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

Coupling lattice and spin degrees of freedom without the use of external magnetic fields allows for energy-efficient spintronic devices. In this context, hybrid materials composed of an oxide layer undergoing a first-order, structural phase transition (SPT), such as V2O3, and a ferromagnetic layer, such as Ni, constitute a very promising paradigm for the development of such applications. In the case of a Ni/V2O3 heterostructure, the structural domains inherent to the first-order phase transition in the V2O3 cause a differential strain pattern in the Ni layer. By virtue of the inverse magnetorestrictive effect, this strain pattern is converted into a magnetic anisotropy distribution in the Ni. This offers a promising alternative to voltage-controlled magnetism or other switching mechanisms without a magnetic field [1,2]. Here we show by direct imaging of the thermal evolution of the Ni spin structure that the magnetic domains can be tuned in both size and orientation upon crossing the structural phase transition (SPT) of the proximal V2O3 layer [3]. We find a drastic temperature-driven reorientation of the Ni magnetic domains across the SPT which is responsible for an abrupt increase in the coercive field (up to 500%). By synchrotron-based X-ray microscopy we show a reconfiguration of the ferromagnetic domain pattern in the Ni layer across the V2O3 SPT together with changes in magnitude and direction of the magnetic anisotropy. These observations are in good agreement with both angular dependent ferromagnetic resonance measurements and micromagnetic simulations. Our simulations successfully replicate the change in the coercivity as well as the reorientation of the magnetization of the Ni domains after the implementation of a simple model of strain-dependent magnetic anisotropies. Furthermore, simulations also exhibit a phase coexistence of different magnetic domains across the SPT in the V2O3. Direct observations of the lateral correlation length of the Ni domains show an increase of almost an order of magnitude at the SPT compared to room temperature as well as a broad spatial distribution of the local transition temperatures. This corroborates the phase coexistence of Ni anisotropies due to the V2O3 structural domains coexistence across the SPT. Our data reveal that the reorientation of Ni domains is controlled by the reconfiguration of the structural domains of the oxide layer across the SPT [4,5], due to strain induced proximity. Our findings reveal a novel pathway to control magnetic domains without a magnetic field through proximity to a material undergoing a first-order structural phase transition, which allows for engineering of coercive fields for novel data storage architectures and novel device concepts based on “straintronics”. Acknowledgements: This work was supported by the Spanish MINECO projects MAT2015-68772-P and PGC2018-097789-B-I00 and European Union FEDER funds. A.F.R. and M.G.M acknowledge financial support from the EU CALIPSO Transnational Access programme. J.G.R. acknowledges support from Colciencias under grant 120471250659. The work by I.V., C.W. and I.K.S. at UCSD were supported by the Office of Basic Energy Science, U.S. Department of Energy, BES-DMS funded by the Department of Energy’s Office of Basic Energy Science, DMR under grant DE FG02 87ER-45332.

**References:**

[1] de la Venta, J., Wang, S., Ramirez, J. G., et al., Appl. Phys. Lett. 102, 1 (2013); [2] de la Venta, J., Wang, S., Saerbeck, T., et al., Appl. Phys. Lett. 104, 062410 (2014). [3] Valmianski, I., Fraile Rodríguez. A., Rodriguez-Alvarez, J., et al., (in review). [4] McLeod, A. S., Van Heumen, E., Ramirez, J. G., et al. Nat. Phys. 13, 80 (2017). [5] Gilbert, D. A., Ramírez, J. G., Saerbeck, T., et al., Sci. Rep. 7 13471 (2017).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/609df7dde4c22e1de9b043b18a9dd651.jpg)
Fig. 1. Top row panels show representative XMCD-PEEM images recorded at temperatures below (150 K), across (170 K) and above (200 K) the SPT of V2O3. The insets show their Fourier transforms. The initial state (blue contrast) at low temperature (LT) splits into a pattern of small domains (yellow) which progressively rotate and grow across the V2O3 SPT as the high-temperature (HT) phase is reached. Bottom row panels show histograms of the absolute value of the average rotation angle of the Ni magnetization, Df, at selected temperatures (calculated from the images in the top row). Sharp Df distributions are found for the LT (150 K) and HT (200 K) phases in contrast to broader distributions in the middle of the V2O3 SPT (170 K).

![](https://assets.underline.io/uploads/markdown_image/1/image/0691162ae0a39dfe6b44c065dd312054.jpg)
Fig. 2. Coercivity as a function of temperature of Ni(10 nm)/V2O3(100 nm) across the V2O3 SPT recorded while cooling the sample from 300 K to 120 K (only data from 200 K are shown). A sharp rise in the coercive field, Hc, of about a factor of four, is found as the temperature goes below the V2O3 SPT. The garnet circles represent the coercivity values obtained from the experimental loops while the green triangles correspond to those obtained from micromagnetic simulations.

 Nanoscale manipulation of magnetic domains by interfacial strain-induced proximity

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. 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. 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.

**References:**

[1] S. Y. Srinivasan, K.M. Paknikar, D. Bodas, et al. Applications of Cobalt Ferrite Nanoparticles in Biomedical Nanotechnology. Nanomedicine. Future Medicine Ltd. May 2018, pp 1221–1238. https://doi.org/10.2217/nnm-2017-0379. [2] S. Amiri, H. Shokrollahi, The Role of Cobalt Ferrite Magnetic Nanoparticles in Medical Science. Materials Science and Engineering C. Elsevier January 2013, pp 1–8. https://doi.org/10.1016/j.msec.2012.09.003. [3] C. Moya, M. D. P.Morales, X. Batlle, A. Labarta, Phys. Chem. Chem. Phys. 17, 13143 (2015). [4] C. Moya, G. Salas, M. D. P. Morales, et al. J. Mater. Chem. C 3, 4522 (2015). [5] C. Moya, A. Fraile Rodríguez, M. Escoda-Torroella, et al., J. Phys Chem. C (2020) (in press). https://dx.doi.org/10.1021/acs.jpcc.0c06657.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/176a07ed0dc34fb44fc96344b80c0136.jpg)
Fig. 1. Monodisperse samples of 8 nm NPs with a tunable amount of structural defects prepared by thermal decomposition of Fe(III) and Co(II) acetylacetonates with a variable concentration of 1,2-hexadecanediol. HRTEM images and element-specific XMCD loops of the Co2+ cations corresponding to highly defective (a, c) and highly crystalline nanoparticles (b, d).

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

**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

Nanoscale manipulation of magnetic domains by interfacial strain-induced proximity

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

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