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Which statement correctly describes the ferrimagnetism shown by ferrites such as magnetite (Fe₃O₄)?

  1. AAll atomic moments are aligned parallel, so it is essentially the same as ferromagnetism
  2. BThe moments of two sublattices are aligned antiparallel and are equal in size, so the net spontaneous magnetization is zero
  3. CThe moments of two sublattices are aligned antiparallel but differ in size, so they do not cancel and a spontaneous magnetization remains
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Answer: C. The moments of two sublattices are aligned antiparallel but differ in size, so they do not cancel and a spontaneous magnetization remains

In a ferrimagnet, the magnetic moments on two kinds of sites in the crystal (sublattices) are aligned antiparallel to each other by the exchange interaction. Because the moments on the two sublattices differ in size or number, they do not cancel completely, and a spontaneous magnetization equal to the difference remains. Macroscopically a ferrimagnet shows domains and hysteresis just as a ferromagnet does, but its saturation magnetization is smaller than that of ferromagnetic metals such as iron. Parallel alignment of all moments is ferromagnetism, and antiparallel moments of equal size that cancel is antiferromagnetism. Ferrites are oxides with a high electrical resistivity, so eddy currents hardly flow in them, and they are used as cores at high frequencies.

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More Magnetic materials questions

  1. 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.
  2. 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?
  3. What is the main reason a ferromagnet splits into many magnetic domains even without an external field?
  4. As particles of a ferromagnet are made smaller, from a few µm down to nm sizes, which statement correctly describes how their coercivity changes?
  5. Take the energy barrier for thermal reversal of the magnetization of a single-domain particle of volume V as KV (K is the magnetic anisotropy constant); for a measurement time of about 100 s, KV = 25kT is a guide to the boundary for superparamagnetism. With K = 1.1×10⁴ J/m³, a temperature of 300 K and the Boltzmann constant k = 1.38×10⁻²³ J/K, which value is closest to the upper limit of the diameter of a spherical particle that becomes superparamagnetic?