A tiny squeeze dramatically changes the magnetism of exotic materials

The anomalous Hall effect is so exotic that I'd never heard of it.

I suppose I should have done - it seems to have been known about since

When I was a graduate student in chemistry a colleague in the engineering department got me to read up on tellurium in the chemistry library and I managed to miss that effect, which didn't help his social life, or my self-esteem.

I suppose Mu-metal doesn't count, as it has been known about for at least half a century, so that doesn't make it exotic any more.

Mu-metal's extraordinarily high permeability does depend on it being very carefully annealed. It's popular for magnetic shielding, in the sense that people do use it, but it's so easy to wreck the permeability that nobody I knew was ever willing to try to use it to solve any practical problem.

IBM has made extensive use of mu metal starting in the 1950's to my knowledge - maybe before then.

Ed

Even after mild mechanical damage, it is still a lot better at magnetic shielding than most other materials.

It is used a lot, in the form of thick-walled pots, as the magnetic shielding for low-signal audio transformers. I presume they are 'drawn' and cut to size first, then annealed as the last stage in the process.

I came across an amusing fault where one of these transformers in a piece of BBC equipment was picking up magnetic hum fron the mains transformer. There were two lengths of brass studding running through the pot, holding the lid on at one end and holding the pot to a mounting plate at the other end. Someone had replaced one of the brass pieces with an almost-identical steel one, which was conducting the external field right through the pot.