I read that hard disk read heads use "giant magnetoresistance" so I assumed that I could measure a resistance change in the presence of a magnet. Unfortunately I failed.
The read head came from a 3 headed Fujitsu laptop HDD. It had 4 connections so I assumed two were read and two were write. Once wired I measured about 5 ohms and 1/2 micro henry on both pairs and got no change placing or wiggling a magnet in front of it. Connections were very difficult to make (microscope and air heater), but I am reasonably sure they were good. There was no connection between pairs.
Should I be able to measure a resistance change this way?
Best Answer
In a word, no.
First of all, GMR heads need to be biased to a particular operating point in order to see the GMR effect at all.
Secondly, the heads are physically designed to be most sensitive to a magnetic field imposed across a tiny gap that rides very close to the surface of the disk. On a modern HDD, that gap is extremely narrow — it's actually smaller than the tiniest transistors in a modern VLSI chip! To whatever extent is possible, the head is designed to be insensitive to all other imposed magnetic fields.