Surface spin-glass freezing and blocking in nickel ferrite nanoparticles

摘要

We prepared single-phase nickel ferrite nanoparticles separated by silicon dioxide using sol-gel method with tetraethyl orthosilicate (TEOS) as a precursor for SiO_2. The magnetic properties are investigated by using SQUID-magnetometry over a broad temperature range (4.2 - 350 K), magnetic field (2 - 70,000 Oe) and frequency (0.1 - 1000 Hz) range. The particle size is in the range 8 - 12 nm. Exchange bias and spin disorder appear at the core-shell interface due to broken bonds on the surface. Disorder and core-shell interaction induces spin-glass freezing which is manifested by a low temperature peak in the AC susceptibility well separated from magnetic blocking peak. This low temperature peak is assigned to spin-glass freezing. The proof of spin-glass freezing is managed by zero field cooled/field cooled (ZFC/FC), frequency and DC field dependence of AC susceptibility, low temperature hysteresis loop and time dependent thermoremanent magnetization at different temperatures. All the measurements stated above signify blocking/unblocking at higher temperatures and surface spin-glass freezing at low temperatures. The aim of our work is to contribute to a better understanding of "spin-frozen" magnetic ferrite nanoparticles at diameters 8 - 12 nm which could be important in future for stabilizing the magnetic state of "core-shell"-structured nanomagnets.