Engineering Magnetic Anisotropy and Emergent Multidirectional Soft Ferromagnetism in Ultrathin Freestanding LaMnO₃ Films

The combination of small coercive fields and weak magnetic anisotropy makes soft ferromagnetic films extremely useful for nanoscale devices that need to easily switch spin directions. However, soft ferromagnets are relatively rare, particularly in ultrathin films with thicknesses of a few nanometers or less. We have synthesized large-area, high-quality, ultrathin freestanding LaMnO₃ films on Si and found unexpected soft ferromagnetism along both the in-plane and out-of-plane directions when the film thickness was reduced to 4 nm. We argue that the vanishing magnetic anisotropy between the two directions is a consequence of two coexisting magnetic easy axes in different atomic layers of the LaMnO₃ film. Spectroscopy measurements reveal a change in Mn valence from 3+ in the film interior to approximately 2+ at the surfaces where considerable hydrogen infiltration occurs due to the water dissolving process. First-principles calculations show that protonation of LaMnO₃ decreases the Mn valence and switches the magnetic easy axis from in-plane to out-of-plane as the Mn valence approaches 2+ from its 3+ bulk value. Our work demonstrates that ultrathin freestanding films can exhibit functional properties that are absent in homogeneous materials, concomitant with their convenient compatibility with Si-based devices.