Origin of the large dispersion of magnetic properties in nanostructured oxides: FexO/Fe3O4 nanoparticles as a case study

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.-- et al.The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.This work was supported by the 2014-SGR-1015 and 2009-SGR-35 projects of the Generalitat de Catalunya, by the MAT2010-20616-C02, MAT2011-27380-C02-01, MAT2010-16407, MAT2013-48628-R and CSD2009-00013 projects of the Spanish Ministerio de Economía y Competitividad (MINECO), the ONDA and COEFNANO projects (no. FP7-PEOPLE-2009-IRSES-247518 and no. FP7-PEOPLE-2012-IRSES-318901) of the European Union, the Russian grants RFBR 13-02-00121, 13-02-12429 and RG 14.B25.31.0025, the Brazilian grants CNPq-304368/2010-5 and FAPEMIG-PPM 00319-11, the Argentine grants PIP 1333 (CONICET) and SECTyP 06/C404 (Univ. Nac. de Cuyo) and the Swedish Research Council (VR). Research at NCSR “D” was supported by the HSF-EU program ARISTEIA, grant COMANA/22. GSA was partially supported by the Knut and Alice Wallenberg Foundation (Project: 3DEM-NATUR). I.V.G. thanks the Generalitat de Catalunya for his sabbatical fellowship (2010 PIV 00096). M.D.B. was partially supported by an ICREA Academia award. M.E. acknowledges the Spanish Ministry of Science and Innovation through the Juan de la Cierva Program. A. G. Roca would like to thank Generalitat de Catalunya for financial support under the Beatriu de Pinós fellowship program (2011 BP_B 00209). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, grant SEV-2013-0295).Peer Reviewe

Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe3O4/Mn3O4 Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of FeO/MnO core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the FeO and MnO magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the MnO shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the FeO cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials

Elucidating individual magnetic contributions in bi-magnetic Fe3O4/Mn3O4 Core/Shell nanoparticles by polarized powder neutron diffraction

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3O4/Mn3O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3O4 and Mn3O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials.I.V.G. acknowledges financial support from the Russian Foundation for Basic Research under Grant No 20-02-00109. A.G.R. and J.N. acknowledge financial support from the grants PID2019-106229RB-I0 funded by MCIN/AEI/10.13039/50110001103 and 2021-SGR-00651 from Generalitat de Catalunya. I.K. and A.G. acknowledge the European Union's H2020 reserach and innovation program, Grant agreement No 871072. A.G.R. acknowledges financial support from RYC2019-027449-I funded by MCIN/AEI/10.13039/501100011033. ICN2 is funded by the CERCA programme/Generalitat de Catalunya. The ICN2 is supported by the CEX2021–001214–S grant funded by MCIN/AEI/10.13039/501100011033. M.E. acknowledges the grants RYC2018-024396-I and PID2019-106165GB-C22 funded by MCIN/AEI/ 10.13039/501100011033 and by “ESF Investing in your future.” A.L.O. acknowledges financial support from the grants PID2021-122613OB-I00 funded by MCIN/AEI/ 10.13039/501100011033 and PJUPNA2020 from Universidad Pública de Navarra

Remanence plots as a probe of spin disorder in magnetic nanoparticles

Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the M plots of γ-FeO nanoparticles isolated by thick SiO shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and δM plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles

Strongly exchange coupled inverse ferrimagnetic soft/hard, Mn(x)Fe(3-x)O(4)/Fe(x)Mn(3-x)O(4), core/shell heterostructured nanoparticles

Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of Mn xFe 3-xO 4/Fe xMn 3-xO 4 with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (∼0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific application

Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles

The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive. Here we show a robust antiferromagnetic (AFM) coupling in core/shell nanoparticles which, in turn, leads to the foremost elucidation of positive exchange bias in bi-magnetic hard-soft systems and the remarkable regulation of the resonance field and amplitude. The AFM coupling in iron oxide manganese oxide based, soft/hard and hard/soft, core/shell nanoparticles is demonstrated by magnetometry, ferromagnetic resonance and X-ray magnetic circular dichroism. Monte Carlo simulations prove the consistency of the AFM coupling. This unique coupling could give rise to more advanced applications of bi-magnetic core/shell nanoparticles

Origin of the large dispersion of magnetic properties in nanostructured oxides: FexO/Fe3O4 nanoparticles as a case study

The intimate relationship in transition-metal oxides between stoichiometry and physiochemical properties makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxide

Origin of the large dispersion of magnetic properties in nanostructured oxides: FexO/Fe3O4 nanoparticles as a case study

The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides

Study of the influence of surface and interface on the magnetic behaviour of nanoparticles

The magnetic nanoparticles (NP) have magnetic behavior different from that of the bulk materials. Their magnetic properties which are new or modified from that of the bulk materials have created new challenges for their study and novel technological applications. In the present work, an atomic three-dimensional model of a spherical nanoparticle has been developed taking into account explicitly each atomic magnetic moment. Furthermore a computational code has been developed using the Metropolis Monte Carlo algorithm to simulate the magnetic behavior of antiferromagnetic, ferrimagnetic and composite NP with a core/shell morphology. Our simulations have demonstrated the dominant role of the surface to the magnetic behavior of antiferromagnetic and ferrimagnetic NP due to its anisotropy and the number of 'uncompensated' magnetic moments. The behavior of the coercive field Hc for the ferrimagnetic NP depends on the details of their surface and their size. The experimental findings for nanoparticles NiO and γ-Fe₂O₃ confirmed our simulation results. Our study has also extended to the case of composite NP with ferromagnetic core and a ferrimagnetic disordered shell. We have demonstrated the important role of the interface in their magnetic behavior. We showed that Hc, exchange bias field HE and remanent magnetization depend on the cooling field. Also the increase of the ferrimagnetic shell thickness causes at first the increase and the stabilization of the Hc and the HE, and the increase of the core radius causes the reversed dependence of the Hc on the temperature above a critical temperature. In all cases Hc and HE decrease exponentially with temperature due to the disordered shell. The training and aging effects, which are characteristic of disordered systems, have also been studied for the composite NP. The simulation results are in agreement with the experimental ones. Finally a novel model for an assembly of composite NPs with ferromagnetic core/antiferromagnetic shell morphology has been developed to study the interface effects together with the interparticle dipolar interaction effects using the Monte Carlo technique. In our study the comparison of the magnetic behavior of the assembly of the composite NPs with that of the assembly of ferromagnetic NPs demonstrated the important role of the exchange anisotropy along the interface. Our results are in agreement with the experimental findings for an assembly of Co NPs embedded in Mn and Ag matrices.Η μελέτη νέων αλλά και διαφοροποιημένων σε σχέση με αυτές των συμπαγών μαγνητικών υλικών ιδιοτήτων των συστημάτων μαγνητικών νανοσωματιδίων ανοίγει νέους ορίζοντες στην ανάπτυξη τεχνολογικών εφαρμογών. Στην παρούσα εργασία μελετήθηκε ένα τρισδιάστατο μοντέλο σφαιρικού νανοσωματιδίου στο οποίο κάθε ατομική μαγνητική ροπή λαμβάνεται ρητά υπόψη. Αναπτύχθηκε υπολογιστικά ένας κώδικας που χρησιμοποιεί τον αλγόριθμο Metropolis Monte Carlo για την προσομοίωση της μαγνητικής συμπεριφοράς αντισιδηρομαγνητικών, σιδηριμαγνητικών νανοσωματιδίων καθώς και σύνθετων νανοσωματιδίων με μορφολογία σιδηρομαγνητικού πυρήνα/σιδηριμαγνητικού φλοιού. Οι προσομοιώσεις μας ανέδειξαν τον κυρίαρχο ρόλο της επιφάνειας στη μαγνητική συμπεριφορά των αντισιδηρομαγνητικών νανοσωματιδίων, τόσο εξαιτίας του είδους και μεγέθους της ανισοτροπίας της όσο και εξαιτίας του αριθμού των 'μη αντισταθμισμένων' μαγνητικών ροπών. Στα σιδηριμαγνητικά νανοσωματίδια η συμπεριφορά του συνεκτικού πεδίου (Hc) εξαρτάται τόσο από την επιφάνεια όσο και από το μέγεθος τους. Τα αποτελέσματα μας επιβεβαιώθηκαν από αντίστοιχα πειραματικά για νανοσωματίδια NiO και γ-Fe₂O₃. Η μελέτη μας επεκτάθηκε στα σύνθετα νανοσωματίδια με σιδηρομαγνητικό πυρήνα και άτακτο σιδηριμαγνητικό φλοιό όπου τα φαινόμενα διεπιφάνειας επηρεάζουν την μαγνητική συμπεριφορά τους. Βρέθηκε ότι το Hc, το πεδίο ανταλλαγής (HE) και η παραμένουσα μαγνήτιση εξαρτώνται από το πεδίου ψύξης. Επίσης η αύξηση του πάχους του σιδηριμαγνητικού φλοιού οδηγεί αρχικά στην αύξηση και μετά στη σταθεροποίηση των πεδίων Hc και HE ενώ η αύξηση της ακτίνας του πυρήνα προκαλεί αντιστροφή στην εξάρτηση του Hc από τη θερμοκρασία πάνω από μια κρίσιμη θερμοκρασία. Σε όλες τις περιπτώσεις τα πεδία Hc και HE μειώνονται εκθετικά με τη θερμοκρασία εξαιτίας του άτακτου φλοιού. Ακόμα μελετήθηκαν τα φαινόμενα παίδευσης και γήρανσης που οφείλονται στον άτακτο φλοιό. Τα αποτελέσματα μας συμφωνούν με τα αντίστοιχα πειραματικά. Τέλος προχωρήσαμε στην ανάπτυξη ενός νέου μοντέλου συλλογής σύνθετων νανοσωματιδίων με μορφολογία σιδηρομαγνητικού πυρήνα/αντισιδηρομαγνητικού φλοιού για τη μελέτη των φαινομένων διεπιφάνειας και διπολικών αλληλεπιδράσεων με τη μέθοδο Monte Carlo. Η σύγκριση της μαγνητικής συμπεριφοράς της παραπάνω συλλογής με αυτής μιας συλλογής απλών σιδηρομαγνητικών νανοσωματιδίων ανέδειξε το ρόλο της ανισοτροπίας ανταλλαγής της διεπεφάνειας. Τα αποτελέσματα μας συμφωνούν με τα πειραματικά αποτελέσματα για συλλογές νανοσωματιδίων Co σε μήτρες Ag και Μn

Enhanced magnetic properties in antiferromagnetic-core/ferrimagnetic-shell nanoparticles

Bi-magnetic core/shell nanoparticles are gaining increasing interest due to their foreseen applications. Inverse antiferromagnetic(AFM)/ferrimagnetic(FiM) core/shell nanoparticles are particularly appealing since they may overcome some of the limitations of conventional FiM/AFM systems. However, virtually no simulations exist on this type of morphology. Here we present systematic Metropolis Monte Carlo simulations of the exchange bias properties of such nanoparticles. The coercivity, H C, and loop shift, H ex, present a non-monotonic dependence with the core diameter and the shell thickness, in excellent agreement with the available experimental data. Additionally, we demonstrate novel unconventional behavior in FiM/AFM particles. Namely, while H C and H ex decrease upon increasing FiM thickness for small AFM cores (as expected), they show the opposite trend for large cores. This presents a counterintuitive FiM size dependence for large AFM cores that is attributed to the competition between core and shell contributions, which expands over a wider range of core diameters leading to non-vanishing H ex even for very large cores. Moreover, the results also hint different possible ways to enhance the experimental performance of inverse core/shell nanoparticles for diverse applications