US3713212A

US3713212A - Method of making a transducer head

Info

Publication number: US3713212A
Application number: US00090773A
Authority: US
Inventors: M Camras
Original assignee: IIT Research Institute
Current assignee: IIT Research Institute
Priority date: 1970-11-18
Filing date: 1970-11-18
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30

Abstract

A method of making a magnetic transducer head or element thereof from a binary alloy of silicon and iron, preferably containing from about 3 percent to 7 percent of silicon by weight, including provision of a grain-oriented silicon-iron core, annealing the core at a temperature above the Curie point in a dry hydrogen atmosphere and during cooling from that temperature subjecting the core piece to a magnetic field applied in the direction of the working flux at the core. The annealed core piece is formed so as to provide a gap-defining face for coupling to a magnetic record medium, the gap being of the order of about 1 micron.

Description

United States Patent 1 Camras 51 Jan.30,1973

[54] METHOD OF MAKING A TRANSDUCER HEAD Marvin Camras,Glencoe, 111.

[52] U.S. Cl. ..29/603, 29/609, 148/112, 179/100.2 C [51] Int. Cl ..Gllb 5/42, H01f7/06 [58] Field of Search ..29/603, 609; 179/1002 C; 340/174.1 F; 346/174 MC; 148/112 3,171,107 2/1965 Rogers, Jr. ..340/174.l F 3,311,711 3/1967 Maryatt et a1. ..179/100.2 C 2,694,754 11/1954 Connell ..179/100.2 C 3,504,134 3/1970 Miyata ....29/603 X 3,240,638 3/1966 Wiener et a1. ..148/112 2,736,776 2/1956 Camras ..29/603 UX Primary ExaminerCharles W. Lanham Assistant ExaminerCarl E. Hall Attorney-Hill, Sherman, Meroni, Gross & Simpson [57] ABSTRACT A method of making a magnetic transducer head or element thereof from a binary alloy of silicon and iron, preferably containing from about 3 percent to 7 percent of silicon by weight, including provision of a grain-oriented silicon-iron core, annealing the core at a temperature above the Curie point in a dry hydrogen atmosphere and during cooling from that temperature subjecting the core piece to a magnetic field applied in the direction of the working flux at the core. The annealed core piece is formed so as to provide a gapdefining face for coupling to a magnetic record medium, the gap being of the order of about 1 micron.

4 Claims, 12 Drawing Figures ween r aouec METHOD OF MAKING A TRANSDUCER HEAD CROSS REFERENCES TO RELATED APPLICATIONS This application is a division of my co-pending application Ser. No. 746,651, filed July 22, 1968 now Pat. No. 3,582,572.

SUMMARY OF THE INVENTION Contrary to earlier conclusions I have now found that superior heads can be made of binary alloys of silicon and iron, the silicon being preferably in the range from about 3 percent to 7 percent by weight, and the balance, except for the usual impurities, being essentially iron. In the fabrication of such binary alloys into pole pieces advantage is taken of the grain-orientation in the strips of silicon-iron alloy used. In accordance with my method, the grain-oriented strips of siliconiron alloy are subjected to annealing at temperatures above the Curie point, and during cooling of the strips from the annealing temperature used they are subjected to a magnetic field at temperatures below the Curie point, with the direction of the magnetic lines of force in the same direction as the grain-orientation.

Heads such as disclosed herein give better defined gaps than obtained using Permalloy or the like when the gap size is below 100 microinches as for example 40 (1 micron), 20, or microinches or even smaller. The gaps are maintained after long tape usage while gaps of Permalloy may smear across and become magnetically short circuited.

When the head pole pieces are of preferred material as taught herein and are mounted in a support made of material that wears more rapidly, the tape support area of the head acquires and maintains a laterally crowned contour with the pole pieces at the highest point. This insures excellent contact between head and tape at the gap, which is extremely important at short wave lengths. Wave lengths less than 0.000,025 inch long were recorded and played back with these heads.

The silicon-iron alloy poles remain clean after passage of tape across them and do not have an affinity for metallic and non-metallic accumulations that separate the tape from the gap.

Silicon-iron pole pieces have a longer life than those made of Permalloy by a factor of several times. They can be made with a lesser gap depth while the poles are still adequately supported because the metal is stronger, and will still have a long life before wearing through. The heads described herein may be made with a gap depth of 1 mil (.001 inch), increasing their efficiency especially at gap lengths of less than 40 microinches (1 micron), which is considered to be the microgap region of special interest in the present specification.

Magnetic saturation densities of kilogauss or more are obtainable with silicon-iron alloys in the lower range of silicon content. Even at the higher limits of silicon-content the saturation density exceeds 10 kilogauss. By comparison, the saturation density is 5 to 8 kilogauss in Mumetal" and Permalloy, and with about 1.5 to 3 kilogauss in ferrites. A high saturation density did not prove advantageous in prior art heads and was therefore not considered important. In

microgap heads which are to be used for recording it v has been found that pole pieces according to this invention allow higher signal levels to be recorded than with prior art heads of the same microgap size.

The improved heads are especially suitable for economical video recorders of the fixed head longitudinal scan type, since they give high performance in the megahertz range, are economical to construct, and have a long life. A single head can be used for both recording and playback. At inches per second tapeto-head speed, a response to 2.5 megahertz was obtained.

Other objects, features and advantages of the present invention willbe apparent from the following detailed description taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a transverse view showing a magnetic head in accordance with the present invention in end elevation and indicating the tape record medium in crosssection;

FIG. 2 is a somewhat diagrammatic side elevational view illustrating a first magnetic core configuration in accordance with the present invention;

FIG. 3 is an end elevational view of a core piece for the core configuration of FIG. 2;

FIG. 4 illustrates apparatus for applying a magnetic field to a magnetic core part such as illustrated in FIG. 3 during the annealing of such core part;

FIG. 5 is a somewhat diagrammatic cross-sectional view taken generally along the line VV of FIG. 4;

FIG. 6 is somewhat diagrammatic side elevational view of a second form of core configuration in accordance with the present invention;

FIG. 7 represents an end elevational view of one of the core parts of the core configuration of FIG. 6;

FIG. 8 illustrates a third form of core configuration in accordance with the present invention,

FIG. 9 illustrates a further form of magnetic transducer head in accordance with the present invention;

FIG. 10 is an enlarged somewhat diagrammatic detail view showing the gap region of the head of FIG. 9;

FIG. 11 illustrates a still further form of head construction in accordance with the present invention; and

FIG. 12 illustrates approximate comparative flux densities and perrneabilities as a function of frequency for Supermalloy," 6.5 percent silicon-iron and 3 percent oriented silicon-iron.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a magnetic transducer head 10 having a tape record medium 11 with its magnetizable surface layer lla in sliding contact with the head. The head is shown as comprising a housing 12 having tape contacting surfaces such as indicated at 12a and having an aperture 12b exposing a magnetic core 13. As illustrated in FIG. 1, when the tape contacting surfaces 13a of the magnetic core 13 are made of my preferred silicon-iron alloy and the housing surface 12a is of a material that wears more rapidly, the tape support area of the head acquires and maintains a laterally crowned contour as shown in FIG. 1, with the tape engaging surface l3a at the highest point. This insures excellent contact between the head 10 and tape 11 at the gap (the plane of the gap being parallel to the plane of FIG.

1). Wavelengths less than 0.000,025 inch long were recorded and played back with a head construction such as shown in FIG. 1. In this embodiment, the head core 13 is embedded in insert blocks such as of silver or other highly conductive material. The tape contacting surfaces 12a of housing 12 may be of silver, brass, Mumetal, or other material that wears more rapidly than the silicon-iron alloy providing the tape engaging surface 13a of core 13.

FIGS' 2 and 3 illustrate a head core 15 including

core pieces

16 and 17. As indicated in FIG. 3, each of the core pieces may have a gap defining face 16a and may have a generally channel cross-section with integral lateral flanges 16b and 160. The gap defining faces such as 16a define a non-magnetic gap 17' for coupling of the core with a magnetic record medium traveling successively across the polar surfaces 18 and 19, for example in the same way as contemplated in FIG. 1.

The

core pieces

16 and 17 are made of a 3 percent grain-oriented silicon-iron sheet material having a thickness of about 2 to 4 mils (1 mil equals 0.001 inch) obtainable from Allegheny Ludlum Steel Company under the name Silectron." The curved portions of the core pieces are in the direction of grain orientation with the preferred direction being indicated by arrows such as 21 and 22 in FIG. 2.

FIGS. 4 and 5 illustrate apparatus for applying a magnetic field to the

core pieces

16 and 17 during annealing treatment. The

core pieces

16 and 17 may be assembled on a loop 30 of non-magnetic stainless steel wire of about one-sixteenth inch diameter held inside a heat treatment chamber indicated at 31. As indicated in FIG. 5, a gap spacer 32 is inserted between the polar edge faces such as 16a, FIG. 3, and the C-

shaped core pieces

16 and 17 are nested inside a ring 34 of nonmagnetic material such as stainless steel, held securely by means of a wedge such as indicated at 35 if necessary. One or more of these nests are strung onto the loop 30 as indicated at 37 and 38 in FIG. 4 (the nest shown in FIG. 5 being designated by the reference numeral 37). The loop 30 is electrically connected by means of conductors 40 and 41 with an electrical current source 42 which maybe capable of supplying an electrical current of 8 to 12 amperes as measured by an ammeter 43.

FIGS 6 and 7 illustrate ahead core 50 made of 6% percent silicon-iron of composition 6.5 percent silicon and the balance iron and minor impurities. Such material is too brittle to be formed as in FIGS. 2 and 3. It is therefore built up by bending strips of silicon-iron into C-shaped side pieces such as indicated at 55 and 58 in FIG. 6 and at 55, 56 and 57 in FIG. 7. The side pieces are spot welded to the generally arcuate interior surface of core pieces 51 and 52 as indicated at 61, 62, 61 and 62', for example, in FIG. 6. Good welds should be obtained as close to the gap 63 as possible. The core pieces 51 and 52 provide polar end faces such as indicated at 51a in FIG. 7 defining the opposite boundaries of the non-magnetic gap 63. Polar tape engaging surfaces are indicated at 64 and 65, and a tape record medium is indicated at 66 having a magnetizable oxide layer at 67 which is in sliding contact with surfaces 64 and 65 and moves in the direction indicated by arrow 68. The core pieces 51 and 52 may be of identical construction and are preferably filled with laminations 70,

FIG. 7, whose configuration corresponds to that of the side pieces such as 55 and 58, FIG. 6. The laminations 70 are of high permeability material such as Supermalloy having a composition of Spercent molybdenum, 79 percent nickel and the remainder iron and minor constituents. The material of the laminations has the lowest possible coercive force and the laminations are insulated with magnesium oxide from each other and from the silicon-iron material.

The core pieces 51 and 52 may be formed from grain oriented sheet material such as the silicon-iron binary alloy above described, having a thickness of 4 mils and having a rolling direction as indicated by the arrows such at 72 and 73 (FIG. 6), so that when the core pieces are bent into the generally arcuate shape the preferred direction follows the circumference of the core (which is in conformity with the direction of the working magnetic flux path in the core). The side pieces such as 55 and 58 (FIGS. 6 and 7) may also be formed from oriented sheet material with a preferred direction as indicated by arrows such as 75 and 76.

FIG. 8 illustrates a head construction wherein core pieces and 81 are formed from the above-mentioned oriented silicon-iron sheet material with preferred directions as indicated by arrows such as 82 and 83. The

core pieces

80 and 81 may be built up from a series of C-shaped laminations of identical configuration having respective extremities such as indicated at 85 and 86 defining a gap 87 across which a tape record medium moves in sliding contact with surfaces such as indicated at 85a and 86a as in the previous embodiments. The polar portions 85 and 86 have a width dimension (in the vertical direction as viewed in FIG. 8) which progressively increases in the direction away from the gap 87. The core pieces have intermediate portions as indicated at 90 and 91 which are of substantially greater width (here measured in the horizontal direction as viewed in FIG. 8) in comparison with the width dimension of the extremities 85 and 86. At the intermediate portions 90 and 91, the preferred direction as indicated by

arrows

92 and 93 is cross-wise to the direction of the working magnetic flux path of the core, and electric windings 95 and 96 are preferably located on the intermediate portions 90 and 91 of the

core pieces

80 and 81.

FIGS. 9 and 10 illustrate a further head construction wherein core pieces 101 and 102 define a coupling gap 103 and have

pole pieces

104 and 105, FIG. 10, across which a tape record medium 107 moves in the direction of arrow 108. As in each of the other embodiments, the tape 107 has a magnetizable layer 108 which is in sliding contact with polar surfaces of the pole pieces 104 and in the vicinity of the gap 103. In each of the embodiments, the layer 108 may be formed of iron oxide particles in a non-magnetic binder and exhibiting a coercivity of at least 200 oersteds. In each of the embodiments, the tape may move in the direction of the arrow 108' at a speed of 30 inches per second or at a speed of 60 inches per second. The gap 103 is considered to have a depth which is measured in the vertical direction as viewed in FIG. 10 and is determined by the vertical dimension of the gap defining faces 104a and 105a of the

pole pieces

104 and 105. The direction of the working magnetic flux path of the core in the gap region is indicated by the flux line 110, FIG. 10.

The core pieces 101 and 102 (FIG. 9) may be formed from a series of generally C-shaped laminations which are stamped from grain oriented flat sheet silicon-iron material. In this embodiment, the preferred direction of magnetization is at an oblique angle, for example about 45 to the horizontal as viewed in FIG. 9, and pointed toward the gap edges as indicated by

arrows

115 and 116, for example. As shown in FIG. 10, the working magnetic flux path in the

pole tips

104 and 105 generally conforms with the preferred direction of magnetization as indicated by

arrows

115 and 116, and the direction of

arrows

115 and 116 is in close conformity with the magnetic field direction inside the record medium as indicated by flux line 110 (FIG.

In FIGS. 9 and 10, the core pieces 101 and 102 maybe made of 3 percent grain oriented silicon-iron and may comprise a stack of identical laminations 1 mil thick with the direction of orientation as indicated by the

arrows

115 and 116. The exterior perimeter of the core formed by the assembly of core pieces 101 and 102 may have an outer perimeter 121 which is circular, and may have an inner perimeter as indicated at 122 which is also circular that is offset in the direction towards the gap 103 relative to the circle defined by the outer perimeter 121. The result is a configuration with a base region 123 of substantially greater cross-sectional area and flux carrying capacity than other portions of the core, the preferred direction as indicated by

arrows

124 and 125 in the base region being generally transverse to the direction of the working magnetic flux path. The circular window defined by inner perimeter 122 accommodates a doughnut-shaped winding 127 which is of larger volume at the region thereof within the inner perimeter 122 than the volume of the

pole pieces

104 and 105 considering the volume of the pole pieces which is directly above the cross-section of the winding 127 as viewed in FIG. 9.

To reduce the reluctance of the base portion 123 of the core of FIG. 9 still further, a channel-shaped piece may be added as indicated in dash-outline at 130 in FIG. 9. In the embodiment of FIG. 9, the channelshaped piece 130 would have a bight portion conforming in contour to the external perimeter 121, and would have respective planar portions extending vertically as viewed in FIG. 9 at opposite sides of the core and in contacting relation with the outer faces of the respective end laminations forming the core pieces 101 and 102. The channel-shaped piece 130 may bridge across the back gap 132 between core pieces 101 and 102, or it may be cut and lapped so as to provide a gap corresponding in configuration with the back gap 132 of the core.

As indicated in FIG. 9, the core pieces 101 and 102 may each be mounted in acavity such as indicated at 140 in an electrically conductive mounting block 141. The generally arcuate face 141a of the block 141 has the same radius of curvature as the outer perimeter of 121 so as to provide a layer of electrically conductive material in close conforming relation to the exterior perimeter of the core piece 101, the core surface being insulated to avoid excessive eddy current loss. The lateral surfaces such as 141k may also be disposed in close conforming relation with the core particularly at the upper portion thereof at each lateral side of the core piece 101. The mounting block 141 may be suitably recessed as indicated at 141c to accommodate the winding 127 and the channel-shaped piece 130. The portion of the mounting block 141 indicated at 145 may be removed during the final finishing operation so as to expose the tape engaging surfaces of the

pole pieces

104 and 105.

FIG. '11 illustrates diagrammatically a single pole piece having a polar extremity 151 for coupling with a magnetic tape record medium 152 moving in the direction of arrow 153. The pole extremity 151 has a polar tape-engaging surface in sliding contact with the surface of the magnetizable layer of the record medium 152. The core piece 150 may be made of a flat strip of oriented silicon-iron, the preferred direction of orientation being in the direction of arrow 155. The properties may be further enhanced by heat treatment in a linear magnetic field also in the direction of arrow 155 using an alternating current magnetic field with a frequency of 60 cycles per second and providing an r.m.s. value of magnetizing force of about 15 oersteds. Substantially higher frequencies may be used instead, or direct current magnetic field may be applied to provide a direct current of magnetizing force of about 15 oersteds.

FIG. 12 compares typical magnetic characteristics of supermalloy6.5 percent silicon-iron, and 3 percent oriented silicon-iron, at various frequencies useful in magnetic heads. The measurements were taken using flat washer-like samples of the material which were stamped from flat sheetmaterial having a thickness of 4 mils. The permeability (u) measurements were taken at a field of 0.1 oersted r.m.s. in all cases, and the B readings represent the highest level (in kilogauss) where an undistorted output was obtained. The oriented 3 percent silicon-iron sample had an orientation somewhat analagous to that indicated by the arrows associated with side pieces 55 and 58 in FIG. 6. A curve 161 shows the permeability variation of the Supermallov sample; curve 162 shows the permeability variation for 6.5 percent silicon-iron; and curve 163 shows the permeability variation for 3 percent oriented silicon-iron. Curve 164 shows the B variation for Supermalloy; curve 165 shows the B variation for 6.5 percent silicon-iron; and curve 166 shows the B variation for 3 percent oriented silicon-iron.

Although Supermalloy has the highest permeability, the lower permeability of the silicon irons has proved to be adequate in actual heads. It will be noted that the B values for the silicon-iron materials are much better than are those for the Supermalloy. The readings for the oriented silicon-iron in FIG. 12 are for a sample having all directions of orientation with respect to the working flux path of the sample, so that these readings would be higher if taken for a sample where the preferred direction conformed with the direction of the working magnetic flux path throughout the sample.

Apparently the silicon-iron heads having a relatively high magnetic saturation are of best advantage when used with high coercive record materials as represented by modern magnetic oxide surfaces with coercive forces above 200 oersteds, for example 225 oersteds or higher, and make it possible to record efficiently on record materials with coercive forces above 350 oersteds which have not been practical except in very thin coatings.

Iron-cobalt alloys of 50 percent iron and 50 percent cobalt composition, and improved versions containing 49 percent iron, 49 percent cobalt, and 2 percent of vanadium also have exceptionally high magnetic saturation, but are more expensive and difficult to produce than are silicon-iron alloys.

The following Table I gives some properties of silicon-iron alloys, the values being intended to be typical for the percentages of silicon indicated. It is notable that the resistivity is a maximum at about 11 percent silicon content; a high resistivity being advantageous for low eddy current losses. The Curie point is useful in specifying the temperature below which the magnetic field must be applied, for example with the apparatus of FIGS. 4 and 5 during the magnetic anneal process; for example l,380 F. for 3 percent silicon material, and l,270 F. for 6% percent silicon material. Also the annealing temperature should be above the Curie point for the material. Saturation magnetization (for an applied direct current magnetizing force) decreases with increasing silicon content, but is high in all cases compared to previously used 79 percent nickel material such as Permalloy. Values not specified in Table I can be interpolated.

TABLE I Typical Characteristics of Silicon Iron Materials By way of comparison, typical values for permal loy are a resistivity of 55 microhm-centimeters, a Curie point of l,l00 F., a B, of 7,300 gauss and a Rockwell Hardness of 30.

It has been found that at frequencies of 100 kilohertz and above there is a marked skin effect in the pole pieces, with only the outer layers carrying the magnetic flux. Advantage has been taken of this effect in the illustrated head designs where the high saturation capability of the silicon iron material increases the high frequency flux at the tape surface. These heads operate at high megahertz frequencies which' were previously considered possible only with ferrite cores.

While oriented materials as for example American Iron and Steel Institute M-4 through M-lO silicon irons are preferred, successful experiments have also been made with non-oriented silicon irons. Examples of nonoriented silicon irons are AISI types M-l5, M-19 M-22, M27 and M-36.

Filler laminations such as indicated at 25 in FIG. 3, and at in FIG. 7 may range from 0.5 to 4 mils in thickness and are helpful in reducing residual magnetization and improving the flux carrying capacity. They may be omitted for economy.

It has been considered that a high initial permeability is essential for pole pieces used in magnetic heads. Heads described herein have given excellent results, however, although the initial permeability was not especially good. The reasons for this may relate to local effects due to illustrated head constructions including skin effects at high frequencies, and to other properties inherent in the head configurations herein specifically disclosed.

When annealing the illustrated core constructions in a magnetic field, for example as represented in FIGS. 4 and 5, it is desirable to place the core pieces in a magnetic circuit configuration similar to that of the final assembly and to include a gap spacer as indicated at 32 in FIG. 5 so that the field during annealing follows the direction of the working magnetic flux path of the head in actual use even down to the fine details as at the gap. The gap spacer may be of a non-magnetic stainless steel or of platinum, or of Inconel (which has a composition of approximately percent nickel, 14 percent chromium, and 6 percent iron), so as to withstand the high annealing temperatures.

For the recommended anneals to be effective, it is important that the silicon iron having a low carbon content of less than 0.05 percent, and preferably about 0.02 percent or below.

A typical winding for the illustrated core constructions would have a top portion with 50 turns of No. 40 AWG wire, and a bottom portion of I50 turns of No. 42 AWG wire, both wound together in a doughnut shape such as shown at 127 in FIG. 9.

As illustrated in the Table I, magnetic saturation densities of 20 kilogauss or more are obtainable with silicon-iron in the lower range of silicon content. Even at the higher limits of silicon content the saturation density exceeds 10 kilogauss. These values compare with saturation densities of 5 to 8 kilogauss in Mumetal and 79 percent material such as Permalloy and with saturation densities of about 1.5 to 3 kilogauss in ferrites. A high saturation density did not prove advantageous in prior art heads and was therefore not considered important. In microgap heads (where the gap dimension is of the order of 1 micron or less) which are to be used for recording, it has been found that pole pieces formed according to this invention allow higher signal levels to be recorded than with prior art heads of the same microgap size.

The improved heads are especially suitable for economical video recorders of the fixed head longitudinal scan type, since they give high performance in the megahertz range, are economical to construct, and have a long life. A single head such as those illustrated in any of the embodiments herein, can be used for both recording and playback. At 60 inches per second tape to head speed, a response to 2.5 megahertz was obtained.

Heads made according to the following examples give better defined gaps than Permalloy" when the gap size is below microinches as for example 40, 20 or 10 microinches or even smaller. The gaps are maintained after long tape usage while gaps ofPermalloy may smear across the gap so as to provide a magnetic short circuit between the pole pieces. Wavelengths less than 0.00(),O25 inch (25 microinches) long were recorded and played back with the heads described herein.

The following examples illustrate preferred practice; features of the various examples may be combined or used separately if desired.

EXAMPLE 1 (FIGS. 2 and 3) The core 15 comprises

core pieces

16 and 17 of a 3 percent grain-oriented silicon-iron alloy 4 mils thick obtainable from Allegheny Ludlum Steel Company under the name Silectron. The curved portions of the

core pieces

16 and 17 are in the direction of grain orientation with the preferred direction as indicated by arrows 21 and 22 and have an outer diameter of H mils and an inner diameter of 70 mils. The 3 percent silicon material is quite malleable and is readily shaped to the channel cross section as shown in FIG. 3. The inner portion of the channel is filled with crescents of Su permalloy, as indicated at 25 of 1 mil thickness insulated from each other by thin layers of magnesium oxide. In the same way the channels themselves are also insulated on all surfaces. The filled

core pieces

16 and 17 are then annealed in an atmosphere of dry hydrogen, holding at a temperature of 2,000 F. for minutes or more, and cooled slowly over a period of several hours. The

core pieces

16 and 17 are cemented into close fitting cavities such as indicated at 140 in FIG. 9 provided by insert blocks such as indicated at 141 in FIG. 9 made of silver. The silver insert blocks may in turn fit into recesses in mating mounting blocks of brass which in turn may be enclosed within a onepiece housing shell of magnetic material similar to that indicated at 12 in FIG. 1 with a separate bottom closure part. As indicated in FIG. 1, the silver insert blocks corresponding to the material 14 in FIG. 1 may be recessed below the tape engaging surfaces 12a and 130. In this construction, the

core pieces

16 and 17 are secured in separate mounting blocks with the mating faces lapped to an optical finish, then assembled with a beryllium copper gap spacer at 17, FIG. 2, having a thickness of approximately 18 microinches and placed between the upper gap defining faces such as indicated at 16a, FIG. 3, with a coil of 200 turns placed around the lower extremities of the

core pieces

16 and 17 generally as indicated for the winding 127 in FIG. 9. The assembly is mounted in a casing which may be of- Mumetal corresponding to casing 12, FIG. 1, the projecting tape engaging surfaces 18 and 19 of the pole pieces being lapped to expose the gap 17' to the active surface such as indicated at 11a of the tape record medium 11 indicated in FIG. 1.

A head made with two side-by-side cores each 21 mils wide, to provide a 42 mil wide track has an inductance of about 2 millihenries with a 200 turn coil when measured at a frequency of l kilohertz. This is one-half the inductance of a similar head made of Supermalloy," yet the output of this head equals or exceeds that of the Supermalloy head under conditions of bias signal optimized for Supermalloy. More turns can thus be used on the silicon-iron head for increased output without exceeding limitations caused by head resonance. A high saturation value is noted, with permeability as a function of magnetizing force still rising at an excitation of 4 ampere-turns.

For video recording, the above head with a 50 turn tap on the 200 turn winding to provide a 50 turn recording winding, gives excellent response to signal frequencies up to 2 megahertz and up to bias frequencies of 7 megahertz using quarter inch tape at 60 inches per second tape-to-head velocity.

EXAMPLE II (FIGS. 6 and 7) A head of overall dimensions the same as in Example I is made of an alloy of 6.5 percent silicon, and 93.5 percent iron and minor constituents, which is 4 mils thick, obtainable from Westinghouse Electric Corp. This material is not malleable enough to bend into a form such as that illustrated in FIG. 3. It is therefore built up by welding as described in connection with FIGS. 6 and 7 heretofore. The rolling direction of the core pieces 51 and 52 is as indicated by the arrows such as 72 and 73, the core pieces being bent to their curvature with the preferred direction following the circumference. The inner C-shaped side pieces such as indicated at 55 and 58 in FIG. 6 are cut or punched from a fiat sheet and are oriented with a preferred direction favoring the gap flux as indicated by the arrows such as 75 and 76. Other directions, however, are operative. Spot welds such as those indicated at 61 and 62, FIG. 6, fasten the side pieces such as 55-58 to the respective core pieces 51 and 52. This silicon-iron material welds very easily using miniature equipment such as is common for transistors and integrated circuits. Welds close to the top and bottom of the side pieces are important, welds near the base of the core of FIG. 6 being indicated at 61' and 62. More sides pieces can be welded in positions between the side pieces such as indicated at 55, 56 and 57 in FIG. 7 to improve the strength and magnetic efficiency of the head. The pockets between the side pieces 5558 are filled with laminations of Supermalloy,and the core configuration is heat treated in hydrogen as in Example I. Other details of finishing are also as in Example I.

The inductance of this head is higher and its saturation flux density lower than that of Example I, but still considerably better than for a comparable head of Supermalloy. High resistivity improves the loss factor at high frequencies. The extra hardness improves gap definition and wear resistance. Response is to 2.5 megahertz at 60 inches per second head-to-tape velocity.

In each of the examples, the various forming operations are preferably carried out before annealing, except for the final polishing or lapping. Good welds should be obtained as close to the gap 63 as possible, so as to give maximum support to the gap defining surfaces such as indicated at 51a, FIG. 7. the laminations 70 for Example II preferably have the lowest possible coercive force, for example, as provided by Superm alloy material. The gap spacer providing the gap 63 is of beryllium copper having a thickness from 60 microinches down to 12 microinches or less in Example II.

The permeability of this silicon-iron material is close enough to that of Supermalloy so that performance of the Example II silicon-iron head in the video range is defining faces, causing the gap to be bridged where the tap rides over it. Soft materials such as Permalioy are susceptible to this effect when the gap is very small. Microscopic observation confirmed these effects.

Another reason for better operation with extra small gap dimension is that the saturation flux density of the 6.5 percent silicon-iron (approximately 10 kilogauss or more) is considerably higher than with the Supermalloy" material, so that higher recording fields are achieved for very small gap sizes where the leakage flux directly between the confronting gap defining faces causes the core to saturate at relatively modest fields during recording. The high resistivity of 6.5 percent silicon-iron (approximately 80 microhm-centimeters) reduces eddy current losses at high frequencies.

The above advantages are most evident with small gap heads of a gap thickness of I microinches or below, and especially with microgap heads with gap dimensions, according to Example II, below 40 microinches (1 micron). An experimental head, according to Example II, with a 14 microinch gap of beryllium copper gives excellent results for video recording at 30 inches per second and at 60 inches per second head-to-tape velocity, using a bias frequency of 6.5 megahertz and video frequencies up to 2.5 megahertz.

A range of heat treatment temperatures was tried for the materials of Example II, above and below the specific temperature of 2,000 F. but the results were optimum at 2,000 F. It is advantageous to cool the core while a magnetic field is applied to it in the direction of magnetization of the working flux as described in connection with FIGS. 4 and 5. This may be accomplished by supplying a direct current of about l0 amperes to conductor loop 30, FIG. 4, while the core cools from l,500 F. in a hydrogen atmosphere.

Examples I and II utilize the magnetic tapes with oxide surface layers having a coercive force on the orderof 225 oersteds.

EXAMPLE III The pole piece material and procedure are the same as in Example [1, except that for the annealing of the core parts they are nested inside a ring of non-magnetic stainless steel as indicated at 34 in FIG. 5, and held securely with a wedge 35, if necessary. The stainless steel is type 302 (18 percent chromium, 8 percent nickel and the balance iron). The loop of stainless steel wire has a cross-section about one-sixteenth inch in diameter and is held within the heat treatment chamber 31 as illustrated in FIG. 4. Sixty cycle per second alternating current having an r.m.s. value of 8 to 12 amperes as measured by ammeter 43 passes through the loop 30, especially during the cooling cycle until the core parts have cooled to below 500 F. This gives a magnetizing force of about l oersteds r.m.s. in the pole pieces, the exact value being non-critical.

Annealing by this procedure gives a higher magnetic permeability than in Example II where the magnetic field treatment is omitted. High frequencies may also be used for the current source 42 with beneficial results.

EXAMPLE IV The same process is used as in Example III but with a direct current having a value of the order of 8 to 12 amperes. A higher magnetic permeability is also obtained, in comparison to the results in Example II without the magnetic field treatment.

EXAMPLE V A head structure is made having laminations of configuration corresponding to that of side pieces 55 and 58 in FIG. 6, with the preferred directions of magnetization in successive laminations arranged generally at right angles to each other, such that preferred directions as illustrated by arrows and 76 in FIG. 6 for one lamination are generally horizontal, while the preferred directions for the adjacent laminations on either side are generally vertical, so that the permeability of the overall assembly is improved at the intermediate portion of the core parts where the orientations 75 and 76 are crosswise to the direction of the working magnetic flux path. The laminated head of this Example is similar to the head of FIG. 9 except for the exact configuration of the laminations. Twenty laminations of 2 mil thickness are used to form each core piece.

The laminations are of 6.5 percentsilicon-iron and have a thin surface layer of magnesium oxide for insulation and are annealed in hydrogen at 2,050 F. When cool they are transferred to a block such as indicated at 141, FIG. 9, having a close fitting cavity such as 140 of high electrical conductivity, the cavity being coated on its surface with a thin layer of epoxy resin, such that the laminations are held at their outer edge and are not subjected to stresses that occur when the sides of the laminations are adhered together. A pair of blocks such as 141 are finished as indicated previously, using an 18 microinch gap spacer. The head made in this way is very efficient at high bias frequencies exceeding 7 megahertz, and gives excellent signal response at 2 megahertz.

Where oriented material 'is used with the preferred direction lined up as illustrated in FIG. 8, the overall permeability is improved by increasing the cross-section of the core pieces at the intermediate regions and 91. The coils and 96 are preferably associated with the core pieces at the intermediate regions 90 and 91 where the direction of orientation as indicated by

arrows

92 and 93 is transverse to the working magnetic flux path of the core, so that the magnetomotive force loss of these poorer regions 90 and 91 is supplied by the immediate source of magnetomotive force; while the best permeability portions are used at the pole pieces 85 and 86 so as to channel the magnetic flux to the working gap 87 with minimum loss.

The directional properties are used to advantage in the construction of FIG. 9 where the preferred direction of magnetization is at an oblique angle (as for example about 45") as indicated byarrows l 15 and l 16 which as indicated in FIG. 10 are pointed toward the gap edges of

pole pieces

104 and 105. As shown in FIG. 10 the arrows H5 and 116 have direction which is in close conformity with the magnetic flux path l 10 inside the magnetizable layer 108.

EXAMPLE VI The head core pieces are made of 3 percent grain oriented siliconairon (Silectron) in laminations 1 mil thick, with the direction of orientation as indicated in FIGS. 9 and 10. The procedure is generally as in Example V. The result is a head with exceptionally good resolution at short wavelengths, and with superior efficiency at megacycle frequencies. Annealing is effected with the direct current magnetic field applied to the core as described in Example IV.

Referring to FIG. 4, the nested assemblies such as 37 may be removed and replaced from the loop 30 by disconnecting the leads 40 and 41 from the ends of the loop 30, and removing the ends of the loop 30 from the end seal indicated at the right-hand end of chamber 31. The seal also has a duct extending therethrough to provide an atmosphere inlet and outlet.

Although in the past silicon-iron has been noted as a possible head material, to applicants knowledge no commercially successful head has been made of such material, presumably because if tried the performance was found to be unsatisfactory.

Transducer heads made of other materials may also benefit by a magnetic anneal as taught herein. Such heads may be made of ferrites, the various materials such as Permalloy, and cobalt iron materials such as Supermendur.ln each instance, there is usually a preferred composition that responds best to such treatment. For the materials such as Permalloy this is the 65 percent nickel, 35 percent iron range. For cobalt iron an alloy of 49 percent cobalt, 49 iron and 2 vanadium is optimum. For silicon-iron a preferred composition is 6.5 percent silicon, and the balance iron and minor impurities.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

I claim as my invention:

1. The method of making a magnetic transducer head of the type having two opposed pole pieces defining a gap therebetween, and of the type wherein an electrically conductive winding is magnetically linked with said magnetic head which comprises forming a core of magnetically permeable material into a shape defining at least said pole pieces and said gap, energizing said shape with a magnetic field corresponding in shape and direction to the field associated with said winding during use in a magnetic recording system, and annealing said shape while exposed to said magnetic field.

2. The method of claim 1 in which said magnetic field is established by linking said shape with an electric current.

3. The method of claim 1 in which said core is formed of relatively brittle material by combining a section of magnetic material having its grain oriented in the direction of the said field of said winding section of magnetic material and another with its grain not oriented in such direction.

4. The method of claim 3 in which said sections are combined by welding.