Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames, Iowa -- Scientists at the U. S. Department of Energy's (DOE's) Ames Laboratory have discovered a material that may be simultaneously superconducting and weakly ferromagnetic.
According to an article published today (May 20, 1996) in the scientific journal Physica C, erbium nickel borocarbide, a member of a family of magnetic superconductors discovered in 1994, appears to have a low-temperature (2.3 Kelvin) phase transition to a state where superconductivity coexists with long-range magnetic order that has a ferromagnetic component.
This may be "the first example of superconductivity coexisting with ferromagnetism over a large temperature range," says Paul Canfield, a physicist at the Ames Laboratory. The novel mixed state is known to persist at 1.8 K and gives every indication of persisting at still lower temperatures.
The coexistence of superconductivity and weak ferromagnetism might manifest itself in any of several theoretically predicted ground states. The simplest of these is a spontaneous vortex lattice, a regular pattern of quantized fluxoids, or vortices of magnetic flux. Physicists are now planning detailed measurements that should reveal the novel phase' s ground state.
In most materials, magnetism and superconductivity are incompatible; the presence of even trace amounts of a magnetic element destroys a material's ability to become superconducting.
In the past 30 years, however, several families of compounds have been discovered in which superconductivity can coexist with magnetic order. These include the rare-earth rhodium borides and rare-earth molybdenum selenides and sulfides, as well as the more recently discovered rare-earth nickel borocarbides.
Although the magnetic superconductors can remain superconducting when they order magnetically, they do so only if the net local field associated with the magnetic order remains below a critical field designated Hc2.
Erbium nickel borocarbide differs from other magnetic superconductors in that only a twentieth of the atomic moments align at the 2.3-K transition. The average magnetization per erbium atom is half a Bohr magneton, a twentieth of the full 10-Bohr-magneton erbium moment. The field associated with this ordered moment is about 500 Gauss, well below the Hc2 for this material, which is about 10,000 Gauss .
In erbium rhodium boride, by contrast, the entire erbium sublattice aligns at the ferromagnetic transition, quickly destroying the material's superconductivity. As a result, ferromagnetic ordering and superconductivity coexist over a temperature range of less than 0.1 degree K.
Ground states that would allow superconductivity and weak ferromagnetism to accommodate one another have been predicted by a number of physicists, including M. Tachiki of Tohoku University in Sendai, Japan, C.J. Kuper of Technion-Israel Institute of Technology in Haifa, Israel, and C. M. Varma of Bell Labs.
Erbium nickel borocarbide is expected to provide data that will decide between these theories -- or perhaps require the elaboration of an entirely new one.
Ames Lab is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-performance computing, environmental cleanup and restoration, and synthesis and study of new materials.
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