We use a range of lithography techniques (electron-beam lithography, interference lithography, block copolymer lithography, zone-plate array lithography) to produce arrays of ‘nanomagnets’. These tiny structures have thicknesses of a few nanometers and lateral dimensions between tens of nm and a few microns. Arrays of these elements can be made with spatial periods of 100 nm and above. The nanomagnets are made by evaporation or sputtering combined with liftoff or etching. We measure the switching mechanisms of the particles, the thermal stability of their magnetization, and interparticle interactions, and we consider their suitability for various data-storage schemes. The behavior of individual particles can be measured using magnetic-force microscopy, while the collective behavior of arrays of particles can be measured using magnetometry. We also measure magnetotransport of patterned structures, and use micromagnetic modeling to understand the hysteresis behavior and remanent states of small magnetic structures. Recently we have concentrated on thin film multilayer structures such as pseudo-spin valves, exchange-pinned bilayers, or spin valves.

Nanomagnets have potential uses in ‘patterned media’, magnetic random access memories (MRAM) and other magneto-electronic applications. Current MRAM devices rely on bar-shaped multilayered magnetoresistive nanomagnets in which a bit of data is stored depending on the relative orientation between the magnetization of the different magnetic layers in the structure. An alternative possibility for high-density MRAMs is to use a ring-shaped nanomagnet, in which a bit of information is stored by magnetizing the ring clockwise or counterclockwise (a ‘vortex’ state) or with two domain walls (an ‘onion’ state). We have characterized the behavior of circular and elliptical rings of various materials and dimensions. We have made electrical connections to these structures and have characterized the anisotropic magnetoresistance of NiFe single-layer rings, and the giant magnetoresistance of NiFe/Cu/Co multilayer rings.

Multilayer rings show unique behavior due to the interactions between domain walls in the two layers. While the resistance of a bar-shape element displays two resistance levels on cycling the free (NiFe) layer, the rings display additional intermediate resistance levels reached through abrupt transitions. Additionally, the storage (Co) layer can be cycled into different states, allowing for profoundly different device responses when the free layer is switched. We have explored the switching mechanisms of PSV ring structures using micro-magnetic simulations, as well as the effect of the contact configuration on the device response. The sharp, low-field resistance changes in these PSV rings, which can be tailored by choice of ring dimensions and multilayer stack, will make them attractive for magneto-electronic applications such as memories or logic devices that require multiple stable resistance levels. Most recently we are pursuing the option of operating these devices using current pulses, rather than with an applied magnetic field. In addition, in elliptical rings, we have shown how shape and exchange bias can allow control of the vortex circulation direction.