Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1999-11-01
2001-08-21
Ometz, David L. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06278588
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a magnetoresistive magnetic field sensor comprising a bilayer with a first soft magnetic layer and in direct contact therewith a second soft magnetic layer, said layers being exchange coupled to one another, the electrical resistivity of the first soft magnetic layer being higher than that of the second soft magnetic layer.
In such a magnetoresistive magnetic field sensor it is of advantage to obtain a magnetoresistance ratio (MR-value) which, also with a relatively small external magnetic field, is sufficient to perform an accurate measurement of the change in the resistivity due to a rotation of the magnetization vector in the bilayer by the external magnetic field. This magnetoresistance ratio is deducted from the resistance value of the sensor in both the absence (R
o
) and the presence (R
s
) of an external magnetic field:
MR
=
&LeftBracketingBar;
R
o
-
R
s
&RightBracketingBar;
R
s
From Kamiguchi et al; Giant magnetoresistance and soft magnetic properties of Co
90
Fe
10
/Cu spin-valve structures, J. Appl. Phys. 79 (8), Apr. 15, 1996, it is known to deposite a NiFe/CoFe layer on a soft magnetic layer, e.g. a CoNbZr layer. In such a bilayer structure with a NiFe/CoFe layer deposited on a CoNbZr layer, the NiFe/CoFe layer has a relatively low electrical resistivity, while the CoNbZr layer has an electrical resistivity which is about a factor of 4 larger than that of the NiFe/CoFe layer. Although the NiFe/CoFe material demonstrates a relatively large anisotropic magnetoresistance effect, the relatively low electrical resistivity of these materials forms a limiting factor in the measurement thereof.
SUMMARY OF THE INVENTION
The object of the invention is to increase the sensitivity of the measurement of the magnetoresistance ratio and to obtain a more accurate magnetoresistive magnetic field sensor.
This object is achieved with the magnetoresistive magnetic field sensor according to the invention, which is characterized in that the second soft magnetic layer is located below, on or in the first soft magnetic layer in a meandering, spiraling or suchlike structure and in that the differences in electrical resistivity between both soft magnetic layers is at least a factor of 10, preferably at least a factor of 100. By the meandering, spiraling or suchlike structure, the electrical resistance of the second soft magnetic layer is increased, while the first soft magnetic layer has such a high electrical resistivity that short circuits of the meandering, spiraling or suchlike structure are prevented. The sensor according to the invention has an increased sensitivity with regard to and is more accurate than the known sensor. It is preferred that the first soft magnetic layer is of a ferrite material.
Although the second soft magnetic layer is preferably deposited on the first soft magnetic layer, it is possible to manufacture the bilayer with the second soft magnetic layer below or in, particularly recessed in, the second magnetic layer.
In many applications the ferrite material is used in the form of a polycrystalline thin film. It is known that the soft magnetic properties of bulk ferrite material are not preserved when this material is deposited in the form of a thin film. This is due to the reduced grain size of the thin film, compared to the bulk material, together with either a “hard”- or “non”-magnetic grain boundary. Hitherto, most studies focused attention on trying to increase the grain size of the film by either growing on a seeded substrate or post deposition annealing steps. Thin ferrites have been grown by MOCVD (modified chemical vapour deposition), pulsed laser ablation, MBE, Sol-gel and sputtering techniques. All of these methods have failed to produce a thin polycrystalline ferrite film with considerable magnetic properties.
The relative permeability of thin film ferrites is strongly dependent on the grain size of the film. This dependency may be described by either the non-magnetic grain boundary (NMGB) model (see: M. T. Johnson and E. G. Visser, IEEE Trans. Mag., Mag.-26, 1987 (1990)), which ascribes the increase in anisotropy to local demagnetizing fields, or the model of Pankert (see: J. Pankert, JMMM 138, 45 (1994), which attributes the low permeability to a lack of exchange coupling across the grains. For example, in a MnZn-ferrite film with a grain size of about 20 nm and a non-magnetic grain boundary of about 2 nm, the relative permeability is about 12 in both models. The relative permeability further increases with the grain size and diminishes with the non-magnetic grain boundary thickness.
It has already been discovered that a relatively hard thin ferrite film can be made considerably softer by depositing a closed thin film of soft magnetic material on top of the ferrite layer; see U.S. Pat. No. 4,610,935. Such an effect may be attributed to interactions between the soft magnetic layer and the individual grains of the ferrite film. Although the coupling between the two layers as indicated in said US patent specification is diminished by an open structure, such as a meandering, spiraling or suchlike structure, and therefore the magnetic properties of the bilayer structure are made worse, these properties, particularly with relatively small distances between the loops of the meandering, spiraling or suchlike structure, are still sufficient to realize a bilayer structure with such magnetoresistive properties that it can be successfully applied in a magnetoresistive magnetic field sensor according to the present invention.
In a particular embodiment a MnZn-ferrite layer is used, in combination with a permalloy or a CoNiFe soft magnetic layer. In a preferred embodiment the composition of the ferrite layer is given by Mn
x
Zn
y
Fe
z
O
w
, the atomic ratios preferably being x=0.48, y=0.31, z=1.79 and w=4.4, on which ferrite layer a Ni
p
Fe
q
layer is deposited with p=80 at. % and q=20 at. %.
In a preferred application, the bilayer structure according to the invention forms the layer in which the magnetization vector rotates under the influence of an external magnetic field in a magnetoresistive magnetic field sensor of an anisotropic type or a giant type.
The invention not only relates to a magnetoresistive magnetic field sensor as indicated above, but also to a magnetic read head device comprising such a magnetoresistive magnetic field sensor and to a system for recording information, comprising such a magnetic read head device.
In the following description, the general structure of a bilayer structure according to the invention and the application in a magnetoresistive magnetic field sensor will be elucidated, by way of example, with reference to the accompanying drawings.
REFERENCES:
patent: 4047236 (1977-09-01), Lee
patent: 4438470 (1984-03-01), Sawada et al.
patent: 4477794 (1984-10-01), Nomura et al.
patent: 5589278 (1996-12-01), Kamijo
Altman, III Franklin D.
Ometz David L.
Treacy David R.
U.S. Philips Corporation
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