The invention relates to the general field of magnetic disks with particular reference to GMR read heads and use of longitudinal bias leads therewith.
The present invention is concerned with the manufacture of the read element in a magnetic disk system. This is a thin slice of material, located between two magnetic shields, whose electrical resistivity changes on exposure to a magnetic field. Magneto-resistance can be significantly increased by means of a structure known as a spin valve (SV). The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are two magnetic layers separated by a non-magnetic layer. The thickness of the non-magnetic layer is chosen so that the magnetic layers are sufficiently far apart for exchange effects to be negligible but are close enough to be within the mean free path of conduction electrons in the material. If the two magnetic layers are magnetized in opposite directions and a current is passed through them along the direction of magnetization, half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing the non magnetic layer. Once these electrons have crossed the non-magnetic layer, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one of the layers must be permanently fixed, or pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic (AFM) material. The other layer, by contrast, is a xe2x80x9cfree layerxe2x80x9d whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface of a magnetic disk). Structures in which the pinned layer is at the top are referred to as top spin valves. Similarly, in a bottom spin valve structure the pinned layer is at the bottom.
Although not directly connected to the GMR effect, an important feature of spin valve structures is a pair of longitudinal bias stripes that are permanently magnetized in a direction parallel to the long dimension of the device. Their purpose is to prevent the formation of multiple magnetic domains in the free layer portion of the GMR sensor, particularly near its ends. Thus longitudinal bias is responsible for the stability of a spin-valve recording head. It is usually achieved by an abutted-type junction formed during hard bias and lead deposition. Longitudinal bias layers may be implemented as permanent magnets by using a hard magnetic material or they may be compounded of a magnetically soft material whose magnetization is maintained by means of a contiguous antiferromagnetic layer.
As the dimensions of spin valve heads continue to be reduced, a number of difficulties in the associated fabrication processes are being encountered. When permanent magnets are used to supply contiguous junction stabilization, the junction region is always a major source of noise and instability due to the uncertain shape and coercivity reduction of the hard bias film at the junction area. It is known that the longitudinal bias permanent magnet is not as hard at the junction region as it is in the bulk material because of its long tapered tail. An external magnetic field can cause irreversible reversal of the hard magnet, thereby causing a hysteric response by the head. This coercivity reduction becomes more severe as the hard magnet becomes thinner.
In U.S. Pat. No. 5,664,316 it was shown that a ferromagnetic/antiferromagnetic coupled layer could be used to replace a permanent magnet. It was also claimed that when a ferromagnetic/antiferromagnetic coupled layer is used, magnetic instability in the junction area will be reduced. In U.S. Pat. No. 5,528,440 it was proposed to apply this structure to provide longitudinal bias in a spin valve head.
In both of the above-cited patents, the ferromagnetic/antiferromagnetic bi-layer was deposited directly after the removal of the side region of the sensor stack. This approach requires very precise control of the end point of the etching process. As an example, consider a bottom spin valve structure in which the GMR stack is ordered from top to bottom as: free/non-magnetic/AP1/Ru/AP2/AFM, where AP1 and AP2 are two ferromagnetic layers magnetized to be anti-parallel to one another, AP2 being the layer closest to the AFM.
When such a structure is being made, etching (to form the GMR pedestal) cannot be allowed to proceed beyond the nonmagnetic layer. Etching down to AP1, AP2, or the AFM layer will result in head instability due to the transverse exchange coupling field from the AP or the AFM layer. As sensor thickness is reduced even further, this scheme becomes more and more difficult to control due to the criticality of the end point.
Another approach to avoiding the above-discussed problem has been to over etch so all GMR stack layers get removed along the pedestal""s sides. However, this method introduces a new problem viz. an increase in the incidence of shorts between the sensor to the bottom shield since some of the lower dielectric layer must also get removed.
A routine search of the prior art was performed with the following references of interest being found.
In U.S. Pat. No. 6,185,078, Lin et al. disclose using a layer of nickel oxide as a pinning layer for a NiFe bias layer. Since the NiO is an insulator, any insulation lost as a result of removing too much of the lower dielectric gets replaced. A glue layer of Ta is used under the lead layer. U.S. Pat. No. 5,664,316 (Chen et al.), U.S. Pat. No. 5,705,973 (Yuan et al.) are all related patents. U.S. Pat. No. 5,856,897 (Mauri) shows a stabilization layer under the lead layer.
It has been an object of the present invention to provide a bottom spin valve structure with longitudinal bias of improved stability.
Another object has been to provide a process for manufacturing said structure.
A further object has been that said process allow substantial latitude for end point control during the etching of the GMR pedestal.
These objects have been achieved by inserting a decoupling layer between the antiferromagnetic layer that is used to stabilize the pinned layer of the spin valve itself and the soft ferromagnetic layer that is used for longitudinal biasing. This soft ferromagnetic layer is pinned by a second antiferromagnetic layer deposited on it on its far side away from the first antiferromagnetic layer. The presence of the decoupling layer ensures that the magnetization of the soft layer is determined only by the second antiferromagnetic layer.