Abstract:
Disclosed is a magnetoresistive magnetic field sensor wherein the sense current flows perpendicular to the plane of the layers of the sensor. The sensor is biased into a linear operating region of the transfer curve using the sense current or a permanent magnet in conjunction with the sense current.

Description:
RELATED APPLICATIONS 
     Referenced-applications 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/267,297 filed on Feb. 28, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention involves the reading of magnetically recorded data and, specifically, deals with new designs for magnetic sensors which can act as magnetic read heads utilizing the giant magneto-resistance effect. 
     BACKGROUND OF INVENTION 
     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 multi-layer device is usually mounted in the read head so that the magnetic axis of the ferromagnetic layers are transverse to the direction of rotation of the disc and parallel to the plane 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 or CIP configuration. 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 component or CPP. 
     The types of sensors commonly in use today for magnetic read heads for a magnetic disc can be categorized as Current-In-Plane (CIP) sensors. Such a sensor is shown schematically in FIG.  1 . In FIG. 1, the current is represented by arrow  8  and is shown flowing parallel to layers  9  of the sensor. 
     CPP sensors are known but are not commonly used for the reading of data from a magnetic disc. A CPP sensor is shown schematically in FIG. 2, wherein the current is represented by arrow  8  and is shown flowing perpendicular to layers  9 . 
     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 CPP sensor is therefore capable of producing a higher output voltage than the CIP sensor, which results in a more precise and sensitive read head. The larger change in resistance comes about because all of the current needs to pass through every ferromagnetic/non-magnetic/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 desirable to have all of the layers and interfaces contribute to the overall Δ R of the device. 
     One example of a CPP sensor is a GMR multi-layer, which consists of a series of FM/NM bi-layers, as shown in FIG.  2 . Every series of interfaces is an opportunity for interfacial spin-dependent scattering and every FM material is a opportunity for bulk spin-dependent bulk. 
     An example of a transfer curve from a CPP GMR multi-layer made of 20 bi-layers of (Cu 18 Å\CoFe 10 Å) is shown FIG.  3 . In the quiescent state, the magnetization of adjacent layers in this sample are oriented 180° with respect to each other, due to RKKY coupling, as is well known in the art. The thickness of the Cu layers was chosen such that the coupling between the CoFe layers would be anti-ferromagnetic (AFM). 
     It can be seen from FIG. 3 that if this type of sensor is to be used in a magnetic read head, it will be necessary to bias the sensor such that it operates in a linear region, such as the regions denoted by reference numbers  1  and  2  in FIG.  3 . This will be necessary if detection schemes that depend on linearity are to be used. 
     It is critical that the GMR multi-layer sensor be biased properly in its linear region of operation. If the sensor is either over or under biased, the signal will become non-linear and create a loss of amplitude, signal asymmetry, detection problems and tracking problems. 
     One way of biasing a GMR multi-layer sensor is to place a permanent magnet (PM) nearby, such that the magnetizations of adjacent FM layers are oriented 90° with respect to each other. This would be similar to applying a DC magnetic field of ˜500 Oe to the sensor whose transfer curve is shown in shown in FIG.  3 . The sensor could then be used to sense the field from a magnetic recording media. 
     One possible CPP read head design using a permanent magnet to bias the sensor into the linear operating region is shown schematically in FIG.  4 . This design uses a permanent magnet and uses the shields as the current carrying leads. 
     One problem with this configuration is that the positioning of the permanent magnet to properly bias the GMR layer is not a trivial task. It may be critical to control the spacing between the GMR multi-layer and the permanent magnet and to center the GMR multi-layer between the ends of the permanent magnet. 
     When building the read head, the actual bias point cannot be determined exactly until after the read head is almost completely built. If the bias point is not correct and there is no way of adjusting the bias, the head will need to be scrapped and all of the previous processing will have been wasted. 
     To avoid expensive manufacturing costs associated with the positioning of the permanent magnet and to avoid scrapping a high percentage of the sensors due to inability to properly bias them, it would be desirable to have a way to adjust the biasing on the sensor such that it operates in a linear region of the transfer curve. 
     SUMMARY OF INVENTION 
     This invention describes an effective way of biasing the GMR multi-layer using the sense current. This biasing may be able to be used in lieu of a permanent magnet, such that the bias is adjustable, which would greatly simplify the sensor manufacturing process, or it may be used in conjunction with a permanent magnet to “fine tune” the biasing provided by the permanent magnet. 
     In this invention, the sense current will be used to induce spin-momentum transfer from one FM layer to the next FM layer to cause the magnetization of these layers to align 90° with respect to each other, thereby providing the proper biasing of the sensor. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a prior art CIP type sensor. 
     FIG. 2 shows a prior art CPP type sensor. 
     FIG. 3 shows the transfer curve from a CPP-GMR multi-layer. 
     FIG. 4 shows one possible configuration of a GMR multi-layer sensor using a permanent magnet for biasing. 
     FIG. 5 shows the transfer curve for a CPP-GMR multi-layer with various sense currents passing therethrough, 
     FIG. 6 shows various configurations of sensors contemplated by this invention. 
    
    
     DETAILED DESCRIPTION 
     Many theories and experiments have shown that spin-momentum can be transferred from one magnetic layer to another. The electrons leaving a FM layer will have their spins polarized to a certain degree depending on the FM material. If these spins then encounter a second FM layer before their spins relax, the second FM layer will exert a torque on the spins in an attempt to align them with the magnetization orientation of the first FM layer. According to Newton&#39;s law, the electrons must exert an equal and opposite torque on the magnetization of the second FM layer. If the volume of the second FM layer is small, this torque can dominate over other intrinsic and extrinsic torques on the moment, allowing the torque exerted from the electrons to change the magnetization of the second FM layer. If the orientation of the first magnetic layer and the current density is chosen correctly, this effect causes the magnetic moments of each layer of a GMR multi-layer to align 90° with respect to each other, as is needed for proper biasing of the multi-layer. 
     FIG. 5 shows transfer curves for three different sense currents for a 100 nm square GMR multi-layer configured and operated in the CPP mode, that is, with the sense current flowing perpendicular to each layer of the multi-layer structure. The trace corresponding to a sense current of 5 mA is centered (symmetric) with respect to the applied field. The traces corresponding to a sense current of +/−25 mA are offset with respect to the applied field. The amount of spin-momentum that is transferred from one layer to the next is directly proportional to the current, so it is expected that this offset effect would be more pronounced at high currents but not for low currents. 
     FIGS.  6 ( a-f ) show several different configurations of sensor layouts that will operate according to this invention, taking advantage of the spin-momentum transfer type of sensor biasing. FIG.  6 ( a ) is a simple configuration comprising leads/shields  10  and GMR multi-layer  12 , having any number of bi-layers. The sensors of FIGS. 6 ( b-d ) all utilize GMR multi-layer  12  with a spin polarizing layer or layers  14  somewhere within the sensor. The spin polarization layer could be an FM layer pinned by an AFM layer, a permanent magnet, an FM layer pinned by a permanent magnet or any combination of these. The purpose of the spin polarizing layers is to orient the electrons passing though the polarizer with the same spin to maximize the spin momentum transfer. Spin polarizers may be used on both sides of the device to effect not only transmitted electrons but also electrons which are reflected back into the device. If the spin polarizing layer is chosen and positioned properly, the transmitted electrons could bias the sensor in one direction (up) and the reflected electrons could bias the sensor in the other direction (down) yielding a differential type sensor. 
     The sensors shown in FIGS.  6 ( e-f ) are extensions of those shown in FIGS.  6 ( b-c ) with only one freelayer  18  that may be de-coupled from the spin polarizing layer or layers  14  via a conductive layer  16 . Freelayer  18  is composed of a ferromagnetic material. The configuration shown in FIGS.  6 ( e-f ) operate as simple spin valves. The combination of the spin polarizer and freelayer/conductor combination provides a GMR effect with out a multi-layer GMR structure. Again, spin polarizers may be used on both ends of the device because of the tendency of some electrons to be reflected back into the device. 
     Many other sensor layouts using this spin-momentum biasing idea are contemplated, all of which would work in a manner similar to those shown in FIGS.  6 ( a-f ) and in accordance with this invention. 
     It is also contemplated that the adjustable biasing of the sensors could be accomplished in several different ways. One way would be for the biasing to be set at the factory as part of the manufacturing process. A second method would be for circuitry in the unit to detect variations in the biasing and to adjust the biasing dynamically by varying the sense current. Such circuitry could be designed by anyone of ordinary skill in the art. 
     Several configuration for sensors using the spin momentum transfer theory are presented in FIG.  6 . However, the invention is not meant to be limited thereby. The bounds of the invention, captured in the following claims, is meant to encompass any magnetoresistive structure wherein the sense current passes perpendicularly through all layers of the sensor and is used to bias the sensor into a linear operating region of the transfer curve, or wherein the main biasing of the sensor is provided by a permanent magnet in close proximity to the magnetoresistive structure and wherein the sense current is used to fine tune the biasing of the sensor.