Induced anisotropy

Oneway of inducing uniaxial anisotropy is to anneal certain alloys in a magnetic field, Fig. 1. A good example is permalloy, Ni₈₀Fe₂₀, which is an fcc alloy with the property that K₁ ≈ 0. When annealed at ∼800 K, atomic diffusion proceeds in the sense of favouring head-to-tail pairs of iron atoms, which have a larger moment than nickel. The iron pairs tend to align with the field, Fig. 2. This built-in texture produces aweak anisotropy. Amorphous ferromagnets may likewise acquire a uniaxial anisotropy due to pairwise texture when annealed in a magnetic field. Similar texture can be achieved by atomic deposition of thin films in a magnetic field.
Another way to induce uniaxial anisotropy in a ferromagnetic solid is to apply uniaxial stress σ. The magnitude of the stress-induced anisotropy is K_uσ = 3/2σλ_s , where λ_s is the saturation magnetostriction discussed in the next

Figure 1: Magnetization of a thin film with induced anisotropy created by annealing in a magnetic field. The ‘sheared’ loop on the left is found when the measuring field H' is applied perpendicular to the annealing field direction. The open loop on the right is obtained when the two directions are parallel.

Figure 2: Pairwise texture induced by magnetic annealing. Pairs of atoms represented by the larger dots tend to be aligned vertically.

section. Both the single-ion and two-ion anisotropy contribute to the stressinduced effect. The largest values of uniaxial anisotropy are found in hexagonal and other uniaxial crystals. Smallest values are found in certain cubic alloys and amorphous ferromagnets.