pubs.acs.org/NanoLett Letter Direct Evidence of a Graded Magnetic Interface in Bimagnetic Core/ Shell Nanoparticles Using Electron Magnetic Circular Dichroism (EMCD) Daniel del-Pozo-Bueno,* María Varela,* Marta Estrader, Alberto López-Ortega,* Alejandro G. Roca,* Josep Nogués, Francesca Peiró, and Sònia Estradé Cite This: Nano Lett. 2021, 21, 6923−6930 Read Online See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Interfaces play a crucial role in composite magnetic materials and particularly in bimagnetic core/shell nanoparticles. However, Downloaded via 79.152.29.203 on December 1, 2022 at 10:33:46 (UTC). resolving the microscopic magnetic structure of these nanoparticles is rather complex. Here, we investigate the local magnetization of antiferromagnetic/ferrimagnetic FeO/Fe3O4 core/shell nanocubes by electron magnetic circular dichroism (EMCD). The electron energy-loss spectroscopy (EELS) compositional analysis of the samples shows the presence of an oxidation gradient at the interface between the FeO core and the Fe3O4 shell. The EMCD measurements show that the nanoparticles are composed of four different zones with distinct magnetic moment in a concentric, onion-type, structure. These magnetic areas correlate spatially with the oxidation and composition gradient with the magnetic moment being largest at the surface and decreasing toward the core. The results show that the combination of EELS compositional mapping and EMCD can provide very valuable information on the inner magnetic structure and its correlation to the microstructure of magnetic nanoparticles. KEYWORDS: EMCD, EELS, Magnetic Nanoparticles, Core/Shell Nanoparticles, Graded Interfaces ■ INTRODUCTION Understanding the interface structure between the different antiferromagnetic).3,7,14−16 Here, not only is the structural/ morphological interface important but also the magnetic components of nanocomposite structures constitutes the arrangement at the interface (e.g., strength of the exchange keystone to harness the functional properties of the materi- coupling, ferromagnetic versus antiferromagnetic interface al.1−8 In particular, graded interfaces (where there is some coupling, graded anisotropy or proximity effects)12,17−19 can degree of chemical intermixing between the two counterparts play a crucial role in the properties and performance of the at the interface; also denoted as rough or interdiffused nanoparticles. However, obtaining information on the interface interfaces) have been shown to play a critical role in tuning magnetic structure in nanoparticles is particularly challenging. the properties of diverse materials1−7 and specifically in core/ Indirect information on the magnetic structure at the interface shell nanoparticles.3,9−12 However, despite the paramount can sometimes be inferred from magnetization, X-ray magnetic importance of interfaces, the process of gaining detailed circular dichroism (XMCD), Mössbauer spectroscopy, and information about them can be rather complex, since other techniques.20−22 However, these techniques give specialized tools must be used to characterize interfaces of information on the whole sample, thus their lack of direct core/shell nanoparticles at the atomic level in real space, spatial resolution precludes establishing a one-to-one correla- including compositional information (e.g., transmission elec- tion between the magnetic behavior and the structural/ tron microscopy, TEM, combined with electron energy-loss morphological features at the atomic scale. To obtain a spectroscopy, EELS; anomalous small-angle X-ray scattering, straightforward correlation between the structural and ASAXS; extended X-ray absorption fine structure, EXAFS; or complex X-ray diffraction analysis).11,13−15 Received: May 27, 2021 The degree of complexity increases in bimagnetic core/shell Revised: July 28, 2021 nanoparticles, a widespread type of core/shell particles due to Published: August 9, 2021 their remarkable fundamental properties and the broad range of applications. In these systems, both the core and the shell exhibit magnetic properties (i.e., ferromagnetic, ferrimagnetic, © 2021 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.nanolett.1c02089 6923 Nano Lett. 2021, 21, 6923−6930 Nano Letters pubs.acs.org/NanoLett Letter magnetic interface, small angle neutron scattering (SANS) can be used, although it has been used only occasionally in bimagnetic core/shell nanoparticles.23,24 However, SANS averages over long lateral distances (typically, the whole sample), thus certain specifics of the structural−magnetic correlation can be easily overlooked. To gain a more microscopic insight of the structural−magnetic interface correlation, high-resolution magnetic transmission electron microscopy techniques like e-holography could potentially be applied, although it has been rarely used to study bimagnetic core/shell nanoparticles.25,26 A very specific TEM-based technique to study the magnetic information at an atomic/nanometric length scale is energy- loss chiral magnetic dichroism, or electron magnetic circular dichroism (EMCD).27,28 EMCD is based on studying the difference between two EEL spectra acquired under the appropriate scattering conditions inside the TEM.27,29,30 Although this approach allows obtaining the local magnetic moment with subnanometer precision, it has seldom been utilized on nanoparticles.29,31 Here, we demonstrate that EMCD can disentangle the magnetic properties of iron oxide-based core/shell nanocubes, consisting of a ferrimagnetic magnetite (Fe3O4) shell and an antiferromagnetic wüstite (FeO) core. Combining a high spatial resolution (0.45 nm) with clustering techniques for EELS data analysis,32 the existence of a graded magnetic interface, associated with an oxidation gradient between the shell and core, is unambiguously established. It is interesting to note that an in-depth comprehension of the magnetic structure in bimagnetic nanoparticles3,16,18,20 and, specifically, in FeO/ Fe3O4 core/shell systems is essential not only from a more fundamental magnetic point of view but also for the optimization of their magnetic properties for their use in diverse fields like biomedical applications (e.g., magnetic hyperthermia or magnetic bioassays) or microwave absorb- ers.33−36 ■ RESULTS AND DISCUSSION Morphological Characterization. The FeO/Fe3O4 core/ shell nanoparticles, synthesized by thermal decomposition (see Methods in Supporting Information), show a rather regular square shape and a high crystallinity with an average particle Figure 1. (a) ABF STEM image at low magnification of the FeO/ size of 29 ± 3 nm (Figure 1a). The core/shell structure was Fe3O4 core/shell nanocubes. (b,c) ABF STEM images of an individual core/shell nanocube at higher magnification. verified by means of annular bright-field (ABF) and annular dark-field (ADF) scanning transmission electron microscopy (STEM) images, evidencing a rather homogeneous morphol- corresponds to a neighboring particle. Although in the image ogy with a shell thickness of about 5−6 nm (Figure 1b,c) along they appear to be in contact, the particles are not overlapping with a cubic shape core, in agreement with previous studies on as shown in Figure S2. Additional analysis of the EELS data similar nanoparticles.37,38 In contrast to other oxide core/shell using alternative calculation approaches40−47 are presented in nanoparticles,39 it is worth noting that the core/shell structure Figures S3a−c. All of the four methods used show very similar (and oxidation states; see below) remains very stable under the results (see Methods in Supporting Information for details). current imaging conditions (200 kV acceleration voltage, beam The core/shell structure can be clearly identified in these currents of tens of pA), even after prolonged electron beam oxidation state maps. The profile of the oxidation state exposure. Pure Fe3O4 nanoparticles were also grown by presented in Figure 2b (left axis) has been calculated from the thermal decomposition to serve as a reference. They also show mean values obtained for the four calculation methods with an a good cubic morphology with an excellent crystallinity, and additional vertical average within the dotted box in Figure 2a, the same particle size of 29 ± 3 nm as the FeO/Fe3O4 core/ to improve signal-to-noise ratio. The oxidation state of the shell nanoparticles (Figure S1). shell is close to the expected +2.65 for Fe3O4. On the other Oxidation State and Compositional Analysis. Figure hand, within the core regions the value of the oxidation state is 2a depicts the spatial map of the Fe oxidation state obtained closer to +2.2, a value that is larger than the one corresponding from the distance between the oxygen K edge and the L3 iron to FeO (+2.0). This finding is probably a consequence of the white line (method ii, see Methods in Supporting Informa- measurements representing a 2D projection of 3D structure of tion). Note that the signal at the left-hand side of the image the particles, that is, the shell covering the core. Since the 6924 https://doi.org/10.1021/acs.nanolett.1c02089 Nano Lett. 2021, 21, 6923−6930 Nano Letters pubs.acs.org/NanoLett Letter electrons must go through the whole particle (core and both upper and lower shells), the shells also contribute to the EELS signal represented in the core regions. An elemental quantification was also extracted from the EELS data. The results are shown in the color map of Figure S3d, where it is presented the oxygen elemental composition in atomic percent (at. %). This map also shows the core/shell structure, exhibiting an oxygen composition of about 57 at. % for the shell regions (in agreement to the magnetite stoichiometry), and an oxygen composition of around 52 at. % for the core region, which is a slightly higher than the expected stoichiometry for wüstite (50 at. %) but consistent with having the upper and lower magnetite shells. The oxygen concentration profile presented in Figure 2b (right axis) has been also calculated by averaging the corresponding values inside the dotted box in Figure 2a. The results of these averaged quantifications (oxidation state and oxygen composition) definitely confirm the presence of an oxidation gradient near the core/shell interface extending about 3.0 ± 0.5 nm from the interface (as depicted in Figure 2b). In addition, there is an almost perfect agreement between the oxidation gradient and the composition gradient. Thus, the sample could be better described as FeO/FexOy/Fe3O4. Note that although the presence of such a graded interface had already been proposed for FeO/Fe3O4 nanoparticles using diverse approaches (lattice parameter examination, X-ray analysis, and Mö ssbauer analysis).13,25,48−50 The EELS oxidation and compositional maps give a definitive proof of the origin of the gradient, which is mainly related to the change of the oxidation state. To further confirm the graded interface in the FeO/Fe3O4 nanocubes, we used principal component analysis (PCA)51 (to reduce the noise) and k-means clustering algorithm32 (to identify different zones in the nanoparticle) (see Methods in Supporting Information). This latter technique allows detecting areas in the nanoparticle with analogous EELS characteristics. Interestingly, the clustering algorithm identifies four concentric areas of distinct properties, that is, inner core/ outer core/inner shell/outer shell (Figure 2c). When comparing these clusters with the averaged oxidation state, the second and third clusters coincide with the oxidation and compositional gradients (Figure 2). Thus, the interface region (where the gradients are found) is identified by the clustering algorithm as two clusters, which is independently verified by the elemental quantification and oxidation state analysis. Consequently, the gradient zone is composed of two dissimilar regions, which can be interpreted as the nanoparticle being formed by a multishell, “onionlike”, structure, FeO/Fe1+xO/ Fe3−δO4/Fe3O4, as proposed by Ichikawa et al.13 Finally, note that the pure Fe3O4 nanoparticles exhibit a very homogeneous oxidation state (+2.65 ± 0.05; using the four analysis approaches) as well as a homogeneous oxygen composition over the whole particle (Figure S1d,e), corrob- orating that the graded oxidation interphase in the bimagnetic system is an intrinsic configuration at the core−shell interface rather than to a spurious self-reduction/oxidation process of Figure 2. (a) Color map of the iron oxidation states of a FeO/Fe3O4 the Fe3O4 phase or the surrounding medium.52,53 core/shell nanocube obtained from the distance between the oxygen Electron Magnetic Circular Dichroism (EMCD). In K edge and the L3 iron white line (method ii, see Methods in Supporting Information). (b) Line profiles of the averaged oxidation order to investigate the effect of the graded interface on the state (black dots) and the oxygen elemental composition (green dots; magnetic structure of the FeO/Fe3O4 nanocubes, we carried see Figure S3d) (in both cases averaged vertically within the dotted out an EMCD study at low temperature. Two EEL spectra box in (a), and in the first case for the four analysis methods; see with different chirality were obtained in two-beam conditions Figure S3). (c) Color map with the results of the clustering analysis. for each nanocube (see Methods in Supporting Information). 6925 https://doi.org/10.1021/acs.nanolett.1c02089 Nano Lett. 2021, 21, 6923−6930 Nano Letters pubs.acs.org/NanoLett Letter Figure 3. Color maps resulting from the clustering analysis for (a) chiral (+) and (b) chiral (−) configuration of a core/shell nanocube. (c−f) Spectra of the + and − chiralities for each section identified in the clustering and the corresponding EMCD signal: (c) outer shell (violet); (d) inner shell (orange); (e) outer core (green); and (f) inner core (blue). The spectra correspond to the “chiral +” (right circular different chiralities. The spectra are clearly different (Figure polarization) and the “chiral − ” (left circular polarization), S4). Subtracting the + from the − spectra, we obtain the respectively. The + and − spectra were subject to the same EMCD signal corresponding to the Fe3O4 nanocubes (Figure previous corrections and clustering algorithms (see Methods in S4). The EMCD results obtained for the different particles are Supporting Information). quite similar and comparable to literature data on Fe3O4 films First, we analyzed several pure Fe3O4 nanocubes to serve as and nanoparticles.29−31 reference. The clustering analysis of the + and − spectra Next, we analyzed the + and − EEL spectra for the core/ revealed homogeneous EEL spectra over the whole sample. shell nanocubes. Importantly, since a pixel-by-pixel comparison Figure S4 shows the EEL spectra corresponding to the two would be extremely complex in our case due to the weak and 6926 https://doi.org/10.1021/acs.nanolett.1c02089 Nano Lett. 2021, 21, 6923−6930 Nano Letters pubs.acs.org/NanoLett Letter noisy signal and the small thermal drifts during the acquisition of the two different chiralities (resulting from small temper- ature fluctuations), we used the clustering algorithm to identify equivalent regions for each chirality. Note that the clustering procedure allows in a simple and efficient way, to classify the large quantity of spectra from the nanocubes and to reduce the intrinsic noise in data by averaging all of the spectra within a given cluster. As expected, the clustering results (i.e., areas with analogous characteristics) were very similar for both chiralities (Figure 3a,b). Similar to the pure Fe3O4 nanocubes, when comparing the EEL spectra of equivalent clusters in the two chiralities it can be clearly seen that the ± EEL spectra are slightly different (Figure 3c−f). By subtracting the + and − spectra for the equivalent regions, we obtain the EMCD spectra of that given area (see Figure 3c−f).54,55 Remarkably, the EMCD spectra for all of the four regions are distinctly different. This implies that the oxidation gradient induces Figure 4. Area of the EMCD spectra for the different clusters. The unambiguous changes in the magnetic structure of the blue region depicts the area calculated for the pure Fe3O4 nanocubes nanocubes. Notably, although the intensity decreases the (see Figure S4) where the width of the band corresponds to the shape of the spectra for the four regions is somewhat similar standard deviation of the results for the different particles. (and similar to the Fe3O4 reference sample). This is somewhat expected for the surface and inner shell since they are both effect that can be inferred for the magnetic moment is the mainly Fe3O4. However, the outer and inner regions of the unusually low moment in the outer Fe1+xO core. Namely, due core are mostly FeO, thus some changes in the EMCD spectra to vacancy clustering Fe1+xO is expected to have larger could potentially be anticipated. The inner FeO core should be moment than FeO. In contrast, the moment of the Fe1+xO antiferromagnetic at 100 K (Néel temperature TN ∼ 200 K; region seems to be slightly smaller than the one in the FeO Figure S5),37 thus no circular dichroism would be expected. region. This implies that the intrinsic moment of the Fe1+xO Hence, the dichroic signal should arise mainly from the outer core should be opposite to the concomitant contribution contribution of the Fe3O4 shells (since the electrons have to go of the Fe3−δO4/Fe3O4 shells (arising from the path of the through the shell before and after they probe the core). electrons). The simplest way to account for this effect is Concerning the Fe1+xO outer core, it is well-known that the probably to assume an antiferromagnetic coupling between the vacancies in Fe1+xO tend to cluster, forming Fe3O4-like regions Fe3O4 shell and the vacancy-induced ferrimagnetic-like spins of inside an FeO matrix.56 Consequently, it is not surprising that the core (as predicted theoretically and hinted experimentally the EMCD spectrum of the Fe1+xO outer core is similar to the for FeO/Fe3O4 and actually observed for other FeO-based shell signals. systems63−65). In fact, this assumption could be supported by To get further insight into the magnetic structure, we tried the unusually large downturn in M(T) below TN of FeO to quantify the dichroic signal of the different regions. (Figure S5). Note that antiferromagnetic core/shell interface Unfortunately, the measuring conditions do not allow to coupling has actually been reported in other iron oxide-based obtain the absolute magnetic moment for each region.57 bimagnetic core/shell nanoparticles.17,66 However, since all of the measurements were carried out in the In addition, the overall results of the same type of analysis same conditions, relative magnetic moments can be obtained applied to other particles (by studying a section of the by integrating the area of the EMCD peaks and compared nanoparticle rather than the whole nanoparticle) were among them. As can be seen in Figure 4, a clear trend emerges comparable, confirming the presence of a gradual change of from the relative moments. The outer shell has the largest the oxidation state at the interface leading to a magnetic magnetic moment, which decreases toward the inner shell gradient (see Supporting Information; Figure S6). becoming the smallest in the two core regions (Figure 4), Notably, this STEM-based EELS approach merging evidencing that the oxidation gradient induces a magnetic compositional and magnetic information is not only limited gradient in the nanoparticles. However, two unusual features to oxide-based core/shell nanoparticles, but it could also be can be identified. First, the magnetic moment of the shell applied to other types of core/shell structures (e.g., metal/ seems to be larger than the reference pure Fe3O4 nanocubes. metal, metal/oxide) and to other morphologies (e.g., Janus This is somewhat unexpected since in pure Fe3O4 nano- nanoparticles) or even inhomogeneous nanoparticles (e.g., phase segregation) or nanostructured composites. ■ particles the surface tends to have a lower magnetic moment than the bulk due to surface disorder.29 Nevertheless, it is important to emphasize that magnetic moments considerably CONCLUSIONS larger than bulk values have been often reported in Fe3O4 (and In summary, we have demonstrated that the combination of other ferrites) thin films.58−61 These enhanced moments are EELS compositional mapping and EMCD is an excellent typically reported to occur for very thin films or at surfaces and option to unravel the correlation between the structural/ are usually linked to specific defects such as grain boundaries, morphological and magnetic structure in bimagnetic core/shell vacancies, or antiphase boundaries.58−61 Thus, common nanoparticles. Importantly, by using a clustering algorithm the defects often observed in this type of (and similar) nano- analysis of the EMCD can be greatly simplified but still achieve particles, such as cation inversion, lattice distortions, Fe2+ nanometer resolution. The results show that the FeO/Fe3O4 vacancies, or grain boundaries could also contribute to the nanocubes have an interdiffused interface leading to a FeO/ enhanced moment.37,49,62,63 The second possible uncommon Fe1+xO/Fe3−δO4/Fe3O4 onionlike structure. This structure 6927 https://doi.org/10.1021/acs.nanolett.1c02089 Nano Lett. 2021, 21, 6923−6930 Nano Letters pubs.acs.org/NanoLett Letter results in a graded magnetic configuration with some unusual Notes features, like an enhanced moment for the surface Fe3O4 shell. The authors declare no competing financial interest. ■ These results open new avenues for studying the magnetic properties of not only complex magnetic nanoparticles but also ACKNOWLEDGMENTS nanostructured materials. The authors acknowledge the financial support from the ■ * ASSOCIATED CONTENT sı Supporting Information Spanish Minister of Science and Innovation (MICINN) through the projects PID2019-106165GB-C21, PID2019- 106165GB-C22, and PID2019-106229RB-I00. They also The Supporting Information is available free of charge at acknowledge funding from Generalitat de Catalunya through https://pubs.acs.org/doi/10.1021/acs.nanolett.1c02089. the 2017-SGR-292 and 2017-SGR-776 projects. In addition, research at UCM was supported by MINECO/FEDER Descriptions and details of the experimental methods: MAT2015-66888-C3-3-R and RTI2018-097895-B-C43 grants. nanoparticles synthesis, morphological characterization, ICN2 is funded by the CERCA programme/Generalitat de electron energy-loss spectroscopy, determination of Catalunya. The ICN2 is supported by the Severo Ochoa oxidation state, determination of the elemental compos- Centres of Excellence programme, funded by the Spanish tion, the electron magnetic circular dichroism technique, Research Agency (AEI, Grant SEV-2017-0706). M.E. thanks and magnetic characterization; Figures S1 to S9 (PDF) the Spanish MICINN and AEI/FSE for Ramón y Cajal ■ AUTHOR INFORMATION Corresponding Authors contract (RYC2018-024396-I). A.L.O. acknowledges support from the Universidad Pública de Navarra (Grant PJUP- NA2020). STEM-EELS observations carried out at the Centro Nacional de Microscopía Electrónica at Universidad Complu- Daniel del-Pozo-Bueno − LENS-MIND, Department tense de Madrid, Spain (ICTS ELECMI). ■ Enginyeries Electrònica i Biomèdica, Universitat de Barcelona, E-08028 Barcelona, Spain; Institute of Nanoscience and Nanotechnology of the University of Barcelona (IN2UB), E- REFERENCES 08028 Barcelona, Spain; orcid.org/0000-0003-1819- (1) Wilks, S. P. 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