1. Field of the Invention
This invention relates generally to the field of magnetoresistive sensors, and more particularly to multilayer magnetoresistive sensors, such as giant magnetoresistive multilayer sensors.
2. Description of the Related Art
A conventional magnetoresistive sensor operates on the basis of the anisotropic magnetoresistive effect. Such conventional magnetoresistive sensors provide an essentially analogue signal output wherein the resistance and hence signal output is directly related to the strength of the magnetic field being sensed.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a non-ferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr, Co/Cu, or Co/Ru multi-layers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers. This GMR effect has also been observed for these types of multilayer structures, but wherein the ferromagnetic layers have a single crystalline structure and thus exhibit uniaxial magnetic anisotropy, as described in U.S. Pat. No. 5,134,533 and by K. Inomata, et al., J. Appl. Phys. 74 (6), Sep. 15, 1993.
The physical origin of the GMR effect is that the application of an external magnetic field causes a reorientation of all of the magnetic moments of the ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus a change in the electrical resistance of the multilayered structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. Magnetoresistive sensors based on the GMR effect also provide an essentially analogue signal output. A magnetoresistive sensor is known for example from U.S. Pat. No. 5,585,986.
The sensor's multilayer stack 110 is preferably formed from ferromagnetic layers 121-129 of cobalt (Co) or permalloy (NixFe1-x), and nonferromagnetic metallic spacer layers 131-138 of copper (Cu). Alternative ferromagnetic materials are binary and ternary alloys of Co, nickel (Ni) and iron (Fe) and alternative non-ferromagnetic metals are silver (Ag), gold (Au) and alloys of Cu, Ag and Au. Such multilayer structures exhibit GMR in that the ferromagnetic layers are antiferromagnetically coupled across the spacer layers and the relative alignments of the magnetizations of the ferromagnetic layers vary in the presence of an external magnetic field.
The stack 110 is a crystalline multilayer grown in such a manner that each of the ferromagnetic layers 121-129 exhibits an intrinsic in-plane uniaxial magnetic anisotropy. This means that in the absence of an external magnetic field the crystalline structure of each ferromagnetic layer induces the magnetization to be aligned either parallel or antiparallel to a single axis. Molecular beam epitaxy (MBE) can be used to prepare the crystalline multilayer. However, it has been shown that a crystalline multilayer can be formed by the simpler process of sputter deposition, as described for example by Harp and Parkin, Appl. Phys. Lett. 65 (24), 3063 (Dec. 12, 1994).
As shown by arrows 170-174 and oppositely directed arrows 180-183, alternate ferromagnetic layers 121-129 have their magnetizations oriented antiparallel in the absence of an external magnetic field. This antiparallel alignment is due to the intrinsic uniaxial anisotropy and the antiferromagnetic coupling across the Cu spacer layers 131-138. The Cu (or other spacer layer) thickness has to be chosen to lie within limited ranges for which the permalloy, Co, or related ferromagnetic layers are coupled antiferromagnetically. For such ranges of spacer layer thickness, GMR is observed.
The present invention provides for a magnetoresistive multilayer sensor having a net magnetic moment. The net magnetic moment of the multilayer sensor stack reduces or eliminates the hysteresis effects which are otherwise observed for low magnetic field strengths whereby the sensor output signal is smoothened for such low field strengths. This way the ambiguity of the sensor output signal for low field strengths is removed and the magnetoresistive sensor delivers precise measurement signals also for low magnetic field strengths.
In accordance with an embodiment of the invention, the ferromagnetic layers have different thicknesses in order to create a net magnetic moment of the multilayer stack.
In accordance with an alternative preferred embodiment of the invention, the thicknesses of juxtaposed ferromagnetic layers differ by a maximum factor of 2. Preferably the thicknesses differ by a factor of 1.2 to 1.3.
In accordance with a further alternative embodiment of the invention, juxtaposed ferromagnetic layer in the multilayer stack consist of different ferromagnetic materials having different magnetizations in order to create a net magnetic moment of the multilayer stack.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description.
For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings. In the following drawings, like reference numerals designate like or similar parts throughout the drawings.
The sensor 300 comprises a substrate 301, a seed layer 303 formed on the substrate 301 and a stack 310 of alternating ferromagnetic layers 321-329 and nonferromagnetic metal spacer layers 331-338 formed on seed layer 303. The seed layer 303 may comprise first and second film layer 304 and 305. There are nine ferromagnetic layers 321-329 separated by eight nonferromagnetic metal layers 331-338. The sensor 300 includes a protective or capping layer 340 and electrical leads 350, 352. The leads 350, 352 provide electrical connection to a current source 360 and a signal sensing circuit 362.
As shown by arrows 370-374 and oppositely directed arrows 380-383, alternate ferromagnetic layers 321-329 have their magnetizations oriented antiparallel in the absence of an external magnetic field. This antiparallel alignment is due to the intrinsic uniaxial anisotropy and the antiferromagnetic coupling across the Cu spacer layers 131-138. The Cu (or other spacer layer) thickness has to be chosen to lie within limited ranges for which the permalloy, Co, or related ferromagnetic layers are coupled antiferromagnetically. For such ranges of spacer layer thickness, GMR is observed.
Multilayer stack 310 has a net magnetic moment. The net magnetic moment of stack 310 results from the magnetic moments of the ferromagnetic layers 321 to 329. In the example considered here first ferromagnetic layers 322, 324, 326 and 328 have lower magnetizations 380, 381, 382 and 383 than the magnetizations 370, 371, 372, 373 and 374 of second ferromagnetic layers 321, 323, 325, 327 and 329. Hence a net magnetization results from the superposition of the magnetizations of the ferromagnetic layers 321 to 329.
It will be apparent to those of ordinary skill in the art that the multilayer stack is not limited to the number of ferromagnetic layers of the illustrative sensor 300 shown in
Preferably, stack 310 is a polycrystalline multilayer grown in such a manner that each of the ferromagnetic layers 321-329 exhibits an intrinsic induced in-plane uniaxial magnetic anisotropy. If no external magnetic field is applied, the magnetizations of the individual ferromagnetic layers are oriented in antiparallel directions.
There are various options for creating a stepped magnetization profile in the multilayer stack:
While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit, scope and teaching of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited only as specified in the appended claims.
Number | Date | Country | Kind |
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03102151.2 | Jul 2003 | DE | national |