Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2001-02-16
2002-12-31
Ho, Hoai (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S158000, C324S252000
Reexamination Certificate
active
06501678
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to the field of magnetic devices. More in particular, a magnetic data storage system and a sensing system of magnetic characteristics, the systems having a magnetization direction that is irreversible in an external magnetic field, are disclosed. A method of manufacturing, a method of resetting or changing or repairing and a method of operating such systems are also disclosed.
BACKGROUND OF THE INVENTION
Magnetic devices are known in the art. Spin-valve structures such as Giant Magneto Resistance (GMR) and Spin-tunnel Magneto Resistance (TMR) devices recently have been extensively studied and were subject of a vast number of disclosures. GMR- and TMR-devices comprise as a basic building stack two ferromagnetic layers separated by a separation layer of a non-magnetic material. This structure in the sequel is referred to as the basic GMR- or TMR-stack of the magnetic device, or is referred to as the GMR- or TMR-structure. Such structure has magneto resistance characteristics and shows the GMR- or TMR-effect. The separation layer is a non-ferromagnetic metallic layer for GMR-devices, and is a non-metallic, preferably insulating, layer for TMR-devices. Over the separation layer, there is a magnetic coupling between the two ferromagnetic layers. The insulating layer in the TMR-devices allows for a significant probability for quantum mechanical tunneling of electrons between the two ferromagnetic layers. Of the two ferromagnetic layers, one is a so-called free layer, and one is a so-called pinned or hard layer. The free layer is a layer whose magnetization direction can be changed by applied magnetic fields with a strength lower, preferably substantially lower, than the strength of the field required for changing the magnetization direction of the pinned layer. Thus the pinned layer has a preferred, rather fixed magnetization direction, whereas the magnetization direction of the free layer can be changed quite easily under an external applied field.
The hard layer can consist of a hard magnetic material or of a relatively soft magnetic material pinned by exchanged biasing to an Anti-Ferromagnetic (AF) layer, or it can consist of an Artificial-Anti-Ferromagnet (AAF) consisting of two or more magnetic layers coupled in an anti-parallel direction by an appropriate intermediate non-magnetic coupling layer. The AAF can be biased by an AF layer to make it even more rigid and to define a single-valued magnetization direction of the AAF.
A change of the magnetization of the free layer changes the resistance of the TMR- or GMR-device. This results in the so-called magneto resistance effect or GMR/TMR effect of these devices. The electrical resistance of the TMR- or GMR-device changes in a predictable manner in response to a varying magnetic field, making the devices suitable for use as magnetic-electrical transducers in a sensing system of a magnetic field. The characteristics of these magnetic devices or systems can be exploited in different ways. For example a spin valve read-out element utilizing the GMR-effect can be used for advanced hard disk thin film read heads. Also stand-alone magnetic memory systems (MRAMs) or non-volatile embedded memory systems can be made based on the GMR- or TMR-devices.
A further application is a sensor device or system for magnetic characteristics. Such sensing systems are used for example in anti-lock braking (ABS) systems or other automotive applications.
It is often required in a number of applications to clearly distinguish between the response of the sensor system (resistance changes) due to (varying) magnetic field and the response of the sensing system (resistance changes) due to environmental factors such as temperature variations. One approach in solving this problem consists in connecting a number of GMR- or TMR-devices in a Wheatstone bridge arrangement. If a pair of GMR or TMR devices can be magnetically biased in such a manner as to have opposite responses (in the sense of opposite polarity) to a given magnetic field but not to other environmental factors, then subtractive comparison of the electrical resistances of the two GMR or TMR devices will cause cancellation of any unwanted response to spurious environmental factors, while exposing any response to magnetic field.
Magnetic field sensing system employing a Wheatstone bridge in this manner are known from the prior art. However, among the sensing system thus known, there are various different approaches when it comes to magnetically biasing the magneto-resistive devices.
For example: in Japanese patent application (Kokai) No. 61-711 (A), each of the resistive devices in the Wheatstone bridge is magnetically biased in a given direction using an appropriately poled permanent magnet positioned in the vicinity of that device; on the other hand, in an article in Philips Electronic Components and Materials Technical Publication 268 (1988) entitled “The magnetoresistive sensor” the individual resistive devices are biased using a so-called “barber pole” (a term generally known and understood in the art, and thus receiving no further elucidation here).
The use of biasing on the basis of permanent magnets as in case (a) above is highly unsatisfactory: not only is very careful tuning of the strength and position of the permanent magnets required, but the permanent magnets are themselves unacceptably sensitive to temperature variations. In addition, the use of permanent magnets necessarily makes any such biased magnetic field sensor bulky, and sets a limit on the attainable degree of miniaturization. On the other hand, while the biasing method in case (b) may be suitable for resistive devices demonstrating the so-called Anisotropic Magneto-Resistive (AMR) effect, it cannot be employed in conjunction with resistive devices demonstrating the GMR or TMR effect.
In the prior art document J. Daughton, J. Brown, E. Chen, R. Beech, A. Pohm and W. Kude, “Magnetic field sensors using GMR multilayer”, IEEE Trans. Magn. 30, 4608 (1994), two (of the four) bridge elements are magnetically shielded, the shields may be used as flux concentrators for the two sensitive device.
Freitas in “Giant magnetoresistive sensors for rotational speed control”, J. Appl. Phys. 85, 5459 (1999) suggests that two (of the four) bridge devices are “inactivated” by depositing them on a roughened part of the substrate.
Another approach devolves on integrating an isolated conductor below or over the sensor elements (consisting of exchange-biased spin valves) to induce a magnetic field that “sets” the exchange-biasing direction of the device in opposite directions, while the devices are heated above the blocking temperature of the exchange-biasing material R. Coehoorn and G. F. A van de Walle, “A magnetic field sensor, an instrument comprising such a sensor and a method of manufacturing such a sensor”, patent application EP 95913296.0, now granted, and J. K. Spong, V. S. Speriosu, R. E. Fontana Jr., M. M. Dovek and T. L. Hylton, “Giant and magnetoresistive spin valve bridge sensor”, IEEE Trans. Magn.32, 366 (1996); M. M. Dovek, R. E. Fontana Jr., V. S. Speriosu and J. K. Spong, “Bridge circuit magnetic field sensor having spin valve magnetoresistive elements formed on common substrate”, U.S. Pat. No. 5,561,368. A comparable method with an integrated conductor has been proposed for elements based on an Artificial Antiferromagnet (AAF) by W. Schelter and H. van den Berg in “Magnetfeldsensor mit einer Brückenschaltung von magnetoresistiven Brückenelementen”, DE 19520206 (01.06.95).
In the patent application WO 9638738-A1 “Magnetoresistive thin-film elements-uses adjustment current at high temp. to regulate magnetization distribution of bias layer of sensor elements arranged in bridge circuit, and includes cooling body” (Ger) it is suggested that in the factory the magnetizations are set in opposite directions in different branches of the bridges by exposing a wafer with sensor structures to an external magnetic field that is induced by a kind of “stamp” comprising a pattern of current car
Adelerhof Derk Jan
Kuiper Antonius Emilius Theodorus
Lenssen Kars-Michiel Hubert
Somers Gerardus Henricus Johannes
Van Zon Joannes Baptist Adrianus Dionisius
Auduong Gene N.
Biren Steven R.
Ho Hoai
Koninklijke Philips Electronics , N.V.
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