Spin Stand Study of the Longitudinal Field Effect on Servo PES

In a perpendicular recording environment, when reading at an offtrack position, the head can experience a local field in the longitudinal direction from the written track. This field can change head sensitivity depending on the polarity of the written magnet and the offtrack direction. This effect is particularly strong when reading null servo position error signal (PES) pattern, where two adjacent servo tracks are written with opposite polarity. This paper studies the effect of this longitudinal field on the PES signal using a spin stand, which permits multiple PES patterns to be evaluated side-by-side. The result shows that the effect can distort the PES linearity. The extent of its impact depends on the servo PES transition density and head design. As a result, design compromise must be made.