Single-crystalline HfO2 nanostructures with novel ferromagnetic properties

Hafnium oxide (HfO₂) has special technological significance due to its superior properties such as high dielectric constant (κ∼25), wide bandgap (∼5.7 eV), and superb thermal and chemical stabilities. Its room-temperature ferromagnetism and excellent CMOS technology compatibility make it a promising candidate for seamless CMOS-spintronics integration. Low-dimensional single-crystalline HfO₂ nanostructures, particularly one dimensional (1D) nanostructures, are expected to exhibit enhanced ferromagnetic properties due to large specific surface areas and potentially more surface defects. To date, the synthesis of single-crystalline HfO₂ 1D nanostructures has, however, remained elusive. Here, single-crystalline dopant-free HfO₂ nanostructures with notable morphologies, including HfO₂ 1D nanostructures, are grown using catalyst-assisted pulsed laser deposition. It is shown that Sn plays a crucial role in producing these 1D nanostructures by increasing both the VLS nucleation and growth rates. Magnetization measurements reveal room-temperature ferromagnetism in HfO₂ nanowires, contrasting with weak paramagnetic responses in HfO₂ nanostructured films. We also provide the first direct evidence of oxygen vacancies as the source of room-temperature ferromagnetism in HfO₂. To account for the observed magnetic property, we employ a modified bound magnetic polaron-band ferromagnetism hybrid model, which is also generally applicable to dopant-free nanostructures of other metal oxides. This work provides new insights into the growth of novel metal oxide 1D nanostructures and the design of new dilute magnetic semiconducting oxides for potential integrated CMOS-spintronics applications.