Tunnel magnetoresistance (TMR) read sensors, such as current-perpendicular-to-plane (CPP) TMR read sensors, are frequently employed as magnetoresistive sensors in the magnetic read heads of hard disk drives. Thermal magnetization fluctuations, also known as magnetic noise, in the ferromagnetic free layer of TMR read sensors have been recognized as a significant source of intrinsic head noise. In a TMR read sensor, the magnetic noise scales inversely with the read sensor's volume. It is believed that the magnetic noise poses a fundamental limit to signal-to-noise ratio (SNR) of a magnetic recording system.
One of major problems in future generation of TMR read sensor devices is to obtain the highest ΔR/R of the magneto-resistance or high utilization factor while further reducing the magnetic noise. However, the magnetic noise increases as the read sensor's dimension is scaled down, and the enhancement of permanent magnet (PM) strength leads to reduction of the utilization factor.
Recently, one of the thrust areas in the TMR read sensor development has been in the reduction of sensor dimensions while maintaining a sizable ΔR/R (e.g., >100%) by optimizing fabrication process. However, as the read sensor dimensions are further reduced, the magnetic noise issue becomes more and more significant in terms of the read sensor's SNR performance.