Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1998-09-24
2001-11-06
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C338S03200R
Reexamination Certificate
active
06313627
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a sensor device for detecting the direction of an external magnetic field through the use of at least one sensor element, including a multilayer system having a very great magnetoresistive effect (GMR) and having at least one soft magnetic measurement layer, at least one relatively harder bias layer with a predetermined direction of magnetization and at least one nonmagnetic intermediate layer disposed therebetween. A corresponding sensor device can be inferred from International Publication No. WO 94/17426, corresponding to U.S. Pat. No. 5,650,721.
In layers of ferromagnetic transition metals such as Ni, Fe or Co and alloys thereof, there may be a dependence of the electrical resistance on the magnitude and the direction of a magnetic field permeating the material. The effect occurring in such layers is called the “anisotropic magnetoresistance (AMR)” or “anisotropic magnetoresistive effect”. In physical terms, it is based on the differing scatter cross sections of electrons with differing spin and the spin polarity of the D band. The electrons are designated as majority or minority electrons. In the case of corresponding magnetoresistive sensors, in general, a thin layer of such a magnetoresistive material having a magnetization in the layer plane is provided. The change in resistance upon rotation of the magnetization with respect to the direction of a current inducted through the sensor may then amount to a few percent of the normal isotropic (=Ohmic) resistance.
Furthermore, magnetoresistive multilayer systems are known which include a plurality of ferromagnetic layers that are disposed in a stack and which are separated from one another in each instance by metallic, nonmagnetic intermediate layers and the magnetizations of which in each instance preferably lie in the layer plane. In that case, the thicknesses of the individual layers are markedly smaller than the mean free path of the conduction electrons. In such multilayer systems, in addition to the mentioned anisotropic magnetoresistive effect AMR, it is now possible for a so-called “giant magnetoresistive effect” or “giant magnetoresistance (GMR)” to occur (see, for example, Published European Patent Application 0 483 373 A1). Such a GMR effect is based on a scattering of majority and minority conduction electrons “of different strengths” at interfaces between the ferromagnetic layers and the intermediate layers adjacent thereto as well as on scattering effects within those layers. In that case, the GMR effect is an isotropic effect. It may be considerably greater than the anisotropic effect AMR. In general, reference is made to a GMR effect (at room temperature), if it adopts values which are markedly above those of AMR single layer elements.
In a first type of corresponding multilayer systems showing a GMR effect, in the absence of an external magnetic field, adjacent magnetic layers are oriented to be magnetically antiparallel by reason of a mutual coupling. That orientation can be converted, by an external magnetic field, into a parallel orientation. In contrast, a second type of GMR multilayer systems has a so-called bias layer, which is magnetically harder than an existing measurement layer which is magnetically as soft as possible. In that case, the measurement layer and/or the bias layer may also be replaced in each instance by a plurality of layers stacked to form a packet. However, in the text which follows only individual layers will be assumed in each instance.
In the case of such a layer system of the second type, the measurement layer and the bias layer are mutually magnetically decoupled by a nonmagnetic intermediate layer. In the absence of an external magnetic field, the magnetizations of the two magnetic layers then have some relationship to one another, for example antiparallel. Under the influence of an external magnetic field H
m
(which is the component of the measurement field in the layer plane of the measurement layer), the magnetization M
m
of the soft magnetic measurement layer then becomes oriented in a manner corresponding to the direction of the magnetic field, while the orientation of the magnetically harder bias layer remains virtually unchanged. In that case, the angle &phgr; between the magnetization directions of the two layers determines the resistance of the multilayer system: In the case of a parallel orientation, the resistance is low, and in the case of an antiparallel one, it is high. That follows from the fact that an unambiguous relation exists between the quantities M
m
and H
m
. In that connection, the following applies in the simplest case:
M
m
·H
m
=M
m
H
m
.
(In this case, the vector quantities are identified by bold script and the scalar quantities by non-bold scripts).
The magnetoresistance signal &Dgr;R of such a GMR multilayer system is then given by:
&Dgr;
R=&Dgr;
(1−cos &phgr;).
It is evident from this equation that &Dgr;R adopts the same values for &phgr;=&phgr;
0
and &phgr;=−&phgr;
0
. However, that means that the angle &phgr; can be unambiguously determined only within a sector of 180°. In addition, the angle sensitivity d&Dgr;R/d&thgr;=&Dgr;sin&thgr; is very low for &thgr;=0 and &thgr;=&pgr;, where &thgr; is the angle between the direction of the external magnetic field H
m
and the reference direction stipulated by the magnetization of the bias layer (see the initially mentioned International Publication No. WO 94/17426, corresponding to U.S. Pat. No. 5,650,721).
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a sensor device for detecting the direction of an external magnetic field using a magnetoresistive sensor element, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, which permits a 360° angle detection and which reduces the problem of an excessively low sensitivity.
With the foregoing and other objects in view there is provided, in accordance with the invention, a sensor device for detecting the direction of an external magnetic field, comprising a substrate; at least one sensor element including at least two element parts; the at least two element parts having magnetization directions enclosing an angle unequal to 0° or unequal to 180°; the at least two element parts having measurement signals to be evaluated in common; the at least two element parts having multilayer systems with a very great magnetoresistive effect; the multilayer systems constructed in common on the substrate; and the multilayer systems each having at least one soft magnetic measurement layer, at least one relatively harder bias layer with a predetermined direction of magnetization, and at least one nonmagnetic intermediate layer disposed between the measurement layer and the bias layer.
In this case, the invention is based on the consideration that with the two preferably identically constructed sensor element parts, two sensor part signals which are to be considered or evaluated in common are to be obtained. The signals create a possibility of an unambiguous distinction between the ranges 0° to 180° and 180° to 360°.
In accordance with another feature of the invention, the sensor element parts are connected, separately from one another, to a common signal evaluating device.
In accordance with a further feature of the invention, the angle enclosed by the magnetization directions of two of the sensor element parts of the at least one sensor element is between 20° and 160° or between 200° and 340°.
In accordance with an added feature of the invention, the angle enclosed by the magnetization directions is at least approximately n·45°±10° where n=1,2,3,5,6 or 7.
In accordance with an additional feature of the invention, the angle included by the magnetization directions is at least approximately 90° or 270°.
In accordance with yet another feature of the invention, the sensor element parts have the same layer structure and the same geo
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Strecker Gerard R.
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