Vortex Chirality Research Articles

Effect of controlled asymmetry on the switching characteristics of ring-based MRAM design

It was suggested that the shape effects are responsible for the vortex formation and chirality in circular ring elements. We found that the introduction of well-controlled asymmetry into permalloy rings provides precise control of the switching mechanism and leads to improved switching characteristics.

Precise control of the vortex chirality in magnetic nano-rings with deliberately introduced asymmetry

Recent studies on Co and permalloy rings have shown that a totally flux-closed magnetic vortex state could be stable at remanence. The two chiralities of the vortex, clockwise and counterclockwise, have been proposed as the carriers for the stored information in MRAM devices. Our micromagnetic simulations demonstrated that asymmetry in the shape of the ring was responsible for the vortex formation and chirality.

Determination of magnetic vortex chirality using lateral spin-valve geometry

We demonstrate the determination of the vortex chirality using a nonlocal spin-valve measurement technique in a lateral spin valve consisting of a Permalloy (Py) disk 1μm in diameter and a Py wire 100nm in width. The vortex chirality is determined under two probe configurations; the disk as the current probe and the disk as the voltage probe. The vortex chirality is found to depend on the probe configuration because of the current-induced Oersted field.

Vortex-chirality control in mesoscopic disk magnets observed by photoelectron emission microscopy

We demonstrate the vortex-chirality control in mesoscopic disk magnets using photoelectron emission microscopy (PEEM). Micrometer-sized vortex-chirality-control devices of permalloy were designed and fabricated by electron-beam lithography and lift-off. The magnetic images were obtained by PEEM using the x-ray magnetic circular dichroism (XMCD) with circularly polarized synchrotron radiation. The XMCD-PEEM observation reveals that the vortex chirality in the designed devices was perfectly controlled by an external applied magnetic field.

Vortex chirality control in mesoscopic disk magnets observed by a newly developed mobile PEEM system

We have developed a compact and mobile photoelectron emission microscope (PEEM) system to probe the magnetic vortex structure in mesoscopic magnets. The spatial resolutions of the PEEM system are measured to be less than 60 nm with a mercury lamp and 130 nm with synchrotron radiation. The magnetization distribution patterns have been imaged using the X-ray magnetic circular dichroism (XMCD) technique with circularly polarized synchrotron radiation. The chirality of the vortices was successfully imaged. Micron-sized vortex-chirality-control magnetic devices were fabricated by electron beam lithography and lift-off techniques. The PEEM observation experiments reveal that the vortex chirality in designed devices was perfectly controlled by an external applied magnetic field.

Correlated vortex chiralities in interacting permalloy dot patterns

Magnetostatic interactions in different polycrystalline permalloy dot patterns for dots with diameter 900nm and thickness 30nm are investigated through magnetic force microscopy imaging, magneto-optic Kerr effect measurements, and micromagnetic simulations. Magnetization reversal occurs through vortex nucleation∕annihilation. Vortex nucleation and annihilation fields for different interacting arrays were observed to vary with pattern type and with the applied field direction and were shown to follow the prediction of a simple point-dipole model. The vortex chirality was established by imaging vortices in a small applied field. The vortex chirality distribution was determined for different arrays and a well-defined ordering phenomenon of the vortex chirality was observed in two-dot and zigzag-dot chain patterns as well as in a honeycomb array of dots. Micromagnetic simulations well account for the type of vortex chirality ordering observed and attribute it to magnetostatic interactions that induce correlated C-state magnetization configurations among nearest neighbor dots prior to vortex nucleation.

Physics of vortex core in chiral p-wave superconductor

We describe the essence of quantum effects inside s-wave and chiral p-wave vortices, without explicit use of quasiclassical Green function formalism. Physical quantities such as the impurity scattering rate and nuclear spin relaxation rate contain the coherence factor of the Andreev bound states in the matrix element of the transitions. The coherence factor of the Andreev bound state is different from that of a quasiparticle in bulk superconductors. Consequently, the physics within vortex core is different from that in normal state or that in spatially uniform superconducting state.

Relation Between Vortex Pinning Energy and Anderson's Theorem

We discuss the elementary vortex pinning in type-II superconductors in connection with the Anderson's theorem for nonmagnetic impurities. We address the following two issues. One is an enhancement of the vortex pinning energy in the unconventional superconductors. This enhancement comes from the pair-breaking effect of a nonmagnetic defect as the pinning center far away from the vortex core (i.e., the pair-breaking effect due to the non-applicability of the Anderson's theorem in the unconventional superconductors). The other is an effect of the chirality on the vortex pinning energy in a chiral p-wave superconductor. The vortex pinning energy depends on the chirality. This is related to the cancellation of the angular momentum between the vorticity and chirality in a chiral p-wave vortex core, resulting in local applicability of the Anderson's theorem (or local recovery of the Anderson's theorem) inside the vortex core.

Numerical Study of Impurity Effects on Quasiparticles within S-wave and Chiral P-wave Vortices

The impurity problems within vortex cores of two-dimensional s-wave and chiral p-wave superconductors are studied numerically in the framework of the quasiclassical theory of superconductivity and self-consistent Born approximation under a trial form of the pair potential. The dispersion and impurity scattering rate (the inverse of the relaxation time) of the Andreev bound state localized in vortex cores are deduced from the angular-resoloved local density of states. The energy dependence of the impurity scattering rates depends on the pairing symmetry; particularly, in the chiral p-wave vortex core where chirality and vorticity have opposite sign and hence the total angular momentum is zero, the impurities are ineffective and the scattering rate is vanishingly small. Owing to the cancellation of angular momentum between chirality and vorticity, the chiral p-wave vortex core is similar to locally realized s-wave region and therefore non-magnetic impurity is harmless as a consequence of Anderson's theorem. The results of the present study confirm the previous results of analytical study (J. Phys. Soc. Jpn. {\bf 69} (2000) 3378) in the Born limit.

Dynamic aggregation of chiral spinners.

An object spinning at the surface of a liquid creates a chiral vortex. If the spinning object is itself chiral, its shape modifies the characteristics of the vortex; interactions between that vortex and other vortices then depend on the chirality of the objects that produce them. This paper describes the aggregation of millimeter-sized, chiral magnetized plates floating at a liquid-air interface and rotating under the influence of a rotating external magnetic field. This external field confines all the plates at densities that cause the vortices they generate to interact strongly. For one set of plates investigated, plates of one chirality attract one another, and plates of the other chirality repel other plates of both chiralities.

Theory of chiral modulations and fluctuations in smectic-A liquid crystals under an electric field

Chiral liquid crystals often exhibit periodic modulations in the molecular director; in particular, thin films of the smectic-C* phase show a chiral striped texture. Here, we investigate whether similar chiral modulations can occur in the induced molecular tilt of the smectic-A phase under an applied electric field. Using both continuum elastic theory and lattice simulations, we find that the state of uniform induced tilt can become unstable when the system approaches the smectic-A-smectic-C* transition or when a high electric field is applied. Beyond that instability point, the system develops chiral stripes in the tilt, which induce corresponding ripples in the smectic layers. The modulation persists up to an upper critical electric field and then disappears. Furthermore, even in the uniform state, the system shows chiral fluctuations, including both incipient chiral stripes and localized chiral vortices. We compare these predictions with observed chiral modulations and fluctuations in smectic-A liquid crystals.

Finite-size-scaling analyses of the chiral ordert in the Josephson-junction ladder with half a flux quantum per plaquette

Chiral order of the Josephson-junction ladder with half a flux quantum per plaquette is studied by means of the exact diagonalization method. We consider an extreme quantum limit where each superconductor grain (order parameter) is represented by S=1/2 spin. So far, the semi-classical S -> \infty case, where each spin reduces to a plane rotator, has been considered extensively. We found that in the case of S=1/2, owing to the strong quantum fluctuations, the chiral (vortex lattice) order becomes dissolved except in a region, where attractive intrachain and, to our surprise, repulsive interchain interactions both exist. On the contrary, for considerably wide range of parameters, the superconductor (XY) order is kept critical. The present results are regarded as a demonstration of the critical phase accompanying chiral-symmetry breaking predicted for frustrated XXZ chain field-theoretically.

Supersolid phase in fully frustrated Josephson-junction arrays

We study the phase diagram and the excitation spectra of an array of small Josephson junctions at $f=1/2$ and arbitrary charge frustration. We find that the supersolid region in the phase diagram is larger than the correspondent region at $f=0$ and it includes two different phases.In the chiral supersolid (SS) charges and vortices are arranged in a checkerboard pattern on a $2\:\times \:2$ supercell analogously to the unfrustrated case. We find a new phase, which we term {\em non-chiral supersolid} (NCSS), which has no corresponding phase at $f=0$. The excitation spectra in the supersolid regions show particle like dispersions which is related to the defectons. The defecton condensation leads to superfluidity in the presence of charge ordered background.