Introduction - Hard Disk Drive: Mechatronics and Control - page 10

Figure 1.4: Thin fi lm inductive head and MR sensor.

The write head is a thin fi lm coil structure that puts out a magnetic fi eld
when current is passed through the coil. This head is commonly known as thin
film inductive head or TFI head. The core of this head has a tiny gap which
fl ies above the disk in close proximity (Figure 1.4). The magnetic fi eld created
in this gap polarizes the medium in the area of the disk that passes under the
write head when current is allowed to fl ow through the coil. Polarity of the
magnetic fi eld, and therefore the magnetization of the media, can be reversed
by changing the polarity of the current fl owing through the coil. Whenever such
reversal is made, a transition of magnetization is created on the medium. Since
the disk is spinning, these polarized areas or bits are arranged along concentric,
circular tracks. In older generation drives, inductive heads were also used for
reading the recorded data bits. Once a sequence of transitions is created on the
track (using inductive write head), it becomes a spatial distribution of series
of tiny magnets with alternating polarity. Changes in the polarity reverse the
fl ux emanating from these tiny magnets. When the disk spins, this spatial
distribution becomes a temporal distribution sensed by the read head placed
above the track. If an inductive head is used as the read sensor, it experiences
the fl ux reversal as function of time. According to the principles of magnetic
induction, a voltage is produced between the two terminals of a coil when it
is placed in a time-varying magnetic fi eld. The voltage induced in the coil of
the inductive read sensor can be expressed mathematically as V ind = −N d dt ,
where N is the number of turns in the coil and Φ is the magnetic fl ux. Since
the fl ux has a spatial distribution, we can say that

dt = d dx d dt = v d dx ,where
'v' is the relative velocity between head and medium. So the output of the
inductive read sensor is proportional to the velocity of data track with respect
to the head.
The read head is usually made narrower than the width of the data track to
avoid interference from adjacent tracks while reading. The dimension is about
60%-80% of the track width. As the track pitch continues to get smaller, so
does the width of the read head. Unfortunately, sensitivity of an inductive
read head decreases with reduction in its dimension as narrower head allows
fewer turns of the coil and, therefore, smaller amplitude of the induced voltage.
The Magneto-resistive (MR) sensing provides the solution for designing narrow

dΦ