Hardening of Cobalt Ferrite Nanoparticles by Local Crystal Strain Release: Implications for Rare Earth Free Magnets

Abstract

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets.

Defect‐Engineering by Solvent Mediated Mild Oxidation as a Tool to Induce Exchange Bias in Metal Doped Ferrites

“…This marks an important difference of the approach here proposed, which involves the topotaxial oxidation of CS monoxide@ferrite to spinel ferrite, with respect to standard decomposition method, that, relying on a different reaction mechanism, directly leads to ordered cobalt ferrite NPs even below 300°C. [ 39,48 ]…”

“…[ 27 ] This cation disorder induces changes in the electronic and magnetic interactions across the interface boundaries and lead to canted spin structure and exchange coupled regions also in the oxidized NPs. [ 33 ] These effects also explain the reduced magnetization value at 5 T. It is worth noting that hysteresis loops recorded at 5 K after zero‐field‐cooled (ZFC) and FC procedures for CoFeO‐oxy650 display all the features typical of assemblies of spin‐ordered cobalt ferrite NPs, [ 39 ] without signature of EB (see Figure S8, Supporting Information). Interestingly, the comparison between the oxidized NPs and the CS precursors (see Figure ) highlighted a qualitative increase of the EB effect when the single phase is attained.…”

“…However, here we want to stress that it is only combining the introduction of lattice defects, by mild oxidation, and the doping with divalent metal ions that it is possible to induce specific magnetic properties obtaining novel nanostructures. Indeed, Co y Fe 3‐y O 4 [ 39 ] and Ni z Co y Fe 3‐z‐y O 4 NPs obtained by one‐pot thermal decomposition of metal precursor, do not display the same peculiar magnetic behavior (for more details see Tables S3 and S4 and Figure S9, Supporting Information).…”

“…This marks an important difference of the approach here proposed, which involves the topotaxial oxidation of CS monoxide@ferrite to spinel ferrite, with respect to standard decomposition method, that, relying on a different reaction mechanism, directly leads to ordered cobalt ferrite NPs even below 300°C. [39,48] The structural disorder induced by the defective Td metal occupancy is expected to strongly affect the magnetic properties. Indeed, the hysteresis loops recorded at 300 and 5 K for FeOoxy display a high magnetization saturation (86 kAm 2 kg −1 at 5 K) and low coercive field, typical features of ordered magnetite NPs Figure 6.…”

“…The different site occupancy and the formation of the lattice defects in the fully oxidized nickel‐cobalt and cobalt‐substituted ferrites lead to the appearance of EB, increasing the coercive field value and consequently, the energy that can be stored in the materials. [ 37 ] Importantly, this peculiar variation is inherent to the transformation that occurs during the oxidation process, since the same behavior is not observed when the same doped‐ferrite nanoparticles are synthesized by one‐pot thermal decomposition, [ 38,39 ] sol–gel method [ 40 ] or in single‐crystal thin films. [ 41–43 ] The peculiar magnetic behavior observed in the Co y Fe 3‐y O 4 and Ni z Co y Fe 3‐y‐z O 4 NPs confirmed that the combination of lattice defects, induced by solvent‐mediated oxidation, and divalent metal ion doping is a promising method to impose novel magnetic properties to nanostructured materials, as a matter of fact generating a novel class of nanomaterials.…”