Magneto-optical memory apparatus and optical memory apparatus

Abstract: The present invention discloses a magneto-optical memory apparatus comprising: an optical head; a magneto-optical recording medium relatively mova­ ble with respect to the optical head, into which recording marks, each having two ends, are recorded as information; the optical head being arranged such that, (i) at the time of recording into the magneto-opti­ cal recording medium, the recording marks are recorded such that the ends of each recording mark correspond to a second code of digital information composed of two dif­ ferent types of codes, (ii) at the time of reproduction, a light beam having a condensing spot is projected to the magneto-opti­ cal recording medium and, when a reproduction light con­ taining reproduction information is entered to the optical head from the magneto-optical recording medium, a repro­ duction signal is supplied, and (iii) when the light beam is projected to a record­ ing mark, an information pulse corresponding to the second code is generated in the reproduction signal, only when the reproduction light from the recording mark interferes with the reproduction light from the non-marking part; and a signal detector circuit for detecting the informa­ tion pulse in the reproduction signal obtained from the optical head and for converting the reproduction signal into a digital reproduction signal. Each of the recording marks contains a number of information to enhance the recording density of the mag­ neto-optical recording medium, thereby to increase the recording capacity. Further, the recording marks may be made in a large size and in a suitable shape. This facili­ tates the transfer of the recording marks made by plastic forming and the like. Futher, the optical system may be simplified in arrangement. The optical memory apparatus in accordance with the present invention includes the arrangement above-mentioned and is so arranged as to reproduce the recorded informa­ tion by diffraction of the lights reflected from the opti­ cal information recording medium having recording marks formed therein.

Description: FIELD OF THE INVENTION The present invention relates to a magneto-optical memory apparatus in which a light beam is projected to a magneto-optical recording medium for recording information therein, erasing and reproducing the information recorded therein. BACKGROUND OF THE INVENTION A magneto-optical memory apparatus of the type above-mentioned is arranged such that the optical head projects a laser light condensed to a spot having a diameter of about 1 µm, to a magneto-optical recording medium capable of recording and erasing information, causing the medium to record information, as well as to read and erase the information recorded therein, and that the signal detector circuit produces a digital reproduction signal based on a reproduction signal supplied from the optical head. The signal detector circuit has a peak position detector circuit for detecting the peak positions of information pulses of the reproduction signal, or an amplitude detector circuit for detecting the width and interval of the information pulses. By either one of these circuits, a digital reproduction signal may be obtained from the reproduction signal. As shown in Fig. 11, a conventional magneto-optical memory apparatus includes a magneto-optical disk 101 as the magneto-optical recording medium, and an optical head 102 for recording information in the magneto-optical disk 101, reading and erasing the information recorded therein. The optical head 102 includes a semiconductor laser 111, a beam splitter (half-mirror) 112, an objective lens 113, a half-wave plate 116, an analyser 117, a condensing lens 114 and a photo detector 115. A light beam emitted from the semiconductor laser 111 passing through the beam splitter (half-mirror) 112 is condensed by the objective lens 113 and projected to the magneto-optical disk 101. The reproduction light reflected from the magneto-optical disk 101 passes through the objective lens 113 and is vertically turned, at a portion thereof, by the beam splitter (half-mirror) 112. This light is inclined at a predetermined angle in the polarization direction by the half-wave plate 116. Then, the light passes through the analyser 117 and is condensed by the condensing lens 114. Thereafter, the light is incident on the photo detector 115. In the photo detector 115, the reproduction light is converted into an electric signal, which is then sent to a signal detector circuit 23. In the circuit 23, the data are reproduced. The magneto-optical disk 101 as the magneto-optical recording medium, includes a magnetic thin film having a magnetization facilitating axis in a direction at a right angle to the surface of the thin film. Data may be recorded in the magneto-optical disk 101 as outlined below. When the optical head 102 projects a laser light to the magnetic thin film of the magneto-optical disk 101, the portion of the thin film irradiated by the laser light, is locally increased in temperature to lower the coercive force thereof. When a magnetic field is externally applied to this portion, the magnetization direction thereof may be inverted in a desired direction. Data are recorded by such inversion of the magnetization direction. On the other hand, data reproduction may be carried out as outlined below. When a laser light of linear polarization weaker than that used at the time of recording, is projected to the magneto-optical recording medium, the polarization plane of the reproduction light which is a reflected light or a transmission light from the magneto-optical recording medium, is rotated by a desired angle according to the magnetization direction of the magneto-optical recording medium. The rotation of the polarization plane results from the Faraday effect for the transmission light and from the Kerr effect for the reflected light. For example, it is now supposed that the vector of the reflected light in a magnetization direction of the magneto-optical recording medium is expressed by R+, while the vector of the reflected light in the inverted magnetization direction with respect to the first-mentioned magnetization direction is expressed by R-. In this case, the rotation angles of the polarization planes corresponding to these reflected lights R+ and R- with respect to the incident polarization plane, are respectively expressed by +ϑ K and -ϑ K , as shown in Fig. 12. Provision is made such that the reproduction signal as an electrical signal is obtained by detecting the detection polarization plane components Rα+ and Rα- of the reflected lights R+ and R- on the detection polarization plane set at 45° with respect to the incident polarization plane. At this time, Rα- and Rα+ are in low and high levels of a binary code, respectively. The following description will discuss in more detail the operation of the magneto-optical memory apparatus having the principle above-mentioned, with reference to Fig. 13 (a). Corresponding to digital data (a) shown in Fig. 13 (a), recording marks (b) are recorded in the magneto-optical disk 101 by inversion of magnetization. It is now supposed that the vectors of the reflected lights from the recording marks are expressed by R-, while the vectors of the reflected lights from other non-marking parts than the recording marks are expressed by R+. Then, a reproduction signal (c) is obtained from the optical head 102 which has received the reflected lights from the magneto-optical disk 101. In the reproduction signal (c), negative-direction pulses are generated by the reflected lights R- from the recording marks, and high-level pulses are generated by the reflected lights R+ from the non-marking parts. When the signal detector circuit 23 has a peak position detector circuit (not shown), the signal detector circuit 23 to which the reproduction signal (c) above-mentioned has been entered, supplies a digital reproduction signal (d) of which rise portions are identical with the negative-direction peak positions of the reproduction signal (c). On the other hand, when the signal detector circuit 23 includes an amplitude detector circuit (not shown), the signal detector circuit 23 supplies a digital reproduction signal (e) which is in a high level for a predetermined width to the negative-direction peak positions of the reproduction signal (c). Based on the digital reproduction signal (d) or (e), digital data (f) may be obtained as reproduction information. From EP-A- 0 220 023 there is known a magneto-optical memory apparatus in which a data signal is extracted by detecting portions of the recording medium between recording marks. A conventional optical memory apparatus comprises: an optical information recording medium in which information is recorded by forming physical concavo-convex portions therein; an optical head for reading the information recorded in the optical information recording medium; and a signal detector circuit for generating a digital reproduction signal from the information read by the optical head. The description of the optical head and the signal detector circuit in the optical memory apparatus is here omitted since they have the same functions and the like as those of the optical head and the signal detector circuit in the magneto-optical memory apparatus above-mentioned. The following description will discuss the operation of the optical memory apparatus with reference to Fig. 13 (b). As shown in Fig. 13 (b), recorded in the optical information recording medium are recording marks (b), as recording information, corresponding to the digital data (a). In reading the recording marks by the optical head, the amounts of the reflected lights are increased or decreased by the diffraction thereof due to the concavo-convex portions above-mentioned. Thus, a reproduction signal (c) is generated. When the signal detector circuit has a peak position detector circuit, the signal detector circuit to which the reproduction signal (c) has been entered, generates a digital reproduction signal (d) of which rise portions are identical with the negative-direction peak positions of the reproduction signal (c). On the other hand, when the signal detector circuit includes an amplitude detector circuit, the signal detector circuit supplies a digital reproduction signal (e) which is in a high level for a predetermined width to the negative-direction peak positions of the reproduction signal (c). Based on the digital reproduction signal (d) or (e), digital data (f) may be obtained as reproduction information. The magneto-optical memory apparatus mentioned earlier generally employs a magneto-optical recording medium in which are formed (i) physical concavo-convex portions presenting track address information and the like and (ii) recording areas for recording information by inverting the magnetization direction of the magneto-optical recording medium. The conventional arrangement above-mentioned presents the problem that it is difficult to enhance the recording density in the magneto-optical recording medium to increase the recording capacity. More specifically, to enhance the recording density of the magneto-optical recording medium in the arrangement above-mentioned, the recording marks need to be further reduced in size. However, as the recording marks are reduced in size, it becomes difficult to record such recording marks into the magneto-optical recording medium. For example, when a laser light is condensed on the magneto-optical recording medium to record the recording marks, it is more difficult to properly form smaller recording marks in view of the degree of diaphragming the laser beam, the characteristics of the magneto-optical recording medium, the ambient temperature and the like. Accordingly, it is difficult to enhance the recording density by forming small-size recording marks. Thus, the conventional arrangement is not suitable to enhance the recording density. For convenience' sake, each recording mark is shown as an independent mark corresponding to one code. However, the recording density may not be satisfactorily enhanced even in an arrangement in which each recording mark corresponds to a plurality of codes and the recording mark is made in size identical with other part than the recording mark. A pit-edge recording technique is described in Japanese Journal of Applied Physics ; Vol.26, 1987, Supplement 26-4, Tokyo, Japan ; pages 243-248 ; Sukeda et al : "High-Density Magneto-Optic Disk Using Highly Controlled Pit-Edge Recording". This document is considered to represent the closest prior art. SUMMARY OF THE INVENTION It is an object of the present invention to provide a magneto-optical memory apparatus in which each recording mark contains a number of information to enhance the recording density in the magneto-optical recording medium, enabling to increase the recording capacity. It is another object of the present invention to provide a magneto-optical memory apparatus in which the recording marks are made in a large size to facilitate the formation of the recording marks. It is a further object of the present invention to provide a magneto-optical memory apparatus in which the number of component elements of the optical system is reduced to simplify the optical system in arrangement. In a first aspect, the invention provides a magneto-optical memory apparatus for recording to and reproducing from a magneto-optical recording medium a digital information signal comprising a sequence of first and second codes, the apparatus being arranged for recording the information signal by forming in the recording medium elongate recording marks which are mutually separated in a track direction, said recording marks having a different magnetization direction to the surrounding non-marked portions of the recording medium, the ends of each recording mark in the track direction representing respective second codes of the information signal, and for reproducing the recorded information signal by scanning the spot of a beam of light having a predetermined polarization plane over the recording medium in the track direction, and detecting the light beam transmitted through or reflected by the recording medium, characterised in that : the second codes represented by the ends of each recording mark are detected using detection means which detects interaction of respective polarization components of light arising from a part of said light beam spot which impinges on a non-marked portion of the recording medium adjacent the end of the recording mark and a part of said light beam spot which impinges on an end portion of the recording mark, said light components having opposing directions. The ends of each recording mark may be detected by virtue of light components which exist in a polarization plane which is orthogonal to the predetermined polarization plane of the incident light beam. In this case, the apparatus may include a polarization beam splitter arranged to direct the reproduction light beam to the recording medium and to direct the reflected light beam towards a photodetector whilst selecting therefrom the light components in said orthogonal polarization plane. The apparatus may be arranged to form each of the recording marks with a length which is such as to represent two of said codes of the information signal sandwiched between the two second codes represented by the ends of the recording mark. In this case, the apparatus may be arranged to form each of said recording marks with a width in the intermediate portion between the two ends which is not smaller than the diameter of the reproduction light beam spot, whereby the intermediate portion of the recording mark represents two first codes of the information signal. Alternatively, the apparatus may be arranged to form two types of said recording marks, a first type having a width in the intermediate portion between the two ends which is not smaller than the diameter of the reproduction light beam spot, whereby the intermediate portion of the first recording mark represents two first codes of the information signal, and a second type having a width in the intermediate portion between the two ends which is smaller than the width of the intermediate portion of the first recording mark and smaller than the diameter of the reproduction light beam spot, whereby the intermediate portion of the second recording mark represents two second codes of the information signal which are detected using said detection means. The magneto-optical memory apparatus may include a photodetector which receives the detected light beam and produces an output reproduction signal having information pulses whose peaks correspond with the second codes represented by the ends of the recording marks. In the apparatus which is arranged to form two types of recording mark as aforesaid, the difference between the diameter of the reproduction light beam spot and the width of the intermediate portion of the second recording mark may be set such that said output reproduction signal has a level below said peaks by not more than a predetermined amount when the reproduction light beam spot illuminates the intermediate portion of the second recording mark. The magneto-optical memory apparatus may include a peak position detector circuit for detecting said peaks of the reproduction signal pulses. The apparatus may include an amplitude detector circuit for detecting the width and interval of said information pulses having a level not more than said predetermined amount below said peaks. The magneto-optical memory apparatus may be arranged to form each of the recording marks such that each second code of the digital information signal represented by an end of the recording mark is sandwiched in between two first codes of the information signal, the apparatus including a two-division photodetector having two independent light-receiving portions arranged to receive respective portions of said detected light beam, and means for detecting the second codes represented by the ends of each recording mark by virtue of asymmetric intensity distribution between said portions of the detected light beam caused by different degrees of said interaction for those portions when the reproduction beam spot illuminates an end of a recording mark, said means for detecting comprising a differential amplifier whose inputs are supplied with the respective electrical outputs of said light-receiving portions of the photodetector so as to output a difference signal containing an information pulse which corresponds to the second code, and a signal detector circuit for detecting each said information pulse so as to generate a digital reproduction signal containing pulses which correspond to the second codes of said information signal. In this case, the apparatus may be arranged to form said recording marks with a length and a width which are greater than the diameter of the reproduction beam spot. The two light-receiving portions of the two-division photodetector may be divided in the direction perpendicular to scanning tracks on the recording medium. The signal detector circuit may include a positive direction detector circuit for detecting a positive-direction information pulse in said difference signal, a negative direction detector circuit for detecting a negative-direction information pulse in said difference signal, and an OR-gate for calculating a logical sum of the outputs of said detector circuits. The positive direction detector circuit and the negative direction detector circuit may include a peak position detector circuit and/or an amplitude detector circuit. In a second aspect, the invention provides a magneto-optical memory apparatus for recording to and reproducing from a magneto-optical recording medium a digital information signal comprising a sequence of first and second codes, the apparatus being arranged for recording the information signal by forming in the recording medium recording marks which are mutually separated in a track direction, said recording marks having a different magnetization direction to the surrounding non-marked portions of the recording medium and each of the recording marks representing one or a sequence of said second codes, and for reproducing the recorded information signal by scanning the spot of a beam of light having a predetermined polarization plane over the recording medium in the track direction, and detecting the light beam transmitted through or reflected by the recording medium, the reproduction light beam spot having a diameter which is greater than the width of the recording marks, characterised in that : the length of the recording mark in the beam scan direction determines the number of second codes represented by the mark, and the recording marks are detected using detection means which detects interaction of respective polarization components of light arising from a part of said light beam spot which impinges on a non-marked portion of the recording medium adjacent the recording mark being detected and a part of said light beam spot which impinges on a portion of the recording mark, said light components having opposing directions. The recording marks may be detected by virtue of light components which exist in a polarization plane which is orthogonal to the predetermined polarization plane of the incident light beam. In this case, the apparatus may include a polarization beam splitter arranged to direct the reproduction light beam to the recording medium and to direct the reflected light beam towards a photodetector whilst selecting therefrom the light components in said orthogonal polarization plane. The apparatus may include a photodetector which receives the detected light beam and produces an output reproduction signal whose level is reduced as the result of said interaction when the beam spot illuminates the recording mark. According to both the first and second aspects of the invention, the magneto-optical memory apparatus may be arranged to form said recording marks by inverting the magnetization direction of a magnetic film layer of said recording medium, the magnetic film layer having an easy axis of magnetization which is perpendicular to the surface thereof. The magneto-optical recording medium may be made of plastic or glass in the shape of disk or a card, and having said magnetic film layer formed thereon. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic block diagram of main portions of magneto-optical memory apparatus in accordance with an embodiment of the present invention; Figure 2 is a view illustrating the arrangement of the magneto-optical memory apparatus in Figure 1; Figure 3 is a view illustrating a reproducing operation of an optical head in which the incident polarization plane of the polarization beam splitter is disposed at a right angle to the detection polarization plane thereof; Figure 4 is a view illustrating the relationship between recording marks and the waveforms of signals at respective parts of the magneto-optical memory apparatus; Figure 5 is a view illustrating the relationship between a recording mark and a light condensing spot in a computer simulation as to reproduction signals of the magneto-optical memory apparatus shown in Figure 6; Figure 6 shows results of a computer simulation in the form of a graph which illustrates the relationship between the position of the light condensing spot on a recording mark and the intensity of the reproduction light incident on a photo detector; Figure 7 and Figure 8 are views illustrating the relationship between different recording marks and the waveforms of signals at the respective parts of the apparatus when such different recording marks are recorded in the magneto-optical recording medium; Figure 9 is a block diagram illustrating the arrangement of main portions of the magneto-optical memory apparatus using a two-division photo detector; Figure 10 is a view illustrating the relationship between recording marks and the waveforms of signals at respective parts when the magneto-optical memory apparatus having the arrangement in Figure 11 is used; Figure 11 is a view illustrating main portions of a conventional magneto-optical memory apparatus; Figure 12 is a view illustrating the reproducing operation of the optical head in which the incident polarization plane of the analyser is disposed at 45° with respect to the detection polarization plane thereof; Figure 13 (a) is a view illustrating the relationship between recording marks and the waveforms of signals at respective parts of the magneto-optical memory apparatus; and Figure 13 (b) is a view illustrating the relationship between recording marks and the waveforms of signals at respective parts of the optical memory apparatus. DESCRIPTION OF THE EMBODIMENTS The following description will discuss embodiments of the present invention with reference to Fig. 1 to Fig. 10. As shown in Fig. 1, the magneto-optical memory apparatus in accordance with the present invention uses a magneto-optical disk 1 as a magneto-optical recording medium. The apparatus comprises an optical head 2 for recording information in the magneto-optical disk 1, reading and erasing the information recorded therein, and a signal detector circuit 3 for converting a reproduction signal obtained from the optical head 2 into a digital reproduction signal. The magneto-optical disk 1 has a magnetic thin film having a magnetization facilitating axis in a direction at a right angle to the surface of the thin film. By inverting the magnetization direction, first and second recording marks as information may be recorded, erased and so on. The magneto-optical disk 1 is movable with respect to the optical head 2. In the optical head 2, a laser light emitted from a semiconductor laser 11 passes through a polarization beam splitter 12 and is condensed by an objective lens 13, as shown in Fig. 2. The laser light is then incident on the magneto-optical disk 1. The reflected light passes again through the objective lens 13 and is partially dispersed in optical path by the polarization beam splitter 12. The polarization beam splitter 12 is arranged such that the detection polarization plane thereof is at a right angle to the incident polarization plane. The polarization beam splitter 12 also serves as an analyser. The reflected light is condensed by a condensing lens 14 and converted into an electrical signal by a photo detector 15. This signal is then sent to the signal detector circuit 3. The optical head 2 is arranged such that, when a laser light as a light beam is prejected to the magneto-optical disk 1, recording marks A as first recording marks and recording marks B as second recording marks shown by (i) in Fig. 4 are recorded for digital data (h) in Fig. 4 as digital information. In each recording mark A, each end or edge thereof in the relative moving direction with respect to the optical head 2, corresponds to one "1" sandwiched in between two "0"s. It is here noted that "0" represents a first code and "1" represents a second code. In each recording mark B, each edge and the intermediate portion between both edges thereof correspond to "1". Each recording mark B corresponds, in its entirety, to "1111". Each recording mark B has a width narrower than that of each recording mark A. The part of each recording mark A between both edges thereof, and other non-marking parts than the recording marks A and B, correspond to "0". Each recording mark A is made in the form of an ellipse of which length and width are greater than the diameter of a condensing spot 4 of a laser light used at the time of reproduction. Each recording mark B has a length longer than the diameter of the condensing spot 4. This facilitates the generation of information pulses, to be discussed later. At the time of reproduction, the optical head 2 is adapted to supply a reproduction signal when a laser light having the condensing spot 4 having a diameter smaller than the width of each recording mark A and greater than the width of each recording mark B, is projected to the magneto-optical disk 1 and the reflected light is entered into the optical head 2. Only when the optical head 2 receives the reflected light generated at the time an end of a recording mark A or B is irradiated by the laser light, the optical head 2 generates, in the reproduction signal, an information pulse corresponding to "1" as the second code due to interference of the reflected light from the recording mark A or B with the reflected light from the non-marking part. When the laser light is projected to the intermediate portion between both ends of a recording mark B, the optical head 2 is adapted to supply a reproduction signal of which the level is lower than a reference value, due to interference of the reflected light from the recording mark B with the reflected light from the non-marking part. According to this embodiment, the optical head 2 includes the polarization beam splitter 12 (serving also as an analyser) in which the incident polarization plane is disposed at a right angle to the detection polarization plane thereof, and the photo detector 15 for converting the output of the polarization beam splitter 12 into an electrical signal, as shown in Fig. 3. When a light beam is projected to an edge of a recording mark A or B, the polarization planes of the reflected lights from the recording mark A or B and the non-marking part are rotated in opposite directions on the laser light incident polarization plane, as shown in Fig. 3. The detection polarization plane components of both reflected lights are separated by 180° in direction. This produces, in a reproduction signal supplied from the photo detector 15, an information pulse or a portion having a level lower than a reference value. The signal detector circuit 3 detects, as "1" or the second code, (i) information pulses contained in the reproduction signal supplied from the optical head 2, and (ii) parts between information pulses of which the level is not greater than the reference value. The signal detector circuit 3 then converts the reproduction signal into the digital reproduction signal. More specifically, the signal detector circuit 3 has a peak position detector circuit (not shown) for detecting the peak positions of information pulses in the reproduction signal, and an amplitude detector circuit (not shown) for detecting the width and interval of information pulses of which the level is not greater than the reference value. By both circuits above-mentioned, the digital reproduction signal may be obtained from the reproduction signal. At the time of recording operation of the magneto-optical memory apparatus having the arrangement above-mentioned, the optical head 2 projects a light beam to the magneto-optical disk 1, and by inverting the magnetization of the part irradiated by this light beam, the recording marks A, B shown by (i) in Fig. 4 corresponding to the digital data (h) in Fig. 4 are recorded in the magneto-optical disk 1. The recording marks A are recorded such that each edge thereof corresponds to "1" sandwiched in between two "0"s in the digital data (h). Each recording mark B is recorded such that each edge and the part between both edges thereof correspond to "1" and the width thereof is smaller than that of each recording mark A. At the time of reproduction, the optical head 2 projects, to the magneto-optical disk 1, a light beam having a light condensing spot with a diameter smaller than the width of each recording mark A and greater than the width of each recording mark B. The reflected light thereof is entered to the optical head 2, which produces a reproduction signal (j). When the light beam from the optical head 2 is projected to other non-marking part than the recording marks A, B of the magneto-optical disk 1, the Kerr effect causes the polarization plane of the reflected light to be rotated according to the magnetization direction of the non-marking part. It is now supposed that the polarization plane is rotated by +ϑ K with respect to the incident polarization plane as shown in Fig. 3. Then, the vector of the reflected light is R+. Since the detection polarization plane of the polarization beam splitter 12 is disposed at a right angle to the laser light incident polarization plane, the detection polarization plane component Rα+ of the reflected light R+ is obtained by detection at the polarization beam splitter 12, and the reproduction signal (j) obtained from the photo detector 15 is in a high level. When the light beam is projected to the part of a recording mark A between both edges thereof, the polarization plane of the reflected light is rotated by -ϑ K and the vector of the reflected light is R-, since the recording mark A is inverted in magnetization with respect to the non-marking part. Then, the detection polarization plane component Rα- of the reflected light R- is obtained by detection at the polarization beam splitter 12, and the reproduction signal (j) from the photo detector is in a high level. At this time, the reproduction signal (j) is slightly low in level. This is because the reflected light from the non-marking part slightly interferes with the reflected light from the recording mark A. When the light beam is projected to an edge of a recording mark A as shown in Fig. 4, the polarization planes of the reflected lights from the recording mark A and the non-marking part are respectively rotated by -ϑ K and +ϑ K . Accordingly, the detection polarization plane components of both reflected lights are separated by 180° in direction. Accordingly, the output of the photo detector 15 is minimized and a negative-direction information pulse is generated in the reproduction signal (j), at the point where the recording mark A and the non-marking part are equally irradiated by the condensing spot 4 of the light beam. On the other hand, when an edge of a recording mark B of the magneto-optical disk 1 is irradiated by the light beam projected by the optical head 2, a negative-direction information pulse is generated in the reproduction signal (j), in like manner for an edge of the recording mark A as above-mentioned. When the part of a recording mark B between both edges thereof is irradiated, the reflected light from the recording mark B interferes with the reflected light from the non-marking part and the reproduction signal (j) is lower in level than the reference value, since the condensing spot of the light beam has a diameter greater than the width of the recording mark B. Figs. 5 and 6 show an example of the results of computer simulations of the reproduction signal. Fig. 5 shows the relationship between a recording mark C and the light condensing spot 4 in a computer simulation of reproduction signals in Fig. 6. In Fig. 5, the recording mark C is made in the form of an ellipse having a length of 4 µm and a width of W, and has at each edge thereof a semi-circular portion having a radius of W/2. The light condensing spot 4 is a Gaussian beam of which the radius is 0.65µm, that is a radius at which the beam intensity is equal to 1/e². The light condensing spot 4 scans in the direction of the line of apsides of the recording mark C. The distance in centre between the recording mark C and the light condensing spot 4 is shown at the upper portion in Fig. 5. The substrate of the magneto-optical disk 1 having the recording mark C formed thereon has a refractive index of 1.5. The pitch between adjacent tracks in the longitudinal direction on the drawing paper is equal to 1.6 µm. The objective lens has an aperture ratio of 0.55, and the laser light forming the condensing spot 4 has a wavelength of 0.78 µm. Fig. 6 shows the results of a simulation. The axis of abscissa represents the scanning direction of the light condensing spot 4 and shows the distance between the centre of the recording mark C and the centre of the light condensing spot 4. The axis of ordinate represents the intensity of a reproduction light incident on the photo detector. This intensity is set to 1.0 when the light condensing spot 4 is located perfectly in other position than the recording mark C, and set to 0.0 when no reproduction light is generated. The photo detector 15 generates a reproduction signal corresponding to such a reproduction light. A simulation was made of the reproduction waveform for seven different widths W (twice the radius of the edge arcuate portion) of the recording mark C, i.e. 0.2 µm, 0.4 µm, 0.6 µm, 0.8 µm, 1.0 µm, 1.4 µm and 1.8 µm. There are obtained remarkable pulses at the edges of the recording mark C for W=1.4 µm and 1.8 µm. As to the width W of the recording mark C (twice the radius of the edge arcuate portion thereof), remarkable pulses are obtained for W=1.8 µm and 1.4 µm. However, even when W is equal to 1.0 µm and 0.8 µm, pulses are still present at the edges and the centre portion of the recording mark C is gradually lowered in level. The simulations were made under the conditions above-mentioned. It is apparent that the optimum width, depth or height of the recording mark C vary with changes in, for example, the refractive index of a glass substrate, the wavelength λ of the laser light, the aperture ratio of the objective lens and the like. The following description will discuss how to record data with the width W of the recording mark C altered. For example, the intensity of an externally applied magnetic field is weakened with the diameter of the light condensing spot 4 maintained constant. This enables data to be recorded with the width of the recording mark C reduced. On the contrary, if the externally applied magnetic field is increased, the width of the recording mark C is increased. Instead of alteration of the intensity of the magnetic field, the light intensity of the light condensing spot 4 used at the recording time may vary. That is, as the light intensity is lowered, the width of the recording mark C is reduced, and vice versa. Alternatively, the same effect may be achieved by electrically shifting the focus in a focus servo system for condensing the light condensing spot 4 on the magneto-optical disk 1. That is, the addition of offset to the focus enables the light condensing spot 4 to be widened. The reproduction signal (j) is entered into the signal detector circuit 3. A peak position detector circuit (not shown) of the circuit 3 generates a digital reproduction signal (k) of which rise portions are identical with the negative peaks of the information pulses in the reproduction signal (j). An amplitude detector circuit (not shown) of the circuit 3 generates a digital reproduction signal (l) in a high level for a predetermined width, to (a) the negative peak positions and (b) the parts having a level lower than the reference value, of the information pulses in the reproduction signal (j). The pulses of these digital reproduction signals (k) and (l) correspond to "1" of the digital data (h). Thus, digital data (m) as reproduction information may be obtained from both signals (k) and (l). The arrangement for obtaining the digital data (m) may be made such that the information pulses generated for the edges of the recording marks A, B are utilized as reproduction clocks and that the reproduction of information is made by detecting the level between edges of a recording mark B. The description has been made of the embodiment in which reproduction is made by utilizing the fact that the polarization plane of the reflected light from the magneto-optical disk 1 is rotated by the Kerr effect. However, the present invention may be arranged such that reproduction is made by utilizing the fact that the polarization plane of the transmission light of the magneto-optical disk 1 is rotated by the Faraday effect. Also, the description has been made of the embodiment in which the detection polarization plane of the polarization beam splitter 12 is disposed at a right angle to the laser light incident polarization plane. However, these planes are not necessarily disposed at a right angle to each other, as far as provision is made such that there occurs, at the edges of the recording marks A, B and between the edges of each recording mark B, interference in the reflected lights or transmission lights of the recording marks A, B and the non-marking parts. The recording marks above-mentioned may, alternatively, be so recorded as to correspond only to the second code ("l") of the digital information composed of two different types of codes (0, 1). Accordingly, the recording marks may be those as shown in Fig. 7 (b). More specifically, at the time of recording operation of the magneto-optical memory apparatus, a laser light is projected from the semiconductor laser 11 of the optical head 2, and this laser light is converted into a linear polarization light by the polarization beam splitter 12. The linear polarization light is then incident on the objective lens 13, which condenses the light on the magneto-optical disk 1. The part of the magneto-optical disk 1 irradiated by the laser light is inverted in magnetization and, as shown in Fig. 7, the recording marks (b) are recorded for the digital data (a). Recording is made such that the recording marks corresponds to "1" in the digital data (a). At the time of reproduction, a laser light projected from the semiconductor laser 11 is condensed, by the objective lens 13, into the condensing spot 4 having a diameter greater than the width of each recording mark. The condensing spot 4 is then projected to the magneto-optical disk 1. The reflected light from the magneto-optical disk 1 is incident on the polarization beam splitter 12 through the objective lens 13, and then taken out as a detection polarization plane component corresponding to the magnetization direction of the magneto-optical disk 1. The light thus taken out is condensed onto the photo detector 15 by the condensing lens 14 and then converted into an electrical signal, which is then generated as a reproduction signal (c). When the laser light from the optical head 2 is projected to a non-marking part of the magneto-optical disk 1, the polarization plane of the reflected light is rotated according to the magnetization direction of the non-marking part by the Kerr effect. If the polarization plane is rotated by +ϑ K as shown in Fig. 3, the vector of the reflected light is R+. Since the detection polarization plane of the polarization beam splitter 12 is disposed at a right angle to the laser light incident polarization plane, a detection polarization plane component Rα+ of the reflected light R+ is obtained by detection at the polarization beam splitter 12. The detection polarization plane component Rα+ is incident on the photo detector 15. At this time, since no light interference occurs, a large amount of the light is incident on the photo detector 15. Accordingly, the reproduction signal (c) supplied from the photo detector 15 is in a high level. When the magneto-optical disk 1 is moved, causing the laser light condensing spot 4 to be projected to a recording mark, the polarization planes of the reflected lights from the recording mark and the non-marking part are respectively rotated by -ϑ K and +ϑ K , since the recording mark is inverted in magnetization with respect to the non-marking part. Thus, the detection polarization plane components of both reflected lights are separated by 180° in direction. This causes the reflected lights from the recording mark and the non-marking part to interfere with each other. This reduces the amount of the reflected light incident on the photo detector 15. Accordingly, the reproduction signal (c) supplied from the photo detector 15 is in a low level in a range according to the length of the recording mark. As another example, the recording marks may be formed as shown by (i) in Fig. 8. More specifically, at the time of recording, the optical head 2 projects a laser light to the magneto-optical disk 1. By inversion in magnetization of the part of the magneto-optical disk 1 irradiated by the laser light, recording marks as shown in Fig. 8 (i) may be recorded in the magneto-optical disk 1 for digital data in Fig. 8 (h). Such recording is made such that each edge of the recording marks corresponds to "1" sandwiched in between two "0"s in the digital data (h). At the time of reproduction, the optical head 2 projects, to the magneto-optical disk 1, a laser light having the condensing spot 4 of which the diameter is smaller than the width of each recording mark. The reflected light is entered into the optical head 2, which generates a reproduction signal (j). When the laser light from the optical head 2 is projected to a non-marking part of the magneto-optical disk 1, the vector of the reflected light is R+, provided that the polarization plane of the reflected light is rotated by +ϑ K according to the magnetization direction of the non-marking part by the Kerr effect, as shown in Fig. 3. Since the detection polarization plane of the polarization beam splitter 12 is disposed at a right angle to the laser light incident polarization plane, the detection polarization plane component Rα+ of the reflected light R+ is obtained by detection at the polarization beam splitter 12, and a reproduction signal (j) obtained from the photo detector is in a high level. When the light beam is projected to the part of a recording mark between both edges thereof, the polarization plane of the reflected light is rotated by -ϑ K and the vector of the reflected light is R-, since the recording mark is inverted in magnetization with respect to the non-marking part. Then, the detection polarization plane component Rα- of the reflected light R- is obtained by detection at the polarization beam splitter 12 and the reproduction signal (j) from the photo detector is in a high level. At this time, the reproduction signal (j) is slightly low in level. This is because the reflected light from the non-marking part slightly interferes with the reflected light from the recording mark. When the laser light is projected to an edge of a recording mark as shown by (i) in Fig. 8, the polarization planes of the reflected lights from the recording mark and the non-marking part are respectively rotated by -ϑ K and +ϑ K . Accordingly, the detection polarization plane components of both reflected lights are separated by 180° in direction. Accordingly, the output of the photo detector 15 is minimized and a negative information pulse is generated in the reproduction signal (j), at the point where the recording mark and the non-marking part are equally irradiated by the laser light condensing spot 4. Thereafter, digital data (m) are reproduced from digital reproduction signals (k) and (l) through the signal detector circuit 3, as described previously. With reference to Figs. 9 and 10, the following description will discuss an example using a two-division photo detector, instead of the photo detector 15 in the optical head 2 above-mentioned. It is noted that like parts in Figs. 9 and 10 are designated by like numerals used in the embodiment above-mentioned. Fig. 9 shows an example of the magneto-optical memory apparatus using a two-division photo detector as a photo detector. In Fig. 10, a differential amplifier 16 is adapted to supply a difference signal containing an information pulse corresponding to "1" as the second code, based on a difference between output signals from light receiving portions 15′a, 15′b of a two-division photo detector 15′. The differential amplifier 16 has a non-invertting input terminal 16a connected to the light receiving portion 15′a of the two-division photo detector 15′ in the optical head 2, and an invertting input terminal 16b connected to the light receiving portion 15′b. A signal detector circuit 3 includes: a positive-direction signal detector circuit 17 for detecting a positive-direction signal in the difference signal supplied from the differential amplifier 16; a negative-direction signal detector circuit 18 for detecting a negative-direction signal in the difference signal; and an OR-gate 19 for supplying a digital reproduction signal upon reception of the output signals from the positive-direction signal detector circuit 17 and the negative-direction signal detector circuit 18. The positive-direction signal detector circuit 17 is adapted to detect, as "1" of digital data, the positive-direction information pulse in the difference signal supplied from the differential amplifier 16, while the negative-direction signal detector circuit 18 is adapted to detect, as "1" of digital data, the negative-direction information pulse. The positive-direction signal detector circuit 17 and the negative-direction signal detector circuit 18 may be formed by, for example, a peak position detector circuit or an amplitude detector circuit. An output from either the positive-direction signal detector circuit 17 or the negative-direction signal detector circuit 18 may be regarded as a digital reproduction signal, from which digital data may be obtained. In the arrangement above-mentioned, at the time of recording operation of the magneto-optical memory apparatus, a laser light is projected from a semiconductor laser 11 of an optical head 2 and converted into a linear polarization light by a polarization beam splitter 12. The laser light is then incident on an objective lens 13, which condenses the light onto a magneto-optical disk 1. The part of the magneto-optical disk 1 irradiated by the laser light is inverted in magnetization, and recording marks shown by (i) in Fig. 10 are recorded for digital data shown by (h) in Fig. 10. Recording is made such that each edge of the recording marks corresponds to "1" sandwiched in between two "0"s in the digital data (h). At the time of reproduction, the laser light projected from the semiconductor laser 11 is condensed, by the objective lens 13, to a condensing spot 4 having a diameter equal to or smaller than the width of each recording mark. The light thus condensed is then projected to the magneto-optical disk 1. The reflected light from the magneto-optical disk 1 is incident on the polarization beam splitter 12 through the objective lens 13, and taken out as a detection polarization plane component corresponding to the magnetization direction of the magneto-optical disk 1. The light thus taken out is condensed onto the light receiving portions 15′a, 15′b of the two-division photo detector 15′ by the condensing lens 14. The light thus condensed is converted into an electrical signal. The outputs from the light receiving portions 15′a, 15′b are entered into the differential amplifier 16. The differential amplifier 16 supplies a difference signal (j) as a reproduction signal. When the laser light from the optical head 2 is projected to a non-marking part of the magneto-optical disk 1, the polarization plane of the reflected light is rotated according to the magnetization direction of the non-marking part by the Kerr effect. At this time, if the polarization plane is rotated by +ϑ K as shown in Fig. 3, the vector of the reflected light is R+. Since the detection polarization plane of the polarization beam splitter 12 is disposed at a right angle to the laser light incident polarization plane, the detection polarization plane component Rα+ of the reflected light R+ is obtained by detection at the polarization beam splitter 12. The light of the detection polarization plane component Rα+ is incident on the two-division photo detector 15′. At this time, since no light interference occurs, the intensity distributions or light receiving patterns of the light incident on the light receiving portions 15′a and 15′b of the two-division photo detector 15′ are symmetric with each other. Accordingly, the output signals of the light receiving portions 15′a, 15′b are equal to each other, and the difference signal (j) supplied from the differential amplifier 16 is at a reference level. When the magneto-optical disk 1 is moved and the laser light condensing spot 4 is moved from the non-marking part to one edge of a recording mark such that this edge is irradiated by the laser light, the polarization planes of the reflected lights are respectively rotated by -ϑ K and +ϑ K , since the recording mark is inverted in magnetization with respect to the non-marking part. Thus, the detection polarization plane components of both reflected lights are separated by 180° in direction. This causes the reflected lights from the recording mark and the non-marking part to interfere with each other. Accordingly, the light receiving patterns in the light receiving portions 15′a, 15′b are asymmetric with each other. At this time, if the amount of the light incident on the light receiving portion 15′a is greater, the output from the light receiving portion 15′a is greater than the output of the light receiving portion 15′b, and a positive-direction information pulse is generated in the difference signal (j). When the part between both edges of a recording mark is irradiated by the laser light, the polarization plane of the reflected light is rotated by -ϑ K and the vector of the reflected light is R-. Then, the detection polarization plane component Rα- of the reflected light R- is obtained by detection at the polarization beam splitter 12 and is incident on the two-division photo detector 15′. At this time, since light Interference hardly occurs, the light receiving patterns in the light receiving portions 15′a, 15′b are symmetric with each other and the difference signal (j) is at the reference level, as in the case mentioned earlier. In such a case too, there are instances where the reflected lights from the recording mark and the non-marking part interfere more or less with each other, resulting in a slight variation of the level of the difference signal (j). When the magneto-optical disk 1 is moved and the laser light condensing spot 4 is moved to the other edge of the recording mark such that this edge is irradiated by the laser light, the polarization planes of the reflected lights from the recording mark and the non-marking part are respectively rotated by -ϑ K and +ϑ K . Accordingly, both reflected lights interfere with each other, and the amount of the light incident on the light receiving portion 15′b is greater, unlike the case mentioned earlier. Thus, a negative-direction information pulse is generated in the difference signal (j). Then, the difference signal (j) is entered into the positive-direction signal detector circuit 17 and negative-direction signal detector circuit 18. The positive-direction signal detector circuit 17 supplies a positive-direction detection signal (k) of which rise portions are identical with the positive-direction peak positions of the information pulses of the difference signal (j). The negative-direction signal detector circuit 18 supplies a negative-direction detection signal (l) of which rise portions are identical with the negative-direction peak positions of the information pulses of the difference signal (j). Both detection signals (k), (l) are entered into an OR-gate 19, which, in turn, generates a digital reproduction signal (m) obtained by logical sum of these detection signals (k), (l). Each pulse of the digital reproduction signal (m) corresponds to "1" in the digital data (h). Accordingly, digital data as reproduction information may be obtained from this signal (m). The description has been made of the arrangement in which reproduction is made by detecting the peak positions of the information pulses of the difference signal (j). However, the arrangement may be made such that reproduction is made by detecting, for example, the amplitude of each information pulse. In the magneto-optical memory apparatus described and illustrated in the foreging, the polarization beam splitter 12 is arranged such that the detection polarization plane is at a right angle to the laser light incident polarization plane. However, it is not necessarily required that these polarization planes be at a right angle to each other. When these planes are not at a right angle to each other, it is merely required to dispose an optical system for producing a difference in direction between the reflected light components from the recording mark and the non-marking part which is such as to cause a detectable cancellation. Further, the description has been made of the arrangement in which reproduction is made by utilizing the fact that the polarization plane of the reflected light from the magneto-optical disk 1 is rotated by the Kerr effect. However, provision may be made such that reproduction is made by utilizing the fact that the polarization plane of the transmission light of the magneto-optical disk 1 is rotated by the Faraday effect. Further, the description has been made of the arrangement in which the division direction of the two-division photo detector 15′ is perpendicular to the scanning track direction of the magneto-optical disk 1. However, if the asymmetry of the intensity distributions of the light incident on the two-division photo detector 15′ is detected, it is sufficient. Accordingly, the present invention may not be limited to the arrangement in which the division direction is perpendicular to the scanning track direction. The description has been made of the arrangement in which the condensing spot 4 has a diameter not greater than each recording mark. However, the diameter of the condensing spot 4 may be greater than each recording mark, so long as the light receiving patterns at the edges are asymmetric. In the foregoing, description has been made of magneto-optical memory apparatus. It should be noted that the present invention may also be applied to a magneto-optical memory apparatus in which the magneto-optical recording medium previously contains physical concavo-convex portions presenting the track address information.

Claims: A magneto-optical memory apparatus for recording to and reproducing from a magneto-optical recording medium a digital information signal comprising a sequence of first and second codes, the apparatus being arranged for recording the information signal by forming in the recording medium elongate recording marks which are mutually separated in a track direction, said recording marks having a different magnetization direction to the surrounding non-marked portions of the recording medium, the ends of each recording mark in the track direction representing respective second codes of the information signal, and for reproducing the recorded information signal by scanning the spot (4) of a beam of light having a predetermined polarization plane over the recording medium in the track direction, and detecting the light beam transmitted through or reflected by the recording medium, characterised in that : the second codes represented by the ends of each recording mark are detected using detection means (12) which detects interaction of respective polarization components of light (Rα+, Rα-) arising from a part of said light beam spot (4) which impinges on a non-marked portion of the recording medium adjacent the end of the recording mark and a part of said light beam spot which impinges on an end portion of the recording mark, said light components having opposing directions. A magneto-optical memory apparatus according to claim 1, wherein said light components (Rα+, Rα-) exist in a polarization plane which is orthogonal to the predetermined polarization plane of the incident light beam. A magneto-optical memory apparatus according to claim 2, including a polarization beam splitter (12) arranged to direct the reproduction light beam to the recording medium and to direct the reflected light beam towards a photodetector (15) whilst selecting therefrom the light components in said orthogonal polarization plane. A magneto-optical memory apparatus according to any one of claims 1 to 3, arranged to form each of the recording marks with a length which is such as to represent two of said codes of the information signal sandwiched between the two second codes represented by the ends of the recording mark. A magneto-optical memory apparatus according to claim 4, arranged to form each of said recording marks with a width in the intermediate portion between the two ends which is not smaller than the diameter of the reproduction light beam spot, whereby the intermediate portion of the recording mark represents two first codes of the information signal. A magneto-optical memory apparatus according to claim 4, arranged to form two types of said recording marks, a first type (A) having a width in the intermediate portion between the two ends which is not smaller than the diameter of the reproduction light beam spot (4), whereby the intermediate portion of the first recording mark represents two first codes of the information signal, and a second type (B) having a width in the intermediate portion between the two ends which is smaller than the width of the intermediate portion of the first recording mark and smaller than the diameter of the reproduction light beam spot (4), whereby the intermediate portion of the second recording mark represents two second codes of the information signal which are detected using said detection means (12). A magneto-optical memory apparatus according to any one of the preceding claims, wherein a photodetector (15) which receives the detected light beam produces an output reproduction signal having information pulses whose peaks correspond with the second codes represented by the ends of the recording marks. A magneto-optical memory apparatus according to claim 7, as appendant to claim 6, wherein the difference between the diameter of the reproduction light beam spot (4) and the width of the intermediate portion of the second recording mark (B) is set such that said output reproduction signal has a level below said peaks by not more than a predetermined amount when the reproduction light beam spot illuminates the intermediate portion of the second recording mark. A magneto-optical memory apparatus according to claim 7 or claim 8, including a peak position detector circuit for detecting said peaks of the reproduction signal pulses. A magneto-optical memory apparatus according to claim 9, as appendant to claim 8, including an amplitude detector circuit for detecting the width and interval of said information pulses having a level not more than said predetermined amount below said peaks. A magneto-optical memory apparatus according to any one of claims 1 to 3, arranged to form each of the recording marks such that each second code of the digital information signal represented by an end of the recording mark is sandwiched in between two first codes of the information signal, the apparatus including a two-division photodetector (15′) having two independent light-receiving portions (15′a, 15′b) arranged to receive respective portions of said detected light beam, and means for detecting the second codes represented by the ends of each recording mark by virtue of asymmetric intensity distribution between said portions of the detected light beam caused by different degrees of said interaction for those portions when the reproduction beam spot illuminates an end of a recording mark, said means for detecting comprising a differential amplifier (16) whose inputs (16a, 16b) are supplied with the respective electrical outputs of said light-receiving portions of the photodetector so as to output a difference signal containing an information pulse which corresponds to the second code, and a signal detector circuit (3) for detecting each said information pulse so as to generate a digital reproduction signal containing pulses which correspond to the second codes of said information signal. A magneto-optical memory apparatus according to claim 11, arranged to form said recording marks with a length and a width which are greater than the diameter of the reproduction beam spot. A magneto-optical memory apparatus according to claim 11 or claim 12, wherein said two light-receiving portions (15′a, 15′b) of the two-division photodetector (15′) are divided in the direction perpendicular to scanning tracks on the recording medium. A magneto-optical memory apparatus according to any one of claims 11 to 13, wherein the signal detector circuit (3) includes a positive direction detector circuit (17) for detecting a positive-direction information pulse in said difference signal, a negative direction detector circuit (18) for detecting a negative-direction information pulse in said difference signal, and an OR-gate (19) for calculating a logical sum of the outputs of said detector circuits. A magneto-optical memory apparatus according to claim 14, wherein the positive direction detector circuit (17) and the negative direction detector circuit (18) include a peak position detector circuit and/or an amplitude detector circuit. A magneto-optical memory apparatus for recording to and reproducing from a magneto-optical recording medium a digital information signal comprising a sequence of first and second codes, the apparatus being arranged for recording the information signal by forming in the recording medium recording marks which are mutually separated in a track direction, said recording marks having a different magnetization direction to the surrounding non-marked portions of the recording medium and each of the recording marks representing one or a sequence of said second codes, and for reproducing the recorded information signal by scanning the spot (4) of a beam of light having a predetermined polarization plane over the recording medium in the track direction, and detecting the light beam transmitted through or reflected by the recording medium, the reproduction light beam spot (4) having a diameter which is greater than the width of the recording marks, characterised in that : the length of the recording mark (Fig. 7(b)) in the beam scan direction determines the number of second codes represented by the mark, and the recording marks are detected using detection means (12) which detects interaction of respective polarization components of light (Rα+, Rα-) arising from a part of said light beam spot which impinges on a non-marked portion of the recording medium adjacent the recording mark being detected and a part of said light beam spot which impinges on a portion of the recording mark, said light components having opposing directions. A magneto-optical memory apparatus according to claim 16, wherein said light components light (Rα+, Rα-) exist in a polarization plane which is orthogonal to the predetermined polarization plane of the incident light beam. A magneto-optical memory apparatus according to claim 17, including a polarization beam splitter (12) arranged to direct the reproduction light beam to the recording medium and to direct the reflected light beam towards a photodetector (15) whilst selecting therefrom the light components in said orthogonal polarization plane. A magneto-optical memory apparatus according to any one of claims 16 to 18, wherein a photodetector (15) which receives the detected light beam produces an output reproduction signal whose level is reduced as the result of said interaction when the beam spot illuminates the recording mark. A magneto-optical memory apparatus according to any one of the preceding claims, arranged to form said recording marks by inverting the magnetization direction of a magnetic film layer of said recording medium, the magnetic film layer having an easy axis of magnetization which is perpendicular to the surface thereof.

Inventors: DEGUCHI TOSHIHISA, FUJI HIROSHI, KOJIMA KUNIO, OHTA KENJI, TAKAHASHI AKIRA, TERASHIMA SHIGEO, YAMAGUCHI TAKESHI, DEGUCHI, TOSHIHISA, FUJI, HIROSHI, KOJIMA, KUNIO, OHTA, KENJI, TAKAHASHI, AKIRA, TERASHIMA, SHIGEO, YAMAGUCHI, TAKESHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B11
IPC codes: G11B7, G11B11