR and has potential applications in precision measurement of subtle physical interactions, chemical analysis, and characterization of local mesoscale structure in materials.

The Role of Electron–Hole Separation in Thermally Activated Delayed Fluorescence in Donor–Acceptor Blends

Thermally activated delayed fluorescence (TADF) is becoming an increasingly important OLED technology that extracts light from nonemissive triplet states via reverse intersystem crossing (RISC) to the bright singlet state. Here we present the rather surprising finding that in TADF materials that contain a mixture of donor and acceptor molecules the electron–hole separation fluctuates as a function of time. By performing time-resolved photoluminescence experiments, both with and without a magnetic field, we observe that at short times the TADF dynamics are insensitive to magnetic field, but a large magnetic field effect (MFE) occurs at longer times. We explain these observations by constructing a quantum mechanical rate model in which the electron and hole cycle between a near-neighbor exciplex state that shows no MFE and a separated polaron-pair state that is not emissive but does show magnetic field dependent dynamics. Interestingly, the model suggests that only a portion of TADF in these blends comes fr...

Large magnetic field effects in electrochemically doped organic light-emitting diodes

Large negative magnetoconductance (MC) of \ensuremath{\sim}12$%$ is observed in electrochemically doped polymer light-emitting diodes at sub-band-gap bias voltages (${V}_{\mathrm{bias}}$). Simultaneously, a positive magnetoefficiency (M\ensuremath{\eta}) of 9$%$ is observed at ${V}_{\mathrm{bias}}$ $=$ 2 V. At higher bias voltages, both the MC and M\ensuremath{\eta} diminish while a negative magnetoelectroluminescence (MEL) appears. The negative MEL effect is rationalized by triplet-triplet annihilation that leads to delayed fluorescence, whereas the positive M\ensuremath{\eta} effect is related to competition between spin mixing and exciton formation leading to an enhanced singlet:triplet ratio at nonzero magnetic field. The resultant reduction in triplet exciton density is argued to reduce detrapping of polarons in the recombination zone at low-bias voltages, explaining the observed negative MC. Regarding organic magnetoresistance, this study provides experimental data to verify existing models describing magnetic field effects in organic semiconductors, which contribute to better understanding hereof. Furthermore, we present indications of strong magnetic field effects related to interactions between trapped carriers and excitons, which specifically can be studied in electrochemically doped organic light-emitting diodes (OLEDs). Regarding light-emitting electrochemical cells (LECs), this work shows that delayed fluorescence from triplet-triplet annihilation substantially contributes to the electroluminescence and the device efficiency.

Two-component jets from 3-dimensional magnetohydrodynamic jet simulations of disk winds at sub-parsec scales

We explore the effect of large scale magnetic field on the formation of two-component jets in magnetohydrodynamic disk winds simulations. Our simulations show a one-component and two-component jets develop depending on the magnetic field distribution along the surface of the accretion disk. Magnetic field configurations with the least steep gradient along the disk lead to a well defined two-component jet with the self-similar (Blandford-Payne) configuration separating the two regimes. Our results have direct implications to jets models of AGN and GRBs if indeed two-component jets emanate directly from the accretion disk. Our findings imply that a three-component jets may exist in AGN jets if one takes into account a Blandford-Znajek component in the innermost, relativistic, regions.

Impact of Strong Magnetic Fields on Collision Mechanism for Transport of Charged Particles

One of the main applications in plasma physics concerns the energy production through thermo-nuclear fusion. The controlled fusion is achieved by magnetic confinement i.e., the plasma is confined into a toroidal domain (tokamak) under the action of huge magnetic fields. Several models exist for describing the evolution of strongly magnetized plasmas, most of them by neglecting the collisions between particles. The subject matter of this paper is to investigate the effect of large magnetic fields with respect to a collision mechanism. We consider here linear collision Boltzmann operators and derive, by averaging with respect to the fast cyclotronic motion due to strong magnetic forces, their effective collision kernels.

RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

We have carried out a numerical study of the effect of large scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on time scales much shorter than the dynamical time scale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor ~ 2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor ~ 0.2 \beta^{1/2} compared to the Bondi value, where \beta is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendixes for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

Space-charge-limited magnetoresistance in organic semiconductors

We present a two-site model for large magnetic field effect on current of nonmagnetic organic semiconductor, based on bipolaron formation in the presence of both hyperfine field and an applied magnetic field. Under the framework of space-charge-limited current, we derived our current density and magnetoresistance expression. The magnetoresistance fit well with the Lorentzian empirical law. We discuss all parameters involved in, and elucidate the origin of positive and negative magnetoresistance under our model.

Using CMBR tools for flow anisotropies in relativistic heavy-ion collisions

Abstract Recently we have shown that there are crucial similarities in the physics of cosmic microwave background radiation (CMBR) and relativistic heavy-ion collision experiments (RHICE). We have argued that following CMB analysis technique, a plot of the root mean square value of the flow coefficient v ˜ n r m s calculated in lab fixed frame will give important information about the initial state fluctuations and also can be used to directly probe elliptic flow in non-central collisions. Here we demonstrate how following the analysis techniques used in CMBR can give important information about the initial state and evolution of the plasma formed in heavy ion collisions. We further explore the similarities in the physics of CMB and RHICE by studying the effect of large magnetic field on flow coefficients.

Magnetic field effects on exciplex-forming systems: the effect on the locally excited fluorophore and its dependence on free energy

This study addresses magnetic field effects in exciplex forming donor-acceptor systems. For moderately exergonic systems, the exciplex and the locally excited fluorophore emission are found to be magneto-sensitive. A previously introduced model attributing this finding to excited state reversibility is confirmed. Systems characterised by a free energy of charge separation up to approximately -0.35 eV are found to exhibit a magnetic field effect on the fluorophore. A simple three-state model of the exciplex is introduced, which uses the reaction distance and the asymmetric electron transfer reaction coordinate as pertinent variables. Comparing the experimental emission band shapes with those predicted by the model, a semi-quantitative picture of the formation of the magnetic field effect is developed based on energy hypersurfaces. The model can also be applied to estimate the indirect contribution of the exchange interaction, even if the perturbative approach fails. The energetic parameters that are essential for the formation of large magnetic field effects on the exciplex are discussed.

Anomalous magnetic field effects during pulsed injection metal-organic chemical vapor deposition of magnetite films

We report on large external magnetic field effects during pulsed injection metal-organic chemical vapor deposition of magnetite films on MgO(001). The application of a 1 T field during the growth process significantly increases the saturation magnetization of magnetite by 150%–220% at a deposition temperature of 550 and 600 °C, while the enhancement of the remanent magnetization is even larger. This anomalous magnetic field effect does not drastically alter the crystalline texture, surface morphology, and film thickness of magnetite, but is explained by a suppression of antiphase-boundary formation during film growth.

Observations of electron density and electron temperature during large scale magnetic fields in the dayside Venus ionosphere and lesson for Mars

We first present several dayside electron density (Ne) and electron temperature (Te) profiles observed by the Langmuir probe experiment aboard Pioneer Venus Orbiter when the Venus ionosphere was in a magnetised state and then examine the effect of large scale magnetic fields on the Venus ionosphere. We find that for the magnetised ionospheres, the “top” moves down to altitudes near 200 km and the ionopause layers with steep altitude gradients in Ne and Te start above this altitude. No significant change in electron density and electron temperature is seen within the ionosphere. Occasionally a second ionopause layer is also seen at higher altitudes and the ionospheric region between the two ionopause layers, which is seen to extend to altitudes up to 250 km, too does not indicate any significant change in Ne and Te for these cases. These results have direct relevance to Mars where large scale ionospheric fields have also been observed.

Some developments of hot accretion flow theory in the past ten years

The hot accretion flow model was re-discovered in 1994 by Narayan and collaborators. Intensive theoretical works have been conducted and significant progresses have been achieved. In this paper, we review several developments in the past ten years. This mainly includes the finding of outflow and convection and its dynamical effect on inflow; the direct electron heating by viscous dissipation; the effect of large scale toroidal magnetic fields in the inner region of the accretion flow; and the effect of global Compton scattering. Their observational applications are also introduced very briefly.

Nanoscale Heterogeneous Structure of Ionic Liquid as Revealed by Magnetic Field Effects

Large magnetic field effects (MFEs) observed for photoinduced hydrogen abstraction reaction between benzophenone and thiophenol in an ionic liquid (N,N,N,-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)amide, TMPA TFSA) are analyzed by using the stochastic Liouville equation for the first time. The sphere cage model can well reproduce the observed MFEs and the nanoscale heterogeneous structure with a cage radius of 1.8 +/- 0.3 nm, and an effective viscosity in the cage of 1-2 cP is found to be formed in TMPA TFSA.

The Magnetic Field Effects on Photochemical Reactions in Ionic Liquids

The magnetic field effects (MFEs) on photoinduced hydrogen abstraction reactions of benzophenone (BP) with thiophenol (PhSH) in the ionic liquids (ILs) N,N,N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl) imide (TMPA TFSI), N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (P13 TFSI), and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide (PP13 TFSI) were investigated at 296 K by using a nanosecond laser flash photolysis technique under magnetic fields of 0-1.7 T. Large MFEs were observed for the first time in the ILs. In TMPA TFSI, the yield of the benzophenone ketyl radical gradually decreased with increasing magnetic field strength from 0 to 1.7 T, producing a 20% decrease at 1.7 T.