Effect of magnetic domain configuration on read-back amplitude variations in inductive film heads (abstract)

Abstract

Narrow track width, high-performance inductive film heads exhibit a noise phenomenon in which the read-back peak amplitude varies after writing with the head. This noise phenomenon was studied in 6- to 8-μm track width, 31-turn double layered coil, film heads using a scanning Kerr effect microscope. These heads, made from electroplated permalloy films with a magnetostrictive coefficient of − 1.6 × 10−6, were specially selected for the study to have varying levels of read-back amplitude variation by using the percent variation in the peak amplitude after repeated writes as a quantitative noise figure. A statistical sampling method was used to quantitatively measure the magnetic domain patterns in the top NiFe layer of the heads under simulated operating conditions. Quantitative correlation was found between the read-back variation and the magnetization orientation in the narrow-sloped pole region. The images show that a single 180° domain wall orientated longitudinally to the flux conduction direction in the narrow pole piece is the major source of read-back variation in these types of heads. The statistical sampling method shows that the pole domain configurations are reproducible in both the low-noise and high-noise heads. Patterned NiFe films on planar alumina substrates have the desired transverse oriented 180° domain wall configuration in the pole-shaped region, as found in the low-noise heads. These results imply that the variation of the domain configuration in the pole region is caused by easy axis reorientation due to variation in the stress-induced magnetic anisotropy in that region. We also find that the read-back noise is not sensitive to the longitudinal 180° walls found near the back closure region of the heads. This is probably due to the lower flux densities found in the back closure as compared to the pole region and the existence of other flux paths that circumvent the domain walls in the back closure region. We conclude that read-back distortion noise in 6-μm track width heads can be drastically reduced by maintaining a transverse magnetic easy axis in the sloped pole piece by either employing fabrication methods that control the stress state of the pole region or by using very small magnetostriction values in the NiFe films.1