Building Composite IronâManganese Oxide Flowerlike

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Building Composite Fe-Mn Oxide FlowerLike Nanostructures: A Detailed Magnetic Study Efisio Zuddas, Sergio Lentijo-Mozo, Alberto Casu, Davide Deiana, and Andrea Falqui J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04915 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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The Journal of Physical Chemistry

Building Composite Fe-Mn Oxide Flower-like Nanostructures: a Detailed Magnetic Study Efisio Zuddas†,△, Sergio Lentijo-Mozo†,△, Alberto Casu†,*, Davide Deiana‡,* and Andrea Falqui†* †

King Abdullah University of Science and Technology (KAUST), Biological and Environmental Sciences and Engineering (BESE) Division, Nabla Lab, Thuwal 23955-6900, Saudi Arabia ‡ Centre Interdisciplinaire de Microscopie Électronique (CIME), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland △

These authors contributed equally to the work.

ABSTRACT: Here we show that it’s possible to produce different magnetic core-multiple shells heterostructures from monodispersed Fe3O4 spherical magnetic seeds by finely controlling the amount of a manganese precursor and using in a smart and simple way a cation exchange synthetic approach. In particular, by increasing the amount of precursor we were able to produce nanostructures ranging from Fe3O4/Mn-ferrite core/single shell nanospheres to larger, flower-like Fe3O4/Mn-ferrite/Mn3O4 core-double shell nanoparticles. We first demonstrate how the formation of the initial thin manganese-ferrite shell determines a dramatic reduction of the superficial disorder in the starting Fe3O4, bringing to nanomagnets with lower hardness. Then, the growth of the second and most external manganese oxide shell causes the magnetical hardening of the heterostructures, while its magnetic exchange coupling with the rest of the heterostructure can be antiferromagentic or ferromagnetic, depending on the strength of the applied external magnetic field. This response is similar to that of an iron oxide-manganese oxide core-shell system but differs from what observed in multiple-shell heterostructures. Finally, we report as the most external shell becomes magnetically irrelevant above the ferrimagnetic-paramagnetic transition of the manganese oxide and the resulting magnetic behavior of the flower-like structures is then studied in-depth.

1. Introduction Magnetic nanoparticles (NPs) are a well-known and very attractive class of materials, whose application spans very different fields, going from catalysis and magnetic energy storage to biomedical applications.1-5 The constant development and refinement of chemical methods allowed the preparation of narrow size-distributed magnetic NPs with an ever growing range of compositions and morphologies; however, this steady push also implies a demand for finer chemo-structural tuning to prepare novel, advanced magnetic nanostructures. In fact, among the different possible routes to enhance the magnetic NPs properties, the development of different asymmetric shapes6 and the preparation of composite heterostructures7 represent a widely known and powerful approach to achieve advanced materials with diverse functionalities. In this context, core/shell systems constitute an interesting example of multi-component structure, where the core (inner material) is surrounded by one or more shells (outer layer material). Although the shells were initially developed as protecting layers8 for the core NPs, they were subsequently used as an active component of the heterostructures, providing additional functions or properties that could not be obtained in individual materials.9 The bi-magnetic core-shell nanostructures can be rightfully set in this framework, since the combination of a

core and a shell both exhibiting magnetic properties can trigger interaction-based effects (e.g. exchange bias) that strongly affect the overall magnetic response of such composite systems.7,10,11 In fact, starting from the study of the exchangebiased ferromagnetic/antiferromagnetic Co/CoO core-shell system,11 several chemical approaches have been proposed to synthesize different bi-magnetic core-shell systems. The seeded growth method, which consists in using pre-made NPs as seeds for the deposition of a shell, emerged as the most efficient strategy for the synthesis of core-shell bi-magnetic oxide nanocrystals and different wet chemistry approaches have been used to grow the shell, going from thermal decomposition method,12-14 to co-precipitation,15 microemulsion,16 and sol-gel methods.17 Alternatively, surface treatment by oxidation18 or reduction19 processes of the seed NPs could result in the formation of one or more external layers with different phases, leading to composite systems ranging from simple core-shell to more complex multi-shell ones.20,21 All these most promising synthetic methods rely on a first step (i.e. the seed formation) that cannot guarantee the synthesis of NPs with the desired shape and/or composition. In addition, post-synthetic cation exchange (CE) reactions offer a strategy to partially or completely replace the cations of parent NPs while concomitantly preserving the anionic sublattice. In such a case, the possible outcomes are the formation of new

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compounds (for ubiquitous CE reactions)22 or composite, structured systems (for localized and/or partial CE reactions).23 With this in mind, we moved from recent works24,25 on the structure of bi-magnetic Fe, Mn oxide-based core-shell NPs synthesized by a seeded-growth approach on pre-synthesized magnetite NPs, to develop a bi-magnetic core/shell system based on a mixed CE/seeded growth approach. Starting from single crystalline, colloidal Fe3O4 NPs and with the aim to substitute the Fe2+ with Mn2+ ions by a post-synthetic CE protocol we obtain a mixed Fe/Mn shell, while preserving their fcc lattice, shape and size, while dramatically reducing the superficial disorder of the starting Fe3O4 NPs. We also show that this in turn makes the core-shell structure magnetically softer. Besides, we show that pushing the reaction over a critical Mn concentration threshold results in the growth of an external Mn3O4 shell over the cation-exchanged composite cores/seeds with a concomitant increase in the NPs magnetic hardness. In particular, we observe that the most external manganese oxide shell could be magnetically switched on/off by keeping the core-double shell nanoparticles either below or above the ferrimagnetic-paramagnetic Mn3O4 transition temperature, respectively. However, while in the work developed by Estrader et al.,24 this phenomenon was shown to occur when the Fe3O4 core was in direct contact with a discontinuous manganese oxide shell, here we report that it could occur also where an intermediate thin shell of manganese ferrite is present between them. Finally we show that below such a temperature, the exchange coupling between the core and the Mn3O4 shell is antiferromagnetic at low temperature. This is different from what reported by Salazar-Alvarez et al.,20 where the presence of intermediate layers among the core and the most external layers brought to a ferromagnetic coupling. However, similarly to what reported by Estrader et al.,24 we show that such an antiferromagnetic coupling can be reversed to ferromagnetic by just increasing the strength of the external magnetic field. 2. Experimental section 2.1 Materials. Oleic acid (OA, 90%), 1-octadecene (90%), oleylamine (OLA, 70%), the iron precursor FeCl3.6H2O (98%), and the manganese precursor MnCl2 (99%) were purchased from Sigma-Aldrich. Sodium oleate (97%) was purchased by TCI, and tri-octylphospine (TOP, 97%) was purchased from STREM chemicals. Organic solvents like acetone, ethanol, and hexane were of analytical grade and obtained from various sources. All chemicals were used as received without any further purification. All experiments were carried out using standard airless techniques: a vacuum/dry nitrogen gas Schlenk line was used for synthesis and an Ar glove-box for storing and handling air-and moisture-sensitive chemicals. 2.2 Synthesis of the Materials 2.2.1 Preparation of the iron oleate precursor. FeCl3.H2O and sodium oleate were dissolved in a 1.5:1:2.5 ratio solution of ethanol, deionized water, and hexane. The resulting solution was heated at 70°C for 4 h. When the reac-

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tion was complete, the brown mixture was transferred to a separatory funnel and the aqueous layer was removed. The organic phase that contains the iron-oleate complex was washed three times with deionized water. After washing, the brown solution was dried with Na2SO4 anhydrous, and hexane was removed by evaporation. The final product was dried under vacuum, obtaining a brown waxy solid. Finally, a stable stock solution of the precursor in 1-octadecene was prepared for the subsequent synthesis. 2.2.2 Colloidal synthesis of Fe3O4 spherical NPs. A solution containing iron oleate in 1-octadecene, and oleic acid (molar ratio iron oleate to oleic acid was 1:0.5) were mixed in a three-neck round bottom flask at room temperature. The reaction solution was heated to 120°C and degassed under vacuum for 30 min. After being filled with a nitrogen atmosphere, the mixture reaction was heated up to 320°C with a heating rate about 3.3°C/min and aged for 45 min. The resulting black mixture was cooled to room temperature, and acetone was added to precipitate the NPs. They were separated by centrifugation and washed, first with acetone, and second with a mixture of combined solvents hexane/acetone several times until the supernatant was colorless. After washing, the NPs were dried, redissolved and stored in hexane. For simplicity, in the following this sample will be called FeO_pure. 2.2.3 Synthesis of Mn2+xFe2+1-xFe3+2O4 NPs by a cation exchange protocol. To proceed with the cation exchange (Mn2+ substituting the Fe2+) a mixture of MnCl2 (44 mg, 0.35 mmol) in OLA was heated at 140°C for 20 min to completely dissolve the MnCl2. After that, Fe3O4 spherical NPs dissolved in hexane were added to the yellow pale solution under N2 atmosphere, and the reaction mixture was degassed under vacuum to remove the hexane. Then, TOP (1 mL) was injected under N2 atmosphere, and the reaction mixture was heated to 200°C for 30 min. The resulting black mixture was cooled down to room temperature (RT), and acetone was added to precipitate the NPs. Again, they were separated by centrifugation and washed with acetone several times until the supernatant was colorless. Such a NPs synthesis procedure was repeated two more times, adding 132 mg of MnCl2 (1.05 mmol), and 176 mg of MnCl2 (1.40 mmol). In all the three cases the same amount of NPs Fe3O4 spheres was kept and the final black solid was dissolved and stored in hexane. Again for simplicity the three samples of which the synthesis is described above, in the following will be called MnFeO_0.35, MnFeO_1.05, and MnFeO_1.40, respectively. 2.3 TEM, STEM and STEM/EDS characterization of the materials. Conventional transmission electron microscopy (TEM) imaging was performed by a FEI Tecnai Spirit microscope, equipped with a lanthanum hexaboride thermionic electron source, a Twin objective lens, a Gatan Orius CCD camera and operating at an acceleration voltage of 120 kV. Spherical aberration (Cs) corrected HRTEM and STEM, along with EDS elemental mapping were carried out by a double spherical corrected FEI Titan Themis cubed microscope, equipped with


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an ultra bright Schottky (XFEG) electron source, a SuperX EDS spectrometer with 0.7 srad collection angle, a FEI Ceta CMOS camera and operating at an acceleration voltage of 200 kV. 2.4 Study of the magnetic behavior of the materials. Magnetic characterization was performed on a Quantum Design VSM SQUID magnetometer, equipped with a superconducting magnet producing fields up to 70 kOe (7 T) and a Helium Quantum Design Evercool liquefier. Zero-field-cooled (ZFC) and field-cooled (FC) magnetizations were collected in the range of temperatures 4÷400 K. ZFC curves were measured by cooling samples in a zero magnetic field and by subsequent increase of the temperature under an applied field of 50 Oe. FC curves were recorded by cooling the samples while maintaining the same applied field of 50 Oe. ZFC hysteresis loops were recorded up to ±70 kOe (7 T) at 4 K and 70 K, while FC hysteresis loops were recorded in the same field range and at the same temperature after cooling the samples under a magnetic field of 30 kOe (3T). To prepare the samples, the solutions were dried and the resulting compounds were measured by thermogravimetric analysis (TGA) to assess the percentage of magnetic phase effectively formed. The dried compounds were subsequently redispersed in hexane in order to obtain new solutions with a fixed, common percentage of magnetic phases with respect to the hexane volume. Finally, fixed volumes of the new solutions were drop-casted and then embedded in teflon tape. The data was subsequently normalized by the effective mass of the magnetic phase. Saturation magnetization values (MSAT) were determined from the hysteresis loops by extrapolation of M values vs. 1/H for 1/H→0. 3. Results and discussion 3.1. Synthesis of Mn-doped magnetite nanoparticles by a cation exchange protocol The synthesis and control composition of different Mndoped Fe3O4 magnetic NPs we show in this work were achieved by a colloidal synthetic approach, followed by a cation exchange treatment. Briefly, as more extensively aforementioned in the experimental section, this synthetic route is based on a two-steps process. First, we synthesized highly monodispersed spherical Fe3O4 NPs (FeO_pure) with a mean diameter

d=14.9±0.9 nm as starting material, by adapting the approach developed by Park et al.26 This preparation route was based on the synthesis and subsequent thermal decomposition of iron oleate precursor in a high boiling point organic solvent, and in presence of surfactants. In the second step, we used a cation exchange synthetic approach on the starting Fe3O4 NPs by adapting and modifying what described by Stynyk et al.27 Then, the manganese precursor was dissolved in oleylamine, which was used as both solvent and surfactant. After the complete precursor solubilization, a solution of FeO_pure in hexane, followed by TOP, was hot-injected in the solution of the metal precursor under inert atmosphere, which produced the extraction of part of Fe2+ from the starting crystal structure of the FeO_pure sample NPs, and the expected substitution with Mn2+ cations. Depending on the concentration of the manganese precursor employed in the cation exchange reaction, and while keeping constant the amount of FeO_pure NPs and the rest of the synthesis parameters, it is possible to modify the content of manganese introduced in the system, keeping unaltered the NPs size and shape only if the manganese atomic content introduced is kept below a certain threshold. Conversely, above this threshold a further magnetic shell is formed and its thickness increases with the Mn atomic content. In Figure 1 (A-D) conventional TEM images of the FeO_pure, MnFeO_0.35, MnFeO_1.05, and MnFeO_1.40 sample are reported, respectively. The starting FeO_pure sample (Figure 1A) displays monodispersed spherical NPs, with mean diameter d=14.9±0.9 nm. Both size and shape are retained in the MnFeO_0.35, which also shows spherical NPs with mean diameter d=15.3±1.4 nm (Figure 1B). When the amount of Mn doping is further increased to get the sample MnFeO_1.05, the overall NPs aspect starts to show differently shaped and more dispersed in size nanoparticles populations (Figure 1C). Measuring the nanoparticles mean size of the sample MnFeO_1.05 a diameter d=15.3±2.2 nm is then determined. Finally, the sample MnFeO_1.40, where the highest amount of manganese chloride was used to promote the reaction, looks dramatically changed: the spherical NPs are no longer observed, and much larger irregularly shaped NPs are found, with a mean diameter d=23.5±2.3 nm (Figure 1D). In any case, all the Mn-doped samples show that the size monodispersity observed for the starting material FeO_pure is basically retained. The NPs size histograms of all the samples are reported in Figure SI1.



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Figure 1. Conventional TEM images of the samples: (A) FeO_pure, (B) MnFeO_0.35, (C) MnFeO_1.05, and (D) MnFeO_1.40. Scale bar: 100 nm.

3.2 Structural and chemical characterization Structural and chemical characterization by HRTEM (High Resolution TEM) and HAADF STEM - EDS (High Angle Annular Dark Field Scanning TEM – Energy Dispersive XRay Spectroscopy) offers a deeper insight on the evolution of the NPs according to the increasing Mn doping regimes. In fact, HRTEM imaging of the starting material (FeO_pure) displayed in Figure 2A highlights the formation of single crystals with a high degree of crystalline strain, as pointed out by the analysis reported in Figure 2B, which is consistent with superficial disorder. A decrease of such a disorder can be then appreciated in the MnFeO_0.35 NPs, as shown in Figure 2C, D. In both cases the lattice spacings and angular relationships are consistent with the magnetite phase. This indicates that the Mn-exchanged NPs still exhibit single crystal features and that

the introduction of Mn by CE does not affect the anionic sublattice of the FeO_pure sample while, conversely, dramatically improves the superficial ordering of the NPs (Figure 2 A-D), as indicated by the shear strain mapping. The effect of the Mn introduction can also be observed by STEM-EDS mapping, with the final nanoparticle remaining unaltered in terms of shape and size (Fig 2 E, F) with respect to the starting ones made just by Fe3O4, but now with Mn EDS signal coming together with of the Fe from an external MnxFe3-xO4 shell having a thickness of about 1 nm, as shown in Figure 2 F. The EDS quantitative analysis performed on the thin Mn-ferrite allowed to estimate an average x value being equal to 0.49, which takes into account its intrinsically mediated nature, starting from a Fe-rich and Mn-poor state at the interface and evolving towards a Fe-poor and Mn-rich state at the surface.

Figure 2. HRTEM images and corresponding shear strain maps of samples FeO_pure (A, B) and MnFeO_0.35 (C, D). The color bars indicate shear strain values corresponding to a -/+10% variation; (E) STEM-EDS elemental mapping of sample MnFeO_0.35 revealing the distribution of Fe and Mn; (F) STEM-EDS intensity profile of the elemental distribution of Fe (red) and Mn (blue) measured along the yellow dashed line in (E).

A further increase in concentration of the manganese precursor yields a more pronounced effect on the resulting sample (MnFeO_1.05), which also affects its shape and size distribution. In fact, the increase in manganese (Mn+2) content triggers

the formation of a thicker Mn shell, with two possible different outcomes in terms of NPs: spherical NPs showcasing a slight increase in size and a second population of bigger-sized, irregular NPs. While the Fe3O4 core is not affected in any case,


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keeping the same diameter of about 15 nm regardless of the overall NPs shape and size, it is the Mn-based shell that determines the features of the final NPs, displayed in Figure 3A, B. In fact, both the spherical and faceted NPs present a thicker shell, which is composed of two distinct phases, the first MnxFe3-xO4 oxide shell already observed in the sample MnFeO_0.35 and formed in correspondence to the few most external layers of the Fe3O4 nanoparticles, and a pure manganese oxide external shell. Depending on the local availability of Mn precursor after the formation of the CE-based mixed shell, the second, more external Mn3O4 shell first grows epitaxially on top of the first MnxFe3-xO4 mixed shell, to gradually taking the Mn3O4 structure, as shown in Figure 3 B, C. This synthetic route is also corroborated by structural results obtained by HRTEM analysis, with the spherical NPs still showcasing only lattice set spacings and angular relationships corresponding to a cubic phase and the irregular NPs showing an additional tetragonal phase compatible with hausmannite (Mn3O4) in correspondence with the thicker external shell

(Figure 3A). Such a nucleation and heterogeneous growth of a manganese oxide phase on the surface of the CE NPs28 becomes more evident by pushing further the Mn-to-Fe ratio during the reaction. The overabundance of Mn precursor triggers the growth of differently shaped, bigger NCs in sample MnFeO_1.40, as shown in Figure 3D, E. Structural analysis and elemental mapping show that these newfound NCs are in fact a core/double shell system arranged in a flower-like shape, with an external shell composed by extensive, irregular Mn3O4 crystallites grown over the CE MnxFe3-xO4 intermediate shell. In particular, while HRTEM mainly shows lattice sets spacings and angular relationships that are consistent with the Mn3O4 phase, EDS elemental mapping provides a fundamental insight on the spatial distribution of Mn and Fe, thus proving that the size of the Fe3O4 core is not affected even in this case, despite the major growth of the Mn3O4 external shell, as clearly displayed in Figure 3E, F.

Figure 3. HRTEM images of samples MnFeO_1.05 (A) and MnFeO_1.40 (D). Magnetite (Fe3O4) lattice distances are indicated in white color, hausmannite (Mn3O4) are indicated in green. (B, C) STEM-EDS elemental mapping and STEM-EDS intensity line profile showing the elemental distribution of Fe (red) and Mn (blue) in samples MnFeO_1.05 and (E, F) in sample MnFeO_1.40. The yellow dashed lines in panel (B) and (E) indicate the linear path used to get the elemental profiles shown in panel (C) and (F), respectively.

3.3. Study of the magnetic behavior The effects of introducing diverse Mn-based magnetic shells were studied by measuring the DC magnetization as a function of temperature upon low field condition (50 Oe) according to the Zero-Field Cooled and Field Cooled (ZFC and FC, respectively) protocols, the latter also measured under high magnetic

field condition (70 kOe). Isothermal hysteresis loops were then recorded at 4 and 70 K within an extended field range while the NPs were in a total or partial magnetically blocked state. With the aim to again elucidate the effects of both the surface spin disorder and the magnetic coupling among nanoparticles core and shells, hysteresis loops were measured after cooling the samples to 4 K in both zero (ZFC hysteresis) and strong (3T, FC hysteresis) magnetic field. 3.3.1. Low magnetic field regime



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The ZFC-FC magnetization curves displayed in Figure 4A are superimposed above a threshold temperature in all samples, indicating their transition from a blocked state to superparamagnetic relaxation within the experimental temperature range. Specifically, the temperatures corresponding to the maximum of the ZFC curve and to the point of superposition between the ZFC and FC curves are indicated as blocking temperature (TB) and irreversibility temperature (TIRR), respectively. TB and TIRR should tend to be very close for highly monodispersed NPs and their difference hints at the extent of size distribution in the NPs population. In the present case, an increase of both TB and TIRR can be observed when going from the FeO_pure sample to the ones containing increasing amount of Mn, along with a secondary step-like transition in the low temperature region for the Mncontaining samples, as shown in Figure 4A and Table 1. The explanation of the diverse trends affecting TB and TIRR on one side and the low temperature transition should be sought in the formation of the core/shell at first (sample MnFeO_0.35) and, subsequently, of the double-shell system (sample MnFeO_1.05 and MnFeO_1.40), with the TB and TIRR being mainly related to the main magnetic components of the system, and the low temperature transition observed at about 45 K being caused by the formation of a Mn-rich shell. In fact, the increase in TB and TIRR with respect to FeO_pure is caused by the formation of the aforementioned cationexchanged substoichiometric Mn-ferrite shell with fcc structure, which considerably diminishes the superficial disorder, as evidenced by the strain maps reported in Figure 2, thus also strongly increasing the effective volume of the non-defective central region of the NPs that has to be considered as giving rise to the magnetic moment blocking. Then, the further availability of Mn causes the growth of the second, thicker shell that takes the tetragonal structure of Mn3O4 and hinders the formation of the intermediate cubic shell. As a consequence, if at first the increase of the effective volume that goes to blocked state is indicated by the brisk increase in TB and TIRR for the MnFeO_0.35 sample with respect to FeO_pure one, then their subsequent decrease reflects the growth’s stop of the fcc MnxFe3-xO4 shell caused by the formation of the external Mn3O4 shell in the Mn-richer MnFeO_1.05 and MnFeO_1.40 samples, being all the corresponding values reported in Table 1. A similar trend can also be observed by analyzing the peaks of the derivative d(MZFC–MFC)/dT, reported in Figure 4B. In fact, since TB and TIRR values are affected by magnetic interaction effects, the derivative d(MZFC–MFC)/dT offers an effective

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way to observe variations in the anisotropy energy barrier as a function of temperature.29,30 The occurrence of peaks (TMAX) still hints at the mean size of the magnetically active nanoparticles, but these distributions are more strictly correlated to the actual magnetic size distribution of the NPs rather than their convolution with interaction contributions that conversely affect the ZFC curves. Then, the increase in temperature corresponding to the maxima of the d(MZFC–MFC)/dT for the Mncontaining samples with respect to the case of FeO_pure confirms the previously observed trend, with the difference between TMAX and TB values proving the role of interactions in affecting the samples overall magnetic response. Furthermore, a low temperature magnetization’s change can be clearly observed at around 45 K in the ZFC-FC curves of MnFeO_0.35, MnFeO_1.05 and MnFeO_1.40, with a modulation in magnetization of the FC curve that goes from a weak boost (MnFeO_0.35) to a strong decrease (MnFeO_1.05 and MnFeO_1.40), as observed in Figure 4A. This can be explained by a double and concomitant effect: a) the formation of a multi-shell system, and b) the concurring shift in balance between them and the core in terms of magnetic coupling. In fact, in the sample MnFeO_0.35 the magnetite core dominates the magnetic coupling with the thin MnxFe3-xO4 shell at low temperature, giving rise to a ferrimagnetic core/shell coupling. The gradual growth of the thicker Mn3O4 shell increases its net magnetization; therefore the shell cannot be forced anymore to a ferrimagnetic coupling below its Curie temperature and an antiferromagnetic coupling appears. The resulting decrease in magnetization becomes more pronounced as the thickness of the external Mn3O4 shell increases, leading to a more pronounced effect going from the MnFeO_1.05 to the MnFeO_1.40 sample, similarly to what reported by Estrader et al.,24 or NPs with Fe3O4 core and a single and discontinuous manganese oxide shell. The common transition temperature observed in the samples containing Mn-rich shells corresponds to the ferrimagnetic transition of Mn3O4, which takes place at TC=42 K for the material in bulk form31 and indicates that below that temperature the multi-shell system shows magnetic features strongly depending on the Mn3O4 phase, as also confirmed by the derivative (MZFC–MFC)/dT curves. In particular, the shallow fluctuation observed at low temperature for MnFeO_0.35 and corresponding to the thin fcc MnxFe3-xO4 shell leaves place to a sharp negative peak at the same temperature for the Mn-richer samples, caused by the presence of the well-formed tetragonal Mn3O4 shell.


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Figure 4. ZFC-FC magnetization and d(MZFC – MFC)/dT curves. (A) Zero Field Cooled (empty symbols) and Field Cooled (full symbols) curves measured in a magnetic field of 50 Oe. The dashed red line indicates the ferromagnetic to paramagnetic transition of Mn3O4; (B) Calculated anisotropy barrier variation with the temperature for all samples according to d(MZFC – MFC)/dT.

Sample

TB (K)

TIRR (K)

TMAX (K)

FeO_pure

181

200

120

MnFeO_0.35

230

240

138 (shoulder at 55)

MnFeO_1.05


232

120 (transition at 39)

MnFeO_1.40


235

137(transition at 37)

Table 1. Magnetic parameters obtained by both the ZFC-FC magnetization and d(MZFC–MFC)/dT curves reported in Figure 4: TB: temperature of the maximum in the ZFC curve; TIRR: temperature at which the ZFC and FC curves superimpose; TMAX: temperature of the maximum in the d(MZFC–MFC)/dT curve. The absolute error on each thermal value is equal to 1K.

3.3.2. High magnetic field regime The low-temperature transition observed at 45 K in the ZFCFC curves highlights the presence of two different regimes below the blocking temperature TB: the whole system, composed by the Fe3O4 core and the Mn-containing shells, is totally blocked below 45 K, while it is in an intermediate condition above it: in fact, the external Mn3O4 shell is in a paramagnetic state above the low-temperature transition, while the core and the intermediate shell with fcc structure are still blocked, being below the blocking temperature TB. In order to understand how the external magnetic field determines the magnetic coupling between core and external Mn3O4 shell below 45 K, further FC magnetization curves were recorded under a 70 kOe applied field. This allowed studying whether and how the lowtemperature transition previously observed in the ZFC-FC curves also affects the magnetization upon high magnetic field conditions. The corresponding results are shown in Figure 5. They clearly indicate that decreased magnetization, previously observed in the MnFeO_1.05 and MnFeO_1.40 samples and caused by the antiferromagnetic core/shell coupling, leaves

place for an additive step-like transition in all the samples. This implies that the coupling between core, MnxFe3-xO4 intermediate shell and Mn3O4 thicker external shell, upon the influence of a magnetic field of sufficient strength, becomes ferromagnetic, i.e. with the magnetic moment of the Mn3O4 shell pointing in the same direction of those of the core and intermediate shell.



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emphasize the effect of the magnetic coupling taking place between the magnetically blocked components of the composite systems, thus providing a further insight on the effect of Mn introduction on the magnetic features of the NPs in their blocked state, below the transition occurring at 45 K. All the magnetic parameters obtained by both ZFC and FC isothermal hysteresis loops at 4K are reported in Table 2.

Figure 5. Field Cooled (FC) magnetization curves measured in a magnetic field of 70 kOe (7 T). The dashed red line indicates the ferromagnetic to paramagnetic transition of Mn3O4.

Figure 6 displays hysteresis loops recorded at 4 K, showing the magnetic response of the whole system (core plus shells). FC (H=3T) hysteresis loops recorded at the same temperature

Figure 6. Zero Field Cooled (dashed lines) and Field Cooled (H=3T, 30kOe) (dotted lines) hysteresis loops recorded at 4 K: the first row shows the whole field region (-70kOe, +70kOe), while the in the second row the low field regions of corresponding hysteresis loops are displayed. The corresponding magnetic parameters are reported in Table 2.

The MSAT value of FeO_pure is consistent with the bulk value of Fe3O4 (92 emu/g)32, which offers a further indication that other iron oxide phases were not formed within the NPs, while the decrease in saturation magnetization observed when going

from FeO_pure to MnFeO_1.40 sample can be put in relation with the presence of Mn3O4, considering the saturation values of bulk Fe3O4 and Mn3O4 (45 emu/g).33 Then, the contribution of Mn3O4 gradually becomes larger and lowers the overall


Page 9 of 21

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saturation of the whole core/double shell system. The remanence (MR) and mean coercivity (HC, defined as (|HC1|+|HC2|)/2, where HC1 and HC2 are the negative and positive coercive fields, respectively) values (i.e., the values that indicate magnetization measured in absence of external field, and the external field leading to null magnetization, respectively) clearly show the magnetic variation resulting from the structural evolution from bare Fe3O4 NPs (FeO_pure sample) to double-shell system (MnFeO_1.05 and MnFeO_1.40 samples). At low temperature, remanence values are quite affected by the introduction of different regimes of Mn, as indicated by the decrease of MR (see Figure 6 and Table 2). These variations can be explained by taking into account the decrease in net magnetization when going from Fe3O4 to Mn3O4, as previously highlighted also by their bulk saturation magnetization values. Besides, a decrease of the coercivity is measured going from FeO_pure to MnFeO_0.35, while a subsequent increase can be observed in MnFeO_1.05 and MnFeO_1.40, as reported in Table 2. These two trends indicate that the external Mn3O4 shell is the first cause of the magnetic hardening of the system, as expected considering its higher magnetocrystalline anisotropy. Coupling between the core and the shell(s) should be expected within the Fe/Mn system and asymmetries/loop shifts in the hysteresis loops give an immediate indication of exchange bias (EB) between different layers. Given the ferrimagnetic nature of all the phases constituting the NPs and the presence of interfaces between compositionally different volumes (Fe3O4 core, MnxFe3-xO4 and Mn3O4 shells) and structurally different volumes (central region along with the thin fcc MnxFe3-xO4 shell and the tetragonal Mn3O4 external shell), ZFC and FC hysteresis loops help in discriminating the different components. The superficial disorder of the FeO_pure sample manifests in the form of a stronger shift in both its ZFC and FC hysteresis loops, as shown by their ∆HC value (Table 2, Figure 6), the latter being defined as (HC1+HC2)/2 and quantitatively measuring the loop shift. An EB effect can also be observed in the ZFC and FC hysteresis of the other samples with a proper core/shell system. In fact, all the layers constituting the heterostructures are in a magnetically blocked state at 4 K, so they will actively contribute to the hysteresis loops. First, the initial decrease in ∆HC values when going from the FeO_pure to the MnFeO_0.35_sample is the further consequence of the reduced superficial spin disorder occurring in correspondence of the MnxFe3-xO4 shell formation, as indicated in Figure 2 B-D. Conversely, the following increase in ∆HC values can be put in relation with the increasing presence of Mn going from MnFeO_0.35 to MnFeO_1.40, but while the loop shift of FC can be attributed to conventional EB due to interaction between core and shells, the spontaneous EB of ZFC hysteresis loops indicates an inherently disordered interface. In fact, the formation of thicker Mn-based shells gradually varies the magnetic features of the whole core/shell system from magnetically soft to hard, but it also gives rise to the growth of a double-shell interacting system characterized by an increasingly well-developed external shell that drives the interaction effects and determines the loop shift of FC hysteresis. Finally, some further considerations can be done about the non-null ∆HC values measured for the sample MnFeO_0.35, MnFeO_1.05 and MnFeO_1.40. First, given the isostructural nature of the MnxFe3-xO4 shell with respect to the core in the

sample MnFeO_0.35, in it the presence of a loop shift should be attributed to the presence of smaller disordered domains at the surface of the NPs, not yet completely ordered by the formation of the shell. Then, these domains still act as a spin glass-like shell, interacting with the main body of the NPs and then giving rise to the spontaneous EB. Differently, the ∆HC values of MnFeO_1.05 and MnFeO_1.40 at 4K should be attributed to the presence of a disordered interface between the fcc MnxFe3-xO4 shell and the tetragonal Mn3O4 shell, the latter becoming magnetically relevant only when both the shells are magnetically blocked, i.e. for temperatures lower than 45K. As well, the presence of such a different disordered parts in the diverse samples is also the cause of the unsaturated hysteresis loops always observed in the high field region. Above that temperature, as indicated by the ZFC and FC magnetization curves shown in Figure 4, the external manganese oxide goes from the ferrimagnetic to the paramagnetic state, thus becoming irrelevant in terms of contribution to the overall magnetic properties. Then, the ZFC and FC hysteresis loops were also collected at 70 K, well above the manganese oxide transition temperature, to understand in depth what happens in such a case. The hysteresis loops and the related parameters are reported in Figure SI2 and in Table SI1. These data highlight some important features of the sole central region of the heterostructures, thus shedding light on the role played by the intermediate substoichiometric manganese ferrite layer. First, the analogous mild depletion of MR values observed at 70 K for MnFeO_0.35, MnFeO_1.05 and MnFeO_1.40 samples is due to the formation of the fcc MnxFe3-xO4 shell, while the strong variation observed at 4 K in MnFeO_1.40 is caused by the presence of the thick Mn3O4 external shell. Secondly, the slight variations of HC recorded at 70 K show that, while the progressive introduction of Mn influenced the magnetic hardness of the heterostructures at 4 K through the growth of the blocked Mn3O4 shell, the presence of Mn in the intermediate fcc MnxFe3-xO4 shell does not affect the overall hardness of the system, since the Mn3O4 shell is paramagnetic at that temperature. Also, hysteresis loops recorded at 70 K only present a mild asymmetry in coercivity after a 30 kOe (3 T) field cooling, with the notable exception of heavier asymmetry of FeO_pure, clearly observable in Figure SI2 and table SI1. In the latter case an asymmetry can be registered already in the ZFC hysteresis and this feature is further enhanced after the field cooling. This confirms the formation of a defective surface that boosts the superficial spin canting and enters a spin glass state at low temperature, acting as a sort of magnetic shell with respect to the rest of the NP,34 also triggering the spontaneous EB in the ZFC hysteresis due to its intrinsically disordered nature.35 As already observed in the ZFC and FC hysteresis loops recorded at 4K, the superficial disorder is dramatically reduced by the formation of the first fcc MnxFe3-xO4 shell grown on the Fe3O4 core, thus minimizing the asymmetry of the hysteresis loops. On the other hand, the small loop shifts observed in the FC hysteresis of MnFeO_0.35, MnFeO_1.05 and MnFeO_1.40 indicates a slight EB and proves that the intermediate fcc MnxFe3-xO4 shell is still blocked at 70 K and follows the behavior of the Fe3O4 core rather than that of the Mn3O4 shell, which at that temperature has become paramagnetic.


FC hysteresis (H=3T)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ZFC hysteresis


Sample

HC1 (Oe)

HC2 (Oe)

HC (Oe)

∆Hc (Oe)

Page 10 of 21

MR (emu g-1)

MS (emu g-1)

MR/MS

FeO_pure

-603

521

562

-41

15.5

92.4

0.17

MnFeO_0.35

-453

406

430

-23

13.8

83.0

0.17

MnFeO_1.05

-764

703

733

-31

14.3

75.4

0.19

MnFeO_1.40

-1413

1317

1365

-48

9.4

70.5

0.13

FeO_pure

-888

330

609

-279

24.9

91.0

0.27

MnFeO_0.35

-565

315

440

-125

19.7

82.9

0.24

MnFeO_1.05

-925

643

784

-141

20.3

75.2

0.27

MnFeO_1.40

-1465

1199

1332

-133

12.1

69.5

0.17

Table 2. Magnetic parameters obtained by hysteresis loops recorded at 4 K with Zero Field Cooling (top) and Field Cooling with a magnetic field strength of 3T. HC1: negative coercive field (left side); HC2: positive coercive field (right side); HC=(|HC1|+|HC2|)/2: average coercive field; ∆HC=(HC1+HC2)/2: loop shift; MR: remnant magnetization; MS: saturation magnetization extrapolated at high fields; MR/MS: remnant reduced magnetization. Absolute error on HC values: 5 Oe. Absolute error on MR and MS values: 0.1 emu g-1. Absolute error on MR/MS values: 0.01

3.3.3. Discussion about the samples complex magnetic behavior While the study of complex systems based on ferrimagnetic oxides represents a wide area of interest for developing novel materials for different applications, either by obtaining composite structures,20-21,24 varying the chemo-structural features36 or exploiting the collective properties of clusters,37 these results show how simple variations in the synthetic conditions can strongly impact on the chemical composition, the structural complexity and in turn on the physical properties of the final composite system. In fact, starting from spherical monodispersed magnetite NPs with makeshift magnetic core/shell, where the magnetic shell is constituted by the superficial disorder observed by HRTEM and strain analysis, shown in Figure 2A, B and acting as a sort of spin glass-like layer, a proper core/shell structure was developed by the progressive introduction of Mn. It enters the system at first by strongly ordering the structure at the surface of the NPs as displayed in Figure 2C, D and subsequently forming a proper Mn3O4 shell, shown in Figure 3. Effects of the Mn introduction are diverse and can be pointed out by the resulting structural and magnetic variations. In fact, the first step (i.e. the ordering of the Fe3O4 surface) leads to the concomitant formation of a thin MnxFe3-xO4 shell that keeps the fcc structure of the Fe3O4 core (sample MnFeO_0.35), while improving the superficial order of the NPs, as shown by HRTEM and strain analysis along with variations in ZFC and FC hysteretic behavior (Figures 2C-D, 6B and SI2B). This in turn brings to a core/shell system that is structurally similar and keeps the same size of the starting material (FeO_pure) (Figure 2A-D) but magnetically softer (Figures 6A-B and SI2A-B). In fact, the introduction of Mn determines a decrease in remanence and coercivity (Tables 2, SI1), and the slight increase observed below 45 K in the FC magnetization curve under a low magnetic field regime is not observed in the starting sample FeO_pure (Figure 4). Such a small increase highlights the fact that, despite the fcc structure, the MnxFe3-xO4 shell gives rise to a ferromagnetic coupling

with the blocked Fe3O4 core due to its mediated nature as described in paragraph 3.2. In fact, the disordered nature of the Fe3O4 surface gives rise to a superficial introduction of Mn in MnFeO_0.35, which we described as giving rise to an MnxFe3xO4 shell but, as specified above, it is however keeping a certain grade of disorder. Due to that, it can be also thought as featuring a distribution of Mn-poor (i.e., Fe-rich) and Mn-rich (i.e., Fe-poor) zones. Then, the Mn-rich zones at the surface of the shell start to partake the features of Mn3O4, thus presenting its typical transition at 45 K. Given their very small size and not homogenously distribution, their interaction with the Fe3O4 core is commanded by the latter into a ferromagnetic coupling, which gives rise to a shallow boost in magnetization at T

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