Thermal evolution of Pt-rich FePt/Fe3O4 heterodimers studied using x-ray absorption near edge spectroscopy

TL;DR

This study uses in situ XANES to investigate the thermal evolution of Pt-rich FePt/Fe3O4 heterodimers, revealing phase transformations and interdiffusion processes that influence their magnetic properties during annealing.

Contribution

It introduces an in situ XANES method to monitor intermediate phases and mechanisms during the thermal annealing of FePt/Fe3O4 heterodimers, providing detailed insights into phase evolution.

Findings

Fe3O4 reduces to FeO with increasing temperature

Fe3Pt begins to form above 550 K

FeO increase at 840 K drives phase stabilization

Abstract

FePt/Fe3O4 nanoparticles can be used as building blocks to obtain, upon thermal annealing, magnetic nanocomposites with combined magnetic properties. Although the pre- and post-annealed samples are usually well characterized, a detailed investigation during annealing is necessary to reveal the role of intermediate processes to produce a desirable composite. We present an alternative method using in situ XANES to investigate the thermal evolution of oleic acid and oleylamine coated Pt-rich FePt/Fe3O4 heterodimers. As the temperature increases, a progressive reduction of Fe3O4 to FeO occurs helped by the thermolysis of the surfactants; while above 550 K Fe3Pt starts to be formed. At 840 K an abrupt increase of FeO further drives the phase transformation to stabilize the iron platinum soft phase. Thus, the Fe3O4 reduction acts as catalyst that promotes the Fe and Pt interdiffusion between the Pt-rich FePt and Fe3O4/FeO to form Fe3Pt instead of exchange coupled FePt/Fe3O4 with hard magnetic properties. In addition, the role of the interface of the heterodimer ends is discussed. The pre- and post-annealed samples were also characterized by TEM, XRD, EXAFS, magnetometry and M\"ossbauer spectroscopy.