Thermal Evolution of Pt-Rich FePt/Fe₃O₄ Heterodimers Studied Using X-ray Absorption Near-Edge Spectroscopy

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 postannealed 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 postannealed samples were also characterized by TEM, XRD, EXAFS, magnetometry, and Mössbauer spectroscopy.