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 Fe3O4 nanoparticles. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed magnetite particles with size similar or equal to 8 mn. Magnetization (M) versus temperature studies yielded blocking temperature T-B similar or equal to 170 K in H = 50 Oe. For T > T-B, M scales approximately as H/T with mu(p) similar or equal to, 5400 mu(B) as the magnetic moment/particle. At 5 K, the magnitudes of hysteresis loop parameters viz. coercivity H-c similar or equal to 37 Oe and saturation magnetization M-s = 62 emu/g are considerably smaller than the reported values for larger 150 nm Fe3O4 particles. In EMR studies, a single symmetric line at g similar or equal to 2.11 with linewidth Delta H = 850 Oe is observed at 300 K with the g-value and Delta H increasing with decreasing T. These results are interpreted in terms of a disordered surface spin layer of 0.5 nm thickness.