High moment shield structures for improved linear density capability in thin film recording heads
An essential part of Hard Disk Drive (HDD) Areal Density Capability (ADC) growth is improving the linear resolution of the reader. Increasing reader shield saturation magnetisation is an attractive means to enable this as dynamic permeability would be expected to improve, thereby providing greater shielding from down-track bits. However, candidate materials tend to have poorly controlled magnetic properties, leading to the risk of reader instability. The use of thick seedlayers to control the magnetic properties of high moment shields is impractical due to reader shield-to-shield spacing constraints. In this work, ultra-thin (≤1nm) underlayers of Ni79Fe21 and Ru are shown to provide a pronounced reduction in the coercivity of both Ni45Fe55 and Co70Fe30 and establish a uniaxial anisotropy. Film stress is also significantly reduced. TEM and Fresnel-mode Lorentz TEM imaging of Ni79Fe21 seeded Ni45Fe55 reveal significant reductions in grain size and magnetic ripple dispersion, no significant texture is measured. It is proposed that initial underlayer island growth provides nucleation sites for the high moment material, resulting in a refinement of the grainsize distribution and an averaging of the local anisotropies. Ni45Fe55 layers seeded with thin Ni79Fe21 incorporated into synthetically antiferromagnetically (SAF) coupled shield structures produce reversal behaviour that matches that of SAF shields comprising Ni79Fe21. Read heads with high moment shields were fabricated and exhibit a 2.7% gain in linear resolution, in agreement with micromagnetic simulations. It is proposed that the reduced magnetisation deflection required by higher moment shields results in an improved dynamic response to media field. It has been demonstrated that use of ultra-thin Ni79Fe21 underlayers provides a practical path to realising highly permeable thin films of Ni45Fe55 with a well-defined uniaxial anisotropy, that when used in SAF shield structures provides improved HDD reader linear resolution. Thesis embargoed until 31 July 2027.
HDD, magnetism, read-write heads, permeability, magnetic shields, magnetisation ripple, magnetic recording
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