Ferrite Head Instability

Abstract

Polycrystalline MIG-ferrite heads were investigated for unstable isolated transition responses. Some of the heads exhibited a severe left-right pulse asymmetry in the form of a “foot” on the leading edge of the transition response. The foot could be attributed to crystallites, magnetically defining the leading gap edge, having an unfavorable magnetization direction. The asymmetry was found to not change with write excitation; it did not contribute head instability. Another phenomenon observed was perturbations (wiggles) in the readback signal around and during the isolated impulses. This was attributed to “illumination”, by passing transitions, of the micro-magnetic granularity of the ferrite at the airbearing surface directly adjoining the gap, as well as to hysteretic domain wall motion and delayed relaxation. The wiggle patterns changed upon write excitation; they gave rise to head instability. Pulse width distribution measurements in the presence of write excitation of the heads were found to give a good criterion for the degree of instability: the coefficient of variation σ/µ of the distributions was found to closely track error rates. It is proposed to alleviate these undesirable responses by a second (thin) Sendust layer on the leading gap edge, by smaller crystallites with a good size control, or by magnetically orienting the crystallites at the airbearing surface in a direction parallel to this surface during manufacturing. © 1992 IEEE