English
Menu

Magnetic materials

What is the main material reason that neodymium magnets, with Nd₂Fe₁₄B as the main phase, can have a large coercivity?

  1. AIt is cubic with almost no magnetocrystalline anisotropy, so the magnetization is stable in any direction
  2. BIts saturation magnetization is as large as about 1.6 T, and a larger saturation magnetization always means a larger coercivity
  3. CIt is tetragonal, with a strong uniaxial magnetocrystalline anisotropy along the c axis
Show answer and explanation

Answer: C. It is tetragonal, with a strong uniaxial magnetocrystalline anisotropy along the c axis

A guide to the upper limit of coercivity is the anisotropy field HA = 2K/(µ₀Ms); the larger K is, the harder it is to turn the magnetization away from the easy axis. Nd₂Fe₁₄B is tetragonal and, owing to the contribution of the Nd 4f electrons, has a strong uniaxial magnetocrystalline anisotropy along the c axis, with µ₀HA reaching about 7.5 T. The actual coercivity is much smaller than HA because reversed domains nucleate at places such as grain surfaces, and it is raised by controlling the microstructure, such as the grain-boundary phase. A cubic crystal with almost no anisotropy is the condition for soft magnetic materials such as permalloy. The saturation magnetization of Nd₂Fe₁₄B, about 1.6 T, is large and raises the remanent flux density, but Ms is in the denominator of the formula for HA, so the coercivity does not necessarily increase.

Report an error

Practice this topic Back to topic

More Magnetic materials questions

  1. What is the main purpose of adding Dy or Tb to neodymium magnets used at high temperatures, such as in electric-vehicle motors?
  2. Which type of magnetism does a material show if its magnetic susceptibility χ is negative and very small, on the order of 10⁻⁵?
  3. Which statement correctly describes the ferrimagnetism shown by ferrites such as magnetite (Fe₃O₄)?
  4. 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.
  5. Magnetite Fe₃O₄ has the inverse spinel structure: the A sites (tetrahedral) hold one Fe³⁺, the B sites (octahedral) hold one Fe³⁺ and one Fe²⁺, and the moments on the A and B sites are antiparallel. Taking Fe³⁺ as 5 µB and Fe²⁺ as 4 µB, with only spin contributing, what is the magnetic moment per Fe₃O₄ formula unit?