# Magnetic Moment of a Rare-Earth Magnet?

1. Aug 6, 2011

### fasc

I was trying to find the value of the magnetic field at a distance x from a rare-earth magnet and came across this equation:

$B=\mu_o*2\mu/(4\pi*d^3)$

Does anyone happen to know the magnetic moment, $\mu$, of a rare-earth magnet? Nothing specific, an order of magnitude value is fine.

Thank you!

2. Aug 7, 2011

### Enthalpy

For a neodymium magnet that has permeability near 1 and produces around 1.1T in short-circuit, you could:
multiply its volume by 1.1T/(4pi*10-7)
that is, volume * 875kA/m.

With neodymium, this approximation is not bad. True data there for instance:
http://www.cy-magnetics.com/CY-Mag-NdFeB.pdf
remember 1 G = 100 µT and 1 Oe produces 1 G in vacuum. These were the CGS units.

AlNiCo magnets, as well as iron magnets, have an important permeability, so you can't convert their short-circuit induction to the coercivity as if they were vacuum plus a coil. But with ferrite magnets, the approximation holds more or less.