Giant magnetoresistive (GMR) spin valve structures employing an exchange-biased layer are being extensively studied for the next generation head of high density recording system. FeMn is believed to be still one of the most promising AF(antiferromagnetic) materials, in spite of its poor corrosion resistance, resulting from the large exchange field $(H_ex)$ and the simplicity of the nonreactive process.
However, exchange coupling occurs only when the FeMn film is grown onto an appropriate face centered cubic (fcc) underlayer, eg. NiFe. The equilibrium structure of FeMn is the so-called α-phase, which is a bcc structure. Films with thickness up to a few tens of nm can be stabilized in the antiferromagnetic γ-phase by epitaxial growth on a lattice matched underlayer. Extensive attempts either by changing fcc underlayers or by utilizing buffer layers have been made to stabilize the fcc γ phase of FeMn, which is responsible for the high Hex. The materials used in an FeMn-based spin valve structure have similar crystal structures and lattice constants; namely, face centered cubic (fcc) structures with close lattice constants.(Cu : a=3.61Å, NiFe : a=3.57Å, γ-FeMn : a=3.63Å) Thus, an epitaxial growth can be obtained in FeMn-based spin valve structure. Due to an epitaxial growth, the orientation of the grains containing the upper layers (pinned NiFe layer and FeMn layer) is determined by that of the grains in the free layer. The texture of this free layer can be varied by an appropriate seed or buffer layer. It has been observed that the introduction of thin Ta seed layer can, under certain circumstances, induce a very strong (111) texture in a NiFe film grown on top of it, regardless of the substrate.
The structure and crystallographic orientation of Ta films deposited onto Si (100) substrate with different deposition power were investigated. A significant influence of the deposition power on the structure of the Ta film has been observed. A deposition power ranging from 27.2W to 55.6W was applied during deposition of the Ta layer at a constant Ar pressure of 3mtorr. By increasing deposition power, the β-Ta (002) texture increases, and therefore enhances NiFe (111) texture. Enhanced NiFe (111) texture can be explained by an epitaxial relation between the β-Ta and the NiFe structure. The (002) planes of β-Ta consist of regular rectangles with side length 10.149Å. The (111) planes of NiFe consist of regular triangles with side length 5.048Å. Thus, in one direction the Ta and the NiFe atom rows fit extremely good, differing only by 0.5%.
To examine the effect of crystallographic texture on the exchange field strength, NiFe(120Å)/Cu(30Å)/NiFe(50Å)/FeMn(100Å) multilayers were deposited onto Ta films fabricated using different deposition power. The exchange field strength tended to increase with the high deposition power applied to Ta underlayer. It is believed that the enhancement of (111) texture of epitaxially grown multilayers promotes the exchange anisotropy field.
Application of DC Substrate bias also enhanced NiFe (111) texture. To study bias effects, Ta/NiFe/FeMn films were deposited onto Si(100) substrates in a dc magnetron sputtering system. A dc substrate bias from -150V to -500V was applied during deposition of the NiFe layer at a constant Ar pressure of 3mTorr. The fcc NiFe (111) texture significantly enhanced with the substrate bias which results in a promotion of γ-FeMn (111) texture. This is because the enlarged d-spacing of NiFe(111) plane resulting from the $Ar^+$ ions bombardment reduces the misfit with the (111) plane of γ-FeMn layer. This also had an effect to promote the exchange field strength.
The magnetoresistance (MR) value of a spin valve was reported to be typically about 4% in present multilayer system. However the presently measured MR value showed only about 2%. During film growth, the magnetoresistance can be affected by defect, contamination and oxidation in the film. the role of defects is the reduction of spin-dependent scattering. By increasing Ta deposition power. defects in Ta films may also increase, It is supposed There are contamination and oxidation on the surface of a spin valve in the 1×10-6 base pressure. More higher base pressure is required. The presently measured magnetoresistance (MR) value was 2-2.5%, which is significantly lower than the maximum value( about 4%) reported in the literature. Due of the probable reasons may be the introduction of defects and impurities during processing of multilayer thin films.
