Browse Prior Art Database

In-situ Track Misregistration Measurement in Disk Drives

IP.com Disclosure Number: IPCOM000121358D
Original Publication Date: 1991-Aug-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 6 page(s) / 191K

Publishing Venue

IBM

Related People

Eaton, RE: AUTHOR [+4]

Abstract

A new method for TMR measurements is proposed. This method, referred to as the Harmonic Ratio Track Misregistration Measurement (HRTMM) method, is a non-invasive, in-situ TMR measurement method, based solely on the analogue read-back signal from the magnetic recording heads in the disk drive under test. The method is inherently calibrated. The calibration factor is only determined by the wavelength of the repetitive waveform written on the track with respect to which the TMR is measured.

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In-situ Track Misregistration Measurement in Disk Drives

      A new method for TMR measurements is proposed.  This
method, referred to as the Harmonic Ratio Track Misregistration
Measurement (HRTMM) method, is a non-invasive, in-situ TMR
measurement method, based solely on the analogue read-back signal
from the magnetic recording heads in the disk drive under test.  The
method is inherently calibrated.  The calibration factor is only
determined by the wavelength of the repetitive waveform written on
the track with respect to which the TMR is measured.

      The HRTMM method is based upon a one-time recording of a single
repetitive waveform along an entire track using a square-wave write
current with frequency fw at a linear velocity vw.  This results in a
magnetization pattern imbedded in the disk with a distance q between
two adjacent transitions of the same polarity (the so-called written
wavelength q).  The adjacent tracks are DC or AC erased.  If the read
head is somewhere near the written track, there will be an analogue
readback signal V(t) which is a periodic signal with a fundamental
frequency f1 = q/v, where v is the linear velocity of the head with
respect to the disk during readback.  This velocity is allowed to be
completely different from the velocity during writing.  The spectrum
of the readback signal V(t) consists predominantly of odd harmonic
lines at frequencies fi = i f1 (i = 1,3,5...). From this spectrum,
the instantaneous amplitude V(fm) of an odd harmonic at frequency fm
and the instantaneous amplitude V(fn) of an odd higher harmonic at
frequency fn are simultaneously measured. The first odd harmonic
frequency fm is preferably the fundamental frequency f1 (m = 1), and
the higher odd harmonic frequency fn is preferably the third harmonic
frequency f3 (n = 3).  In the following, it is assumed that the
frequencies are chosen to be f1 and f3.

      The instrument according to the HRTMM principle produces an
instantaneous output signal Vo(t) equal to a constant K times the
logarithm of the ratio of the two detected amplitudes V(f1) and V(f3)
of the two spectral lines f1 and f3 in the readback signal V(t):

      To show that this signal Vo(t) is indeed a function of the TMR,
we recall the equations by Van Herk [*].  Van Herk calculated for a
filamentary track the harmonic sensitivity C(d,x,k), where d is the
head- to-medium clearance, x is the lateral spacing between the
filamentary track and the recording head, as shown in Fig. 1, and k
is the wave number (for the fundamental wavelength, k is given by k1
= 2 pi / q).  He finds:

      Here, e is the head efficiency, N the number of turns, g the
gaplength and t the medium thickness.  The harmonic transfer function
from the magnetization M(k) on the disk to the output voltage V(k) of
the amplifier (Fig. 2) can now be determined using the above
expressions.

      The magnetization M(k) is proportional to the remanent
magnetization of the...