Reduced exchange coupling in perpendicular multilayers
Liu, Wen Hong
Lairson, Bruce M.
Doctor of Philosophy thesis
The ideal magnetic switching mechanism for magnetic recording is isolated domain coherent rotation. However, in typical Pd/Co and Pt/Co multilayers with high coercivity, the dominant switching mechanism is domain wall motion, which causes noise in the readback signal. Wall motion, however requires exchange coupling between grains. Oxidation and carbon incorporation into Pd/Co multilayers have been investigated as ways of reducing the exchange coupling among adjacent grains. This reduction of magnetic coupling reduces the length scale over which incoherent switching occurs. Kelly-Hankel plots and switching radius ratio have been used to compare interactions and domain sizes of oxidized and carbon segregated Pd/Co multilayers. Both oxidation and carbon segregation reduce the coupling between domains, but the multilayers nevertheless remain more strongly coupled than perpendicular alloy films. A Pt/CoCrTa multilayer structure has been invented which display little intergranular exchange coupling and nearly 100% remanence in the perpendicular direction. Magnetic recordings of continuous square wave patterns have been made and characterized using an inductive contact recording transducer and by magnetic force microscopy. Written transitions appear to retain good fidelity at more than 300,000 transitions per inch. With exchange isolation of the grains, media noise becomes negligible relative to thermal noise from the recording system electronics. Analysis of magnetic force microscopy images yields a signal to integrated noise ratio of 17dB at 309,000 transitions per inch written in a four micron wide track, which is comparable to a measured signal to noise ratio of 8dB at 350,000 reversals per inch measured using an inductive transducer at 100 inches/sec. These results are contrasted with observations on exchange coupled Pt/Co multilayers.
Engineering, Electronics and Electrical; Engineering, Materials Science