Onion State Research Articles

Magnetic vortices in nanocaps induced by curvature

Magnetic nanoparticles with room temperature remanent magnetic vortices stabilized by their curvature are very intriguing due to their potential use in biomedicine. In the present study, we investigate room temperature magnetic chirality in 100 nm diameter permalloy spherical caps with 10 nm and 30 nm thicknesses. Micromagnetic OOMMF simulations predict the equilibrium spin structure for these caps to form a vortex state. We fabricate the permalloy caps by sputtering permalloy on both close-packed and sparse arrays of polystyrene nanoparticles. Magnetic force microscopy scans show a clear signature of a vortex state in close-packed caps of both 10 nm and 30 nm thicknesses. Alternating gradient magnetometry measurements of the caps are consistent with a remnant vortex state in 30 nm thick caps and a transition to an onion state followed by a vortex state in 10 nm thick caps. Out-of-plane measurements supported by micromagnetic simulations shows that an out-of-plane field can stabilize a vortex state down to a diameter of 15 nm.

Nanomorphology Effects in Semiconductors with Native Ferromagnetism: Hierarchical Europium (II) Oxide Tubes Prepared via a Topotactic Nanostructure Transition.

Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unknown. This lack impedes the exploration of a new class of materials characterized by a direct combination of effects on the electronic system caused by quantum confinement effects with magnetism. A good example is EuO for which currently no reliable routes for nanoparticle synthesis can be established. Bottom-up approaches applicable to other oxides fail because of the labile oxidation state +II. Instead of targeting a direct synthesis, the two steps-"structure control" and "chemical transformation"-are separated. The generation of a transitional, hybrid nanophase is followed by its conversion into EuO under full conservation of all morphological features. Hierarchical EuO materials are now accessible in the shape of oriented nanodisks stacked to tubular particles. Magnetically, the coupling of either vortex or onion states has been found. An unexpected temperature dependence is governed by thermally activated transitions between these states.

Micromagnetic simulation study of magnetization reversal in torus-shaped permalloy nanorings

Using micromagnetic simulation, the magnetization reversal of soft permalloy rings of torus shape with major radius R varying within 20–100 nm has been investigated. The minor radius r of the torus rings was increased from 5 nm up to a maximum value r[Formula: see text] such that R- r[Formula: see text] = 10 nm. Micromagnetic simulation of in-plane hysteresis curve of these nanorings revealed that in the case of very thin rings (r [Formula: see text] 10 nm), the remanent state is found to be an onion state, whereas for all other rings, the remanent state is a vortex state. The area of the hysteresis loop was found to be decreasing gradually with the increment of r. The normalized area under the hysteresis loops (A[Formula: see text]) increases initially with increment of r. It attains a maximum for a certain value of r = r0 and again decreases thereafter. This value r0 increases as we decrease R and as a result, this peak feature is hardly visible in the case of smaller rings (rings having small R).

Geometry induced phase transitions in magnetic spherical shell

Equilibrium magnetization states in spherical shells of a magnetically soft ferromagnet form two out-of-surface vortices with codirectionally magnetized vortex cores at the sphere poles: (i) a whirligig state with the in-surface magnetization oriented along parallels is typical for thick shells; (ii) a three dimensional onion state with the in-surface meridional direction of the magnetization is realized in thin shells. The geometry of spherical shell prohibits an existence of spatially homogeneous magnetization distribution, even in the case of small sample radii. By varying geometrical parameters a continuous phase transition between the whirligig and onion states takes place. The detailed analytical description of the phase diagram is well confirmed by micromagnetic simulations.

Magnetization switching process in a torus nanoring with easy-plane surface anisotropy

We have studied the effects of surface shape anisotropy in the magnetization behavior of a torus nanoring by means of Monte Carlo simulations. Stable states (vortex and reverse vortex states) and metastable states (onion and asymmetric onion states) were found in the torus nanoring. The probability of occurrence of the metastable states (stable states) tends to decrease (increase) as the amount of Monte Carlo steps per spin, temperature steps and negative values of the anisotropy constant increase. We evaluated under which conditions it is possible to switch the magnetic state of the torus nanoring from a vortex to a reverse vortex state by applying a circular magnetic field at certain temperature interval. The switching probability (from a vortex to a reverse vortex state) depends on the value of the current intensity, which generates the circular magnetic field, and the temperature interval where the magnetic field is applied. There is a linear relationship between the current intensity and the minimum temperature interval above which the vortex state can be switched.

Mapping magnetoelastic response of terfenol-D ring structure

The magneto-elastic response of a Terfenol-D (Tb.3Dy.7Fe1.92) ring has been experimentally investigated and analyzed. Ring structures give rise to complex behavior based on the interaction of the magnetic field with the material, which is further compounded with anisotropies associated with mechanical and magnetic properties. Discrete strain measurements were used to construct magnetostriction maps, which are used to elucidate the non-uniformity of the strain distribution due to geometrical factors and magnetic field interactions, namely, magnetic shielding and stable onion state in the ring structure.

Magnetization Reversal in Ferromagnetic Nanorings of Fourfold Symmetries

Magnetic nanoparticles offer a broad spectrum of magnetization reversal processes and respective magnetic states, such as onion, horseshoe, or vortex states as well as various states including domain walls. These states can be correlated with stable intermediate states at remanence, enabling new quaternary memory devices storing two bits in one particle. The stability of these intermediated states was tested with respect to shape modifications, variations in the anisotropy axes, and rotations and fluctuations of the external magnetic field. In our micromagnetic simulations, 6 different stable intermediate states were observed at vanishing magnetic field in addition to the remanence state. The angular region of approx. 5°–12° between nanoring and external magnetic field was identified as being most stable with respect to all modifications, with an onion state as technologically best accessible intermediate state to create quaternary memory devices.

Switching behavior and novel stable states of magnetic hexagonal nanorings

Micromagnetic simulations for Cobalt hexagonal shape nanorings show onion (O) and vortex state (V) along with new state named “tri-domain state”. The tri-domain state is observed in sufficiently large width of ring. The magnetic reversible mechanism and transition of states are explained with help of vector field display. The transitions from one state to other occur by propagation of domain wall. The vertical parts of hexagonal rings play important role in developing the new “tri-domain” state. The behaviors of switching fields from onion to tri-domain (HO-Tr), tri-domain to vortex state (HTr--V) and vortex to onion state and “states size” are discussed in term of geometrical parameter of ring.

Micromagnetic simulation of hysteresis loop of elliptic permalloy nanorings

Magnetic hysteresis behavior of isotropic permalloy elliptic nanorings of outer semi-major axis length [Formula: see text] 100 nm and thickness [Formula: see text] 20 nm were studied with respect to the variation of two parameters: outer semiminor axis length [Formula: see text] and the difference between outer and inner dimensions [Formula: see text]. The outer semiminor axis length [Formula: see text] varied from 90 nm to 20 nm which covers from nearly circular nanoring to elliptic nanoring of high aspect ratio. The value of [Formula: see text] varied in steps of 10 nm. Micromagnetic simulation of in-plane hysteresis curve of these nanorings revealed that the remanent state of all of these elliptic rings are onion states if the magnetic field is applied along the longer side of the elliptic rings. If the magnetic field is applied along the shorter side, then the remanent states turn out to be vortex state. The hysteresis loss indicated by area of the hysteresis loop was found to be decreasing gradually with the increment of either [Formula: see text] or [Formula: see text]. On the other hand, the remanent magnetization increased with increment of [Formula: see text] but decreased with the increment of [Formula: see text]. The changes were attributed to three parameters mainly: inner curvature, exchange energy and demagnetization energy. The changes in loop area were discussed in light of variation of these three parameters.

Dynamic susceptibility of onion in ferromagnetic elliptical nanoring

Micromagnetic simulation was performed to investigate the equilibrium state and dynamic susceptibility spectra of magnetic elliptical nanoring. There are two equilibrium states (onion and vortex) obtained in elliptical nanoring. The onion state can be used to record information in MRAM. And it is important to investigate the dynamic susceptibility spectra of onion state, which is closely related to writing and reading speed of magnetic memory devices. Those results show that two or three resonance peaks are found under different thickness of elliptical nanoring with onion state, respectively. The low resonance frequency of two resonance peaks is increasing with the arm width of the elliptical ring, but is decreasing with the thickness. However, the high frequency of two resonance peaks is decreasing with the arm width of the elliptical ring.

缺陷钴纳米环磁化动力学研究

利用Monte Carlo (蒙特卡洛)方法结合快速傅立叶变换，研究了不同缺陷环半径、不同缺陷位置的纳米环磁特性，模拟结果表明：纳米环系统缺陷程度较小时，其磁特性与对称纳米环系统接近；而当缺陷程度较大时，由于其几何形状与对称单纳米环相差甚远，系统的磁特性与对称纳米环系统差别较大。此外，研究还发现，缺陷程度较大时，系统虽保留了对称纳米环系统的主要特征(涡旋态和洋葱态)，但相比起来，涡旋态的稳定性更低，过渡状态增加，整体的磁化过程也更为复杂。 Based on the Monte-Carlo simulation and fast Fourier transformation micro-magnetism method, the magnetic properties of Co nanorings with different defect locations are studied. The simula-tion results show that the magnetic properties of the nanorings with small defect degree are similar to those of symmetric nanorings. With the defect degree increasing, the magnetic dynamic behavior of defect nanorings is obviously different from that of symmetric nanorings. The results also indicate that the defect system keeps the main magnetic characteristic of the symmetric nanorings (such as vortex state and onion state). Compared to the symmetric nanorings, the stability of defect nanorings is weak. Furthermore, with the defect degree increasing, the number of transition states in nanoring system increases, and magnetization process becomes complex.

Polarization dependent switching of asymmetric nanorings with a circular field

We experimentally investigated the switching from onion to vortex states in asymmetric cobalt nanorings by an applied circular field. An in-plane field is applied along the symmetric or asymmetric axis of the ring to establish domain walls (DWs) with symmetric or asymmetric polarization. A circular field is then applied to switch from the onion state to the vortex state, moving the DWs in the process. The asymmetry of the ring leads to different switching fields depending on the location of the DWs and direction of applied field. For polarization along the asymmetric axis, the field required to move the DWs to the narrow side of the ring is smaller than the field required to move the DWs to the larger side of the ring. For polarization along the symmetric axis, establishing one DW in the narrow side and one on the wide side, the field required to switch to the vortex state is an intermediate value.

Deterministic Switching of the Magnetization States in Cobalt Nanorings

We studied the magnetoresistance of symmetric cobalt nanorings with nanowires attached at diametrically opposite positions. Deterministic switching, induced by an external magnetic field between the stable magnetization states, is demonstrated using histograms of the resistance changes as a function of an external magnetic field. The changes of the switching fields were studied as the lateral dimensions of these elements were varied systematically. The field required to switch between the vortex state and the onion state of the rings decreases as the line widths of the elements were increased. The vortex state is unstable at the larger line widths. Our experimental observations are explained using micromagnetic simulations.

Confinement of Magnetic Vortex and Domain Walls in Dipolar-Coupled Concentric Nanocylinders

We report a theoretical study of the magnetic phases of core-shell nanocylinders, consisting of a Py cylindrical core, dipolar coupled to a coaxial Fe cylindrical shell. A few nanometers thick nonmagnetic cylindrical layer separates the core from the shell, and controls the magnitude of the core-shell dipolar interaction. New magnetic phases emerge from the dipolar interaction, and may consist of either the combination of the intrinsic magnetic phases or new phases that are not seen in isolated cylinders and shells. We discuss typical examples. The magnetic phases of a 21 nm-height nanocylinder composed of a 57 nm-diameter Py core coupled to a 12 nm-thick Fe shell may be set to be a Py vortex with the same chirality of the Fe shell circular state, or a Py uniform domain coupled to a pair of domain walls of the Fe shell onion state. A magnetic vortex may be stabilized in a 6 nm-height, 42 nm-diameter Py cylinder coupled to a 6 nm-thick Fe shell.

Torsion-induced effects in magnetic nanowires

Magnetic helix wire is one of the most simple magnetic systems which manifest properties of both curvature and torsion. There exist two equilibrium states in the helix wire with easy-tangential anisotropy: a quasi-tangential magnetization distribution in case of relatively small curvatures and torsions, and an onion state in opposite case. In the last case the magnetization is close to tangential one, deviations are caused by the torsion and curvature. Possible equilibrium magnetization states in the helix magnet with different anisotropy directions are studied theoretically. The torsion also essentially influences the spin-wave dynamics, acting as an effective magnetic field. Originated from the curvature induced effective Dzyaloshinskii interaction, this magnetic field leads to the coupling between the helix chirality and the magnetochirality, it breaks mirror symmetry in spin-wave spectrum. All analytical predictions on magnetization statics an dynamics are well confirmed by the direct spin lattice simulations.

Magnetic configurations in a Co/Cu/NiFe multilayered rhombic ring

We have studied the magnetization reversal of Co/Cu/NiFe giant-magnetoresistance (GMR) rhombic-ring structures by using electrical measurements and analytical calculations. GMR rhombic rings include magnetic domain walls (DWs) at the remanent state owing to their shape anisotropy. Therefore, the magnetization reversal evolves via DW motion caused by an external magnetic field. The rhombic ring has multiple intermediate states, such as vortex, hard-axis onion, and horseshoe states, making it difficult to recognize its specific magnetic configuration in the GMR measurements. Simple GMR calculations were employed to understand the magnetization reversal based on several prospective magnetic states, and the results were consistent with the experimental data.

Dynamic domain wall chirality rectification by rotating magnetic fields

We report on the observation of magnetic vortex domain wall chirality reversal in ferromagnetic rings that is controlled by the sense of rotation of a magnetic field. We use time-resolved X-ray microscopy to dynamically image the chirality-switching process and perform micromagnetic simulations to deduce the switching details from time-resolved snapshots. We find experimentally that the switching occurs within less than 4 ns and is observed in all samples with ring widths ranging from 0.5 μm to 2 μm, ring diameters between 2 μm and 5 μm, and a thickness of 30 nm, where a vortex domain wall is present in the magnetic onion state of the ring. From the magnetic contrast in the time-resolved images, we can identify effects of thermal activation, which plays a role for the switching process. Moreover, we find that the process is highly reproducible so that the domain wall chirality can be set with high fidelity.

Domain wall movement in a rhombic Co thin film ring

The magnetization switching of micron-scale thin film Co rhombic rings is studied by magnetic force microscopy and electrical measurements. The rhombic rings have a 1.4 μm/0.7 μm major/minor diameter with 200 nm width. A magnetic field ranged from 130 Oe to 200 Oe causes motion of domain walls (DWs) around the ring in a direction controlled by the magnetic field orientation. In addition to the well-known ‘onion’ and ‘vortex’ states, intermediate states such as horseshoe and hard-axis onion states were formed for certain field cycles, in agreement with micromagnetic simulations and anisotropic magnetoresistance calculations. DWs induce a 0.4% resistance change when the external magnetic field is cycled.

In-plane hysteresis of permalloy nanorings: a study of micromagnetic simulation

Magnetic hysteresis of isotropic permalloy nanorings with outer diameter 200 nm and thickness 20 nm has been studied. The inner diameter is varied from 0 to 190 nm to accommodate wide range of samples from nanodisk to thin nanorings. Micromagnetic simulation of in-plane hysteresis curve of these nanorings reveals that the magnetic properties change gradually with the change of inner diameter. The hysteresis loss indicated by the area of the hysteresis loop, increases gradually with the increase in inner radius up to d in = 174 nm. For inner diameter of 176 nm, the loop area decreases drastically and remains so for up to d in = 180 nm. After that, a small increment of d in results in a large increment of loop area. The remanent states are found to be vortex states for d in = 0–180 nm and onion states for d in > 180 nm. The changes are attributed to two parameters mainly: exchange energy and demagnetization energy. These two parameters depend on inner curvature of the ring, which is treated as a variable in this simulation work. The changes in loop area have been discussed in light of variation of these parameters.

Distortion of Magnetic Domain Wall Measured by Magneto-Resistance Changes in a Co Nanoring.

The electrical anisotropic magneto-resistance (AMR) measurements were performed to see the formation of a 360 degree magnetic domain wall (360 DW) and distortion of the magnetic moments in a Co nanoring structure. Since the 360 DW is consisted of two 180 degree DWs, a decrease of the resistance was found in the switching process from the vortex to reverse onion state by the AMR effects, which is consistent with micromagnetic simulations. In addition, a decrease of the resistance in the switching process from the onion to vortex state was observed by the distortion of the local magnetic moments due to an applied magnetic field. The stochastic behavior in the switching process is caused by thermally induced magnetic moments changes.