Cubic FeCo alloy nanoparticles (NPs) with body-centered cubic (bcc) phase were prepared using sputter based gas-condensation method. When the NPs formed long chain assemblies, the magnetic properties were quite different from that of well-dispersed NPs. Most of the well-dispersed NPs were superparamagnetic at room temperature while the long chain NP assemblies were ferromagnetic with coercivities around 765 Oe, which displayed quite different magnetic properties. The ferromagnetism of long chain NPs was from the exchange coupling between NPs, which eventually led to the transition from superparamagnetism (SPM) to superferromagetism (SFM). Zero-field-cooled (ZFC) and field-cooled (FC) curves were obtained and long chain NP assemblies displayed ferromagnetism at the temperature ranging from 10 K to 400 K. Time-dependent remanent magnetic moment curves also indicated that the long chain structure had better thermal stability due to the strong exchange coupling.
Bibliographical noteFunding Information:
Parts of this work were carried out using the Characterization Facility, which receives partial support from NSF through the NSF Minnesota MRSEC program under Award Number DMR-0819885. Authors thank the partial support from the Institute for Rock Magnetism, Department of Earth Science, University of Minnesota, Twin Cities, for the use of instruments. The authors thank Prof. Bin Ma and Dr. Junyang Chen for enlightening discussions.
© 2016 Author(s).