1. Field of the Invention
This invention relates in general to giant magnetoresistive (GMR) sensors for reading information signals from a magnetic medium and, in particular, to a spin valve sensor having an antiparallel coupled free layer having a low intrinsic uniaxial anisotropy, and to magnetic storage systems that incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Leads 140 and 145 formed in the end regions 104 and 106, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect.
Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG. 1. This improved exchange coupling increases the stability of the AP-pinned SV sensor at high temperatures which allows the use of corrosion resistant and electrically insulating antiferromagnetic materials such as NiO for the AFM layer.
Referring to FIG. 2, an AP-pinned SV sensor 200 comprises a free layer 210 separated from a laminated AP-pinned layer structure 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 230. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic layer 226 and a second ferromagnetic layer 222 separated by an antiparallel coupling (APC) layer 224 of nonmagnetic material (usually ruthenium (Ru)). The two ferromagnetic layers 226, 222 (FM1 and FM2) in the laminated AP-pinned layer structure 220 have their magnetization directions oriented antiparallel, as indicated by the arrows 227, 223 (arrows pointing out of and into the plane of the paper respectively).
As magnetic storage density increases in order to meet the demands of high storage capacity disk drives, it is increasingly important to increase the GMR coefficient of SV sensors in order to improve the sensitivity and signal-to-noise characteristics of the signal readback system and to decrease the thickness of the free layer to meet the higher areal density requirements. Sense current shunting around the spacer layer and the pinned layer and spacer layer interfaces with the spacer layer results in reduces GMR coefficient since most of the spin dependent scattering giving rise to the GMR effect occurs in this region. The free layer of SV sensors usually consists of Coxe2x80x94Fe and Nixe2x80x94Fe layers. The Coxe2x80x94Fe is used to obtain a high GMR coefficient, and the Nixe2x80x94Fe is added to achieve a free layer with soft magnetic properties. However, the Nixe2x80x94Fe has a low electrical resistivity which contributes to sense current shunting resulting in a decrease of the GMR coefficient. Reduction of the free layer thickness for high areal density applications results in degradation of magnetic properties and a reduced GMR coefficient. The use of an antiparallel coupled structure for the free layer is a method to reduce the free layer magnetic thickness without degrading the magnetic properties and the GMR coefficient. However, the intrinsic uniaxial anisotropy Hk of the free layer increases by antiparallel coupling making this structure unattractive for free layer application.
Therefore, there is a need for an improved antiparallel coupled free layer to reduce free layer thickness, reduce sense current shunting and to increase the GMR coefficient of a spin valve sensor while maintaining a very low value of Hk for the free layer.
Accordingly, it is an object of the present invention to disclose a spin valve sensor having an antiparallel (AP)-coupled free layer with low intrinsic uniaxial anisotropy Hk.
It is another object of the present invention to disclose a spin valve sensor having a free layer of high electrical resistivity, soft ferromagnetic material.
It is yet another object of the present invention to disclose a spin valve sensor having an improved GMR coefficient due to reduced current shunting by the ferromagnetic free layer.
It is a further object of the present invention to disclose a spin valve sensor having an AP-coupled free layer comprising a third ferromagnetic layer of Coxe2x80x94Fe and a fourth ferromagnetic layer of Coxe2x80x94Fexe2x80x94Hfxe2x80x94O separated by an antiferromagnetic coupling layer of ruthenium (Ru),.
In accordance with the principles of the present invention, there is disclosed a spin valve (SV) sensor having an AP-pinned layer, a laminated AP-coupled free layer and a non-magnetic electrically conductive spacer layer sandwiched between the AP-pinned layer and the free layer. The AP-pinned layer comprises first and second ferromagnetic layers separated by an antiparallel coupling (APC) layer. The AP-coupled free layer comprises a third ferromagnetic layer of Coxe2x80x94Fe adjacent to the spacer layer and a fourth ferromagnetic layer of Coxe2x80x94Fexe2x80x94Hfxe2x80x94O separated from the third ferromagnetic layer by an antiparallel coupling (APC) layer. During the AFM anneal process step of the sensor, the easy axis of the Coxe2x80x94Fe layer becomes transverse while the easy axis of the Coxe2x80x94Fexe2x80x94Hfxe2x80x94O remains longitudinal due to the higher thermal stability of this nano-crystalline material. The 90 degree angle between the easy axis of the Coxe2x80x94Fe layer and the the easy axis of the Coxe2x80x94Fexe2x80x94Hfxe2x80x94O material results in a very low Hk for the AP-coupled free layer. The Coxe2x80x94Fexe2x80x94Hfxe2x80x94O material of the fourth ferromagnetic layer has high resistivity resulting in reduced sense current shunting by the free layer. In addition, the metal oxide material of the fourth ferromagnetic layer is known to cause specular scattering of electrons. The reduced sense current shunting and the specular scattering of electrons both contribute to improving the GMR coefficient of the SV sensor.
The above, as well as additional objects, features and advantages of the present invention will become apparent in the following detailed written description.