August 21, 1998
A silver compound controls electric current via magnetism
Of recent interest to researchers has been the development of new types of magnetoresistive devices. Such materials can be of large practical importance, as they will change their electrical resistance in the presence of a magnetic field. Examples of magnetoresistive materials already in use would be the recording heads of tape and video, which use the material permalloy. However, most materials only exhibit appreciable magnetoresistance under extreme conditions, such as high magnetic fields or low temperatures. Researchers are challenged to find or create materials that are magnetoresistive at room temperature or in modest magnetic fields, for such a substance would be widely useful.
Recently a group at the Materials Center and Argonne National Laboratory led by Thomas F. Rosenbaum and Marie-Louise Saboungi has entered the pursuit for new magnetoresistive materials, using an interesting approach. Instead of examining known magnetoresistive materials, they have studied substances that heretofore were not known to exhibit any significant magnetoresistance whatsoever. If one were to ask how they came up with the idea, serendipity would be their frank answer. While studying how electric current flows in certain materials, they found a temperature where the current was at a minimum. Suspecting magnetism had something to do with it, they applied a magnetic field. When the minimum got even more dramatic, they knew they had something interesting on their hands.
The research group began their studies with a class of compounds called the silver chalcogenides (AgS, Ag Se, Ag Te). At room temperature these compounds conduct only a tiny amount of current, regardless of the presence or absence of a magnetic field. In other words, the ordinary silver chalcogenides do not meet the criteria for a magnetoresistive material... that is, as long as the ratio of silver to its partner element is two to one.
The group found that samples of silver chalcogenides with a slightly altered ratio (Ag, where the optimal value is of order 0.01) exhibited increases in magnetic activity by massive amounts. Specifically, resistance increased by 200% in AgSe and AgTe when their ingredients were slightly changed. Even fields no larger than that in a common loudspeaker were enough to increase the resistance by a percent or so. In recent years other researchers have discovered materials with "colossal" and "giant" magnetoresistance. The new silver chalcogenides showed resistance increases comparable to "colossal" magnetoresistance, but at lower magnetic levels and by a different, as yet unknown, mechanism.
Perhaps even more interesting than the discovery of a new magnetoresistant material is something quite unusual for magnetoresistant materials: the resistance changes in direct proportion to the magnetic field. In most cases the change of resistance is proportional to the square of the field instead. A few theories have been offered to account for this challenge to the classical model of magnetoresistance: maybe the magnetic field alters the electronic structure of the compound on the scale of many repeat spacings, producing large-scale patterns. Changing the field could change the size of these patterns, thus changing the resistance in proportion to the field strength. Currently, neutron and x-ray diffraction studies are being carried out to investigate whether modulation is responsible for the origin of the magnetoresistance. Further variations of the material are currently being tested, such as Se doped with Ni, in the hopes of increasing the magnetoresistance.
- "Large Magnetoresistance in Non-magnetic Silver Chalcogenides" R. Xu, A. Husmann, T. F. Rosenbaum, M. L. Saboungi, J. E. Enderby and P. B. Littlewood, Nature. 390 57-59 (6 Nov. 1997).