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Showing papers on "Magnetic field published in 2007"


Journal ArticleDOI
TL;DR: In this article, the basic principles of modern optical magnetometers, discuss fundamental limitations on their performance, and describe recently explored applications for dynamical measurements of biomagnetic fields, detecting signals in NMR and MRI, inertial rotation sensing, magnetic microscopy with cold atoms, and tests of fundamental symmetries of nature.
Abstract: Some of the most sensitive methods of measuring magnetic fields use interactions of resonant light with atomic vapour. Recent developments in this vibrant field have led to improvements in sensitivity and other characteristics of atomic magnetometers, benefiting their traditional applications for measurements of geomagnetic anomalies and magnetic fields in space, and opening many new areas previously accessible only to magnetometers based on superconducting quantum interference devices. We review basic principles of modern optical magnetometers, discuss fundamental limitations on their performance, and describe recently explored applications for dynamical measurements of biomagnetic fields, detecting signals in NMR and MRI, inertial rotation sensing, magnetic microscopy with cold atoms, and tests of fundamental symmetries of nature.

1,489 citations


Journal ArticleDOI
TL;DR: It is experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field, revealing an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.
Abstract: We experimentally demonstrate that the magnetization can be reversed in a reproducible manner by a single 40 femtosecond circularly polarized laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal previously believed impossible is the combined result of femtosecond laser heating of the magnetic system to just below the Curie point and circularly polarized light simultaneously acting as a magnetic field. The direction of this opto-magnetic switching is determined only by the helicity of light. This finding reveals an ultrafast and efficient pathway for writing magnetic bits at record-breaking speeds.

1,208 citations


Journal ArticleDOI
TL;DR: In this article, a general hydrodynamic theory of transport in the vicinity of superfluid-insulator transitions in two spatial dimensions described by ''Lorentz''-invariant quantum critical points was presented.
Abstract: We present a general hydrodynamic theory of transport in the vicinity of superfluid-insulator transitions in two spatial dimensions described by ``Lorentz''-invariant quantum critical points. We allow for a weak impurity scattering rate, a magnetic field $B$, and a deviation in the density $\ensuremath{\rho}$ from that of the insulator. We show that the frequency-dependent thermal and electric linear response functions, including the Nernst coefficient, are fully determined by a single transport coefficient (a universal electrical conductivity), the impurity scattering rate, and a few thermodynamic state variables. With reasonable estimates for the parameters, our results predict a magnetic field and temperature dependence of the Nernst signal which resembles measurements in the cuprates, including the overall magnitude. Our theory predicts a ``hydrodynamic cyclotron mode'' which could be observable in ultrapure samples. We also present exact results for the zero frequency transport coefficients of a supersymmetric conformal field theory (CFT), which is solvable by the anti--de Sitter (AdS)/CFT correspondence. This correspondence maps the $\ensuremath{\rho}$ and $B$ perturbations of the $2+1$ dimensional CFT to electric and magnetic charges of a black hole in the $3+1$ dimensional anti--de Sitter space. These exact results are found to be in full agreement with the general predictions of our hydrodynamic analysis in the appropriate limiting regime. The mapping of the hydrodynamic and AdS/CFT results under particle-vortex duality is also described.

662 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate that a magnetic vortex, isolated within a nanoscale spin-valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized d.c. current.
Abstract: Transfer of angular momentum from a spin-polarized current to a ferromagnet provides an efficient means to control the magnetization dynamics of nanomagnets. A peculiar consequence of this spin torque, the ability to induce persistent oscillations in a nanomagnet by applying a d.c. current, has previously been reported only for spatially uniform nanomagnets. Here, we demonstrate that a quintessentially non-uniform magnetic structure, a magnetic vortex, isolated within a nanoscale spin-valve structure, can be excited into persistent microwave-frequency oscillations by a spin-polarized d.c. current. Comparison with micromagnetic simulations leads to identification of the oscillations with a precession of the vortex core. The oscillations, which can be obtained in essentially zero magnetic field, exhibit linewidths that can be narrower than 300 kHz at ∼1.1 GHz, making these highly compact spin-torque vortex-oscillator devices potential candidates for microwave signal-processing applications, and a powerful new tool for fundamental studies of vortex dynamics in magnetic nanostructures.

593 citations


Journal ArticleDOI
TL;DR: The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first-order structural transition are studied by magnetization, strain, and neutron-diffraction studies under magnetic field as mentioned in this paper.
Abstract: Applying a magnetic field to a ferromagnetic Ni{sub 50}Mn{sub 34}In{sub 16} alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field, giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first-order structural transition are studied by magnetization, strain, and neutron-diffraction studies under magnetic field.

476 citations


Journal ArticleDOI
TL;DR: In this article, a Sweet-Parker-type scaling analysis for asymmetric antiparallel reconnection (in which the reconnecting magnetic field strengths and plasma densities are different on opposite sides of the dissipation region) is performed.
Abstract: A Sweet-Parker-type scaling analysis for asymmetric antiparallel reconnection (in which the reconnecting magnetic field strengths and plasma densities are different on opposite sides of the dissipation region) is performed. Scaling laws for the reconnection rate, outflow speed, the density of the outflow, and the structure of the dissipation region are derived from first principles. These results are independent of the dissipation mechanism. It is shown that a generic feature of asymmetric reconnection is that the X-line and stagnation point are not colocated, leading to a bulk flow of plasma across the X-line. The scaling laws are verified using two-dimensional resistive magnetohydrodynamics numerical simulations for the special case of asymmetric magnetic fields with symmetric density. Observational signatures and applications to reconnection in the magnetosphere are discussed.

438 citations


Book ChapterDOI
TL;DR: Magnetoelasts as discussed by the authors represent a new type of composite and consist of small magnetic particles, usually in the nanometer to micron range, dispersed in a highly elastic polymeric matrix.
Abstract: The combination of polymers with nano- or microsized solid materials displays novel and often enhancedproperties compared to the traditional materials. They can open up possibilities for new technologicalapplications. Materials whose physical properties can be varied by application of magnetic fields belongto a specific class of smart materials. The broad family of magnetic field-controllable soft materialsincludes ferrofluids, magneto-rheological fluids, magnetic gels, and magnetic elastomers. The magneticgels and elastomers (magnetoelasts) represent a new type of composite and consist of small magneticparticles, usually in the nanometer to micron range, dispersed in a highly elastic polymeric matrix.The magnetic particles can be incorporated into the elastic body either randomly or in ordered structure.If a uniform magnetic field is applied to the reactive mixture during the cross-linking process, particlechains form and become locked into the elastomer. The resulting composites exhibit anisotropic properties.

416 citations


Journal ArticleDOI
TL;DR: The observation of dynamo action in the von Kármán sodium experiment, i.e., the generation of a magnetic field by a strongly turbulent swirling flow of liquid sodium, is reported.
Abstract: We report the observation of dynamo action in the von Karman sodium experiment, i.e., the generation of a magnetic field by a strongly turbulent swirling flow of liquid sodium. Both mean and fluctuating parts of the field are studied. The dynamo threshold corresponds to a magnetic Reynolds number R-m similar to 30. A mean magnetic field of the order of 40 G is observed 30\% above threshold at the flow lateral boundary. The rms fluctuations are larger than the corresponding mean value for two of the components. The scaling of the mean square magnetic field is compared to a prediction previously made for high Reynolds number flows.

395 citations


Journal ArticleDOI
TL;DR: In this paper, a theoretical model for the formation of hot corona around strongly magnetized neutron stars was developed, and it was shown that the corona self-organizes quickly (on a millisecond timescale) into a quasi-steady state, with voltage 108-109 V along the magnetic lines.
Abstract: We develop a theoretical model that explains the formation of hot coronae around strongly magnetized neutron stars—magnetars. The starquakes of a magnetar shear its external magnetic field, which becomes nonpotential and threaded by an electric current. Once twisted, the magnetosphere cannot untwist immediately because of its self-induction. The self-induction electric field lifts particles from the stellar surface, accelerates them, and initiates avalanches of pair creation in the magnetosphere. The created plasma corona maintains the electric current demanded by ∇ × and regulates the self-induction EMF by screening. This corona persists in dynamic equilibrium: it is continually lost to the stellar surface on the light crossing time ~10-4 s and replenished with new particles. In essence, the twisted magnetosphere acts as an accelerator that converts the toroidal field energy to particle kinetic energy. Using a direct numerical experiment, we show that the corona self-organizes quickly (on a millisecond timescale) into a quasi-steady state, with voltage 108-109 V along the magnetic lines. The voltage is maintained near the threshold for e± discharge. The heating rate of the corona is ~1036 ergs s-1, in agreement with the observed persistent, high-energy output of magnetars. We deduce that a static twist that is suddenly implanted into the magnetosphere will decay on a timescale of 1-10 yr. The particles accelerated in the corona impact the solid crust, knock out protons, and regulate the column density of the hydrostatic atmosphere of the star. The transition layer between the atmosphere and corona may be hot enough to create additional e± pairs. This layer can be the source of the observed 100 keV emission from magnetars. The corona emits curvature radiation and can supply the observed IR-optical luminosity.

390 citations


Journal ArticleDOI
TL;DR: In this article, the authors demonstrate the electrical switching of the core magnetisation by utilizing the current-driven resonant dynamics of the vortex; the core switching is triggered by a strong dynamic field which is produced locally by a rotational core motion at a high speed of several hundred m/s.
Abstract: A magnetic vortex is a curling magnetic structure realized in a ferromagnetic disk, which is a promising candidate of a memory cell for future nonvolatile data storage devices. Thus, understanding of the stability and dynamical behaviour of the magnetic vortex is a major requirement for developing magnetic data storage technology. Since the experimental proof of the existence of a nanometre-scale core with out-of-plane magnetisation in the magnetic vortex, the dynamics of a vortex has been investigated intensively. However, the way to electrically control the core magnetisation, which is a key for constructing a vortex core memory, has been lacking. Here, we demonstrate the electrical switching of the core magnetisation by utilizing the current-driven resonant dynamics of the vortex; the core switching is triggered by a strong dynamic field which is produced locally by a rotational core motion at a high speed of several hundred m/s. Efficient switching of the vortex core without magnetic field application is achieved thanks to resonance. This opens up the potentiality of a simple magnetic disk as a building block for spintronic devices like a memory cell where the bit data is stored as the direction of the nanometre-scale core magnetisation.

373 citations


Journal ArticleDOI
TL;DR: In this article, a class of strongly interacting conformal field theories in a transverse magnetic field can be studied using the AdS/CFT duality, and zero momentum hydrodynamic response functions of maximally supersymmetric Yang-Mills theory at the conformal fixed point, in the large $N$ limit, are computed.
Abstract: A class of strongly interacting $2+1$ dimensional conformal field theories in a transverse magnetic field can be studied using the AdS/CFT duality. We compute zero momentum hydrodynamic response functions of maximally supersymmetric $2+1$ dimensional $SU(N)$ Yang-Mills theory at the conformal fixed point, in the large $N$ limit. With background magnetic field $B$ and electric charge density $\ensuremath{\rho}$, the Hall conductivity is found to be $\ensuremath{\rho}/B$. The result, anticipated on kinematic grounds in field theory, is obtained from perturbations of a four-dimensional AdS black hole with both electric and magnetic charges.

Journal ArticleDOI
TL;DR: In this article, the authors consider the effect of preexisting, large-scale, broadband turbulent density fluctuations on propagating hydromagnetic shock waves and find that the density fluctuations not only distort the shape of the shock front and lead to a turbulent postshock fluid, but also produce a number of important changes in the post-shock magnetic field.
Abstract: We consider the effect of preexisting, large-scale, broadband turbulent density fluctuations on propagating hydromagnetic shock waves. We present results from several numerical simulations that solve the two-dimensional magnetohydrodynamic equations. In our simulations, a plasma containing large-scale, low-amplitude density and magnetic field turbulence is forced to flow into a rigid wall, forming a shock wave. We find that the density fluctuations not only distort the shape of the shock front and lead to a turbulent postshock fluid, but they also produce a number of important changes in the postshock magnetic field. The average downstream magnetic field is increased significantly, and large fluctuations in the magnetic vector occur, with the maximum field strength reaching levels such that magnetic stresses are important in the postshock region. The downstream field enhancement can be understood in terms of the stretching and forcing together of the magnetic field entrained within the turbulent fluid of the postshock flow. We suggest that these effects of the density fluctuations on the magnetic field are observed in astrophysical shock waves such as supernova blast waves and the heliospheric termination shock.

Journal ArticleDOI
TL;DR: In this paper, an axisymmetric magnetic disk field configuration with reversals inside the solar circle fits available observations better than other 3D-model over a wide frequency range, and a coupling factor between thermal electrons and the random magnetic field component is proposed, which in addition to the small filling factor of thermal electrons increases small-scale RM fluctuations and thus accounts for the observed depolarization at 1.4 GHz.
Abstract: (Abridged) We constrain simulated all-sky maps in total intensity, linear polarization, and rotation measure (RM) by observations. We test a number of large-scale magnetic field configurations and take the properties of the warm interstellar medium into account. From a comparison of simulated and observed maps we are able to constrain the regular large-scale Galactic magnetic field in the disk and the halo of the Galaxy. The local regular field is 2 microG and the average random field is about 3 microG. The known local excess of synchrotron emission originating either from enhanced CR electrons or random magnetic fields is able to explain the observed high-latitude synchrotron emission. The thermal electron model (NE2001) in conjunction with a proper filling factor accounts for the observed optically thin thermal emission and low frequency absorption by optically thick emission. A coupling factor between thermal electrons and the random magnetic field component is proposed, which in addition to the small filling factor of thermal electrons increases small-scale RM fluctuations and thus accounts for the observed depolarization at 1.4 GHz. We conclude that an axisymmetric magnetic disk field configuration with reversals inside the solar circle fits available observations best. Out of the plane a strong toroidal magnetic field with different signs above and below the plane is needed to account for the observed high-latitude RMs. Our preferred 3D-model fits the observations better than other models over a wide frequency range.

Journal ArticleDOI
TL;DR: In this article, the authors reviewed measurements of edge plasma turbulence in toroidal magnetic fusion devices with an emphasis on recent results in tokamaks, showing that the dominant feature of edge turbulence is a high level of broadband density fluctuations with a relative amplitude δn/n ~ 5-100%.
Abstract: This paper reviews measurements of edge plasma turbulence in toroidal magnetic fusion devices with an emphasis on recent results in tokamaks. The dominant feature of edge turbulence is a high level of broadband density fluctuations with a relative amplitude δn/n ~ 5–100%, accompanied by large potential and electron temperature fluctuations. The frequency range of this turbulence is ~10 kHz–1 MHz, and the size scale is typically ~0.1–10 cm perpendicular to the magnetic field but many metres along the magnetic field, i.e. the structure is nearly that of 2D 'filaments'. Large intermittent bursts or 'blobs' are usually observed in the scrape-off layer. Diagnostic and data analysis techniques are reviewed and the main experimental results are summarized. Recent comparisons of experimental results with edge turbulence theory are discussed, and some directions for future experiments are suggested.

Journal ArticleDOI
TL;DR: In this article, the authors calculate the electrical and thermal conductivities and the thermoelectric coefficient of a class of strongly interacting 2+1-dimensional conformal field theories with anti-de Sitter space duals.
Abstract: We calculate the electrical and thermal conductivities and the thermoelectric coefficient of a class of strongly interacting 2+1-dimensional conformal field theories with anti-de Sitter space duals. We obtain these transport coefficients as a function of charge density, background magnetic field, temperature, and frequency. We show that the thermal conductivity and thermoelectric coefficient are determined by the electrical conductivity alone. At small frequency, in the hydrodynamic limit, we are able to provide a number of analytic formulas for the electrical conductivity. A dominant feature of the conductivity is the presence of a cyclotron pole. We show how bulk electromagnetic duality acts on the transport coefficients.

Journal ArticleDOI
TL;DR: Particle-in-cell simulations of collisionless magnetic reconnection are presented that demonstrate that reconnection remains fast in very large systems and that the electron dissipation region develops a distinct two-scale structure along the outflow direction.
Abstract: Particle-in-cell simulations of collisionless magnetic reconnection are presented that demonstrate that reconnection remains fast in very large systems. The electron dissipation region develops a distinct two-scale structure along the outflow direction. Consistent with fast reconnection, the length of the electron current layer stabilizes and decreases with decreasing electron mass, approaching the ion inertial length for a proton-electron plasma. Surprisingly, the electrons form a super-Alfvenic outflow jet that remains decoupled from the magnetic field and extends large distances downstream from the x line.

Journal ArticleDOI
12 Jan 2007-Polymer
TL;DR: In this paper, the elastic modulus of highly elastic magnetic elastomers has been studied by three different experimental techniques: elongation, static and dynamic shears, and it has been shown that the material elasticity increases considerably in an external homogeneous magnetic field of up to 0.3 T (100-fold increase of the tangential modulus has been observed at small 1-4% deformations).

Journal ArticleDOI
TL;DR: In this article, the authors have carried out MHD simulations of solar surface convection, including the effects of strong stratification, compressibility, partial ionization, radiative transfer, as well as an open lower boundary.
Abstract: Context: Observations indicate that the `quiet' solar photosphere outside active regions contains considerable amounts of magnetic energy and magnetic flux, with mixed polarity on small scales. The origin of this flux is unclear. Aims: We test whether local dynamo action of the near-surface convection (granulation) can generate a significant contribution to the observed magnetic flux. Methods: We have carried out MHD simulations of solar surface convection, including the effects of strong stratification, compressibility, partial ionization, radiative transfer, as well as an open lower boundary. Results: Exponential growth of a weak magnetic seed field (with vanishing net flux through the computational box) is found in a simulation run with a magnetic Reynolds number of about 2600. The magnetic energy approaches saturation at a level of a few percent of the total kinetic energy of the convective motions. Near the visible solar surface, the (unsigned) magnetic flux density reaches at least a value of about 25 G. Conclusions: A realistic flow topology of stratified, compressible, non-helical surface convection without enforced recirculation is capable of turbulent local dynamo action near the solar surface.

Journal ArticleDOI
TL;DR: In this paper, the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5 were performed using the FeH band near 1 μm, and the integrated surface magnetic flux was classified as low (well under 1 kG), intermediate (between 1 and about 2.5 kG) or strong (greater than about 3 kG).
Abstract: We present the first direct magnetic field measurements on M dwarfs cooler than spectral class M4.5. Utilizing a new method based on the FeH band near 1 μm, we categorize the integrated surface magnetic flux as low (well under 1 kG), intermediate (between 1 and about 2.5 kG), or strong (greater than about 3 kG) for a set of more than 20 stars ranging from M2 down to M9. Along with the field, we also measure the rotational broadening (v sin i) and Hα emission strength. Our goal is to advance the understanding of how dynamo field production varies with stellar parameters for very low mass stars, how the field and emission activity are related, and whether there is a connection between the rotation and magnetic flux. We find that fields are produced throughout the M dwarfs. In the mid-M stars, there is a clear connection between slow rotation and weak fields. In the late-M stars, rotation is always measurable, and the strongest fields are associated with with the most rapid rotators. Interestingly, these very cool rapid rotators appear to have the largest magnetic flux in the whole sample (greater than in the classical dMe stars). Hα emission is found to be a good proxy for magnetic fields, although the relation between the fractional emission and the magnetic flux varies with effective temperature. The drop-off in this fractional emission near the bottom of the main sequence is not accompanied by a drop-off in magnetic flux. It is clear that the methodology we have developed can be further applied to discover more about the behavior of magnetic dynamos and activity in cool and fully convective objects.

Journal ArticleDOI
TL;DR: In this article, the authors applied state-of-the-art nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA Active Region 10930 around the time of a powerful X3.4 flare.
Abstract: Solar flares and coronal mass ejections are associated with rapid changes in field connectivity and powered by the partial dissipation of electrical currents in the solar atmosphere. A critical unanswered question is whether the currents involved are induced by the motion of pre-existing atmospheric magnetic flux subject to surface plasma flows, or whether these currents are associated with the emergence of flux from within the solar convective zone. We address this problem by applying state-of-the-art nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA Active Region 10930 around the time of a powerful X3.4 flare. We compute 14 NLFFF models with 4 different codes and a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We discuss the relative merits of these models in a general critique of present abilities to model the coronal magnetic field based on surface vector field measurements. For our application in particular, we find a fair agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, (3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the ~10^32 erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection.

Journal ArticleDOI
TL;DR: In this paper, a review of the superconducting properties of MgB2 that are relevant for power applications is presented, focusing on the reversible mixed state parameters, which define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current.
Abstract: This review focuses on the superconducting properties of MgB2 that are relevant for power applications. The reversible mixed state parameters are the most important, since they define the limiting conditions for loss-free currents: the transition temperature, the upper critical field and the depairing current. They also determine the flux pinning energy, the pinning force and the elastic properties of the flux line lattice and, therefore, strongly influence the critical current densities. The magnetic properties of magnesium diboride are anisotropic and influenced by the two different energy gaps of the σ- and π-bands. Whereas the transition temperature could not be enhanced significantly during the past five years, the upper critical field was considerably increased by impurity scattering or doping. Flux pinning is very weak in MgB2 single crystals and was only improved by irradiation techniques so far. In polycrystalline samples, grain boundary pinning seems to play the dominant role. High critical currents close to the theoretical limit were found in c-axis oriented thin films. The anisotropy of the upper critical field strongly reduces the critical currents in untextured MgB2 at high magnetic fields, where the supercurrents become highly percolative, since not all grains are superconducting anymore. The performance of polycrystalline wires and tapes was significantly improved during the past few years by increasing the upper critical field and by reducing its anisotropy. Pinning seems to be nearly optimized in many forms of this material, but the connectivity between the grains might be further improved.

Journal ArticleDOI
01 Jun 2007-EPL
TL;DR: In this article, the velocity of a single 180° domain wall in a magnetic structure with reduced thickness and/or lateral dimension under the combined action of an external applied magnetic field and an electrical current was analyzed.
Abstract: We present an analytical calculation of the velocity of a single 180° domain wall in a magnetic structure with reduced thickness and/or lateral dimension under the combined action of an external applied magnetic field and an electrical current. As for the case of field-induced domain wall propagation in thick films, two motion regimes with different mobilities are obtained, below and far above the so-called Walker field. Additionally, for the case of current induced motion, a Walker-like current density threshold is defined. The threshold field and current density, stating the wall's internal structure stability, differ from those in thick films; both are reduced by the same geometrical demagnetising factor which accounts for the confinement. This points out the fact that the velocity dependence over an extended field/current range and the knowledge of the Walker breakdown are mandatory to draw conclusions about the phenomenological Gilbert damping parameter tuning the magnetisation dynamics.

Journal ArticleDOI
Mark Wardle1
TL;DR: In this article, the authors examined the magnetic diffusivity in a minimum solar nebula model and presented calculations of the ionisation equilibrium and magnetic diffusion as a function of height from the disk midplane at radii of 1 and 5 AU.
Abstract: Magnetic fields likely play a key role in the dynamics and evolution of protoplanetary disks. They have the potential to efficiently transport angular momentum by MHD turbulence or via the magnetocentrifugal acceleration of outflows from the disk surface. Magnetically-driven mixing has implications for disk chemistry and evolution of the grain population, and the effective viscous response of the disk determines whether planets migrate inwards or outwards. However, the weak ionisation of protoplanetary disks means that magnetic fields may not be able to effectively couple to the matter. I examine the magnetic diffusivity in a minimum solar nebula model and present calculations of the ionisation equilibrium and magnetic diffusivity as a function of height from the disk midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling by soaking up electrons and ions from the gas phase and reducing the conductivity of the gas by many orders of magnitude. However, once grains have grown to a few microns in size their effect starts to wane and magnetic fields can begin to couple to the gas even at the disk midplane. Because ions are generally decoupled from the magnetic field by neutral collisions while electrons are not, the Hall effect tends to dominate the diffusion of the magnetic field when it is able to partially couple to the gas, except at the disk surfaces where the low density of neutrals permits the ions to remain attached to the field lines. For a standard population of 0.1 μm grains the active surface layers have a combined column Σactive≈2 g cm−2 at 1 AU; by the time grains have aggregated to 3 μm, Σactive≈80 g cm−2. Ionisation in the active layers is dominated by stellar X-rays. In the absence of grains, X-rays maintain magnetic coupling to 10% of the disk material at 1 AU (i.e. Σactive≈150 g cm−2). At 5 AU the Σactive≈Σtotal once grains have aggregated to 1 μm in size.

Journal ArticleDOI
TL;DR: The room-temperature control of magnetization reversal with an electric field in an epitaxial nanostructure consisting of ferrimagnetic nanopillars embedded in a ferroelectric matrix is demonstrated and a proposed electric field to assist magnetic recording in multiferroic systems with high perpendicular magnetic anisotropy is proposed.
Abstract: We demonstrate the room-temperature control of magnetization reversal with an electric field in an epitaxial nanostructure consisting of ferrimagnetic nanopillars embedded in a ferroelectric matrix. This was achieved by combining a weak, uniform magnetic field with the switching electric field to selectively switch pillars with only one magnetic configuration. On the basis of these experimental results, we propose to use an electric field to assist magnetic recording in multiferroic systems with high perpendicular magnetic anisotropy.

Journal ArticleDOI
TL;DR: The torque and force generated by an arbitrary magnetic field on an axially symmetric soft-magnetic body is calculated using a continuous model that unifies two disparate magnetic models, and the field direction is provided to maximize torque for a given field magnitude.
Abstract: We calculate the torque and force generated by an arbitrary magnetic field on an axially symmetric soft-magnetic body. We consider the magnetization of the body as a function of the applied field, using a continuous model that unifies two disparate magnetic models. The continuous torque and force follow. The model is verified experimentally, and captures the often neglected region between weak and saturating fields, where interesting behavior is observed. We provide the field direction to maximize torque for a given field magnitude. We also find an absolute maximum torque, for a given body geometry and material, which can be generated with relatively weak applied fields. This paper is aimed at those interested in systems-level analysis, simulation, and real-time control of soft-magnetic bodies.

Journal ArticleDOI
TL;DR: In this paper, a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields, was used to explore the weak part of the magnetic field distribution of Ap/bp stars.
Abstract: Aims. We investigated a sample of 28 well-known spectroscopically-identified magnetic Ap/Bp stars, with weak, poorly-determined or previously undetected magnetic fields. The aim of this study is to explore the weak part of the magnetic field distribution of Ap/Bp stars. Methods. Using the MuSiCoS and NARVAL spectropolarimeters at Telescope Bernard Lyot (Observatoire du Pic du Midi, France) and the cross-correlation technique Least Squares Deconvolution (LSD), we obtained 282 LSD Stokes V signatures of our 28 sample stars, in order to detect the magnetic field and to infer its longitudinal component with high precision (median σ = 40 G). Results. For the 28 studied stars, we obtained 27 detections of StokesV Zeeman signatures from the MuSiCoS observations. Detection of the Stokes V signature of the 28th star (HD 32650) was obtained during science demonstration time of the new NARVAL spectropolarimeter at Pic du Midi. This result clearly shows that when observed with sufficient precision, all firmly classified Ap/Bp stars show detectable surface magnetic fields. Furthermore, all detected magnetic fields correspond to longitudinal fields which are significantly greater than some tens of G. To better characterise the surface magnetic field intensities and geometries of the sample, we phased the longitudinal field measurements of each star using new and previously-published rotational periods, and modeled them to infer the dipolar field intensity (Bd, measured at the magnetic pole) and the magnetic obliquity (β). The distribution of derived dipole strengths for these stars exhibits a plateau at about 1 kG, falling off to larger and smaller field strengths. Remarkably, in this sample of stars selected for their presumably weak magnetic fields, we find only 2 stars for which the derived dipole strength is weaker than 300 G. We interpret this “magnetic threshold” as a critical value necessary for the stability of large-scale magnetic fields, and develop a simple quantitative model that is able to approximately reproduce the observed threshold characteristics. This scenario leads to a natural explanation of the small fraction of intermediate-mass magnetic stars. It may also explain the near-absence of magnetic fields in more massive B and O-type stars.

Journal ArticleDOI
TL;DR: In this article, the authors describe experimental and theoretical investigations of the magnetic domain formation and the field reversal behavior in antiferromagnetically coupled perpendicular anisotropy multilayers that mimic A-type Antiferromagnetic (AF) structures.

Journal ArticleDOI
TL;DR: In this article, a smoothed particle hydrodynamics (SPH) method was used to simulate the collapse and fragmentation of molecular cloud cores using a new algorithm for MHD within the smoothed PSH method that enforces the zero magnetic divergence constraint.
Abstract: We have performed magnetohydrodynamics (MHD) simulations of the collapse and fragmentation of molecular cloud cores using a new algorithm for MHD within the smoothed particle hydrodynamics (SPH) method that enforces the zero magnetic divergence constraint. We find that the support provided by magnetic fields over thermal pressure alone has several important effects on fragmentation and the formation of binary and multiple systems, and on the properties of massive circumstellar discs. The extra support suppresses the tendency of molecular cloud cores to fragment due to either initial density perturbations or disc fragmentation. Furthermore, unlike most previous studies, we find that magnetic pressure plays the dominant role in inhibiting fragmentation rather than magnetic tension or magnetic braking. In particular, we find that if the magnetic field is aligned with the rotation axis of the molecular cloud core, the effects of the magnetic field on fragmentation and disc structure are almost entirely due to magnetic pressure, while if the rotation axis is initially perpendicular to the magnetic field, magnetic tension plays a greater role and can actually aid fragmentation. Despite these effects, and contrary to several past studies, we find that strongly perturbed molecular cloud cores are able to fragment to form wide binary systems even in the presence of quite strong magnetic fields. For massive circumstellar discs, we find that slowing of the collapse caused by the magnetic support decreases the mass infall rate on to the disc and, thus, weakens gravitational instabilities in young massive circumstellar discs. This not only reduces the likelihood that they will fragment, but also decreases the importance of spiral density waves in providing angular momentum transport and in promoting planet formation.

Journal ArticleDOI
TL;DR: In this article, electrical injection, detection, and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts is presented, where Fe∕Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel.
Abstract: We present the electrical injection, detection, and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts Fe∕Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel Nonlocal detection techniques show that the spin current detected after transport through the silicon is sensitive to the relative orientation of the magnetization of the injecting and detecting contacts Hanle effect measurements demonstrate that the spin current can be modulated by a perpendicular magnetic field, which causes the spin to precess and dephase in the transport channel

Journal ArticleDOI
TL;DR: In this article, the authors examined the MHD instabilities arising in the radiation zone of a differentially rotating star, in which a poloidal field of fossil origin is sheared into a toroidal field.
Abstract: Context. We examine the MHD instabilities arising in the radiation zone of a differentially rotating star, in which a poloidal field of fossil origin is sheared into a toroidal field. Aims. We focus on the non-axisymmetric instability that affects the toroidal magnetic field in a rotating star, which was first studied by Pitts and Tayler in the non-dissipative limit. If such an instability were able to mix the stellar material, it could have an impact on the evolution of the star. According to Spruit, it could also drive a dynamo. Methods. We compare the numerical solutions built with the 3-dimensional ASH code with the predictions drawn from an analytical study of the Pitts & Tayler instability. Results. The Pitts & Tayler instability is manifestly present in our simulations, with its conspicuous m = 1 dependence in azimuth. But its analytic treatment used so far is too simplified to be applied to the real stellar situation. Although the instability generated field reaches an energy comparable to that of the mean poloidal field, that field seems unaffected by the instability: it undergoes Ohmic decline, and is neither eroded nor regenerated by the instability. The toroidal field is produced by shearing the poloidal field and it draws its energy from the differential rotation. The small scale motions behave as Alfven waves; they cause negligible eddy-diffusivity and contribute little to the net transport of angular momentum. Conclusions. In our simulations we observe no sign of dynamo action, of either mean field or fluctuation type, up to a magnetic Reynolds number of 10 5 . However the Pitts & Tayler instability is sustained as long as the differential rotation acting on the poloidal field is able to generate a toroidal field of sufficient strength. But in the Sun such a poloidal field of fossil origin is ruled out by the nearly uniform rotation of the deep interior.