Magnetism Engineering in Antiferromagnetic beta-FeOOH Nanostructures via Chemically Induced Lattice Defects

Elongated akaganeite (beta-FeOOH) nanostructures were synthesized through a simple hydrothermal route, in which a careful selection of the experimental conditions allows for a tunable length and aspect ratio and concomitantly predetermines the magnetic response. An in-depth structural characterization using transmission electron microscopy, X-ray diffraction, and Raman spectroscopy, jointly with DC magnetic measurements, reveals a complex scenario where the interstitial Cl(- )content dictates the beta-FeOOH thermal stability and leads to the formation of bulk uncompensated spins along the inner channels. The coexistence of different magnetic contributions is observed to result in a non-monotonic dependence of the coercivity and exchange bias field on both temperature and size, posing major limitations for the archetypical magnetic core-shell model generally assumed for nanostructured antiferromagnets. As a proof of concept, we further show how the beta-FeOOH internal microstructure can be chemically manipulated through Cl- anion exchange, giving rise to a superparamagnetic component that comes along with an almost 20-fold increase in the coercivity at low temperature. The evaluation of these results reveals the potential of controlling the interplay between the crystal and magnetic structure via intercalation chemistry in antiferromagnets, expanding fundamental science knowledge and supporting practical applications, given their huge role in the technological fields of spintronics and magnonics.