Surface Spin Disorder in Fe$_{3}$O$_{4}$ Nanoparticles Probed by Electron Magnetic Resonance Spectroscopy and Magnetometry

Abstract

SQUID magnetometry and electron magnetic resonance (EMR) spectroscopy at 9.28 GHz were employed to probe magnetism and surface spin disorder in Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanoparticles. 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 T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> sime 170 K in H= 50 Oe. For T>T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> , M scales approximately as H/T with mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> sime5400 mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> as the magnetic moment/particle. At 5 K, the magnitudes of hysteresis loop parameters viz. coercivity H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> sime 37 Oe and saturation magnetization M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> = 62 emu/g are considerably smaller than the reported values for larger 150 nm Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> particles. 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