Synthesis and Magnetic Properties of FePt@ MnO Nano-heteroparticles

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Synthesis and Magnetic Properties of FePt@MnO Nanoheteroparticles Thomas D. Schladt,†,§,∥ Tanja Graf,†,§,∥ Oskar Köhler,† Heiko Bauer,† Michael Dietzsch,†,§ Jörn Mertins,† Robert Branscheid,‡ Ute Kolb,‡ and Wolfgang Tremel†,* †

Institute für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität, Duesbergweg 10-14, D-55099 Mainz, Germany. ‡ Elektronenmikroskopiezentrum Mainz, Institut für Physikalische Chemie, Johannes Gutenberg-Universität, Welderweg 11, D-55099 Mainz, Germany § Graduate School Materials Science in Mainz, Staudinger Weg 9, D-55128 Mainz, Germany ∥ IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, United States S Supporting Information *

ABSTRACT: Monodisperse FePt@MnO nano-heteroparticles with different sizes and morphologies were prepared by a seed-mediated nucleation and growth technique. Both size and morphology of the individual domains could be controlled by adjustment of the synthetic parameters. As a consequence, different particle constructs, including dimers, dumbbell-like particles, and flowerlike particles, could be obtained by changing the polarity of the solvent. The FePt@MnO nano-heteroparticles were thoroughly characterized by high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD) analyses and superconducting quantum interference device (SQUID) magnetometry. Due to a sufficient lattice match, the MnO nanoparticles (NPs) preferentially grow on the (111) surfaces of the fccFePt seeds. Furthermore, the surface spins of the antiferromagnetic MnO domains pin the magnetic moments of the ferromagnetic FePt NPs, which leads to an exchanged biased magnetic hysteresis. KEYWORDS: FePt, manganese oxide, magnetic nanoparticles, magnetic interaction, nanocomposite, heteroparticles, Janus-particles properties that are promising for a variety of applications.13,16 FePt occurs in two different modifications: an ordered fct (or L10) phase, in which the Fe and Pt atoms form alternating atomic layers in the (100) direction of the crystal lattice, and a disordered fcc phase, in which the atoms are randomly distributed.25−27 Much research effort has been devoted to the preparation of ferromagnetic fct FePt NPs, because they exhibit a high magneto-crystalline anisotropy (K), high saturation magnetization, as well as a high maximum energy product.28−30 This makes them ideal candidates for many different applications including high density data storage devices,13 nanocomposites for permanent magnets,31 or even biomedical probes.32−36 However, because as-prepared FePt NPs usually exhibit the disordered fcc structure, their magnetocrystalline anisotropy and saturation magnetization are reasonably low, leading to super-paramagnetic behavior at room temperature.27 Several approaches were investigated in the past to improve the magnetic characteristics of fcc-FePt NPs or to transform them into ordered fct-FePt NPs without losing the small

1. INTRODUCTION Efforts in nanoscale science and technology are focused on the development of novel multifunctional materials that exhibit several beneficial properties at the same time. In this context, the synthesis and design of hybrid nanomaterials, which consist of two or more individual inorganic components has attracted considerable interest. The controlled combination of different materials, which themselves exhibit distinct chemical and physical properties, offers several advantages.1,2 In fact, the combination of magnetic nanoparticles (NPs) with optically active materials, such as gold,3−5 silver6,7 or semiconductor NPs,8 has led to many interesting discoveries.9−11 Magnetic NPs have been among the most intensively studied material classes for more than two decades. Owing to their interesting properties, they have found applications in a vast range of different scientific areas, including mass data storage, catalysis,12−15 protein separation,16−18 specific cell targeting, drug delivery, 19,20 and magnetic resonance imaging (MRI).4,21−24 Among these magnetic nanomaterials, FePt NPs containing a near-equal atomic percentage of Fe and Pt, play an outstanding role, not only because they were one of the first nanomaterials that could be prepared with an exceptionally narrow particle size distribution but also because they have interesting magnetic © 2012 American Chemical Society

Received: October 11, 2011 Revised: December 31, 2011 Published: January 5, 2012 525

dx.doi.org/10.1021/cm2030685 | Chem.Mater. 2012, 24, 525−535

Chemistry of Materials

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Synthesis of 3 nm FePt NPs. The preparation of monodisperse FePt NPs, with an average size of 3 nm, was carried out according to the procedure by Sun et al.13 Under standard airless conditions, platinum acetylacetonate (Pt(acac)2, 196.7 mg, 0.5 mmol), 1,2hexadecanediol (387.7 mg, 1.5 mmol), oleic acid (159 μL, 0.5 mmol), and oleylamine (165 μL, 0.5 mmol) were mixed in dioctyl ether (20 mL) and degassed at 70 °C for one hour with intermittent argon backfilling. After that, the mixture was heated to 120 °C and iron pentacarbonyl (Fe(CO)5, 132 μL, 1 mmol) was added before raising the temperature rapidly to reflux (298 °C). After 30 min, the reaction mixture was cooled to room temperature and the product was washed by three cycles of precipitation with ethanol, centrifugation (9000 rpm for 10 min), and resuspension in hexane. The final product was stabilized with 25 μL of oleic acid and 25 μL of oleylamine and stored in hexane in a fridge. Synthesis of 6 nm FePt NPs. For the preparation of monodisperse FePt NPs with an average size of 6 nm, a reported method by Chen et al. was employed.55 In a typical reaction, Pt(acac)2 (196.7 mg, 0.5 mmol) was mixed with 10 mL of benzyl ether and 5 mL of 1-octadecene and degassed at 70 °C for one hour and regularly backfilled with argon. Subsequently, the solution was heated to 120 °C and Fe(CO)5 (264 μL, 2 mmol) and oleic acid (1588 μL, 5 mmol) were added simultaneously. After 5 min, oleylamine (1650 μL, 5 mmol) was injected and the mixture was heated to 205 °C and kept at this temperature for 1 h. After cooling to room temperature, the product was washed and stored in the same way as mentioned above. Synthesis of 13 nm FePt Nanocubes. Larger FePt nanocubes were prepared according to a procedure by Chou et al.56 Pt(acac)2 (40 mg, 0.1 mmol) was mixed with 1,2-hexadecanediol (1033.8 mg, 4 mmol), oleic acid (4 mL, 12.6 mmol), oleylamine (4 mL, 12.1 mmol), and 4 mL dioctyl ether. The mixture was degassed and backfilled with argon at 80 °C for 45 min and subsequently heated to 115 °C. Fe(CO)5 (66 μL, 0.5 mmol) was injected, and the reaction mixture was heated to 240 °C for 60 min. After cooling to room temperature, the FePt nanocubes were isolated and washed with ethanol. Finally, the NPs were stored in hexane with oleic acid and oleylamine (50 μL). Preparation of Manganese Oleate. Manganese oleate was prepared according to a published procedure.47,57 Briefly, 40 mmol of MnCl2 × 4H2O and 80 mmol oleic acid were dissolved in 200 mL methanol before 80 mmol NaOH (in 200 mL methanol) was added dropwise to precipitate Mn-oleate. The product was washed with water, ethanol, and acetone and subsequently dried in high vacuum at 120 °C to produce a waxy deep red solid. Synthesis of FePt@MnO Nanodimers. For the synthesis of FePt@MnO nanodimers, FePt NPs of the desired size (5−25 mg) were used as seeds. The size of each domain could be controlled by selecting the initial seed size (for FePt) and/or the amount of Mn oleate (for MnO). For instance, to obtain 3 nm@17 nm nanodimers, 5 mg of 3 nm FePt NPs were dissolved in 0.5 mL 1-octadecene with 25 μL of oleic acid and oleylamine and subsequently mixed with manganese oleate (309 mg, 0.5 mmol), oleic acid (953 μL, 3 mmol), oleylamine (1980 μL, 6 mmol), and 15 mL 1-octadecene. The reaction mixture was degassed/backfilled with argon at 80 °C for 30 min. After that, the reaction mixture was heated to reflux (315 °C) within 8 min and kept at this temperature for 30 min. After cooling to room temperature, the product was washed by repeated precipitation with acetone, centrifugation (9000 rpm for 10 min), and redispersion in hexane. Synthesis of FePt@MnO Nanomultimers. According to principle synthetic mechanisms mentioned in the Results and Discussion, further nucleation sites on the FePt seeds can be obtained by increasing the polarity, that is, the amount of available electron density in the solvent. For this purpose the FePt NPs (5−25 mg) were dissolved in benzyl ether by ultra sonication and added to a Mn-oleate, oleic acid, oleylamine, benzyl ether (15 mL) mixture. Subsequently, the reaction mixture was treated in the same way as described above. Synthesis of FePt@MnO Nano-heterostructures Based on 13 nm FePt NPs. In principle, the preparation follows the same procedure as described above. However, the amount of Mn oleate must be significantly lower (10%. Dynamic light scattering (DLS) experiments were carried out on pure FePt NPs and FePt@MnO nanodimers to evaluate the hydrodynamic radii (RH) of the NPs (see the Supporting Information). The DLS results basically confirm the narrow size distributions obtained in the TEM measurements; however, the RH values are larger, which is due to the surfactant “corona” covering the NPs (for a more detailed discussion please see the Supporting Information). 3.2.2. FePt@MnO Nanomultimers. By changing the polarity of the solvent, different particle morphologies could be obtained. Figure 3 shows TEM images of flowerlike FePt@ MnO NPs with multiple MnO domains being attached to each

different synthetic parameters, such as FePt NP size, solvent polarity, and precursor ratio, was investigated, as described in the following sections. 3.1. Synthesis of FePt Nanocrystals. FePt NPs of 3, 6, and 13 nm in size were prepared by reported procedures involving the decomposition of platinum(II) acetylacetonate in the presence of iron pentacarbonyl.13,55,56 The size of the NPs is tunable by varying the initial precursor ratio, temperature, or surfactant ratio. In fact, the evolution of the FePt nanoparticle shape and composition is kinetically controlled, and a suitable mechanism was proposed by Chen et al.55 In general, the size of the FePt nanocrystals increases with the amount of capping agents added to the reaction mixture. Figure 1 shows representative TEM images of monodisperse FePt NPs with average sizes of 3, 6, and 13 nm. The 3 nm FePt particles (Figure 1a) are approximately spherical, whereas the larger particles tend to grow into cube-like, rectangular, or even octapod-like shapes with pronounced faceting (Figure 1b and c). 3.2. Synthesis of FePt@MnO Nano-heteroparticles. In the second step, manganese oxide domains were nucleated heterogeneously on the FePt seeds by the decomposition of a manganese oleate precursor.47 As reported by Wang et al., a successful formation of nano-heteroparticles can only be achieved by suppressing homogeneous nucleation and simultaneously promoting heterogeneous nucleation.58 In fact, suppression of homogeneous nucleation can be obtained by keeping the precursor concentration under the critical supersaturation value. This can be achieved by precise control of the precursor ratio, amount of seed particles and heating profile. Additionally, the resulting morphology of the nano-heterostructures significantly depends on the solvent used during the synthesis. Depending on the polarity of the solvent, either dimers or multimers with several MnO particles being attached to one FePt seed are obtained. 3.2.1. FePt@MnO Nanodimers. The application of 1octadecene as solvent led to the formation of mainly dimeric nanoparticles, consisting of one MnO domain attached to one FePt seed. Examples of such FePt@MnO nanodimers are displayed in the TEM images in Figure 2. As a result of the higher atomic number of platinum, the FePt NPs appear darker compared to MnO, which makes a distinction of both material domains easier. The three images on the top (Figure 2a−c) show nano-heterodimers prepared with 3 nm FePt NPs as seeds, whereas the bottom images (Figure 2d−f) represent samples synthesized with 6 nm FePt seeds. It is obvious that the overall size and shape of the seed nanoparticles was retained and that despite some multimers with more than one MnO domain being present, FePt@MnO nanodimers constitute the vast majority of the observed nanostructures. Furthermore, the results presented in Figure 2 demonstrate that the size of the MnO domain can easily be varied by changing the amount of added Mn precursor. For instance, a reduction of Mn oleate from 0.5 to 0.1 mmol leads to a decrease of the MnO NP size from 17 to 9 nm. The particle sizes of each domain were determined from TEM images of various FePt and FePt@MnO nanodimer samples. In general, the size distribution of each domain is comparably narrow. However, the FePt NPs have a particularly narrow size distribution with standard deviations ranging 5− 10%, in both “naked” FePt and FePt@MnO nanodimer samples, as obtained from measuring the diameters of >100 particles. On the other hand, the MnO domains appear to be

Figure 3. TEM images of FePt@MnO multimer NPs prepared in benzyl ether. Besides dimers, more complex morphologies with two or more MnO domains on the FePt particles are observed.

FePt core. These particles were prepared in the same way as those presented in Figure 2; however, benzyl ether was used as solvent instead of 1-octadecene. It is obvious that the change of solvent had a major influence on the particle morphology. A similar behavior was reported by Yu et al. for Au@Fe3O4 nanoheteroparticles.3 These authors proposed that the nucleation of Fe3O4 on the Au surface leads to a charge polarization at the nucleation interface. However, the free electrons of the Au nanoparticle must compensate the electron loss at the interface and are therefore withdrawn from the remaining surface. On the other hand, further nucleation is only possible if a sufficient charge density is available on the Au surface. Therefore, further growth of the metal oxide domain will lead to the formation of dimeric structures, if the electron deficiency cannot be compensated. By using a solvent that carries a considerable amount of electron density by itself, part of this charge density can be provided from the solvent to the metal surface and, therefore, allow additional nucleation. As a result, multiple nucleation sites can occur on each metal seed leading to the formation of flowerlike nano-heteroparticles. The results shown in Figures 2 and 3 confirm these assumptions in an impressive way. Similar to Au, FePt NPs possess free electrons that are depleted by the nucleation of MnO on the seed surface. If 1-octadecene is used as solvent during the reaction, the electrons deficiency cannot be compensated and the growth of the MnO domain results in FePt@MnO dimers. Benzyl ether, on the other hand, is a typical example for a polar solvent that contains a sufficient amount of electron density, owing the delocalized π-electron system in the benzyl ring. It can provide electron density to the FePt surface, leading to further nucleation of MnO on suitable crystal facets and, therefore, the formation of flowerlike FePt@ MnO NPs. In addition to electron compensation on the FePt 528



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seeds, for benzyl ether, a strong surface coordination effect on the MnO domains has to be taken into account. This leads to the stabilization of the MnO NP surface and hence a slower growth of MnO, which, in turn, allows the nucleation of further metal oxide domains. Concerning the average particle size of each domain, detailed TEM analysis revealed a more polydisperse nature of the MnO NPs with sizes ranging from 10 to 25 nm. 3.3. XRD Analysis. The phase composition of the asprepared samples was investigated by powder X-ray diffraction. Figure 4 displays typical powder XRD patterns of FePt@MnO nanodimers with 3 nm FePt@17 nm MnO (Figure 4a) and 6

Figure 5. (a) HR-TEM image of a 6 nm@12 nm FePt@MnO dimer showing the preferred [111] growth orientation of MnO on FePt. (b) FFT analysis of the blue-framed region. (c) Inverse FFT of the respective area: Because of a slight lattice mismatch (d(111)FePt ≈ 0.85 d(111)MnO), a two-dimensional defect structure arises in which an extra lattice plane is inserted every 8−9 rows (yellow arrows).

interplanar distance of 1.37 Å, which matches well with the (220) set of planes in fcc FePt. Further comprehension on the relative orientation of both particles can be derived from the corresponding FFT image (see Figure 5b). The circled diffraction spots can be assigned to the (−202) and (−220) planes in both FePt (blue circles) and MnO (red circles). Additionally, the measured angle of 61.2° between the indicated lattice fringes in both samples is in good agreement with the theoretical value of 60° between the (−202) and the (−220) planes in an fcc lattice. These results confirm that both particles are co-oriented along the crystallographic [111] axis. However, a lattice mismatch between the (111) plane distances in both materials (d(111)FePt ≈ 0.85 d(111)MnO) leads to the formation of a distinct defect structure. More precisely, this means that an extra atomic layer has to be inserted into FePt every 8−9 layers to keep up the periodicity of the MnO lattice. A better impression of this behavior can be gained from the inverse fast Fourier transform (FFT) image in Figure 5c, where the appearance of the two-dimensional defect structure is indicated by the yellow arrows. In fact, this behavior is an example for domain matching epitaxy (DME) in nanoparticles, a phenomenon that is commonly known from the heteroepitaxial growth of thin film systems.59 Unlike conventional lattice matching epitaxy (LME), where epitaxial growth is only possible for heterosystems exhibiting lattice misfits of less than 7%, DME allows epitaxial growth of large lattice mismatch systems by matching of domains where integral multiples of major lattice planes match across the interface. Another example of the epitaxial relationship between the FePt and MnO domains is presented in Figure S1 in the Supporting Information. The image shows a FePt@MnO nano-heterodimer oriented in [110] direction, revealing a coherent interface between both materials. The lattice spacing of 2.22 Å in the larger particle matches well with the (002) planes in rock salt MnO, whereas the interplanar distance of 1.95 Å observed in the smaller particle can be assigned to the (002) planes of fcc FePt. Again, the lattice mismatch (d(002)FePt ≈ 0.9 d(002)MnO) leads to a defect structure with an additional plane being inserted in FePt every 9 rows (see yellow arrows in Figure S1b in the Supporting Information). It should be noted, that although different relative orientations of FePt and MnO domains were observed in other particles (for representative

Figure 4. Powder XRD pattern of FePt@MnO nano-heteroparticles: (a) 3 nm@17 nm and (b) 6 nm@17 nm. The expected reflection positions of fcc FePt (■) and MnO (×) are indicated by the corresponding symbols.

nm FePt@17 nm MnO (Figure 4b) domains. The positions and relative intensities of all reflections match well with expected patterns for fcc FePt and rock-salt MnO. The reflection width represents the average particle size distribution of both components: The shape of the FePt reflections is much broader than that of the MnO reflections, which is in accordance with the expected appearance derived from Scherrer’s equation. 3.4. HR-TEM Analysis. HR-TEM measurements were carried out to investigate the exact nanocomposite structure and to gain insight into how both materials are connected at the interface. Although different types of heterointerfaces were found, the most frequently observed cases will be described in the following. 3.4.1. HR-TEM Analysis of FePt@MnO Nano-heterodimers. Figure 5 shows a representative high resolution TEM image of a 6 nm@12 nm FePt@MnO nanodimer. The presence of distinct lattice fringes throughout each particle confirms the single crystalline nature of both components. Distance measurements of the adjacent parallel planes in the larger particle shown in Figure 3a revealed a lattice spacing of 1.57 Å, corresponding to the (220) set of planes in cubic MnO. The respective analysis in the smaller particle of Figure 3a yielded an 529



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examples see Figure S2 in the Supporting Information), coorientation along a common crystallographic axis was the predominant case in the nano-heteroparticles investigated in this study. In addition to that, the connection between both preferentially occurs via the (111) faces of FePt. Consequently, the generation of FePt@MnO nano-heterodimers can be derived by estimating an epitaxial growth of MnO on the (111) faces of the FePt seeds. 3.4.2. HR-TEM Analysis of FePt@MnO Nano-heteromultimers. HR-TEM investigation of FePt@MnO multimers led to a similar observation. A representative HR-TEM image of a FePt@MnO nanotrimer is shown in Figure 6. Measurement of the distances between the perpendicular lattice fringes in

with the (111) and (022) planes in a cubic lattice. The FFT image in Figure 6b emphasizes this assumption and leads to the conclusion that the entire nanotrimer is oriented along the [211] zone axis. Therefore, it can be concluded that both MnO domains and the FePt seed particle are connected along the same crystal faces. Again, the slight lattice mismatch leads to the propagation of a defect structure, which is indicated by the arrows in Figure 6c. 3.5. FePt@MnO Nano-heteroparticles from Large FePt Cubes. The formation of nano-heterostructures was also studied for 13 nm FePt seed particles. As shown in Figure 7, larger FePt nanoparticles have the tendency to grow into cubelike shapes, such as truncated nanocubes or nano-octapods, depending on the growth conditions. For instance, Chou et al. demonstrated that the morphology of the FePt particles can be controlled by the amount of capping agent and the reaction time.56 In any case, formation of either cubes or octapods requires a faster growth rate of the (111) facets compared to the (100) crystal faces. However, if these FePt nanoparticles are used as seeds for the preparation of FePt@MnO nanoheterostructures, mainly nanomultimers are obtained with several MnO domains attached to each FePt core. A characteristic TEM image is presented in Figure 7b. Besides nano-heterostructures carrying two, three, and five MnO domains, pentamers consisting of four MnO nanoparticles attached to one FePt seed are the predominant morphology observed in TEM analysis (see Figure S3 in the Supporting Information for additional examples). Additionally, the MnO domains appear to preferably grow on the edges and corners of the FePt particles rather than on the faces. This behavior may be explained by the higher surface energy of edges and corners in a cubelike structure and the consequent preferred nucleation on these sites compared to the faces. The HR-TEM analysis of a representative FePt@MnO pentamer is shown in Figure 7c. Determination of the lattice spacing in each particle confirmed the assumption of a FePt@MnO pentamer with four MnO nanoparticles surrounding a FePt core. However, to investigate a possible epitaxial relationship between both kinds of particles, the relative orientation of each particle was derived from the corresponding FFT images and the propagation of the corresponding lattice plains. A comparison of the orientation of the (111) planes in the opposite MnO domains (i.e., particles 2 and 5 and particles 3 and 4) revealed that particles 2 and 5 are oriented similarly, whereas a distinct tilt is observed

Figure 6. HR-TEM analysis of a [211] oriented nano-heterotrimer (particles 1 and 2 are MnO; particle 3 is FePt). (a) The observed distances in each particle match well with the respective (111) and (022) set of planes. (b) FFT image of the trimer showing the coorientation of both FePt and MnO nanoparticles. (c) The inverse FFT image of the blue framed area in part a reveals the propagating defect structure which originates from the lattice mismatch between fcc FePt and MnO (the arrows indicate the insertion of an extra plane every 8− 9 rows).

particles 1 and 2 in Figure 6a revealed a d-spacing of 2.56 and 1.56 Å, respectively. These distances are consistent with (111) and (022) set of planes found in MnO. On the other hand, the respective distances in particle 3 are 2.20 and 1.35 Å, corresponding to the same planes in cubic FePt. Moreover, the observed angle of 90° between the planes is also consistent

Figure 7. (a)TEM image of 13 nm FePt nanocubes and nano-octapods. (b) TEM image of FePt@MnO nanomultimers synthesized with FePt NPs from part a. Although structures with two, three, and five MnO domains are observed, the predominant morphology consists of pentamers with four MnO particles surrounding one FePt core. (c) HR-TEM analysis of a single FePt@MnO pentamer with the FFT images of the corresponding nanoparticles (1 − 5). 530



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loop is recorded after cooling the sample in an external field of 2 T (field cooled (FC) state). As can be seen in Figure 8, the FC hysteresis loop (open triangles) is wider (|HC| = 5 kOe) and shifted by the exchange bias field |Hex| = 2.5 kOe from the origin (with |Hex| = |He1| − (|He1| + |He2|)1/2, refer to Figure 8 for the definitions of Hc1 and Hc2). This behavior is due to magnetic exchange anisotropy interactions, which occur in systems where ferromagnetic and antiferromagnetic materials share a common interface. More precisely, the magnetic moment of the antiferromagnet MnO, which originates from uncompensated spins on the NP surface, is aligned parallel to the external field. On the other hand, the magnetic moment of the FePt domains is pinned to the MnO moment as a result of the magnetic exchange interaction, and thus, the spins of FePt are frozen. A reversal of their magnetization direction now needs a higher force, that is, a higher antiparallel field, which leads to an enhanced coercive field Hc and, consequently, to a relative shift of the m(H) loop by Hex. It should also be noted that dipolar coupling of nonjoined particles, which is often observed in magnetic measurements of powder samples, may also contribute to these magnetic properties.62 However, this coupling should not qualitatively alter our observation of strong exchange coupling 3 nm@9 nm Dimers. In principle, the 3 nm@9 nm FePt@ MnO NPs show a similar behavior as the 6 nm@9 nm particles, although the values are slightly smaller. Field-dependent magnetization measurements carried at 5 K in the zero field cooled (ZFC) state revealed a ferromagnetic hysteresis loop with |HC| = 3.5 kOe (see Figure S6 in the Supporting Information). Again, performing these measurements after cooling the sample in an applied field of 2 T led to a distinct shift of the hysteresis loop due to exchange interactions at the interface. However, compared to that of 6 nm@9 nm FePt@ MnO nanodimers, |Hex| is reduced to 1.5 kOe. It is reasonable to assume that this reduction is caused by the smaller effective interface area provided in 3 nm@9 nm compared to 6 nm@9 nm FePt@MnO nanodimers. Consequently, this smaller interface area leads to a weaker exchange coupling and hence, a reduced exchange bias field. 6 nm@17 nm and 3 nm@17 nm Dimers. Interestingly, an anomalous vertical shift of the FC hysteresis loop is observed for the 6 nm@17 nm and 3 nm@17 nm nano-heteroparticles; that is, the magnitude of magnetization in positive field direction, relative to the cooling field direction, is larger than that for applied field in the opposite direction (see Figure 9 and Figure S7 in the Supporting Information). This vertical loop shift indicates that a part of the magnetic moments is frozen in the FC direction and cannot be reversed by the applied field. The magnitude of the vertical shift can be defined as δm = (m1 − m2)/2, where m1 and m2 are the remanent magnetizations of the sample as depicted in Figure 9. For these 6 nm@17 nm FePt@MnO NPs, a δm of 0.364 emu g−1 was observed, which corresponds to a relative magnetization shift of ∼21% at μ0H = 0. Surprisingly, these particles do not show a significant exchange bias behavior, that is, a horizontal shift of the hysteresis loop, as discussed above for particles with smaller MnO domains. These results suggest that the exact exchange coupling mechanism is very complex. However, an even more pronounced vertical shift, accompanied by a negative exchange bias effect, was observed for pure MnO NPs, as reported by us previously.47 In general, such a vertical shift of the hysteresis loop is often observed in FM/AFM bilayer systems63,64 or in NPs with FM

between 3 and 4. However, these results again demonstrate the preference of MnO to grow epitaxially on equivalent crystal faces of FePt. A further example demonstrating the epitaxial relationship between the MnO domains and FePt octapods is shown in Figure S4 in the Supporting Information. 3.6. Magnetic Properties. Magnetic measurements performed on 3 and 6 nm FePt NPs and on 3 nm@9 nm, 6 nm@9 nm, 3 nm@17 nm, and 6 nm@17 nm FePt@MnO dimer NPs, respectively, revealed super-paramagnetic behavior at room temperature in all investigated samples. However, the individual magnetic properties differ substantially from each other in measurements carried out at 5 K, which indicates a considerable magnetic interaction between the FePt and MnO domains. The results are summarized in Table 1 (see Supporting Information). It was reported previously that such interactions can lead to enhanced magnetic properties, because the thickness of the magnetic dead layer in the FePt domain (or “canted spin” layer, which occurs due to broken symmetry at the FePt nanoparticle surface) is reduced.38,60,61 An explanation for this behavior can be derived by considering two key contributions: (1) the ferromagnetic−antiferromagnetic (FMAFM) interaction between the surface spins of FePt and MnO and (2) the magnetic volume of each sample. 3.6.1. Field-Dependent Measurements. 6 nm@9 nm Dimers. As an example, Figure 8 shows field-dependent

Figure 8. Comparison of the magnetic hysteresis loops of 6 nm@9 nm FePt-MnO NPs measured under different conditions: at 5 K after ZFC (filled triangles), at 5 K after FC with 2 T (open triangles), and at 300 K (circles). Because of magnetic anisotropy interactions between the FePt and MnO domains, the FC hysteresis is shifted by the exchange bias field Hex by 2.5 kOe from the origin.

magnetization curves of a 6 nm@9 nm FePt@MnO nanodimer sample conducted under different conditions. In contrast to the measurements carried out at room temperature, these NPs exhibit a pronounced ferromagnetic hysteresis loop at 5 K with a coercive field (HC) of 4.2 kOe. Furthermore, saturation is not reached even in applied fields of up to 5 T. Indeed, this feature is not uncommon in systems where an antiferromagnetic component with linear susceptibility, such as MnO, is added.47 It can be ascribed to canting of the antiferromagnetic moments in MnO toward the field direction. However, the applied field is insufficient to saturate the antiferromagnet. On the other hand, naked FePt NPs with a diameter of 6 nm are magnetically softer with HC = 3.4 kOe (see Figure S5a in the Supporting Information). However, the strongest evidence for a direct interaction between both magnetic domains can be observed if the magnetic hysteresis 531



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behavior, we carried out a series of field-dependent magnetization experiments with various cooling fields ranging from 0.1 kOe to 10 kOe and determined the corresponding vertical magnetization shift δm. Figure S8 in the Supporting Information shows plots of δm versus the applied cooling fields for 6 nm@17 nm and 3 nm@17 nm FePt@MnO nanodimers. Also, the shift is larger in the 6 nm@17 nm sample, which supports the assumption of a strong magnetic interface interaction between FePt and MnO because the 6 nm@17 nm particles exhibit a larger interfacial area. In both cases, it appears that δm reaches saturation for higher cooling fields, indicating that for a certain cooling field all interface spins are frozen in direction of the applied field and, therefore, higher cooling fields lead to a saturation effect. However, it should be noted these nano-heterodimers show a complex magnetic structure depending on the sizes of the individual domains, as well as on their interface, and that more detailed investigations are necessary to thoroughly explain this phenomenon. Indeed, further experiments to examine the magnetic structure of the present nanoparticle system by stroboscopic polarized small-angle neutron scattering (SANS) are currently in progress. Mixture of 6 nm FePt and 17 nm MnO NPs. Control measurements with pure 6 nm FePt NPs and with thoroughly mixed but nonconnected 6 nm FePt and 9 nm MnO NPs were carried out to ensure that the observed effects can only be ascribed to the interface interaction in connected FePt@MnO nano-heteroparticles (see Figure S5 in the Supporting Information). In fact, these measurements revealed no change in the hysteresis loop recorded in the FC and ZFC states in neither pure FePt NPs nor mixed FePt/MnO NPs. This clearly indicates that the exchange bias effect and the vertical shift in the hysteresis loop are caused by the interface interaction of the MnO and the FePt domain. 3.6.2. Temperature-Dependent Measurements. Temperature-dependent magnetization measurements in FC/ZFC mode display a distinct blocking temperature TB, which is typical for super-paramagnetic particles (compare Figure 10).

Figure 9. Field-dependent magnetization measurements of 6 nm@17 nm FePt@MnO nanodimers carried out in a ZFC (filled triangles) and 2 T FC (open triangles) state at a temperature of 5 K. The fact that MnO NPs by themselves can be considered to be a magnetic core− shell system leads to a vertical splitting of the FC hysteresis. Because of the additional interactions with ferromagnetic moments of the FePt domain, the whole FC-hysteresis is shifted vertically by δm = 0.364 emu g−1.

core and a spin glass-like shell.65 However, in most cases, the magnitude of this shift is only in the range of a few percent,65,66 while it reaches values of up to 21% in our case. On the other hand, both vertical and horizontal loop shifts were reported in AFM NiO and MnO NPs,43,47 in oxidized cobalt NPs67,68 as well as in nanostructured composite materials consisting of ferromagnetic Fe and antiferromagnetic MnO2.69 Here, the authors found that the magnitude of both the exchange bias and δm depends strongly on the molar ratio of Fe to MnO2, and indeed, they observed a larger vertical shift for samples with a low Fe content. Several models exist to explain the complex interface coupling of FM and AFM materials,69,70 which can be qualitatively transferred to our FePt@MnO nanodimer system. First of all, MnO NPs themselves have an AFM core with uncompensated surface spins causing a FM behavior, if the particle size is small. Thus, the MnO NPs are magnetically an AFM/FM core shell system, which leads to weak but measurable exchange bias effects.47 When pure MnO NPs are cooled below the Néel temperature (TN = 122 K) in an externally applied magnetic field, the uncompensated surface spins are pinned to the AFM core. Thus, the magnetization of these spins is harder to reverse by an opposite field, which leads to an enhanced coercive field and a weak exchange bias effect. Moreover, in MnO NPs, the AFM core experiences the magnetic field of the FM shell in addition to the external field resulting in a shifted interface magnetization and thus, in a vertically shifted hysteresis loop. If both the external field and the exchange field point in the positive direction, the loop is shifted upward, corresponding to a FM interface interaction.70 In fact, this vertical shift can become reasonably large in MnO NPs, as demonstrated by our previous findings.47 Based on these considerations, the magnetic structure of the FePt@MnO nanodimers can be described as a FM (FePt) connected with a FM/AFM core/shell system (MnO) via a common interface; that is, a detailed description of the exchange interactions in these particles is nontrivial. Additionally, the fact that this vertical shift was only observed for those nanodimers with a relative large MnO domain clearly indicates that the magnetic spin structure of MnO plays a major role, for example, the ratio of noncompensated/canted surface moments to bulk moments within each particle. To further investigate the observed

Figure 10. Temperature-dependent ZFC magnetization measurements of 6 nm FePt NPs and 6 nm@9 nm and 6 nm@17 nm FePt@ MnO nano-heterodimers in an applied field of 100 Oe, revealing magnetic blocking temperatures TB of 32, 47, and 77 K, respectively. The corresponding magnetization values were normalized for better comparison. The shoulder in the 6 nm@17 nm sample between 20 and 40 K is probably due to the presence of free FePt NPs.

Above TB, the thermal energy of the system kBT is sufficient to freely flip the particle moments. At lower temperatures, on the other hand, kBT is too small and the magnetic moments of the 532



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connected to conductive metals (such as FePt or Au) might therefore exhibit interesting properties that require thin transparent semiconductors, such as optoelectronics and energy harvesting.

particles are aligned in direction of the external magnetic field. Consequently, the system behaves like a ferromagnet. The latter case is often referred to as the “blocked” state, and therefore, the temperature, at which the transition from the super-paramagnetic to the ferromagnetic state occurs, is called magnetic blocking temperature. TB is 78 and 48 K for the 6 nm@17 nm and 6 nm@9 nm FePt@MnO NPs, respectively, while the pure FePt (6 nm) seeds exhibit a TB of 32 K. The increase in TB in the FePt@MnO samples with increasing the MnO size can be ascribed to two factors. First, the pinning effect caused by the presence of an AFM MnO domain, which is connected via a common interface, results in a higher stability against thermal fluctuations. A similar observation was reported for FePt-MnO core−shell NPs with FePt domains of 3 nm in size.38,71 Second, magnetic dipolar interaction between the FePt and MnO domains could increase the overall magnetic anisotropy of the system, thus leading to higher blocking temperatures, a phenomenon that has recently been observed in Fe nanocubes.72 Furthermore, the present ZFC plots of the 6 nm FePt NPs and the 6 nm@9 nm FePt@MnO NPs exhibit considerably narrow shapes, which is a clear indication for a narrow particle size distribution. Only the curve for the 6 nm@ 17 nm FePt@MnO dimers is slightly broadened and exhibits a weak shoulder at low temperature, which might be caused by a small amount of remaining pure FePt seeds. The magnetic blocking temperatures of FePt@MnO nanoheterodimers based on 3 nm FePt are comparable to those of the 6 nm particles (see Figure S9 in the Supporting Information). However, the corresponding values are slightly lower with TB = 16, 45, and 65 K for pure 3 nm FePt NPs and 3 nm@9 nm and 3 nm@17 nm FePt@MnO nanodimers, respectively. These lower values for TB can be ascribed to the lower particle volume of the FePt domains. However, independent of the FM grain size, the blocking temperature seems to be mainly dependent on the size of the MnO grains. Information on the impact of shape anisotropy and exchange interactions on the magnetic anisotropy in the FePt@MnO nano-heterodimers nanoparticles would be highly desirable. This information could be achieved with the aid of stroboscopic SANS measurements on nano-heterodimers of different subunit sizes. With different frequencies of an oscillating magnetic field, samples could be studied as a function of temperature down to the known blocking temperatures. Low-temperature measurements in a magnetic field and zero field would help with analyzing the influence of particle alignment. The change of contrast for polarized neutrons at the reversal of the magnetic field could allow to precisely determine the phase shift of the scattering response with respect to the inducing field and hence to determine the delay of the moment relaxation when the characteristic frequency is approached. The binding of metal and metal oxide nanoparticles to chalcogenide layers on the basis of Pearson hardness73,74 may be used for the fabrication of “molecular” analogs of metal oxide gated layered chalcogenide transistors. Traditional metaloxide-semiconductor (MOS) structures are obtained by growing a layer of metal oxide on top of a semiconductor substrate and depositing a layer of metal (or silicon). Recently, Kis and co-workers used a metal oxide gate dielectric to demonstrate a room-temperature single-layer MoS2 mobility, similar to that of graphene nanoribbons, and to demonstrate transistors with room-temperature on/off current and ultralow standby power dissipation.75 Chalcogenide layers “gated” by metal oxide (such as MnO or Fe3O4) particles epitaxially

4. CONCLUSIONS In summary, we were able to prepare monodisperse FePt@ MnO nano-heteroparticles of different shapes and sizes by a two-step seed-mediated growth technique. The size of each domain could be controlled by adjusting the individual synthetic parameters. Moreover, the particle morphology could be varied from dimers to flowerlike particles by changing the polarity of the solvent from nonpolar to polar. Additionally, FePt@MnO nano-heteroparticles prepared with large (13 nm) FePt nanocubes as seeds showed a predominant formation of multimers with two or more MnO domains. HR-TEM investigations revealed that both materials appeared to be connected in a co-oriented way. Furthermore, magnetic measurements gave evidence of considerable interactions between both magnetic domains, resulting in increased remanent field, coercive field, and blocking temperature compared to naked FePt NPs. Finally, a shift in the fieldcooled hysteresis could be observed, which is due to magnetic exchange anisotropy across the FM/AFM FePt-MnO interface.

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ASSOCIATED CONTENT

* Supporting Information S

Additional figures and tables and the DLS results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 6131 39-25135. Fax: +49 6131 39-25605. E-mail: [email protected].

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ACKNOWLEDGMENTS We are grateful to Center for Complex Matter (COMATT) for support. T.D. Schladt is a recipient of a Feodor Lynen Fellowship from the Alexander von Humboldt Foundation. M. Dietzsch is a recipient of a Carl-Zeiss Fellowship. T. Graf is a recipient of a fellowship through funding of the Excellence Initiative (DFG/GSC 266). The Electron Microscopy Center in Mainz (EZMZ) is operated through the Center for Complex Matter (COMATT). DLS experiments were carried out in the group of Prof. M. Schmidt, Institute for Physical Chemistry at the Johannes Gutenberg University in Mainz. We also want to thank Prof. M. Schmidt for fruitful discussions.

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REFERENCES

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Synthesis and Magnetic Properties of FePt@ MnO Nano-heteroparticles

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