Mn3O4 Hybrid Nanocrystals with Unique Magnetic

May 12, 2017 - seeds with two different orientations are shown in Figure 1a. TEM image of the .... Information). The M−T curve of the H-O-P samples ...
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Anisotropic Fe3O4/Mn3O4 Hybrid Nanocrystals with Unique Magnetic Properties Maowei Jiang and Xiaogang Peng* Center for Chemistry of Novel and High-Performance Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *

ABSTRACT: This work explores novel nanomagnets by siteand facet-selective epitaxy of Mn3O4 nanodomains onto colloidal Fe3O4 nanoprisms in solution. At 190 °C, the Mn3O4 nanodomains epitaxially grow at three vertexes of the Fe3O4 nanoprisms in solution and form horns-on-prism hybrid nanocrystals. At 240 °C and in the same reaction solution, the epitaxy occurs on the top facet of the Fe3O4 nanoprisms, which results in prism-on-prism hybrid nanocrystals. As the temperature increases from 190 to 240 °C, the ratio between the prism-on-prism and horns-on-prism nanocrystals increases. A possible formation mechanism of Fe3O4/Mn3O4 hybrid nanocrystals is proposed. Novel magnetic behaviors, such as compensation point, large positive (or negative) exchange bias, and unusual hysteresis loop character (constricted at low field and expanded at high field), have been observed for both types of anisotropic hybrid nanomagnets. These unique magnetic properties are consistent with controlled switch of relative magnetization orientations between Fe3O4 and Mn3O4 nanodomains from parallel to antiparallel exchange-coupled configurations. KEYWORDS: Seeded synthesis, anisotropic hybrid nanocrystal, spin flop, compensation point, exchange bias

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might result in hybrid nanostructures with unique magnetic properties. Fe3O4/Mn3O4 soft/hard hybrid magnetic structures have been studied for layered thin-film structures17 and core/shell nanocrystals,11 which was selected as our model system for synthesis and study of hybrid magnetic nanocrystals with anisotropic configuration. Generally, it still remains difficult to control the site-selective epitaxy of secondary domains onto the exposed surfaces of nanocrystal seeds,18−30 especially for anisotropic two-dimensional building block.31−33 In this report, the unique structural features of Fe3O4 nanoprisms coupled with control of epitaxial temperature allowed us to siteselectively grow Mn3O4 nanodomains onto either three vertexes or one top face of the preprepared Fe3O4 triangular nanoprisms. Synthesis of quasi-triangular Fe3O4 nanoprisms followed a modified procedure34 through the solvothermal reaction of iron acetylacetonate under the existence of dodecanoic acid, oleylamine, and toluene at 200 °C (see Experimental Section in Supporting Information). This simple procedure resulted in Fe3O4 nanoprisms with side length of 27.5 nm and thickness of 4.1 nm (Figures 1a and S1). Site-specific epitaxy of Mn3O4 nanodomains onto the Fe3O4 nanoprisms was carried out at

oth bulk and nanosized magnetic materials with single composition are well understood, which have been widely exploited and are playing a key role in the modern society. Rapid advancement of technology demands miniaturized magnetic components and magnetic structures with unique properties. Thin-film magnetic structures with hybrid magnetic materials have been explored in diverse fields in past decades, such as new magnets, data storage, and spin valve.1−3 Colloidal hybrid nanocrystals may compensate the thin-film hybrid magnetic structures due to their flexibility in synthesis and miniaturization of magnetic components. At present, colloidal core/shell nanocrystals composed of two or more magnetic compounds have been explored, displaying novel magnetic properties such as tunable blocking temperature and coercivity,4−9 exchange bias,9−12 proximity effect,13 and magnetic thermal induction.14 However, existing colloidal hybrid nanocrystals in core/shell configuration can only promote isotropic magnetic interaction between core and shell domains. Many magnetic properties observed for two-dimensional hybrid magnetic thin films have not yet been experimentally observed for the corresponding core/shell nanocrystals.15,16 For single-composition magnetic structures, anisotropic magnetic effects are well-known. We anticipate that in comparison to isotropic core/shell hybrid nanocrystals colloidal anisotropic nanocrystals as seeds epitaxially grown with anisotropic second magnetic domain(s) © 2017 American Chemical Society

Received: February 18, 2017 Revised: May 5, 2017 Published: May 12, 2017 3570

DOI: 10.1021/acs.nanolett.7b00727 Nano Lett. 2017, 17, 3570−3575

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Figure 1. (a) TEM images of the Fe3O4 seeds with two orientations, (b) horns-on-prism hybrid nanocrystals and (c) prism-on-prism hybrid nanocrystals. (d) Electron diffraction patterns of seeds with two orientations and two types of hybrid nanocrystals. (e) Dark-field STEM image and corresponding EDS-mapping image of single H-O-P hybrid nanocrystal. (f) Dark-field STEM image and corresponding EDS-mapping image of single P-O-P hybrid nanocrystal. Mn, green; Fe, red.

microscopy (STEM) image and corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping image of single hybrid nanocrystal grown at 190 °C are shown in Figure 1e. These results were found to be consistent with the assignment that the nanohorns were Mn3O4 and the triangle part was Fe3O4 in the hybrid nanocrystal. Highresolution TEM images of these hybrid nanocrystals in Figure S2 further revealed the epitaxial relationship between the Mn3O4 nanohorns and Fe3O4 nanoprism. For convenience, Fe3O4/Mn3O4 nanohorns/nanoprism hybrid nanocrystals may be called as “horns-on-prism” (H-O-P) nanocrystals in this report. Results in Figures 1c,f and S2 confirmed that at 240 °C epitaxial growth of Mn3O4 occurred at one of the top facets of the triangular Fe3O4 nanoprisms. Specifically, TEM image in Figure 1c and high-resolution TEM image in Figure S2 revealed that the hybrid nanocrystals retained the triangular nanoprism shape of the Fe3O4 seeds with similar side length but doubled thickness. The STEM image and corresponding EDS elemental mapping image (Figure 1f and Figure S5) of single hybrid nanocrystal grown at 240 °C indicated the epitaxy occurred at one of the top facets of the Fe3O4 nanoprisms. For simplicity, this type of hybrid Fe3O4/Mn3O4 nanocrystals may be called as “prism-on-prism” (P-O-P) nanocrystals. Temperature-dependent growth pattern implied possible mechanisms of the site-selective epitaxy. As pointed out above, epitaxial growth of Mn3O4 occurred at the vertexes and the top

ambient pressure. A mixture of Fe3O4 nanoprisms, manganese decanoate, dodecanoic acid, and tetradecane was heated up to a designated temperature. Subsequently, a mixture of oleylamine and dodecanol was injected into the solution to facilitate decomposition of manganese decanoate for the epitaxial growth (see Experimental Section in Supporting Information for details). To achieve site-selective epitaxial growth, 190 and 240 °C were identified as the optimal injection temperature of the mixture of oleylamine and dodecanol to initiate the growth respectively at the vertexes and at the top face of the Fe3O4 nanoprisms (Figures 1b,c and S2). To avoid possible contamination of the products, the Fe3O4 nanoprism seeds and Fe3O4/Mn3O4 hybrid nanostructures were purified by several times of magnetic separation and centrifugation separation, which should leave behind nonmagnetic and weak magnetic impurities (see Figure 1a−c for purified nanocrystals). The electron diffraction patterns in Figure 1d further confirm crystalline purity of the nanocrystals (also see Figure S4, Supporting Information). Experimental details were provided in Supporting Information. Transmission electron microscopy (TEM) images of the seeds with two different orientations are shown in Figure 1a. TEM image of the products synthesized at 190 °C is given in Figure 1b. Different from the Fe3O4 triangular nanoprisms in Figure 1a, the Fe3O4/Mn3O4 hybrid nanocrystals in Figure 1b were triangular with nanohorns sticking out from three vertexes of each triangle. The dark-field scanning transmission electron 3571

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Figure 2. M−T measurements of two types of hybrid nanocrystals. M−T curves for H-O-P (a) and P-O-P (b) hybrid nanocrystals obtained at different applied fields. Inset: enlarged M−T curve at 5 kOe for H-O-P samples (a) and at 30 kOe for P-O-P samples (b).

facet of a Fe3O4 nanoprism at 190 and 240 °C, respectively. Systematic studies revealed that two types of epitaxy were in direct competition. Specifically, upon increasing the epitaxy temperature from 180 to 240 °C, the ratio between prism-onprism and horns-on-prism hybrid nanocrystals increased gradually. For example, when the epitaxy was carried out at 225 °C, growth of Mn3O4 nanodomain(s) occurred without site-selectivity (Figure S6). These results indicated that siteselectivity was not solely determined by the structure of the seeds. Instead, it is temperature-assisted site-selective epitaxy. A Fe3O4 nanoprism possesses three vertexes, three ridges, six small side (111) facets, and two large (111) basal planes (Figure S1c,d).29 By purely considering surface bonding geometry for epitaxy, three vertexes of a nanoprism are suggested as the most reactive sites, ridges are the least reactive sites, and the (111) facets are between the other two types.35 Surface passivation by ligands should also play an important role for determining reactivity of different sites. In principle, all surface sites should be terminated with iron ions that are further coordinated with carboxylate ligands. At relatively low reaction temperatures (∼190 °C), two factors would all make the vertexes as the favorable epitaxial sites. Consequently, growth of horns-on-prism hybrid nanocrystals would be dominating. As the temperature increased to beyond 200 °C, the carboxylate ligands with relatively short hydrocarbon chains (decanoates and dodecanoates used in this work) would gradually switch to a dynamic bonding mode, rapidly switching between bonding onto and detaching from the surface of nanocrystals.36 When the surface ligands were detached from the surface, it should be most likely in the neutral form of iron caoboxylates. This would leave the sites on the (111) facets being substantially more reactive for the coming Mn ions by providing nearly ideal coordination environment. As a result, at ∼240 °C each (111) basal plane would possess comparatively better (or similar) coordination environment and vastly larger number/area of epitaxial sites than the vertexes, which would convert the epitaxial products from horns-on-prism to prismon-prism. It is interesting to notice that epitaxial growth of prism-onprism hybrid nanocrystals only occurred on one of two (111) basal planes (Figure 1f). After formation of two-dimensional nuclei onto one of two basal planes at high temperatures, the following growth should be very rapid to form complete coverage of Mn3O4 on one basal plane of the seeds. Given a significant lattice mismatching between Mn3O4 and Fe3O4 (∼3%, Figure S2), a lattice strain would be induced and

might make two-dimensional nucleation of Mn3O4 on the opposite basal plane be difficult.24 In addition to this tentative mechanism, we noticed that the ⟨111⟩ direction of Fe3O4 is polar with alternating iron and oxygen layers. The irons in one type of layers are located in the mixed tetrahedral and octahedral voids, and the irons in the other type of layers are located in the octahedral voids (Figure S3). Such atomic arrangement might result in asymmetric surface, and one possible asymmetric pattern is given in Figure S3. Such asymmetric top and bottom basal planes should also promote rapid nucleation and growth onto one of two (111) basal planes of the seeds. Magnetic properties of Fe3O4 nanoprisms and two types of Fe3O4/Mn3O4 nanocrystals were characterized by the moment versus temperature (M−T) and moment versus field (M−H) measurements. The M−T and M−H curves for Fe 3 O 4 nanoseeds revealed that the Fe3O4 nanoseeds displayed typical features of anisotropic Fe3O4 nanocrystals, such as roomtemperature ferrimagnetism (Figure S7). In comparison, two types of Fe3O4/Mn3O4 hybrid nanocrystals showed substantially different temperature- and field-dependences from the pure Fe3O4 nanoseeds. For the M−T measurements, the samples were initially magnetized with a 5 T field at 5 K before all the measurements were carried out (see Experimental Section in Supporting Information). The M−T curve of the H-O-P samples at H = 0 (remanence curve) in Figure 2a showed a sign change at 35 K for the magnetic moment, indicating antiparallel spin-offset state for Fe3O4 and Mn3O4 counterparts in the H-O-P hybrid nanocrystals. Similar behavior was previously observed in Fe 3 O 4 /Mn 3 O 4 thin-film systems and interpreted as a compensation point.17 At high temperatures, the M−T curve should reveal the net moment of ferrimagnetic Fe3O4 and paramagnetic Mn3O4. (The Curie temperature of Mn3O4 is about 40 K.37−40) Ferrimagnetic Mn3O4 moment should decrease rapidly due to thermal disturbance, which would result in rapid decrease of the net moment as temperature was increased.11,17 When the magnetic field increased to 0.4−1 kOe, the M−T curves of the H-O-P samples were qualitatively varied (Figure 2a). While the rapid decrease remained at low-temperature range, the moment possessed a defined minimum at ∼30 K. After this minimum, further warming up would result in a fast increase stage (Figures 2a and S8), suggesting Fe3O4 and Mn3O4 may experience a spin-flop process to form certain angles with external field, which was similar to the magnetic 3572

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Figure 3. M−H measurements of two types of hybrid nanocrystals. (a−f) M−H curves at typical temperaturesof 5 K (a), 20 K (b), 33 K (c), 45 K (d), 60 K (e), and 120 K (f) for H-O-P and P-O-P samples. (g,h) Hc−T curves (coercivity field versus temperature) (g) and Mr−T curves (remanence versus temperature) (h) for H-O-P and P-O-P samples.

to parallel state in the H-O-P samples under the influence of external field. The temperature-dependence of magnetic properties of the P-O-P samples in Figure 2b was qualitatively similar to that for the H-O-P samples in Figure 2a. However, two types of hybrid nanostructures differed quantitatively from each other on their unique properties. The compensation point (40 K) in the remanence measurement of the P-O-P samples in Figure 2b was slightly higher than that in Figure 2a. In comparison to those in Figure 2a, the minimum for the low-field measurements was less defined and the ascent stage at about 30 K < T < 60 K lasted for much larger field window (H < 3 T) in Figure 2b. While the moment peak in the M−T curve of the H-O-P samples appeared at 5 kOe in Figure 2a, it occurred at 3 T for the P-O-P samples in Figure 2b. These quantitative differences suggested that, P-O-P hybrid nanocrystals should have larger exchange energy than the H-O-P ones. As a result, a relative high field was necessary to twist the spins in the Fe3O4 and Mn3O4 nanodomains of the P-O-P hybrid nanocrystals from antiparallel to parallel coupling. The M−H curves for both H-O-P and P-O-P hybrid nanocrystals are given in Figure 3. The samples were precooled down to 5 K under zero field and the measurements were

state in multidomain structure and could be understood as a twisted phase.17 Formation of twisted phase should be a result of balance of the exchange energy and Zeeman energy17,41 in the Fe3O4/Mn3O4 hybrid nanostructures. As the external field further increased to beyond 5 kOe, all M−T curves of the H-O-P samples displayed two slowly decreased stages (∼5−30 K and above 60 K respectively) and one rapidly decreased stage (∼30−60 K) as shown in Figure 2a. The signals above 60 K of the H-O-P samples were similar to the moment response tendency with pure Fe3O4 nanoseeds which decreased persistently at all the measurement temperatures of 5−300 K (Figure S7f). The initial decrease of the net moment between 5 and 60 K should be mainly caused by the decrease of the Mn3O4 submoment. Overall, the M−T curves of the H-O-P samples above 5 kOe displayed characters in accordance with a composite magnet of two components with different Curie temperatures that were ferromagnetic/ferromagnetic parallel-coupled (Figure 2a). Specifically, this result indicated parallel-coupling state for the Fe3O4 and Mn3O4 nanodomains. Particularly, the M−T curve of the H-O-P samples at H = 5 kOe displayed a peak at ∼37 K (see inset in Figure 2a), which should be an indication of complete phase transition for the spin-fop of Fe3O4 and Mn3O4 nanodomains 3573

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The remanence (Mr) at different temperatures in the M−H curves of both types of hybrid nanostructures are summarized in Figure 3h. The Mr of the H-O-P hybrid nanocrystals at 5 K after the respective magnetization of positive or negative field possessed an asymmetry with obvious negative shift. To our knowledge, such a large negative Mr shift (1.7 emu/g, see Figure S9b) should be the record value so far, and the first report especially under the measurement after zero field cooling for either Fe3O4/Mn3O4 layered thin-film or hybrid nanocrystal systems. At 120 K, Figure 3h showed that the Mr of the H-O-P hybrid nanocrystals was zero. As a comparison, Mr shift was much smaller or unobservable at all the experiment temperatures (Figure S9b) for the P-O-P samples. In addition, the Mr of the P-O-P hybrid nanocrystals had a maximum at 45 K, and either temperature decrease or increase would result in the decrease of Mr. Comparison of the hysteresis features (both Hc and Mr) between two types of hybrid nanocrystals would reveal a few interesting points. At low-temperature range (5−20 K), Figure 3g,h indicates that the magnetic properties of the H-O-P hybrid nanocrystals seemed significantly more robust. When temperature increased to the range between 38 and 240 K, larger Hc and Mr values were recorded for the P-O-P samples. While temperature was beyond 240 K, two types of hybrid nanocrystals were similar to each other. Particularly, results in Figure 3g,h revealed that both Hc and Mr for the H-O-P hybrid nanocrystals were vanished at 120 K due to the complete spinoffset of the magnetic counterparts while neither Hc nor Mr for the P-O-P samples vanished in the entire temperature range (Figure 3g,h). In summary, the results above confirm that it is possible to synthesize hybrid magnetic nanocrystals with defined anisotropic magnetic domains to realize unique magnetic properties. As the first example, with proper lattice structures of both magnetic components Mn3O4 nanodomains were epitaxially grown onto the anisotropic Fe3O4 nanoprisms with different types of specific site-selectivity. Also, a possible formation mechanism for the growth of different types of Fe3O4/Mn3O4 hybrid nanocrystals was proposed. The resulting anisotropic Fe3O4/Mn3O4 hybrid nanocrystals in this work demonstrated some unique magnetic properties that were not yet reported in corresponding colloidal core/shell hybrid magnetic nanocrystals, including compensation point between Fe3O4 and Mn3O4 nanodomains in the hybrid nanocrystals, hysteresis loop with contraction at low field and expansion at high field, and positive hysteresis loop shift after zero-field-cooling. Possible origins of the different magnetic behaviors for two types of hybrid nanocrystals should be related to their structural differences, such as the morphology (size and shape) of Mn3O4 submagnet, the relative locations of Mn3O4 and Fe3O4 submagnets, the new-constructed interface (coherent interphase and interfacial contact area) between Mn3O4 and Fe3O4, and so on. The results reported here indicate that colloidal synthesis shall provide flexible low-cost means to develop novel hybrid nanomagnets for various advanced magnetic devices.

carried out at designated temperatures along the warming process. Figure 3a−f illustrates the M−H curves for both types of hybrid nanocrystals at different temperatures, which exhibit very diverse patterns for both types of hybrid nanocrystals. Each plot in Figure 3a−f includes both types of hybrid nanocrystals for comparison. The M−H curve of the H-O-P hybrid nanocrystals at very low temperature (see 5 K curve in Figure 3a) showed a novel hysteresis loop, which was contracted at low field and expanded at high field. While the temperature was increased to ∼20 K, a similar pattern but with reduced expansion at high field was identified for the H-O-P samples (Figure 3b). When the temperature was further increased to beyond 33 K, the highfield expansion disappeared for the H-O-P hybrid nanocrystals (Figure 3c−f). Similar high-field expansion was reported previously in Fe3O4/Mn3O4 layered thin-film systems17 though it has not yet been observed for Fe3O4/Mn3O4 nanocrystal systems. This phenomenon was related to the switch of relative magnetization orientation between the Fe3O4 and Mn3O4 counterparts under external field from antiparallel twisted to parallel exchange-coupled state. Consistent with the M−T measurements discussed in the above subsection, the M−H curves of H-O-P hybrid nanocrystals in Figure 3 further illustrated the Fe3O4 and Mn3O4 counterparts possessed a parallel coupling state at high field, and an antiparallel state at low field. Different from those of the H-O-P samples, the M−H curves of the P-O-P hybrid nanocrystals at low temperatures (see 5 K curve in Figure 3a) displayed a very weak high-field expansion. Furthermore, the P-O-P hybrid nanocrystals possessed a relatively small hysteresis loop area at 5 K than the H-O-P samples. However, at relatively high temperatures (T > 38 K), the hysteresis of the P-O-P hybrid nanocrystals was much more persistent than that of the H-O-P samples (Figure 3d−f). Figure 3g summarizes the coercivity field (Hc) observed after respective demagnetization by negative and positive field at different measurement temperatures for two types of hybrid nanocrystals. The Hc of the H-O-P hybrid nanocrystals decreased rapidly from 5 to 38 K, which was followed by a slow decrease to Hc equal to zero at ∼120 K. When the temperature was higher than 120 K, Hc increased slowly up to room temperature (300 K), implying a nonzero Hc at room temperature. The remarkable hysteresis-loop shifts with temperature below 38 K should be the so-called exchange bias, which was widely observed in antiferromagnetic/ ferromagnetic or ferromagnetic soft/hard systems.9,42 This was found to be consistent with existence of an antiparallel exchange-coupled state at low field discussed above. Particularly, the Hc showed large positive shifts. For example, the shift at 5 K was positive 90 Oe (Figure S9a), which is the first report for the measurements after zero field cooling to show such a large positive shift for either Fe3O4/Mn3O4 layered thin-film or hybrid nanocrystal systems. To our knowledge, this is the largest value reported so far. The Hc of the P-O-P samples in Figure 3g showed a maximum at around 38 K. From the maximum, Hc decreased progressively for both increasing and decreasing temperature but never reached zero. Exchange bias (Figure S9a) was also evidenced for the P-O-P samples. The M−H curve at 5 K displayed a negative shift of 13 Oe. Interestingly, as temperature increased to 33 K, the loop shifts to the opposite direction with a positive shift of 6 Oe. The M−H curves at 20 K and above 38 K showed very weak exchange bias.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b00727. 3574

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Experimental section; additional TEM images, HRTEM images, XRD curves, STEM-EDS elemental mapping images; brief analyses of the crystalline structures of Fe3O4 nanoprisms, horns-on-prism and prism-on-prism hybrid Fe3O4/Mn3O4 nanocrystals; additional magnetic measurements (M−H curves, M−T curves) and analyses of Fe3O4 nanoprisms, horns-on-prism and prism-onprism Fe3O4/Mn3O4 hybrid nanocrystals(PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Maowei Jiang: 0000-0001-8364-2938 Xiaogang Peng: 0000-0002-5606-8472 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21233005). REFERENCES

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DOI: 10.1021/acs.nanolett.7b00727 Nano Lett. 2017, 17, 3570−3575