Biasing materials for anisotropic magnetoresistive and spin-valve read heads

As recording densities rapidly rise, traditional inductive heads are no longer suited for read-back of data due to their lower signal levels and lower sensitivities. Dedicated heads, optimized for read-back, using the anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) effects are more attractive. The output signal from devices based on both of these phenomena is, however, susceptible to noise arising from domain wall motion in the sensor element. One of the more common methods of suppressing this ‘Barkhausen noise’ employs the exchange coupling between antiferromagnetic and ferromagnetic layers to ensure that the ferromagnetic sensor is in a single domain state. Another popular method uses the magnetostatic fields from permanent magnets to saturate the sensor. An additional, and increasingly important, application for exchange biasing materials is for pinning one of the ferromagnetic layers in a spin-valve type of sensor. FeMn has traditionally filled the role of exchange biasing material, but due to its poor corrosion resistance and poor thermal stability, it is proving inadequate. Thus, there is great interest in the storage industry for a suitable replacement.; In this thesis, the performance of CoNiO, NiO, NiMn and IrMn as exchange biasing materials and CoCrPt as a permanent magnet has been evaluated. The significant material properties investigated were biasing fields, thermal stability, thickness effects and corrosion resistance. The oxide materials had excellent corrosion resistance, but NiO had poor exchange fields and CoNiO had a poor blocking temperature. NiMn had the best exchange field with an interfacial exchange coupling of 0.25 erg/cm2, but it required high temperature annealing in order to realize this. It also had good thermal and corrosion resistance properties. IrMn was found to have a good combination of these properties, the weakest being its corrosion resistance, which was nevertheless better than FeMn. Spin-valves fabricated with IrMn as the pinning material showed excellent magnetic and thermal properties with MR ratios as high as 10% and good spin-valve responses up to 210°C. The blocking temperatures of the oxides and IrMn were found to diminish with decreasing thicknesses of the antiferromagnets. This effect was determined to be consistent with a finite-size-scaling phenomenon. Substrate bias was found to enhance the coercivity of CoCrPt permanent magnets when deposited on 25 Å Cr underlayers. These magnets had coercivities in the range of 1400–1900 Oe and a remanent magnetization of 700 emu/cm3.; In order to better evaluate the performance of these materials, small trackwidth devices utilizing them were fabricated. Permanent magnet stabilized sensors in the abutted configuration had large biasing fields of 50–120 Oe for sensors in the 5–1.5 $m$m trackwidth range. These values showed good agreement with finite element modeling results of the permanent magnet junction. Stabilized elements of NiO with 1.6 $m$m trackwidths showed some signs of Barkhausen noise, while NiMn stabilized elements were noise free. IrMn pinned spin-valves had very good performance for narrow trackwidth elements, with MR ratios as high as 5% and pinning fields of 650 Oe for 0.5 $m$m trackwidth devices.