Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1999-07-26
2001-09-18
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C338S03200R, C428S332000
Reexamination Certificate
active
06291993
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a magnetic field sensor and to a process for fabricating such a sensor.
Such a sensor may be employed for reading magnetic recordings such as magnetic tapes or magnetic disks or for producing magnetic memory with integrated reading (Magnetic Random Access Memory) and in general for the detection of weak magnetic fields with high spatial and angular resolution.
DISCUSSION OF THE INVENTION
It is known that the resistance of a tunnelling junction composed of a very thin layer of insulator between two ferromagnetic metal electrodes depends on the relative orientation of the magnetization in the two electrodes. This effect was discovered by Jullière in 1975 (see document [1] at the end of the description), but it is only recently that high (up to 30%) and fairly reproducible magnetoresistance values have been obtained (see, for example, document [2] at the end of the description). We will refer to this type of junction as a “planar junction”. Typically, in the planar junctions studied by Moodera et al. [2], a 1.5 to 2.5 mm layer of insulator (for example Al
2
O
3
) separates two electrodes of cobalt or cobalt alloy. Deposition through masks makes it possible to produce the conventional crossed junction geometry in which the crossover of the two electrodes separated by the insulating layer defines the tunnelling effect zone. Compared with magnetic multilayers with giant magnetoresistance (see document [5]), tunnelling junctions have the advantage of much higher resistance, which is beneficial for a number of applications. Their drawback is the difficulty of fabricating them: roughness, porosity (“pinhole”) and other imperfections of the ultra-thin insulating layer establish direct contacts between electrodes which short-circuit the junction, and it is generally difficult to obtain reproducible results. The very small thickness of the insulating layers also makes planar junctions sensitive to breakdown effects.
A magnetoresistance effect resulting from a tunnelling effect between ferromagnetic metals may also be obtained in granular materials consisting of small particles (aggregates) of ferromagnetic metal which are enclosed in an insulating matrix (see documents [3] and [4]). Conduction takes place by tunnelling transport of electrons from one aggregate to the neighbouring aggregate, and the resistance of the material varies when a magnetic field modifies the relative orientation of the magnetic moments of the aggregates. These materials are easier to fabricate than planar junctions, are robust and can exhibit significant variations in resistance [3 and 4]. However, their disadvantage derives from the high field needed for orienting the moments of small particles and obtaining the magnetoresistance effect. This field is particularly high in the superparamagnetic range, that is to say when the temperature is above the barrier temperature (T
B
) for the thermal fluctuations of the moments.
SUMMARY OF THE INVENTION
The invention relates to a junction which combines the properties of planar junctions (weak field response) and those of granular material (ease of production and robustness).
The invention therefore relates to a magnetic sensor comprising two conductive layers, at least one of which is a ferromagnetic material, these two layers being separated by a layer of a nonmagnetic insulating material which has an almost uniform thickness and contains particles of ferromagnetic material that lie in a plane parallel to the layer of ferromagnetic material, the magnetization characteristics of the ferromagnetic conductive layer and of the ferromagnetic particles being different.
The invention also relates to a process for producing a sensor, characterized in that it comprises the following steps:
producing the ferromagnetic conductive layer on one face of a substrate:
producing a first layer of nonmagnetic insulator;
producing, by sputtering, a thin layer of a ferromagnetic conducting material so that the said material aggregates to form aggregate particles;
producing a second layer of nonmagnetic insulator;
producing the layer of a ferromagnetic conducting material.
REFERENCES:
patent: 5313186 (1994-05-01), Schuhl et al.
patent: 5474833 (1995-12-01), Etienne et al.
patent: 5521500 (1996-05-01), Schuhl et al.
patent: 5617071 (1997-04-01), Daughton
patent: 5644455 (1997-07-01), Schultz
patent: 5661449 (1997-08-01), Araki et al.
patent: 5686879 (1997-11-01), Schuhl et al.
patent: 5738929 (1998-04-01), Maeda et al.
Fert Albert
Petroff Frédéric
Schelp Luiz Fernando
Schuhl Alain
"Thomson-CSF"
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Oda Christine
Zaveri Subhash
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