WO2004032116A1

WO2004032116A1 - Read only storage system

Info

Publication number: WO2004032116A1
Application number: PCT/IB2003/004128
Authority: WO
Inventors: Hendrik Van Houten
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2002-10-03
Filing date: 2003-09-17
Publication date: 2004-04-15
Anticipated expiration: 2005-04-03
Also published as: AU2003260892A1; TW200418000A

Abstract

A record carrier of a removable type has an information plane (12) with bit locations that constitute at least one track. The presence or absence of electro-magnetic material at the information plane represents a value of a bit location. A storage device has a head (41) with a sensor element (42,54,56) that is sensitive to the presence or absence of said electro-magnetic material within a near-field working distance. The record carrier has a scanning surface (18) that is substantially flat for scanning the track within the near-field working distance via the head. The device has a drive unit (45) for rotating or moving the record carrier while the head is scanning the track within the near-field working distance.

Description

READ ONLY STORAGE SYSTEM

The invention relates to a storage system comprising a record carrier and a storage device, which record carrier can be inserted in and removed from the storage device, the record carrier having an information plane that is provided with a pattern of an electromagnetic material at bit locations that constitute at least one track.

The invention further relates to a record carrier for use in the system comprising an information plane that is provided with a pattern of an electro-magnetic material at bit locations that constitute at least one track.

The invention further relates to a storage device for use in the system.

A storage system, record carrier, and a device for storing information are known from patent US 5,956,216. Data storage systems using magnetic material on a disctype record carrier are well known, for example a removable type magnetic record carrier like the floppy disk. The document describes a magnetic record carrier of a patterned type. The record carrier has an information plane that is provided with a magnetic layer that can be magnetized by a suitable magnetic field from a write head. In particular the information plane is provided with a non-magnetic substrate and magnetic domain elements that can have two magnetization values. The magnetic domain elements constitute bit locations for storing a single bit of data. The device has a head and a write unit for recording information in a track constituted by the bit locations on the record carrier. The value of a bit location must be set or retrieved by positioning a read/write head opposite the bit location, e.g. by scanning the track A problem of the known magnetic storage system is that setting the values of the bit locations via the magnetic state of the magnetic domain elements requires scanning the surface with a write unit.

Therefore it is an object of the invention to provide a system comprising a record carrier and a device for storing information that allow efficient setting of the information at the bit locations. According to a first aspect of the invention the object is achieved with a storage system as defined in the opening paragraph, the presence or absence of said material at the information plane representing a value of a bit location, and the device having a head that is provided with at least one electro-magnetic sensor element that is sensitive to the presence of said electro-magnetic material within a near-field working distance, the record carrier comprising a scanning surface that is substantially flat for scanning the track within the near-field working distance via the head, and the device comprising scanning means for scanning the track via the head within the near-field working distance between a bit location and the sensor element during said scanning. According to a second aspect of the invention the object is achieved with a record carrier as defined in the opening paragraph, characterized in that the presence or absence of said electro-magnetic material at the information plane represents a value of a bit location, and in that the record carrier comprises a scanning surface that is substantially flat for scanning the track within the near-field working distance via a head. According to a third aspect of the invention the object is achieved with a storage device as defined in the opening paragraph, characterized in that the device comprises a head that is provided with at least one electro-magnetic sensor element that is sensitive to the presence of said electro-magnetic material within a near-field working distance, and scanning means for scanning the track via the head within the near-field working distance between a bit location and the sensor element during said scanning.

The effect of the presence or absence of said material at the information plane representing a value of a bit location is, that a fixed pattern of material can be applied to the record carrier in a low-cost manufacturing process, e.g. by mechanically embossing a pattern. This has the advantage that data can be distributed at a low cost. The invention is also based on the following recognition. The known magnetic storage system provides a record carrier that can be recorded by magnetizing a material in a layer or pattern in a user recording device. Further there is a substantial need for data storage and distribution of data that is fixed. However the well known optical discs that provide cheap data distribution are relatively slow. The inventors have seen a record carrier can be produced by mechanical manufacture techniques for providing a pattern of electro-magnetic material on a substrate. The material is called electro-magnetic because its presence or absence is detectable via an electrical and/or magnetic field (also called bias field). It is noted that the detection of the value of a bit location does not depend on the magnetic state of the material, but on the presence or absence of the material itself within the working distance of the electro-magnetic element. The electro-magnetic element detects disturbances in the bias field within a predefined near-field working distance, which is in practice in the same order of magnitude as the mi mum dimensions of the bit location. Scanning at close range is performed to bring the electro-magnetic element opposite and close to the bit locations within the near-field working distance. Suitable electro-magnetic elements are produced using solid state production methods, e.g. known from producing MRAM magnetic storage devices.

It is to be noted that US 6,347,016 discloses a master information carrier. The master disc has a substrate with an embossed surface corresponding to an information signal. Protruding portions of the surface comprise a ferromagnetic material. The information signal is written to a magnetic record medium with a ferromagnetic thin film or coating by putting the surface of the magnetic record medium into contact with the master disc so as to write a magnetized pattern from the protruding portions into the record medium. The master disc is a manufacture tool for well-known magnetic discs, and is different in that the surface is not suitable for scanning. Further preferred embodiments of the device and server entity according to the invention are given in the further claims.

These and other aspects of the invention will be apparent from and elucidated further with reference to the embodiments described by way of example in the following description and with reference to the accompanying drawings, in which

Figure la shows a record carrier (top view),

Figure lb shows a record carrier (cross section),

Figure 2a shows a patterned information carrier part, Figure 2b shows an embossed information carrier part,

Figure 2c shows an information carrier part having embedded particles,

Figure 3 shows an information carrier in a cartridge,

Figure 4 shows a storage device for scanning an information carrier,

Figure 5 shows a sensor elements at a near field working distance of an information plane, and

Figure 6 shows a sensor element in detail. In the Figures, elements which correspond to elements already described have the same reference numerals. Figure la shows a disc-shaped information carrier 11 having a track 9 and a central hole 10. The track 9 constituted by a series of marks representing information, is arranged in accordance with a spiral pattern of turns constituting substantially parallel tracks on an information layer. A mark is a physical area of the track, also called bit location, that represents a bit of information by the absence or presence of material on an information plane. Hence the information carrier, as complete product also called record carrier, is of the read-only type, and the information content is provided during manufacture. The marks on the information plane are electro-magnetically readable, as described below with reference to Figure 5. The surface of the information carrier is substantially flat for allowing scanning by a head having a sensor element within a near-field working distance of the information layer.

Figure lb is a cross-section taken along the line b-b of the information carrier

11, in which a substrate 15 is provided with an information plane 12 at the upper level of the substrate 15, constituted by a layer of electro-magnetic material 16. The depressed parts 14 of the substrate are a cross section of the track 9, e.g. of marks as described below with reference to Figure 2b. A protective layer 17 covers the information plane for constituting a scanning surface 18 of the information carrier that is flat for allowing a head to scan the track very close to the scanning surface, i.e. within a near-field working distance of a sensor element. The information plane may alternatively be covered with a lubricant. Further ways of providing a substantially flat scanning surface depend on the type of information plane, e.g. as described below with reference to Figure 2.

In an embodiment of the record carrier the track 9 has an embossed track structure provided during manufacture. The track structure is constituted, for example, by a groove which enables a read/write head to follow the track during scanning. The groove may be implemented as an indentation or an elevation of the substrate 15 material, or as a material property deviating from its surroundings, and may be detectable via the sensor element or via a separate tracking sensor, e.g. an optical sensor using a laser beam and photo-detector. In an embodiment the track structure comprises position information, e.g. addresses, for indication the location of units of information, usually called information blocks. The position information includes specific synchronizing marks for locating the start of such information blocks.

In an embodiment the record carrier 11 is carrying information in files representing digitally encoded real-time information like PCM, MP3, or video according to a standardized format like MPEG2. Figure 2a shows a patterned information carrier part in a cross section view along the track. The information carrier has a substrate 21. An information plane 12 is constituted on the top side of the substrate 21 by a pattern of electro-magnetic material, the pattern being arranged to constitute the track via a series of bit locations. In a first bit location 22 the material is present for example indicating the logic value 1, and in a second bit location 23 the material is absent for example indicating a logic value 0. The material has a soft magnetic property for being detectable by said sensor elements. The pattern of material can be applied by well known manufacturing methods for patterned magnetic media, although it is to be noted that no permanent magnetizations are required. Suitable methods are sputtering and locally etching, ion beam patterning or pressing using a mask.

Figure 2b shows an embossed information carrier part in a cross section view along the track. The information carrier has a substrate 25. An information plane is constituted on the top side of the substrate 25 by a continuous layer of electro-magnetic material that has depressed portions. The shape of the layer the pattern is arranged to constitute the track via a series of bit locations. In a first bit location 26 the material is present on the information plane within the near-field working distance of the intended read-out unit, for example indicating the logic value 1. In a second bit location 27 the material is absent from the information plane by a depressed portion which brings the material outside the near- field working distance, for example indicating a logic value 0. The embossed pattern can be applied to the substrate (or to the layer itself) by well known manufacturing methods, like pressing using a stamp similar to producing optical discs of the CD type. For example for production first fabricate a resist mask on a bare Si wafer by means of electron-beam lithography and use this as a master. If desired, holes are etched in the Si for storing the information in the 2D hole pattern. Then, using the master, replicate the pattern on a foil, or via injection moulding, or via embossing, or via 2P. Then deposit a thin magnetic layer on the replica, and, optionally, magnetize the material in a uniform external magnetic field. The electro-magnetic material can be formed as a layer on a substrate by a usual method for creating a thin film, such as sputtering, vacuum vapor deposition, plating, or chemical vapor deposition. It is to be noted that there are various possibilities for the exact operation principle. The information plane merely functions as a flux guide (using soft-magnetic material, and hence no magnetization step required); the information plane uses shape anisotropy, resulting e.g. in a perpendicular magnetization of the inverted holes; or the information plane has been magnetized uniformly, resulting in stray fields at the edges of the holes. The first principle, as further described with Figure 5, has the advantage that it is most simple to realize, and it circumvents the limitations on bit size imposed by the super paramagnetic limit.

Figure 2c shows an information carrier part having embedded particles in a cross section view. The information carrier has a substrate 28. An information plane is constituted at the top side of the substrate 28 by embedding particles 29. The particles are arranged to constitute the track via a series of bit locations. At a bit location there is either a particle of the material embedded or no particle, indicating the logical value. The particles present the material within the near-field working distance of the intended read-out unit. Obviously, instead of embedding a single particle at a bit location, a number of smaller particles may be used also. The information carrier is manufactured by incorporating a pattern of beads in the substrate or attaching beads to the substrate using a glue mask. Alternatively the beads can be positioned by applying spatially modulated magnetic fields.

Figure 3 shows an information carrier in a cartridge. The record carrier is formed by the information carrier 11 enclosed by a cartridge 31. The cartridge is closable by a slider 32, which slider is moved away when the cartridge is inserted in a device for making the information carrier surface accessible for a read head. The centre hole 10 is visible for coupling to a driving unit in the device, but may alternatively be covered by the slider 32. The cartridge and slider constitute a substantially closed box around the information carrier 11 when not inserted in a device. A storage device has an opening mechanism (not shown) for moving the cover aside during said inserting. Several options for slidable covers are known from optical or magnetic recording disc cartridges and cooperating devices.

In an embodiment the cartridge is provided with a contamination collecting unit 33 for collecting dust which has accidentally has entered the cartridge. In an embodiment the contamination collecting unit 23 is constituted by the inner walls of the cartridge being covered with a layer of material to which particles adhere, e.g. a material with a high surface energy being highly reactive such as (chemically treated) activated carbon. In an embodiment the contamination the collecting unit 33 is a cleaning pad. The pad or other cleaning units such as a brush may be placed on the cartridge itself. Alternatively the pad is located on and/or moved by the cover 32 for wiping the information carrier surface when the cover is moved.

Figure 4 shows a storage device for scanning an information carrier. The apparatus is arranged for reading the information carrier 11, which information carrier is identical to the information carriers shown in Figure 1 or 2. The device is provided with read means comprising a read head 41 for scanning the track on the information carrier, a drive unit 45 for rotating the information carrier 11, a read signal processing unit 48 for example comprising a channel decoder and an error corrector and a system control unit 40. The read head comprises a sensor element 42 detecting the presence or absence of the electro-magnetic material on a track of the information carrier via an electro-magnetic field 38 at an interface surface of the sensor element. The field 38 is generated by the sensor element itself or by a separate field generator. The read head 41 further comprises a tracking actuator 37 for fine positioning of the sensor element in radial direction on the center of the track opposite the bit locations. The field 38 is affected by the material in the bit locations as described below with reference to Figure 5. A read signal 47 is generated from the variations in the field 38 as affected by said material. During reading, the read signal 47 is converted into output information, indicated by arrow 49, in the read signal processing unit 48.

A tracking error signal is generated by a tracking monitor 46 indicating the deviation of the sensor element from the center of the track, for example by monitoring the overall amplitude of the read signal 47. The tracking error signal is applied to a tracking control unit 44 that provides a drive signal for the tracking actuator 37. The apparatus has a positioning mechanism 43 for coarsely positioning the read head 41 in the radial direction on the track, e.g. a sledge or rotating arm. The fine positioning is performed by the tracking actuator 37, e.g. comprising coils for radially moving the sensor element, hi an embodiment the coarse tracking and fine tracking are combined in a single tracking actuator, for example a voice coil motor controlling the radial position of the head 41 and sensor element 42.

The total of the drive unit 45, the tracking monitor 46, the tracking control unit 44, the tracking actuator 37, the positioning mechanism 43 and the system control unit 40 constitute the scanning means that allow scanning of the track. A system bus 39 connects the system control unit 40 to the units to be controlled, such as the positioning mechanism 43, the tracking control unit 44, the drive unit 45, the tracking monitor 46 and the read signal processing unit 48.

In an embodiment the tracking monitor 46 is arranged for detecting a misalignment as follows. A substantial misalignment of the sensor element with respect to the center of the track results in covering bit locations of two adjacent tracks. Read-out signals of adjacent locations having the same value will be different from read-out signals of adjacent locations having differing values. During scanning of the track it is well known to synchronize a detector unit to the centre of the bit locations in the direction along the track for detecting the peak value of the read signal at the time the sensor element is maximally covering the bit location. Such peak values should have either a first value or a second value in dependence of the presence of material on the bit location. Hence if misalignment occurs intermediate values will be detected. A control signal is generated to activate the actuator 37 until such intermediate values are minimized. In an embodiment the record carrier is provided with optical marks for enabling tracking, and the device is provided with separate optical sensors for detecting the optical marks for generating a misalignment signal. In an embodiment of the scanning device the read processing unit 48 is provided with processing circuitry for analyzing the read signal for eliminating influences of neighboring bit locations. The sensor element will be influenced by adjacent bit locations due to remaining misalignment resulting from a (residual) tracking error, which results in a read signal disturbed by inter-symbol interference. By analyzing the read signal electronic compensation for the inter-symbol interference is provided. For example the read signal may be filtered based on the detected inter-symbol interference, hi an embodiment further additional read signals of neighboring tracks are generated by additional sensor elements, and are combined to compensate any inter-symbol interference. In an embodiment of the scanning device the head is provided with an array of sensor elements. A linear array is positioned perpendicular to the track direction. Several tracks are scanned at the same time during a single scan. A multitude of read signals is generated by the array. The read processing unit 48 has parallel processing circuitry for deriving the information in parallel from the multitude of read signals. In an embodiment of the scanning device the head is provided with a high density array of sensor elements for generating a multitude of read signals for a single track. The high density array can be a linear array that has a smaller pitch than the track pitch, e.g. having n sensor elements spanning m tracks, n being larger than m, e.g. m=2 and n=8. The linear array is positioned on the head perpendicular to the track. In a further alternative the high density array can be achieved by using a two-dimensional array of sensor elements, e.g. having 4 columns that are shifted in 1/4 fractions of the pitch in the direction perpendicular to the track. Alternatively the high density array can effectively be achieved by positioning an array of sensor elements in a slanted direction with respect to the track.

The high density array has the effect that some of the sensor element of the array will be substantially centered on the track while other elements are partly off track or in between tracks. The read processing unit 48 has a parallel read signal selection circuit for selecting the read signals from the sensor elements that are centered on the track. The read processing unit may include a combination unit for combining several read signals in dependence of a detected misalignment. For example the signal of two sensor elements located on both sides of the track may be combined and interpolated to generate a single improved read signal, thereby reducing the inter-symbol interference of neighboring tracks.

In an embodiment having the high density array the device does not have a fine tracking actuator. It is noted that the misalignment of a high density array having effectively n sensor elements for scanning a track has at least one read signal at maximally a misalignment of the track pitch divided by 2*n, e.g. using n = 4 results in a maximal misalignment of 1/8 of the track pitch. By using the read signal of the sensor that is aligned best above the track the read signal will fully acceptable. It is to be noted that due to eccentricity of the track or the record carrier the track to be scanned will be shifting along the array, and the selection circuit is arranged to (periodically) adjust the selection of the best read signal.

In an embodiment the scanning device is provided with a height actuator for controlling a height of the interface surface of the sensor element above the scanning surface of the record carrier. The tracking monitor 46 is provided with a height signal generator. For example the height signal may be derived from the slope of the transitions of the read signal when traveling from a bit location having material to a bit location having no material. At a larger height the slope will be less steep. The height signal is coupled to the tracking control unit 44 for generating a drive signal for the height actuator.

In an embodiment the scanning device is provided with a pressure system for bringing the interface surface of the sensor element on a predefined distance from the substrate of the record carrier, for example by creating a pressure between the substrate and the interface surface that depends on the flying height. The head is of by a sliding type having aerodynamically shaped guiding elements. The pressure is created by the speed of the surface of the record carrier relative to the sliding head. The head derives its position above the scanning surface of the record carrier from the pressure, e.g. by resilient hinges that provide a balance to said pressure. hi an embodiment the device is provided with a generator for generating an electrostatic field for attracting the information carrier to the interface surface. The attractive force of the electrostatic field may be balanced against the air pressure as described in the previous embodiment. In an embodiment of the record carrier the information plane is provided on a flexible substrate. The flexible substrate may be attracted to the interface surface by said electrostatic field, or may be pushed to the interface surface, e.g. by air pressure or mechanical elements on the other side of the substrate. Figure 5 shows sensor elements at a near field working distance of an information plane. Two sensor elements 54, 56 are shown. It is noted that the Figure depicts either two elements as part of an array, or a single sensor element at two scanning positions. Above the sensor elements 54, 56 an information carrier part is shown having a substrate 51 and a layer of a magnetic material 52. At a bit location 53 the material is present at the level of the information plane, close to the sensor element 56 and within its near-field working distance. At the adjacent bit location the material is outside the near-field working distance of the next sensor element 54, because that portion of the substrate is depressed. The sensor elements are arranged for generating magnetic fields 55,57, for example as shown by guiding an electric current via a lead 58 beneath the element 56. The magnetic field is influenced by the absence or presence of the magnetic material as shown in the resulting magnetic fields 55,57, which result in a different magnetic direction in a top layer of the sensor element. The direction is detected in sensor elements having a multilayer or single layer stack by using a magneto-resistive effect, for example GMR, AMR or TMR. The TMR type sensor is preferred for resistance matching reasons for the read-only sensor element in the read head of this invention.

As shown in the Figure the vicinity of a portion of the magnetic layer on the information carrier forces the field lines of a bias field away from the TMR-element. The material acts as a flux guide: the field lines go through the material instead of through the free layer of the spin-tunnel junction. If the stack of the spin-tunnel junction is designed such that the interlayer magnetostatic coupling results in an antiparallel magnetization configuration if no external magnetic field is applied, the vicinity of a protrusion of the magnetic layer results in a high resistance, while otherwise the bias field will cause a low resistance state. In an embodiment a current carrying conductor is used as field generating strap for the bias field. Alternatively this may be a permanent magnet. Many variants are possible for the bias fields and also stray fields may be used, as will be clear for the person skilled in the art. The bias field in the media can be in the plane of the substrate (as shown in the Figure), but one could alternatively also consider bias fields perpendicular to the substrate resulting in stray fields from the magnetic layer that have components in the plane of the layers of the spin-tunnel junctions. While the given examples use magnetoresistive elements with in-plane sensitivity it is also possible to use elements that are sensitive to perpendicular fields. For a further description of sensors using magnetoresistive effects refer to "Magnetoresistive sensors and memory" by K.-M.H. Lenssen, as published in "Frontiers of Multifunctional Nanosystems", page 431-452, ISBN 1-4020-0560-1 (HB) or 1-4020-0561-X (PB).

In the storage system data are represented by magnetization directions occurring at a sensor element due to the bit location opposite the sensor on the information plane. The read-out is done by a resistance measurement which relies on a magnetoresistance (MR) phenomenon detected in a multilayer stack. Sensors can be based on the anisotropic magnetoresistance (AMR) effect in thin films. Since the amplitude of the AMR effect in thin films is typically less than 3%, the use of AMR requires sensitive electronics. The larger giant magnetoresistance effect (GMR) has a larger MR effect (5 a 15%), and therefore a higher output signal. The magnetic tunnel junctions use a large tunnel magnetoresistance (TMR) effect, and resistance changes up to «50% have been shown. Because of the strong dependence of the TMR effect on the bias voltage, the useable resistance change in practical applications is at present around 35%. In general, both GMR and TMR result in a low resistance if the magnetization directions in the multilayer stack are parallel and in a high resistance when the magnetizations are oriented antiparallel. In TMR multilayers the sense current has to be applied perpendicular to the layer planes (CPP) because the electrons have to tunnel through the barrier layer; in GMR devices the sense current usually flows in the plane of the layers (CIP), although a CPP configuration might provide a larger MR effect, but the resistance perpendicular to the planes of these all-metallic multilayers is very small. Nevertheless, using further miniaturization, sensors based on CPP and GMR are possible. Figure 6 shows a sensor element in detail. The sensor has a bit line 61 of an electrically conductive material for guiding a read current 67 to a multilayer stack of layers of a free magnetic layer 62, a tunneling barrier 63, and a fixed magnetic layer 64. The stack is build on a further conductor 65 connected via a selection line 68 to a selection transistor 66. The selection transistor 66 couples said read current 67 to ground level for reading the respective bit cell when activated by a control voltage on its gate. The magnetization directions 69 present in the fixed magnetic layer 64 (also called pinned layer) and the free magnetic layer 62 determine the resistance in the tunneling barrier 63, similar to the bit cell elements in an MRAM memory. The magnetization in the free magnetic layer is determined by the material at the bit location opposite the sensor as described above with Figure 5, when such material is within the near-field working distance indicated by arrow 60.

In an embodiment no additional means are needed to generate the bias field, but the bias field is effectively built-in in the spin-tunnel junction. This might, for example, be accomplished in the following ways. A built-in permanent magnet is achieved by an additional hard-magnetic layer underneath or above the spin-tunnel junction, or by an "over- dimensioned" pinned layer, e.g. an exchange-biased layer, or the hard-magnetic layer in the case of a "pseudo-spin valve like" MR-element. It is important that the resulting magnetostatic coupling dominates any direct exchange coupling between pinned and free layer, as is generally the case for a spin-tunnel junction. The effect of the magnetostatic coupling on the free layer should be reduced sharply when the soft-magnetic layer of the information carrier is close to the element, i.e. inside the near-field working distance. This can be accomplished by making the distance sufficiently small and the thickness of this layer sufficiently large. In an embodiment the material in the information plane is permanent magnetized in a direction parallel to the magnetization direction of the free layer in the sensor element. Because of flux closure protrusions in the information carrier will lead to a reversal of the magnetization of the free layer, provided the coupling to the carrier is stronger than the coupling with the other layers within the MR element.

For the sensor elements, because of the different requirements compared to those for MRAM, the composition and characteristics of the spin-tunnel junctions are adapted compared to those used for MRAM. While for MRAM two stable magnetization configurations (i.e. parallel and antiparallel) are essential for the storage, this does not have to be the case for the proposed sensor element. Here read sensitivity is crucial, while a bi-stable magnetization configuration is in general not relevant. Of course the direction of the reference magnetization, e.g. in the pinned or exchange-biased layer should be invariant.

Hence for the free layer, which acts as detection layer, materials with a low coercivity can be chosen. In an embodiment the sensor element is positioned to detect magnetic field components perpendicular to the information carrier by rotating the element 90° with respect to the position of the sensor element as shown in Figure 5. In an embodiment a number of sensor elements are read at the same time. The addressing of the bit cells is done by means of an array of crossing lines. The read-out method depends on the type of sensor. In the case of pseudo-spin valves a number of cells (N) can be connected in series in the word line, because the resistance of these completely metallic cells is relatively low. This provides the interesting advantage that only one switching element (usually a transistor) is needed per Ν cells. The associated disadvantage is that the relative resistance change is divided by N. The read-out is done by measuring the resistance of a word line (with the series of cells), while subsequently a small positive plus negative current pulse is applied to the desired bit line. The accompanying magnetic field pulses are between the switching fields of the two ferromagnetic layers; thus the layer with the higher switching field (the sensing layer) will remain unchanged, while the magnetization of the other layer will be set in a defined direction and then be reversed. From the sign of the resulting resistance change in the word line it can be seen whether a '0' or a ' 1 ' is stored in the cell at the crossing point the word and the bit line, hi an embodiment spin valves with a fixed magnetization direction are used and the data is detected in the other, free magnetic layer. In this case the absolute resistance of the cell is measured. In an embodiment the resistance is measured differentially with respect to a reference cell. This cell is selected by means of a switching element (usually a transistor), which implies that in this case one transistor is required per cell. Besides sensors with one transistor per cell, alternatively sensors without transistors within the cell are considered. The zero-transistor per cell sensor elements in cross-point geometry provide a higher density, but have a somewhat longer read time.

The memory device according to the invention is in particular suitable for the following applications. The record carrier can be used as a storage medium for content distribution. A further application is a memory that is very well copyright-protected. The protection benefits from the fact that no recordable/rewritable version of the record carrier exists and a consumer cannot easily copy the read-only information carrier, and from the fact that without the (correct) bias field reading the information carrier is not possible. For example this type of memory is suitable for game distribution. In particular the record carrier is easily replicable.

Although the invention has been mainly explained by embodiments using soft magnetic material and flux guidance, any type of near-field interaction can be used, e.g. capacitive coupling. Further a disc shaped record carrier and rotational scanning has been described in the examples, but obviously any shape of the record carrier, such as rectangular is possible. Further the scanning may be linear or any suitable movement of the record carrier with respect to a head comprising (an array of) sensor element(s), for example the user pushing the record carrier along the head. It is noted, that in this document the verb 'comprise' and its conjugations do not exclude the presence of other elements or steps than those listed and the word 'a' or 'an' preceding an element does not exclude the presence of a plurality of such elements, that any reference signs do not limit the scope of the claims, that the invention may be implemented by means of both hardware and software, and that several 'means' or 'units' may be represented by the same item of hardware or software. Further, the scope of the invention is not limited to the embodiments, and the invention lies in each and every novel feature or combination of features described above.

Claims

CLAIMS:

1. Storage system comprising a record carrier and a storage device, which record carrier can be inserted in and removed from the storage device,

- the record carrier having an information plane that is a pattern of an electro-magnetic material on a substrate at bit locations that constitute at least one track, the presence or absence of said material at the information plane representing a value of a bit location, and

- the device having a head that is provided with at least one electro-magnetic sensor element that is sensitive to the presence of said electro-magnetic material within a near-field working distance,

- the record carrier comprising a scanning surface that is substantially flat for scanning the track within the near-field working distance via the head, and the device comprising scanning means for scanning the track via the head within the near-field working distance between a bit location and the sensor element during said scanning.

2. Record carrier for use in the system as claimed in claim 1 , the record carrier comprising an information plane that is a pattern of an electro-magnetic material on a substrate at bit locations that constitute at least one track, characterized in that the presence or absence of said electro-magnetic material at the information plane represents a value of a bit location, and in that the record carrier comprises a scanning surface that is substantially flat for scanning the track within the near-field working distance via a head.

3. Record carrier as claimed in claim 2, wherein the information plane is covered with a protective layer for constituting the scanning surface.

4. Record carrier as claimed in claim 2, wherein the pattern at the information plane is constituted by a continuous layer of the electro-magnetic material on the substrate having depressed portions that bring the material of the layer outside the near-field working distance.

5. Record carrier as claimed in claim 2, wherein the pattern at the information plane is constituted by the substrate covered by a pattern of areas of the electro-magnetic material.

6. Record carrier as claimed in claim 2, wherein the pattern at the information plane is constituted by the presence or absence of particles of the electro-magnetic material embedded in the substrate.

7. Record carrier as claimed in claim 2, wherein the electro-magnetic material has a soft magnetic property for being detectable by said sensor element, or wherein the electro-magnetic material has an electrically conductive property for being detectable by said sensor element.

8. Record carrier as claimed in claim 2, wherein the substrate is of a flexible material for allowing positioning of the bit locations near the sensor elements within the near- field working distance between a bit location and the corresponding sensor element.

9. Storage device for use in the system as claimed in claim 1, characterized in that the device comprises a head that is provided with at least one electro-magnetic sensor element that is sensitive to the presence of said electro-magnetic material within a near-field working distance, and scanning means for scanning the track via the head within the near-field working distance between a bit location and the sensor element during said scanning.

10. Device as claimed in claim 9, wherein the device comprises means for generating an electro-magnetic field near the sensor element in at least one of the following ways:

- generating a magnetic field and the sensor element being arranged for detecting the magnetic field as influenced by the presence of absence of the electro-magnetic material via a soft magnetic property;

- generating an electrical field and the sensor element being arranged for detecting the electrical field as influenced by the presence or absence of the electro-magnetic material via a capacitive coupling; - generating a fluctuating magnetic field and the sensor element being arranged for detecting the magnetic field as influenced by the presence of absence of the electro-magnetic material via eddy currents.

11. Device as claimed in claim 10, wherein the means for generating an electromagnetic field are included in the sensor element.

12. Device as claimed in claim 9, wherein the scanning means comprise a driving unit for rotating the record carrier and positioning means for positioning the head opposite the track.

13. Device as claimed in claim 9, wherein the scanning means comprise means for positioning the head in dependence of a read-out signal from the sensor elements.

15. Device as claimed in claim 9, wherein the scanning means comprise means for generating an electrostatic field for attracting the record carrier.

16. Device as claimed in claim 9, wherein the head comprises an array of electromagnetic sensor elements for generating a multitude of read signals, and the device comprises a read signal processing unit that has parallel read signal means for processing said multitude for deriving a read signal.