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What is the main reason permalloy, Fe with about 78% Ni, shows an extremely high permeability?

  1. AAdding Ni roughly doubles the saturation magnetization relative to pure iron, giving a large flux density quickly even in a weak field
  2. BIts magnetocrystalline anisotropy and magnetostriction are both nearly zero, so domain walls move and magnetization rotates easily
  3. CRapid quenching from the melt refines its grains to nanometer sizes, so that the magnetocrystalline anisotropy averages out and cancels
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Answer: B. Its magnetocrystalline anisotropy and magnetostriction are both nearly zero, so domain walls move and magnetization rotates easily

A high permeability requires the magnetization to change greatly in a weak field. Magnetization rotation is hindered by magnetocrystalline anisotropy, and if there is magnetostriction, the magnetoelastic energy coupled to internal stresses hinders domain-wall motion. In the Fe–Ni system, at compositions around 80% Ni both the magnetocrystalline anisotropy constant and the magnetostriction constant approach zero, and heat treatment that adjusts the degree of ordering can make the anisotropy even smaller, so a very high permeability is obtained. The saturation flux density of permalloy is considerably lower than that of pure iron (about 2.2 T), so the option citing a saturation magnetization larger than that of pure iron is wrong. Making grains nanometer-sized so that the anisotropy averages out is the idea behind nanocrystalline soft magnetic alloys, not the reason for permalloy.

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More Magnetic materials questions

  1. Grain-oriented electrical steel (Fe with about 3% Si) used for transformer cores has especially low loss and high permeability when magnetized along the rolling direction. Which reason is correct?
  2. Assume the eddy-current loss of electrical steel sheet is proportional to t²f²Bm², where t is the sheet thickness, f the frequency and Bm the peak flux density (with the same resistivity). At the same frequency and flux density, by roughly what factor does the eddy-current loss change if the thickness is reduced from 0.50 mm to 0.35 mm?
  3. An alternating field at 50 Hz is applied to electrical steel sheet with an electrical resistivity of 4.8×10⁻⁷ Ω·m and a relative permeability of 5000. Which value is closest to the skin depth δ = √(2ρ/(ωµ))? Take ω = 2πf, µ = µᵣµ₀ and µ₀ = 4π×10⁻⁷ H/m.
  4. On the demagnetization curve of a permanent magnet, let the magnitude of the reverse field that brings the flux density B to zero be the coercivity HcB, and the magnitude of the reverse field that brings the magnetization M to zero be the intrinsic coercivity HcJ. From the relation B = µ₀(H + M), which relationship between them is correct?
  5. For an ideal permanent magnet whose demagnetization curve is the straight line B = Br + µ₀H (with a sufficiently large HcJ) and whose remanent flux density is Br = 1.40 T, which value is closest to the maximum energy product (BH)max? Take µ₀ = 4π×10⁻⁷ H/m.