Patent Publication Number: US-2004057154-A1

Title: Magnetic head having first core and second core bonded together and manufacturing method therefor

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a magnetic head primarily used with a magnetic recording/reproducing apparatus of video equipment for recording/reproducing recording signals onto/from magnetic tape or a data magnetic recording/reproducing apparatus for a computer. More particularly, the present invention relates to a magnetic head capable of improving planar bondability of a first core and a second core and also capable of providing evenly improved bonding strength, and a manufacturing method for the same.  
       [0003] 2. Description of the Related Art  
       [0004] In a magnetic recording apparatus in video equipment or a magnetic recording/reproducing apparatus or the like for storing data for a computer, a magnetic head is mounted on a rotary drum of a rotating head apparatus, and a magnetic tape runs in contact with the rotary drum along a helical track, and information is recorded onto a magnetic tape by helical scanning as the rotary drum rotates.  
       [0005] Recently, in magnetic recording/reproducing apparatuses of video equipment, magnetic recording/reproducing apparatuses for storing data for computers, or other similar apparatuses, efforts have been focused on making further narrower tracks with reduced track widths or using higher frequencies to achieve recording of information onto a magnetic recording medium with a higher density. To accomplish narrower tracks, a track width Tw of a magnetic gap must be decreased.  
       [0006] To achieve such a narrower track, the use of a thin-film magnetic head formed by a thin film forming process has been proposed.  
       [0007]FIG. 16 is a perspective view showing an example of a magnetic head using a conventional thin-film magnetic head. The magnetic head shown in FIG. 16 has a reproducing magneto-resistive (MR) thin-film magnetic head  2 , a recording inductive head  3  and an insulating layer  6  serving as a protective film, which are deposited on a first core  1 . A second core  5  is bonded onto the insulating layer  6  by an adhesion layer  4 . Reference numeral  7  denotes electrodes.  
       [0008] In the magnetic head shown in FIG. 16, entire bonding surfaces  1   a  and  5   a  of the first core  1  and the second core  5 , respectively, are planar. The bonding surfaces  1   a  and  5   a  are bonded to each other by the adhesion layer  4 . However, the bonding surfaces  1   a  and  5   a  of the first core  1  and the second core  5 , respectively, have large areas, making it difficult to machine the bonding surfaces  1   a  and  5   a  to planar surfaces with high accuracy. This tends to lead to failure of highly accurate planar bonding between the bonding surfaces  1   a  and  5   a  of the first core  1  and the second core  5 . Hence, there has been a problem in that the poor planar bondability frequently results in uneven thickness of the adhesion layer  4  between the bonding surfaces  1   a  and  5   a , causing deteriorated bonding strength.  
       [0009] Furthermore, in the magnetic head shown in FIG. 16, the adhesion layer  4  between the first core  1  and the second core  5  is exposed on a medium opposing surface H 2 A. If, therefore, magnetic particles come off a magnetic tape when the magnetic tape slides against the medium opposing surface H 2 A, the magnetic particles adhere to the adhesion layer  4  exposed on the medium opposing surface H 2 A, leading to deteriorated characteristics of the magnetic head.  
       [0010]FIG. 17 shows a magnetic head shown in FIG. 1 of Japanese Unexamined Patent Application Publication No. 2000357304 (hereinafter referred to as “Patent Document 1”). FIG. 17 is a partial perspective view of the magnetic head. The components assigned the same reference numerals as those in FIG. 16 denote the same components shown in FIG. 16.  
       [0011] According to Patent Document 1, the thickness of the adhesion layer  4  in the tape traveling direction (direction Z in the figure) increased toward a height direction (direction Y in the figure) from a medium opposing surface H 3 A. It is described that the adhesion layer  4  is not exposed on the medium opposing surface H 3 A.  
       [0012] According to Patent Document 1, in order to gradually increase the thickness of the adhesion layer  4  in the height direction, a groove  8  is formed in the bonding surface  1   a  of the first core  1  (referred to as a substrate in the publication) that is in contact with the second core  5 , the groove  8  gradually becoming deeper in the height direction as the distance from the medium opposing surface H 3 A increases. In addition, a groove  9  is formed in the bonding surface  5   a  of the second core  5  (referred to as a protective substrate in the publication) that is in contact with the first core  1 , the groove  9  gradually becoming deeper in the height direction as the distance from the medium opposing surface H 3 A increases. An adhesive agent is injected between the grooves  8  and  9  to gradually increase the thickness of the adhesion layer  4  in the height direction.  
       [0013] The magnetic head described in Patent Document 1, however, poses the following problem. First, when the first core  1  and the second core  5  are provided with the grooves  8  and  9  that gradually become deeper in the height direction or direction Y in the drawing as the distance from the medium opposing surface H 3 A increases, as shown in FIG. 17, then the first core  1  and the second core  5  will not have any portion that would be in surface contact when they are abutted against each other in a manufacturing process. This prevents the first core  1  and the second core  5  from being positioned and bonded with high accuracy. Especially because the first core  1  and the second core  5  must be supported with high accuracy by jigs shown in FIG. 17 until the adhesion layer  4  is fixed. This requires highly accurate support of the jigs, inevitably leading to an extremely complicated manufacturing process.  
       [0014] Furthermore, since the adhesion layer  4  is formed to become thicker in the height direction, so that the bonding strength of the adhesion layer  4  is not even in the height direction. The bonding strength near the medium opposing surface H 3 A where the adhesion layer  4  is thin is particularly low. At a height side where the adhesion layer  4  is thick, the bonding strength tends to decrease if the adhesion layer  4  is excessively thick, because the bonding strength is the strength of a resin itself.  
       [0015] Thus, in the magnetic head according to Patent Document 1, it is impossible to bond the first core  1  and the second core  5  with high accuracy, and the bonding strength tends to be uneven and poor.  
       SUMMARY OF THE INVENTION  
       [0016] Accordingly, the present invention has been made with a view toward solving the problem with the prior art described above, and it is an object of the present invention to provide a magnetic head that permits the planar bondability of a first core and a second core, in particular, to be improved and allows bonding strength to be evenly improved, and a manufacturing method for the same.  
       [0017] To this end, according to one aspect of the present invention, a magnetic head is provided, which includes a first core equipped with a thin film magnetic head, and a second core bonded to a surface of the first core whereon the thin film magnetic head is formed, a magnetic gap of the thin film magnetic head being exposed on a medium opposing surface of the first core and the second core, wherein a bonding surface of at least one of the first core and the second core is provided with at least one abutting plane that juts out toward the other bonding surface and a groove formed to have a predetermined depth with a step provided between itself and the abutting plane, the abutting plane and the bonding surface of the other core are butted against each other, an adhesion layer of a predetermined thickness is provided at least between the groove and the bonding surface of the other core, and the first core and the second core are bonded.  
       [0018] Thus, the abutting plane partly jutting out is formed on the bonding surface of at least one of the first core and the second core and the abutting plane is butted against the bonding surface of the other core, allowing the first core and the second core to be partly in surface contact. Moreover, the planar machining of the abutting plane can be accomplished with high accuracy, so that the planar bondability of the first core and the second core can be improved.  
       [0019] The groove formed with the step provided between itself and the abutting plane has a predetermined depth, and the adhesion layer of a predetermined thickness is formed between the groove and the bonding surface of the other core. This arrangement allows the adhesion layer to have even bonding strength, making it possible to firmly bond the first core and the second core.  
       [0020] Thus, the abutting plane is formed on a part of the bonding surface, so that more accurate planar machining than that in the conventional example shown in FIG. 16 can be accomplished. Hence, the planar bondability can be improved, allowing the first core and the second core to be bonded with uniform and greater bonding strength.  
       [0021] Preferably, the abutting plane is formed such that it includes the region formed on the first core wherein the thin film magnetic head is formed. This arrangement makes it possible to prevent the adhesion layer from being exposed on the medium opposing surface, thus solving the problem of adhesion or the like of magnetic particles to the adhesion layer. Moreover, since the groove is not formed in the region where the thin film magnetic head is formed, the thin film magnetic head is not damaged when the groove is formed. Thus, a thin film magnetic head with outstanding reproducing and recording characteristics can be secured.  
       [0022] Preferably, the thickness of the adhesion layer ranges from 0.05 μm to 0.3 μm. The experimental results to be discussed hereinafter have revealed that a core transverse rupture strength of 2N or more can be obtained even in an adverse environment with high humidity.  
       [0023] Preferably, thin film magnetic head is formed to have an MR thin film magnetic head.  
       [0024] Preferably, the thin film magnetic head and the first core are covered with a protective film made of an insulating material, and the front surface of the protective film provides the bonding surface.  
       [0025] The adhesion layer is preferably formed of an epoxy-based adhesive agent or a low-melting, glass-based adhesive agent. The heating temperature required for curing the epoxy-based adhesive agent or the like is 300° C. or less. The upper limit temperature that the MR thin film magnetic head can survive in the curing process is about 300° C. at the most; therefore, using an epoxy-based adhesive agent for the adhesion layer makes it possible to adequately prevent deterioration of the reproducing characteristic of the MR thin film magnetic head.  
       [0026] According to another aspect of the present invention, a manufacturing method for a magnetic head is provided, the method including the steps of (a) forming a plurality of thin film magnetic heads on a first substrate, then cutting the first substrate into a bar with a plurality of thin film magnetic heads aligned thereon in the longitudinal direction to form a first bar, (b) cutting a second substrate into a bar to form a second bar, (c) defining the surface of the first bar whereon the thin film magnetic heads are formed as the surface to be bonded to the second bar, protuberantly forming at least one or more abutting planes on the boding surface of at least one of the first bar or the second bar at positions where they will remain in cores when the bars are cut into individual cores in a subsequent step, and forming a groove to a predetermined depth with a step provided between itself and the abutting plane, (d) butting the abutting plane formed on at least one bar against the bonding surface of the other bar, setting the bars parallel to each other, and forming an adhesion layer of a predetermined thickness between the groove formed in at least one bar and the bonding surface of the other bar to bond the first bar and the second bar, and (e) cutting the first bar and the second bar into cores between the individual thin film magnetic heads to produce a magnetic head having the first core and the second core bonded through the intermediary of the adhesion layer and a magnetic gap of the thin film magnetic head being exposed on the medium opposing surface of the first core and the second core.  
       [0027] As set forth above, in step (c), the abutting plane is protuberantly formed on the bonding surface of at least one of the first bar and the second bar, and the groove is formed to a predetermined depth with a step provided between itself and the abutting plane. With this arrangement, the abutting plane of one bar and the bonding surface of the other bar can be butted against each other to secure surface contact in step (d) described above. Moreover, the abutting plane can be formed in a predetermined small area, so that the abutting plane can be machined with high accuracy, making it possible to improve the planar bondability of the abutting plane of one bar and the bonding surface of the other bar. In addition, the surface-abutting of the abutting plane against the bonding surface allows the first bar and the second bar to be stably disposed in parallel, and the adhesion layer of the predetermined thickness can be formed in the groove between the first bar and the second bar. This makes it possible to easily and properly fabricate a magnetic head with enhanced bonding strength.  
       [0028] Preferably, the abutting plane is formed in step (c) described above such that it includes the region wherein the thin film magnetic heads of the first bar are provided.  
       [0029] Preferably, the abutting plane is formed in each region wherein the thin film magnetic heads are formed, and the groove formed between the abutting planes is exposed up to the front end surface of the first bar that will provide a medium opposing surface. This prevents the groove from being formed in the region wherein the thin film magnetic heads are formed, making it possible to properly protect the thin film magnetic heads from damage caused by forming the groove. In addition, since the groove is partly open at the front end surfaces of the first bar and the second bar, when the first bar and the second bar are pressed against each other, an adhesive agent injected into the groove between the first bar and the second bar will evenly spread in the groove due to the capillary phenomenon or the like, allowing the first bar and the second bar to be firmly bonded and fixed. Alternatively, the first bar and the second bar may be butted against each other and positioned, then an adhesive agent may be injected into the groove exposed at the front end surfaces.  
       [0030] Alternatively, in step (c) described above, the abutting planes may be formed in a part of the region between the thin film magnetic heads arranged lengthwise on the first bar. The abutting plane formed in a part of the region between the thin film magnetic heads may be a dummy pad positioned on a cutting line for cutting the first bar and the second bar into cores in step (e) described above and completely removed or partly removed in the cutting step.  
       [0031] Preferably, the groove is formed to a depth ranging from 0.05 μm to 0.3 μm in step (c) above, and the adhesion layer formed in step (d) above is formed to a thickness ranging from 0.05 μm to 0.3 μm.  
       [0032] Preferably, an epoxy-based adhesive agent or a lowmelting, glass-based adhesive agent is selected as an adhesive agent in step (d) above. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0033]FIG. 1 is a perspective view of a magnetic head according to an embodiment of the present invention;  
     [0034]FIG. 2 is a perspective view of a first core shown in FIG. 1;  
     [0035]FIG. 3 is a partial front view of a different first core from that shown in FIG. 2;  
     [0036]FIG. 4 is a partial sectional view of the magnetic head shown in FIG. 1 observed from a medium opposing surface;  
     [0037]FIG. 5 is a top plan view of a rotating-head magnetic recording/reproducing apparatus using the magnetic head shown in FIG. 1;  
     [0038]FIG. 6 is a process diagram illustrating a manufacturing method for the magnetic head shown in FIG. 1;  
     [0039]FIG. 7 is another process diagram illustrating the manufacturing method for the magnetic head shown in FIG. 1;  
     [0040]FIG. 8 is still another process diagram illustrating the manufacturing method for the magnetic head shown in FIG. 1;  
     [0041]FIG. 9 is yet another process diagram illustrating the manufacturing method for the magnetic head shown in FIG. 1;  
     [0042]FIG. 10 is a further process diagram illustrating the manufacturing method for the magnetic head shown in FIG. 1;  
     [0043]FIG. 11 is another process diagram illustrating the manufacturing method for the magnetic head shown in FIG. 1;  
     [0044]FIG. 12 is a diagram illustrating an experimental method for measuring a preferable thickness range of an adhesion layer;  
     [0045]FIG. 13 is a view of the assembly shown in FIG. 12 observed from a direction indicated by an arrow I;  
     [0046]FIG. 14 is a graph showing a relationship between the thickness of the adhesion layer and the transverse rupture strength of cores immediately after a first core and a second core are bonded;  
     [0047]FIG. 15 is a graph showing a relationship between the thickness of the adhesion layer and the transverse rupture strength of the cores after the first core and the second core bonded by the adhesion layer are left for 72 hours in an environment wherein the room temperature is 40° C. and humidity is 95%;  
     [0048]FIG. 16 is a perspective view of a conventional magnetic head; and  
     [0049]FIG. 17 is a perspective view showing a conventional magnetic head having a different construction from that shown in FIG. 16. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0050]FIG. 1 a perspective view of a magnetic head according to a first embodiment of the present invention. FIG. 2 is a perspective view of a first core of the magnetic head shown in FIG. 1, the first core being observed from the surface whereon thin film magnetic heads are formed. FIG. 3 is a partial front view of a first core according to a second embodiment observed from the surface whereon thin film magnetic heads are formed. FIG. 4 is a partial sectional view of the magnetic head shown in FIG. 1 observed from a medium opposing surface.  
     [0051] A magnetic head H 1  is a sliding magnetic head constituting a magnetic recording/reproducing apparatus of video equipment for recording/reproducing recording signals onto/from, for example, a magnetic tape, or a data magnetic recording/reproducing apparatus for a computer.  
     [0052] The sliding type thin film magnetic head shown in FIG. 1 can be installed on a rotating head apparatus, as shown in FIG. 5.  
     [0053] In a rotating head apparatus  50  provided on a magnetic recording/reproducing apparatus shown in FIG. 5, a fixed drum (not shown) is secured, a rotary drum  50   a  coaxial with the fixed drum is rotatively supported on the fixed drum, and the rotary drum  50   a  is rotatively driven in the direction of the arrow by motor power. A magnetic tape T, which is a magnetic recording medium, is wound onto the rotating head apparatus  50  at a predetermined angle along a helical track to run in the direction of the arrow. Meanwhile, the rotary drum  50   a  rotates, and sliding type thin film magnetic heads H 1  mounted on the rotary drum  50   a  scan the magnetic tape T.  
     [0054] In FIG. 5, a pair of sliding type thin film magnetic heads H 1  is mounted on the rotary drum  50   a  at positions where they oppose each other. Alternatively, however, three or more sliding type thin film magnetic heads H 1  may be installed.  
     [0055] In the magnetic head H 1 , thin film magnetic heads  12  and an insulating layer  24  composed of Al 2 O 3  to serve as a protective film are deposited by a thin film forming process, through the intermediary of a ground layer composed of an insulating material, such as Al 2 O 3  or SiO 2 , on a surface  11   a  of a first core  11  composed of nonmagnetic alumina-titanium carbide, the surface  11   a  having magnetic reproducing heads formed thereon.  
     [0056] Referring to FIG. 4, the thin film magnetic head  12  is a compound thin film magnetic head combining an MR thin film magnetic head  22  and an inductive head  23 .  
     [0057] The MR thin film magnetic head  22  includes a lower shielding layer  22   b , a lower gap layer  22   c , an MR element layer  22   d , a hard bias layer  22   e , electrode layers  22   f , an upper gap layer  22   g  and an upper shielding layer  22   h , which are deposited, via an insulating layer, which is the ground layer, on the first core  11  composed of alumina-titanium carbide by a thin film forming process. The portion sandwiched by the lower shielding layer  22   b  and the upper shielding layer  22   h  and opposing a magnetic tape provides a magnetic gap Ga of the MR thin film magnetic head  22 .  
     [0058] Referring to FIG. 4, the recording inductive head  23  provided on the MR thin film magnetic head  22  includes a gap layer  23   b , a coil layer  23   c  and an upper core layer  23   d  deposited on the lower core layer  23   a  also serving as an upper shielding layer by the thin film forming process as in the case of the MR thin film magnetic head  22 . The portion sandwiched by the lower core layer  23   a  and the upper core layer  23   d  and opposing a magnetic tape provides a magnetic gap Gb of the inductive head  23 .  
     [0059] The lower gap layer  22   c , the upper gap layer  22   g  and the gap layer  23   b  are formed of Al 2 O 3  or SiO 2 . The lower shielding layer  22   b , the upper shielding layer  22   h  (the lower core layer  23   a ) and the upper core layer  23   d  are formed of a soft magnetic material, such as Permalloy. The electrode layer  22   f  and the coil layer  23   c  are formed of an electrically conductive material, such as Cu. The hard bias layer  22   e  is formed of a hard magnetic material, such as PtCo.  
     [0060] The MR element layer  22   d  is formed of a GMR element or AMR element, such as a spin-valve type thin film element.  
     [0061] The insulating layer  24  functioning as a protective film is deposited on the inductive head  23 .  
     [0062] As shown in FIGS. 1 and 4, the second core  25  is bonded to the first core  11  such that it faces toward the surface  11   a  on which the thin film magnetic head  12  is formed. The second core  25  is composed of alumina-titanium carbide or the like, as in the case of the first core  11 . Referring to FIG. 4, the insulating layer  26 , which is a protective film composed of an insulating material, such as Al 2 O 3 , is formed to a thin film on the surface of the second core  25  that opposes the first core  11  by, for example, sputtering or the like. The insulating layer  26  is formed by, for example, sputtering, and the sputtering enhances the force of bonding between the second core  25  and the insulating layer  26 . This makes it possible to protect the second core  25  from damage at the interface between the second core  25  and the insulating layer  26  when the magnetic tape slides on a medium opposing surface H 1 A.  
     [0063] In the embodiment shown in FIGS. 1 and 4, the front surface of the insulating layer  24  formed on the surface  11   a  of the first core  11  with the thin film magnetic heads  12  formed thereon provides a bonding surface  11   b  to be bonded to the second core  25 . The front surface of the insulating layer  26  formed on the second core  25  provides a bonding surface  25   a  to be bonded to the first core  11 .  
     [0064] Referring to FIG. 1, the medium opposing surface H 1 A of the first core  11  and the second core  25  is curved in a radius shape in direction Z in the drawing, which is the tape sliding direction. A magnetic gap G of the thin film magnetic head  12  is exposed at the medium opposing surface H 1 A, and the magnetic gap G is positioned substantially at the middle of the medium opposing surface H 1 A in direction Z in the drawing. The magnetic gap G in this embodiment refers to both the magnetic gap Ga of the MR thin film magnetic head  22  and the magnetic gap Gb of the inductive head  23  shown in FIG. 4.  
     [0065] As shown in FIG. 1, the length of the first core  11  in direction Y in the drawing, which will be referred to as “the height direction” in some cases hereinafter, is set to be greater than the length of the second core  25  in direction Y. The front surface of the insulating layer  24  formed on the first core  11  extends in direction Y beyond a rear end surface  25   b  of the second core  25  at the opposite side of the medium opposing surface H 1 A. Furthermore, a plurality of electrodes  13  is provided on the front surface of the insulating layer  24  of the first core  11  that juts out in direction Y beyond the second core  25 . The electrodes  13  is connected in conduction with the MR thin film magnetic head  22  and the inductive head  23  by a leading layer or the like (not shown) inside the insulating layer  24 . Currents flow from the electrodes  13  to the MR thin film magnetic head  22  and the inductive head  23 .  
     [0066] Referring to FIGS. 1 and 2, the first core  11  and the second core  25  are provided with recessed portions  19  and  20 , respectively, formed via steps B in the height direction (direction Y). The recessed portions  19  and  20  extend from both side ends  17  and  18  in the width direction (direction X) of the medium opposing surface H 1 A. Due to the recessed portions  19  and  20 , the medium opposing surface H 1 A of the first core  11  and the second core  25  projects beyond the rest of the assembly. With this arrangement, the magnetic gap G exposed at the medium opposing surface H 1 A comes in contact with the magnetic tape sliding on the medium opposing surface H 1 A under an appropriate surface pressure, making it possible to properly improve frequency characteristics or the like of the magnetic head.  
     [0067] Referring to FIG. 2, the bonding surface  11   b  of the first core  11  is provided with abutting planes  14  and  15  jutting out in direction Z toward a bonding surface  25   a  of the second core  25 . For convenience sake, the abutting plane  14  will be referred to as “the first abutting plane” and the abutting plane  15  as “the second abutting plane.” 
     [0068] As shown in FIG. 2, the first abutting plane  14  is formed in a region A of the thin film magnetic head  12 . FIG. 3 shows an embodiment different from the one shown in FIG. 2. However, the position, shape, etc. of the first abutting plane  14  of the embodiment shown in FIG. 3 are the same as those shown in FIG. 2; therefore, the explanation of the first abutting plane  14  may use FIG. 3, as necessary.  
     [0069]FIGS. 2 and 3 show the MR element layer  22   d  and the electrode layers  22   f  located on both sides of the MR element layer  22   d  of the thin film magnetic head  12  observed from the bonding surface  11   b  of the first core  11  (the MR element layer  22   d  and the electrode layers  22   f  being indicated by the dashed line in the drawings. The term “region A” means a planar region having a size for including the denotation of all layers of the shielding layers  22   b  and  22   h , the MR element layer  22   d , the bias layer  22   e , the electrode layers  22   f , the coil layer  23   c , the core layers  23   a  and  23   d , which constitute the thin film magnetic head  12 . Among these layers, the electrode layers  22   f , for example, have the largest planar areas, so that FIGS. 2 and 3 show the planar shape of the electrode layers  22   f , in particular, to indicate the region A.  
     [0070] The first abutting plane  14  is formed to extend in direction Y from the medium opposing surface H 1 A to have a predetermined width (the dimension in direction X) and a predetermined length (the dimension in direction Y) so that the area of the abutting plane  14  completely includes the region A of the thin film magnetic head  12 .  
     [0071] Referring to FIG. 2, a groove  16  is formed to a predetermined depth, via a step, from a surface  14   a  of the first abutting plane  14 , the surface  14   a  being on the farther end from the medium opposing surface H 1 A. In this embodiment, the groove  16  is formed to have a predetermined length in direction Y in the drawing and to extend from a left end  11   c  to a right end lid of the first core  11 .  
     [0072] As shown in FIG. 2, the first core  11  is provided with a second abutting plane  15  that extends, via a step, from an end  16   a  of the groove  16  on the farther end from the medium opposing surface H 1 A in direction Y and juts out toward the bonding surface  25   a  of the second core  25 . The foregoing electrodes  13  are formed on the second abutting plane  15 .  
     [0073] The first abutting plane  14  and the second abutting plane  15  are formed to have the same height.  
     [0074] In this embodiment, the bonding surface  25   a  of the second core  25  is not provided with the abutting planes  14 ,  15  and the groove  16  formed on the bonding surface  11   b  of the first core  11 . The entire bonding surface  25   a  of the second core  25  is planar.  
     [0075] According to this embodiment, the surfaces of the abutting planes  14  and  15  and the bonding surface  25   a  of the second core  25  are butted to bond the first core  11  and the second core  25 . As shown in FIG. 1, the entire surface of the first abutting plane  14  is butted to the bonding surface  25   a  of the second core  25 , while only a part of the second abutting plane  15  is butted to the bonding surface  25   a  of the second core  25 .  
     [0076] At this time, the gap shown in FIG. 1 is formed between the groove  16  formed in the first core  11  and the bonding surface  25   a  of the second core  25 , and an adhesion layer  30  is provided in the gap.  
     [0077] As described above, the groove  16  is formed to have a predetermined depth, and the abutting planes  14  and  15  formed on the first core  11  are formed to the same height. These abutting planes  14  and  15  and the bonding surface  25   a  of the second core  25  are butted to each other so that the first core  11  and the second core  25  are disposed in parallel to each other and the gap is formed to a predetermined thickness between the groove  16  and the bonding surface  25   a  of the second core  25 . Hence, the adhesion layer  30  buried in the gap is also formed to have a predetermined thickness.  
     [0078] The magnetic head shown in FIGS. 1 and 2 allows the first abutting plane  14  formed on the first core  11  to have a smaller predetermined area and the planar machining of the first abutting plane  14  to be accomplished with high accuracy. This makes it possible to improve the planar bondability of the first core  11  and the second core  25  and also to achieve uniformly enhanced bonding strength at any portion of the adhesion layer  30  since the adhesion layer  30  is formed to a predetermined thickness, as previously described. Moreover, the second abutting plane  15  formed on the first core  11  is formed to have a larger area than that of the first abutting plane  14 , whereas only a part of the second abutting plane  15  is butted to the bonding surface  25   a  of the second core  25 . This means that highly accurate planar machining is not required for the entire second abutting plane  15 , and the planar bondability of the first core  11  and the second core  25  can be further improved by carrying out high-accuracy planar machining only on the portion of the second abutting plane  15  that will be butted to the second core  25 .  
     [0079] As in the case of the embodiment shown in FIG. 1, the first abutting plane  14  is preferably formed to include the region A formed on first core  11  wherein the thin film magnetic head  12  is formed. This arrangement prevents the thin film magnetic head  12  from being damaged by etching or the like when the groove  16  is formed, so that deterioration of the reproducing characteristic or the recording characteristic of the thin film magnetic head  12  can be prevented. In addition, although an edge of the first abutting plane  14  is exposed on the medium opposing surface H 1 A, the adhesion layer  30  formed in the groove  16  is located at a position retreated in the height direction from the medium opposing surface H 1 A, so that the adhesion layer  30  is not exposed on the medium opposing surface H 1 A. This makes it possible to prevent a problem of adhesion of magnetic particles attributable to exposure of the adhesion layer  30  at the medium opposing surface H 1 A.  
     [0080] An adhesive agent injected into the gap between the groove  16  formed in the first core  11  and the bonding surface  25   a  of the second core  25  may slightly oozes out, due to the capillary phenomenon or the like, between the first abutting plane  14  formed on the first core  11  and the bonding surface  25   a  of the second core  25 . In such a case also, exposure of the adhesive agent on the medium opposing surface H 1 A can be prevented.  
     [0081] Thickness t1 of the adhesion layer  30  preferably ranges from 0.05 μm to 0.3 μm. The experimental results to be discussed hereinafter have revealed that a core transverse rupture strength of 2N or more can be obtained even in an adverse environment with high humidity if the adhesion layer  30  is formed to a thickness within the aforesaid range.  
     [0082] In the embodiment shown in FIG. 1, the MR thin film magnetic head has been used as the reproducing head for the thin film magnetic head  12 . The present invention, however, is not limited to MR thin film magnetic heads and any other magnetic reproducing means may be used for the thin film magnetic head  12 . In the embodiment shown in FIG. 4, the recording inductive head  23  made of a thin film is deposited on the MR thin film magnetic head  12 . However, the inductive head  23  may be omitted, or the MR thin film magnetic head  12  may be omitted and only the inductive head  23  may be formed.  
     [0083] As shown in FIGS. 1, 2 and  4 , the thin film magnetic head  12  and the first core  11  whereon the thin film magnetic head  12  is not formed are covered by the insulating layer  24  providing a protective film. This arrangement prevents the bonding surface  25   a  of the second core  25  from being directly butted onto the thin film magnetic head  12 , thus protecting the thin film magnetic head  12  from damage or the like when the first core  11  and the second core  25  are butted to each other. The groove  16  is formed within the thickness of the insulating layer  24 , as shown in FIG. 2. The groove  16 , however, may alternatively be extended to the first core  11  made of alumina-titanium carbide under the insulating layer  24 .  
     [0084] Since the adhesion layer  30  is formed of an epoxy-based adhesive agent, the bonding process can be implemented at 300° C. or less, thus restraining deterioration of the characteristics of the MR thin film magnetic head  12 . The adhesion layer  30  may be formed using a low-melting, glassbased adhesive agent in place of an epoxy-based adhesive agent.  
     [0085]FIG. 3 shows an embodiment in which the abutting plane on the bonding surface  11   b  of the first core  11  is located at a different position from that shown in FIGS. 1 and 2.  
     [0086] As described above, in the embodiment shown in FIG. 3, the first abutting plane  14  extends in the height direction (direction Y in the drawing) from the medium opposing surface H 1 A to have a predetermined width (the dimension in direction X) and a length (the dimensional in direction Y) to include the region A wherein the thin film magnetic head  12  is formed, the first abutting plane  14  jutting out toward the bonding surface  25   a  of the second core  25 , as in the embodiment shown in FIGS. 1 and 2.  
     [0087] Referring to FIG. 3, the bonding surface  11   b  of the first core  11  has three abutting planes  30 ,  31  and  32  in addition to the first abutting plane  14 . The abutting plane  30  is formed toward the inside of the first core  11  from a left end  11   c  of the first core  11 , while the abutting plane  31  is formed toward the inside of the first core  11  from a right end lid of the first core  11 . The abutting plane  32  is formed substantially at the middle between the left end  11   c  and the right end lid of the first core  11 .  
     [0088] These abutting planes  14 ,  30 ,  31  and  32  are all formed to have the same height, and an electrode surface  34  which is flush with the abutting planes is protuberantly formed at a position away from the abutting planes in the height direction (direction Y in the drawing). The electrodes  13  shown in FIGS. 1 and 2 are formed on the electrode surface  34 . A groove  33  is formed to a predetermined depth through the intermediary of steps from the abutting planes  14 ,  30 ,  31  and  32  and the electrode surface  34 .  
     [0089] The one-dot chain line shown in FIG. 3 denotes the position where a rear end surface  25   b  of the second core  25  at the farther side from the medium opposing surface H 1 A is disposed when the first core  11  and the second core  25  are bonded. The rear end surface  25   b  of the second core  25  is positioned more closely to the medium opposing surface H 1 A than the electrode surface  34  formed on the first core  11 . As a result, the four abutting planes  14 ,  30 ,  31  and  32  formed on the first core  11  are butted to the bonding surface  25   a  of the second core  25 , and the first core  11  and the second core  25  are bonded by an adhesive agent injected into the groove  33 .  
     [0090] A plurality of the small abutting planes  14 ,  30 ,  31  and  32  shown in FIG. 3 and the bonding surface  25   a  of the second core  25  are planarly bonded, and the bonding surface  25   a  of the second core  25  does not overlap the electrode surface  34  having a large planar area. These small abutting planes  14 ,  30 ,  31  and  32  can be planarly machined with high accuracy, allowing the first core  11  and the second core  25  to be planarly bonded with high accuracy.  
     [0091] The minimum required number of the abutting planes is one, and the abutting plane or planes may be formed at an arbitrary position or positions. Furthermore, in FIG. 1 through FIG. 3, the abutting planes and the groove are formed on the bonding surface  11   b  of the first core  11 . The abutting planes and the groove may alternatively be formed on the bonding surface  25   a  of the second core  25 , or the abutting planes and the groove may be provided on both bonding surfaces  11   b  and  25   a  of the first core  11  and the second core  25 , respectively.  
     [0092]FIG. 6 through FIG. 11 illustrate the steps of a manufacturing method for the magnetic head shown in FIG. 1. Referring first to FIG. 6, a ground layer composed of an insulating material, such as Al 2 O 3  or SiO 2 , is formed to a thin film by sputtering on a first substrate  40  composed of alumina-titanium carbide. Then, the thin film magnetic head  12  constructed of the MR thin film magnetic head  22  and the inductive head  23  explained in conjunction with FIG. 4 is formed to a thin film on the ground layer. Alternatively, only the MR thin film magnetic head  22  may be formed to a thin film. Furthermore, the magnetic head used is not limited to the MR thin film magnetic head; a different type may be used as long as it is a magnetic reproducing head.  
     [0093] After the inductive head  23  is formed, the insulating layer  24 , which is a protective film composed of Al 2 O 3 , is formed to a thin film by sputtering. Furthermore, as illustrated in FIG. 6, the electrodes  13  connected in conduction to the MR thin film magnetic heads  22  and the inductive heads  23  are formed on the insulating layer  24 .  
     [0094]FIG. 6 shows only some of the thin film magnetic heads  12  and the electrodes  13  formed on the entire surface of the first substrate  40  with predetermined intervals provided among them.  
     [0095] Then, the first substrate  40  is cut into bars along dotted lines C shown in FIG. 6 to make a plurality of first bars  41  shown in FIG. 7. As can be understood from FIG. 7, the first bar  41  has a plurality of the MR thin film magnetic heads  22  and the inductive heads  23  arranged in alignment in the lengthwise direction (direction X in the drawing).  
     [0096] Subsequently, the surface of the first bar  41  shown in FIG. 7 on which the electrodes  13  have been formed (the surface will be referred to as a “bonding surface  41   c ” hereinafter) is machined as shown in FIG. 8. In this embodiment, the bonding surface  41   c  denotes the surface of the insulating layer  24 .  
     [0097]FIG. 8 is a partial top plan view of the first bar  41  shown in FIG. 7 observed from the direction indicated by an arrow D. As shown in FIG. 8, the first abutting plane  14  that includes the region A wherein the thin film magnetic heads  12  have been formed is protuberantly formed. In addition, dummy pads  42  are protuberantly formed at positions in the regions that are located in direction X with respect to the thin film magnetic heads  12  and are away in direction Y from the surface that will provide the medium opposing surface H 1 A in a later step. The definition of the region A is as given above.  
     [0098] An electrode surface  43  is protuberantly formed at position further away in direction Y than the dummy pads  42  from the surface that will provide the medium opposing surface H 1 A in a later step. On the electrode surface  43 , the electrodes  13  shown in FIG. 7 are formed at positions distanced in direction Y. The first abutting plane  14 , the dummy pads  42  and the electrode surface  43  are all formed to be flush.  
     [0099] Referring to FIG. 8, the first abutting plane  14 , the dummy pads  42  and the electrode surfaces  43  are protuberantly formed by first depositing a resist layer on a surface on which they are to be formed, then etching the surface of the insulating layer  24  that is not covered by the resist layer to a predetermined depth. A groove  44  of a predetermined depth is formed in the etched insulating layer  24 .  
     [0100] As shown in FIG. 8, it is preferable that the first abutting plane  14  be protuberantly formed to include the region A in which the thin film magnetic heads  12  have been formed.  
     [0101] Forming the first abutting plane  14  to include the region A protect the thin film magnetic heads  12  from the aforesaid etching, making it possible to maintain good reproducing characteristics of the MR thin film magnetic heads  22  and good recording characteristics of the inductive heads  23 .  
     [0102] Furthermore, the first abutting plane  14  is formed at a position away from a front end surface  41   a  of the first bar  41 , which will be the medium opposing surface H 1 A in a subsequent process, in direction Y and also machined so that the groove  44  will not be exposed at the medium opposing surface H 1 A in a subsequent step. Hence, the adhesive agent injected into the groove  44  will not be exposed at the medium opposing surface H 1 A, making it possible to prevent magnetic particles sticking to the adhesive agent at the medium opposing surface H 1 A.  
     [0103] Preferably, as shown in FIG. 8, the first abutting plane  14  is individually formed in the region A of each thin film magnetic head  12  formed on the first bar  41 , and a groove  44   a  formed in direction X between the first abutting planes  14  is made exposed up to the front end surface  41   a  of the first bar  41 . With this arrangement, when the first bar  41  and the second bar  46  are butted against each other to position them, an adhesive agent injected into the groove  44  easily spreads evenly in the groove  44  due to the capillary phenomenon, allowing the first bar and the second bar to be firmly secured by bonding. Furthermore, an adhesive agent can be easily injected through the exposed grooves  44   a , so that it is unnecessary to apply the adhesive agent to a bonding surface of either the first bar  41  or the second bar  46  beforehand when butting the two bars to each other. This makes it possible to butt the first bar  41  and the second bar  46  to each other to position them with high accuracy, then to inject the adhesive agent.  
     [0104] Referring again to FIG. 8, as in the case of the first abutting plane  14 , the dummy pads  42  that have planes of a predetermined area and are flush with the first abutting plane  14  are formed between the thin film magnetic heads  12  arranged in the longitudinal direction (in direction X) of the first bar  41 . The dummy pads  42  are on cutting lines E for along which the first bar  41  and the second bar  46  are cut into cores in a subsequent process, meaning that the dummy pads  42  are removed in the subsequent cutting process. Providing the dummy pads  42  in the longitudinal direction, in which the thin film magnetic heads  12  are arranged, between the thin film magnetic heads  12  makes it possible to evenly distribute the force applied when the first bar  41  and the second bar  46  are butted against each other, restraining an undue force from being applied to the first abutting plane  14  on which the thin film magnetic heads  12  are formed. Thus, good reproducing and recording characteristics of the thin film magnetic heads  12  can be maintained, and the planar bondability of the first bar  41  and the second bar  46  can be improved. The dummy pads  42  provided between the thin film magnetic heads  12  in the longitudinal direction in which the thin film magnetic heads  12  are arranged make it possible to form the groove  44 , which is defined by the first abutting plane  14 , the dummy pads  42  and the electrode surface  43 , into a substantially rectangular shape with a large area. This arrangement allows the adhesive agent to uniformly spread in the entire groove  44  due to the capillary phenomenon.  
     [0105] The dummy pads  42  shown in FIG. 8 are all removed when the first bar  41  and the second bar  46  are cut into cores. The dummy pads  42 , however, can be partly left in the first core  11  by setting the width of the dummy pads  42  in direction X to be larger than the interval between the cutting lines E.  
     [0106] If the abutting plane  14  and the dummy pads  42  shown in FIG. 8 are provided at least two or more at the same height adjacently to the first bar  41 , and the abutting plane and the dummy pads are preferably arranged at regular intervals in the longitudinal direction (direction X), then the planar bondability for butting the first bar  41  and the second bar  46  to each other can be further improved. This allows the bars to be set parallel to each other easily and properly, and also allows adhesion layers  47  to be easily formed to a predetermined thickness when producing it by injecting an adhesive agent.  
     [0107] In the step illustrated in FIG. 9, an insulating layer  26  is deposited by sputtering on a second substrate  45  composed of alumina-titanium carbide, then the second substrate  45  is cut along dotted lines F shown in FIG. 9 to make a plurality of bar-shaped second bars  46 .  
     [0108] Referring to FIG. 10, the surface of the first bar  41  on which the thin film magnetic heads  12  are formed is defined as the bonding surface  41   c , while the surface of the insulating layer  26  of the second bar  46  is defined as a bonding surface  46   c . These bonding surfaces  41   c  and  46   c  are butted to each other. In the butting step, for example, an adhesive agent is injected into the groove  44  formed in the first bar  41  beforehand, and the first abutting plane  14  and the dummy pads  42  formed on the bonding surface  41   c  of the first bar  41  and the electrode surface  43  are partly butted to the bonding surface  46   c  of the second bar  46 . This causes the adhesive agent in the groove  44  to evenly spread in the groove  44  due to the capillary phenomenon.  
     [0109] As shown in FIG. 10, the groove  44   a  constituting the groove  44  is opened at the front end surface  41   a  of the first bar  41 , so that the adhesive agent injected into the groove  44  spreads in the groove  44  more evenly and quickly by the capillary phenomenon when the first bar  41  and the second bar  46  are butted to each other. Alternatively, the first bar  41  and the second bar  46  may be first positioned with high accuracy, and then an adhesive agent may be injected through the grooves  44   a  exposed on the front end surface  41   a . Thereafter, the adhesive agent is cured by heating so as to bond the first bar  41  and the second bar  46  by the adhesion layers  47 .  
     [0110] In this embodiment, since an epoxy-based adhesive agent is used as the aforesaid adhesive agent, the bonding process can be implemented at 300° C. or less, thus restraining deterioration of the characteristics of the MR thin film magnetic heads  12 . The adhesion layers  47  may be formed using a low-melting, glass-based adhesive agent in place of an epoxy-based adhesive agent.  
     [0111] In the step illustrated in FIG. 8, the groove  44  is formed to have a constant depth ranging from 0.05 μm to 0.3 μm, so that the adhesion layers  47  can be formed to have a constant thickness ranging from 0.05 μm to 0.3 μm accordingly. The experiment results to be discussed later have indicated that a core transverse rupture strength of 2N or more can be obtained even in a highly humid environment if the adhesion layers  47  are formed to a thickness within the aforesaid range. Therefore, high bonding strength of the adhesion layers  47  can be maintained.  
     [0112] In the step illustrated in FIG. 11, the grooves  44   a  with their ends exposed on the front end surface  41   a  of the first bar  41  have been filled with the adhesive agent, so that the adhesion layers  47  are exposed on the front end surface  41   a . The first bar  41  and the second bar  46  are ground along dotted lines G such that the exposed portions of the adhesion layers  47  are also ground so as to form the recessed portions  19  and  20  (shown in FIG. 1) in the first bar  41  and the second bar  46 , respectively. Once the recessed portions  19  and  20  are formed, the front end surfaces  41   a  and  46   a  of the first bar  41  and the second bar  46 , respectively, no longer have any portion where the adhesion layers  47  are exposed.  
     [0113] Then, the first bar  41  and the second bar  46  are cut into cores along one-dot chain lines E shown in FIG. 11 so as to produce magnetic heads, each including the first core  11  and the second core  25  bonded by the adhesion layer  47 . Furthermore, the medium opposing surface H 1 A of the magnetic head is subjected to cylindrical grinding or copy grinding to a radium shape. This fabricates a magnetic head having the magnetic gap G of the thin film magnetic head  12  exposed on the medium opposing surface H 1 A of the first core  11  and the second core  25 .  
     [0114] The following will describe the experiments carried out to determine a preferable range of thickness of the adhesion layer lying between the first core and the second core.  
     [0115] First, as shown in FIG. 12, a first core shaped in a rectangular parallelepiped and a second core that is also shaped in a rectangular parallelepiped but is shorter than the first core were bonded with an adhesion layer provided therebetween. The shapes of the bonding surfaces of the first core and the second core are the same as those shown in FIGS. 1 and 2. About 95% to 96% of the area of the bonding surface of the first core is occupied by a groove, and the remaining area of 5% to 4% is occupied by a first abutting plane that includes the region wherein thin film magnetic heads are formed. The first core was produced as explained in conjunction with FIG. 8. The first bar, as shown in FIG. 8, was formed, and the first core was made from the first bar. When the area of the surface to be bonded to the second bar is taken as 100%, the area of the abutting plane in the first bar occupies about 20% and the groove occupies the remaining 80%.  
     [0116] Referring to FIG. 12, both side surfaces of the first core were secured by fixing jigs, and a force was applied in the direction indicated by the arrow H to a side surface of the second core to measure the pressure at which the first core and the second core broke, which will be referred to as “the transverse rupture strength of the core.” Thickness t2 of the first core and the second core was set to 0.23 μm. An epoxy-based resin was used for the adhesion layer.  
     [0117]FIG. 13 is a partial side view of the portion of the first core clamped between the fixing jigs shown in FIG. 12, which is observed from the direction indicated by the arrow I. As shown in FIG. 13, the first core and the second core have azimuths. The force was applied to the side surface of the second core at about 0.1 μm above the center of the adhesion layer. The direction H in which the force was to be applied was set such that the azimuths gradually shift away in relation to the direction H.  
     [0118]FIG. 14 is a graph showing the relationship between the thickness of the adhesion layer and the transverse rupture strength of the cores immediately after the first core and the second core were bonded. FIG. 15 is a graph showing a relationship between the thickness of the adhesion layer and the transverse rupture strength of the cores after the first core and the second core bonded by the adhesion layer were left for 72 hours in an environment wherein the room temperature is 40° C. and humidity is 95%.  
     [0119] The graph shown in FIG. 14 indicates that the highest transverse rupture strength of the cores is observed when the thickness of the adhesion layer is about 0.15 μm, and that the transverse rupture strength of the cores gradually decreases as the adhesion layer is thinner or thicker than 0.15 μm.  
     [0120] The graph shown in FIG. 15 indicates that the transverse rupture strength of the cores is the highest when the adhesion layer is about 0.10 μm, and gradually decreases as the adhesion layer is thinner or thicker than 0.10 μm.  
     [0121] The transverse rupture strength of the cores decreases as the thickness of the adhesion layer reaches a certain point probably because moisture infiltrates into the adhesion layer more easily especially in a highly humid environment.  
     [0122] The transverse rupture strength of the cores of 2N or more will protect the cores from breakage, and it should not be 1N or less when the magnetic head is actually used. Hence, the thickness of the adhesion layer was set to the range of 0.05 μm to 0.3 μm. This allows the transverse rupture strength of the cores to be maintained at 2N or more.