Large magnetic anisotropy in Ferrihydrite nanoparticles synthesized from reverse micelles

TL;DR

This study synthesizes ferrihydrite nanoparticles within reverse micelles, revealing large magnetic anisotropy and size-dependent magnetic properties, with potential implications for nanomagnetism and material design.

Contribution

It demonstrates a method to produce ferrihydrite nanoparticles with high magnetic anisotropy and characterizes their size-dependent magnetic behavior.

Findings

Effective anisotropy energy $K_{eff} = 312 extpm10$ kJ/m$^3$

Blocking temperatures increase with particle size

Magnetic contributions from core and surface are distinguishable

Abstract

Six-line ferrihydrite(FH) nanoparticles have been synthesized in the core of reverse micelles, used as nanoreactors to obtain average particle sizes $<d>$ $\approx$ 2 to 4 nm. The blocking temperatures $T_B^m$ extracted from magnetization data increased from $\approx 10$ to 20 K for increasing particle size. Low-temperature \MOS measurements allowed to observe the onset of differentiated contributions from particle core and surface as the particle size increases. The magnetic properties measured in the liquid state of the original emulsion showed that the \FH phase is not present in the liquid precursor, but precipitates in the micelle cores after the free water is freeze-dried. Systematic susceptibility $\chi_{ac}(\emph{f},T)$ measurements showed the dependence of the effective magnetic anisotropy energies $E_{a}$ with particle volume, and yielded an effective anisotropy value of $K_{eff} = 312\pm10$ kJ/m$^3$.