Electricity: measuring and testing – Magnetic – Magnetometers
Patent
1995-06-19
1997-11-11
O'shea, Sandra L.
Electricity: measuring and testing
Magnetic
Magnetometers
32420721, 360113, G01R 3306, H01L 4308
Patent
active
056868383
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a magnetoresistive sensor and a method for fabricating it.
In ferromagnetic transition metals such as nickel (Ni), iron (Fe) or cobalt (Co) and in alloys containing these metals, the electrical resistance depends on the magnitude and direction of a magnetic field permeating the material. This effect is referred to as anisotropic magnetoresistance (AMR) or anisotropic magnetoresistive effect. Physically, it is based on the different scattering cross sections of electrons having different spins which are referred to as majority electrons and minority electrons of the D band. A thin layer made of a magnetoresistive material having a magnetization in the plane of the layer is generally used for magnetoresistive sensors. The change in resistance as the orientation of an applied magnetic field rotates with respect to the direction of the current may amount to several percent of the normal isotropic resistance.
Multilayer systems are known which comprise a plurality of ferromagnetic layers which are arranged in a stack and are separated from one another by metallic interlayers, and whose magnetizations in each case coincide in orientation with the plane of the layer. The respective layer thicknesses in this arrangement are chosen to be considerably smaller than the mean free path of the conduction electrons. Each layer displays not only the anisotropic magnetoresistive effect described above, but also the so-called giant magnetoresistive effect or giant magnetoresistance (Giant MR). Giant MR arises due to the differential scattering of majority and minority conduction electrons in the bulk of the layers, especially in alloys, and at the interfaces between the ferromagnetic layers and the interlayers. This Giant MR is an isotropic effect and may be considerably larger than the anisotropic MR, with values of up to 70% of the normal isotropic resistance.
Two basic types of such Giant-MR multilayer systems are known. In the first type, the ferromagnetic layers are anti-ferromagnetically coupled to one another via the interlayers, so that the magnetizations of two adjacent ferromagnetic layers (which coincide with the planes of the layers) align themselves antiparallel with respect to one another in the absence of an external magnetic field. An example of this type are iron-chromium superlattices (Fe-Cr superlattices) having ferromagnetic layers consisting of Fe and anti-ferromagnetic interlayers consisting of Cr. An applied external magnetic field causes the magnetizations of adjacent ferromagnetic layers to rotate against the anti-ferromagnetic coupling forces and to align themselves in parallel. This reorientation of the magnetizations by the magnetic field results in a steady decrease of the Giant MR. The magnitude of this decrease corresponds to the magnitude of the external applied magnetic field. Once a saturation field strength H.sub.s is reached, no further change in the Giant MR takes place, because all magnetizations are then aligned in parallel with respect to one another. The Giant MR in this situation depends solely on the magnitude of the field strength ("Physical Review Letters", Vol. 61, No. 21, Nov. 21, 1988, pages 2472-2475).
This type of system comprising anti-ferromagnetically coupled ferromagnetic layers has also been the subject of theoretical calculations which show that the current coefficients and the transmission coefficients for spin-up and spin-down electrons scattered at the interfaces depend on the angle between the magnetizations in adjacent ferromagnetic layers. According to these calculations, the Giant MR increases steadily as the angle between the two magnetizations increases from 0.degree. to 180.degree., and is greatest at an angle of 180.degree. ("Physical Review Letters", Vol. 63, No. 6, August 1989, pages 664 to 667).
In the second type of a Giant-MR multilayer system, ferromagnetic layers whose magnetizations in the planes of the layers are, on average, parallel to one another, are separated from one another by diamagnetic
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O'Shea Sandra L.
Patidar Jay M.
Siemens Aktiengesellschaft
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