Room-temperature ferromagnetism in an amorphous alumina on the surface of aluminum nanoparticles

Spherical Al-O nanoparticles ranging in average particle size from 20 up to 400 nm were prepared by using the levitation-jet aerosol synthesis through condensation of metal aluminum vapor in helium gas flow with an air injection. The nanoparticles have been characterized by Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Brunauer-Emmett-Teller method (BET), X-ray diffraction (XRD), Ultraviolet–visible spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Vibrating sample magnetometry (VSM). In these nanoparticles, room-temperature ferromagnetism (RTFM) with maximum magnetization above 2.2 emu/g and a critical temperature of ferromagnetic ordering about 980 K was recorded. NPs with maximum magnetization possessed a certain unit cell volume of metallic Al, the maximum transmittance of FTIR, and increased Raman intensity. The results indicate a predominant role of the amorphous alumina shell on the surface of Al nanoparticles in their properties. By using XPS and VSM data it was discovered a pronounced increase in the maximum magnetization of NPs when a ratio of the thickness of alumina shell to average particle size tends to zero. It is suggested that the observed ferromagnetic ordering may be related to intrinsic oxygen defects in the Al/Al₂O₃ interface.