This is a sensor arm of a hard disk containing a GMR or TMR magnetic sensor and a coil for creating the write field (1-3 GHz at around 2 Tesla as far as I know).

enter image description here

I'm counting 6 conduction lines, GMR sensor needs a four point measurement, the coil 2 lines. But what I'm unsure about, is this the "top" of the arm pointing towards the magnetic plate (or is this the backside view) and sit the GMR sensor and coil both in the silver rectangle at the front or only the coil, and what kind of electronics is embedded in that little black box, all conduction lines lead firstly to this, than some seem to go from there to the silver top.

It seems by measuring with a multimeter, that even small DC currents degrade the coil, so this thing is normally driven with AC current and skin effect decreases the heating. What kind of magnetic field is created by this coil (narrow band or broad band, does someone know some values?). Sorry for many small questions, just wondering if someone can shed some light on the details of such a electronic device besides the data I mentioned.

  • \$\begingroup\$ I would guess that there is active signal processing logic at the tip. I don't think you can assume it's completely passive. As such, simply multimeter measurements may not tell you anything meaningul. \$\endgroup\$ Apr 17, 2013 at 4:30
  • \$\begingroup\$ @ConnorWolf Actually you can simply identify the coil with a ohmmmeter and drive it with a ac current, measure a Tesla signal with a AC Gaussmeter. What kine of active signal processing to you assume? E.g. amplifier for maybe small GMR voltages,... \$\endgroup\$
    – James Last
    Apr 17, 2013 at 7:55
  • \$\begingroup\$ the black box is apparently a preamplifier \$\endgroup\$
    – pault
    Apr 17, 2013 at 11:34
  • \$\begingroup\$ @JamesLast - I was just guessing. It seems surprising to me that they run the tiny signal from the read head along more then an inch of wire instead of immediately amplifying it. \$\endgroup\$ Apr 18, 2013 at 1:23
  • \$\begingroup\$ engineerguy.com/videos/video-harddrive.htm This should get you starting. \$\endgroup\$ Apr 23, 2013 at 22:35

1 Answer 1


Assuming this is a TMR head, I would guess the 6 lines are for: 2 for the write coil, 2 for the the read head, and 2 for a heater. 2 lines are needed for each feature to form a complete electrical circuit.

1st the heater: just like it sounds, it is used to heat up the slider, which is the black box you are referring to. That is a purely mechanical device that controls the aerodynamics of the head (think airplane wing). Since the disk drive platter can spin pretty fast (5400 rpm ~ 60 mph I think) a lot of windage is created. The slider actually has indentations in the bottom that pull the head towards the platter just like the fins on the front of formula 1 cars pull them down into the race track. Your photo is of the bottom of the slider. Since many materials expand when they heat up, the slider has a coil that can be heated up to make the slider (and attached read/write heads) come a little bit closer to the platter. This is for fine tuning the vertical head position.

2nd the write circuit: there is a simple ferromagnetic material wrapped in a coil to change the direction of magnetization on the end of the slider (on the side facing the loading ramp extension). DC current in 1 direction will cause an up magnetization (for perpendicularly oriented magnetic media), and DC current in the other direction will cause a down magnetization.

3rd the read circuit: by far the most interesting part of the disk drive arm. This also uses a DC voltage that only needs to be oriented in 1 direction. The magnetization on the media will change the orientation of the TMR free layer and the current going back to the preamp along with it. This is how the orientation of the magnetization on the media is detected. The voltage (and thus current) are incredibly small since the signal will pass through a pre-amplifier before being passed back to the disk drive DSP engine (read channel). There is a lot to learn about TMR and would be slightly off topic but I refer you to the following pages if you want to know more:

TMR (disk drive) = http://en.wikipedia.org/wiki/Disk_read-and-write_head#Tunneling_magnetoresistive_.28TMR.29

TMR (theory) = http://en.wikipedia.org/wiki/Magnetic_tunnel_effect

GMR (industry page) = http://www.pcguide.com/ref/hdd/op/heads/techGMR-c.html

Exchange bias = http://en.wikipedia.org/wiki/Exchange_bias

Antiferromagnetism = http://en.wikipedia.org/wiki/Antiferromagnetism

Domain wall pinning = http://en.wikipedia.org/wiki/N%C3%A9el_wall#N.C3.A9el_wall

Great lecture about why pinning works = https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCUQFjAB&url=http%3A%2F%2Fneon.mems.cmu.edu%2Frollett%2F27302%2F302.L10.ppt.pin.ppt&ei=MBD-U8fWI8j0iQKSpYDoBw&usg=AFQjCNFBdP1gVrYJVou9vibDg8KZb5gFkQ


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