Patent Application
20050195535
The present invention relates to magnetoresistive sensors and more particularly to the use of electromagnetic biasing of a free layer in a magnetoresistive sensor.
A computer system requires the storage of large amounts of data. Such storage is usually provided by a magnetic disk drive that includes a magnetic disk, a slider where a magnetic head assembly including write and read heads is mounted, a suspension arm, and an actuator arm. When the magnetic disk rotates, air adjacent to the disk surface moves with the disk under the surface (ABS) of the slider, causing the slider to fly on an air bearing. The actuator arm swings the suspension arm to place the magnetic head assembly over selected circular tracks on the rotating magnetic disk, where signal fields are written and read by the write and read heads, respectively. The write and read heads are connected to processing circuitry that operates according to a computer program to implement write and read functions.
With reference to
The magnetization of the pinned layer can be pinned as indicated by arrows 114, 116 by exchange coupling with an anitferrmagnetic material layer 118 such as PtMn. While antiferromagnetic materials are not magnetic in and of themselves, when exchange coupled with a ferromagnetic material, they can strongly set the magnetization of the ferromagnetic material.
The free layer 104 has a magnetization that is biased in a direction 120 parallel with an ABS surface of the slider in which it is incorporated. Although the magnetization is biased in the direction indicated by arrow 120, the magnetization of the free layer is free to rotate in response to a magnetic field. As those skilled in the art will recognize, the relative directions of magnetization of the free 104 and pinned 102 layers affects the electrical resistance through the sensor. This resistance is greatest when the magnetization of the free layer 104 and the ferromagnetic layer 114 of the pinned layer 102 are antiparallel and is at a minimum when they are parallel. These magnetizations 120 and 114 are set to be perpendicular in the absence of a magnetic field, so that the signal sensed from relative movements about this quiescent state will be at its most linear relationship.
The magnetization of the free layer in a typical magnetoresistive sensor is biased by hard bias layers 122, 124, disposed at either lateral edge of the sensor. Electrically conductive leads 123, 125 disposed over the bias layers provide electrical sense current to the sensor 100 during use. First and second electrically insulating gaps 100, 129, insulate the sensor 100 from a pair of magnetic shields 131, 133. A capping layer 135, such as Ta, can also be provided to prevent corrosion during subsequent manufacturing processes.
The bias layers 122, 124 are constructed of a hard magnetic material (ie. a material with a high coercivity), and are magnetized by placing them in a magnetic field. The amount of biasing achieved by the hard bias layers 122, 124 is critical in that if the free layer is too strongly biased, the sensor will not be sufficiently sensitive. In other words, it would require a very strong field to rotate the direction of magnetization 120 of the free layer. The sensor would therefore be useless. On the other hand, weak biasing of the free layer results in domain fluctuations in the free layer resulting excessive noise in the signal read from the sensor. This too renders the sensor useless.
With reference now to
Therefore, there is a strong felt need for reliable free layer biasing in a magnetoresistive sensor. Such biasing would preferably always, or nearly always maintain the bias field within an acceptable range, eliminating the need to scrap heads due to insensitivity or instability.
The present invention provides a mechanism for reliably biasing a free layer of a magnetoresistive sensor. The invention includes an electromagnet including a ferromagnetic yoke having terminations adjacent opposite ends of a free layer of the sensor. An electrical coil wrapped around a portion of the yoke can induce a magnetic field in the yoke to bias the free layer. The yoke is preferably formed of a soft ferromagnetic material, ie. one having a low coercivity.
The magnetic yoke can be formed as first and second ferromagnetic portions separated by an electrically insulating gap. Forming the yoke as separate first and second portions prevents the yoke from shunting current from one side of the sensor to the other. The yoke portions can be constructed of many soft magnetic materials such as NiFe, FeN, FeXN (X could be Al, Ta, Co, etc.), CoFe, Sendust, CZT, CZN, etc. A pair of electrically conductive leads for providing a sense current to the sensor can be formed on top of the yoke portions.
By controlling the amount of current flowing through the coil, the amount of magnetic bias can advantageously be controlled so that no sensor would have to be rejected for having an improper amount of bias.
The invention can also include first and second front magnetic layers that have portions abutting first and second sides of the sensor, and a third back ferromagnetic layer that is separated from the first and second front ferromagnetic layers by a dielectric layer. A coil can be formed to wrap around the third magnetic layer so as to magnetize the third magnetic layer. The first and second magnetic layer can be formed to have portions overlapping the third magnetic layer, while being electrically insulated therefrom. With this arrangement, the magnetization of the third layer will magnetize the first and second magnetic layer, thereby providing magnetic biasing of the sensor without current shunting.
For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings.
The following description is the best embodiment presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein.
Referring now to
At least one slider 313 is positioned near the magnetic disk 312, each slider 313 supporting one or more magnetic head assemblies 321. As the magnetic disk rotates, slider 313 is moved radially in and out over the disk surface 322 so that the magnetic head assembly 321 may access different tracks of the magnetic disk where desired data are written. Each slider 313 is attached to an actuator arm 319 by way of a suspension 315. The suspension 315 provides a slight spring force which biases slider 313 against the disk surface 322. Each actuator arm 319 is attached to an actuator means 327. The actuator means 327 as shown in
During operation of the disk storage system, the rotation of the magnetic disk 312 generates an air bearing between the slider 313 and the disk surface 322 which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension 315 and supports slider 313 off and slightly above the disk surface by a small, substantially constant spacing during normal operation.
The various components of the disk storage system are controlled in operation by control signals generated by control unit 329, such as access control signals and internal clock signals. Typically, the control unit 329 comprises logic control circuits, storage means and a microprocessor. The control unit 329 generates control signals to control various system operations such as drive motor control signals on line 323 and head position and seek control signals on line 328. The control signals on line 328 provide the desired current profiles to optimally move and position slider 313 to the desired data track on disk 312. Write and read signals are communicated to and from write and read heads 321 by way of recording channel 325.
The above description of a typical magnetic disk storage system, and the accompanying illustration of
With reference to the schematic illustration of
It will be appreciated by those skilled in the art that when an electrical current flows through the coil 408 a magnetic flux will be induced in the yoke 406, which will bias the magnetization of the free layer 104 of the sensor in a desired direction. The amount of biasing of the free layer 104 of the sensor 402 can be controlled by controlling the amount of current in the coil 408. The amount of current in the coil 408 can be controlled in situ by feedback circuitry incorporated into the control unit 329.
An electrically conductive coil is 511 constructed to wrap around the back portion of the yoke 502 which can be laterally elongated to more efficiently accommodate the coil. The coil can be formed of, for example, Cu or Au, and can be formed by plating a first set of generally parallel lines 510 of conductive material before construction of the yoke 502. After a construction of the yoke 502 and leads 506, 508, a second set of generally parallel, electrically conductive lines 512, the ends of which can be joined with ends of the first set 510 of electrically conductive lines by a vias formed therebetween. It should be appreciated that although, for purposes of clarity, only a few coil turns are illustrated in
With reference now to
An electrically conductive coil 612 wraps around the back soft bias layer 606, such that an electrical current flowing through the coil 612, will magnetize the soft bias layer 606. The coil can be constructed in a manner similar to that described above with reference to
It will be appreciated with continued reference to
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Other possible embodiments encompassed within the invention will be apparent to those skilled in the art. For example rather than providing a gap in the yoke described above, the magnetic yoke could be constructed of a soft magnetic, dielectric material such as MnZn-ferrite, NiZn-ferrite, etc. In addition, while the present invention has been described for use in a disk drive system, the present invention could be used in other applications, such as for example tape drive systems. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
