Measurement and analysis of noise sources in giant magnetoresistive sensors Up to 6 GHz

Abstract

This paper describes the electrical and magnetic noise sources prevalent in giant magnetoresistive (GMR) sensors. At lower frequencies (<1 MHz), 1/f noise is generally dominant. Electrical (Nyquist-Johnson) and magnetic thermal fluctuation noise are dominant above 1 MHz. Because the GMR sensor resistance is current dependent (i.e., nonlinear), its electrical noise is higher than would be expected from its dc resistance. Noise measurements on saturated GMR sensors indicate that the ac small-signal resistance is a better indicator of electrical noise. A thermal-electric model is presented for the GMR sensor that is useful for interpreting the ac resistance and electrical noise. Along with midfrequency (1-100 MHz) magnetic thermal noise data, noise spectra (up to 6 GHz) show a magnetic noise resonance around 4-5 GHz. The paper presents a simple single-domain, Landau-Lifshitz model to explain the resonance behavior. In some cases, two significant resonance peaks appear; we believe this stems from nonuniform free-layer magnetization caused by insufficient hard bias. Resonance data with externally applied transverse fields show a wide unsystematic variation in linewidth (corresponding to damping constant α = 0.009-0.033). However, the low-frequency noise power, roughly proportional to the linewidth, does not show such a variation.