Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1998-09-10
2001-11-13
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C428S900000, C438S046000, C430S019000, C430S270210, C430S495100, C430S346000
Reexamination Certificate
active
06316131
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to magnetoresistive compounds and is particularly directed to a class further identified as narrow-gap, self-doping compounds.
BACKGROUND OF THE INVENTION
In recent years, new materials have emerged as potential candidates for magnetoresistive (MR) devices. In the case of “giant” magnetoresistance (GMR), interfacial spins modulate the electron transport both in artificially engineered nanostructural materials consisting of magnetic multilayers and in properly crafted granular alloys. The “colossal” magnetoresistance (CMR) analogs are manganite perovskites with a magnetic-field-driven metal-insulator transition at tens of kOe. These compounds have the general formula A
1−x
B
x
MnO
3+&dgr;
, where A=La, Nd and B=Bu, Sr, Ca, Pb. These compounds undergo a transition into a ferromagnetic conducting state for a narrow composition range around x=0.33 where the resistivity is ~10
4
&mgr;&OHgr;-cm (comparable in magnitude with narrow-gap semiconductors). Near room temperature, the normalized MR can be very large for H of many tens of kOe, the scale of the magnetic-field-driven metal-insulator transition, but &Dgr;&rgr;/&rgr; falls precipitously at low fields.
The silver chalcogenides, Ag
2
S, Ag
2
Se and Ag
2
Te, are classic superionic conductors of technological import as solid electrolytes. In their high temperature (&agr;) phase, electrical transport proceeds via a well-orchestrated coupling of microscopic lattice distortions to the ion migration dynamics. Below T~400 K, there is a structural transformation into the &bgr; phase, where the cation sublattice freezes into a semiconducting state with characteristic energy gaps as small as a few hundredths of an eV. Magnetic fields are commonly applied during the growth process to aid anion vaporization, but have a negligible effect on the electronic response in the solid. In fact, a noise-limited magnetoresistance (MR) in intrinsic &bgr;-Ag
2
Se has been reported.
SUMMARY OF THE INVENTION
Here we report on a previously unexplored class of magnetoresistive compounds: the silver chalcogenides. Ag
2
S, Ag
2
Se, and Ag
2
Te are classic superionic conductors at high temperature. Below T~400 K, the cation sublattice freezes into a non-magnetic, semiconducting state with no appreciable magnetoresistance. We find that slightly altering the stoichiometry leads to a huge increase in the magnetic response. At room temperature, we observe a positive MR of up to 200% in H=55 kOe, making Ag
2+&dgr;
Se and Ag
2+&dgr;
Te highly competitive with the CMR materials. Moreover, our most responsive samples demonstrate an unusual linear dependence on H, indicating both a robust response down to fields of practical importance and a peculiarly long length scale associated with the underlying mechanism.
REFERENCES:
patent: 5215862 (1993-06-01), Suzuki
patent: 6033929 (2000-03-01), Murakami
Husmann Anke
Price David C. L.
Rosenbaum Thomas F.
Saboungi Marie-Louis
Xu Rong
Alwan Joy
Caress Virginia B.
Dvorscak Mark P.
Kiliman Leszek
The United States of America as represented by the United States
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