Surface Magnetic Structure Research Articles

Stokes IQUV magnetic Doppler imaging of Ap stars – II. Next generation magnetic Doppler imaging of α2 CVn★

We present updated magnetic field maps of the chemically peculiar B9p star $\alpha^2$ CVn created using a series of time resolved observations obtained using the high resolution spectropolarimeters ESPaDOnS and Narval. We compare these new magnetic field maps with the original magnetic Doppler imaging maps based on spectra recorded with the MuSiCoS spectropolarimeter and taken a decade earlier. These new maps are inferred from line profiles in all four Stokes parameters using the magnetic Doppler imaging code INVERS10. With the addition of new lines exhibiting Stokes $IQUV$ signatures we have a unique insight into how the derived magnetic surface structure may be affected by the atomic lines chosen for inversion. We report new magnetic maps of $\alpha^2$ CVn created using strong iron lines (directly comparable to the published MuSiCoS maps), weak iron lines and chromium lines, all of which yield a magnetic field structure roughly consistent with that obtained previously. We then derive an updated magnetic structure map for $\alpha^2$ CVn based on the complete sample of Fe and Cr lines, which we believe to produce a more representative model of the magnetic topology of $\alpha^2$ CVn. In agreement with the previous mapping, this new updated magnetic map shows a dipolar-like field which has complex sub-structure which cannot be explained by a simple low order multipolar geometry. Our new maps show that regardless of the atomic line or species choice, the reconstructed magnetic field is consistent with that published previously, suggesting that the reconstructed field is a realistic representation of the magnetic field of $\alpha^2$ CVn. $\alpha^2$ CVn is the first Ap star for which multiple, high resolution magnetic maps have been derived, providing important observational evidence for the stability of both the large and small-scale magnetic field.

Structure Related Magnetic Dead Layer for Ultrathin Fe/Ir(111) Films

Magnetic properties and surface structure of ultrathin Fe/Ir(111) films have been investigated using the surface magneto-optic Kerr effect and low-energy electron diffraction. Layer-by-layer growth of Fe/Ir(111) is observed for the first three monolayers at room temperature. For Fe thinner than three monolayers, pseudomorphic growth of Fe films is observed. The layer distance is close to that of fcc(111) Fe. For Fe thicker than three monolayers, the surface structure can be identified to be related to the bcc(110) arrangement of Fe atoms in Kurdjumov-Sachs orientation. As the Fe thickness increases, the linear increase of the Kerr intensity is observed. The Kerr intensity comes from the bcc-Fe and a thin magnetic dead layer is observed at the interface.

Internal Magnetic Configuration Measured by ECE Imaging on EAST Tokamak

ECE imaging (electron cyclotron emission imaging) is an important diagnostic which can give 2D imaging of temperature fluctuation in the core of tokamak. A method based on ECE imaging is introduced which can give the information of the position of magnetic axis and the structure of internal magnetic surface for EAST tokamak. The EFIT equilibrium reconstruction is not reliable due to the absence of important core diagnostic at the initial phase for EAST, so the information given by ECE imaging could help to improve the accuracy of EFIT equilibrium reconstruction.

Magnetic exchange forces andd-state filling: Antiferromagnetic MnO(001) and NiO(001) surfaces

Magnetic exchange force microscopy (MExFM) is one of the most important methods to investigate the spin structure of magnetic surfaces. However, a deep understanding of the measured spin contrast and the spin-dependent forces is missing. For the prototypical antiferromagnetic MnO(001) and NiO(001) surfaces probed by ferromagnetic Fe tips we demonstrate by means of spin-polarized density functional theory that the differences in the tip-surface interaction are due to the distance-dependent occupation of the transition-metal $3d$ states. While agreement is achieved with recent MExFM measurements of NiO(001), we predict opposite spin-contrast for MnO(001).

Radio pulsar activity and the crustal Hall drift

Models of pulsar radio emission that are based on an inner accelerating region require the existence of very strong and small scale surface magnetic field structures at or near the canonical polar cap. The aim of this paper is to identify a mechanism that creates such field structures and maintains them over a pulsar's lifetime. The likely physical process that can create the required 'magnetic spots' is the Hall drift occurring in the crust of a neutron star. It is demonstrated, that the Hall drift can produce small scale strong surface magnetic field anomalies (spots) on timescales of $10^4$ years by means of non-linear interaction between poloidal and toroidal components of the subsurface magnetic field. These anomalies are characterized by strengths of about $10^{14}$ G and curvature radii of field lines of about $10^6$ cm, both of which are fundamental for generation of observable radio emission.

Validation of single-fluid and two-fluid magnetohydrodynamic models of the helicity injected torus spheromak experiment with the NIMROD code

We present a comparison study of 3-D pressureless resistive MHD (rMHD) and 3-D presureless two-fluid MHD models of the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI). HIT-SI is a current drive experiment that uses two geometrically asymmetric helicity injectors to generate and sustain toroidal plasmas. The comparable size of the collisionless ion skin depth di to the resistive skin depth predicates the importance of the Hall term for HIT-SI. The simulations are run with NIMROD, an initial-value, 3-D extended MHD code. The modeled plasma density and temperature are assumed uniform and constant. The helicity injectors are modeled as oscillating normal magnetic and parallel electric field boundary conditions. The simulations use parameters that closely match those of the experiment. The simulation output is compared to the formation time, plasma current, and internal and surface magnetic fields. Results of the study indicate 2fl-MHD shows quantitative agreement with the experiment while rMHD only captures the qualitative features. The validity of each model is assessed based on how accurately it reproduces the global quantities as well as the temporal and spatial dependence of the measured magnetic fields. 2fl-MHD produces the current amplification ItorIinj and formation time τf demonstrated by HIT-SI with similar internal magnetic fields. rMHD underestimates ItorIinj and exhibits much a longer τf. Biorthogonal decomposition (BD), a powerful mathematical tool for reducing large data sets, is employed to quantify how well the simulations reproduce the measured surface magnetic fields without resorting to a probe-by-probe comparison. BD shows that 2fl-MHD captures the dominant surface magnetic structures and the temporal behavior of these features better than rMHD.

Unwinding of a Skyrmion Lattice by Magnetic Monopoles

Skyrmion crystals are regular arrangements of magnetic whirls that exist in a wide range of chiral magnets. Because of their topology, they cannot be created or destroyed by smooth rearrangements of the direction of the local magnetization. Using magnetic force microscopy, we tracked the destruction of the skyrmion lattice on the surface of a bulk crystal of Fe(1-x)Co(x)Si (x = 0.5). Our study revealed that skyrmions vanish by a coalescence, forming elongated structures. Numerical simulations showed that changes of topology are controlled by singular magnetic point defects. They can be viewed as quantized magnetic monopoles and antimonopoles, which provide sources and sinks of one flux quantum of emergent magnetic flux, respectively.

Magnetic field topology of the RS CVn star II Pegasi

The dynamo processes in cool active stars generate complex magnetic fields responsible for prominent surface stellar activity and variability at different time scales. For a small number of cool stars magnetic field topologies were reconstructed from the time series of spectropolarimetric observations using the Zeeman Doppler imaging (ZDI) method. In this study we follow a long-term evolution of the magnetic field topology of the RS CVn binary star II Peg. We collected high-resolution circular polarisation observations of II Peg using the SOFIN spectropolarimeter at the Nordic Optical Telescope. These data cover 12 epochs spread over 7 years. A multi-line diagnostic technique in combination with a new ZDI code is applied to interpret these observations. Magnetic inversions using these data reveals evolving magnetic fields with typical local strengths of 0.5-1.0 kG and complex topologies. Despite using a self-consistent magnetic and temperature mapping technique, we do not find a clear correlation between magnetic and temperature features in the ZDI maps. Neither do we confirm the presence of persistent azimuthal field rings found in other RS CVn stars. Reconstruction of the magnetic field topology of II Peg reveals significant evolution of both the surface magnetic field structure and the extended magnetospheric field geometry. From 2004 to 2010 the total field energy drastically declined and the field became less axisymmetric. This also coincided with the transition from predominantly poloidal to mainly toroidal field topology. A qualitative comparison of the ZDI maps of II Peg with the prediction of dynamo theory suggests that the magnetic field in this star is produced mainly by the turbulent alpha^2 dynamo rather than the solar alphaOmega dynamo. Our results do not show a clear active longitude system, nor is there an evidence of the presence of an azimuthal dynamo wave.

THE EFFECT OF LIMITED SPATIAL RESOLUTION OF STELLAR SURFACE MAGNETIC FIELD MAPS ON MAGNETOHYDRODYNAMIC WIND AND CORONAL X-RAY EMISSION MODELS

We study the influence of the spatial resolution on scales of $5\deg$ and smaller of solar surface magnetic field maps on global magnetohydrodynamic solar wind models, and on a model of coronal heating and X-ray emission. We compare the solutions driven by a low-resolution Wilcox Solar Observatory magnetic map, the same map with spatial resolution artificially increased by a refinement algorithm, and a high-resolution Solar and Heliospheric Observatory Michelson Doppler Imager map. We find that both the wind structure and the X-ray morphology are affected by the fine-scale surface magnetic structure. Moreover, the X-ray morphology is dominated by the closed loop structure between mixed polarities on smaller scales and shows significant changes between high and low resolution maps. We conclude that three-dimensional modeling of coronal X-ray emission has greater surface magnetic field spatial resolution requirements than wind modeling, and can be unreliable unless the dominant mixed polarity magnetic flux is properly resolved.

Spin reorientation transitions and structures of electrodeposited Ni/Cu(100) ultrathin films with and without Pb additives

Magnetic properties and surface structures of Ni/Cu(100) ultrathin films are studied by means of magneto-optical Kerr effect and in situ scanning tunneling microscopy in combination with cyclic voltammetry. At the initial stage of Ni deposition on a Cu(100) electrode, nickel atoms attach onto the steps and the surface shows single atomic steps corresponding to a layer-by-layer growth. For thicker Ni/Cu(100) films, nanometer-size clusters are randomly distributed on the surface showing a three-dimensional island growth. For thinner Ni layers in the coherent region, the magnetic anisotropy energy of the Cl-electrolyte/Ni interface is small. The reduction of squareness of the hysteresis loops is related to the inhomogeneous growth of the Ni layers. For thicker Ni layers in the incoherent region, the negative value of interface anisotropy for the Cl-electrolyte/Ni interface has a strong impact on perpendicular magnetic anisotropy and plays an important role on the reduction of the Ni thickness for spin reorientation transition in the electrolyte condition. By adding Pb additives, the deposition of a Pb wetting layer causes a defaceting phenomenon and the hydrogen evolution reaction is reduced. As the Ni thickness increases, the growth of Ni changes from layer-by-layer to quasi-two-dimensional islands with a flat top layer. With a Pb additive, the spin reorientation transitions of the Ni/Cu(100) system are not significantly influenced. However, due to the change of the growth mode by Pb atoms as a surfactant, the squareness of the hysteresis loops is enhanced for all the Ni thicknesses.

Stokes IQUV mapping of α2CVn & other Ap stars using ESPaDOnS and NARVAL

New spectral line polarisation observations of 7 bright Ap stars have been obtained with the ESPaDOnS and Narval high resolution spectropolarimeters (Silvester et al. 2012). The aim of this data set is produce a series of surface magnetic field and surface chemistry maps for these Ap stars. We present new magnetic maps for the Ap star α2 CVn using these new data and the MDI inversion code INVERS10. α2 CVn is the first Ap star to be observed during two separate epochs using high resolution phase resolved spectropolarimetric IQUV observations and as such allows us an insight into how stable the surface magnetic structure is over a decade timescale. We show that the new maps give a magnetic field structure consistent with the previous maps obtained by Kochukhov and Wade (2010) from lower quality MuSiCoS spectra taken a decade ago and that the field topology cannot be described by a dipolar or quadrupolar field.

Ab initio study of the surface properties of austenitic stainless steel alloys

Using ab initio calculations we investigated the surface energies of paramagnetic Fe1−c−nCrcNin random alloys within the concentration range of 0.12≤c≤0.32 and 0.04≤n≤0.32. These alloys crystallize mainly in the face centred cubic (fcc) structure and constitute the main building blocks of austenitic stainless steels. It is shown that all alloys have the lowest surface energies along the most close packed crystal orientation, namely the fcc (111) surfaces. The amount of Ni seems to have little effect on the surface energy, while almost all composition-driven change may be attributed to the changes in the Cr content. Within the studied compositional range, the change of the surface energy with the composition is of the order of 10%. Trends of the surface energy can be related to the magnetic structure of surfaces. Using the total energy as a function of the concentration, we determine the effective chemical potentials in bulk and at the surface, which can be used to estimate the surface segregation energies.

Surface and internal magnetic domain structures of Fe-Ga alloy rods

The magnetic domain structures of the 〈100〉 oriented Fe81Ga19 polycrystalline alloys with rapid quenching (RQ) and post-annealing (PA) thermal treatments are investigated by using magnetic force microscopy (MFM). The surfaces of the RQ alloy rods take on the dendritic domains after undergoing a standard mechanical polishing. While after PA processing, the distinct domain structures are observed at different temperatures. The wide stripe-like domain patterns appear in the surface of those PA rods at 550°C while the ramous domains remain at 400°C annealing. X-ray diffraction patterns indicate that a thin iron layer is formed on the surface of the specimens annealed at higher temperature. The soft magnetic iron layer, actually acting as a shield for the stray fields emerging from the internal magnetic structures underneath, brings about the presentation of the wide internal domains.

Atomic arrangement and magnetism of iron silicide on Fe(1 0 0) surface

We investigated surface magnetic structure in the initial stage of Si deposition on an Fe(100) surface by spin-polarized ion scattering spectroscopy (SP-ISS). We found silicidation at the Si/Fe interface after Si deposition followed by annealing at 823K. The silicidation occurs by the incorporation of silicon into the Fe substrate via the substitutional site of bcc Fe. After annealing, the incorporated Si atoms are distributed in surface layers several nanometers thick. The SP-ISS analysis revealed that the average magnetic moment of Fe in the silicide surface layer is about 70 % of that of Fe in the Fe(100) surface layer, whereas that of Si is almost zero. These surface magnetic moments are discussed in terms of the local magnetic environment. It is likely that the outermost surface of the silicide layer has an atomic arrangement similar to that of Fe3Si(100) with surface termination by the Fe–Si plane.

Giant enhancement in ferromagnetic properties of Pd nanoparticle induced by intentionally created defects

The important central question related to origin of ferromagnetic properties in the non-magnetic materials at nano-dimensions has been investigated by a novel approach of studying the evolution of magnetic properties by intentionally creating defects. The ferromagnetic response of Pd is found to increase by 20 times for nanoparticle (NP) dispersed in carbon matrix and increase by about 3.5 times in case of Pd nanoparticles dispersed in SiO2 matrix on exposure to swift heavy ion irradiation. Ferromagnetic response is found to increase by about 9.3 times on subjecting the Pd nanoparticles to hydrogen loading-deloading cycle. Ferromagnetic properties of Pd nanoparticles dispersed in carbon and SiO2 matrices, despite having same size and concentration, are observed to be vastly different due to matrix effect. These changes in ferromagnetic properties are correlated to the change in the electronic structure due to matrix, nanoparticle size, and creation of defects in the nanoparticle core and at NP-matrix interface during post deposition treatments. Giant enhancement in the magnetic properties and change in electronic properties point toward a core and surface magnetic structure in metal nanoparticle.

A versatile ray-tracing code for studying rf wave propagation in toroidal magnetized plasmas

A new ray-tracing code named C3PO has been developed to study the propagation of arbitrary electromagnetic radio-frequency (rf) waves in magnetized toroidal plasmas. Its structure is designed for maximum flexibility regarding the choice of coordinate system and dielectric model. The versatility of this code makes it particularly suitable for integrated modeling systems. Using a coordinate system that reflects the nested structure of magnetic flux surfaces in tokamaks, fast and accurate calculations inside the plasma separatrix can be performed using analytical derivatives of a spline-Fourier interpolation of the axisymmetric toroidal MHD equilibrium. Applications to reverse field pinch magnetic configuration are also included. The effects of 3D perturbations of the axisymmetric toroidal MHD equilibrium, due to the discreteness of the magnetic coil system or plasma fluctuations in an original quasi-optical approach, are also studied. Using a Runge–Kutta–Fehlberg method for solving the set of ordinary differential equations, the ray-tracing code is extensively benchmarked against analytical models and other codes for lower hybrid and electron cyclotron waves.

A permalloy zigzag structure based magnetic bio-sensor

A magnetic fluid consisting of Fe3O4 magnetic nanoparticles is embedded inside cells by intracellular uptake. A micro-fabricated magnetic zigzag-shaped surface structure is studied for use as a biosensor. We have developed a MOKE magnetometer based methodology to measure the different hysteresis loop signals between cells with and without being placed on zigzag sensors. Adding the magnetic cells on the structure decreases the coercivity from the magneto-optical Kerr effect (MOKE) signal of zigzag magnetic thin films because of the magnetic properties of superparamagnetic nanoparticles. The magnetoresistance measurement observed is that the switching fields of the zigzag structure with magnetic cells are significantly increased compared to the case without cells in the hard axis of the external field applied.

Resonant photoelectron diffraction with circularly polarized light

Resonant angle scanned x-ray photoelectron diffraction (RXPD) allows the determination of the atomic and magnetic structure of surfaces and interfaces. For the case of magnetized nickel the resonant L2 excitation with circularly polarized light yields electrons with a dichroic signature from which the dipolar part may be retrieved. The corresponding L2MM and L3MM Auger electrons carry different angular momenta since their source waves rotate the dichroic dipole in the electron emission patterns by distinct angles.

Microscopic investigation of surface and interfacial magnetic domain structure of Fe–NiO(1 0 0) system

We report our experimental observation of microscopic modification, reconstruction and evolution of the antiferromagnetic domain structure of the NiO(1 0 0) surface in view of the exchange bias effect. Some domain patterns (called non-equilibrium domains) as observed on the as-cleaved surface do not follow the well-known bulk symmetry traces on the (1 0 0) surface. But, bulk-terminated domains (called equilibrium domains) are found to be renucleated up on cooling the sample from above the Néel temperature, unless domains are strongly pinned by crystalline defects. We also observe certain domain evolution after annealing the crystal at various temperatures above the Néel temperature. Influence of growth condition on the ferromagnetic domain structure of the Fe film deposited on NiO(1 0 0) has been followed in situ. While for thin film case (7 ML), no influence of growth has been observed, for thicker film (21 ML) the Fe domain structure is found to be determined by both growth-induced and exchange anisotropy. Thus, our observations depict a thickness-dependent interplay between growth-induced and exchange anisotropy in ferromagnetic film grown on the antiferromagnetic substrate.

ChemInform Abstract: Magnetic Surface Nanostructures

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