The invention relates to the field of the magnetic recording of data, and, in particular, to the recording of data on a magnetic disc. Specifically, this invention discloses a novel sensor for the reading of data from a magnetic disc.
Devices utilizing the giant magneto-resistance (GMR) effect have utility as magnetic sensors, especially as read sensors in read heads used in magnetic disc storage systems. The GMR effect is observed in thin, electrically conductive multi-layer systems having multiple magnetic layers. One sensor type that utilizes the GMR effect is the GMR multilayer. The GMR multilayer typically comprise a series of bi-layer devices, each of which comprise a thin sheet of a ferromagnetic material and a thin sheet of a non-magnetic material. The bi-layers are stacked to form a multi-layer device. The magnetization of each ferromagnetic layer in the multi-layer device is approximately orthogonal to the magnetization of adjacent ferromagnetic layers and would be oriented in a plane perpendicular to the plane of the disc. The multi-layer device is typically mounted in the read head so that the magnetic axis of the ferromagnetic layers are transverse to the direction of rotation of the disc.
In operation, a sense current is caused to flow through the read head and therefore through the sensor. The magnetic flux from the disc causes a rotation of the magnetization vector in at least one of the sheets, which in turn causes a change in the overall resistance of the sensor. As the resistance of the sensor changes, the voltage across the sensor changes, thereby producing an output voltage.
The output voltage produced by the sensor is affected by various characteristics of the sensor. The sense current can flow through the sensor in a direction that is parallel to the planes of the layers or stacked strips. This is known as a current-in-plane (CIP) configuration. This configuration is shown in FIG. 1, wherein the sense current is represented by arrow 8 and is shown flowing parallel to layers 9 of the sensor. Reference numbers 5, 6, and 7 show bottom shield, an insulating layer and permanent magnets, respectively. Typically, the types of sensors used today for the reading of magnetically recorded data can be categorized as current-in-plane sensors.
Alternatively, the sense current can flow through the sensor in a direction that is perpendicular to the planes of the layers or stacked strips that comprise the sensor. This configuration is known as a current-perpendicular-to-plane (CPP) configuration. A CPP sensor is shown schematically in FIG. 2, wherein the sense current is represented by arrow 8 and is shown flowing perpendicular to layers 9 of the sensor through shields 5 and non-magnetic electrical conducting layers 4.
The CPP sensor is interesting because of its potentially larger giant magneto-resistance (GMR) or change in resistance when a magnetic field is applied. The larger change in resistance comes about because all of the current needs to pass through every ferromagnetic/nonmagnetic/ferromagnetic (FM/NM/FM) series of interfaces and none of the current is shunted around the interfaces. Because every film and interface leads to additional resistance, it is desired to have all of the films and interfaces contribute to the overall xcex94R. One such sensor is a GMR multilayer, which consists of a series of FM/NM bi-layers. Every series of interfaces is an opportunity for interfacial spin-dependent scattering and every FM material is an opportunity for bulk spin-dependent scattering.
An example of a transfer curve from a CPP-GMR multilayer made of 15 bi-layers of (Cu 18 xc3x85 CoFe 10 xc3x85) is shown in FIG. 3. In the quiescent state, the magnetization of adjacent layers in this sample are oriented 180xc2x0 with respect to each other, due to RKKY (Ruderman-Kittel-Kasuya-Yosida) coupling. The Cu thickness was chosen such that the RKKY coupling between the CoFe layers would be antiferromagnetic.
It can be seen from FIG. 3 that if this type of sensor is used in a magnetic recording head, it will need to be biased such that it operates in a linear region, denoted by A and B on the graph. This will be necessary to use detection and tracking schemes that depend on signal linearity. One way of biasing a GMR multilayer sensor is to place a permanent magnet (PM) nearby, such that the magnetizations of adjacent FM layers are approximately orthogonal to each other. This would be similar to applying a DC magnetic field of xcx9c500 Oe to the sensor shown in FIG. 3. The sensor could then be used to sense the field from the magnetic recording media.
FIG. 4 shows a schematic representation of one possible design for a CPP read head using a GMR multilayer 10 that is biased into the linear operating region using permanent magnet 12 and which uses shields 11 as the current carrying leads. Layers 13 and 14 are non-magnetic conductors.
The transfer curve response that the head of FIG. 4 would have to perpendicular media may resemble a square wave similar to the diagram shown in FIG. 5. This type of response is difficult for a read back channel to handle due to the fact that it""s impulse response contains DC components.
One suggested solution to this problem is to differentiate the signal, which may result in a signal resembling that shown in FIG. 6. This would make the signal much more compatible with the read back channels used today. A problem with this solution is that the process of differentiating the signal may add high frequency noise to the read back signal.
It would therefore be desirable to provide a sensor which outputs a signal compatible with contemporary read back channels without the high frequency noise.
The solution disclosed herein is to make a head that effectively differentiates the flux from the media. The output from such a head may also resemble the signal shown in FIG. 6.
The invention described here is a CPP-GMR design that would act as a differential read back sensor. A differential sensor could be made by biasing part of the sensor in region A shown in FIG. 3 and part of the sensor in region B shown in FIG. 3. This can be accomplished by providing a pair of GMR multilayers separated by a non-magnetic interlayer. The magnetizations of the GMR multilayers would be biased such that they point in opposite directions, for example, one pointing toward the media and one pointing away from the media. As such, when exposed to a magnetic field, the resistance of the GMR multilayers will vary inversely.
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