Abstract:
A single-element magnetoresistive (MR) read head with reduced susceptibility to noise is disclosed. In particular, the present invention addresses the problem of noise generated thermally through contact with the recording medium. The present invention solves this problem by keeping the temperature of the read head at a level that minimizes the noise level. According to a preferred embodiment of the present invention, the shielding material used in the read head is recessed with respect to the magnetic-medium-bearing surface. In an alternative embodiment of the present invention, a thin coating of metal on the read head surface is applied. In yet another embodiment, the read element is operated with a low bias current so as to minimize the thermal effect of power consumption due to electrical resistance. In still another embodiment, the read element makes use of insulating material that is both an electrical insulator and a high quality thermal insulator.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for reducing noise due to head-tape contact in a single-ended magnetoresistive read element. 
   2. Background of the Invention 
   Information is written onto a magnetic tape by magnetizing tape elements. These magnetized tape elements produce a magnetic field that can be detected and converted to an electrical signal by a read head. A common type of read head for carrying out this conversion is the magnetoresistive (MR) read head. 
   A simple MR head consists of a thin film of magnetoresistive material, such as permalloy, between two insulating layers. When the MR layer is formed, a magnetic field is typically applied in a direction parallel to the plane of the thin layer. Thus, the MR layer exhibits a uniaxial anisotropy with an easy-axis of magnetization parallel to the direction of the applied field. If an external magnetic field, such as from a magnetic tape, is applied normal to the easy-axis, the magnetization direction of the MR layer will rotate away from the easy-axis and toward the direction of the applied magnetic field. This magnetization rotation causes a change in resistance in the MR layer. When no external field is applied, the resistance is greatest. The resistance decreases with increasing applied field. For practical geometries of the MR layer, resistance as a function of applied field traces a bell-shaped curve. The MR head is often biased with an applied current such that a zero magnitude applied field results in a resistance near an inflection point on the resistance curve. Thus, small changes about a zero magnitude applied external field result in nearly linear changes in resistance. 
   To accommodate increasing densities of data stored on magnetic tape, the geometries of read heads continue to shrink. As read head geometries become smaller, however, MR read heads become increasingly susceptible to noise. Dual-element read heads may be used in a differential manner to counteract some of this susceptibility by eliminating common-mode noise, but at a cost of slightly increased head size and loss of data density on the recording medium. A need exists, therefore, for a read head that allows for a smaller physical size than conventional double-element read heads, but with less susceptibility to noise. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward a single-element magnetoresistive (MR) read head with reduced susceptibility to noise. In particular, the present invention addresses the problem of noise generated thermally through contact with the recording medium. The present invention solves this problem by keeping the temperature of the read head at a level that minimizes the noise level. According to a preferred embodiment of the present invention, the shielding material used in the read head is recessed with respect to the magnetic-medium-bearing surface. In an alternative embodiment of the present invention, a thin coating of metal on the read head surface is applied. In yet another embodiment, the read element is operated with a low bias current so as to minimize the thermal effect of power consumption due to electrical resistance. In still another embodiment, the read element makes use of insulating material that is both an electrical insulator and a high quality thermal insulator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a diagram depicting the main components of a magnetic tape drive in which the teachings of the present invention may be applied; 
       FIG. 2  is a diagram of a magnetoresistive read element and associated read circuitry as may be incorporated into a preferred embodiment of the present invention; 
       FIG. 3  is a diagram of a magnetoresistive read element in which the shielding material is recessed with respect to the active surface of the read element in accordance with a preferred embodiment of the present invention; and 
       FIG. 4  is a diagram of a magnetoresistive read element with thin metal layer in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention is directed generally toward a magnetoresistive (MR) read element for reading information from a magnetic storage medium in a storage device.  FIG. 1  depicts the primary components of a magnetic tape drive, which is one type of storage device in which the present invention may be implemented. 
   Magnetic tape  108  moves from source spool  110  to take-up spool  112  in a pulley action from force applied by motor  114 . Source spool  110  and take-up spool  112  may exist separately, or may be incorporated into an integrated package, such as a tape cartridge or cassette. 
   Data recorded to magnetic tape  108  will preferably be written in the form of several parallel tracks extending longitudinally along a surface of magnetic tape  108 . Read/write assembly  106  will preferably contain multiple read heads and write heads for reading and writing to/from these tracks simultaneously. Read/write assembly  106  maintains alignment with magnetic tape  108  by way of a servo control  124 , which uses solenoid  126  to position read/write assembly  106  vertically with respect to magnetic tape  108 . Read/write assembly  106  is generally positioned so that its read and write heads are kept at a very small distance from magnetic tape  108  in order to detect the small magnetic flux reversals on magnetic tape  108  that encode digital data or other information (e.g. analog signals or audio). 
   Because read/write assembly  106  is positioned at only a small distance from magnetic tape  108 , which is a moving medium, intermittently the read/write assembly  106  and the read and write elements it contains will make contact with magnetic tape  108 . This intermittent contact with magnetic tape  108  causes the temperature of the read and write elements in read/write assembly  106  to fluctuate. When the read elements are very small in size, this contact with magnetic tape  108  can result in a significant amount of thermally-generated noise being injected into the signal read from magnetic tape  108 . 
   One of ordinary skill in the art will recognize that this thermally-generated noise resulting from contact with the recording medium (hereinafter referred to as “thermal contact noise”) is not a phenomenon limited to magnetic tapes, but may also occur in other magnetic media without limitation. Examples of other magnetic media include floppy disks, hard disks, and magnetic drums. 
     FIG. 2  is a diagram depicting a single-element magnetoresistive (MR) read element  200  as may be incorporated into a preferred embodiment of the present invention. MR read element  200  comprises a layer of magnetoresistive material  202 , such as permalloy, sandwiched between two layers of an electrically insulating material  204  to form a generally rectangular block, which is in turn sandwiched between two layers of magnetic shielding material (shields)  211 . One side of MR read element  200  that contains an exposed portion of magnetoresistive material  202  is designated as an active side  206  of MR read element  200  and is the side of MR read element  200  that faces recording medium  208 . 
   In the presence of a magnetic field, such as that provided by recording medium  208 , magnetoresistive material  202  changes in electrical resistance. Thus, magnetoresistive material  202  acts as a variable resistor or rheostat that varies in resistance in response to changes in the local magnetic field. Thus, a signal may be read from recording medium  208  via MR read element  200  by incorporating magnetoresistive material  202  into a circuit to fulfill the role of a rheostat. In  FIG. 2 , magnetoresistive material  202  is used as a variable feedback resistor in an operational amplifier (op-amp) circuit operating in an inverting configuration. A voltage source  210  is connected through a fixed resistor  212  to inverting input  216  of an op-amp  214 , while non-inverting input  218  of op-amp  214  is grounded (optionally through a resistor  220 , as shown). Magnetoresistive material  202  is connected in a feedback path from output  222  of op-amp  214  to inverting input  216  of op-amp  214 . This arrangement allows a bias current to flow through magnetoresistive material  202  and allows the resistance of magnetoresistive material to control the gain of the resulting inverting amplifier circuit provided by op-amp  214 . Thus, a magnetic signal recorded on recording medium  208  in converted by the resulting variable-gain amplifier circuit into a corresponding voltage level at output  222 . 
   Returning attention now to the physical characteristics of MR read element  200 , it can be seen from  FIG. 2  that active side  206  of MR read element  200  forms a rectangle in two dimensions. The width of each constituent component in the MR read element sandwich is the “element width” of MR read element  200  (dimension  224 ). The length of the entire MR read element  200  across the sandwich layers is called the “total shield distance” of MR read element  200  (dimension  226 ). It can be shown experimentally that thermal contact noise in an MR read element such as MR read element  200  becomes significant when element width  224  is equal to or less than total shield distance  226 , the element&#39;s operating temperature is substantially higher than the ambient temperature, and when the thermal conduction from the MR element to the media is too high. 
   Thermal contact noise can be reduced by minimizing the temperature change of the read element by tape contact. The tape cools the element by the following process: First, the tape transfers heat from the shields, cooling them. Then, since the shields are in thermal equilibrium with the element the element will cool also. 
   We can therefore minimize element temperature changes by either minimizing heat flow between the shields and tape, or providing a means to make the heat flow more constant. 
   Minimizing heat flow between shields and tape can be done by
         A) Keeping the element temperature close to the tape temperature, thereby not heating the shields with the element and thus lowering the temperature differential between shields and tape. We accomplish this by either maintaining a bias current below a pre-determined amount, or lowering resistance below a pre-determined amount, or both.   B) Thermally isolating the element from the shields, thereby not heating the shields with the element and thus lowering the temperature differential between shields and tape. We accomplish this by either using a high-quality thermal insulator for the insulating material  204 , or by increasing the thickness of this material, or both.
 
Making the heat flow between shields and tape more constant can by done by
   A) Enhancing thermal contact between tape and shields with a coating on the head surface, thereby keeping heat transfer between shields and tape more constant and less susceptible to intermittent, discrete cooling events. This is depicted in  FIG. 4 .   B) Reducing thermal contact between the tape and shields by recessing the shields from the tape bearing surface, thereby minimizing heat transfer between the shields and tape. This is depicted in  FIG. 3 .       

     FIG. 3  is a diagram of an MR read element  300  in accordance with a preferred embodiment of the present invention. MR read element  300  contains magnetoresistive material  302  surrounded by shields  310 , separated from magnetoresistive material  302  by layers of insulating material  304 . Shields  310  are then enclosed by closure layers  315 , which are separated from shields  310  by insulating layers  313 . Closure layers  315  are, in a preferred embodiment, made from an alloy such as an aluminum-titanium-carbon (AlTiC) alloy, both other materials may be substituted without departing from the scope and spirit of the present invention. Closure layers  315  have medium-bearing surfaces  320  that make contact with magnetic medium  317  (e.g., magnetic tape or disk). Shields  310  (as well as magnetoresistive material  302  and insulating layers  304  and  313  in this example) are recessed from the plane of tape bearing surfaces  320 . This configuration avoids heat transfer between shields  310  and magnetic medium  317 , and thus reduces thermal contact noise. 
     FIG. 4  is a diagram depicting an alternative embodiment of an MR read element  400  in accordance with a preferred embodiment of the present invention. MR read element  400 , in addition to having a layer of magnetoresistive material  402 , layers of insulative material  404 , and layers of shielding material  411 , also has a thin layer of metal applied to the active side of MR read element  400 . The metal layer allows for more even cooling of MR read element  400  and helps to stabilize the temperature of MR read element  400  to avoid noise. The metal layer in one particular embodiment consists of a 10 Angstrom layer of gold. The alternative embodiment of the present invention depicted in  FIG. 4  is particularly useful in single-use or limited-use magnetic reading devices, where erosion of the metal layer over time is not a problem. 
   As stated previously, additional alternative embodiments of the present invention reduce thermal contact noise by lowering a bias current of the magnetoresistive material or steady state resistance of the magnetoresistive material to beneath a pre-determined amount. This prevents heating of the shields by the magnetoresistive material by keeping the current density of the magnetoresistive material low and thus lowers the temperature differential between the shields and magnetic medium (e.g., tape). This can also be accomplished by using a high-quality insulating material and/or increasing the thickness of the insulating material to prevent the magnetoresistive material portion of the MR read element from heating the shields. 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.