Method to measure signal decay and a device therefor

Details Method to measure signal decay and a device therefor Method to measure signal decay and a device therefor

2001-06-14

2003-10-07

Barlow, John (Department: 2863)

Electricity: measuring and testing

Magnetic

Magnetic information storage element testing

C360S025000

Reexamination Certificate

active

06630824

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an apparatus and method for measuring signal decay of magnetic recordings.
BACKGROUND
While the areal density in magnetic hard disk recording has increased, it is understood that thermal activation processes may impose a limit on the ultimate recording density that can be achieved. Thin film media have a granular structure with grains well below the single domain size. There is a general agreement that maintaining adequate signal-to-noise ratio, SNR, at higher recording density requires a reduction of grain size. Very small grains become superparamagnetic. See C. P. Bean and J. D. Livingston, “Superparamagnetism,”
J. Appl. Phys,
vol. 30 Suppl., pp 120S-129S, April 1959. That is, they remain (internally) magnetically ordered, but lose hysteresis. When the superparamagnetic regime is approached, hysteresis vanishes gradually and one observes a decrease in coercivity with grain size, as reported by Kneller and Luborsky. See E. F. Kneller and F. E. Luborsky, “Particle size dependence of coercivity and remanence of single domain particles,”
J. Appl. Phys.
vol. 34, pp 656-658, March 1963. For magnetic recording applications, the onset of superparamagnetism manifests itself in two ways: (1) the coercivity becomes time dependent and (2) the recorded information becomes unstable over time. See H. J. Richter, “Longitudinal recording at 10 to 20 Gbit/inch
2
and beyond,”
IEEE Trans. Magn.
vol. 35, pp. 2790-2795, September 1999. Thus, the proper choice of grain size for high density recording media reflects a trade-off between the stability of the recorded information and achievable SNR.
Since a recording must not decay significantly over the lifetime of a recording device, a realistic assessment of the stability requires an extremely precise measurement of signal decay. If the signal decays at a rate of 1% per order of magnitude (decade) in time, for example, a signal change of only a fraction of 1% has to be detected during a measurement ranging from 0.1 to 10,000 seconds. In this invention, “long term” means a decade or more while “short term” means 10,000 seconds or less. The theory of thermal activation predicts that elevated temperature accelerates signal decay. Magnetic fields, such as demagnetizing fields, also accelerate signal decay.
For practical use, the information stored magnetically typically has a lifetime of several years. The magnetization or signal decay varies in most cases logarithmically with time. See R. Street and J. C Woolley, “A study on magnetic viscosity,” Proc. Roy. Soc., A62, pp 562, 1949. In order to assess a priori what the signal decay of recorded information would be after several years, one has to measure the signal loss at a reasonable time period after recording and extrapolate thereafter. Media that could exhibit a significant amount of signal decay after several years of recording are obviously not practically useful. On the other hand, “good” media that would exhibit little signal decay after several years of recording would likely exhibit almost negligible signal decay at a reasonably short period after recording. Therefore, prior to this invention, the challenge facing a person trying to develop a “good” recording medium was to reliably measure short-term signal decay to accurately predict the long-term performance of the medium. Prior to this invention it was not possible to reliably measure extremely small changes of the recorded signals because of the following two experimental problems:
(1) Thermal drift: Over time, the head moves across the written track due to temperature changes of the environment.
(2) Sensitivity changes of the transducer, i.e. the recording head: Typical sensitivity changes are of the order of a few percent. These sensitivity changes add to noise of the measurements and, therefore, one is no longer able to detect small changes in the signal.
To eliminate the thermal drift effects, one has to reposition the head using a scan of a reference track, which is also called control track (CT). To account for the transducer sensitivity change, the prior art teaches the CT should be aged. See P. Dhagat, R. S. Indeck, M. W. Muller, “Spin-stand measurements of time and temperature dependence of magnetic recordings,”
J. Appl. Phys,
vol. 85, pp 4994-4996, April 1999 and Y. Zhang and H. N. Bertram, “Thermal decay in high density disk media,”
IEEE Trans Magn,
vol. 34, pp 3786-3793, Sep. 1998. The aging process assures that the signal change of the aged reference track can be neglected and one has a stable reference signal.
However, the prior art teaches that the CT and the decaying track, also called the track of interest (TOI), should be separate tracks. Therefore, by the prior art methods, any correction of the signal of the CT requires swapping tracks and performing a track scan of the CT while a track scan of the TOI has been stopped. During this time period, typically, of a few seconds, when the track scan of the TOI has been stopped, the head sensitivity could change. Although the referencing technique improves the quality of the data, the data still remain corrupted by noise and small magnetization changes remain undetected.
The present invention describes a method for measuring very small changes in recorded signals on a spin-stand tester and an apparatus therefore. Particularly, this invention focuses on correcting sensitivity changes of the transducer. The present invention deals with a technique that allows the measurement of signal changes much more sensitively than that possible by the prior art methods. In addition, data could be acquired one order of magnitude faster than that done using the prior art methods.
SUMMARY OF THE INVENTION
This invention is directed to an apparatus and a method for measuring the decay of the magnetic properties of recording media. Particularly, the invention is directed to an apparatus and a method for measuring the short-term signal decay of disk recording media.
One embodiment is a system for measuring signal decay, comprising means for positioning a recording head and a recording track comprising a control track and a track of interest located on a track of the recording track. The means for positioning a recording head could comprise a head positioner such as that disclosed in U.S. Pat. No. 6,166,536.
Another embodiment is a method for measuring signal decay, comprising reading an information signal from a control track and reading an information signal from a track of interest, wherein the control track and the track of interest are located on a track of a recording track. The method could further comprise writing data onto the control track. The method could further comprise recording left and right pilot tones on each side of the control track. The method could further comprise locating a position where the left pilot tone reduces to 30% of a maximum value of the left pilot tone and locating a position where the right pilot tone increases to 30% of a maximum value of the right pilot tone. The method could further comprise aging the control track after writing data to the control track. The method could further comprise monitoring the left and right pilot tones while aging the control track. The method could further comprise aligning a recording head with respect to the control track. The method could further comprise writing data onto the track of interest. The method could further comprise calculating a normalized signal S(TOI)/S(CT). In a preferred embodiment of the method of this invention, the reading of an information signal from a control track and the reading of an information signal from a track of interest are done without swapping a recording head between two different recording tracks. The method could further comprise finding a track center using a track scan comprising a positioning section.
Another embodiment is a system for measuring signal decay, comprising a control track and a track of interest located on a track of a recording track. The system could further comprise a read gate and a write gate. The sy

Affiliated with

Also associated with

Rating

Method to measure signal decay and a device therefor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Method to measure signal decay and a device therefor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method to measure signal decay and a device therefor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3113809