Development of nanostructured magnetoresistive devices

The defining parameters in ferromagnetic metallic films have length scales on the order of 10 nm or less. The structure of ferromagnetism in single thin films is controlled by interatomic exchange coupling occurring across spacings less than 1 nm. Multilayered films have interlayer exchange coupling occurring across spacings less than 10 nm. Because the conduction electrons in these materials are also the "magnetic" electrons, the electron transport properties have fascinating properties at nano length scales. The mean free path for spin up and spin down electrons is about 10 nm and 1 nm, respectively. Ballistic hot electrons have a decay length of 10 nm or so. These facts underlie the richness of emerging phenomena and devices in the area of nanomagnetics and spin transport. The evolution of vacuum deposition equipment for ferromagnetic thin films has permitted the exploration of effects due to layer thickness on the order of one rim. Prime examples of such effects are Giant Magnetoresistance (GMR) and Spin Dependent Tunneling (SDT). The continual reduction of the lateral dimensions at which experimenters can work in these materials and structures presents both challenges and opportunities. One of the fundamental challenges is decreasing ferromagnetic thermal stability with size. Technical challenges are numerous, and include fabrication and analytical issues. How does one know if the proper structure was made if one has no way of "seeing" it? Opportunities arise when fundamental limitations are turned into advantages. Good examples of this are: the use of superparamagnetic films (nonferromagnetic due to thermal instability) as sensing layers; and turning the thermal instability of a magnetoresistive random access memory (MRAM) bit into a read-write mechanism. This paper examines the construction of magnetoresistive data devices in the regime, where largest dimension is less than 1000 nm. A history closed-flux structures is presented. Based on this history a device design trend is extrapolated into the future. A case is made for investigating the use of nonferromagnetic structures for data storage.