Read heads in planar monolithic integrated circuit chips

Details Read heads in planar monolithic integrated circuit chips Read heads in planar monolithic integrated circuit chips

2001-03-22

2002-06-11

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Magnetic

Magnetometers

C324S207210, C360S100100

Reexamination Certificate

active

06404191

ABSTRACT:

BACKGROUND OF THE INVENTION
Digital data magnetic recording systems store digital data by recording same in a moving magnetic media layer using a storage, or “write”, electrical current-to-magnetic field transducer, or “head”, positioned immediately adjacent thereto. The data is stored or written to the magnetic media by switching the direction of flow in an otherwise substantially constant magnitude write current that is established in coil windings in the write transducer in accordance with the data. Each write current direction transition results in a reversal of the magnetization direction, in that portion of the magnetic media just then passing by the transducer during this directional switching of the current flow, with respect to the magnetization direction in that media induced by the previous in the opposite direction.
Recovery of such recorded digital data is accomplished through positioning a retrieval, or “read” magnetic field-to-voltage transducer, or “head”, to have the magnetic media, containing previously stored data, pass thereby. Such passing by of the media adjacent to the transducer permits the flux accompanying the magnetization reversal regions in that media either to induce a corresponding voltage pulse in forming an analog output read signal for that retrieval transducer or, alternatively, change a transducer circuit parameter based on magnetoresistive sensing of magnetic conditions therein to thereby provide such an output signal voltage pulse.
Such transducers or sensors can often be advantageously fabricated using ferromagnetic thin-film materials. 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 for an electrically conductive material intermediate layer or a “spin dependent tunneling effect” being present for an electrically insulative material intermediate layer. 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.
Another magnetic field sensor suited for fabrication with dimensions of a few microns or less can be fabricated that provides a suitable response to the presence of external magnetic fields and low power dissipation by substituting an electrical insulator for a conductor in the nonmagnetic layer. This sensor can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the outer magnetic films provided in a “sandwich” structure on either side of an intermediate nonmagnetic layer which ferromagnetic films maybe composite films, but this insulating intermediate nonmagnetic layer conducts electrical current therethrough based primarily on a quantum electrodynamic effect “tunneling” current.
This “tunneling” current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a “magnetic valve effect”. Such an effect results in an effective resistance, or conductance, characterizing this intermediate layer with respect to the “tunneling” current therethrough.
In addition, shape anisotropy is often used in such a sensor to provide different coercivities in the two ferromagnetic layers, and by forming one of the ferromagnetic layers to be thicker than the other. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such sensor device and the operating circuitry therefor.
A “sandwich” structure for such a sensor, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the “magnetic valve effect” if the materials for

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