4 DISCRETE MAGNETIC STATES OF KEEPERED MEDIA

Keepered recording media includes a high permeability (mu) keeper laye r overlying a conventional longitudinal thin-film magnetic layer. Vibr ating sample magnetometer (VSM) hysteresis loops show four discrete ma gnetic states in the superposition of two exchange decoupled loops: on e is the high coercivity storage layer and the other is the low coerci vity keeper. The keeper layer provides an adjustable reluctance shunt across the head gap and a closure between recorded transitions. Keeper ed media output power is attenuated more than 20 dB until a bias curre nt is applied to the reproduce head windings. Head bias current and re corded wavelength, lambda, have a direct linear effect on both head ou tput level and mirror-symmetric peak pair offset within a defined vari able reluctance zone. Head output level within the variable reluctance zone is 1 dB per mA turn of bias and normalized peak pair offset is /-1% or +/-3 degrees per +/-mA turn of bias, with phase reversal at ze ro current. Variable reluctance amplitude and phase functions are line ar until keeper layer saturation at +/-17 mA turns bias. A method of e ncoding binary data into the four magnetic states is described. Eight user bits are coded into five flux changes and recorded at 90 degrees increments. Reproduce circuitry uses a 2F phase locked loop keeper hea d bias which decodes the four magnetic states into four voltage levels at 90 degrees spacing. The described (1,5) four level code requires o ne less flux change per 8 user bits than (1,7) code, and is stable onl y within the variable reluctance zone of keepered media. (C) 1997 Amer ican Institute of Physics.