Abstract:
A method of flux conduction comprising providing a structure with anisotropy not purely in the transverse in-plane direction such that flux spreading into the transverse in-plane direction can occur by rotation. 
     This method of flux conduction is used in magnetic devices having coupled magnetic layers or coupled magnetic domains. Devices disclosed which utilize this method can function as a magnetic recording head, bubble memory, magnetic field structure, magnetic field sensor, transformer, laminated magnetic memory element or magnetic shield.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to rotational flux conduction in thin film magnetic devices. 
     Magnetic devices, such as thin film recording heads, whether read, read/write, or write, are designed to have particular magnetic orientations, or domains, within the active regions of the device material. The static state of each of these domains can be oriented along a given axis, known as the easy axis (or axis of anisotropy), as desired, by control of the manufacturing process. Performance is critically linked to domain formation, and, therefore, control can be exerted over device performance by controlling domain formation. 
     Conduction of flux, such as in a thin film recording head, is achieved by two mechanisms: domain wall motion and domain rotation. Domain wall motion results in flux being transmitted through a thin film head along the walls of a domain, or domains, the flux spreading out along the domain walls as the flux seeks to return to equilibrium. However, while domain wall motion facilitates conduction of flux at low frequencies, it is a poor vehicle for conduction of flux at high frequencies. Furthermore, defects in the material in a magnetic head can be the source of Barkhausen noise during conduction of flux by wall motion as the flux is perturbed by these defects. Such noise can result in erroneous readings of recorded data. 
     A conventional prior art pole of a magnetic recording head is shown in FIG. 1(a), where medium M will be seen as inserting signal flux into a first transversely oriented domain dl of a series of transversely oriented domains dl-dn. The rest state flux of the domains is in the X-axis (transverse direction), which leaves a rotational option of flowing into the Y axis (longitudinal direction) or Z axis (vertical direction) in reaction to the signal flux from the medium. The Z-axis is out of plane and is a high reluctance path. Hence the flux will rotate into the Y-axis, as indicated by the rotated arrows of each of domains dl-dn. 
     In view of the foregoing, it will be understood that conduction of flux by domain rotation may be achieved by configuring a series of neighboring transverse domains axially aligned along parallel axes, where signal flux impinged upon the first domain at a given angle will cause the magnetization of that domain to rotate from its static orientation by that angle radially into and impinging upon the neighboring domain. Such rotation can be transmitted in a like manner along an axis of flux transmission by each neighboring domain of the series. Hence, flux can be conducted in the pole and through the yoke of a thin film head in a series of domain rotations. The yoke enables conduction of flux through a transducer which produces electrical signals proportional to flux levels or the rate of flux change. The transducer might be an induction coil or other flux sensing device. An induction coil is shown schematically in FIG. 1(a) as it might overlie the bottom pole of a two pole structure. 
     Further discussion of the flux conduction mechanisms will be found in co-pending patent application entitled: &#34;Perpendicular Anisotropy In Thin Film Devices&#34;, U.S. Ser. No. 07/227,813, filed Aug. 3, 1988, which is incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention relates to a method and apparatus for providing rotational flux spreading into the transverse in-plane direction in a flux conduction structure with a preferred rest state not purely in the transverse in-plane direction. 
     One feature of the invention is to enable rotational flux spreading in the yoke of a magnetic device. 
     In one preferred embodiment of the invention, a magnetic device has an axis of preferred rest state not purely in the transverse in-plane direction, such that flux spreading into the transverse in plane direction can occur by rotation. 
     In some embodiments of the present invention, a magnetic conduction device has two active layers capable of both longitudinal and lateral flux conduction by rotation. These layers are sufficiently isolated from each other at rest such that the domain state of one layer is independent of the other. However, the layers are sufficiently coupled to each other such that signal flux can readily flow therebetween. 
     In other embodiments of the present invention, the gap between the two layers is small enough to pass rest state domain flux. Also, the preferred orientation of the two layers is mainly transverse but is canted at angles of +/- theta relative to the transverse direction. The coupling of the signal flux between these two layers allows the flux to spread in a wedge with an opening angle of about twice theta for small signal levels. 
     In still other embodiments of the present invention, four active magnetic layers (A, B, C and D) comprise two pairs of layers (AB and CD). The layers A and B are oriented in the same direction and the gap between A and B is small enough to couple rest state domain flux between them. The layers C and D are similarly configured but their orientation is at a different angle. The gap from B to C is large enough to not conduct rest state domain flux but small enough to conduct signal flux. For example, A and B could be longitudinally oriented while C and D are transversely oriented. Alternatively, A and B could be oriented at plus theta to the transverse direction while C and D are at minus theta. 
     In yet other embodiments of the present invention having three active layers (A, B and C), A and B are paired as described above and are transversely oriented The gap from B to C is large enough to block rest state domain flux but small enough to conduct signal flux. Layer C is oriented longitudinally. The pair A and B conducts flux longitudinally even for narrow track widths. The layer C spreads the flux laterally thus to avoid the high impedance flux beam effect. Alternatively A and B would be oriented longitudinally and could be oriented transversely. 
     Other advantages and features will become apparent from the following description of the preferred embodiment and from the claims. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     We first briefly describe the drawings. 
     FIG. 1(a) is a schematic view of the top layer of a prior art pole; FIG. 1(b) is a cross-sectional view of a two layer configuration of the prior art pole of FIG. 1(a). 
     FIG. 2 is a schematic of a two layer pole of a preferred embodiment of the present invention. 
     FIG. 3 is a schematic of the top layer of the pole of FIG. 2. 
     FIG. 4 is a schematic of the bottom layer of the pole of FIG. 2. 
     FIG. 5 is a schematic in two views of a unidirectional anisotropy prior art head. 
     FIGS. 6a, 6b and 6c are three schematic views of an alternative embodiment of the present invention showing a flux beam comprised of angular and rotational components. 
     FIG. 7 is a schematic of an alternative laminated embodiment of the present invention having two layers, each layer having a parallel sublayer. 
     FIG. 8 is a conventional laminated two pole geometry. 
     FIG. 9 is a schematic of an alternative embodiment of the present invention where poles P1 and P2 are in the same horizontal plane. 
     FIG. 10 is a schematic of another alternative embodiment of the present invention where poles P1 and P2 are stacked. 
    
    
     It is desirable that signal flux be efficiently conducted from the pole tip through the yoke of a magnetic conduction device. Thus a low reluctance flux conduction mechanism is desired. The present invention enables low reluctance flux conduction by magnetization rotation at high frequencies. This is achieved by provision of a structure allowing both longitudinal and lateral rotational conductivity Such multi-dimensional conductivity obviates the need for domain wall propagation, thus avoiding low response at high frequencies and Barkhausen noise 
     The invention is applicable to a wide variety of devices relying upon magnetic conduction, including transformers, magnetic field sensors, bubble memory magnetic structures, thin film magnetic memory elements, magnetic shields, and recording heads, and may be embodied in both single and multi-layered devices. 
     FIG. 1(b) shows in cross section a two-layer configuration of a prior art pole. Here, layers L1 and L2 are separated by a very narrow gap g to enable coupling between layers L1, L2 of rest state domain flux (i.e., D.C. flux of the domains at rest). The anisotropy directions of layers L1, L2 are parallel to each other and transverse to the pole, as indicated by the horizontal arrows a, b in layers L1, L2, respectively. Also, the thicknesses of L1 and L2 are by design equal. 
     Turning to the schematic drawing of FIG. 2, a preferred embodiment of the present invention is shown from a magnetic recording medium looking into the tip of a pole 10 of a magnetic recording head having two active layers, L1, L2, one with in-plane anisotropy (i.e., rest state domain orientation) transversely oriented, and the other layer longitudinally oriented These layers are separated by a gap layer g. The gap layer is thick enough such that the domain state of each layer will remain independent from each other, but yet is thin enough such that signal flux can transmit readily between the two layers of the pole. In present thin film configurations, perhaps a range of three hundred to two thousand angstroms would be a reasonable target for gap size. In a typical two pole recording head, each pole could have a like configuration. Layers L1 and L2 are preferably configured where L1 thickness t1 does not equal L2 thickness t2. These layers are preferably configured with t1 greater than t2. 
     A top view of pole 10 of FIG 2 is shown in FIG. 3, where the first layer L1 of pole 10 has a preferred easy axis transverse to the longitudinal axis of the pole. The domain structure is configured to have relatively large and uniform central domains 12 with adjacent relatively small edge domains 16 in the region of pole tip 20, and with extended edge domains 22 in the yoke region 24. In this configuration, flux from the medium will be read at the first domain d1 at pole tip 20. As a result, flux will be conducted by rotation, serially propagating down adjacent domains 12, as it heads toward yoke section 24. This conducting mechanism may be referred to as a flux pipe or flux beam, and is schematically indicated by the parallel vertical dotted lines running up the pole from the tip and through the yoke. 
     A top view of layer L2 of FIG. 2 is shown in FIG. 4. Layer L2 has a longitudinal easy axis, parallel to the longitudinal axis of the pole and transverse to the easy axis of layer L1. The domain structure is substantially longitudinal in orientation. As a result it will conduct flux transversely by rotation. 
     In FIGS. 3 and 4, the yoke section 24 is shown to increase in width as it extends away from pole tip 20. If the yoke section were not spread, the boundaries of the flux beam would be defined as shown in dotted lines and the high reluctance effect mentioned above would degrade operation. However, by having this region spread out, the flux beam will somewhat spread out, thus adding to the area within the dotted lines by including at least a portion of the regions indicated by the hash marks 23. This effectively results in a lower reluctance path of conduction through the yoke. 
     It should be appreciated, however, that mere broadening of the yoke alone is not sufficient to accomplish substantial flux spreading in this embodiment. Rather, it is the cooperation between differently oriented layers L 1 , L 2  of pole piece 10, which facilitates substantial flux spreading, and the yoke is broadened therefore to facilitate interaction of these layers. A more detailed description of the flux spreading mechanism as applied to the embodiment of FIGS. 2, 3 and 4 follows. 
     Flux will be introduced from the magnetic medium into the first transverse domain dl domain at pole tip 20 perpendicular to the easy axis of the transversely magnetized layer L1. The flux will be propagated longitudinally by rotation down that layer, domain 12 to domain 12, for as long as a low path of reluctance is experienced by the flux. As path length and thus reluctance accumulates, a portion of the flux crosses the gap g and propagates into the longitudinally oriented domain L2, thus lowering the reluctance of the system. The flux will propagate transversely in layer L2 until it accumulates reluctance as will cause it to seek a path of lower reluctance and to jump the gap to re-enter into the transverse layer L1. Once in the transverse layer, the flux propagates longitudinally until accumulated reluctance again causes the flux to seek a lower path of reluctance in the longitudinal active layer on the other side of the gap. This process will continue until the flux has propagated through the yoke, but by the virtue of the process of adding a transverse rotational component to flux flow, the flux will have spread out within the widened yoke region, thus reducing total system reluctance. 
     Thin film heads are being used at higher bit densities. One way to achieve this higher bit density is by reduction of track width. Hence, it is desirable to produce a thin film head having a narrow track width but which also operates at high frequencies. Conduction by rotation can enable such achievements. 
     A narrow track should have a high anisotropy field. This high anisotropy field will act to keep the flux path open by keeping edge domains relatively small. As a result, the rotational mechanism may be kept alive even where the structural magnetic reluctance is lowered in the yoke section according to the present invention, although the high anisotropy field will reduce permeability of the structure. 
     In the design of a narrow track, laminated films often are used to eliminate edge domains and 180° walls. Therefore, these laminated films will not spread flux well in conventional configurations. 
     Referring to FIG. 5 (A and B), which includes schematic complementary views of a unidirectional anisotropy prior art head not having the cooperating layers of the present invention, it will be seen that the flux imposed at an angle phi (or 180° minus phi) from the medium upon the transversely oriented magnetization will cause a rotation of the magnetization by such angle. In all other aspects a uniform beam will be conducted back through the pole into and through the yoke, except that it will be canted by one half of phi. As a result, a high magnetic impedance will be experienced at the yoke because the flux beam does not widen significantly. 
     The present invention, in an alternative preferred embodiment, achieves flux spreading by orienting the easy axes of the top and bottom layers of laminated poles at plus and minus angles, theta, relative to the transverse magnetic direction. As a result, the flux will then spread out in a wedge angle of twice the value of theta, plus the additional angular component phi caused by the flux rotation. The wedge creating embodiment of the present invention is shown conceptually in FIG. 6 (A, B and C), where in 6A the first active layer L1 has been imparted a static angular displacement of its magnetic orientation at an angle theta measured from the transverse axis. The second active layer L2 is shown in 6B having a complementary magnetic orientation at rest at an angle 180° minus theta. When flux is impinged upon the L1 and L2 layers normal to the transverse axis, then the resulting flux beam will be spread equivalent to twice theta plus the rotational component phi for each layer (i.e., one-half phi plus one-half phi). Thus, the flux beam will be the summation of the two angularly and rotationally displaced flux beams of each layer, amounting to two theta plus phi, as shown in 6C. It is preferred in this configuration that the gap between the two layers is small enough to pass rest state domain flux. 
     In operation, flux from the magnetic medium will enter the pole tip and be conducted back to the yoke juncture by rotation in both layers. As the flux enters the yoke juncture, the top layer directs a flux beam off at an angle theta relative to the axis of symmetry of the pole, and the bottom layer directs the flux beam off at an angle 180° minus theta. Conduction of flux between the layers allows the top layer to insert flux into the bottom one in positive theta regions and vice versa. 
     In other embodiments of the invention utilizing laminated films, two or more active magnetic layers can be employed. In a particular embodiment, one pole of which is illustrated in FIG. 7, four active magnetic layers (A, B, C and D) comprise two pairs (AB and CD) of layers. The layers A and B are oriented in the same direction and the gap between A and B is small enough to couple rest state domain flux between them. The layers C and D are similarly configured but their orientation is at a different angle. The gap from B to C is large enough to not conduct rest state domain flux but small enough to conduct signal flux. For example, A and B could be longitudinally oriented while C and D are transversely oriented. Alternatively, A and B could be oriented at plus theta to the transverse direction while C and D are at 180° minus theta. 
     An alternative configuration of the embodiment of FIG. 7 is provided having three active layers (A, B and C), A and B are paired as described above and are transversely oriented. The gap from B to C is large enough to block rest state domain flux but small enough to conduct signal flux. Layer C is oriented longitudinally. The pair A and B conducts flux longitudinally even for narrow track widths. The layer C spreads the flux laterally thus to avoid the high impedance flux beam effect. Alternatively, A and B could be oriented longitudinally and C could be oriented transversely. 
     FIG. 8 shows a conventional two pole yoke geometry where the top and bottom halves of the yoke are symmetrical, and the tips are centered about gap g. The foregoing configurations of the invention can be applied to this geometry. By doing so, flux spreading by rotation will be achieved while preserving flux conduction by rotation in the longitudinal direction. Each pole is shown to have been laminated into two active magnetic layers E,F and E1,F1. 
     These above-described configurations of the invention can also be applied to the geometries shown in FIGS. 9 and 10. Here, the invention not only aids in flux spreading by rotation, but also facilitates transverse flux conduction by rotation. For example, in the configuration shown in FIG. 9, poles P1 and P2 are horizontal. Their first layer L1 is longitudinally oriented. Layer L2 is transversely oriented. Flux entering the right hand pole tip near the gap in region RP is initially conducted to the right in the top longitudinally oriented layer. When the flux reaches the juncture with the right hand leg in region RJ, it will transfer to the bottom layer and be conducted toward the back of that pole in that transversely oriented layer. When the flux reaches the back section of the yoke, it must jump back into the top layer in order to travel leftward by rotation. The flux travels in a complementary manner to complete the flux path back to the left hand pole tip. 
     In FIG. 10, poles P1 and P2 are stacked. The first layer L1 extends over each pole and is longitudinally oriented Layer L2 is transversely oriented and extends over each pole. Vias are provided in the yoke to assure coupling between respective portions of each layer. In this embodiment, the same process occurs as in the embodiment of FIG. 9, but the turns on the path are not abrupt. Also, the pole tips are stacked. 
     It will be appreciated that a transducer must be associated with the yoke in order to generate electrical signals from the signal flux in the device. Various transducer configurations would be operable with practice of the present invention. 
     Manufacturing 
     In a dry vacuum deposition approach, the transversely oriented layer can be deposited on a wafer substrate by placing the wafer in a transversely oriented magnetic field during deposition. (Alternately, this can be accomplished by growing the film with a crystal orientation that places a crystallographically oriented axis in the transverse direction.) 
     The transversely oriented magnetic field can be supplied by coils or by the magnetron target magnets of a sputtering machine, for example. Also, the deposition can be made at angles normal or other than normal to the surface, where the incident direction establishes the preferred axis. For example, one layer might have a field direction at plus theta to the transverse direction, and the other layer might be oriented at minus theta. During subsequent heat treatment, exclusion of all fields would allow the two layers to self bias. Furthermore, in a configuration where an external transverse field is needed to be present to orient other head layers, then the easy axis in both layers could be established at some angle greater than theta, such that after anneal they would relax to angle theta. 
     After the first layer is laid down, then a buffer layer (non-magnetic) can be deposited. For dry deposition, it is convenient to use a ceramic which can be made thinner than a metal and will stop exchange coupling between the first and the next layer. In orthogonal configurations, for example, the second magnetic layer will be deposited either with the wafer rotated 90° to the first deposition position, or the magnetic field rotated 90°. After these layers are deposited, the yoke may be formed by means of a masked ion milling process. 
     Where it is believed advantageous to sub-laminate each of the oriented layers, orientation direction of magnetization in the sublayers paired together preferably would be in the same direction. It may be found that dry deposition is more suited to this type of heavily laminated structure. 
     For wet processing, each layer is deposited through a plating mask in a bias field to establish appropriate magnetic domain orientation. The non-magnetic layer can be plated if it is a metal, for example Ni-P, Au, Cu, etc If a ceramic is used for the buffer layer, then this must be sputtered after the mask has been removed. The structure is now ready to receive the next magnetic layer, whereupon a seed layer and mask will be laid down, and the layer will be plated thereon. 
     A problem with the technology described above resides in that any exposure to very high temperatures (for example, greater than 200° C. for greater than half an hour for NiFe poles) will degrade the induced anisotropy obtained by depositing in a field. This sensitivity can be overcome by using the angle of incidence approach to inducing anisotropy as discussed above in regard to dry deposition. In particular, it is possible to deposit the first layer of the bottom of the yoke in a longitudinally oriented field. This layer can be annealed in the field at a temperature that is much higher than the process temperatures used in the rest of the process. Next, the top layer of the bottom of the yoke and the bottom layer of the top of the yoke may be deposited in a transverse field, which field is maintained during high temperature processing. Thereafter the top layer of the top yoke may be deposited in a longitudinal field and which is not annealed. Only the bottom layer then has experienced an anisotropy-lowering anneal, and it will retain only some of its orientation. Nevertheless, orientation of this lowest layer, while important for yoke conductivity, is not critical for pole tip conductivity, and therefore such conductivity can be diminished. 
     Other embodiments are within the following claims. 
     Pole materials which may be used in the above processes include alloys of nickel, iron and cobalt, and magnetic garnet and ferrite crystals, to name a few alternatives.