The ferromagnetic anisotropy constant $K_1$ has been accurately measured by the magnetic torque method for a group of iron alloys containing up to 13.7 atomic percent silicon. The points fall on two intersecting straight lines given by the equations: $\{K_1\times {10}^{-5\mathrm{}}=5.29-0.279\mathrm{}\mathrm{A} A<\mathrm{}9.86\mathrm{}\}\{K_1\times {10}^{-5\mathrm{}}=4.43-0.1915\mathrm{}\mathrm{A} A>\mathrm{}9.86\mathrm{}\}$ $K_1$ is expressed in ergs/${\mathrm{cm}}^3$ and $A$ is the atomic percentage of silicon. The value for iron is considerably higher than the one due to Akulov which has often been quoted in the literature. It is shown that the latter is unreliable because it is an average value based on the highly discordant results of several investigators. Torque measurements were made on the identical ellipsoid of iron for which Piety measured the magnetization curves along the simple crystallographic directions and a large discrepancy was found between the values of $K_1$ calculated by these two methods. This leads to the belief that the present-day theory of ferromagnetic anisotropy is in need of some revision, possibly in connection with the higher order terms in the magnetic energy equation. When $K_1$ is plotted against the atomic percentage of silicon, the change in slope occurs at very nearly the same concentration as the similar change in slope found by Jette and Greiner for the lattice parameter $a_0$. An attempt is made to explain the occurrence of these breaks in terms of a change from a hypothetical superlattice with a silicon concentration of 3/32 to a partially formed ${\mathrm{Fe}}_3$Si superlattice.

• Received 14 September 1939

DOI:https://doi.org/10.1103/PhysRev.56.1231