3d Metal Doping of Core@Shell Wüstite@ferrite Nanoparticles as a Promising Route toward Room Temperature Exchange Bias Magnets

Abstract

Nanometric core@shell wüstite@ferrite (Fe1−xO@Fe3O4) has been extensively studied because of the emergence of exchange bias phenomena. Since their actual implementation in modern technologies is hampered by the low temperature at which bias is operating, the critical issue to be solved is to obtain exchange‐coupled antiferromagnetic@ferrimagnetic nanoparticles (NPs) with ordering temperature close to 300 K by replacing the divalent iron with other transition‐metal ions. Here, the effect of the combined substitution of Fe(II) with Co(II) and Ni(II) on the crystal structure and magnetic properties is studied. To this aim, a series of 20 nm NPs with a wüstite‐based core and a ferrite shell, with tailored composition, (Co0.3Fe0.7O@Co0.8Fe2.2O4 and Ni0.17Co0.21Fe0.62O@Ni0.4Co0.3Fe2.3O4) is synthetized through a thermal‐decomposition method. An extensive morphological and crystallographic characterization of the obtained NPs shows how a higher stability against the oxidation process in ambient condition is attained when divalent cation doping of the iron oxide lattice with Co(II) and Ni(II) ions is performed. The dual‐doping is revealed to be an efficient way for tuning the magnetic properties of the final system, obtaining Ni‐Co doped iron oxide core@shell NPs with high coercivity (and therefore, high energy product), and increased antiferromagnetic ordering transition temperature, close to room temperature.

Magnetic Coupling in Cobalt-Doped Iron Oxide Core–Shell Nanoparticles: Exchange Pinning through Epitaxial Alignment

“…First, native core–shell magnetic nanoparticles are characterized by a lattice matching of core and shell materials that is beneficial for the magnetic exchange interaction. In addition, transition-metal doping aiming for ferrite materials with higher magnetocrystalline anisotropy such as cobalt ferrite has been highlighted very recently to enhance exchange bias properties …”

“…In addition, transition-metal doping aiming for ferrite materials with higher magnetocrystalline anisotropy such as cobalt ferrite has been highlighted very recently to enhance exchange bias properties. 40 We accordingly center our study on cobalt-doped iron oxide nanoparticles synthesized by a modified thermal decomposition of a mixed cobalt−iron oleate complex as described in our previous work. 60 The oleate decomposition route originally established by Hyeon and co-workers provides slightly reducing conditions during particle nucleation and growth 59 and therefore commonly leads to native wustite@spinel core− shell nanoparticles when carried out in inert atmosphere (here under N 2 ).…”

“…Even in the fully oxidized nanoparticles, a significant exchange bias field is induced by antiphase boundaries formed in the topotaxial oxidation process. , Similarly, strong exchange bias fields have been observed for core–shell nanoparticles prepared by cation exchange strategies . Transition-metal doping of native iron oxide core–shell nanoparticles has further been found as a promising strategy to enhance the exchange bias properties. , …”

“…39 Transition-metal doping of native iron oxide core−shell nanoparticles has further been found as a promising strategy to enhance the exchange bias properties. 9,40 Up to now, the observed exchange bias in native core−shell bimagnetic nanoparticles has been exclusively attributed to the coupling of an antiferromagnetic (AFM) particle core with a ferromagnetic (FM) or ferrimagnetic (FiM) shell. Based on the bulk material and the common observation of a Neél transition in macroscopic magnetization measurements, 41,42 assuming antiferromagnetic properties for a wustite-like nanoparticle core is a priori intuitive.…”

“…The exact composition of the core and shell, however, often remains out of reach, in particular for the particle core, as typically a cross-section through the particle is seen in microscopy. In a very recent study, Muzzi et al reported an extensive composition analysis of mixed Co–Ni–Fe oxide core–shell nanoparticles that is important to understand the effect of doping on the transition temperature and associated exchange bias behavior . Similarly, EELS and EMCD have recently been applied to show a clear core–shell picture of the Fe oxidation state and its distribution in individual nanoparticles. , The antiferromagnetic behavior of the core is typically assumed, although not unambiguously accessible, as a projection of the magnetization and composition within the particle is probed that naturally includes a contribution of the shell materials together with the core signal.…”