Large-Pore Mesoporous Ho₃Fe₅O₁₂ Thin Films with a Strong Room-Temperature Perpendicular Magnetic Anisotropy by Sol–Gel Processing

We report the evaporation-induced self-assembly synthesis of large-pore mesoporous thin films of ferrimagnetic holmium iron garnet (Ho₃Fe₅O₁₂) from nitrate salt precursors and a polyisobutylene-<i>block</i>-poly­(ethylene oxide) polymer structure-directing agent. The phase composition, atomic bonding configuration, and pore structure of the top surface and the interior of the films were investigated by microscopy, scattering, and spectroscopy techniques, including synchrotron-based grazing incidence small-angle X-ray scattering, X-ray photoelectron spectroscopy, X-ray diffraction (including Rietveld refinement), and others. The data provide evidence that the sol–gel-derived material is single-phase garnet with 27 nm diameter crystallites and few defects after being heated to 850 °C in air, and the continuous network of pores averaging 23 nm in diameter is preserved to a large extent, despite a solid–solid conversion from metastable <i>h</i>-HoFeO₃ to Ho₃Fe₅O₁₂ during the crystallization process. Furthermore, dc magnetometry measurements show the thin films are magnetically stable with a room-temperature coercivity of ∼170 Oe and exhibit an out-of-plane easy axis with a significant perpendicular magnetic anisotropy. A strong preference for out-of-plane magnetic alignment in solution-processed mesostructured films is unique, making them attractive for application in spintronics and nanomagnetics