D. Morecroft

Mesoscopic thin-film magnetic rings (invited)

The magnetic properties and magnetoresistance of thin-film circular and elliptical magnetic rings made from Co, NiFe, NiFe∕FeMn, and Co∕Cu∕NiFe have been explored. Single-layer rings show stable onion and vortex states and metastable twisted states containing a 360° wall. For NiFe rings, four-point magnetotransport results can be explained quantitatively by anisotropic magnetoresistance. NiFe∕FeMn exchange-biased rings show offset hysteresis loops, and the easy axis is determined by a combination of the ring ellipticity and the exchange coupling. In Co∕Cu∕NiFe multilayer rings the behavior is dominated by the magnetostatic coupling between the domain walls in the Co and NiFe. In the major loop the giant magnetoresistance varies between three distinct levels corresponding to combinations of onion and vortex states in the NiFe and Co layers.

Magnetism in multilayer thin film rings

The magnetic and magnetoresistive behaviour of circular and elliptical thin film multilayer rings is discussed. Rings are particularly interesting because they can adopt several stable and metastable magnetic states characterized by different numbers of domain walls. Electrically contacted rings made of Co/Cu/NiFe or IrMn/Co/Cu/NiFe, with diameters of ~0.5–5 µm and widths of ~100 nm were fabricated by electron-beam lithography, sputtering and liftoff processing. We show that multilayer rings switch their magnetization by a qualitatively different mechanism compared with that of a single-layer ring; how the magnetization circulation direction around the ring can be controlled; how the rings can be electrically contacted to show large fractional field-induced changes in resistance and how these structures may be used in magnetic random access memories or magnetic logic devices.

Magnetization reversal in single-layer and exchange-biased elliptical-ring arrays

Arrays of elliptical rings with long axis of 3μm, short axis of 1.8μm, and widths of 400nm and above were fabricated by zone-plate-array lithography and lift-off processing. Hysteresis loops of NiFe elliptical rings with different widths indicate the “vortex” state is more stable for narrower rings. Micromagnetic simulations reveal that the magnetization configurations of wider rings are more complicated than that of narrower ones. Elliptical-ring arrays fabricated from exchange-biased thin film structures display shifted hysteresis loops and the same width dependence of the stability of the vortex state.

Reading and writing of vortex circulation in pseudo-spin-valve ring devices

The authors present a simple method of reading the circulation direction of vortex states in pseudo-spin-valve ferromagnetic ring devices via magnetoresistance measurements. It is shown that by placing the current contacts asymmetrically onto the structure, the circulation of a vortex state in the hard layer may be read directly from the total resistance of the device. Furthermore, they show that by choosing the direction in which the ring is initially saturated prior to obtaining the vortex state, the vortex circulation may be selectively written to the structure, creating the basis of a working memory element.

Influence of contact geometry on the magnetoresistance of elliptical rings

Room temperature magnetotransport measurements have been carried out on NiFe single layer and NiFe∕Cu∕Co∕Au multilayer elliptical rings. The shape of the magnetoresistance response is strongly dependent on the contact configuration and the direction of the applied field with respect to the easy axis of the ellipse. The magnetization states and magnetoresistance can be quantitatively modeled.

Angular dependence of the giant magnetoresistance in multilayer rings with different contact configurations

We report on the effects of electrical contact configuration on the giant magnetoresistance (GMR) response of NiFe∕Cu∕Co pseudo-spin-valve elliptical rings. Room temperature GMR measurements show different approaches to remanence, depending on the position of the voltage leads with respect to the curvature of the rings. Computational analysis of micromagnetic simulations reproduces these experimental trends, which result from changes in the relative alignment between the NiFe and Co layers in different regions of the rings.

Magnetoresistance of submicrometre multilayer Wheatstone bridges as a probe of magnetic reversal mechanism

The magnetoresistance of elliptical NiFe/Cu/Co multilayer rings with deep submicrometre widths contacted in a Wheatstone bridge configuration has been characterized. The rings show large voltage signals corresponding to magnetic configurations in which either the Co or the NiFe layer is in a vortex state. This result indicates that asymmetrically placed 360° domain walls can be present in the vortex states attained during the reversal of the magnetic layers. Minor loop cycling generates a range of possible remanent resistance levels in the rings, which makes these devices interesting candidates for multiple-bit-per-cell magnetic random access memory or for non-volatile programmable logic devices.

Study of the size dependence of exchange bias using in situ magnetoresistance measurements

Due to the general trend towards miniaturization of magnetic devices, it is important to understand the size dependence of exchange bias. An exchange bias spin valve structure of the form Ta∕NiFe∕Cu∕NiFe∕IrMn∕Ta was used with the IrMn exchange bias layer at the top, which enabled in situ magnetoresistance measurements to be carried out as the antiferromagnetic (AFM) layer was gradually milled away in an argon ion miller. The MR decreased when the thickness was reduced below 3 nm. Optical and focused ion beam (FIB) lithography were used to micro and nano pattern wire arrays in the IrMn layer.

Moment Selective Digital Detection of Single Magnetic Beads for Multiplexed Bioassays

Research into lab‐on‐a‐chip multiplexed bioassays has focused on libraries of biochemical probes, indexed by optically encoded micron‐sized labels. However, few current methods have reconciled large multiplexing capability with a rapid detection system amenable to miniaturization. Magnetic identification of labels provides a strong candidate solution to this problem, yet no proposed single‐label magnetic detection system can both read and encode magnetic labels. We present a magnetic multiplexed assay in lab‐on‐a‐chip format which identifies target biomolecules from the hybridization results by reading encoded magnetic beads. We show that a microfabricated magnetoresistive ring‐shaped sensor can read the magnetic moments of individual commercially available paramagnetic beads using an active digital technique. This work provides proof of principle for a new approach to magnetic labeling of biomolecules for high‐throughput bioassays.