Crucial Role of the Co Cations on the Destabilization of the Ferrimagnetic Alignment in Co-Ferrite Nanoparticles with Tunable Structural Defects

Abstract

Cobalt ferrite nanoparticles (NPs) are competitive candidates in nanomedicine and biomedical applications, especially in the fields of magnetic drug delivery, magnetic resonance imaging, and separation and detection of biomolecules [1,2]. The reason behind is a suitable combination of excellent chemical and magnetic characteristics, such as a high chemical stability, surface active sites, and ease of synthesis and functionalization, together with a high anisotropy constant, a high coercivity, and a moderate saturation magnetization. However, in order to control the functional response of cobalt ferrite NPs, one of the biggest challenges is to quantitatively disentangle the dependence of the composition, structure, or surface chemistry onto the overall magnetic response.In this study we unravel the key role played by the Co2+ cations on the destabilization of the collinear ferrimagnetism in cobalt ferrite NPs, by combining an advanced synthesis approach with a broad set of world-class complementary local probes.A set of samples of monodispersed, 8 nm cobalt ferrite NPs of identical stoichiometry but with a progressive inclusion of structural defects was prepared [3,4]. SQUID magnetometry results show a rapid degradation of the collinear ferrimagnetism as the structural disorder increases within the NPs, and even samples that are almost free from crystallographic defects exhibit relatively large values of the high-field susceptibility suggesting the occurrence of canting at least for some of the cations and sites [5]. As local characterization probes, synchrotron-based, element-, valence- and site- specific X-ray spectroscopy and magnetometry on ensembles of NPs was combined with high resolution transmission electron microscopy of selected, individual NP (Fig. 1) [5].The analysis of element-specific X-Ray Magnetic Circular Dichroism (XMCD) spectra and hysteresis loops for all cationic sites reveals that the collinear alignment of the Co2+ cations in octahedral sites is significantly more affected by the structural disorder than in any other cation. This is because structural defects cause local distortions of the crystal field acting on the orbital component of the cations, yielding effective local anisotropy axes that cause a prevalent Co2+ spin canting through the spin–orbit coupling, owing to the relatively large value of the partially unquenched moment of these cations, as found by XMCD. As the number of structural defects increases, the rest of the cations are progressively dragged off the ferrimagnetic alignment, being the Fe3+ cations in tetrahedral sites the last ones to be affected by the disorder because the canting takes place first in octahedral sites thanks to their smaller number of next-nearest neighbors in the tetrahedral sublattice [5].Our results demonstrate the key role of the Co2+ cations on the destabilization of the collinear ferrimagnetism in Co-ferrite NPs as their crystalline quality worsens and may help clarify the often conflicting, large variability of magnetic properties in the literature of Co-ferrite NPs with slightly different structural features. Our work may provide new avenues to interpret and control the functional response of Co-ferrite NPs, of relevance in the fields of rare-earth free permanent magnets, biological separation and detection, or nanomedicine.AcknowledgementsThe work was supported by Spanish MCIU and AEI (MAT2015-68772-P; PGC2018-097789-B-I00) and European Union FEDER funds. M.E.T. acknowledges Spanish MINECO for the Ph.D. contract BES-2016-077527. A.F.R. and C.M. acknowledge the financial support from the EU CALIPSO Transnational Access programme. Part of this work was performed at the Swiss Light Source, Paul Scherrer Institut, Switzerland. The measurements at FEI TITAN3 of the Laboratorio de Microscopias Avanzadas (LMA), Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza, are also gratefully acknowledged. S.R.V.A. thanks the Swiss National Science Foundation for financial support under project Nr. 169467. **