Studies Of FeCo Thin Films For Ultra-high Density Magnetic Recording Write-head

The use of high-moment pole materials in inductive write heads is an essential requirement to achieve adequate writeability at ultrahigh areal density longitudinal as well as perpendicular magnetic recording. Fe-Co alloy with composition around Fe₆₅Co₃₅ is a competitive candidate because of the highest saturation magnetization 4Ï€M_s of 24.5 kG among ferromagnetic materials. But it has large magnetocrystalline anisotropy (¹0 kJ/m³) and large magnetostriction (4-6×10^-5), which results in potentially high covercivity H_c. In this thesis, The method that uses suitable underlayers to give rise to soft magnetic properties in thin films was studied. The results are as follows:1. Magnetically soft FeCo thin films were prepared successfully using two deposition methods: (1) conventional planar magnetron sputtering. (2) facing targets sputtering.2. By planar magnetron sputtering system, excellent soft magnetic properties were obtained by choosing suitable underlayers: Ni₇₉Fe₂₁, Cu and Co₉₃Fe₇. For FeCo singelayer, the saturation magnetization is as large as 24 kG in the composition range of Fe_100-xCo_x (30≤x≤40). Films have coercivity as large as 150 Oe sputtered at room temperature. With the sputtering temperature increasing, the value of coercivity decreases rapidly to 24 Oe at 360 ℃. The soft magnetic properties of Fe₆₅Co₃₅ films are much improved by using the underlayers: 4Ï€M_s²4 kG, H_ce -11-14 Oe, H_ch 1-3 Oe, H_k 50-80 Oe. It is found that the texture of Fe₆₅Co₃₅ with an underlayer is dependent on the material and the thickness of underlayer. It indicates that soft magnetic properties are much improved with the texture change from (110) type to (200) type.3. By facing targets sputtering system, the effect of Co underlayer on stress, texture, grain size, morphology, initial growth and interface was studied systematically. The FeCo single layer shows almost isotropy with the coercivity larger than 100 Oe. FeCo/Co films exhibit a well-defined in-plane uniaxial anisotropy with coercivities of 10 Oe and 3 Oe along the easy axis and the hard axis. The effective permeability of the films remains flat around 250 up to 800 MHz. The saturation magnetostriction is 5.2 × 10^-5 and the intrinsic stress is 0.8 GPa in FeCo single layer, both are slightly reduced by Co underlayer. The Co underlayer changes the preferred orientation of the FeCo films from (200) to (110), but more significantly, reduces the average grain size from 74 nm to 8.2 nm. It also reduces the surface roughness from 2.35 nm to 0.75 nm. The initial stage and interface diffusion propertiesare also examined by TEM and XPS.4. The mechanism of remarkable reduction of coercivity in FeCo/Co films are grain size effect, which is quantitatively in agreement with the theory models for nanocrystalline ferromagnetic films: Hoffmann' ripple theory.5. In the two sputtering systems, the films exhibit opposite texture variation: In conventional sputtering, the soft magnetic properties are related to the texture transition from (110) to (200) by underlayers. In facing targets sputtering, the FeCo single layer shows (200) texture at lower thickness, as the thickness increase, the (110) starts to appear, eventually the top surface will become (110). Apparently using Co seedlayer eliminates the microstructure variation. The texture evolution was explained based on the surface energy minimum.6. Two methods were developed to tailor the in-plane magnetic anisotropy: (1) based on controlling short range atom order by change the substrate-targets distance drs in facing targets sputtering. The maximum value of/4=180 Oe was obtained at the optimum fas without deteriorating the soft magnetic properties. (2) Based on controlling stress-induced anisotropy by bending substrate. When a compressive stress is applied along the initial easy axis, the Hk is reduced. While that is applied along the hard axis, the Hk is increased. The soft magnetic properties with Ms 24 kG, HCh 2 Oe and Hk as high as 250 Oe is achieved by applying a compressive stress of 2.86 x 10u dyne/cm2 along the hard axis.