Magnetic properties investigation of a ferrofluid with cobalt ferrite nanoparticles using polarized muons

mu SR-spectroscopy method was applied to the investigation of water based ferrofluid samples with 0.5% and 3% vol. concentrations of CoFe2O4 nanoparticles. The measurements were overdone in the (26 divided by 300) K temperature range in an external magnetic field transversal to the muon spin and with the sample cooling: (i) without applied magnetic field (ZFC) and (ii) in the presence of field (FC). It was found that regardless of the cooling regimes in both samples the diamagnetic (muon) fraction in ferrofluid is produced approximately in the same ratio as well in pure H2O. However, the relaxation rate and the precession frequency of the muon spin in the ferrofluid with 3% nanoparticles concentration depends on the cooling conditions of the sample, and they differ significantly from similar data for H2O. The present results show that magnetic nanoparticles of cobalt ferrite have a high coefficient of magnetic anisotropy. The magnetic field, created by single domain nanoparticles in ferrofluid has been found.