Quantum effects in magnetic pickup at high-areal storage densities

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Abstract
This paper reports on a search for physical limitations of the inductive playback mechanism of magnetic recording at high-areal storage densities and high reading frequencies. Such a limitation is set by Johnson noise in the head wire material at about 108–109 bit/cm2 at room temperature. Cooling the head leads to a reduction of the head resistance, which goes far beyond classical expectation because of a quantum-size effect which becomes important when the mean free path of the wire material becomes larger than the wire diameter. At the same time the inductive pickup signal becomes a periodic function of the flux linked by the head with period Φ0/2, where Φ0=h/e is the quantum of flux. No (irreversible) inductive pickup signal can be expected at any temperature when the flux of a magnetic bit unit is less than Φ0/2, or at storage densities larger than 109–1010 bit/cm2.

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