IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011 3147 Off-Transition Noise Characterization During Head Thermal Protrusion Enhao Ed Lin, Shaoping Li, and Sining Mao Western Digital, Inc., Fremont, CA 94538 USA This paper characterizes the read-back off-transition noise (OTN) in spectral, spatial, and time domains during the thermal protrusion of a magnetic head. In perpendicular recording, the OTN is typically lower than the background noise, and shows bathtub-like cross- track profile. The spatial and spectral characteristics of the read-back OTN make it sensitive to the head positing change in cross-track, down-tack, and vertical directions. In contrast to the conventional on-transition signal reading, the OTN monitoring reverses the usage of signal and noise and lowers the requirement on signal quality. Monitoring OTN during head’s thermal control can detect air bearing’s stability, head’s fly-ability, and head-lube/disk contact without Fourier transform (FT) related techniques. A single-track and single read-frequency head-lube/disk contact detection method with high simplicity, high sensitivity, and high repeatability is introduced. Index Terms—Contact detection, magnetic head, off-transition noise, read back, signal and noise, thermal protrusion, transition. I. INTRODUCTION W ITH the increasing of track density and linear density in magnetic recording, the writing field from head and read-back signal from media are getting weaker, hence the head/ disk clearance needs to be reduced to better detect the transitions written on disk. Thermal protrusion technique is widely used in hard drives to control the head/disk clearance [1]–[9]. Detecting head/disk contact due to head’s thermal protrusion is the funda- mental technique in head/disk clearance control. The sensors used for detecting head/disk contact include acoustic emission (AE) sensor, laser Doppler vibrometer (LDV), force transducer, capacitance sensor, temperature sensor, and magnetic reader [2]–[4], [7], [9]. Using magnetic reader as contact detecting sensor is the most common since each head comes with a reader. The reader-based methods usu- ally use servo system for monitoring positioning error signal (PES) [1] or Fourier transform (FT) [2], [9] for monitoring phase modulation during head’s thermal protrusion. In recent years, the reader-based contact detection methods showed more deficiency due to the weaker and less stable read-back signal and the change of air bearing design among the areal density push. The hard disk drive industry is turning to non-reader based contact detection methods although keeps looking for innovation in the reader based contact detection methods. Anyway, reader is indispensable in magnetic head and is the only proved sensor for measured flying height by Wallace equation [6]–[10]. This paper characterizes the read-back off-transition noise (OTN) in spectral, spatial, and time domains during the thermal protrusion of a magnetic head, and demonstrates its application in head-lube/disk contact detection. II. FUNDAMENTAL EXPERIMENT AND ANALYSIS In this study, off-transition noise (OTN) is the noise measured at a frequency offset from the single-tone writing frequency. In contrast to the background noise, OTN is read back after the Manuscript received February 21, 2011; accepted April 11, 2011. Date of current version September 23, 2011. Corresponding author: E. E. Lin (e-mail: enhaolin@yahoo.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2011.2144963 Fig. 1. Spectral view on off-transition noise. transitions are written on media. In this study, OTN is measured by setting read frequency off the write frequency using narrow- band filter and analog spectrum analyzer. In reality, part of tran- sition signal and transition noise may be mixed with OTN and cause jittering or modulation on read back OTN during head’s thermal protrusion. The three-dimensional views on OTN in fre- quency domain, space domain, and time domain are analyzed as follows. A. Spectral Analysis on Down-Track Dimension Fig. 1 shows that the background noise on media erased by 0.65 T alternating current (AC) and the on-track spectrum of a 2 T data pattern written on the media. The above plot shows that the noise floor drops up to several dB after the data pattern is written. In perpendicular recording, the lower frequency and stronger transitions (2 T data pattern in this example) can overwrite the higher frequency and weaker transitions (0.65 T data pattern in this example) and suppress the background noise. OTN magnitude rolls off while the read frequency moving away the write frequency (187.5 MHz). OTN can decrease dozens of dB within 500 kHz away the write frequency. When there is phase shift (either periodically or stochastically) on the signal due to air bearing instability or head-lube/disk contact, the OTN track averaged amplitude (TAA) will jitter or modulate. Because of the large magnitude difference between OTN and on-transition signal (OTS), the OTS quality is no more a concern in OTN measurement. The 0018-9464/$26.00 © 2011 IEEE