Over the past 4 decades, computers have gone from storing a few thousand bits of data per square inch of hard disk space (the standard industry measure) to tens of billions of bits in the same space today. Now, researchers report an innovation that could keep the data-density gains rolling in the years to come.
In the 1 July issue of Physical Review B, materials scientists Harsh Deep Chopra and Susan Hua of the State University of New York, Buffalo, report passing electrons through a cluster of magnetic atoms that bridge two magnetic wires. When the magnetic orientation of those electrons, also known as their spin, is the same as the magnetic orientation of the wires, the electrons travel effortlessly through the cluster, a phenomenon known as ballistic magnetoresistance (BMR). But when the magnetic orientations of the wires point in opposite directions, electrons moving through the cluster from one wire to the other must quickly flip their spin. Because that's hard to do in the nanosized clusters, the measured electrical resistance jumped over 3000%--the largest such effect ever seen (see figure).
A related effect, known as giant magnetoresistance, forms the basis for the magnetic read heads found in nearly all computer hard-disk drives. As a read head moves above bits of magnetic data, changes in the magnetic orientation of those bits alter the electrical resistance of electrons flowing through the sensor, translating the magnetic data into a stream of electrical pulses. Those changes in magnetic orientation produce only about a 100% change in resistance in the read head.
Nicholás García and colleagues at the Consejo Superior de Investigaciones Científicas in Madrid, Spain, have shown that the BMR effect can increase that resistance up to 700%. Chopra and Hua more than quadrupled this number by creating more well-formed connections between the two wires, although the exact mechanism by which this produces the increase in resistivity is still unclear, Chopra says. The larger BMR effect could lead to smaller and more sensitive read heads capable of reading smaller magnetic bits. And that, in turn, could allow diskmakers to boost the storage density of disk drives to a staggering 1 trillion bits per square inch.
"This is a great discovery," says William Egelhoff Jr., a physical chemist at the National Institute of Standards and Technology in Gaithersburg, Maryland. "It's exactly what the disk-drive industry needs if it wants to maintain the growth rates in data-storage density."