METHOD AND APPARATUS FOR CONTROLLING THE FLYING HEIGHT OF THE HEAD IN A MAGNETIC STORAGE UNIT

Abstract: The method involves the steps of lowering the magnetic head (8) to within aerodynamic distance from the surface (16) of a moving recording medium, enforced further lowering of the head (8) until a tunnel current occurs across the gap between head (8) and surface (16), and maintaining said tunnel current constant by using the deviations of the current from a predetermined value (which corresponds to a certain flying height). The apparatus comprises a tunnel electrode (9) forming an integral part of the head assembly which is attached to a distance control mechanism (5) permitting the positioning of the head assembly with a one-nanometer accuracy. The tunnel electrode (9) has a very slightly beveled shoulder (45, 47) ensuring early detection of asperities approaching the head while sitting on the surface (16) of the moving recording medium (3). Grada­ tion of the potential between tunnel electrode (9) and surface (16) of the recording medium (3) is used to enhance the response to asperities.

Description: The invention relates to a method for controlling the flying height of the magnetic head in a magnetic storage unit, and to an apparatus for performing this method. Magnetic head assemblies that fly relative to magnetic storage media have been used extensively. The objectives for improving the non-contact transducing relationship between a magnetic head and a magnetic recording medium, such as a rotary disk, are to attain very close spacing between the head and the surface of the recording medium, and to maintain the attained spacing essentially constant. The head/surface spacing, usually called 'flying height' is one of the parameters which determine the maximum possible storage density. Other important parameters include the narrowness of the transducing gap and the thickness of the magnetic record­ ing medium. Small flying height allows for high spatial frequency signals to be recorded, thereby affording high density recording. Additionally, by having a reduced flying height regulated to a constant value, the ampli­ tude of the signal being recorded or read out is thus improving signal resolution and making signal processing more reliable. Traditionally, the flying of a magnetic head over the surface of a moving recording medium is regulated passively by adjusting the aerodynamic properties of the medium/head assembly, i.e. surface geometry, distance and velocity of relative displacement. While many known magnetic storage units rely on the air cushion that builds up naturally with disk rotation and relative displacement, other storage units use a forced air cushion generated by pressurized air exiting from a plurality of openings on the surface of the head facing the magnetic recording medium. With this air bearing technology flying heights down to about 300 nm have been achieved. However, further reduction of the flying height appears difficult with this technology. The state of the art of magnetic recording on moving media is perhaps still best documented in R.E. Matik, Computer Storage Systems and Technology, John Wiley & Sons, New York, 1977, p. 342 ff. In view of the limitations inherent in the conven­ tional air bearing technology, it is an object of the invention to propose new ways for reducing the flying height in a controlled fashion so as to increase the storage density in moving magnetic storage devices. The invention contemplates the exploitation of (quasi-) vacuum tunneling for controlling the flying height of the magnetic head assembly in magnetic storage units. With this technology, a drastic reduction of the flying height to values on the order of tens of nanome­ ters and below and an ensuing increase of the storage density up to a factor of 10, possibly even larger, as compared to conventional storage devices should be within reach. Tunneling through vacuum or air (= "quasi-vacuum") is excellently suited to control distances in the range of 0,1 to 10 nm. The control mechanism is based on the tunnel resistance which varies exponentially with the size of the gap between two conductors if a potential difference exists between them. This control mechanism is the heart of the well-publicized Scanning Tunneling Microscope, cf. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Appl. Phys. Lett. 40, 178 (1982). Accordingly, the present invention proposes a method for controlling the flying height of the magnetic head above the surface of a moving recording medium in a magnetic storage unit, wherein said magnetic head is supported for random access of the tracks on said record­ ing medium, the method being characterized by the follow­ ing steps: lowering said magnetic head onto the surface of the moving recording medium to within aerodynamic distance, enforced further lowering of the head until a tunnel current starts flowing across the gap between said head and said recording medium as a result of a potential difference existing across said gap, and maintaining said tunnel current at a constant predetermined value corre­ sponding to a predetermined flying height by using the deviations from said predetermined value to control the mechanical head support in a feedback loop. The present invention further proposes an apparatus for controlling the flying height of a magnetic head above the surface of a moving recording medium in a magnetic storage unit, wherein said magnetic head is supported for random access of the tracks on said record­ ing medium, the apparatus being characterized in that said head carries a tunnel electrode on its surface facing the recording medium, said tunnel electrode and said surface of the recording medium being electrically conductive and being connected to a source of electrical potential, and in that means are provided to measure the tunnel current flowing between said tunnel electrode and said surface, and to supply a signal derived from the deviations from a prede- termined value of said tunnel current to the input of a positioning device at the support of said magnetic head. Details of the inventive method and apparatus will be explained in the following description by way of example and with reference to the attached drawings in which: Fig. 1 is a schematic representation of the magnetic head and its distance control mechanism with which the present invention can be used; Fig. 2 shows the tunnel electrode/transducer assemb­ ly; Fig. 3 represents a cross-section through a record­ ing medium to indicate the various types of asperities which may occur; Fig. 4 is a diagram to explain the relations between the flying height of the magnetic head and the height of asperities; Figs. 5 through 7 shows various embodiments of distance control mechanism; Fig. 8 is an embodiment of the fine-approach unit; Fig. 9 is a cross-section through a preferred embodiment of the head/tunnel electrode assembly in thin-film technology; Fig. 10 is a cross-section through the transducer region of the head assembly at extreme magnification; and Fig. 11 shows the transducer region of Fig. 10 with gradated potentials. For the purposes of the following specification, the recording medium will be assumed to consist of a rotary magnetic disk, although the invention can advantageously be applied to other forms of moving storage, such as tape or drum. The problem of constant, and very small, flying height is also present in non-magnetic storage, viz. in optical storage systems. The invention is likewise applicable to those systems. The design of tunnel distance-regulated head/disk assemblies partially dwells on techniques developed for the Scanning Tunneling Microscope (STM). It differs from the STM in the need for a high data rate, namely above 10 Mbit/s. This is compensated by relaxed requirements with respect to variations of the flying height. The latter can be much larger than the tunnel distance, which is at maximum 3 nm; it only must not exceed the maximum allowed magnetic gap spacing lest the signal deterioration should become too strong. As a result, the maximum position regulation frequency can be kept well below the bitrate, for instance at 30 kHz, as will be discussed below. In contrast to the Scanning Tunneling Microscope where the topography of the sample is the main concern, tunnel distance regulation as applied to data storage has to correct for head/recording medium distance variations regardless of their origin, be it that they result from surface asperities or from vibrations within the system. The essential elements of a tunnel distance-regu­ lated head/recording medium (in casu: disk) assembly are shown in Fig. 1. An actuator 1 can be energized to move an arm 2 in a direction radial with respect to a disk 3 which rotates about its axis 4. Arm 2 carries a two-stage distance control mechanism 5 comprising a coarse-approach unit 6 and a fine-approach unit 7. Attached to fine-­ approach unit 7 is a transducer 8 which for the purposes of this description may be assumed to consist of a magnetic head of essentially conventional design but reduced size. Such a head would comprise a magnetizeable core with a coil wound thereon, the core having a gap of appropriate size, and the gap facing the tracks on magnetic disk 3. As in conventional disk storage units, actuator 1 may be activated to extend or retract arm 2 so as to radially align magnetic head 8 with the tracks on disk 3. Coarse-approach unit 6 lowers head 8 roughly into operating position, i.e. within 200 to 500 nm from the surface of disk 3. This corresponds to the order of magnitude of the head/disk spacing which can be achieved with conventional aerodynamic means where the head was made to fly on a cushion of air molecules which move toward the front side of the head as the disk rotates and which provide enough buoyancy to keep the head clear of the disk surface at an essentially constant distance. Fine-approach unit 7 forces head 8 to further lower toward the surface of disk 3 to a predetermined operating distance which may be of the order of 3 nm. During operation, fine-approach unit 7 controls the flying height of head 8 within certain limits by moving the head up and down in approximate correspondence with the profile the surface of disk 3 has along the track which is presented to head 8. Fine-approach unit 7 may consist of several stages if that is advantageous to optimize its response to surface asperities. To control the head/disk spacing at a distance of about 3 nm, head 3 is equipped with a tunnel electrode 9, shown schematically in Fig. 2. The pole tips 10, 11 of the magnetic core of transducer 8 are almost flush with tunnel electrode 9; a little recession of pole tips 10, 11 is in order for safety reasons; it can amount to about 1 nm. Gap 12 between pole tips 10 and 11 is filled with the usual paramagnetic material, and the entire magnetic head 8 is separated from the body 14, of which electrode 9 is a part, by a suitable dielectric 15. As the surface 16 of disk 3 moves in the direction of arrow 17 past the body/head assembly, it is tunnel electrode 9 which first "sees" the asperities of surface 16. The control mecha­ nism associated with electrode 9 will almost instantane­ ously lift or lower the body/head assembly in response to the characteristics of surface 16 to always maintain the predetermined flying height to guarantee uniform writing and reading, and to avoid head-crash. While the tunnel electrodes used in scanning tunnel­ ing microscopy are usually very thin, pointed needles with the radius of curvature at the apex thereof being as small as can be fabricated, it has been found preferable in the application of this invention to use a tunnel electrode with very slightly tilted slopes. The reasons for preferring a truncated electrode will now be ex­ plained with reference to Fig. 3. Electrode 9 has the form of a very flat wedge with slightly rounded apex. The surface of electrode 9 may be considered as smooth; atomic-size microtips may exist but are of minor importance in this connection. The material of the electrode is selected for stability against corrosion and wear; it should be a reasonably good conductor. Good results have been obtained with gold/bis­ muth alloys and with glassy carbon. The electrode mate­ rial should also have a very small workfunction in order to allow for large tunnel distances. A small workfunction can be achieved by certain organic or oxide adhesion layers. They are not shown in Fig. 3. The entire elec­ trode/head assembly (Fig. 2) can be fabricated in thin-­ film technology. Magnetic disk 3 carries a thin layer 18 of magne­ tizeable material disposed in some carrier medium, as is well known in the art. Layer 18 allows for small bit size to exploit the high recording density capability afforded by the tunnel distance-regulation. The diameter of disk 3 can be between 50 and 80 mm; the disk rotates with appro­ ximately 30 revolutions per minute. The disk surface is covered with a stable, non-corrosive, conducting film 19 having a very small work function. A gold film of less than 5 nm thickness, for example, provides electrical conduction but in no way affects the magnetic recording properties of the underlying layer 18 of magnetizeable material. The surface of disk 3 can be made flat, e.g. by polishing, to approximately 20 nm over ranges of 2 to 5 µm, which is standard optical quality. Even at the flatness indicated above, the surface 16 of disk 3 still consists of flat areas 20, gently corru­ gated areas 21, holes 22, and small protrusions 23. If electrode 9 is over a flat area 20, as shown in Fig. 3, its distance from surface 16 of disk 3 is short­ est at the electrode's apex 24, the latter then being the source of the tunnel current indicated by dotted lines in Fig. 3. Over a gently corrugated area 21, the place of shortest distance and, hence, origin of the tunnel current, walks off from the apex 24 by a small increment depending on the inclination of the slope and shape of electrode 9. Over a hole 22, the tunnel current first remains stable at the value it had while electrode 9 passed over point 25, then the tunnel current jumps to the value associated with point 26 while electrode 9 dives into hole 22 by an amount depending on its slope and the width of hole 22. This behavior was verified experimentally. Upon approach of a protrusion 23, the place of origin of the tunnel current locks onto the rim 27 of the protrusion. This locking of the tunnel current occurs whenever the tilt angle of a feature on the disk surface 16 exceeds the apex angle of electrode 9. The dashed line 28 represents the path of tunnel electrode 9 in the coordinates of the spinning disk if all inertial effects are ignored, i.e. when the disk rotates very slowly. Line 28 reproduces the topography of surface 16 with the constraint of a mechanical "slew rate" limitation: Slopes steeper than the apex angle are replaced by ramps as indicated near hole 22 and protru­ sion 23. Under operating conditions, the fast motion of disk 3 necessitates abrupt retract manoeuvers upon the approach of asperities in order to avoid collision. The dynamics requirements for retraction may be summarized as follows: The position control system recognizes the approach of an asperity, through sudden increase of the tunnel current when the distance h z between the asperity 23 and the sidewall of electrode 9 becomes equal to the operating tunnel distance d t . The electrode then has to quickly retract such that h z never becomes smaller than a safety margin h s while the asperity passes under tunnel electrode 9. In response to the recognition of an asperity 23, the entire electrode unit 7, 8, 9, 14 begins to retract with an acceleration B which can be assumed as constant for the purposes of a semi-quantitative estimate. If H(t) is the flying height over time, then h(t) = · t² The value of b is influenced by the condition that the flying height should never be smaller than a certain safety margin, say 1 nm. With a safety factor of 3, h(t) should be at least 3 nm at all times. This leads to a constant acceleration of b = 6 nm/s². Relating b to the maximum response frequency f max and to the maximum amplitude a max of the fine-approach unit 7 in accordance with the equation b = 4π² a max · f max ² and considering that the maximum acceleration b max obtains for wherein β is the wedge angle of electrode 9, v d is the velocity of disk 3 and d s is the safety margin mentioned before. Equalization of these equations yields: Comparing f max with the bit rat N b leads to with L b being the bit length on disk 3. A possible implementation of fine approach unit 7 is a cantilever-mounted piezoelectric "bimorph" or "bender" element. A specific bender which is commercially available under the designation G-1195, thickness 0,5 mm, from Piezoelectric Products, Inc., has an f max = 239 kHz/­ l[mm] and an a max = 1,32 nm/V · U b · l[mm]. Using these values with an l b = 1,6 mm long bender and an operating voltage of 50 V, one obtains f max = 90 kHz and a max = 175 nm. Entering these values and a bit rate of 10Mbit/s into the last equation requires a β ∼ 2°, i.e. a faily flat tunnel electrode which is lasy to manufature. The piezoelectric effect is known to be fast enough for responses up to several hundred kilohertz. Also, there are reserves in the voltage which could be raised up to 180 V. The amplitude a max of almost 200 nm is comfortable with respect to the expected height of asperities on a well-polished surface. Returning to Fig. 1, coarse-approach unit 6 is actuated at the beginning and end of a reading or writing operation or whenever the actual flying height drifts beyond the operating range of fine-approach unit 7. For approaching, transducer 8 is supposed to start at a large distance and be gradually lowered by both coarse-approach unit 6 and fine-approach unit 7 until tunneling occurs. Then coarse-approach unit 6 gets fixed. (In specific embodiments, it may continue to make slow control motions in accordance with the low frequency tail of the tunnel distance error signal.) Coarse-approach unit 6 may take one of three preferred embodiments. a) As a passive aerodynamic slider. Fig. 5 shows the arrangement where coarse-approach unit 6 flies on an air cushion at submicrometer distance like in conventional disk storage units. Fine-approach unit 7 preferably has two stages, designated 28 and 29 respectively, to provide for a large dynamic range. b) As an active aerodynamic slider. In Fig. 6, coarse-­ approach unit 6 is designed as a slider equipped with an aerodynamic wing 30 permitting active regulation of the air cushion thickness through shaping of the body of unit 6. This design is well suited for an implementation in micromechanical technology using silicon. c) As a piezoelectric stepper translator. Fig. 7 shows a schematic representation of the translator 6 which may be a simplified version of the translator disclosed in IBM Technical Disclosure Bulletin Vol. 26, No. 10A, p. 4898 (1984). A support 31 having clamps 32 through 35 receives a gliding element 36 within a channel 37 formed by said clamps. Gliding element 36 consists of a piezoelectric crystal to which fine-approach unit 7 carrying trans­ ducer 8 is attached. Clamps 32...35 are actuated or released in pairs (32, 33; 34, 35) as gliding element 26 contracts or extends. The design in accordance with Fig. 7 is well suited for vacuum operation where the designs of Figs. 5 and 6 for obvious reasons cannot be used. Fine-approach unit 7 has to complete the positioning of the transducer 8 to within one nanometer and minimize variations in flying height which result from corruga­ tions at the disk surface, vibrations of the head/disk assembly, and slow drifts in temperature and pressure, etc. Piezoelectric positioners are most appropriate as they can be made sufficently small, precise, and simple to operate. The design of fine-approach unit 7 follows from the dynamics and amplitude of response as discussed before. A favoured embodiment would use a micromechanical structure processed from a silicon wafer. This technique is described in European Application 85102554.4 (SZ 9-85-001). Fig. 8 shows a more conventional embodiment of fine-approach unit 7 with a pair of cantilever-mounted piezoelectric bimorphs 38, 39 attached to the body of coarse-approach unit 6. The free length of bimorphs 38, 39 is l b = 1,6 mm. The first bending mode, which is the lowest-order elastic resonance, occurs at 90 kHz, as discussed before. The deflection sensitivity is 3,5 nm/V which provides the above-mentioned maximum deflection amplitude of 175 nm at the operating voltage of 50 V. The two benders 38, 39 are oppositely poled so as to generate opposite momenta at any point in time, resulting in a recoil-free operation of fine-approach unit 7. Lower bimorph 39 carries the transducer/tunnel electrode assembly 8...11, 14 at its end. The transducer/tunnel electrode assembly 8...11, 14 may be designed in thin-film technology as shown schema­ tically in Figs. 9 and 10. Its main parts are the flat tunnel electrode 9 and the pole tips 10 and 11. Tunnel contact 40 is well insulated from a rear magnetic layer 41 by dielectric 15 (cf. Fig. 2), windings 42, and front magnetic layer 43. Magnetic layers 41 and 43 together with a paramagnetic layer 13 between them, and windings 42 form a thin-film magnetic head similar to state-of-the-­ art technology. Its dimensions are miniaturized to cope with the decreased bit size on disk 3. The parts making up the transducer/tunnel electrode assembly are embedded in a matrix 44 which provides the necessary stiffness to the assembly. The lower endface thereof, including the beveled shoulder 45 of body 14, tunnel electrode 9 and pole tips 10 and 11, has to be flat within less than 2 nm. The inclination of shoulder 45 should extend over a distance of at least A max /β (A max = maximum asperity height), i.e. ≧ 2 µm. The inclined sidewalls of tunnel electrode 9 need not be completely plane but should be smooth within the safety distance, ca. 1 nm. A cylindrical shape for the tunnel electrode is preferred over a prismatic or rooftop shape because of the better smoothness (no corners). The upper part 46 of the transducer/tunnel electrode assembly may serve for electrical contacting and signal processing purposes. In a preferred embodiment of the transducer/tunnel electrode assembly, shown in Fig. 11, the lower surface of the assembly is given the same inclination β on the rear side that it has on the front side (with respect to the asperities arriving on the rotating disk). This has been found advantageous because it prevents the loss of the tunnel contact after the head has flown over an asperity and, hence, avoids possible overshooting the correction which otherwise might have caused a surface crash. A further improvement of the operating characteris­ tics of the transducer/tunnel electrode assembly, namely a considerable shortening of the response time, is possible if the electrical potential at the shoulder 45 of body 14 is gradated. Fig. 11 shows that this principle is applied to both, the front and rear shoulders 45 and 47, respectively, in that three different potentials with increasing values (of e.g. 0,2 V, 1,4 V and 10 V) are applied at three sections 48 through 50 of said shoulders 45 and 47, with the lowest potential at the innermost section 48 that includes tunnel electrode 9, and the highest potential at those sections 50 which are farest from the tunnel electrode. While the lower potential is sufficient to maintain the tunnel effect operative, the higher potential at the sections 50 most distant from tunnel electrode 9 makes them work in the field-emission mode. This mode is effective at distances above 20 nm and, thus, permits approaching asperities to be recognized much sooner than is possible with the tunnel potential applied alone.

Claims: 1. Method for controlling the flying height of the magnetic head above the surface of a moving recording medium in a magnetic storage unit, wherein said magnetic head is supported for random access of the tracks on said recording medium, characterized by the following steps: - lowering said magnetic head (8) onto the surface (16) of the moving recording medium (3) to within aerodynamic distance, - enforced further lowering said magnetic head (8) until a tunnel current starts flowing across the gap between said head (8) and said recording medium (3) as a result of a potential difference existing across said gap, - and maintaining said tunnel current at a predetermined constant value corresponding to a predetermined flying height by using the deviations from said predetermined value to control the mechanical head support (5) in a feedback loop. 2. Apparatus for controlling the flying height of a magnetic head above the surface of a moving recording medium in a magnetic storage unit, wherein said magnetic head is supported for random access of the tracks on said recording medium, characterized in that said head (8) carries a tunnel electrode (9) on its surface facing the recording medium (3), said tunnel electrode (9) and said surface (16) of the recording medium (3) being electri­ cally conductive and being connected to a source of electrical potential, and in that means are provided to measure the tunnel current flowing between said tunnel electrode (9) and said surface (16), and to supply a signal derived from the deviations from a predetermined value of said tunnel current to the input of a position­ ing device (5) at the support (2) of said magnetic head (8). 3. Apparatus in accordance with claim 2, character­ ized in that the pole tips (10, 11) of the magnetic head (8) are recessed by a distance of between one and five nanometers with respect to the tunnel electrode (9). 4. Apparatus in accordance with claim 2, character­ ized in that the body (14) of the tunnel electrode (9) is separated from the pole tips (10, 11) of the magnetic head (8) by a dielectric (15). 5. Apparatus in accordance with claim 2, character­ ized in that the assembly comprising the magnetic head (8) and the tunnel electrode (9) has its surface facing the recording medium (3) forming part of a cylinder mantle with its center near the gap between the pole tips (10, 11) of the transducer (8) and with its shoulder beveled at an angle between 1 and 10 degrees. 6. Apparatus in accordance with claim 2, character­ ized in that the shoulder (45, 47) of the assembly comprising the magnetic head (8) and the tunnel electrode (9) is divided in a plurality of sections (48...50) which are each connected to a different electrical potential. 7. Apparatus in accordance with claim 6, character­ ized in that the lowest potential is applied to the innermost section (48) comprising the tunnel electrode (9) and that the highest potential is applied to the outermost section (50) of the shoulder (45, 47). 8. Apparatus in accordance with claim 1, character­ ized in that the assembly comprising the magnetic head (8) and the tunnel electrode (9) has its surface facing the recording medium (3) flat to better than 2 nm. 9. Apparatus in accordance with claim 1, character­ ized in that said positioning device (5) comprises a coarse-approach unit (6) and a fine-approach unit (7) attached to the former, the coarse-approach unit (6) being designed as an active aerodynamic slider (Fig. 6) having a remotely controllable wing (30) for regulating the thickness of the air cushion on which the body of the coarse-approach unit (6) flies. 10. Apparatus in accordance with claim 1, character­ ized in that said positioning device (5) comprises a coarse-approach unit designed as a piezoelectric stepper translator consisting of a support (31) having clamps (32...35) actuatable to hold a gliding element (36) received therebetween in any position dictated by the controlled contraction or extension of the gliding element (36), the clamps (32...35) being releasable in pairs (32, 33; 34, 35) to permit creeping of the gliding element (36).

Inventors: GIMZEWSKI, JAMES KAZIMIERZ, DR., MURALT, PAUL RENE, DR., POHL, WOLFGANG DIETER, DR., Muralt, Paul René, Dr.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: G11B5
IPC codes: G11B5, G11B21