Magnetic moments; ferro vs ferrimagnetic materials

Magnetic dipole moment

A magnetic dipole moment $m$ is a measure of the strength of a magnetic dipole (current loop or pair of poles). For a current $I$ circulating in a loop of area $A$:

$$m = IA \quad (\text{unit: A m}^2)$$

It points normal to the loop by the right-hand rule. In a field $B$ the dipole experiences a torque $\tau = m \times B$.

Atomic magnetic moment

The atomic magnetic moment is the net magnetic moment of an atom, arising from (i) the orbital motion of electrons round the nucleus, (ii) the spin of electrons, and (iii) a very small nuclear spin contribution. Filled shells cancel, so only unpaired electrons contribute. Its natural unit is the Bohr magneton:

$$\mu_B = \frac{e\hbar}{2m_e} = 9.27\times10^{-24}\ \text{A m}^2$$

Example: an iron atom has about $2.2\,\mu_B$ in the metal.

Ferromagnetic vs ferrimagnetic materials

 Ferro:  ^  ^  ^  ^       Ferri:  ^  v  ^  v
                                   (large ^, small v)