Giant Exchange‐Bias in Polyol‐Made CoFe₂O₄‐CoO Core–Shell Like Nanoparticles

Abstract

Cobalt ferrite ferrimagnetic nanoparticles (NPs) are prepared and used in this work as seeds to grow a thin antiferromagnetic poly‐ and nanocrystalline CoO shell. The major purpose is to study systematically the characteristics of the as‐produced powders, making emphasis on their internal crystallographic arrangement and their magnetic properties. 57Fe Mössbauer spectrometry evidences an evolution of the cation distribution among the spinel lattice in the cobalt ferrite core during the core–shell NPs processing. High‐resolution transmission electron microscopy shows a perfect epitaxy between the face‐centered cubic lattices of the spinel core and the rock‐salt shell. Finally, the measurements of 7T‐field‐cooled magnetic hysteresis loops at low temperature (5 K) of the composite particles exhibit a strong exchange bias coupling with an exchange field, µ0HE, of 365 mT and an enhanced coercive field, µ0HC, of 1395 mT. These values are very high compared to those of differently prepared CoO‐based oxide composite NPs and for which a giant exchange‐bias is reported. These features are attributed to the favorable material processing conditions offered by the polyol process in terms of crystalline quality, particularly at the interfaces, and for the pinning action exerted by the CoO phase on the magnetization of the CoFe2O4 phase.

Magnetic Traits in CoO‐Core@CoFe2O4‐shell Like Nanoparticles

“…μ 0 H C increases when the magnetic domain size decreases, due to the surface anisotropy energy increase, if it is assumed that the volume magnetocrystalline anisotropy energy is almost constant from one sample to another. Recalling that aCFO, CO‐CFO and bCFO NPs were synthesized under the same heating time and temperature conditions, one may assume an equivalent cation distribution in the spinel lattice and then an almost equal magnetocrystalline anisotropy constant . Within the same logic, the smaller magnetic domain, the lower blocking temperature (Figure ), since it is directly related to the magnetic domain volume, according to Stoner‐Wohlfarth theory…”

“…For instance, in the case of granular systems and particularly core‐shell NPs, a MCM made from a F core coated by an AF shell does not always give the same magnetic response as the AF@F one, in which the previous AF phase forms the core and the F one the shell. Indeed, Flores‐Martinez et al., observed a net hysteresis loop shift on CoFe 2 O 4 @CoO NPs when cooled to low temperature (5 K) under a high magnetic field, while Lima et al., observed only a broadening of the hysteresis loops on CoO@CoFe 2 O 4 NPs, under almost the same experimental magnetic conditions . At the same time, both Omelianchik et al .…”

“…Since polyol process is reputed for the production of MCM particles of epitaxial quality,, and for maintaining grain size uniformity, we used this technique to prepare core‐shell NPs based on submicrometer pseudo‐single CoO crystals coated by a poly‐ and nanocrystalline CoFe 2 O 4 shell. We complemented these investigations by studying the magnetic properties of single‐crystal CoFe 2 O 4 particles ( aCFO ) of the same size as the ferrite spinel crystals which make up the shell of the CoO‐core@CoFe 2 O 4 ‐shell particles ( CO‐CFO ).…”

Polyol-Made Luminescent and Superparamagnetic β-NaY0.8Eu0.2F4@γ-Fe2O3 Core-Satellites Nanoparticles for Dual Magnetic Resonance and Optical Imaging

“…To fill this gap, we propose in the present work to build a superparamagnetic-luminescent architecture based on these two components within the last geometry, using a single material processing route, the polyol process. The polyol process is a versatile, easy-to-achieve and scalable wet chemistry route, which has already demonstrated its power for the synthesis of well-crystallized and shape-controlled granular hetero-nanostructures [28][29][30]. In practice, europium-doped β-NaYF 4 single crystals were precipitated in polyol and subsequently dispersed in a fresh iron salt polyol solution to serve as seeds for iron oxide nanocrystal growth, leading to the formation of β-NaYF 4 :Eu@γ-Fe 2 O 3 core-satellite particles.…”

Magnetic Properties of Fe3O4/CoFe2O4 Composite Nanoparticles with Core/Shell Architecture

“…Currently, there are only a few studies in this direction [3,[8][9][10][11]. Issues giving rise to a full understanding of the influence of the core/shell architecture on the magnetization and the effective anisotropy of magnetic nanoparticles, as well as the role of interface effects in the formation of magnetic properties of composite nanostructures of this type, still remain open.…”