Field Dependence of the Spin Relaxation Within a Film of Iron Oxide Nanocrystals Formed via Electrophoretic Deposition
1 Department of Physics and Astronomy, Vanderbilt University, Nashville, TN, 37235, USA
2 Interdisciplinary Graduate Program in Materials Science, Vanderbilt University, Nashville, TN, 37235, USA
3 Vanderbilt Institute for Nanoscale Science and Engineering, Vanderbilt University, Nashville, TN, 37235, USA
4 National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL, 32310, USA
Nanoscale Research Letters 2010, 5:1540-1545 doi:10.1007/s11671-010-9674-2Published: 20 June 2010
The thermal relaxation of macrospins in a strongly interacting thin film of spinel-phase iron oxide nanocrystals (NCs) is probed by vibrating sample magnetometry (VSM). Thin films are fabricated by depositing FeO/Fe3O4 core–shell NCs by electrophoretic deposition (EPD), followed by sintering at 400°C. Sintering transforms the core–shell structure to a uniform spinel phase, which effectively increases the magnetic moment per NC. Atomic force microscopy (AFM) confirms a large packing density and a reduced inter-particle separation in comparison with colloidal assemblies. At an applied field of 25 Oe, the superparamagnetic blocking temperature is TBSP ≈ 348 K, which is much larger than the Néel-Brown approximation of TBSP ≈ 210 K. The enhanced value of TBSP is attributed to strong dipole–dipole interactions and local exchange coupling between NCs. The field dependence of the blocking temperature, TBSP(H), is characterized by a monotonically decreasing function, which is in agreement with recent theoretical models of interacting macrospins.