F. Q. Zhu

Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings.

Magnetization reversals through the formation of a vortex state and the rotation of an onion state are two processes with comparable probabilities for symmetric magnetic nanorings with a radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and cannot be altered after fabrication. In this work, we report a novel type of nanorings--asymmetric nanorings. By tuning the asymmetry, we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain-wall energy on the local cross-section area for which we have provided theoretical calculations.

Controllable High-Speed Rotation of Nanowires

We report a versatile method for executing controllable high-speed rotation of nanowires by AC voltages applied to multiple electrodes. The rotation of the nanowires can be instantly switched on or off with precisely controlled rotation speed (to at least 1800 rpm), definite chirality, and total angle of rotation. We have determined the torque due to the fluidic drag force on nanowire of different lengths. We also demonstrate a micromotor using a rotating nanowires driving a dust particle into circular motion. This method has been used to rotate magnetic and nonmagnetic nanowires as well as carbon nanotubes.

Manipulation of nanowires in suspension by ac electric fields

Nanowires are potential building blocks for nanoscale devices. Manipulation of nanowires in suspension has been a formidable problem. Using ac electric fields applied to strategically designed microelectrodes, nanowires in suspension can be driven to align, to chain, to accelerate in directions parallel and perpendicular to its orientation, to concentrate onto designated places, and to disperse in a controlled manner with high efficiency despite an extremely low Reynolds number at the level of 10−5. The manipulation of nanowires can also be applied to other small elongated entities such as carbon nanotubes.

Fabrication and Magnetic Properties of Ordered Macroporous Nickel Structures

Here we report on the fabrication of ordered macroporous nickel structures by electrodeposition into colloidal crystal templates formed by self-assembly of polystyrene particles with diameters from 100 nm to 1 μm. The current during deposition into well-ordered colloidal crystals with low defect density exhibits periodic oscillations associated with the change in fractional area within the crystal. Magnetic hysteresis loops and magnetoresistance measurements are performed as a function of pore diameter and film thickness. The magnetic properties can be explained by modeling the ordered macroporous nickel structures as a three-dimensional network structure with magnetic particles connected by narrow ligaments.

Efficiency of assembling of nanowires in suspension by ac electric fields

The authors report a versatile and efficient method for assembling nanowires in suspension into scaffolds using ac electric fields. The electric field and its gradient aligns and transports, respectively, the nanowires into scaffolds according to the electric field distributions. The assembling efficiency strongly depends on the frequency of the applied ac voltages and varies as square of the voltage. The assembly morphology is highly influenced by the frequency regardless of applied voltages.

Exploiting finite size effects in a novel core/shell microstructure

Electrodeposition of Ni–Cu alloys at high Cu(II) concentration and large overpotentials results in phase separation with a unique microstructure characterized by features with a copper-rich core and a nickel-rich shell. By confining deposition to nanoporous channels with dimensions comparable to or smaller than the grain size results in the formation of solid Ni–Cu nanowires with a copper-rich core and a nickel-rich shell. Etching of the copper-rich core results in the formation of Ni-rich nanotubes. The magnetic properties of the Ni–Cu nanowires and the Ni nanotubes are investigated.

Electronic properties of nanoentities revealed by electrically driven rotation

Direct electric measurement via small contacting pads on individual quasi-one-dimensional nanoentities, such as nanowires and carbon nanotubes, are usually required to access its electronic properties. We show in this work that 1D nanoentities in suspension can be driven to rotation by AC electric fields. The chirality of the resultantrotation unambiguously reveals whether the nanoentities are metal, semiconductor, or insulator due to the dependence of the Clausius–Mossotti factor on the material conductivity and frequency. This contactless method provides rapid and parallel identification of the electrical characteristics of 1D nanoentities.

Determination of domain wall resistance in a cobalt thin film by thickness modulation

Inspired by a well-known fact that the magnetic coercivity of a thin film has a strong dependence on its thickness, we have fabricated a 5×60μm and 30nm thick cobalt (Co) strip with thickness modulation along its long axis. The modulation period of 700nm with a depth of 8nm was prepared by a focused ion beam. From magnetic force microscope images, we observed an induced magnetic anisotropy along the short axis of the strip. By comparing out-of-plane magnetoresistance measurements in two magnetic remnant states, we extracted a positive domain wall resistance of 0.03Ω, corresponding to 0.14% magnetoresistance (MR) in a Co thin film.

Determination of multiple easy axes in magnetic multilayers by remanence measurement using a vector magnetometer

The determination of multiple easy axes in magnetic multilayers with noncollinear spin structures using traditional hysteresis loop measurements at different angles is time consuming and problematic. We describe a simple and accurate method for determining the easy axis and remnant magnetization of each individual magnetic layer. By measuring the angular dependence of the x component of the remnant magnetization using a vector vibrating sample magnetometer and numerical fitting based on superposition of the remnant magnetizations, multiple easy axes can be determined with a resolution of about 0.1°. We demonstrate that multilayer samples with one, two, or even three easy axes can be determined with only one measurement of angular dependent remanence.

Parallel fabrication of magnetic tunnel junction nanopillars by nanosphere lithography

We present a new method for fabricating magnetic tunnel junction nanopillars that uses polystyrene nanospheres as a lithographic template. Unlike the common approaches, which depend on electron beam lithography to sequentially fabricate each nanopillar, this method is capable of patterning a large number of nanopillars simultaneously. Both random and ordered nanosphere patterns have been explored for fabricating high quality tunneling junctions with magnetoresistance in excess of 100%, employing ferromagnetic layers with both out-of-plane and in-plane easy axis. Novel voltage induced switching has been observed in these structures. This method provides a cost-effective way of rapidly fabricating a large number of tunnel junction nanopillars in parallel.