Magnetic head

Abstract: The disclosure relates to a magnetic head having a recording capacity for sufficiently deriving performance of a high coercive magnetic tape and capable of recording and reproducing signals of very high frequencies at high effi­ ciency. The magnetic head of the present invention includes ferromagnetic thin films and insulative thin films alter­ nately laminated one upon another to form a multi-layered film magnetic core which constitutes part of whole of magnetic path, with the ferromagnetic thin films being so arranged that directions of their anisotropy are different from each other, and thus, Snoek's limit due to ferromagnetic resonance is broken through for making it possible to record and reproduce signals in an efficient manner up to high frequency band regions.

Description: BACKGROUND OF THE INVENTION The present invention relates to a magnetic head for efficiently recording and reproducing high frequency signals and suitable for systems dealing with such high frequency signals as in a high-definition video tape recorder, digital video tape recorder or the like. US-A-3 611 417 discloses a magnetic head which comprises ferromagnetic thin films and insulative thin films alternately laminated one upon another to form a multi-layered film magnetic core which constitutes part or a whole of a magnetic path. The ferromagnetic thin films are so arranged that the direction of anisotropy thereof are different from each other and are directed alternately in different direction, and that these directions of anisotropy generally intersect at right angles with each other. In the known magnetic head, the face forming the magnetic path and the film face of the ferromagnetic thin film are in the relation intersecting approximately at right angles with each other, with such advantegous as small magnetic path, facilitation for employing a multi-channel construction, and low inductance suitable for high-frequency operation. Consequently, the known magnetic head is mainly applied to the fixed disc apparatus. However, since the cross section of the magnetic path is small due to limitation by the film thickness of the ferromagnetic thin film, the reproducing sensitivity thereof tends to be low, which point is improved by reducing the gap depth. Accordingly, the known head is not employed in video tape recorders in which abrasion of the head is brought into question by the sliding movement of the magnetic tape and the engaging contact made between the head and the tape. From US-A-4 103 315, it is known to use amorphous magnetic alloy for the ferromagnetic layer and SiO₂ for the insulating layer for the manufacture of thin-film magnetic heads. Conventionally, in the apparatuses for recording and reproducing high frequency signals such as video tape recorders, etc., ferrite material with a less high frequency loss has been generally employed as a magnetic material for the video heads. In recent years, however, systems dealing with signals in a still wider band region such as a high-definition video tape recorder, digital video tape recorder or the like are vigorously developed, and recording media thereof are being altered from iron oxide media into high coercive media such as metal powder media, metal evaporated media, etc. in the trend towards high density construction for recording such a large amount of information. Meanwhile, the ferrite head has its maximum magnetic flux density at about 5000 gauss at the most, and for efficiently reproducing short-wave length signals, it is required to adopt narrow gaps, and thus, in the high coercive media having Hc higher than 1000 oersted, the ferrite core at the gap forward end portion is saturated, making it impossible to effect fully perfect recording. Accordingly, development of a magnetic head employing metallic magnetic materials such as sendust and amorphous magnetic alloys having high maximum flux density has been attempted, but the metallic magnetic material in a bulk state as employed has a large high frequency loss due to eddy current, and can hardly be used for the system as described above. Owing to the reason as described above, employment of the metallic magnetic material formed into a thin film has been investigated in order to suppress such a loss as for as possible, and by way of example, it is intended to cope with high frequencies by constituting the main magnetic circuit by a laminated structure of the metallic magnetic thin films and insulative thin films. In the high-definition video tape recorder, and digital video tape recorder, etc., the recording signal band region thereof extends in the range of 30 to 60 MHz, and as the core material for the magnetic head, a material having a high initial permeability in such a high frequency band region is required. In a graphical diagram of Fig. 5, there is shown a frequency characteristic of initial permeability in the case where direction of anisotropy of an amorphous magnetic thin film with respect to a measuring direction is altered, in a laminated structure of CoNb Ta Zr amorphous magnetic thin film and SiO₂ film. The thickness of the magnetic thin film per one layer is set to be 4 µm by taking the eddy current loss into consideration, and SiO₂ film thickness between layers is set at 0.2 µm for lamination in 5 layers. In Fig. 5, a curve (1) represents a random oriented lamination film, in which, although the eddy current loss is improved by the lamination effect, the high frequency characteristic thereof is restricted by Snoek's limit due to ferromagnetic resonance, and the initial permeability at the high frequency band region above 30 MHz falls below 500. Therefore, such a random oriented magnetic film, if employed as a head core, can not correspond to the high frequency system as described earlier. On the other hand, the initial permeability characteristic of a multi-layer film in which amorphous magnetic thin films having uniaxial anisotropy are laminated, with directions of easy axes thereof aligned, is as shown in Fig. 5, and upon measurement in the direction of the easy axis, extremely low initial permeability characteristic is observed over the entire frequency band region as represented by a curve (2), while upon measurement in the direction of the hard axis, high initial permeability is maintained up to high frequencies as represented by a curve (3), with a value above 1000 even at 60 MHz. However, in the case where a magnetic head having a comparatively large coil winding opening as in a video head or the like is constituted by the magnetic core having anisotropy in one direction, the easy axis direction is to be contained in its magnetic path, with a large reduction in a head efficiency. Moreover, constitution of all magnetic paths in the hard axis direction is very difficult from the viewpoint of the head manufacturing method. SUMMARY OF THE INVENTION Accordingly, an essential object of the present invention is to provide an improved magnetic head, which is capable of efficiently recording and reproducing signals up to high frequency band region by breaking through Snoek's limit due to ferromagnetic resonance. Another important object of the present invention is to provide a magnetic head of the above described type, which is simple in construction and stable in functioning, and can be readily manufactured on a large scale at low cost. In accomplishing these and other objects, according to one preferred embodiment of the present invention, there is provided a magnetic head comprising ferromagnetic thin films and insulative thin films alternately laminated one upon another to form a multi-layered film magnetic core which constitutes part or whole of a magnetic path, said multi-layered film magnetic core being constituted by ferromagnetic thin films having anisotropric nature in its film face and so arranged that directions of anisotropy thereof intersect generally at right angles with each other, characterized in that a magnetic gap is constituted by a cross section of said multi-layered film. In the magnetic head of the invention, the permeability characteristic in the direction of the difficult axis can be utilized irrespective of the directions of flow of the magnetic flux, and, thus, a magnetic head supperior in the high frequency characteristic can be obtained. Therefore, the magnetic head of the present invention contributes, to a large extend, to the systems which will require recording and reproduction in the broad band region particularly in video tape recorders like digital VTR, high great digital VTR etc.. According to the present invention, by employing the multi-layered magnetic core including the ferromagnetic thin films with different directions of anisotropy as the head core, it is possible to obtain the initial permeability characteristic exceeding the limit due to the ferromagnetic resonance by which the high frequency characteristics have been conventionally restricted, while, owing to the fact that the initial permeability characteristic may be exhibited in most of the magnetic path of the magnetic head, an improved magnetic head capable of recording and reproducing signals at high efficiency even in the high frequency band region over 30 MHz can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the following description taken in conjuction with preferred embodiment thereof with reference to the accompanying drawings, in which; Fig. 1 is a perspective view partly broken away, of a magnetic head according to one preferred embodiment of the present invention, Fig. 2 is a view similar to Fig. 1, which particularly shows a second embodiment according to the present invention, Fig. 3 are top plan views showing examples of video heads constituted by ferromagnetic thin films having uniaxial anisotropy, Fig. 4 is a graphical diagram showing head output characteristics of the video heads shown in Figs. 3, and of the second embodiment according to the present invention, and Fig. 5 is a graphical diagram showing results of measurements of initial permeability characteristics of ferromagnetic thin films according to directions of anisotropy (already referred to). DETAILED DESCRIPTION OF THE INVENTION Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring now to the drawings there is shown in Fig. 1, a magnetic head HA according to a first embodiment of the present invention which includes ferromagnetic thin films 1 and insulative thin films 2 alternately laminated one upon another to form a multi-layered film magnetic core 3 which constitutes part or whole of magnetic path, with the ferromagnetic thin films 1 being so arranged that directions of anisotropy thereof are different from each other. The ferromagnetic thin films 1 are each made of an amorphous magnetic alloy, sendust alloy, or the like, with a film thickness thereof per one layer being set by taking into account, the eddy current loss in the working frequency band region, and are laminated through the insulative thin films 2 of SiO₂, etc., to form the magnetic core 3 constituted by a multi-layered film. The ferromagnetic thin films 1 respectively having the uniaxial anisotropy in the magnetic path, includes those whose directions of easy axes one disposed generally to intersect at right angles with the face for a magnetic gap 7, and those whose directions of easy axes are directed generally to be parallel with said face of the magnetic gap 7 as shown by arrows in the broken away portions in Fig. 1, thus both films constituting the magnetic core 3. The magnetic core 3 composed of such laminated film is held between non-magnetic substrates 4 of a NiO-MgO-TiO₂ ceramic material, etc., and joined with a confronting core having a coil winding opening 5 by a bonding material, e.g. bonding glass 6, thereby forming the magnetic gap 7. In the multi-layered film in which the amorphous thin films are laminated so that the directions of anisotropy thereof intersect generally at right angles with each other alternately, the initial permeability characteristic as measured in the direction of one of the anisotropy are represented by a curve (4) in Fig. 5. As is seen from the figure, the initial permeability thereof is approximately half of the value as measured in the hard axis direction, but the frequency characteristic thereof is generally equal to that in the hard axis direction, having value of about 1000 even at 40 MHz. When the measuring direction is further rotated through 90° for measurement in the other direction of anisotropy generally similar characteristic is also obtained. Therefore, since most of the magnetic path of the magnetic head in Fig. 1 constituted by such multi-layered film as described above, functions by the initial permeability characteristic as shown by the curve (4) in Fig. 5, a high head efficiency is given even at high frequencies above 30 MHz. For a manufacturing process of such magnetic head HA as described above, in the first place, the ferromagnetic thin films and insulative magnetic thin films are alternately laminated by sputtering on the non-magnetic substrate. In the above case, by sputtering the ferromagnetic thin films in a fixed magnetic field so as to form the laminated layer while rotating the magnet position through 90° per each layer, the multi-layered film in which directions of anisotropy alternately interest generally at right angles with each other as described above can be readily obtained. By piling one upon another, a plurality of head substrates in which the multi-layered film are formed as described above for bonding by crystallized glass, etc. and for subsequent cutting off, core blocks in which the multi-layered films and non-magnetic substrates are alternately laminated are prepared. Thereafter, through similar process as in the conventional manufacturing method for ferrite heads, the magnetic head as shown in fig. 1 is produced. It should be noted here that, for laminating the ferromagnetic thin films having different directions of anisotropy, the ferromagnetic thin films with the same direction of anisotropy may be disposed each in a group of several layers, but a higher efficiency as a head on the whole can be achieved when such ferromagnetic thin films are disposed so that the directions of anisotropy thereof are alternately different from each other, owing to the fact that, when the direction of the magnetic path is changed, the layer with high initial permeability comes at the opposite sides of said layer. Referring further to Fig. 2, there is shown a magnetic head HB according to a second embodiment of the present invention, which includes ferromagnetic thin films 11 and insulative thin films 2 alternately laminated one upon another to form a multi-layered film magnetic core 3B which constitutes part or whole of magnetic path, with said ferromagnetic thin films 11 being so arranged that directions of anisotropy of first and second sets 11a and 11b of said ferromagnetic thin films 11 intersect generally at right angles with each other, and with direction of magnetization easy axis for the first set 11a thereof being adapted to generally intersect at right angles with a recording medium sliding surface, total thickness of said first set 11a of the ferromagnetic thin films 11 whose direction of magnetization easy axis intersect generally at right angles with the recording medium sliding surface, being larger than total thickness of said second set 11b of the ferromagnetic thin films 11. More specifically, the ferromagnetic thin films 11a and 11b similarly made of the amorphous alloy, sendust alloy or the like respectively have the uniaxial anisotropy in the magnetic path, and as shown by the arrows in the broken away portions in Fig. 2, the first set 11a of the ferromagnetic thin films 11 have the direction of the easy axis thereof directed to generally intersect at right angles with the recording medium sliding surface, while the second set 11b of the ferromagnetic thin films 11 have the easy axis thereof directed in the direction generally intersecting at right angles with that of the first set 11a. These ferromagnetic thin films 11a and 11b are laminated through the insulative thin films 2 of SiO₂, etc., to form the magnetic core 3B constituted by the multi-layered film, while the film thickness thereof per one layer is set to be less than the thickness in which eddy current loss in the working frequency band region is taken into account. Moreover, as referred to above, the total thickness of said first set 11a of the ferromagnetic thin films 11 whose direction of magnetization easy axis intersect generally at right angles with the recording medium sliding surface, is set to be larger than the total thickness of said second set 11b of the ferromagnetic thin films (11). In the similar manner as in the magnetic head HA in Fig. 1, the magnetic core 3B composed of such laminated film is held between the non-magnetic substrates 4 of the NiO-MgO-TiO₂ ceramic material, etc., and joined with the confronting core having the coil winding opening 5 by bonding glass 6 and the like,thereby forming the magnetic gap 7. According to experiments carried out by the present inventors, video heads Va and Vb are trially prepared by the amorphous magnetic thin films having uniaxial anisotropy in the same direction as shown in Fig. 3, and head output characteristics thereof are measured, with results as given in Fig. 4. As is seen from Fig. 4, the head Va in which the magnetization easy axis is directed to intersect at right angles with the recording medium sliding surface, provided the head output considerably higher than that of the head Vb in which the easy axis is directed to be parallel with the recording medium sliding surface. This may be attributable to the fact that, in the head Vb, since the initial permeability in the direction intersecting at right angles with the recording medium sliding surface in the vicinity of the magnetic gap, i.e. in the direction parallel with the gap face is high, most of the signal flux generated from signal magnetization on the recording medium flows along the gap face, and accordingly, it is considered that leakage flux at the magnetic gap is increased to reduce the reproducing efficiency. On the contrary, in the head Va, since the initial permeability in the direction parallel with the gap face is considerably small, and most of the signal flux flowing in from the forward end of the gap flows in the direction parallel with the sliding face having high initial permeability, the magnetic flux flowing along the gap face is reduced, and consequently, it is considered that the leakage flux at the magnetic gap portion is reduced for the increasing of the reproducing efficiency. Such a trend may be clearly seen from the magnetic flux diagram showing the results as calculated by the finite element method. In Fig. 4, a curve Vc represents head output characteristics of a video head in the construction of the present invention, in which the thickness of the ferromagnetic thin film 11a whose easy axis direction intersects at right angles with the recording medium sliding surface is set at 3µm, and the thickness of the ferromagnetic thin film 11b whose easy axis direction is parallel thereto, is set at 1µm for the lamination. According to the arrangement of the present invention, as referred to earlier, in the vicinity of the magnetic gap, since the ferromagnetic thin film 11a having the high initial permeability in the direction parallel to the sliding surface is superior in terms of cross sectional area so as to be dominant, the leakage flux is reduced for the improvement of the reproducing efficiency. Moreover, due to the fact that the width of the magnetic path is large at the magnetic path portion in the direction intersecting at right angles with the sliding surface, even if the film thickness is thin, the ferromagnetic thin film 11b whose initial permeability in the direction intersecting at right angles with the sliding surface is high becomes dominant. As a result, since the characteristics in the direction of the hard axis may be effectively utilized in most of the magnetic path, the signal can be recorded or reproduced at a sufficiently high efficiency even in the high frequency region above 30MHz.

Claims: A magnetic head comprising ferromagnetic thin films (1; 11) and insulative thin films (2) alternately laminated one upon another to form a multi-layered film magnetic core (3) which constitutes part or whole of a magnetic path, said multi-layered film magnetic core (3) being constituted by ferromagnetic thin films (1; 11) having anisotropric nature in its film face and so arranged that directions of anisotropy thereof intersect generally at right angles with each other; characterized in that a magnetic gap (7) is constituted by a cross section of said multi-layered film. A magnetic head as claimed in claim 1, characterized in that the direction of magnetization easy axis for a first set (11a) of the ferromagnetic thin films (11) is adapted to generally intersect at right angles with a recording medium sliding surface, and that the total thickness of said first set (11a) of the ferromagnetic thin films (11) whose direction of magnetization easy axis intersect generally at right angles with the recording medium sliding surface, is larger than the total thickness of a second set (11b) of the ferromagnetic thin films (11), said second set (11b) intersecting generally at right angles with said first set (11a). A magnetic head as claimed in claim 2, wherein said ferromagnetic thin films (11) are made of an amorphous magnetic alloy. A magnetic head as claimed in claim 2 or 3, wherein said insulative films (2) are made of SiO₂. A magnetic head as claimed in claim 2, wherein said ferromagnetic thin films (11) are made of sendust alloy.

Inventors: TAKAHASHI KEN, YOHDA HIROSHI, TAKAHASHI, KEN, YOHDA, HIROSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1988
Main classification: G11B5
IPC codes: G11B5