White-noise magnetization fluctuations in magnetoresistive heads

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Abstract
Thermal magnetization fluctuations in magnetoresistive (MR) heads for magnetic hard-disk storage are a fundamental limit on their signal-to-noise ratio. The resultant noise is essentially frequency flat (white), scales with head sensitivity as signal does, but increases inversely with sensor volume. It will impact the present course of industrial R&D efforts toward geometric scaling and increasing raw head sensitivity to achieve increased areal storage densities and data rates. Magnetization noise is shown to exceed Johnson noise in 0.4 μm sensor size, giant-MR spin-valve heads designed for ∼20 Gbit/in.2 areal storage density. The basic physics underlying the experimental results is shown to be consistent with predictions from the fluctuation–dissipation theorem.