B. G. Ng

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.

Current-driven vortex formation in a magnetic multilayer ring

Current-driven domain wall (DW) motion has been studied in the NiFe layer of a Co/Cu/NiFe thin film ring using giant-magnetoresistance measurements in a four-point contact geometry. The NiFe layer is initially in an onion state configuration with two 180° DWs. An electric current drives the walls around the ring so that they annihilate and the NiFe layer forms a DW-free vortex state. The direction of motion of the two DWs is determined by the current polarity, enabling the vortex chirality to be selected.

High-throughput combinatorial study of local stress in thin film composition spreads

We investigate the stresses in thin films with sub-millimeter lateral spatial resolution using a dense array of prefabricated cantilever beams prepared by microelectromechanical-system techniques. Stress induced deflection of the cantilever is interrogated by an optical (laser/position sensitive detector) measurement system. Composition spread films are deposited on the cantilever array using a three gun on-axis magnetron cosputtering system. The position dependent composition is inferred using rate calibrations and verified by electron microprobe/energy dispersive spectroscopy. We demonstrate the function of this system using an Fe-Ni-Al composition spread with approximately 1 at. % resolution. This approach allows for measurement of the composition dependence of other electromechanical properties such as the martensitic phase transition temperature of traditional and ferromagnetic shape-memory alloys, as well as the properties of hydrogen storage materials and the magnetic response of magnetostrictive materials.

Effect of magnetic field direction on the remanent resistance levels and vortex chirality of a multilayered magnetic ring

The effect of applied field direction on the magnetoresistance response of a 5 μm diameter Co/Cu/NiFe thin film ring has been examined. When the Co layer of the ring is placed in a vortex state, four possible remanent magnetization configurations exist, in which the NiFe layer is in one of four states: forward onion, reverse onion, clockwise vortex or counterclockwise vortex. The resistance levels of these four remanent states depend on the field angle with respect to the electrical contact leads, and measured values agree well with an electrical model. The chirality of the Co vortex can be determined from measurements of the minor loop, and the chirality of the NiFe vortex can be set using two-step field cycling at two different field angles.

Angular dependence of ferromagnetic resonance and magnetization configuration of thin film Permalloy nanoellipse arrays

In-plane and out-of-plane angular dependence of ferromagnetic resonance (FMR) and magnetization measurements were performed on arrays of 20, 40, and 60 nm thick, 520 nm long, and 250 nm wide elliptical Permalloy elements. Besides the main FMR “volume” mode resonance, a well-defined second FMR mode was observed, which exhibits a very strong angular dependence. This mode originates from localized regions where the magnetization has a strong component perpendicular to the bias field and to the volume magnetization. These regions of nonuniform magnetization may be associated with magnetization canting at the edges of the ellipses, due to the nonuniformity of the demagnetizing tensor elements, and with magnetization vortices, which are predicted by micromagnetic simulation.

360° domain wall mediated reversal in rhombic Co/Cu/NiFe magnetic rings

The reversal process of thin film micron-scale Co/Cu/NiFe rhombic rings in an in-plane magnetic field is investigated by micromagnetic simulation and magnetoresistance measurements. Simulations show that the impingement of reverse domains leads to the formation of multiple 360° domain walls in the ring during low-field cycling. Two types of reversal process can be identified experimentally which are attributed to the presence or absence of residual 360° domain walls in the ring. The reversal path depends on the field history, which affects the population of walls in the ring.

Electrical observation of asymmetric magnetization configurations in the vortex state of NiFe and Co rings

Anisotropic magnetoresistance (AMR) measurements have been used to probe the detailed reversal mechanism of 3??m diameter, 15?nm thick NiFe and Co rings. In the vortex state, small changes in the resistance are associated with distortion or buckling in the section of the ring magnetized antiparallel to the applied field, and the resistance changes can be similar in magnitude to the domain-wall resistance. Micromagnetic simulations showed that a distorted-vortex state forms just before the vortex?onion transition, and a reversible change between the distorted-vortex state and a fully symmetric vortex state is expected during minor loop magnetic cycling. The distorted-vortex state enables the vortex chirality in a single magnetic ring to be detected using AMR measurements.

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.

Magnetic domain formation within patterned NiFe/Cu/Co ellipses

Neutron reflectometry was used to study the formation and evolution of magnetic domains within a patterned array of NiFe/Cu/Co ellipses. The measurements directly show that domains form upon relaxation away from hard axis magnetic saturation, and their size and shape distributions are invariant throughout the process. Modeling of the data demonstrates that uniform magnetic domains are commensurate with the ellipse structure, but are approximately 75 nm smaller in radius. Together these findings suggest that there is one magnetic domain per nanoparticle whose constituent moments rotate collectively as the field is varied.

Magnetic pinning of flux lattice in superconducting-nanomagnet hybrids

Strong superconducting pinning effects are observed from magnetic landscapes produced by arrays of circular rings with varying magnetic remanent states. The collective and the background pinning of superconducting Nb films is strongly enhanced by the stray magnetic field produced by an array of circular Ni rings magnetized to form “onion” (bidomain) states. On the other hand, when the same rings are magnetized into vortex (flux-closed) states, or are randomly magnetized, the superconducting pinning is much smaller. The greatest pinning is produced when the superconducting vortex lattice motion is along a direction in which there is a strong magnetic field variation.