Spin Canting in Silica-Coated Nickel Ferrite (NiFe2O4@SiO2) Nanoparticles: a Mossbauer Spectroscopy Study

Abstract

We present a comparative study on superparamagnetic uncoated and silica-coated NiFe2O4@SiO2 nanoparticles with identical NiFe2O4 cores and two different silica coating thicknesses, focusing on the surface spin arrangement on NiFe2O4. For the investigation of surface spin dynamics, in addition to the SQUID magnetometry experiments which probe the macroscopic behaviour of particle magnetization, Mossbauer spectroscopy was utilized as being a local technique with superior characteristic measurement time. It was revealed that two common aspects observed in nanoscaled magnetic particles, namely the surface spin canting and interparticle dipolar interactions, are intertwined with respect to their contributions to the total magnetic anisotropy, where the latter is significantly decreased with silica coating thickness and allowed surface effects to manifest itself in Mossbauer spectra. The reduced saturation magnetization of uncoated NiFe2O4 nanoparticles with respect to their bulk counterparts indicated the existence of a magnetically dead layer on NiFe2O4 surface. In opposite to some assumptions in the literature, the Mossbauer spectra confirmed that such reduced magnetization arises from spin disorder, rather than a change in coordination from inverse spinel to the mixed spinel structure. On the other side, emergence of broad sextet peaks at room temperature Mossbauer spectra showed that for NiFe2O4@SiO2 samples, silica coating further enhanced the spin canting on NiFe2O4 surface. Similarly, for NiFe2O4@SiO2 sample, an unusual asymmetric hysteresis loop at spin freezing temperature was observed as a signature of the existence of disordered spins in the SiO2/NiFe2O4 interface. It was shown that the sextet components in Mossbauer spectra can be reproduced with a continuous hyperfine field distribution due to the frustrated local spin arrangement at the surface. The thickness of the magnetically dead disordered surface spin layer was estimated for uncoated NiFe2O4, and average spin canting angle was calculated for NiFe2O4@SiO2 nanoparticles.