Surface Spin Disorder in Fe₃O₄ Nanoparticles Probed by Electron Magnetic Resonance Spectroscopy and Magnetometry

SQUID magnetometry and electron magnetic resonance (EMR) spectroscopy at 9.28 GHz were employed to probe magnetism and surface spin disorder in Fe3O4nanoparticles. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed magnetite particles with size sime8 nm. Magnetization (M) versus temperature studies yielded blocking temperature TBsime 170 K in H= 50 Oe. For T>TB, M scales approximately as H/T with mupsime5400 muBas the magnetic moment/particle. At 5 K, the magnitudes of hysteresis loop parameters viz. coercivity Hcsime 37 Oe and saturation magnetization Ms= 62 emu/g are considerably smaller than the reported values for larger 150 nm Fe3O4particles. In EMR studies, a single symmetric line at gsime2.11 with linewidth DeltaH= 850Oe is observed at 300 K with the g-value and DeltaH increasing with decreasing T. These results are interpreted in terms of a disordered surface spin layer of 0.5 nm thickness