Permalloy Rings Research Articles

Vortex state in magnetic rings

Recent studies on cobalt and permalloy rings have shown that a totally flux-closed magnetic vortex structure is stable at remanence. The two chiralities of the vortex, clockwise and anti-clockwise, have been proposed as the carriers for the stored information that could be read in a magneto-resistance-based device. Here, we present the results of systematic characterization of arrays of micron size narrow permalloy ring elements of different symmetry with magnetic force microscopy (MFM) to determine the magnetic patterns inside the rings and to visualize the reversal process during a magnetization cycle. Those experimental data are in a good agreement with the results of micromagnetic calculations.

Fabrication of nanoscale magnetic ring structures and devices

We have fabricated nanoscale ferromagnetic rings and radially contacted magnetic ring structures using electron beam lithography with a lift-off process for pattern transfer. The resist thickness and electron beam dose were optimised to produce 10 nm-thick permalloy rings with linewidths of 30 nm and outer diameters down to 90 nm. Magneto-optical Kerr effect measurements show that down to the smallest diameters, reproducible magnetic switching into the flux-closure vortex state is maintained. For the radially contacted ring devices, magnetic rings with linewidths of 100 and 200 nm (outer diameters of 1.1 and 1.2 μm) were produced and aluminium contacts added using an electron beam writer overlay procedure. The position of domain walls could be determined by measuring the magnetoresistance between different contacts and so the details of the magnetic switching could be ascertained.

Control of magnetization states in microstructured permalloy rings

Magnetization processes of microstructured NiFe rings are studied by the fringe-field-induced local Hall effect and numerical model calculations. The changes in reversible and irreversible magnetization of single rings are detected with very high resolution. We observe that the type of magnetic transition depends on the ratio between the inner and outer ring diameter. For narrow rings, sharp transitions from so-called “onion” to the “vortex” state are observed. In rings with smaller inner diameter, the transitions are more complex. The creation of local vortices and their spatial movement by applying an external magnetic field are detected.

Hall magnetometry on a ferromagnetic nanoring

We have integrated an individual nanostructured permalloy ring on a Hall sensor and studied the switching processes in a temperature regime between 4.2 and 90 K in in-plane magnetic fields. The nanoring has an outer diameter of 550 nm. In the ring with a small width of 125 nm we find a characteristic two-step switching process in the Hall traces. We show that the switching fields change if we vary the direction of the in-plane field or the temperature. We attribute this to lateral inhomogeneities in the nanoring.

Domain wall motion induced by spin polarized currents in ferromagnetic ring structures

We present an experimental study of the influence of spin-polarized currents on the displacement of domain walls in submicrometer permalloy ring structures. Using magnetoresistance (MR) measurements with multiple nonmagnetic contacts, we can sense the displacement of a domain wall and, by injecting large dc current densities (1011 A/m2), we can increase or decrease the magnetic field needed to move a single domain wall, depending on the direction of the current with respect to the applied field direction. Using rings with and without notches and by measuring the MR with the magnetic field applied along different directions, we show that we can exclude the possibility that the dominating effect is a classical Oersted field. We conclude that our observations can be explained by a directional spin torque effect.

Construction of hysteresis loops of single domain elements and coupled permalloy ring arrays by magnetic force microscopy

Magnetic structure and magnetization reversal of permalloy ring arrays and elongated permalloy particle array were studied by magnetic force microscopy (MFM). For single domain permalloy particles, the hysteresis loop is constructed by counting the percentage of switched elements imaged at remanence. For permalloy ring elements, two different states are energetically stable: a vortex state and an onion state. Their hysteresis loop is obtained by MFM imaging at a field between the switching fields of these two states. The magnetostatic coupling among these ring elements is directly revealed.

Domain wall pinning and controlled magnetic switching in narrow ferromagnetic ring structures with notches (invited)

We present a magnetoresistance study of controlled spin switching and domain wall pinning effects in mesoscopic narrow ferromagnetic Permalloy ring structures containing notches. It is found that notches create an attractive potential well for the transverse domain walls present in these structures. The strength and size of this potential well is measured and found to stretch far beyond the physical dimensions of the notch. Measuring the resistance, a minimum at remanence is found to occur when a transverse domain wall is pinned at the notch, in agreement with the results of numerical simulations of the anisotropic magnetoresistance. Applying a field in the direction corresponding to a potential well edge, a magnetic state with a very wide domain wall is stabilized, leading to a dip in the magnetoresistance at such angles. Using notches of different sizes, it is demonstrated that controlled switching is achieved, which means that the circulation direction of the vortex state can be selected.

Metastable states during magnetization reversal in square permalloy rings

The magnetic reversal process in a two-dimensional array of permalloy square rings is presented. Rings of thickness of 25 nm, of lateral size of 2.1 \ensuremath{\mu}m, and with ring width of 240 nm were microfabricated using electron-beam lithography and lift-off techniques. Analysis of the diffracted magneto-optical Kerr effect hysteresis loops, magnetic force microscopy images, and micromagnetic simulations show that the magnetization reversal path depends on the direction of the in-plane applied magnetic field. On reducing the field from saturation, for fields along an edge or a diagonal of the square, the ``onion'' state is the stable state at remanence. In a narrow field range around reversal we find that the reversal occurs via a metastable intermediate state. For fields along the diagonal this intermediate state is a magnetic vortex. When the field is applied along an edge direction the intermediate state is a ``horseshoe'' state.

Influence of vortex domain walls on magnetoresistance signals in Permalloy rings

The magnetization reversal and the corresponding magnetoresistance of Permalloy rings are investigated here both numerically and experimentally. Micromagnetic simulations reveal that during the reversal process there exist three intermediate metastable states which lead to an unconventional triple-switching characteristic. Size effects are studied as well. The simulation results are in good agreement with our magnetoresistance measurements. Significant differences in the magnetoresistance at two orthogonal directions of the magnetic fields are observed and explained. An important contribution to the magnetoresistance due to the relative position of the domains and the current leads is also investigated.

Domain wall pinning in narrow ferromagnetic ring structures probed by magnetoresistance measurements.

We present a magnetoresistance study of magnetization reversal and domain wall pinning effects in a mesoscopic narrow ferromagnetic Permalloy ring structure containing notches. The size and strength of the attractive pinning potential created by a notch is measured and the resistance minimum at remanence is found to occur when a single transverse domain wall is pinned at the notch, in agreement with the results of numerical simulations of the anisotropic magnetoresistance. When a field is applied in the direction corresponding to a potential well edge, a novel magnetic state with a very wide domain wall is stabilized, giving rise to a characteristic signature in the magnetoresistance at such angles.

Magnetization reversal in nanostructured permalloy rings

Three different stable magnetization configurations of soft-type nanostructured permalloy rings are investigated in detail by micromagnetic simulation. During the reversal process, the abrupt change of the hysteresis loop corresponds to the evolution of these states. Beside the quasiuniform and circular states, we find a new pair-vortices state. We compare the magnetic pole density distributions transformed from the simulation patterns with the magnetic force microscopic (MFM) images and find very good agreement between them.

Controlled magnetic switching in single narrow rings probed by magnetoresistance measurements

We present anisotropic magnetoresistance measurements of magnetic switching processes in narrow ferromagnetic permalloy rings fabricated with six nonmagnetic electrical contacts. We demonstrate that measuring the resistance between different contacts allows the determination of the domain wall positions. Furthermore, these measurements also yield the possibility of determining the local switching fields of different parts of the ring. This provides a very useful tool to explore the complete switching process of a single ring. We show that, by using notches of suitable size and by applying fields along appropriate directions, it is possible to select the circulation direction of the vortex state using a uniform field only.