Nanoscale Magnetic Structure of Non-Joulian Magnets

Strain dependence of magnetic anisotropy energy produces Joule magnetostriction that is a volume conserving process, whereas sensitivity of isotropic exchange energy to interatomic distance is the cause of volume magnetostriction. In a typical magnet, Joule magnetostriction dominates as the volume fraction occupied by regions of uniform spin alignment (domains) is 2-4 orders of magnitude higher than that which is occupied by regions with magnetoelastic gradients (domain walls). Recently, `giant' non-volume conserving or non-Joulian magnetostriction has been discovered in iron-gallium alloys. Here we show using high-resolution polarization-dependent photoelectron microscopy that non-Joulian magnetism arises from an unusual partition of the crystal into nm-scale lamellar domains and domain walls within highly periodic magnetic microcells. High-resolution x-ray circular dichroism measurements at the Fe and Ga L absorption edges further provide evidence of weak iron-induced magnetism on gallium atoms via negative exchange. The results are in excellent agreement with the state-of-the-art theoretical electronic-structure calculations.