Take the magnetic moment per atom of iron (BCC, lattice parameter 0.2866 nm) as 2.22 µB. Which value is closest to the saturation magnetization Ms at 0 K, when all the moments are aligned? Take the Bohr magneton as µB = 9.27×10⁻²⁴ A·m².

Answer: B. About 1.7×10⁶ A/m

A BCC unit cell contains 2 atoms, so the number density of atoms is n = 2/a³ = 2/(0.2866×10⁻⁹ m)³ ≈ 8.50×10²⁸ m⁻³. The saturation magnetization is the magnetic moment per unit volume, so Ms = n × 2.22 µB = 8.50×10²⁸ × 2.22 × 9.27×10⁻²⁴ ≈ 1.7×10⁶ A/m. Expressed as µ₀Ms this is about 2.2 T, in good agreement with the saturation flux density of iron. About 8.7×10⁵ A/m counts the unit cell as containing 1 atom, and about 3.5×10⁶ A/m counts 4 atoms as in FCC. The moment per atom is not a whole number because the 3d electrons responsible for magnetism form a band.