Why doesn't my magnet stick to the hhd magnetic plate?

Pithikos

I opened my harddisk and put a magnet on the plates but the plate acts like a piece of wood. I would expect the magnet to stick to the plate. It is a magnetic plate afterall! My "magnet" is in fact a row of neo cube balls which are quite strong magnets and small. So I would surely feel or see a tiny magnetic behaviour if there was one.

Then some other question arose in my brain. Are all electric conductors influenced by magnetism? Are all electric conductors even heat conductors?

sophiecentaur

The material which makes up your hard disc takes an immensely strong local magnetic field to magnetise it one way or another. The record head produces a high enough field to magnetise, (permanently - until it's re-written), a minute part of the disc (one bit's worth) in the direction it needs. All these recorded bits are magnetised permanently in 'up' and 'down' directions - if you looked with a microscope or read with another 'read' head, you could see this. Your permanent magnet, however, is right next to millions of bits which, on average, have the same number of Ns as Ss so the magnet is neither attracted nor repelled.
I think that a suitably prepared disc could actually be arranged to be attracted to your magnet.

I believe that it was actually years and years ago that floppy disc material was very sensitive to the moderate fields around 'normal' permanent magnets. The dire warnings were very much over-stated for years.

pallidin

Interesting thought.
But, wouldn't the strong neo magnet "flip" the repulsive "bits" and thus causing the iron oxide coated aluminum platter to be attracted?
Not sure, just asking.

sophiecentaur

No. The remnance is too high. The flux of the write head isn't that high but the flux density is. It's the flux density that determines whether a particular grain of ferrite flips or not.

pallidin

Just to clarify:
1) Is the flux density of a write head greater than that of a surface-touching neo magnet?
2) Is the relatively thin iron oxide coating resistant to domain flip from a touching neodymium magnet?

sophiecentaur

I get the feeling that your experiment demonstrated this. If you used the magnetic disc and not some other part, that is. ;-)

uart

I suspect that the domains are flipping when the neodymium magnet touches them, but the lack of noticeable force is purely due to the extremely thin layer of actively magnetic material (on the order of only tens of nanometer).

sophiecentaur

Good explanation. Now you have to put the disc bac in and try to read it.