1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP sensor with an improved ferromagnetic reference layer.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically formed of Cu or Ag. One ferromagnetic layer, typically called the “reference” layer, has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer, typically called the “free” layer, has its magnetization direction free to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as current-perpendicular-to-the-plane (CPP) sensor.
In a magnetic recording disk drive CPP-GMR read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance.
The reference ferromagnetic layer in a CPP-GMR sensor used in read heads may be a single pinned layer (sometimes called a simple pinned layer) or part of an antiparallel (AP) pinned structure. The AP-pinned structure has first (AP1) and second (AP2) ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's electrically conductive spacer layer on the other side, is the reference layer. The AP1 layer, which is typically in contact with an antiferromagnetic layer, such as IrMn, on one side and the nonmagnetic APC layer on the other side, is typically referred to as the pinned layer. The AP-pinned structure minimizes magnetostatic coupling between the reference layer and the CPP-SV free ferromagnetic layer. The AP-pinned structure, also called a “laminated” pinned layer, and sometimes called a synthetic antiferromagnet (SAF), is described in U.S. Pat. No. 5,465,185.
The materials making up the free layer and the reference layer (either the simple pinned layer or the AP2 layer in an AP-pinned structure) are typically crystalline alloys of CoFe or NiFe. Heusler alloys, which are chemically ordered alloys like Co2MnX (where X is one or more of Ge, Si, Sn, Ga or Al) and Co2FeZ (where Z is one or more of Ge, Si, Al, Sn or Ga), are known to have high spin-polarization and result in an enhanced magnetoresistance and are thus desirable ferromagnetic materials to use in one or both of the reference layer and free layer. In the reference layer, Heusler alloys are usually deposited directly on a layer of a crystalline ferromagnetic material, such as Co or CoFe, or on an amorphous ferromagnetic material. Heusler alloys typically require significant post-deposition annealing to achieve chemical ordering and high spin-polarization. However, while Heusler alloy reference layers provide high spin-polarization, they also display low magnetic damping, which makes them particularly subject to spin torque instability due to the spin polarized electrons from the free layer when the sensor is under voltage bias. Thus sensors with Heusler alloy reference layers are especially prone to suffer reduced magnetic stability and reduced sensitivity. This requires further enhanced pinning strength for the Heusler-based reference layers compared to crystalline CoFe alloy reference layers with lower spin polarization.
What is needed is a CPP-GMR sensor with a reference layer that has a Heusler alloy for high spin-polarization and improved magnetoresistance, but that is not prone to magnetic instability and reduced sensitivity with increased voltage bias.