Number 281 (Story #3), July 29, 1996 by Phillip F. Schewe and Ben Stein TUNNEL JUNCTION MAGNETORESISTANCE may lead to higher-density magnetic storage devices. Physicists have known for some time that sandwiches of alternating magnetic and nonmagnetic microlayers can undergo a change in electrical resistance in the presence of an external magnetic field (arising, say, from a tiny domain on a segment of magnetic tape). This magnetoresistance (MR) effect can be used to decode binary data and has been employed in reading heads in computer hard drives. Giant magnetoresistance (GMR), a stronger version of MR, affords even greater data-decoding sensitivity. Prototype hard-drives with read heads using GMR have achieved areal data densities of 3 Gbits/sq.in. Tunnel junction magnetoresistance (JMR) is yet another approach to transforming a tiny magnetic field into a change in resistance. Unlike the all-metal GMR sensor, a room-temperature JMR sensor consists of two metal (ferromagnetic) layers separated by an insulating layer. A JMR trilayer junction tested recently at MIT is only 20 nm thick and the signal (the fractional change in resistance) was 23%, compared to a signal of less than 7% for a 40-nm-thick, 4-layer GMR prototype. MIT physicist Jagadeesh Moodera (moodera@slipknot.mit.edu; 617-253-5423) suggests that the more compact size, relatively larger signal, and the low sub-nanoamp operating current of the JMR sensor could make for easier engineering of devices and lower production costs. An areal density of more than 10 Gbits/sq.in. is possible, he says. (J.S. Moodera et al., Applied Physics Letters, 29 July.)