Cobalt ferrite nanoparticles (NP) are competitive systems in technological and biomedical applications due to 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 Co-ferrite NP, one of the biggest challenges is to quantitatively disentangle the dependence of the overall magnetic response on the composition, structure, or surface chemistry. In this work, we address the key role of the structural defects on the magnetic properties of Co-ferrite NP investigated by element- and site-specific X-ray magnetic circular dichroism (XMCD) combined with high-resolution transmission electron microscopy of individual NP [1]. We studied a series of samples of monodisperse 8 nm NP, prepared by chemical routes [2,3], with similar stoichiometry and cation distribution but with a tunable number of structural defects. Element- specific XAS and XMCD spectra and hysteresis loops reveal that the collinear alignment of Co2+ (Oh) cations is much more affected by the structural disorder than that of the Fe3+ cations. This is because structural defects give rise to effective local anisotropy axes, causing a prevalent Co2+ spin canting through the spin−orbit coupling due to the relatively large value of the partially unquenched moment of these cations. As the structural disorder is further increased, the rest of the cations in the two sublattices are progressively dragged off the ferromagnetic alignment, being Fe3+(Td) cations the least sensitive to defects due to the smaller number of next-nearest neighbors in the Td-sublattice. Our work highlights the importance of combining advanced synthesis with the use of complementary local probes with element, valence, site, and structural sensitivity to enhance the performance of nanoscale magnetic materials.

![](https://assets.underline.io/uploads/markdown_image/1/image/a5aa5eb0e8a24261183ee2e55ee59892.jpg)

HRTEM images for Co-ferriteNP with increasing crystalline quality. Bottom panel: XMCD hysteresis loops measured at 2 K for Co2+ cations in Oh sites for highly-defective (left) and highly-crystalline (right) NP.

Disentangling Atom- and Site- Selective Spin Canting in Co-ferrite Nanoparticles with Tunable Structural Disorder

Coupling between lattice degrees of freedom and the spin systems without the use of magnetic fields allows for efficient spintronic devices. A promising alternative to produce large effects uses materials which exhibit a first order structural phase transition (SPT). In particular, for Ni/V2O3 bilayers, changes in the coercive field of up to 500% have been reported due to interfacial proximity to an archetypal Mott insulator V2O3 layer across its SPT [1]. In this work [2], using photoemission electron microscopy (PEEM) combined with X-ray magnetic circular dichroism (XMCD) we find an abrupt temperature-driven reorientation of the Ni magnetic domains. Angular dependent ferromagnetic resonance (FMR) shows a remarkable change in the magnetic anisotropy of the Ni film across the SPT of V2O3. Direct measurements of the lateral correlation length of the Ni domains from XMCD images show an increase of almost one order of magnitude at the SPT compared to room temperature and a broad spatial distribution of the local transition temperatures. Micromagnetic simulations based on these results are in quantitative agreement with the PEEM data, thereby corroborating the phase coexistence of Ni anisotropies caused by the V2O3 SPT. In summary, we show that the rearrangement of the Ni domains is due to the strain induced by the oxide layers’ structural domains across the SPT [2]. Our results illustrate the use of alternative hybrid systems to manipulate magnetic domains at the nanoscale, which allows for engineering of coercive fields for novel data storage architectures. The work was supported by Spanish MCIU and AEI (MAT2015-68772-P; PGC2018-097789-B-I00) and European Union FEDER funds. Some of this work was supported by by DOE-BES grant number DE-FG02-87ER45332.

![](https://assets.underline.io/uploads/markdown_image/1/image/d9e0eb8f03533ad116b494eac4563db7.jpg)

Temperature-driven reorientation of magnetic domains in a thin Ni film across the structural phase transition of a prximity-coupled V2O3 layer, giving rise to large changes in the magnetic anisotropy of the Ni film.

Strain-controlled ferromagnetic domains induced by interfacial proximity to Mott insulator V2O3

Iron oxide nanoparticles (NPs) have attracted a great attention for biomedical and environmental applications due to their good biocompatibility and magnetic performance. The control over the crystal structure and oxidation state are of paramount importance to optimize their properties [1, 2]. In this work we studied the effect of the amount of both 1,2-hexadecanediol and the solvent 1-octadecene on the thermal decomposition method with iron (III) acetylacetonate. On the one hand, a higher amount of one of the reagents gives rise to single-phase, single-crystal Fe3O4 NPs with diameters below 10 nm and narrow size distributions but with moderate reaction yields. On the other hand, low amounts of either of the two reagents result in large NPs containing both Fe3O4 and FeO phases but with high values of the reaction yield. Consequently, the samples exhibited two distinct magnetic behaviors depending on the amount of these two reagents. The hysteresis loops at room temperature for the small NPs showed the typical features of superparamagnetism: values of the saturation magnetization close to the bulk one for magnetite with no coercive field. On the contrary, larger NPs showed ferrimagnetic behavior with reduced values of the saturation magnetization, as well as shifted hysteresis loops at 5 K after field cooling the sample at 1 T. Zero-field cooling-field cooling (ZFC-FC) curves below 200 K for the small NPs exhibited a peak below room temperature corresponding to the blocking temperature, while those curves for the larger particles displayed two peaks at higher temperatures which can be associated with the Verwey and Neel transitions of magnetite and wüstite phases, respectively. This shows the biphasic nature of the large NPs. With this accurate monitoring of the reaction conditions we have added an extra level of optimization to the synthesis of these NPs. In particular, we have found that, for 1 mmol of iron (III) acetylacetonate, the minimum amounts of 1,2-hexadecanediol and 1-octadecene for the preparation of monophasic, single-crystale Fe3O4 NPs are 2.5 mmol and 5 mL, respectively. This allows us to tune the properties of each sample of iron oxide NPs to its specific application. [3] Acknowledgements The work was supported by Spanish MCIU and AEI (MAT2015-68772-P; PGC2018-097789-B-I00) and European Union FEDER funds. M.E-T. acknowledge Spanish MCIU for BES-2016-077527.

**References:**

[1] A. Fraile Rodríguez, C. Moya, M. Escoda-Torroella, A. Romero, A. Labarta and X. Batlle. J. Mater. Chem. C, 6, 4, 875–882 (2018). [2] C. Moya, M. P. Morales, X. Batlle and A. Labarta. Phys. Chem. Chem. Phys., 17, 19, 13143–13149 (2015). [3] M. Escoda-Torroella, C. Moya, A. Fraile Rodríguez, X. Batlle and A. Labarta. Langmuir (2020) doi.org/10.1021/acs.langmuir.0c02221

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/790431501f371d0ebef8ec3cb20fc3ab.jpg)
Colored HRTEM images of two samples synthetized with increasing amounts of 1,2-hexadecanediol and 1-octadecence and their respective magnetization curves, showing the correlation between low amount of both reagents and a defective crystal structure and a poorer magnetic behavior.

 Selective Control of Reagents as a Tuning Knob for Iron Oxide Nanoparticles with Controlled Morphology, Oxidation State and Magnetic Response

**Session chair:** Victor Prida, University of Oviedo



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Disentangling Atom- and Site- Selective Spin Canting in Co-ferrite Nanoparticles with Tunable Structural Disorder

Strain-controlled ferromagnetic domains induced by interfacial proximity to Mott insulator V2O3

Selective Control of Reagents as a Tuning Knob for Iron Oxide Nanoparticles with Controlled Morphology, Oxidation State and Magnetic Response

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