Electrical resistors – Resistance value responsive to a condition – Magnetic field or compass
Reexamination Certificate
2000-10-23
2002-10-08
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Magnetic field or compass
C360S324100
Reexamination Certificate
active
06462641
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a tunnel effect magnetoresistance, also known as a “magnetic valve” magnetoresistance, and a magnetic sensor using such a magnetoresistance.
Magnetic sensors are sensitive to magnetic fields or fluxes. In this way, the magnetic sensor according to the invention may be used, for example, to read data recorded on magnetic data storage media. In addition, the invention may be used to produce Magnetic Random Access Memory devices.
The magnetic sensor may also be used to determine an electric current flowing in a wire, by measuring the magnetic field applied in the vicinity of said wire.
Finally, other applications of the magnetic sensor, such as a position sensor or a magnetoresistive compass, may also be envisaged.
More generally, the invention relates to any type of sensor or magnetoresistance capable of detecting or measuring magnetic fields, particularly weak fields, i.e. ranging from a few A/m to a few thousand A/m.
STATE OF THE PRIOR ART
Until recently, the magnetoresistive sensors used to detect weak magnetic fields, particularly in the field of magnetic recording, were mostly sensors based on a “magnetoresistance anisotropism” effect.
The magnetoresistance anisotropism effect can be seen in ferromagnetic transition metals such as nickel, cobalt or iron-based alloys. It consists of a variation of the resistivity of the magnetic material as a function of an angle between an electric measurement current flowing through the material and the magnetisation of the material.
The relative change in resistivity &rgr; of the magnetic material, referred to as &Dgr;&rgr;/&rgr;, may reach 4 to 5% at room temperature for fields of the order of 1 kA/m, and in solid ferromagnetic transition metals. However, this amplitude is reduced to 1 to 2% when the same materials are deposited in thin layers with thicknesses of 15 to 30 nanometer. This range of thicknesses is that used to manufacture current magnetoresistive sensors. Therefore, the sensitivity of these sensors is limited. In addition, their response is not linear. Indeed, the variation in the resistivity is proportional to the square of the cosine of the angle between the measurement current and the magnetisation.
Sensors operating according to a “giant magnetoresistance” effect are also known. This effect was first discovered for iron-chromium type multilayer structures and subsequently for other multilayer systems formed by alternating layers of ferromagnetic transition metal and layers of non-magnetic metal.
In these systems, the magnetoresistance effect is essentially linked to a change in the relative orientation of the magnetisations of the successive ferromagnetic layers. This effect is usually referred to using the terms “giant magnetoresistance” or “spin-valve effect”.
In spin-valve type magneto resistances, the sensitive ferromagnetic layer, i.e. the free magnetisation layer, has a thickness between 6 and 12 nm to obtain a maximum magnetoresistance amplitude. Below 6 nm, said magnetoresistances have a response amplitude that decreases considerably. Therefore, this type of magnetoresistance is limited in terms of sensitivity for low flux quantities.
Document 1, the reference of which is given at the end of the present disclosure, gives a very general description of the use of said giant magnetoresistance effect to produce magnetic field sensors.
Finally, it is known that there is a magnetoresistance effect in metal-insulating material-metal tunnel effect junctions wherein a thin layer of insulating material, forming a potential barrier for conduction electrons, is inserted between two layers of magnetic metal.
The magnetic metal is selected, for example, from Fe, Co, Ni or their alloys and the layer of the insulating material, a few nanometer thick, is composed of a material selected, for example, from Al
2
O
3
, MgO, AlN, Ta
2
O
5
, HfO
2
, NiO.
In this type of junction, when electrons are forced to pass through the barrier by means of a tunnel effect, by connecting the junction to a current source or by applying a voltage between the two layers of magnetic metal, it is observed that the conductance G of the junction varies as a function of the relative orientation of the magnetisations of the layers of magnetic material at either side of the barrier formed by the insulating material (in the manner of an optical polariser-analyser system).
This effect, called the “magnetic valve effect”, was first observed only at low temperatures and its amplitude was very low.
However, specific magnetic material/insulating material/magnetic material type structures with Fe/Al/Al
2
O
3
/FeCo type junctions have made it possible to obtain variations in conductance, at room temperature, with an amplitude of the order of 17%.
Magnetic valve effect structures are described, for example, in documents 2, 3 and 4. Similarly, experiments on tunnel effect junctions are described in documents 5 and 6. The references of these documents are given at the end of the present disclosure.
Recently, considerable progress was made in the development of junctions, particularly in relation to the quality control of the insulating barrier.
The insulating barrier is produced, for example, by depositing a thin layer of aluminium on one of the metal electrodes of the junction and then oxidising the aluminium layer with oxygen plasma.
The oxygen plasma oxidation time thus makes it possible to check the thickness and, therefore, the electrical resistance of the insulating barrier.
It is also possible to allow the layer of aluminium to oxidise in air. In this case, the results and the quality of the insulating barrier are less reproducible.
In magnetic valve effect junctions with a magnetic material-oxide-magnetic material type structure, designated M—O—M′, the magnetic materials are selected such that the magnetisation of one of the magnetic layers (e.g. M′a) remains fixed in a given direction, in the range of fields to be measured, while the magnetisation of the other layer (M in this example) is capable of following the variations of the field applied. The first layer is called the “trapped layer” while the second is called the “sensitive layer”. The benefit of magnetic valve junctions in relation to spin-valve structures is that they offer wider measurement amplitudes (17% instead of 5 to 9%).
DESCRIPTION OF THE INVENTION
The invention relates to a tunnel effect magnetoresistance as described above offering a wider conductance variation amplitude.
The invention also relates to magnetoresistances with an increased sensitivity and offering a more compact size.
The invention also relates to a magnetic sensor, particularly for ultra-high density magnetic recording (greater than 10 Gbit/inch
2
), making it possible to read data using very small quantities of magnetic flux.
To achieve these objectives, the present invention more specifically relates to a tunnel effect magnetoresistance comprising, in the form of a stack:
a first layer of free magnetisation magnetic material,
a “barrier” layer, composed of an electrically insulating material, and
a second layer of trapped magnetisation magnetic material.
According to the invention, the thickness of the first layer of magnetic material is less than or equal to 7 nm.
A particularly good magnetoresistance sensitivity may be obtained when the thickness of the first layer of magnetic material is between 0.2 nm and 2 nm.
Thanks to the extreme thinness of the first layer of magnetic material in particular, the magnetoresistance shows wide-amplitude conductance variations for low variation values of the magnetic flux applied.
Such a magnetoresistance is thus suitable for reading data on data media, such as hard disks, with a high data density. Indeed, the greater the density of data stored on a hard disk, the lower the quantity of magnetic flux &phgr; produced by the magnetic transitions between two adjacent data bits, picked up by a read head, is. This magnetic flux induces a rotation &Dgr;&thgr; of the magnetisation of the sensitive magnetic layer given by &ph
Dieny Bernard
Giacomoni Laurence
Vedyaev Anatoly
Commissariat a l'Energie Atomique
Easthom Karl D.
Pearne & Gordon LLP
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