Magnetic Heteronanostructured Nanowires
Magnetic Confinement and Coupling in Narrow-Diameter Au-Ni Nanowires
Small magnetic nanostructures can show large spin torque effects as well as magnetic shape anisotropy. Collaborating with the Mirkin group at Northwestern University, we have synthesized magnetic narrow (45 nm diameter) Ni-Au heteronanostructures with varying aspect ratios. The materials show shape and aspect ratio dependent magnetic anisotropy where the easy-axis of the high aspect ratio rods is along the long axis of the rod. Low aspect ratio disk structures exhibit an easy axis within the plane of the disk. Because these sample shapes do not provide the needed functionality to produce a spin torque device, we also have combined these individual disk and rod elements. Coupled disk-rod systems are shown to exhibit tunability of magnetic remanence, allowing for the production of composite magnetic species with new functionality for use in spin torque devices. Most interestingly, we show that it is possible to couple "pinned" high aspect ratio rod segments to the "free" low aspect disk ratio segments. This coupling results in a three-state spin system that allows the disk to more easily flip its magnetization and could be used as a composite free layer in spin torque devices.
Figure 1. Coupling of high aspect ratio rods and low aspect ratio disks. (A). Schematic of coupling between disk and rod sections. The purple arrows indicate magnetic field direction (perpendicular to the long axis of each nanowire), bold black arrows indicate magnetization of disks, dotted grey arrows indicate the easy axis of each magnetic segment. (B) Regression of remanant magnetization with Au spacer distance. This implies easier switching with decreased spacer distance as remanence measures how strongly spins stay magnetized when the applied magnetic field is removed.