Spin Disorder and Magnetic Anisotropy in Fe3O4 Nanoparticles

TL;DR

This study investigates the magnetic properties of dextran-coated Fe3O4 nanoparticles, revealing bulk-like ferrimagnetic behavior with nearly compensated moments and highlighting the role of interparticle interactions in their magnetic dynamics.

Contribution

It provides new insights into the magnetic structure and anisotropy of Fe3O4 nanoparticles, challenging assumptions about surface effects and spin canting in such small particles.

Findings

Magnetic moments are much smaller than bulk Fe3O4.

No evidence of magnetic irreversibility or non-saturating behavior.

Interparticle interactions influence the blocking temperature behavior.

Abstract

We have studied the magnetic behavior of dextran-coated magnetite (Fe$_3$O$_4$) nanoparticles with median particle size $\left<d\right>=8$ $nm$. Magnetization curves and in-field M\"ossbauer spectroscopy measurements showed that the magnetic moment $M_S$ of the particles was much smaller than the bulk material. However, we found no evidence of magnetic irreversibility or non-saturating behavior at high fields, usually associated to spin canting. The values of magnetic anisotropy $K_{eff}$ from different techniques indicate that surface or shape contributions are negligible. It is proposed that these particles have bulk-like ferrimagnetic structure with ordered A and B sublattices, but nearly compensated magnetic moments. The dependence of the blocking temperature with frequency and applied fields, $T_B(H,\omega)$, suggests that the observed non-monotonic behavior is governed by the strength of interparticle interactions.