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The size of the sensor in spin-valve GMR heads has been reduced to increase the areal magnetic recording density. Thermal magnetic noise, which arises from thermal fluctuation, becomes the main source of head noise and a limitation on recording density. Insufficient abutted permanent magnetic biasing yields an asymmetrical waveform both for signal output and thermal magnetic noise. This is due to the same mechanism as that of Barkhausen noise. In contrast, it has been found that small Hex, which is the exchange coupling strength between the bottom pinned layer (Pin1) and the antiferromagnetic biasing layer, emphasizes thermal magnetic noise without affecting signal output. In a head with small Hex, the magnetization near the air bearing surface and the top of the Pin2 layer tilts in the direction of the track width and randomly flips in the opposite direction. In synthetic ferrimagnetic heads, thermal magnetic noise chiefly depends on Hex rather than unidirectional anisotropy, Hua. The value of Hua does not directly affect thermal magnetic noise. The results indicate that Hex must be considered for reducing thermal magnetic noise.

Media with excellent signal to noise properties and adequate high temperature thermal stability have been developed for over 20 Gbit/in 2 recording. The media are fabricated by dc magnetron sputtering onto textured NiP films on Al-Mg substrates. The magnetic layer is a CoCr-based quaternary alloy with (11.0) preferred orientation on a Cr alloy (002) underlayer. The average grain diameter has been reduced to an average of < 10 nm. TEM-EDX studies revealed that Cr segregation has been enhanced compared to a previous CoCrPtTa media used for 15 Gbit/in 2 recording resulting in improved Co to Cr core composition and increased grain boundary Cr content. Spinstand measurements of the thermal decay after annealing at 65 C indicate sufficient thermal stability at 400 kfci for media with Mr 0 3 memu/cm 2 consistent with magnetometry measurements.Index Terms-Longitudinal magnetic recording media, low noise properties, thermal stability, 20 Gbit/in 2 recording.

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