The use of magnetoresistive (MR) sensing mechanisms is well known. Such sensors can be used in information storage systems such as disk or tape drives, or as measurement devices for test equipment, among other things.
A transducer for reading and writing information on a medium such as a disk or tape, for example, may have a MR mechanism for sensing magnetic signals from the medium, as well as an inductive mechanism for writing magnetic signals to the medium. MR sensors or transducers that are known for disk drive heads include anisotropic magnetoresistive (AMR) sensors, canted current sensors, spin valve (SV) sensors, giant magnetoresistive (GMR) sensors, etc. To optimize sensor performance it is conventional to provide magnetic fields that bias the magnetoresistive layer or layers within the sensor, in order to remove noise that can occur due to boundary domains as well as to provide more easily interpreted signals.
Ravipati et al., in U.S. Pat. No. 5,434,826, discuss various mechanisms for providing a longitudinal bias to a MR sensor. As pointed out in that patent, it is important that the MR sensing layer has a magnetic direction that will change at a lower coercive force than that required to change the magnetic direction of the bias layers. Unfortunately, if the magnetization provided by the bias layers to the MR sensing layer is too strong, the sensing layer will not change magnetic direction under the influence of changing fields from the media. Even for the case where the MR sensing layer is able to change magnetic direction under the influence of media fields, the sensitivity of the sensing layer may be reduced by the longitudinal bias provided by the bias layers.
Moreover, bias layers abutting ends of MR sensor layers are constrained to a thickness approximating that of the sensor layers, which may be less than 1000 .ANG.. Stated differently, a primary determinant of sensor linear resolution is the spacing between shield layers surrounding the sensor, within which bias, lead, gap and any seed layers must also fit. Unfortunately, the quest for higher sensor resolution and reduced thickness may exacerbate these difficulties.
This need for thinner sensor layers differs from the thickness requirements for media layers, for which a primary determinant of resolution is head to media spacing. Thus reducing the thickness of any overcoat that separates a media layer from the head may be important whereas the thickness of layers formed under the media layer may be immaterial, except for the extra time needed to create thicker underlayers. For example, in U.S. Pat. Nos. 5,693,426 and 5,800,931, Lee et al. teach that media coercivity may be enhanced by forming a relatively thick NiAl underlayer, but that such coercivity is dramatically reduced when the NiAl underlayers are less than 100 nm in thickness.