Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1999-01-05
2001-02-20
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S251000, C324S252000
Reexamination Certificate
active
06191581
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a thin-film magnetic field sensor.
2. Discussion of the Background
The production of a reliable magnetic sensor in the 10
−6
-10
−1
oersted magnetic field range remains a strategic objective, both for the detection of magnetic anomalies and for producing read heads for reading high-density magnetic storage media.
Among the physical effects that are sensitive to the magnetic field, magneto-transport phenomena have the advantage of being able to be easily integrated into the read electronics. At the present time, two types of effect are used: on the one hand, the magneto-resistance of magnetic materials and, on the other hand, the Hall effect.
French Patent Application No. 93/15551 describes a sensor for detecting a low magnetic field, based on a planar photosensitive Hall-effect element. This patent application may serve as the basis of the closest prior art. U.S. Pat. No. 4,987,509 describes a system comprising two magnetoresistive elements placed so as to be parallel to each other and a conductor placed between these two elements in order to induce a polarization field and to determine, when not in use, a direction of the magnetization in each element.
Such a sensor relies on the transverse measurement of the anisotropic magnetoresistance effect in a thin ferromagnetic film.
FIG. 1
shows the magneto-sensitive layer. A current I flows in a direction XX′ of the layer and the resistivity is measured in the direction YY′. This resistivity varies with the magnetization M applied to the sensor. In addition, it depends on the angle &thgr; between the magnetization and the direction XX′ of the current:
ρ
=
Δ
ρ
2
sin
2
θ
The voltage measured in the YY′ direction varies with the angle &thgr; between the magnetization of the film and the measurement current, according to the equation below:
V
=
I
Δ
R
2
sin
5
(
2
θ
)
where &Dgr;R is the resistivity anisotropy, which depends essentially on the material and on the thickness of the active part of the sensor.
The two main advantages of planar Hall-effect sensors over longitudinal-measurement magnetoresistive sensors are, on the one hand, a great simplification in the associated technology and, on the other hand, a reduction by approximately four orders of magnitude in the thermal drift, the main noise component at low frequencies (around 1 Hz). By construction, this sensor may be made sensitive only to the magnetic field component perpendicular to its direction of supply. Its size may be reduced to dimensions smaller than those of the magnetic domains, thereby eliminating the source of noise associated with the movements of the walls. Measurements on prototypes of such sensors have shown that these sensors have a linear response over four orders of magnitude (see document A. Schuhl, F. Nguyen-Van-Dau and J. R. Childress, Applied Physics Letters, 66, May 15, 1995).
However, in cases in which the two easy axes of magnetization of the magnetoresistive layer of the sensor are equivalent (that is to say have equivalent values) a problem arises in determining the situation when the sensor is not in use.
SUMMARY OF THE INVENTION
The invention allows this doubt to be dispelled.
The invention therefore relates to a magnetic field sensor comprising a first planar thin-film element made of a crystalline magnetoresistive material exhibiting resistivity anisotropy in the plane, having a first and a second easy axis of magnetization, this element having means of electrical connections allowing a first electrical measurement current to flow through the element in a first direction, as well as two electrical connections allowing the voltage to be measured in a second direction transverse to the first direction, characterized in that the two easy axes of magnetizations are of more or less equivalent values and in that it includes a first electrical conductor placed so as to be parallel to an easy axis of magnetization, insulated from the magnetoresistive material and allowing an electrical control current to flow, which control current induces, in the element, a magnetic field which imposes, when the sensor is not in use, an orientation of the direction of magnetization of the element parallel to the second easy axis of magnetization.
In addition, experimentally such planar Hall-effect sensors have a non-zero zero-field resistance.
There may be two reasons for this offset:
defects in the definition of the geometry of the sensor, making it asymmetrical;
poor alignment of the pattern with respect to the crystal axes of the active layer.
The presence of this offset introduces an isotropic component in the transverse resistivity which drifts with temperature, thereby losing one of the essential advantages of the transverse geometry. Moreover, the output signal is then high, which limits the performance of the sensor, particularly for reading in synchronous detection mode. The proposed invention solves this problem.
Another problem is the production of biaxial compasses. To do this, provision is made to assemble two planar Hall-effect sensors in the same plane. However, this assembly must meet particular conditions.
This is why the invention also relates to an embodiment according to the invention of a sensor characterized in that it comprises:
a second element made of a magnetoresistive material similar to the first element, the two easy axes of magnetization of which are parallel to those of the first element;
means of electrical connections allowing a second electrical measurement current to flow through this second element in a direction parallel to the first direction or to the second direction;
two electrical connections allowing the voltage to be measured in a direction transverse to the direction of flow of the second current and parallel to the second direction or to the first direction;
a second electrical conductor placed so as to be parallel to the second easy axis of magnetization and allowing an electrical control current to flow, which control current induces, in this second element, a magnetic field which imposes, when the sensor is not in use, an orientation of the direction of magnetization of the element parallel to the first easy axis of magnetization.
Each of the two detectors is then sensitive to one component of the magnetic field. It may be seen that the accuracy of such a biaxial sensor will certainly be limited by the accuracy with which the two detectors are assembled.
Moreover, the two components of the magnetic field are measured by two detectors which have been assembled. The measurement accuracy is then limited by the difference in sensitivity between the two sensors. In the case of bulk manufacture, large-diameter substrates will be used. Large differences between the sensitivities of the various sensors may therefore be expected. To solve this problem, it is either necessary to tag the sensors, in order to assemble two neighbouring sensors, or to measure the sensitivity of each sensor. In both cases, this requires lengthy operations and the effect of them will be felt in the cost of the sensor.
According to the invention, provision is therefore made for the two sensors to be made in the same magnetoresistive layer and to be a short distance apart.
REFERENCES:
patent: 4987509 (1991-01-01), Hardayal et al
patent: 0348027 (1989-12-01), None
patent: 0660128 (1995-06-01), None
patent: 57-021883 (1982-05-01), None
Montaigne François
Nguyen Van Dau Frederic
Schuhl Alain
"Thomson-CSF"
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Patidar Jay
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