Nanoscale distribution of magnetic anisotropies in bimagnetic soft core-hard shell MnFe$_2$O$_4$@CoFe$_2$O$_4$ nanoparticles

TL;DR

This study reveals the nanoscale distribution of magnetic anisotropies in core-shell MnFe$_2$O$_4$@CoFe$_2$O$_4$ nanoparticles, showing how a thin hard shell influences the core's magnetic properties using element-specific spectroscopies.

Contribution

It introduces a novel combination of element-selective magnetic spectroscopies to map magnetic anisotropies at the nanoscale in core-shell nanoparticles.

Findings

The coercive field varies within the MnFe$_2$O$_4$ core.

A 0.5 nm CoFe$_2$O$_4$ shell induces strong anisotropy.

Inner core exhibits high coercivity and remanence.

Abstract

The nanoscale distribution of magnetic anisotropies was measured in core@shell MnFe$_2$O$_4$@CoFe$_2$O$_4$ 7.0 nm particles using a combination of element selective magnetic spectroscopies with different probing depths. As this picture is not accessible by any other technique, emergent magnetic properties were revealed. The coercive field is not constant in a whole nanospinel. The very thin (0.5 nm) CoFe$_2$O$_4$ hard shell imposes a strong magnetic anisotropy to the otherwise very soft MnFe$_2$O$_4$ core: a large gradient in coercivity was measured inside the MnFe$_2$O$_4$ core with lower values close to the interface region, while the inner core presents a substantial coercive field (0.54 T) and a very high remnant magnetization (90% of the magnetization at saturation).