Defect-engineering by solvent mediated mild oxidation as a tool to induce exchange bias in metal doped ferrites

RESEARCH ARTICLE www.small-methods.com Defect-Engineering by Solvent Mediated Mild Oxidation as a Tool to Induce Exchange Bias in Metal Doped Ferrites Beatrice Muzzi, Martin Albino, Michele Petrecca, Claudia Innocenti, César de Julián Fernández, Giovanni Bertoni, M. Ricardo Ibarra, Mogens Christensen, Maxim Avdeev, Clara Marquina,* and Claudio Sangregorio* The crystal site occupancy of different divalent ions and the induction of lattice defects represent an additional tool for modifying the intrinsic magnetic properties of spinel ferrites nanoparticles. Here, the relevance of the lattice defects is demonstrated in the appearance of exchange-bias and in the improvement of the magnetic properties of doped ferrites of 20 nm, obtained from the mild oxidation of core@shell (wüstite@ferrite) nanoparticles. Three types of nanoparticles (Fe0.95O@Fe3O4,Co [email protected] [email protected]) are oxidized. As a result, the core@shell morphology is removed and transformed in a spinel-like nanoparticle, through a topotactic transformation. This study shows that most of the induced defects in these nanoparticles and their magnetic properties are driven by the inability of the Co(II) ions at the octahedral sites to migrate to tetrahedral sites, at the chosen mild oxidation temperature. In addition, the appearance of crystal defects and antiphase boundaries improves the magnetic properties of the starting compounds and leads to the appearance of exchange bias at room temperature. These results highlight the validity of the proposed method to impose novel magnetic characteristics in the technologically relevant class of nanomaterials such as spinel ferrites, expanding their potential exploitation in several application fields. B. Muzzi Department of Biotechnology Chemistry and Pharmacy University of Siena Siena I-53100, Italy B. Muzzi, C. Innocenti, C. Sangregorio Istituto di Chimica dei Composti Organometallici (ICCOM) Consiglio Nazionale delle Ricerche (CNR) Sesto Fiorentino (FI) I-50019, Italy E-mail: [email protected].it B.Muzzi,M.Albino,M.Petrecca,C.Innocenti,C.Sangregorio DepartmentofChemistry“U.Schiff” UniversityofFlorenceandINSTM SestoFiorentino,(FI)I-50019,Italy The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smtd.202300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/smtd.202300647 1. Introduction In the last decades, the emergence of new-generation technologies have pushed the research activity toward the development of nanomaterials. Nanosized materials reveal excellent and unique optical, electrical, catalytic, medical, biological and magnetic properties,[1–6] which arise from the fine tuning of the nanostructure characteristics (e.g. size, shape, or a combination of different nano-sized materials). Among others, magnetic nanoparticles (NPs) can be exploited in several applications such as biomedicine,[7,8] catalysis,[9,10] magnetic recording,[11] and as permanent magnets.[12,13] In particular, thanks to the large variety of physical properties (high magnetic permeability, high magnetic anisotropy or high electrical resistance), spinel ferrite-based nanostructures, either as single phase or enclosed in complex architectures, have C. de Julián Fernández Istituto dei Materiali per l’ Elettronica ed il Magnetismo (IMEM) Consiglio Nazionale delle Ricerche(CNR) Parma I-43124, Italy G. Bertoni CNR – Istituto Nanoscienze Modena I-41125, Italy M. R. Ibarra, C. Marquina Instituto de Nanociencia y Materiales de Aragón (INMA) Consejo Superior de Investigaciones Científicas (CSIC)-Universidad de Zaragoza Zaragoza 50009, Spain E-mail: [email protected] M.R.Ibarra,C.Marquina DepartamentodeFísicadelaMateriaCondensada UniversidaddeZaragoza Zaragoza50009,Spain M.R.Ibarra LaboratoriodeMicroscopiasAvanzadas(LMA) UniversidaddeZaragoza Zaragoza50009,Spain Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (1 of 12) www.advancedsciencenews.com www.small-methods.com been extensively investigated for these applications.[14,15] Recent studies on iron-based NPs (e.g. Fe3O4, core@shell Fe1-XO@Fe3O4, etc.), revealed that defect-engineering at the nanoscale can be a novel approach to tailor the magnetic properties of NPs (e.g. high coercive field, exchange bias)[16] Indeed, there is a close correlation between the magnetic properties and the crystal structure (e.g. lattice defects, ions distribution). Lappas et. al.[17] found by Monte Carlo simulations that the magnetic anisotropy energy barrier of defected iron-oxides NPs can be tuned as a function of the lattice defect (vacancies, dislocations, stacking faults and antiphase boundaries) and the uncompensated spins in the defected regions can induce some magnetic phenomena, such as the exchange bias (EB).[18–20] This latter is commonly observed in exchange coupled antiferromagnetic (AFM)/ferr(i)magnetic F(i)M nano-heterostructures, including core@shell (CS) NPs, when the system is cooled from room temperature through the ordering temperature of the AFM component in the presence of an external magnetic field. Although the exact nature and the phenomenology of the EB are still under debate, it is generally accepted that it originates from the pinning force asserted by the AFM phase on the magnetic moment of the F(i)M interface uncompensated spins. The latter will thus require a higher external magnetic field, for reversing the magnetization toward the direction opposite to the cooling field.[12,20–25] As a result, a horizontal shift (EB field, HE) of the hysteresis loops, and the increase of the coercive field (HC)are observed. The controlled oxidation of CS AFM@F(i)M Fe1-xO@Fe3O4 NPs has proven to be an efficient strategy to introduce defects into the ferrite spinel lattice. Wetterskog et al.[26] observed that under controlled oxidation, the lattice strain at the CS interface was released as the Fe1-xO core oxidized, but some defects in the crystal structure, such as antiphase boundaries (APBs)[27] were still retained in the final single phase Fe3O4. Indeed, the wüstite structure is characterized by octahedral (Oh) sites, occupied by Fe(II) only, which, after oxidation, evolves into a cubic spinel structure. In the latter, both Fe(II) and Fe(III) are present and they coordinate with oxygen differently: Fe(II) cations coordinate with Oh symmetry (as in wustite), whereas Fe(III) cations coordinate equally with both Oh and tetrahedral (Td) symmetry. Therefore, the Td sites in the spinel originate new crystal planes, that are not present in the wüstite phase, which causes the oxidationgenerated defects to persist, even when oxidation to single-phase ferrite is complete.[17,26,28–36] This approach was successfully applied by some of us to the synthesis of 10 nm defected cobalt ferrite NPs, which thanks M. Christensen Department of Chemistry and iNANO Aarhus University Langelandsgade 140, Aarhus C DK-8000, Denmark M. Avdeev Australian Nuclear Science and Technology Organisation (ANSTO) New Illawarra Road, Lucas Heights, NSW 2234, Australia M. Avdeev School of Chemistry The University of Sydney Sydney, NSW 2006, Australia to the large contribution to the magneto-crystalline anisotropy from Co(II) ions, in addition to the oxidation-generated defects, exhibited very large EB.[27] However, their magnetic behavior was strongly affected by the contribution of the AFM component. This feature was ascribed to the formation of a large magnetic disorder spinel ferrite domains.[37] A possible strategy to overcome the drawback is to add to the defected cobalt ferrite a second divalent cation able to restore the magnetic order while retaining the lattice defects. Recently, Muzzi et al. demonstrate that the introduction of Ni(II) in the crystal lattice of CS [email protected], allowed for partially recovering the magnetic order in the ferrite shell, leading to a more pronounced F(i)M character in the ferrites moiety. Starting from these considerations, we here present an investigation aimed to rationalize the effect of Co(II) and Ni(II) on the lattice defects induced in spinel ferrite NPs by solvent-mediated mild oxidation of AFM@FiM precursors, and on how they affect the magnetic properties. With this aim, 20 nm CS Fe0.95O@Fe3O4,Co [email protected] [email protected] synthesized by thermal decomposition method,[37] were oxidized to the single phase Fe3-xO4(FeO-oxy), CoyFe3-yO4(CoFeO-oxy), and NizCoyFe3-y-zO4 (NiCoFeO-oxy) nanocrystals. 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 CoyFe3-yO4and NizCoyFe3-y-zO4NPs 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. 2. Results and Discussion The mild oxidation of the CS Fe0.95O@Fe3O4,Co 0.3Fe0.7O@ Co0.8Fe2.2O4,andNi [email protected][37] in a high boiling temperature solvent (310 °C) led to nanocrystals of 20 ±3 nm for FeO-oxy, Figure 1a, and NiCoFeO-oxy, Figure 1c, and 22 ±4 nm for CoFeO-oxy, Figure 1b, with truncated octahedron shape ascribed to (111) faces well-formed and sharp. The size and size distribution remained almost unaltered after the thermal annealing (Figures S1 and S2, Supporting Information) confirming the validity of the approach used for the oxidation step to avoid particle aggregation. For the whole series, the NPs were capped by an oleate surfactant layer, estimated by CHN analysis as ca. 25% w/w. In Figure 2 (left panel) scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) chemical composition mapping, after Principal Component Analysis (PCA) decomposition, for a single NP of FeO-oxy, CoFeO-oxy, and NiCoFeO-oxy, respectively, is shown with the corresponding element distribution. The distribution of cobalt and nickel in these NPs remained homogeneous after the Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (2 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-methods.com Figure 1. Transmission electron microscopy (TEM) images of a) FeO-oxy, b) CoFeO-oxy, and c) NiCoFeO-oxy NPs and the corresponding size distributions. oxidation process, underlining the suitability of the chosen method (Figure S3, Supporting Information). The chemical quantification analysis performed on normalized EELS spectra (Figure 2, right panel) indicated the final stoichiometric compositions: Fe3O4for FeO-oxy, CoFe2O4for CoFeO-oxy, and Ni0.4Co0.6Fe2O4for NiCoFeO-oxy, were attained. The formation of the Fe3O4phase was further confirmed by the magnetization vs temperature curve that displayed the characteristic feature of the Verwey transition at ca. 100 K (Figure S5, Supporting Information). The crystal structure evolution was investigated by acquiring Ultra-High Resolution Transmission Electron Microscope (UHRTEM) images of the NPs before and after oxidation and analyzing their local Fast Fourier Transformation (FFT). In Figure 3 (left panel), the UHRTEM image of a NiCoFeO NP (i.e. [email protected] oxidation) is shown as a representative, while images of FeO (Fe0.95O@Fe3O4) and CoFeO ([email protected]) NPs are reported in the Figures S1 and S2 (Supporting Information). The CS morphology can be clearly distinguished in Figure 3, left. The FFT analysis, performed on the two red squares areas, a(core) and b (shell), revealed, respectively, the rock-salt (RS, Fm  3m) structure of the Ni0.17Co0.21Fe0.62O core, and reflections corresponding to the cubic spinel structure (S, Fd 3m) of the Ni0.4Co0.3Fe2.3O4shell (Figure 3a’,b’).[37] After the oxidation step at 310 °C, only one phase can be distinguished by the UHRTEM analysis (Figure 3, right panel). FFT analysis on the red square carea (Figure 3c’) indeed, revealed reflections related only to the S structure in the whole NP. The annealing-oxidation treatment here proposed, induces the complete transformation of the metal monoxide core into the spinel phase, removing the CS architecture. Although UHRTEM suggests the NPs have a high degree of crystalline order, Bragg filtering was applied to investigate the presence of crystal defects. The (440)SBragg filtering images (Figure 4) obtained from the FFT of the FeO-oxy, CoFeO-oxy, and NiCoFeO-oxy UHRTEM images, show a continuous nondefective S phase for the FeO-oxy NP, while images of the doped ferrites depict a defective structure with a considerable number of APB. This evolution toward a nonhomogeneous crystal structure can be attributed to the presence of the Co(II) in the lattice.[27] On the other hand, also Ni(II) in NiCoFeO-oxy, which coordinates only in Oh symmetry,[44] may hinder the formation of the S cubic structure. In Figure 5, the powder X-Ray diffraction (PXRD) patterns of samples before and after mild oxidation are shown. For the preoxidized samples, the monoxide structure (black pattern), corresponding to the Fm  3mspace group, was observed. The peak (311) at 2𝜃=36.5 degrees, related to the cubic S structure Fd  3m, can be distinguished only for the FeO sample, suggesting the ferrite shell surrounding the monoxide core is thicker in these NPs, while it is too thin to give an appreciable signal for the cobalt and nickel-doped analogs.[37] After the oxidation, all the reflections observed in the PXRD pattern of FeO-oxy (red pattern) match for position and intensity those of the Fe3O4reference pattern and the peak profile suggests a good crystallinity without defects. Conversely, the oxidative treatment carried out on the cobalt and the nickel-cobalt substituted samples led to some peculiarities: despite the position of all the peaks corresponding to those expected for an S structure, the profile and the relative intensities of some peaks present some differences. The three peaks at 2𝜃=37.0°, 43.0°, and 62.5°, indexed as the (222)S, (400)S, and (440)Sreflections of the doped spinel ferrite, respectively, are sharp, while the other four peaks, (220)S, (311)S, (422)S, and (511)S, are much broader. This feature can be explained by considering that the two crystallographic structures, RS and S, share a very similar cubic packing of oxygen anions. Indeed, the (222)S, (400)S, and (440)Sreflections of the S structure correspond to the same families of planes (111)RS, (200)RS, and (220)RS of the monoxide structure. In particular, the progressive growth of the oxidized phase occurs by the displacement of the RS/S interface along (111)RS//(222)S, (200)RS//(400)S,and (220)RS//(440)Splanes. Fe(II) in the monoxide and part of the Fe(II)/Fe(III) in the S occupy the Oh positions, corresponding to Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (3 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-methods.com Figure 2. Left: mapping and distributions of oxygen (O-K edge, red), iron (Fe-L2,3 edge, green), cobalt (Co-L2,3 edge, blue) and nickel (Ni-L2,3 edge, magenta) inside one NP of FeO-oxy (black rectangle, top), CoFeO-oxy (blue rectangle, middle), NiCoFeO-oxy (red rectangle, bottom). The colour intensity is proportional to the relative concentration of the corresponding ion. Right: EELS spectra of FeO-oxy (black line), CoFeO-oxy (blue line) and NiCoFeO-oxy (red line). The same analysis was performed on 10 NPs of each sample, providing the same results. the (200)RS//(400)Sand (220)RS//(440)Splanes. Instead, the formation of the long-range ordered tetrahedral (Td)Fe (III) sublattice generates the (220)Splanes in the S ferrite, which do not have a counterpart in the monoxide, potentially leading to a final defective iron spinel ferrite.[17,34] In fact, along the directions shared by the RS and S structures, the transformation from RS to S requires only a small rearrangement of the oxygen lattice, which can be easily achieved even at low annealing temperature, such as the one adopted in this work. Conversely, the rearrangement along the crystallographic directions not shared by the two cubic phases requires higher energy. Interestingly, this peculiar behavior was observed only for the cobalt and nickel-doped-iron oxide nanocrystals. In fact, the evaluation of the microstrain by a Rietveld refinement of the PXRD patterns, shows that for CoFeO-oxy and NiCoFeO-oxy the values derived from the broad peaks are three orders of magnitude higher than those derived from the sharp ones, while no difference is observed for FeO-oxy. Therefore, it can be argued that Fe(II) replacement in the pristine monoxide NPs plays a crucial role in the formation of a defective lattice during the structural transformation, with a consequent increase of the crystal lattice strain. The strain maps obtained by Geometric Phases Analysis (GPA) measurements calculated for the crystallographic planes (311)Sand (440)S/(220)RS allowed us to estimate how much they are tilted from a reference position along the xand yaxes (Figure 6). The red-green regions are related to the zerostrain value, while the yellow or blue-black areas correspond to the regions where the planes are strongly tilted (up to ±35%). Figure 3. Left: UHRTEM image of a NiCoFeO NP with the CS structure: the two red squares indicates the regions selected to identify the core (a) and the 2 nm thick shell (b); FFT analysis of the selected regions: the labelled spots are related to the crystallographic planes indexed as (a’)RSphase(Fm  3m), in zone axis [001]RS and (b’)Sstructure(Fd  3m), [001]S. Right: UHRTEM image of a NiCoFeO-oxy NP: the core@shell structure disappeared and the FFT analysis of region crevealed the cubic spinel structure (Fd 3m), in zone axis [001]S. Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (4 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-methods.com Figure 4. UHRTEM images of a FeO-oxy (a), CoFeO-oxy (b) and NiCoFeO-oxy (c) NPs oriented in zone axis [001]Sand their corresponding Bragg filtered maps for the (440)Sdiffraction spots. The blue squares are enlargements of the selected area (red square) to highlight dislocation defects (e.g. APB) in (440)s maps. Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (5 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-methods.com Figure 5. PXRD patterns of the three samples before (black pattern) and after (red pattern) solvent mediated oxidation treatment at 310°C; orange and green bars correspond to the reference pattern for Fe3O4(JCPDS PDF #19-0629) and FeO (JCPDS PDF #73-2144), respectively. All the PXRD patterns are recorded using a Cu K𝛼(1.5406 Å) radiation source. The strain mapping confirmed that the structural rearrangement along one of the new directions not shared by the two structures, (311)S, results in a lattice strain higher than that observed when the atoms rearrange along a direction common to the two structures, (440)S/(220)RS.[45] In particular, this difference is more evident for CoFeO-oxy, and to a lower extent for NiCoFeO-oxy. From the structural characterization by PXRD, Bragg filtered images, and lattice deformation profile it can be concluded that in the doped ferrite NPs the topotaxial oxidative growth of the S phase led to the formation of small spinel subdomain (mosaic texture) in a well-defined RS structure, underlining the key role of the Co(II) and Ni(II) in the crystal structure evolution. Previous studies on doped ferrites have shown that Co and Ni occupy different positions in the spinel structure. In CoFe2O4NPs synthesized by thermal decomposition or by annealing, Co(II) is distributed between Oh and Td sites with inversion degrees in the 0.6-0.8 range.[13,44,46] Conversely, in NixFe3-xO4NPs (x=0.04, 0.06 and 0.11), Ni(II) substitutes the Fe(II) in the Oh sites, independently of the stoichiometry.[44,47] To assess the Co(II) distribution, neutron powder diffraction was performed. Experiments were limited to CoFeO-oxy for which the different scattering lengths of Fe and Co (b(Fe) =9.45 fm and b(Co) =2.49 fm) ensures a good elemental contrast. Conversely, since b(Ni) =10.3 fm is relatively close to the scattering length of Fe, NiCoFeO-oxy was not investigated. Therefore, in the following, it will be assumed all Ni(II) ions are coordinated with Oh symmetry. In Figure 7 is reported the neutron powder diffraction pattern and, as a reference, the CoK𝛼X-ray pattern recorded with a high-intensity source. The combined Rietveld refinement of the two patterns (see Supporting Information for details) displayed that Co(II) ions occupy only Oh sites. Moreover, the fitting shows ca. 15% of Td-vacancies, underlining a defective structure occurs in the Td sites lattice, which can be correlated to the presence of stacking fault, APB, and lattice microstrain in the final spinel.[17,26] This observation is in good agreement with the lattice deformation profile corresponding to the (311)Sdirection, which passes through the Td sites, reported in Figure 6. On the contrary, no strain is observed along directions corresponding to Oh sites, in good accordance with the lattice deformation profile corresponding to the (440)S/(220)RS directions. Interestingly, when CoFeO-oxy is treated in an oven at 650°C, CoFeO-oxy650, the 10(1)% of Co(II) ions occupy the Td sites achieving the ideal lattice plane separation in an S structure (see Supporting Information). These data display that in CoFeOoxy the migration of Co(II) ions toward Td cavities does not occur during the short time oxidation at mild temperature (310 °C). However, when a high enough energy is supplied (i.e. annealing at 650°C) Co(II) ions occupy also Td cavities, leading to a high ordered spinel cubic structure (see Supporting Information for further 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 kAm2kg−1at 5 K) and low coercive field, typical features of ordered magnetite NPs Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (6 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-methods.com Figure 6. 𝜖xy (symmetric shear) strain maps calculated for the (311)S(NP with zone axis[1–30]) and (440)S/(220)RS (NP with zone axis [001]) planes of FeO-oxy, CoFeO-oxy and NiCoFeO-oxy. The colour bars refer to the tilting degree (-35% to +35%) of the considered plane from the reference zero position, arbitrarily located in the NP. Corresponding lattice deformation profiles were evaluated along the diagonal of each NP (from the high-left corner of the image to the bottom-right one, inside the white circle) and the deformation percentage values relate to the reference zero position. (Figure 8a,b and Table 1). Conversely, CoFeO-oxy and NiCoFeOoxy exhibit low magnetization at 5 T (M5T) while the hysteresis loops are very far from saturation at high field, pointing out the presence of a strong AFM-like contribution in place of the FiM behavior commonly observed for an S nanostructure.[13,38,39] However, the comparison between the two loops highlights that the introduction of Ni(II) allows for a partial, albeit small, increase of the spin order. Moreover, for NiCoFeO-oxy a kink is observed at the remanence (H=0), which is attributed to the presence of decoupled ferrite subdomains (see Supporting Information). Due to the presence of the high anisotropy Co(II) ions, the coercive field (HC) of CoFeO-oxy and NiCoFeO-oxy is two orders of magnitude larger than that of FeO-oxy at 5 K. Similarly, at room temperature open hysteresis loops are observed for the two doped samples, while HCis null for FeO-oxy. To have a deeper insight into the effect induced by crystal defects on the magnetic properties, the magnetization was measured after field cooling CoFeO-oxy and NiCoFeO-oxy from 380 K in an applied field of 5 T. The field-cooled (FC) loops exhibit the typical horizontal shift toward negative fields (Figure 8c) due to the EB effect, while no shift is observed for FeO-oxy (Figure S6, Supporting Information). As further evidence of the EB effect, HCincreases from 0.8 and 0.9 up to 2.3 and 1.9 T, for CoFeO-oxy and NiCoFeO-oxy, respectively. It is worth noting that magnetization curves recorded by cycling the field between ±9 T allowed us to exclude that the EB originates only from minor loop effect (Figure S7, Supporting Information). The presence of EB in the substituted samples can be attributed to the presence of APB and/or stacking faults formed during the spinel growth.[26] Indeed, these defects modify the Co(II) and Ni(II) local structural and magnetic ordering with respect to that in the bulk materials.[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 feaSmall Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (7 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-methods.com Figure 7. Neutron and X-ray powder diffraction pattern of a) CoFeO-oxy and b) CoFeO-oxy650, respectively. The red circles are the collected data points, while the black line is the model, and the blue line is the difference between data and model. In the neutron patterns, the contributions from the nuclear and the magnetic structure are represented by purple and green profiles, respectively. The orange line is the contribution from the Al-foil sample holder. The Bragg positions and Miller indexes (hkl) are given as a reference. Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (8 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License www.advancedsciencenews.com www.small-methods.com Figure 8. Hysteresis loops recorded at a) 300 K and b) 5 K of all the samples; c) Hysteresis loops at 5 K after FC in a 5 T magnetic field applied. The insets show the low field regions of the loops. Black: FeO-oxy, blue: CoFeO-oxy and red: NiCoFeO-oxy. tures 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 9) highlighted a qualitative increase of the EB effect when the single phase is attained. Even if, due to the high magnetic anisotropy, the hysteresis loops of the samples are recorded under minor loop conditions, for NiCoFeO-oxy aH Eincrease of ca. 50% with respect to the starting NiCoFeO CS NPs can be roughly assessed. Conversely, the vertical shift of the loop of the starting CS CoFeO NPs prevents the estimation of the HEmodification. Nevertheless, the high increase of the coercive field and the high magnetization of CoFeO-oxy compared to the precursor, clearly highlight the improved hardness of the oxidized sample. These results highlight that, although the oxidation removed the AFM@F(i)M interface, where the local uncompensated spins are responsible of the EB in CS NPs, the spin disorder, induced by lattice defects, lead to an even larger EB effect in the Co, Ni single phase NPs. Once again, these results underline the peculiarity of the spinel phases formed in the oxidation process of CoFeO-oxy and NiCoFeO-oxy, with respect to FeO-oxy, confirming the crucial role of the divalent ions in the evolution from the monoxide. 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, CoyFe3-yO4[39] and NizCoyFe3-z-yO4NPs 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). 3. Conclusion Inducing defects in the crystal lattice is a promising strategy for tuning chemical-physical properties of NPs. Here we investigated how the lattice deformations emerging from the incomplete crystal transformation from RS to S in Co, Ni substituted ferrites, modify the magnetic properties of the final NPs. For this study, 20 nm CS RS@S NPs were oxidized to single phase spinel ferrite NPs. The oxidation method chosen allows us to heat the NPs up to a mild temperature (T=310°C) avoiding interparticle aggregation effects, change in the NPs’ morphology, and decomposition of the organic coating. The substituted ferrite NPs are characterized by a mosaic texture with small spinel subdomains. Neutron powder diffraction data evidenced that in the doped NPs Co(II) occupy only Oh sites, leading to the formation of a ferrite phase with a defective occupation of Td sites. It is only by heating at 650 °C that it is possible to obtain a partially inverted singlephase spinel where 10(1) % of Co(II) occupies the Td sites, but at the expense of the size and shape of the NPs. The presence of spinel subdomains and lattice defects (APB and stacking faults), which is more pronounced in CoFeO-oxy, led to EB bias effect in the substituted samples, and, consequently, the materials display a significant increase of the magnetization at 5 T (100% increase) and larger hysteresis loop area with respect to those of the starting CS NPs. Interestingly, the replacement of some Co(II) by Ni(II) allows for a partial recovery of the spin ordering. The characteristic magnetic behavior of the obtained CoyFe3-yO4and NizCoyFe3-y-zO4samples highlighted that the introduction of lattice defects in substituted spinel ferrite NPs, here performed by mild oxidation, is an appealing strategy for inducing magnetic features that cannot be otherwise observed when the same nanomaterials are obtained by standard techniques. Moreover, unlike conventional AFM@F(i)M systems, Small Methods 2023,7, 2300647 © 2023 The Authors. Small Methods published by Wiley-VCH GmbH 2300647 (9 of 12) 23669608, 2023, 11, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smtd.202300647 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [23/01/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License