Magnetic Properties of an Iron—Nickel Single Crystal Ordered by Neutron Bombardment

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Abstract
It has been shown that a new ordered structure can be obtained in an Fe‐Ni (50%–50%) alloy, by irradiating it with neutrons below 320°C. This ordered structure is tetragonal (AuCu type) and leads to a very high magnetic anisotropy. We have studied the magnetic properties of a single‐crystal disk irradiated with neutrons at 295°C, with an applied magnetic field along the [100] direction. The x‐ray superlattice lines show that after irradiation the sample is made of small ordered crystallites with their tetragonal axes distributed along a [100], [010], or [001] direction. The study of the anisotropy energy in the sample plane [011], and the magnetization curves drawn for different directions, show that this sample presents the characteristics of a uniaxial substance. But if we express, in a classical way, the anisotropy energy by K1 sin2θ+K2 sin4θ, we find experimentally K1=3.2·106 ergs/cm3 and K2=2.3·106 ergs/cm3. This high value of K2 is not in agreement with the conception of independent crystallites. One of us (L.N.) has made a theory taking into account the exchange interaction between the crystallites which are in fact smaller than the magnetic domains. The formulas obtained in this way give a good agreement with the experimental results.

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