Method of and apparatus for writing and reading a magneto-optical record carrier

Abstract: A method and apparatus are described for optically writing and reading information in a pattern of magnetic domains (4, 5) in an information layer (3) of a record carrier (1) by means of an optical scanning beam (b) focused to a diffraction-limited radiation spot (V) and a magnetic field which is generated by means of a coil (13) through which a square-wave energising current is passed which is modulated in dependence upon an information signal (S i ). By modulating the amplitude of the energising current in accordance with the information signal to be written, domains (4) having a variable magnetisation ratio are written, and the information density can be increased.

Description: The invention relates to a method of optically writing and reading information in a pattern of magnetic domains in an information layer of a record carrier by means of an optical scanning beam focused to a radiation spot by subjecting, during writing, the portions of the information layer heated by the radiation spot to the influence of a magnetic field which is directed substantially perpendicularly to the information layer and is generated by means of a coil through which an energising current is passed so that said domains are formed, and by detecting, during reading, the variation caused by the magnetic domains in the state of polarisation of the scanning beam. The invention also relates to an apparatus for performing the method. Such a method of recording information is known from United States Patent no. 4,466,004. Generally, a laser beam, for example a diode laser beam, which is focused to a diffraction-limited radiation spot by means of an optical system is used when inscribing a magneto-optical information layer. The still uninscribed information layer is premagnetised in a direction perpendicular to this layer. During writing the portion of the magneto-optical material at the location of the radiation spot is heated to a given temperature, for example, the Curie temperature, so that the coercive force is reduced locally. As a result, this portion can be magnetised by a relatively small external magnetic field in a desired direction perpendicular to the magnetic layer. After the relevant portion of the magneto-optical information layer has cooled down, the magnetical direction of the external magnetic field is frozen, as it were, into the information layer. By moving the radiation spot and the record carrier with respect to each other and by modulating the external magnetic field, a series of magnetic domains, or information areas, having a direction of magnetisation deviating from their surroundings can be written in the information layer, the succession of the information areas in the direction of movement representing the inscribed information. This method is known as the magnetic field modulation method. It is alternatively possible to inscribe magnetic domains by means of a constant external magnetic field and by modulating the intensity of the radiation beam in accordance with the information to be written. In this so-called radiation source modulation method the size of the information areas is determined by the size of the radiation spot. In known systems, in which the radiation spot has a half-value width of approximately 1 µm, the information areas are substantially circular with a diameter of the order of 1 µm. The information density is then of the order of 10⁶ bits per mm². There is an ever increasing need of larger information densities so that more information can be stored in a record carrier of equal dimensions. To this end it must be possible to write and read information areas which are smaller than those hitherto used in a magneto-optical record carrier. In said United States Patent no. 4,466,004 it is proposed to provide information areas in the form of magnetic domains in a magneto-optical record carrier, which areas have a dimension in the scanning direction which is smaller than this dimension of the write-radiation spot, by switching the magnetic field at a high frequency. Firstly, the area of the information layer under the radiation spot is magnetised in a direction opposite to the original direction of magnetisation of the information layer. Then, while the radiation spot is still partly above said area, the magnetic field is reversed so that said part of the area acquires the original direction of magnetisation again. United States Patent no. 4,466,004 does not state how the magnetic domains with their smaller dimension in the scanning direction thus obtained can be read Since each information bit is fixed in an information area, each information area must be read separately. This means that reading must be performed by means of a radiation spot whose dimension in the scanning direction is of the same order as this dimension of the information areas. This read-radiation spot must be considerably smaller than the write-radiation spot. An optical scanning system in which radiation at a given wavelength, λ, and an objective lens having a given numerical aperture, NA, are used, has an optical cut-off frequency f co which is proportional to 2.NA/λ, i.e. inversely proportional to the size of the scanning spot. Such a system can no longer separately observe details of an object, in this case the information areas in the information layer, if the mutual distance between these details or areas is equal to or smaller than 2.NA/λ. Thus, a given spatial frequency f r of the information areas is associated with this optical cut-off frequency. Since the size of the diffraction-limited radiation spot is proportional to λ/NA, in which λ is the wavelength of the radiation used and NA is the numerical aperture of the objective system used, the radiation spot can only be reduced by decreasing the wavelength and/or enlarging the numerical aperture. An enlargement of the numerical aperture involves a decrease of the depth of field of the radiation beam so that the requirements to be imposed on the focusing of the radiation beam become more stringent. Moreover, an objective system having a larger numerical aperture is more sensitive to aberrations so that stricter tolerance requirements must be imposed on the write-read apparatus. If a diode laser is to be maintained as a radiation source, which is necessary in a mass product which the magneto-optical write-read apparatus envisages to be, the reduction of the wavelength of the radiation beam is not a real possibility because there are no short-wavelength diode lasers which yield a sufficiently high power for writing. The present invention provides a novel possibility of increasing the information density in a magneto-optical record carrier, in which the radiation spot need not be reduced. This novel possibility is realised by means of a novel method which is characterized in that, during writing, the amplitude of the energising current is modulated in dependence upon the information signal to be written so that magnetic domains are produced which comprise sub-domains having dimensions which are considerably smaller than the size of the radiation spot, the number of sub-domains per domain and hence the magnetisation ratio of the relevant domain being determined by the information signal. The magnetisation ratio of a domain is defined as the ratio between the total surface of the areas within the domain having a first direction of magnetisation perpendicular to the information layer and the total surface of the areas within the domain having a second direction of magnetisation which is opposed to the first direction of magnetisation. This ratio determines the state of polarisation of the scanning beam. The invention is based on the recognition that by means of a relatively large variation of the strength of the external magnetic field magnetic domains are written, which change the state of polarisation of a read beam to a different extent, and hence represent different signal values of the information signal. More information can then be written per domain. Use is made of the effect, which has hitherto not been used in magneto-optical recording, that when varying the external magnetic field strength from a given negative value to a given positive value, more and more sub-domains with a direction of magnetisation opposed to the original direction of magnetisation are produced in the area of a domain until the relevant area is entirely filled with sub-domains. During reading the variation of the number of sub-domains per domain causes a variation of the polarisation modulation of the read beam issuing from the information layer and passing through an objective system, and hence a variation of the intensity of the radiation received by the radiation-sensitive detection system. This detection system then supplies an electric signal which is modulated in accordance with the inscribed information. The sub-domains within a domain, which sub-domains are not separately detected, jointly determine the intensity of the radiation beam issuing from this domain and received by the detection system. As it were, different grey levels are now written and read, with each grey level representing a given information signal value, instead of a black-white structure as has hitherto been conventional. It is to be noted that it is known per se from European Patent Application no. 0,284,056 to write different signal levels in a magneto-optical information layer so as to increase the information density. In accordance with the known method, however, the intensity of the write beam is varied so that the level within the information layer, for which the material is heated above the Curie point and the direction of magnetisation can be reversed, is also varied. However, a relatively thick information layer must then be used and the radiation beam must have a high intensity. Moreover, such a thermal process is difficult to control due to its thermal diffusion. In accordance with the method described in European Patent Application no. 0,284,056 the written information areas are detected separately. The method according to the invention may be performed with various magnetic materials as an information layer. However, the method is preferably further characterized in that a magnetic material whose compensation temperature is considerably lower than the Curie temperature is used for the information layer. These materials generally comprise rare earth transition metals and are structured in accordance with two sub-lattices having an opposite magnetisation. The compensation point is the point on the temperature scale where the absolute values of the magnetisations associated with the two sub-lattices are equal, while the Curie point is the point on the temperature scale for which both magnetisations are zero. According the compensation point is further away from the Curie point, there is a smaller risk that the domain walls are in motion during the formation of the magnetic domains and that consequently the walls of the domains formed are not well-defined so that noise is produced upon subsequent reading. For the magnetic materials having a compensation temperature above the Curie temperature the signal-to-noise ratio is satisfactory, also for domains which have been inscribed at a small magnetic field strength. These materials also have a satisfactory linear relationship between the strength of the magnetic field applied and the number of sub-domains per domain, or the grey value. A material which is optimal as regards said linear relationship and the signal-to-noise ratio is constituted by an alloy of terbium, iron and cobalt. The compensation point of this material is determined by the ratio between terbium on the one hand and iron-cobalt on the other hand, while the Curie point is determined by the ratio between iron and cobalt. Satisfactory results can alternatively be achieved when using a multi-layer structure of cobalt-platinum as an information layer. The use of such a multi-layer structure for magneto-optical information storage in general is described in, for example "Applied Physics Letters", 54, (1989), page 2481. The above-described, still general method according to the invention may be further characterized in that the amplitude of the energising current is continuously modulated in accordance with the signal value of the signal to be written. Thus, the signal is written in the information layer in an analogous form. The method according to the invention is preferably characterized in that the amplitude of the energising current is varied in n discrete steps, in which n is an integer which is larger than two. The information signal is now recorded, as it were, in n different grey levels. If n = 8, i.e. for eight grey levels on the record carrier, use can be made of an encoding which in addition to the values "1" and "0" also uses 1/7, 2/7, 3/7, 4/7, 5/7 and 6/7. A first, preferably used main embodiment of the last-mentioned method is characterized in that domains are written which adjoin each other in the scanning direction without the interposition of intermediate areas. The entire path traversed by the scanning spot on the information layer is then provided with new information so that a record carrier which has already been inscribed can be directly overwritten without first having to erase this record carrier. A second main embodiment of the method is characterized in that magnetic domains which are separated from each other are written. A first embodiment of this method is characterized in that use is made of a square-wave energising current so as to fix the positions of the magnetic domains, which current has a first level located above a limit value at which magnetic domains having the first direction of magnetisation are written and a second level below this limit value, the first level being varied in accordance with the desired magnetisation ratio of said magnetic domains. The second level may be such that the magnetic field is insufficiently strong for writing magnetic domains. In that case only a record carrier whose information layer is entirely premagnetised in a direction opposite to the first direction of magnetisation can be written. However, said first embodiment of the method is preferably further characterized in that the second level is located below a second limit value at which magnetic domains having a direction of magnetisation which is opposite to the first direction of magnetisation are written. A record carrier which has already been inscribed can then be directly overwritten without first having to erase this record carrier. The first embodiment may be further characterized in that use is made of a radiation beam having a constant intensity. With this embodiment it is achieved in the simplest manner that the information layer is always sufficiently heated locally for writing a domain. Alternatively, the first embodiment may be further characterized in that use is made of a pulsed radiation beam. By supplying the radiation energy in a pulsed manner, the temperature gradient in the boundary region of a magnetic domain to be formed is very high, so that the accuracy with which the boundaries of the magnetic domain are fixed is great. When reading the pattern of magnetic domains, this results in an improved signal-to-noise ratio, notably at high write speeds. A further advantage is that the thermal load of the information layer and hence the rate at which this layer ages is smaller than in the case of using a continuous radiation beam. The method in which use is made of magnetic field modulation and a pulsed write beam is preferably further characterized in that such a phase relation is maintained between the radiation pulses of the radiation beam and the energising current that each radiation pulse does not end until the magnetic field has adjusted to a new final value. This ensures that the magnetic field is well-defined during the local cooling of the information layer and that the domain formed has a maximum quality. A second embodiment of the method according to the invention is characterized in that use is made of a radiation beam which is square-wave modulated so as to fix the positions of the magnetic domains. The magnetic domains are now fixed by means of the so-called radiation beam modulation, while the energising current is only varied in amplitude so as to vary the magnetisation ratio of these domains. A second aspect of the present invention relates to the apparatus for performing the novel method. This apparatus, which comprises a radiation source for supplying a scanning beam, an objective system for focusing the scanning beam to a radiation spot on the information layer, a radiation-sensitive detection system for converting the beam issuing from the information layer into an electric signal, a magnet coil for generating a magnetic field which is directed substantially perpendicularly to the information layer in the scanned portion of the information layer and an energising circuit for generating an energising current in the coil, which current is modulated in dependence upon an information signal, is characterized in that the energising circuit comprises a controllable current source having a control input for receiving a signal which is proportional to the signal to be written and an output for supplying an energising current whose amplitude varies in accordance with the signal to be written. This apparatus may have different embodiments. A first, preferably used main embodiment is characterized in that the energising circuit is adapted to supply a current whose amplitude always has a level above a threshold value at which magnetic domains having the first direction of magnetisation can be written. The magnetic field is now only varied so as to obtain the different magnetisation ratios and no individual magnetic domains separated from each other are written, but the tracks are magnetised in the first direction over their entire length and the information is fixed in the different magnetisation ratios, or grey values, of the successive areas in tracks. A record carrier which has already been inscribed can then be re-inscribed immediately. The first main embodiment may be further characterized in that the radiation source is adapted to supply a write-radiation beam having a constant intensity. A second main embodiment of the apparatus is characterized in that the radiation source is adapted to supply a write-radiation beam whose intensity is square-wave modulated between a first level above a threshold value at which domains are formed, and a second level below the threshold value. While using radiation source modulation, individual magnetic domains which are separated from each other are then formed, with successive domains having different magnetisation ratios. In order to achieve that these domains have well-defined edges and the desired direction of magnetisation, the second main embodiment is characterized in that a synchronising circuit is provided for maintaining a phase relation between the intensity variation of the write-radiation beam and the energising current such that the transitions between the first and the second intensity level of the radiation beam substantially coincide with the instants when the magnetic field has reached a new final value. A third main embodiment of the apparatus is characterized in that the energising circuit is adapted to supply a square-wave energising current having a first, variable level above a threshold value at which magnetic domains having a first direction of magnetisation and a variable magnetisation ratio are written, and a second level below the threshold value. Individual magnetic domains which are separated from each other are now written while using magnetic field modulation. The third main embodiment of the apparatus may be further characterized in that the radiation source is adapted to supply a write-radiation beam having a constant intensity. Alternatively, the third main embodiment may be characterized in that the radiation source is adapted to supply a pulsed write-radiation beam whose pulse duration is considerably shorter than half the period. A synchronising circuit is preferably provided for maintaining a phase relation between the radiation pulses and the square-wave energising current such that the end of each radiation pulse substantially coincides with an instant when the magnetic field has reached a new final value. Embodiments of the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings in which Fig. 1 shows an embodiment of a write-read apparatus for a magneto-optical record carrier, Fig. 2 shows in a cross-section a part of an information track written by means of a known method, Fig. 3 shows the size of the information areas written by means of the known method with respect to the size of the write and read spot used, Figs. 4a and 4b show diagrammatically the principle of optical diffraction reading, Figs. 5a-5d show the principle of writing different grey levels by means of an amplitude-modulated magnetic field, Fig. 6 shows domains with different magnetisation ratios associated with the different strengths of the magnetic field, Fig. 7 shows the variation of the grey level as a function of the magnetic field strength for different materials, Fig. 8 shows the signal which has been read from a record carrier in which a triangular signal is written in an analog form, Fig. 9 shows a first embodiment of an energising circuit, Fig. 10 shows a pattern of discrete domains in which a signal is stored in an analog form and in which the hatch density represents the magnetisation ratio, Fig. 11 shows a pattern of contiguous magnetic domains with an octal grey coding, Fig. 12 shows a pattern of separated magnetic domains with an octal grey coding, Fig. 13 shows diagrammatically an embodiment of an apparatus for writing discrete domains by means of a square-wave modulated write beam, Figs. 14a and 14b show an energising current for writing four grey levels in discrete domains and the domain pattern written by means of this current, respectively, Fig. 15 shows the basic circuit diagram of an apparatus in which magnetic field modulation and a pulsed write beam are used, Fig. 16 shows a radiation pulse, the temperature variation caused thereby in the information layer and an energising current pulse as a function of time, Fig. 17 shows the variation of the energising current and the write beam intensity in the apparatus according to Fig. 15, and the domain pattern obtained, Fig. 18 shows an embodiment of a synchronising circuit for use in the apparatus according to Fig. 15, and Fig. 19 shows an embodiment of a write-read apparatus with two coils. In Fig. 1 the reference numeral 1 denotes a magneto-optical record carrier, comprising a transparent substrate 2 and a magnetic information layer 3. This information layer is irradiated by a radiation beam b which is supplied by a radiation source 10. This source is formed by a diode laser, for example, an AlGaAs laser emitting radiation at a wavelength of the order of, for example 800 nm. A part of the radiation emitted by the diode laser is captured by a collimator lens 11 and focused by an objective system 12, which is shown diagrammatically by means of a single lens, to a diffraction-limited scanning spot V having a half-value width of the order of 1 µm in the information plane. Information areas in the form of magnetic domains are written in the layer 3 by controlling the diode laser in such a way that it emits a beam with a sufficiently high power, for example, 5 mWatt so as to heat the region on the information layer 3 at the location of the spot V to, for example, the Curie temperature and by modulating the magnetic field in accordance with the information signal S i to be written in. This signal is applied to an energising circuit 15. The energising circuit applies an energising current to a coil 13. The magneto-optical layer 3 is, for example, premagnetised in a given direction indicated by the arrow M₁. By heating the layer 3 at the location of the radiation spot V, the coercive force at this location decreases and the direction of magnetisation can be locally reversed by means of a relatively small external magnetic field generated by means of the magnet coil 13, in accordance with the arrow M₂ in Fig. 1. After the local supply of heat is terminated, for example, because the radiation spot has been moved, the material of the layer 3 cools down again and, as it were, the direction M₂ of magnetisation is frozen. By moving the radiation spot V and the record carrier 1 with respect to each other, in the case of a round disc-shaped record carrier, for example, by rotating the record carrier around the shaft 5, a plurality of information areas can be successively written in the scanning direction so that an information track is produced. Fig. 2 shows a small portion of such an information track in a cross-section. The areas of the information layer 3 where the direction of magnetisation is reversed (M₂) are referred to as information areas 4, and the areas which have maintained the original direction of magnetisation (M₁) are referred to as intermediate areas 5. Moreover, by moving the radiation spot V and the record carrier 1 with respect to each other in a direction perpendicular to the plane of the drawing in Fig. 1, in the case of a round disc-shaped record carrier in the radial direction, a plurality of tracks can be written side by side. When reading the written information in the apparatus of Fig. 1, diode laser 10 is also used. However, this laser is then operated at a considerably lower power, for example, ten times lower than during the writing operation so that the stored information is not affected. The record carrier is preferably reflective so that the beam modulated by the information layer in accordance with the written information is reflected to the objective system 12. The radiation path incorporates a partly transmissive element, for example a 70% transmissive mirror or prism 17 which reflects a portion of the reflected and modulated read beam b′ to a radiation-sensitive detection system 18. In the embodiment of Fig. 1 a lens 19 for concentrating the radiation on the detection system 18 is arranged between the element 17 and this system. Reading of the information layer is based on the change which the information areas, or domains, 4 cause in the state of polarisation of the read beam. To detect this change, a polarisation analyser 20 is arranged in the radiation path in front of the detection system 18, which analyser converts the polarisation modulation into an intensity modulation which is converted by the detection system into an electric signal S O . A polarisor 21 whose direction of polarisation effectively extends at an angle of, for example 85° to that of the analyser 20 may be arranged in the radiation path of the ongoing read beam b. In order to be able to ascertain during reading whether the read spot is centered on the information track and/or whether the read beam is focused on the information plane, a partly transmissive mirror, for example, a 90% transmissive mirror or prism 22 may be arranged in the radiation path of the reflected beam b′ reflecting a part of this beam to a second radiation-sensitive detection system 23. The electric signals supplied by this detection system are used to correct the tracking and focusing. The tracking and focus-servo systems may be used also during writing, utilising the portion of the write beam then reflected by the record carrier. For further details about writing and reading of a magneto-optical record carrier and about its arrangement, reference may be made to the article "Erasable Magneto-Optical Recording" in "Philips Technical Review", 42, no. 2, August 1985, pages 37-47. In the conventional method of magneto-optical recording, magnetic domains are written with a surface area of the order of magnitude of that of the radiation spot. Fig. 3 shows the write spot V w of a known magneto-optical apparatus and a plurality of information areas 4 written by means of this spot. The information areas are arranged along an information track 30. This track is only partly written. During writing, the write spot moves with respect to the information plane to the right in accordance with the arrow 32. In the situation shown in Fig. 3 the write spot is present above an unwritten portion and in a position where a subsequent information area can be written. The information track is read at a later stage by means of the read spot V r which is shown at the left in Fig. 3 for the sake of clarity, but in reality the spots V r and V w coincide. In the conventional method of magneto-optical information storage each information bit is fixed in a separate magnetic domain and each domain must be detected separately. The minimum dimensions of the magnetic domains, and hence the maximum information density is then determined by the size of the diffraction-limited scanning spot. As already noted in the opening paragraph, it is not very well possible in practice to increase the information density by reducing the scanning spot. According to the present invention the information density is increased while using a scanning spot of the conventional size by writing magnetic domains having a different number of sub-domains. The information is now fixed in the different magnetisation ratios which become manifest as different grey levels during reading. The sub-domains are not observed separately because their distance is much smaller than the size of the scanning spot and because they have a frequency which is considerably larger than the so-called optical cut-off frequency of the read apparatus. The notion of optical cut-off frequency of a magneto-optical apparatus and the relation between the size of the information areas and the optical cut-off frequency can be explained in the simplest manner by considering the structure of magnetic domains as a structure of areas alternately transmitting radiation and not transmitting radiation in the case of transmission reading, or as a structure of areas alternately reflecting and not reflecting in the case of reflection reading. This is allowed because the polarisation rotation caused by the magnetic domains in combination with the use of polarisation means in the apparatus results in an amplitude modulation of the read beam. The structure consisting of information areas can be considered as a two-dimensional diffraction grating which splits the incident scanning beam into a non-diffracted zero-order sub-beam, a plurality of first-order sub-beams and a plurality of higher order sub-beams. For reading, only the zero-order and first-order sub-beams reflected in the tangential direction are important because the higher order sub-beams have only a small energy. Fig. 4a shows a linear grating g in a cross-section. This grating is illuminated by means of a beam b from an objective of which only the pupil p is shown. The grating g reflects the beam and splits it into a zero-order sub-beam b(0), a +1 order sub-beam b(+1), a -1 order sub-beam b(-1) and a plurality of higher order sub-beams which are not shown. The sub-beams b(+1) and b(-1) are deflected at angles of +α and -α, respectively. Fig. 4b shows the cross-sections of the beams at the location of the pupil. The sub-beam b(0) which has the same aperture angle β and the same direction as the incident beam b falls entirely within the pupil and is passed on to a detector (18 in Fig. 1) in the case of an optical scanning apparatus with which the information grating g is read. The zero-order sub-beam does not comprise any information about the succession of the information areas and intermediate areas. This information is particularly present in the first-order sub-beams b(+1), b(-1). Only the portions denoted by the hatched areas OV₁ and OV₂ of these sub-beams fall within the pupil. In the known read method use is made of the phase variations in the sub-beams b(+1) and b(-1) with respect to the zero-order sub-beam. In the areas OV₁ and OV₂ in Fig. 4b the first-order sub-beams overlap the zero-order sub-beams and interferences occur. When moving the scanning spot across an information track, the phases of the first-order sub-beams change. As a result, the intensity of the total radiation passing through the objective and reaching the detector changes. If the centre of the scanning spot coincides with the centre of an information area, for example a pit, there is a given phase difference ψ between a first-order sub-beam and the zero-order sub-beam. This phase difference is also referred to as phase depth of the information structure. If the scanning spot of a first information area moves to a second area, the phase of the +1 order sub-beam increases and this phase is increased by 2π at the moment when the centre of the scanning spot has reached the second information area. The phase of the -1 order beam then decreases. The phases of the first-order sub-beams with respect to those of the zero-order sub-beam can thus be represented as (+1) = ψ + 2π x p t (-1) = ψ - 2π x p t in which x is the position in the tangential direction of the scanning spot and p t is the local tangential period of the information structure. The electric output signals of two detectors arranged behind the areas of transition OV₁ and OV₂ can then be represented by S₁ = cos (ψ + 2π x p t ) S₂ = cos (ψ - 2π x p t ) By adding these detector signals, the information signal S i is obtained: S i = S₁ + S₂ + 2.cos ψ cos 2π x p t The above applies only as long as there is overlap between the sub-beams b(+1) and b(-1) on the one hand and b(0) on the other hand. The angle α at which the first-order sub-beam is deflected is given by sin α = λ p t = λ.f r in which f r is the spatial frequency of the grating g or the local spatial frequency of the information structure. There is no overlap if α = 2β. Since sin β = NA, the optical cut-off frequency f c is given by: f CO = 2 NA λ Information areas which have such a spatial frequency that α > 2β, can no longer be detected separately so that the information can no longer be read if it is coded in the frequency of these areas. However, if, as proposed in the present invention, use is made of sub-domains having such a high frequency that only the zero- order beam passes through the read objective and the signal value of the information signal is coded in the number of sub-domains per domain, this information can be read. The fact is then utilised that at any moment the amplitude of the zero-order beam is determined by the number of sub-domains present under the read spot at that moment. The magnetic domain structure is now observed, as it were, as a structure of areas having different grey hues, with each grey hue corresponding to a given magnetisation ratio, i.e. the quotient of the total surface of the areas having a first direction of magnetisation and the total surface of the areas having a second opposite direction of magnetisation within the area which is covered by the scanning spot. According to the invention the sub-domains are formed by means of an effect which has hitherto not been used in magneto-optical recording, namely the effect that in a heated portion of a magneto-optical layer which may be premagnetised in a given direction by applying an external magnetic field which is not sufficiently strong to magnetise the entire area in a direction opposite to that of the premagnetisation, sub-areas are magnetised in the said opposite direction anyway. This effect even occurs up to a given negative strength of the external magnetic field. Negative strength is understood to mean that the external magnetic field has the same direction as that of the premagnetisation. As described in the article "Magneto-optic recording materials with direct overwrite capability" in "Applied Physics Letters", 49 (8), 25 August 1986, pages 473-474 in a thin film of an amorphous ferri-magnetic rare earth metal compound such as GdTeFe, TbCo or TbFeCo whose compensation temperature is slightly above the ambient or room temperature, which film is premagnetised in a given direction, an area having an opposite direction of magnetisation can be formed by means of irradiation with a laser beam without the presence of an external magnetic field. Since the area under the laser spot is heated above the compensation temperature, the coercive force in this area is greatly reduced. A demagnetising field then locally occurs in a direction which is opposite to that of the premagnetisation and when the area cools down, this demagnetising field is frozen, as it were, into the layer. In the afore-mentioned article mention is only made of domains of the order of magnitude of 1 µm and no mention is made of considerably smaller sub-domains, nor of the consequences of the use of external magnetic fields with different strengths, both positive and negative. Experiments performed by the Applicants have surprisingly proved that the use of external magnetic fields with field strengths which are smaller than those hitherto used, and which may even be negative, lead to the formation of sub-domains in the irradiated area, which domains are considerably smaller than the conventional domains, and that the number of sub-domains increases with an increasing external magnetic field strength until a full domain is formed. This phenomenon, which may be partly explained by means of the theory described in the article in "Applied Physics Letters" 49 (8) pages 473-474, is illustrated in Figs. 5a-5d. In these Figures the magnetic information layer 3 is shown in a cross-section. The solid-line arrows M indicate the direction of premagnetisation and the broken-line arrows M₂ indicate the direction of magnetisation which is obtained upon heating and with an external magnetic field. Fig. 5a shows the initial situation in which no external magnetic field is present and in which the laser beam is switched off or has only a low intensity. The portion of the magnetic layer shown is then completely magnetised in the direction M₁. Fig. 5b shows the situation in which the laser beam has the write intensity. The irradiated area V is then heated above the Curie point. The material in the area V then becomes paramagnetic, i.e. it has in principle no preference for a given direction of magnetisation. If, as is shown in Fig. 5b, a strong negative magnetic field H = -H₁ is present, the area V remains magnetised in the original direction M₁. If, as shown in Fig. 5c, there is no external magnetic field (H = 0), the influence of the so-called demagnetising field also referred to as internal field is predominant in the information layer. This demagnetising field DF, which originates from the ambiance of the irradiated area, ensures that within the irradiated area sub-areas are magnetised in the direction M₂. In their turn these sub-areas produce demagnetising fields which, within the irradiated area, magnetise further areas in the direction M₂ and so forth. The formation of the sub-areas, or sub-domains, having magnetisation M₂ starts where the cooling begins, thus at the edge of the irradiated area in the case of writing by means of radiation source modulation, as is shown in Fig. 5c. If writing is effected by means of a continuous radiation beam and magnetic field modulation, the formation of the sub-domains with magnetisation M₂ trails the displacement of the radiation spot. However, if magnetic field modulation with a pulsed radiation beam is used, the formation of said sub-domains takes place from the edge of the irradiated area towards its centre. If a positive external magnetic field is applied, more sub-domains are produced, while the number of sub-domains is larger as the external magnetic field is stronger. At a given field strength H=+H₁, the entire area V is magnetised in the direction M₂ and the conventional domain 4 is produced again. The demagnetising field is capable of forming sub-domains with magnetisation direction M₂, also when a negative external magnetic field is applied, as long as the demagnetising field is larger than the external field. The number of sub-domains is then also proportional to the strength of the external field. Figs. 6a-6e are plan views of the magnetic domains formed at different strengths of the external magnetic field. The domains of Figs. 6a, 6c and 6e are produced under the circumstances indicated in Figs. 5b, 5c and 5d, respectively. The domains of Figs. 6b and 6d are produced when a respective negative and positive field strength H₂ is applied which is larger than zero and smaller than H₁. The sub-domains in Figs. 6a-6e are shown as black areas 4′. When an external magnetic field of the strength H=-H₁ is applied, no sub-domains are written (Fig. 6a). The relevant area of the information layer, or the imaginary domain 4₀, may be characterized as a white domain. If the external magnetic field has the strength H=-H₂, a small number of sub-domains is formed (Fig. 6b). The domain 4₁ may be indicated as light grey. If the strength of the magnetic field has been reduced to zero, or in other words, if there is no external magnetic field, approximately half of the domain is filled with sub-domains. This domain, 4₂ in Fig. 6c, behaves as a medium grey area upon reading. When a positive magnetic field of the strength H=+H₂ is applied, approximately three quarters of the domain are filled with sub-domains (Fig. 6d). This domain 4₃ can be considered as dark grey. A black domain 4, which is entirely filled with sub-domains, is produced when applying a magnetic field of the strength H=+H₁. The values of H₁ and H₂ and the grey levels at the different field strength depend on the composition of the magnetic information layer. Fig. 7 shows the grey levels (GL) for three different alloys: GdTbFe, TbFeCo and DyFeCo as a function of the external magnetic field strength H. This Figure shows that the material TbFeCo, which has a compensation point at approximately 180°K, has a satisfactory linear relationship between the strength of the external magnetic field and the grey level. Moreover, this material has the advantage that the write noise is also small for light grey areas. It has been found that when using this material for writing information in domains having different grey levels the noise is practically equal to that produced when writing information in black domains. For the material GdTbFe, with a compensation temperature of approximately 300°K, the curve showing the variation of the grey level as a function of the external magnetic field strength has a steeper and less linear slope, while more noise occurs for the light grey areas. However, this material is in principle suitable for writing different grey levels. However, preferred are TbFeCo or DyFeCo, which has a compensation temperature below 300°K, and which grey level versus magnetic field strength characteristic curve is also shown in Fig. 7. The field strengths H₁ and H₂ of Fig. 6 are 400 Oe and 200 Oe, respectively, for the material TbFeCo. As a further illustration of the possibility provided by the present invention, Fig. 8 shows the output signal S o of the detection system 18 in Fig. 1 when reading a magneto-optical record carrier in which a triangular signal S i is written in an analog form while using the method according to the present invention. It may be stated by way of example that the speed of the scanning spot with respect to the record carrier is 3.75 m/sec, the size of the sub-domains is of the order of 0.1 µm and the period of the shown triangular signal which has been read is of the order of 10 µm, and that the strength of the magnetic field is modulated between -300 Oe and +300 Oe. The write-read apparatus must be provided with a special energising circuit for writing information in different grey levels. Fig. 9 shows a first embodiment of such a circuit 15 which is suitable for writing an analog input signal in an analog manner. The circuit comprises a controllable current source 40 whose input 41 receives the analog input signal Si and whose output 42 is connected to the coil 13 for supplying a modulated energising current to this coil. If the signal Si to be written is not an analog but a digital signal, this signal is firstly converted into an analog signal by a converter 43. When a signal is recorded in an analog form, it is in principle not necessary to form separate magnetic domains, but a continuous magnetic track can be written whose magnetisation ratio continuously varies in accordance with the signal Si. The write-radiation beam must then have a constant and high intensity so that all areas of the track are successively heated sufficiently. The magnetic field is then sufficiently strong at any moment, between -300 Oe and +300 Oe in the given example of TbFeCo, so as to be able to give the entire track the magnetisation direction M₂ with the desired magnetisation ratio. A signal in an analog form may also be recorded in discrete magnetic domains which are separated from each other by intermediate areas having the original direction of magnetisation. Such domains can be formed by square-wave switching the intensity of the radiation beam between a low level and a high level by means of a laser modulation circuit, 16 in Fig. 1, whereby magnetic domains are formed only at those moments, and hence at corresponding positions along a track to be written, when the radiation beam has the high intensity. The energising circuit of Fig. 9 may then be used whereby the analog energising current Ip, and hence the magnetic field are sampled, as it were. A domain pattern as shown in Fig. 10 is then produced, in which the number of hatched lines of a shaded area represents the greyness or degree of magnetisation of said area or domain. Fig. 10 shows the domains 4 of one track portion in two rows, which is due to lack of space in the longitudinal direction, but in reality the domains of the second row are present behind those of the first row. Moreover, Fig. 10 only shows a small number of domains and thus also a small number of grey levels. However, the domains may have any grey level between white and black at this analog writing of the information level. Instead of being written in an analog form, a signal may also be written in a digital form, namely in a discrete number of grey levels, which number is larger than two, at which a higher order, for example octal, coding is used. The amplitude of the energising current is varied in n discrete steps, with n corresponding to the number of values of the coding. The successive magnetic domains with a varying magnetisation ratio in the scanning direction are preferably contiguous with each other so that there are no intermediate areas. This situation is shown symbolically in Fig. 11 for an octal coding at which the number of hatched lines of the shaded areas again indicates given magnetisation ratios or grey levels. For writing the domain pattern of Fig. 11 a circuit analogous to that of Fig. 9 can be used, in which the converter 43 is of the type converting an analog or binary input signal S i into a signal having a plurality of, for example, four or eight signal levels. Apart from being written in a domain pattern of contiguous domains having the same direction of magnetisation, a signal having a plurality of levels may alternatively be coded in a pattern of individual domains which are separated from each other by intermediate areas, or intermediate domains having an opposite direction of magnetisation. Fig. 12 symbolically shows such a pattern for an octal coding, with the information domains being denoted by 4₁ ... 4₈ and the intermediate domains being denoted by 5. Fig. 13 shows the basic circuit diagram of an embodiment of an apparatus for writing and reading a domain pattern according to Fig. 2. The input signal Si, for example a binary signal, is converted by a, for example, three-bit converter 45, into a multilevel signal having, for example, eight digital signal levels, which signal is applied to a controllable current source 40. This current source supplies an energising current to the coil 13, which current may have an amplitude with a plurality of values, for example eight values. The radiation source 10 in the optical write/read head 50 supplies a radiation beam whose intensity is square-wave varied at a fixed frequency by means of the modulation circuit 16 so that discrete areas of the information layer 3 are heated and the intermediate areas are not heated. Magnetic domains whose magnetisation ratio, or grey level, is determined by the instantaneous value of the energising current Ip are formed at the location of the heated areas. The signal stored in the magnetic domains thus obtained can be regained by means of the optical head 50 which may comprise the same elements 10, 11, 12, 17, 18, 19, 20, 21, 22 and 23 as the head shown in Fig. 1. The output signal of the radiation-sensitive detector 18 is applied to a, for example, three-bit converter 51 which converts the multilevel signal having, for example, eight signal levels into a, for example, binary signal So which corresponds to the signal Si. The input signal Si may alternatively be an analog signal. The converter 45 must then be adapted to convert an analog signal into a multivalent signal, while the converter 51 must be adapted to convert the multivalent signal into an analog signal. Instead of radiation beam modulation, the so-called magnetic field modulation is preferably used for fixing the positions of the information areas when writing information areas in the form of discrete magnetic domains. The magnetic field is alternately switched between a level above a threshold value at which domains with a first direction of magnetisation are formed and a level below this threshold value at which domains with a second direction of magnetisation, opposite to the first direction of magnetisation are formed. A magnetic field is then always present, not only during writing of the information areas but also during the intermediate time intervals. It is thereby achieved that the intermediate areas always acquire a different direction of magnetisation than the information areas, independent of the condition of the information layer to be written. A record carrier which has already been inscribed can then be rewritten with new information without having to erase it first. The principle of the magnetic field modulation is described in, inter alia, European Patent Application no. 0,230,325. According to the invention the strength of the magnetic field above the said threshold value is modulated so that the magnetic domains formed with the first direction of magnetisation have different degrees of magnetisation, or grey levels. Fig. 14a shows by way of example the variation of the energising current Ip as a function of time for a quadrivalent signal. The signal with which the controllable current source is controlled is a so-called NRZ signal so that the energising current also has an NRZ character. The zero level of this current is indicated by means of a broken line. Fig. 14b shows the pattern of information areas 4 and intermediate areas 5 formed by means of this energising current, with the different magnetisation ratios of the information areas, or domains 4, being indicated by means of different numbers of hatched lines of the shaded areas. An energising current as shown in Fig. 14a can be directly obtained from the current source 40 by applying a control signal to this current source, which signal has a square-wave variation, alternately with a low, fixed level and a higher, variable level. It is alternatively possible for the input of the controllable current source to be connected to a switch which is controlled at a fixed frequency and which alternately passes the signal from the converter 45 (Fig. 13) and a negative reference signal to the current source. When writing the information by means of magnetic field modulation, use can be made of a laser beam having a constant and high intensity level. It is then ensured in the simplest manner that the material of the information layer is always locally heated to a sufficient extent to vary its direction of magnetisation in dependence upon the magnetic field modulation. A pulsed laser beam may alternatively be used, in which the laser pulses are generated with a pulse duration of, for example, 20 nsec by means of a laser modulation circuit of a conventional type. An advantage of using a pulsed laser beam is that the thermal load of the information layer, and hence the rate at which the record carrier ages, is lower than in the case of writing with a radiation beam of constant intensity. A fixed phase relation must be maintained between the points of reversal of the energising current and the radiation pulses. To this end the apparatus, which operates with radiation pulses and which is shown diagrammatically in Fig. 15, comprises a synchronising circuit 60. This circuit derives control signals S i,1 and S i,2 with the same frequency from the information signal S i for the laser modulation circuit 16 and the energising circuit 15, respectively. The energising circuit 15 supplies a square-wave energising current to the coil 13, the upper level of said current being modulated in accordance with the information to be written. It is preferably ensured that the instant when a radiation pulse ends in principle coincides with the instant when the magnetic field has adjusted to a new value so that the relevant area is written in the desired direction by means of a well-defined magnetic field and the domain formed has well-defined edges. This is illustrated in Fig. 16 in which, on a considerably larger time scale, the reference numeral 65a denotes a radiation pulse and 67 denotes a transition between a maximum negative value, -I m , and, for example, a maximum positive value, +I m , of the magnetic field strength. The reference numeral 70 in Fig. 16 denotes the temperature variation as a function of time for an area of the information layer 3 irradiated by radiation pulse 65a. As a result of the supplied radiation energy, the temperature in this area rises rapidly above the write temperature Ts, which indicates the temperature above which the direction of magnetisation of the recording layer can be changed by the generated magnetic field. The write temperature Ts is generally above the compensation temperature of the material of the information layer. After the end of the radiation pulse 65a the material cools down very rapidly to approximately its ambient temperature due to the transport of heat in the information layer 3. The duration of the radiation pulse 65a and the phase relation between this pulse and the point of reversal from level 66a to level 66b of the energising current are chosen to be such that the information layer cools down at the positive value +I m = 66b of the energising current so that the area 68a is magnetised in the direction of magnetisation associated with this value and an information area 4b is produced with the magnetisation ratio associated with the value +I m , as is illustrated in Fig. 17. After some time, which is determined by the information signal to be written, the energising current changes sign (level 66a). At that moment a second radiation pulse 65b, which has heated an area 68b of the information layer, ends. The area 68b is then magnetised in a direction which is opposite to that in which the area 68a is magnetised and a magnetic domain or intermediate area 5 is produced. Subsequently the energising current is adjusted at the level 66c at which an information area 4c with a lower magnetisation ratio, or grey level, is written. By successively adjusting the energising current at the levels 66c, 66d, 66e, 66f, etc. and by supplying the associated radiation pulses 65d ... 65g, the information areas 4d and 4e with a decreasing magnetisation ratio and the intermediate areas 5 are written. It will be evident that the influence of variations of the write sensitivity on the accuracy with which the boundary of the magnetic domain formed is positioned is smaller as the temperature gradient in the boundary region is larger. It is to be noted that the temperature gradient increases as the required energy is supplied to the information layer within a shorter time. It is advantageous to choose the length of the radiation pulses to be small with respect to the repetition time. Fig. 18 shows an embodiment of the synchronising circuit which is suitable for controlling the recording of a digital information signal S i at a given bit frequency, for example, an NRZ-modulated signal. The synchronising circuit shown in Fig. 18 comprises a circuit for regaining a channel clock signal S cl having the same frequency as the bit frequency of the information signal S i . Such a circuit may comprise a phase detector 80 of a conventional type, which at each zero crossing of the information signal S i determines the phase difference between this zero crossing and the clock signal S cl . The phase detector 80 applies a signal, which is indicative of the given phase difference, to a voltage-controlled oscillator 81 via a loop filter 82. The oscillator generates a periodical signal at a frequency which is an integral multiple of the channel clock signal S cl , from which periodical signal the channel clock signal S cl is derived by means of frequency division which is performed with the aid of a counter 83. The phase detector 80, the loop filter 82, the voltage-controlled oscillator 81 and the counter 83 constitute a phase-locked loop circuit of a conventional type. The count of counter 83 is applied via a bus 84 to the decoding circuit 85 which generates three logic "1" signals 86a, 86b and 86c when three respective successive counts are reached. The signals 86a and 86b are applied to the inputs of a dual-port AND gate 87. The output signal of AND gate 87 is applied to the laser modulation circuit 57 which in response to each pulse of the output signal of AND gate 87 generates a pulsatory control signal for the laser 10. The signals 86b and 86c are applied to a dual-port AND gate 88. The output signal of AND gate 88 serves as a control signal for an electronic switch 89. The information signal S i is applied to a first input of the switch 89, while a second input of the switch 89 is connected to ground potential. The electronic switch 89 connects the output of the switch 87 to the first or the second input of the switch 89, dependent on the logic value of the control signal from AND gate 88. The signal 90 thus obtained at the output of switch 89 is a square-wave signal having a frequency which is equal to the bit frequency of the signal S i , while the polarity is determined by the instantaneous polarity of the information signal S i . The signal 90 is applied to the energising circuit 15. As an alternative for a controllable current source, this energising circuit comprises a power amplifier 91 which generates a voltage which is proportional to the input voltage of the amplifier 91. The output of the amplifier 91 is connected to the coil 13 via a resistor 92, which resistor 92 functions as a limiting resistor for the energising current. The resistance of resistor 92 and the inductance of the coil 13 are adapted to each other in such a way that the time constant of the RL circuit formed is small with respect to the pulse width of the energising current pulse. It is to be noted that the energising current blocks are converted in the synchronising circuit of Fig. 18 into a number of energising current pulses per block, so that the dissipation in the energising coil, the coil core and the electronic control equipment is reduced. Instead of the controllable current source 40 shown in Fig. 9 or the power amplifier 91 shown in fig. 18, which amplifier must supply a high output voltage, one would like to make use of the magnetic field modulator in the write-read apparatus, which modulator is described in European Patent Application no. 0,312,143 and which operates at a considerably lower voltage. This magnetic field modulator has an input for receiving a bivalent control signal, a magnet coil for generating a magnetic field, switching means for coupling, in dependence upon the logic value of the control signal, at least one connection terminal of the magnet coil to a first potential point having a first potential or to a second potential point having a second potential in order to generate an alternating current in the magnet coil, which current has a polarity determined by the logic value of the control signal, and in which the magnet coil is incorporated in a parallel resonant circuit and the switching means are provided with means for blocking, after decoupling the connection terminal from one of the potential points, the coupling with the other potential point during a time interval which substantially corresponds to half the resonance time of the resonant circuit, has been designed for writing first and second magnetic domains having opposite directions of magnetisation. The first domains are the information domains all of which are completely magnetised and thus have a constant and maximum magnetisation ratio. The second domains constitute intermediate areas. In accordance with a further aspect of the present invention the concept of said magnetic field modulator may alternatively be used in an apparatus for writing magnetic information domains having a plurality of magnetisation ratios or grey levels. To this end the device is provided with a plurality of these magnetic field modulators, which plurality is determined by the number of grey levels to be written. If four grey levels must be written, two magnet coils are required, ensuring that the absolute value of the magnetic field strength MFS₁ of the one coil is twice as large as that, MFS₂ of the other coil. Since the total magnetic field strength MFS t is equal to MFS₁+MFS₂ and the magnetic field of each coil can be switched between positive and negative, the total magnetic field may assume the following four values: MFS t = +MFS₁+MFS₂ = +3MFS₂ MFS₁-MFS₂ = + MFS₂ -MFS₁+MFS₂ = - MFS₂ -MFS₁-MFS₂ = -3MFS₂ It will be evident that three coils are required for writing eight grey levels, four coils are required for writing sixteen grey levels, etc. It is of course not necessary to use magnetic field modulators according to European Patent Application 0,312,143 when using a plurality of coils in the write-read apparatus, but it is alternatively possible to use other coil energising circuits. When using two magnetic field coils, these are preferably arranged on both sides of the record carrier. If one of the coils has a transparent core, for example an air-cored coil, the objective lens may be secured to this coil in such a way that the scanning beam is focused on the information layer via the transparent core of the coil. Fig. 19 shows an embodiment of an apparatus including two coils, utilising this possibility. In this Figure the components corresponding to the components shown in Fig. 1 have the same reference numerals. The recording apparatus shown in Fig. 21 has a focusing control of a conventional type, comprising the objective lens 12, the partly transmissive prism 17, the partly transmissive prism 22, a roof prism 121, a system of, for example, four radiation-sensitive detectors 122a, 122b, 122c and 122d, a circuit 123, a control circuit 124 and an actuator 125. The beam b′ reflected by the information layer 3 is passed to the roof prism 121 by means of the prisms 17 and 22. This prism 121 splits the beam b′ into two sub-beams b₁′ and b₂′ which are each incident on a separate pair of detectors 122a, 122b and 122c, 122d, respectively. The output signals of these detectors are applied to the circuit 123 in which the difference between the sum of the output signals of the detectors 122a and 122d and the sum of the output signals of the detectors 122b and 122c is determined. This difference signal is indicative of the degree of focusing of the scanning beam b on the information plane 3. The difference signal S d is applied to the control circuit 124 which generates such a control signal S f for the actuator 125 that the actuator 125 keeps the radiation beam b focused on the information layer 3 by way of movement of the lens 12, which means that the distance between the lens 12 and the information layer 3 is maintained constant. The distance between the coil 13 and the information layer is also maintained constant. The second coil 13′ is present under the record carrier and is controlled by a second energising circuit 15′ which, likewise as the energising circuit 15 for the coil 13, is controlled from a control circuit 14. This circuit ensures that the two coils are energised in a mutually coordinated manner. If three coils must be used, for example, the coil 13′ may be replaced by two coils whose axes extend at, for example, opposite angles to a normal on the information plane, for example, the optical axis of the system.

Claims: A method of optically writing and reading information in a pattern of magnetic domains (4,5) in an information layer (3) of a record carrier (1) by means of an optical scanning beam focused to a radiation spot (V) by subjecting, during writing, the portions of the information layer (3) heated by the radiation spot to the influence of a magnetic field (H ext ) which is directed substantially perpendicularly to the information layer (3) and is generated by means of a coil (13) through which an energising current is passed so that said domains (4,5) are formed, and by detecting, during reading, the variation caused by the magnetic domains (4,5) in the state of polarisation of the scanning beam (b), characterized in that, during writing, the amplitude of the energising current is modulated in dependence upon the information signal to be written so that magnetic domains (4,5) are produced which comprise sub-domains having dimensions which are considerably smaller than the size of the radiation spot (V), the number of sub-domains per domain and hence the magnetisation ratio of the relevant domain being determined by the information signal. A method as claimed in Claim 1, characterized in that a magnetic material whose compensation temperature is considerably lower than the Curie temperature is used for the information layer. A method as claimed in Claim 2, characterized in that use is made of an information layer comprising an alloy of terbium, iron and cobalt. A method as claimed in Claim 2, characterized in that a multilayer structure of cobalt and platinum is used as an information layer. A method as claimed in Claim 1, 2, 3 or 4, characterized in that the amplitude of the energising current is continuously modulated in accordance with the signal value of the signal to be written. A method as claimed in Claim 1, 2, 3 or 4, characterized in that the amplitude of the energising current is varied in n discrete steps, in which n is an integer which is larger than two. A method as claimed in Claim 5 or 6, characterized in that domains are written which adjoin each other in the scanning direction without the interposition of intermediate areas. A method as claimed in Claim 5 or 6, characterized in that magnetic domains which are separated from each other are written. A method as claimed in Claim 8, characterized in that use is made of a square-wave energising current so as to fix the positions of the magnetic domains, which current has a first level located above a limit value at which magnetic domains having the first direction of magnetisation are written and a second level below this limit value, the first level being varied in accordance with the desired magnetisation ratio of said magnetic domains. A method as claimed in Claim 9, characterized in that the second level is located below a second limit value at which magnetic domains having a direction of magnetisation which is opposite to the first direction of magnetisation are written. A method as claimed in Claim 6, 7, 8, 9 or 10, characterized in that use is made of a radiation beam having a constant intensity. A method as claimed in Claim 6, 7, 8, 9 or 10, characterized in that use is made of a pulsed radiation beam. A method as claimed in Claim 9, characterized in that such a phase relation is maintained between the radiation pulses of the radiation beam and the energising current that each radiation pulse does not end until the magnetic field has adjusted to a new final value. A method as claimed in Claim 8, characterized in that use is made of a radiation beam which is square-wave modulated so as to fix the positions of the magnetic domains. An apparatus for performing the method as claimed in Claim 1, which apparatus comprises a radiation source (10) for supplying a scanning beam (b), an objective system for focusing the scanning beam (b) to a radiation spot (V) on the information layer (3), a radiation-sensitive detection system (18) for converting the beam issuing from the information layer into an electric signal (S o ), a magnet coil (13) for generating a magnetic field which is directed substantially perpendicularly to the information layer (3) in the scanned portion of the information layer (3) and an energising circuit (15) for generating an energising current in the coil (3), which current is modulated in dependence upon an information signal, characterized in that the energising circuit (15) comprises a controllable current source (40) having a control input for receiving a signal which is proportional to the signal to be written and an output for supplying an energising current whose amplitude varies in accordance with the signal to be written. An apparatus as claimed in Claim 15, characterized in that the energising circuit is adapted to supply a current whose amplitude always has a level above a threshold value at which magnetic domains having the first direction of magnetisation can be written. An apparatus as claimed in Claim 16, characterized in that the radiation source is adapted to supply a write-radiation beam having a constant intensity. An apparatus as claimed in Claim 15, characterized in that the radiation source is adapted to supply a write-radiation beam whose intensity is square-wave modulated between a first level above a threshold value at which domains are formed, and a second level below the threshold value. An apparatus as claimed in Claim 18, characterized in that a synchronising circuit (60) is provided for maintaining a phase relation between the intensity variation of the write-radiation beam and the energising current such that the transitions between the first and the second intensity level of the radiation beam substantially coincide with the instants when the magnetic field has reached a new final value. An apparatus as claimed in Claim 15, characterized in that the energising circuit is adapted to supply a square-wave energising current having a first, variable level above a threshold value at which magnetic domains having a first direction of magnetisation and a variable magnetisation ratio are written, and a second level below the threshold value. An apparatus as claimed in Claim 20, characterized in that the radiation source is adapted to supply a write-radiation beam having a constant intensity. An apparatus as claimed in Claim 20, characterized in that the radiation source is adapted to supply a pulsed write-radiation beam whose pulse duration is considerably shorter than half the period. An apparatus as claimed in Claim 22, characterized in that a synchronising circuit is provided for maintaining a phase relation between the radiation pulses and the square-wave energising current such that the end of each radiation pulse substantially coincides with an instant when the magnetic field has reached a new final value. An apparatus as claimed in Claim 19 for recording a digital information signal at a given bit frequency, characterized in that the synchronising means are adapted to synchronise the generation of the radiation pulses and energising current pulses at the bit frequency of the information signal, and in that the means for modulating the energising current are adapted to generate energising current pulses having a polarity which is determined by the logic value of the information signal. An apparatus as claimed in Claim 24, characterized in that it comprises an oscillator for generating a periodical signal, in that the synchronising means are adapted to derive the instants of generating the radiation pulses and energising current pulses from the periodical signal, and in that the synchronising means are further provided with means for maintaining a fixed phase relation between the information signal and the periodical signal. An apparatus as claimed in Claim 24, characterized in that it comprises phase-comparing means for determining the phase difference between the periodical signal and the information signal, and means for synchronising the information signal with the periodical signal in dependence upon the phase difference. An apparatus as claimed in Claim 15 or 16, characterized by a number of at least two coils arranged on both sides of the plane of the information layer, by an equally large number of energising circuits for the coils and by a control circuit for energising the coils in a coordinated manner. An apparatus as claimed in Claim 27, characterized in that the core of one of the coils is radiation-transmissive and in that the coil is arranged in such a way that the scanning beam is directed onto the information layer via the core of said coil.

Inventors: GREIDANUS FRANCISCUS JOHANNES, JACOBS BERNARDUS ANTONIUS JOHA, SPRUIT JOHANNES HENDRIKUS MARI, GREIDANUS, FRANCISCUS JOHANNES ANTONIUS MARIA, JACOBS, BERNARDUS ANTONIUS JOHANNUS, SPRUIT, JOHANNES HENDRIKUS MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1990
Main classification: G11B11
IPC codes: G11B11