Experimental observations of thermally excited ferromagnetic resonance and mag-noise spectra in spin valve heads

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Abstract
Thermally excited ferromagnetic resonance (FMR) gives rise to so-called mag-noise in the spin valve read heads used in disk drives. In this article, an experimental method of measuring the mag-noise power spectrum density (PSD) in a frequency range of 2.5–6 GHz is presented. The thermally excited FMR modes in the spin valve and the corresponding resonant frequencies have been observed. It is found that the resonance frequencies occur in the range from 3 to 5 GHz in today’s spin valve heads at the quiescent state. Multiple resonance peaks were observed for some spin valve read heads while others only show a single resonance peak. An external field was applied to the spin valve heads to study the field dependence of the FMR spectrum. The mag-noise level at low frequencies is at the similar level of the head’s Johnson noise for heads of track width W=0.3 μm and 50 Ω resistance. It is suggested that the thermally excited FMR PSD could be used as a rapid diagnostic tool for spin valve heads.

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