Micromagnetic tests of techniques for reducing pole tip remanence of high density perpendicular write heads.

Abstract

A multi-scale fast Fourier transform (FFT) based micromagnetic model has been developed to simulate erase after write (EAW) for a 2.4 T FeCo solid pole writer. The simulated remnant state of the writer shows vortices at the pole tip, break and paddle regions that qualitatively matches an experimental MFM image. Dynamic responses show that EAW worsens with a longer breakpoint. Sensitivity of EAW with breakpoint is in good agreement with the experimental data. Modeling suggests that cross track anisotropy reduces EAW risks; however, perpendicular anisotropy is found to be detrimental to EAW. Simulations show that EAW risks are substantially reduced when a uni-polar demagnetization pulse of polarity opposite to that of the last write is applied to the writer.

Using the model, the response functions of uni-polar demagnetization pulses have been modeled for reducing EAW events. Simulations show that the value of the initial field created by the demagnetization current is the most effective parameter in reducing pole tip remanence. To avoid driving the head in the opposite direction with the demagnetization pulse, it is important to ramp down quickly with a time constant of about 500 psec. In-plane exchange is found to affect EAW quite significantly; just 25% lower exchange reduces EAW fields by 30%. Modeling shows that introduction of antiferromagnetic coupling in the write pole reduces EAW significantly. Modeling also suggests that non-magnetic holes with in plane dimensions of 35 nm in the middle of the breakpoint region reduce EAW by ~35%. The underlying mechanism to reduce EAW is to make vortex formation in the pole tip energetically inexpensive.

Micromagnetics is used to design an unshielded perpendicular writer for an areal density of 1 Tb/in 2 . The head consists of a probe-type tip protruding from a collar. The tip has saturation magnetization ( M S ) of 24 kG while the collar has lower M S . The magnitude and orientation of anisotropy field ( H k ) in the tip is varied to obtain the best recording performance. The combination of high anisotropy write tip with low M S collar is shown to produce effective write fields in excess of 19 kOe and less than 20% track erasure for 10 7 passes. Introduction of in-plane anisotropy within the pole tip reduces head remanence sufficiently that on-track erasure exceeds a benchmark of 10 8 passes. The damping constant in the Landau-Lifshitz-Gilbert equation is varied to improve the frequency response. With damping constant equal to 1.0, simulations show that the head is capable of switching in approximately 0.15 ns.