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A peculiar magnetic domain pattern (so-called cellular domain structure) was recently observed in Fe–Ga alloys, which was claimed to be responsible for desirable hysteresis-free and isotropic magnetization behaviors and to be associated with the reported non-Joulian magnetism [H. D. Chopra and M. Wuttig, Nature 521, 340 (2015)]. So far, such cellular domain phenomenon is observed only in Fe–Ga single crystal specimens of thin disk shape and with special thermal history. In this Letter, we report that the cellular domain structure is also observed in as-cast polycrystalline Fe–Ga alloys that are simply synthesized by arc-melting without special thermal treatment. For comparison, the cellular domains observed in an Fe–Ga single crystal specimen of rectangular prism shape are also presented. The cellular domain structure is discussed in light of a surface domain phenomenon and is found to be well interpreted by the Stephan model [Von W. Stephan, Exp. Tech. Phys. 4, 153 (1956)] based on classical domain theory, contrary to the Chopra–Wuttig model relying on the hypothesized charge density wave theory.

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The so-called cellular magnetic domain structure has been widely observed in Fe–Ga alloys of different compositions and heat treatment. It has attracted attention for producing desirable magnetic properties and also for arousing controversy over its cause and identity. Two existing models, one based on novel charge density waves and the other based on traditional V-lines from the classical magnetic domain theory, give contradictory interpretations of the cellular domains in Fe–Ga alloys, which remain to be clarified. The cellular domains observed in Fe–Ga alloys so far are highly periodic, consisting of parallel chains of rectangular cells. This paper reports on the presence of various deviations in the cellular domains from the previously known ideal periodic cellular structure in Fe–Ga alloys and explores the implications of those variations. The variations include changes in the cell shape, spacing, branching, and nesting. It is shown that the observed variations in the cellular domains can be explained well by the V-line model where the competition between elastic and wall energy drives the pattern formation process. In comparison, the disagreements of the charge density wave model with the experimental observations are addressed. Similar cellular domain variations observed previously in Fe–Si alloys are also discussed, confirming the generic aspects of the cellular domains and their variations in cubic magnetostrictive materials. The findings provide insights into the magnetic domain phenomena in Fe–Ga alloys.