Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
1997-08-08
2002-01-22
Ballato, Josie (Department: 2858)
Electricity: measuring and testing
Magnetic
Magnetometers
C360S112000
Reexamination Certificate
active
06340886
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used to sense externally applied magnetic fields.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as field sensors. Magnetometers and other magnetic field sensing devices are used extensively in many kinds of systems including magnetic disk memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic field sensed thereby in a variety of situations.
Such sensors can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic conditions therein. These devices may be provided on a surface of a monolithic integrated circuit chip to provide convenient electrical connections between the device and the operating circuitry therefor in the integrated circuit chip. Otherwise, they may be mounted on another structure conveniently close to the sensor for this purpose.
Ferromagnetic thin-film sensors can be made very small when so constructed. Such sensors are often provided in the form of an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic thin-film is provided. In such “sandwich” structures, reducing the thickness of the ferromagnetic thin-films in the intermediate layer has been shown to lead to a “giant magnetoresistive effect” being present. This effect can be enhanced by having additional alternating ones of such films and layers, i.e. superlattices. This effect can yield a magnetoresistive response which can be in the range of up to an order of magnitude greater than that due to the well-known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response in ferromagnetic thin-films, varying differences between the direction of the magnetization vector in such a thin-film and the direction of a sensing current passed through that film in turn lead to varying differences in the effective electrical resistance of the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the film and the current direction are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance of such a magnetoresistive ferromagnetic thin-film exhibiting this response can be shown to be given by a constant value, representing the minimum resistance present, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance follows a square of the cosine of that angle.
As a result, external magnetic fields supplied for operating a film sensor of this sort can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film portion. This axis exists in the film because of an anisotropy present therein typically resulting from depositing the film in the presence of an externally supplied magnetic field during deposition of the film that is oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of a sensing device using this resulting film, such externally supplied magnetic fields for operating the film sensor can vary the magnetization vector angle to such an extent as to cause switching of that film's magnetization vector between two stable states which occur as magnetizations oriented in opposite directions along the established easy axis. The state of the magnetization vector in such a film portion can be measured, or sensed, by the change in resistance encountered by a current directed through this film portion.
In contrast to this arrangement, resistance in the plane of either of the ferromagnetic thin-films in the “sandwich” structure is isotropic with respect to the giant magnetoresistive effect rather than depending on the direction of a sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect has a magnetization dependent component to resistance that varies as the cosine of the angle between the magnetizations in the two ferromagnetic thin-films on either side of the intermediate layer. In the giant magnetoresistive effect, the electrical resistance through the “sandwich” or superlattice is lower if the magnetizations in the two separated ferromagnetic thin-films are parallel than it is if these magnetizations are antiparallel, i.e. oriented in opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas very thin films are a fundamental requirement to obtain a significant giant magnetoresistive effect. The total electrical resistance in such a magnetoresistive ferromagnetic thin-film “sandwich” structure can be shown again to be given by a constant value, representing the minimum resistance present, plus an additional value depending on the angle between the magnetization vectors and the two films as indicated above.
One common magnetic field sensing situation is the sensing of magnetization changes along a data recording track selected from many such tracks in the magnetic media of a magnetic data storage system. As these tracks are made narrower and narrower to permit increases in the data density in the magnetic media, inductive sensing of the magnetization changes along any of those tracks becomes less feasible. The smaller magnetization volumes lead to smaller outputs from an inductive sensor, and there is a limit to the number of turns in the coil used in such a sensor which can be provided to increase the output signal. Even in thin-film versions thereof, such inductive sensing structures remain relatively thick which becomes a problem as the tracks are made more narrow. Thus, sensing of the magnetization changes along the track using thin-film magnetoresistive sensors has become attractive.
Such magnetoresistive sensors for detecting magnetization changes along a track in the magnetic media are typically formed with the magnetoresistive sensor film in a rectangular shape, and sensors based on such films in initial designs therefor had such a sensing film positioned between a pair of highly permeable magnetic material shielding poles with a long side of the film's rectangular shape located adjacent the magnetic media to result in what is oftentimes termed a horizontal sensor. More recently, such magnetoresistive sensors have had an alternative construction with such sensing films positioned between the poles with the short side of the rectangle adjacent the magnetic media to form what is often termed a vertical sensor or an “end-on” sensor. These kinds of sensors were both initially based on use of the anisotropic magnetoresistive effect in the sensing films. This effect gives a maximum change in magnetoresistance due to the sensed magnetic fields on the order of 2.5% at room temperature.
As data tracks in the magnetic media grow ever thinner coupled with use of higher densities of magnetization direction changes therealong, the need for a more efficient converter of such magnetization changes in the magnetic medium into a sufficiently large current or voltage output signal becomes greater. Hence, horizontal and vertical magnetoresistive sensors based on the “giant magnetoresistive effect” were introduced because of the greater changes in resistance possible from corresponding changes in externally applied magnetic fields. A vertical or end-on magnetoresistive sensor based on the “giant magnetoresistive effect” is typically formed with a nonmagnetic intermediate conductive metal layer having ferromagnetic layers on opposite sides of the major surfaces t
Daughton James M.
Pohm Arthur V.
Ballato Josie
Kinney & Lange , P.A.
Nonvolatile Electronics Incorporated
Sundaram T. R.
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