Magnetism at the Interface of Magnetic Oxide and Nonmagnetic

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Magnetism at the Interface of Magnetic Oxide and Nonmagnetic Semiconductor Quantum Dots Avijit Saha† and Ranjani Viswanatha*,†,‡ †

New Chemistry Unit and ‡International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India S Supporting Information *

ABSTRACT: Engineering interfaces specifically in quantum dot (QD) heterostructures provide several prospects for developing multifunctional building block materials. Precise control over internal structure by chemical synthesis offers a combination of different properties in QDs and allows us to study their fundamental properties, depending on their structure. Herein, we studied the interface of magnetic/nonmagnetic Fe3O4/CdS QD heterostructures. In this work, we demonstrate the decrease in the size of the magnetic core due to annealing at high temperature by the decrease in saturation magnetization and blocking temperature. Furthermore, surprisingly, in a prominently optically active and magnetically inactive material such as CdS, we observe the presence of substantial exchange bias in spite of the nonmagnetic nature of CdS QDs. The presence of exchange bias was proven by the increase in magnetic anisotropy as well as the presence of exchange bias field (HE) during the field-cooled magnetic measurements. This exchange coupling was eventually traced to the in situ formation of a thin antiferromagnetic FeS layer at the interface. This is verified by the study of Fe local structure using Xray absorption fine structure spectroscopy, demonstrating the importance of interface engineering in QDs. KEYWORDS: interface magnetism, exchange bias, quantum dots, magnetic/nonmagnetic heterostructure, XAFS materials has been extensively studied,12−16 magnetic/nonmagnetic interfaces have largely been ignored, except for a few recent studies on atomically controlled interfaces, such as the linear chain of three Fe atoms on monatomic copper nitride layer.17 Specifically, FM/nonmagnetic QD heterostructure interfaces,18−20 which lie between bulk and atomic limits and play a key role in miniaturization of devices, have not been explored for interface-induced properties. Fundamental understanding of nanomagnetism, specifically single-domain magnetism in confined domains, also known as superparamagnetism, has been identified as an important area in the field of magnetic QDs. Recently, studies have been carried out on magnetic QDs to optimize saturation magnetization, magnetic anisotropy, coercivity, and other properties for applications in magnetic data storage,21 MRI contrast agents,22,23 magnetic hyperthermia,22,24,25 etc. In the case of FM/AFM magnetic nano-heterostructures, one of the few interesting interfacial properties that has been well-studied is the exchange bias effect.7,16,26−29 Exchange bias can be defined

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ell-controlled interfaces between bulk heterostructures have given rise to interesting properties arising out of the surface interaction of the constituent materials at the interface. One of the extensively studied textbook-like examples of the interfacial phenomena is the LaAlO3 and SrTiO3 perovskite oxides.1−5 Electronic reconstructions of these oxides have shown the presence of interfacial conductivity and a large negative magneto-resistance in these otherwise nonmagnetic insulating oxides.1,4,6 Similarly, other materials studied for interface-induced phenomena include magnetism at the interface of half-metallic ferromagnet La2/3Ca1/3MnO3 and superconducting YBa2Cu3O7 as well as characteristics of a ferromagnet/oxide interface in the Fe/MgO system.7,8 Management of the magnetism and magneto-resistance arising out of interfaces is key to the development of multiferroic and spintronics applications.9 Hence, it is important to study these interfaces especially as the devices approach the quantum limit, where the effects are enhanced and compete with their corresponding bulk properties. Atomic level manipulation of spin and charge of an electron in magnetic and nonmagnetic heterostructures can lead to the development of quantum dot (QD)-based spintronic devices10,11 and hence the need to be explored. However, though interface magnetism at the interface of ferromagnetic (FM) and an antiferromagnetic (AFM) © 2017 American Chemical Society

Received: February 1, 2017 Accepted: March 4, 2017 Published: March 4, 2017 3347

DOI: 10.1021/acsnano.7b00711 ACS Nano 2017, 11, 3347−3354

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Figure 1. (a) M vs H hysteresis loop of different Fe3O4−nCdS QDs during initial shell growth, measured at 2 K. (b) Zero-field-cooled (ZFC, solid lines) and field-cooled (FC) magnetization (dashed lines) measured using a field of H = 500 Oe for all the Fe3O4−nCdS samples. Inset shows the zoomed-in view of the FC−ZFC curve of Fe3O4−5CdS at lower temperature.

as a unidirectional anisotropy16,26,27,30 that arises due to the exchange coupling of the soft FM material with a hard AFM interface upon field cooling below the Neel temperature of the AFM material. This gives rise to an additional anisotropy in the system that can overcome the superparamagnetic limit31−33 in small-sized QDs. Since the discovery of exchange bias in the Co/CoO core−shell FM/AFM heterostructure,26,27 exchange bias has been studied on various FM/AFM heterostructures in core−shell nanoparticles, providing more insight into this field.34−36 However, interfaces between magnetic and nonmagnetic materials have largely been ignored as they are not expected to have any exchange coupling interaction at the interface. In fact, nonmagnetic layers have routinely been used as spin insulating layers between two magnetic systems in spin transport applications.37,38 However, recent work7 in bulk Fe/ MgO has shown interesting results at the interface due to the proposed presence of FeO interactions at the surface. In these bulk materials, these effects are very small and largely not observed in bulk magnetic measurements. In fact, the authors had to perform magnetization-induced second harmonic generation to selectively probe the interface and observed a very small exchange bias (∼19 Oe), demonstrating the presence of an antiferromagnetic interface layer but no exchange bias in the bulk. In this work, we study the magnetic property at the interface of ferrimagnetic (FiM) Fe3O4 and nonmagnetic CdS heterostructure QDs with decreasing magnetic cluster size. While encapsulation of Fe3O4 or other ferro(ferri)magnetic nanoparticles in a nonmagnetic matrix has been extensively carried out39−45 for various applications including biocompatibility as well as magnetism enhancement,20,42,46 the interface of this magnetic/nonmagnetic heterostructure has not been probed. In the present work, we focus on the exchange coupling at the interface and demonstrate the in situ formation of an AFM layer at the interface during the growth of the CdS layer. In this system, the magnetic exchange interaction would be more strongly perturbed due to the spatial confinement compared to its bulk counterpart. This can affect the electronic structure and give rise to the exchange of charge carriers between ferromagnetic and semiconducting phases by injecting spins from the ferromagnetic to the semiconducting phase without requiring the atomic layer precision that has become the

hallmark necessary to observe these effects in FM/nonmagnetic systems.2,47 In this work, we have used the colloidal SILAR (successive ionic layer adsorption and reaction) synthesis method48 to obtain Fe3O4 QDs overcoated with a semiconducting CdS matrix. Magnetic ion localization inside the semiconductor matrix is a very crucial factor for both fundamental understanding and application point of view. The samples were characterized using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) measurements. The magnetism at the core and at the interface of this ferromagnetic/nonmagnetic QDs have been studied using standard magnetization measurements using a superconducting quantum interference device (SQUID) magnetometer. Variation of magnetic cluster size is determined from their corresponding variation of blocking temperatures (Tb). Further, to investigate the origin of anomalously high coercivity, we studied the hysteresis loop shift as a function of field leading to the discovery of an exchange bias at a magnetic/nonmagnetic interface. The origin of this exchange bias field has been proposed to arise from the in situ formation of interfacial antiferromagnetic FeS formation. Indirect proof for this interfacial structure has been obtained by modifying the size and annealing temperature to maximize the interface and hence the increase in coercivity as well as the exchange bias. Additionally, a more direct proof is achieved by a local environment study of the Fe ion using X-ray absorption fine structure (XAFS) for a series of CdS overcoating and annealing temperatures. Thus, we report here a magnetic/semiconductor nanoparticle system which is not only an optically active material but also shows a magnetic exchange bias effect at a FiM/nonmagnetic interface.

RESULTS AND DISCUSSION The study of internal structure using HRTEM is the most straightforward proof for the presence of interfacial layers. However, HRTEM shows the presence of only single lattice spacing in Fe 3O 4 /nCdS, as shown in the Supporting Information (SI) Figure S1. From the TEM, while it is evident that the particle size grows systematically with increasing CdS monolayers, clusters of Fe3O4 cannot be distinguished from the lattice parameters due to the presence of a very thin shell. 3348

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the coercivity (Hc) of these nanocrystals does not decrease monotonically as expected. Figure 2a shows the M versus H

Hence, study of an even thinner interface layer using HRTEM is clearly beyond the accuracy of the TEM. XRD measurements in literature19 for these systems show clear evidence of magnetic core/nonmagnetic shell heterostructures in these systems, specifically with thin CdS shells. The strongest peak for the Fe3O4 and CdS is shown in Figure S2 in the SI. From the figure, it is evident that while the characteristic peak intensity of Fe3O4 corresponds to the (311) lattice plane decreases, it also becomes broader as expected for a decreasing core size. On the other hand, the CdS peak corresponding to the (101) lattice plane increases in intensity with an associated decrease in the full width at half-maximum for the peak due to the overall increasing size of the nanocrystals. However, interfaces have not received enough interest in literature due to the lack of characterization techniques in spite of their interesting properties. Property-driven characterization wherein a specific change in property induced by the interface is observed or indirect methods, such as modeling to fit variable emission photoemission data49 or extended X-ray absorption fine structure (EXAFS) data, are some of the possible means of studying the interface. In this work, we have studied magnetic properties of a series of samples with an increasingly thicker nonmagnetic CdS shell to propose the nature of the interface. This is then further evaluated by the study of EXAFS of the Fe edge for these materials. The evolution of magnetic properties has been studied on a series of samples of Fe3O4/nCdS with varying values of n, the number of monolayers of CdS at 2 K and at room temperature, and are shown in Figure 1a and SI Figure S3, respectively. At room temperature, these particles show superparamagnetic behavior with no coercivity (SI Figure S3) as they are above the blocking temperature. However, at 2 K, all of these nanocrystals showed ferromagnetic behavior with significant coercivity (Hc). Figure 1a shows the magnetization (M) versus applied field (H) plots at 2 K for Fe3O4−nCdS samples for differing values of n, whereas Figure 1b shows the DC magnetic susceptibility measured from M versus temperature (T) curves following zero-field-cooled (ZFC) and field-cooled (FC) protocols using an external field of 500 Oe. The inset to Figure 1b shows the zoomed-in plot of the same to demonstrate the blocking temperature (Tb). From Figure 1a, we can observe that as the thickness of CdS increases the saturation magnetization per magnetic ion decreases dramatically. This is not surprising as the overcoating of CdS is done at high temperature for extended periods of time. Hence, as the annealing time increases, the magnetic core slowly diffuses into the nonmagnetic CdS matrix, leading to an effective decrease in the magnetic cluster size as supported by the XRD data. Additionally, if this is indeed true, it is well-known that in superparamagnetic nanoparticles their blocking temperature (Tb) decreases with the decrease of cluster size as we need lesser thermal energy for flipping of spin for a smaller size cluster22,24 from its blocked state to a superparamagnetic state. Hence, we studied the variation of blocking temperature for various thicknesses of CdS, as shown in Figure 1b. From the figure, it is evident that the blocking temperature decreases from 54 to 21 K as we progress from Fe3O4 to Fe3O4−5CdS. Thus, in this case, in spite of an overall increase in the size of the nanocrystal, the size of the magnetic domain, critical for the determination of Tb, reduces with increasing annealing time. However, surprisingly, the evolution of magnetic anisotropy inside the nanocrystal during CdS shell growth quantified by

Figure 2. M vs H (zoomed-in view at low field) hysteresis loops of Fe3O4−0CdS and Fe3O4−3CdS nanocrystals measured at 2 K; inset shows the variation of coercivity obtained from M vs H hysteresis loops for all of the Fe3O4−CdS samples obtained overcoating (a) 4.5 nm Fe3O4 core and (b) 7.3 nm Fe3O4 core.

curve for Fe3O4 (4.5 nm) and Fe3O4−3CdS (6.8 ± 1.5 nm), and the inset shows the variation of coercivity with the increase of CdS shell growth. There is a clear enhancement of coercivity from Fe3O4 (Hc = 240 Oe) to Fe3O4−3CdS (Hc = 540 Oe). This anomalous increase in Hc is surprising as CdS is a nonmagnetic semiconductor and hence would not affect the magnetic behavior of the core. In order to investigate the validity and the generality of this effect, we synthesized a larger magnetic Fe3O4 core (∼7.3 nm) overcoated with a thin shell of CdS (size = 10.5 ± 1.2 nm). These particles were characterized by TEM images shown in SI Figure S4. The M versus H plots for these larger samples at 2 K are shown in Figure 2b. From the figure, it can be observed that the core QDs have larger coercivity (∼310 Oe) at 2 K as expected due to the increase in size. Similar to the earlier case, we not only observe an increase in the coercivity with the overcoating of a thin shell of CdS but actually also observe an almost 5-fold increase in coercivity (∼1570 Oe) that is clearly above the normal error limit. Study of the origin of coercivity points toward an increase in the anisotropy energy inside the QD. This enhancement in anisotropy can arise out of various factors like shape anisotropy, isotropic dipolar effects, or exchange bias. Since our particles are spherical in shape, shape anisotropy is not likely. It is wellknown in literature that exchange bias induces anisotropy, leading to an increase in the coercivity24,26,27 due to exchange interactions between the core and the shell at the interface. In fact, in the case of a FM/AFM interface, it has been shown that an increase in anisotropy can be the effected due to pinning of the domain wall on the surface of the magnetic core, giving rise to the so-called “exchange bias” effect16,26,27,35,36,50 similar to 3349

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Figure 3. (a) M vs H hysteresis loop for the Fe3O4/CdS (larger) sample, measured at 2 K using 70 kOe field-cooled (red) and zero-field-cooled (black) conditions. (b) Zoomed-in version of M vs H hysteresis loop at low field. (c) Variation of HE with various cooling field for large Fe3O4−3CdS (black) and small Fe3O4−3CdS (blue) nanoparticles obtained from a 7.3 and 4.5 nm core, respectively. (d) Variation of Hc and HE as a function of CdS thickness obtained by overcoating 7.3 nm core. The lines are a guide for the eyes.

provide the interaction with the ferromagnetic core, making the observation of exchange bias anomalous. The only possible rationalization of these observations can be obtained by the presence of an AFM FeS layer at the core−shell interface that is formed in situ during the synthesis, as shown in Scheme 1, which would be responsible for this exchange bias.51,52

our observation here in the current scenario. Further, these data also suggest that the effect is enhanced in the presence of larger interfacial surface as in the case of 7.3 nm Fe3O4, attributing these changes to an interfacial phenomenon. Based on this information and from the observed increase in coercivity, we hypothesize the presence of an in situ AFM layer leading to exchange bias in these systems. It is well-known that the classic signatures of the presence of such an exchange coupling is the asymmetric shift in the hysteresis loop by an amount quantified by the exchange shift HE that is dependent on the cooling field. The comparison of M versus H plot for the ZFC and FC condition using a cooling field of 70 kOe for the larger Fe3O4-3CdS is shown in Figure 3a, and the zoomed-in version of the same is shown in Figure 3b, showing a clear asymmetric shift in the hysteresis loop. Quantitatively, we observed a HE of ∼200 Oe upon using a field of 70 kOe during the FC measurement for the larger Fe3O4− 3CdS. To study the effect of CdS overcoating on observed magnetic exchange coupling, we have measured FC/ZFC M versus H measurement (2 K) for different CdS shell thickness (large core), and the asymmetric shift of the loop upon field cooling with 70 kOe for different samples with varying CdS shell thicknesses is shown in SI Figure S5. The quantitative variations of HE with different cooling field for the smaller and larger Fe3O4 cores are shown in Figure 3c, demonstrating a clear monotonic dependence of the exchange field with the applied cooling field. The variation of Hc and HE shown in Figure 3d demonstrates a direct correlation of the exchange field with the observed coercivity of these nanoparticles, attributing the entire increase in coercivity to the presence of exchange bias. The exchange bias in core−shell nanoparticles described in literature is observed in the FM/AFM (metal/ oxide or oxide/oxide) core−shell heterostructure due to the interaction of FM and AFM layers.12,14,15 However, in this work, we do not have any a priori AFM material that can

Scheme 1. Formation of a Thin AFM Layer (FeS) at the Interface Giving Rise to FiM/AFM Coupling

Upon survey of bulk literature, it was observed that, in the case of Fe/MgO, there exists a small exchange bias (∼19 Oe) at the interface, as observed from magnetization-induced second harmonic generation measurements even though no bulk exchange fields were observed.7 This exchange bias was eventually traced to the presence of the AFM FeO layer at the interface. Similarly, in our case, in order to trace the origin of the antiferromagnetic ordering, giving rise to the exchange anisotropy, internal structure at the interface is important. In the current case, XRD and magnetism point toward the diffusion of Fe into the CdS lattice. Based on the observed data and the synthesis methods, we propose the in situ formation of a thin FeS layer at the interface before Fe diffuses into the CdS lattice. However, the short-range ordering of Fe−S−Fe would not be evident from XRD measurements. Hence, if this is indeed true, study of local structure around the Fe atom in 3350

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Figure 4. (a) Magnitude of Fourier transformed Fe K-edge EXAFS spectra (open symbols) and their best fit (solid black line) for (a) Fe3O4− 1CdS, (b) Fe3O4−2CdS, (c) Fe3O4−3CdS, and (d) Fe3O4−4CdS, synthesized using a 7.3 nm Fe3O4 core. Dotted lines show their component fitting paths.

these structures using EXAFS should show the presence of Fe− S−Fe second-order interactions. In order to verify the presence of the FeS layer, we measure the Fe K-edge for a series of samples and trace the local environment change around Fe atoms. We specifically measured those Fe3O4−nCdS samples in which the starting core was larger in size (7.3 nm) in order to observe a higher signal-to-noise ratio for the second-order interactions. These data were processed using ATHENA and then fitted using the theoretical proposed model and generating relevant paths using the ARTEMIS FEFF 6 program. Linear combination fit of XANES spectra for these samples, similar to earlier literature,19 shows the presence of an increasing FeS coordination and a decreasing Fe3O4 coordination (as obtained from standard bulk FeS and Fe3O4 data) with increasing CdS shell thickness, as shown in SI Figure S6 and Table S1. However, FeS interactions can arise from both Fe−S−Cd and Fe−S−Fe interactions. Hence, in order to verify the formation of an in situ AFM layer, we studied the Fe−S−Fe second-order interactions using EXAFS. Figure 4a−d shows the Fe K-edge experimental data and their corresponding fits for Fe3O4−1CdS, Fe3O4−2CdS, Fe3O4−3CdS, and Fe3O4−4CdS. The results obtained from these fits are tabulated in SI Table S2. It can be observed from the spectra (Figure 4a) that Fe3O4−1CdS data look very similar to that of Fe3O4, as observed in previous literature18,19,53 and fitted using Fe−O and Fe−Fe bonds originating from tetrahedral and octahedral interstices in a cubic inverse spinel structure. The peak at 1.4 Å in Figure 4a is due to the Fe−O bond arising with the nearest oxygen atoms, whereas the peaks at 2.52 and 3.01 Å are from Fe−Fe bonds due to same and different interstices, respectively. With the increase of CdS shell thickness, an additional Fe−S path arises at 1.96 Å corresponding to the formation of Fe−S bonds within the shell matrix due to the diffusion of Fe from core to the shell.

The Fe−S peak dominates as the core starts to diffuse inside the CdS matrix, as shown in the Figure 4a−d and SI Figure S7. SI Figure S7 shows the Fourier transformation of Fe K-edge XAFS data of Fe3O4−8CdS, where most of the contributions come from Fe−S with a very little amount of Fe−O, which suggests a complete dispersion of the core inside the thick CdS matrix. More importantly, it is important to observe that as the Fe starts to diffuse into the CdS matrix, we expect an increase in the FeS layer thickness in the proposed model. This increase in the FeS layer can be observed in the coordination number of Fe−Fe interactions, which is plotted in Figure 5. The total coordination number for Fe−O and Fe−S is found to be in the range of 3.5 to 4.2, which is in good agreement with the expected value of 4. Individually, Fe−O decreases from 4.08 to 0.6, whereas Fe−S increases from 0 to 2.8 as expected due to Fe diffusion inside CdS. However, interestingly, the Fe−Fe second shell coordination number at the initial stages is found to be almost constant in spite of the Fe−S formation, suggesting the formation of the FeS layer and not the diffusion of Fe in CdS. However, as the annealing increases, it eventually drops down to zero after much thicker CdS shell formation. This is also in agreement with the peak observed for HE values that is also plotted in Figure 5 for comparison. It is observed that the coordination number of this second shell is much smaller than the theoretically expected value of 12 due to small size as well as the in-built randomness due to the large surface to volume ratio. However, in spite of this quantitative disagreement, it is interesting to note that this Fe−Fe second shell contribution initially arising from the ordered Fe3O4 core does not decrease as the cluster size decreases and in spite of diffusion of Fe ions into the CdS lattice. This might be due to the presence of ordered Fe−S−Fe interactions along with Fe−O−Fe at the interface. This significant contribution of the Fe−Fe second3351

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effect55,56 is expected to diffuse the core in the semiconductor matrix in a controllable manner.57 The reaction is performed in the presence of excess reducing agent such as oleic acid to scavenge the oxygen released from the reduction of Fe3+ in Fe3O4 to Fe2+ in CdS. We carefully monitored the reaction and collected samples at different stages during nanocrystal growth to arrest the diverse internal structure from core−shell Fe3O4/CdS and probe their magnetic properties. We have also synthesized a larger core with a size of 7.3 (±1.3) nm and overcoated CdS following a similar procedure to increase the coercivity of the heterostructure. Characterization. Even though the SILAR synthesis dictates an exact monolayer growth of nanoparticles in the ideal scenario, it is rarely observed in reality. Hence, HRTEM was used to determine the actual size of the nanoparticles, whereas the samples were labeled using the number of monolayers for convenience. Powder X-ray diffraction patterns for the nanocrystals were recorded on a Bruker D8 Advance diffractometer using Cu Kα radiation (1.5418 Å). To determine the Cd and Fe ratio, elemental analysis was carried out using inductively coupled plasma optical emission spectroscopy. Elemental analysis was performed by dissolving the samples in Millipore water with 2−5% HNO3 solution, and the Fe and Cd concentrations were measured against known Fe and Cd standards (high-purity). Magnetic measurements (M vs H) at 2 K and at room temperature (300 K) were carried out in SQUID VSM (Quantum Design). Hysteresis loops were measured in ZFC and FC conditions after being cooled from room temperature to 2 K with 10, 30, and 70 kOe applied field. Magnetization values were initially obtained in emu/g that is inclusive of the QDs as well as the ligands. In order to exclude the effect of ligand weight, we performed thermogravimetric analysis (TGA) to estimate the amount of ligand attached with the nanocrystal surface and to estimate the molecular weight of the QDs. After the ligand weight was subtracted, approximate saturation magnetization values obtained for different nanocrystals were reported here. TGA was performed using a TGA/DSC 2 STAR instrument in the temperature range of 300−1073 K under nitrogen atmosphere with a ramp rate of 5 K/min. XAFS spectroscopy was employed to probe the local structure around Fe atoms for all the nanocrystals. Fe K-edge (7112 eV) for the samples was measured at beamline 2−2 at Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory. The gases in the ion chambers were optimized for adequate absorption of photons. For Fe K-edge, a mixture of 80% helium and 20% nitrogen gas was used in the initial ion chamber I0 placed before the sample and 100% nitrogen gas in transmission and reference ion chambers. Argon gas was used for the fluorescence ion chamber. Data collected were processed using Athena software58 by extracting the EXAFS oscillations χ(k) as a function of photoelectron wavenumber k following standard procedures. The theoretical model consistent with the other characterization techniques and the corresponding paths were generated using FEFF659 followed by fitting to the experimental data using the fitting program Artemis.60 In order to isolate the χ(k) oscillations, the atomic background was subtracted by applying a low distance cutoff equal to 1 Å in the Fourier transform and using a cubic spline. Fourier filtering was accomplished with a k weight equal to 2 in a range from 2 to ∼10 Å−1.

Figure 5. Variation of local coordination number around of Fe considering the nearest oxygen (black symbols), sulfur (green symbols), and iron (violet symbols) atoms from XAFS fittings. Red symbols show the variation of coercivity Hc as a function of CdS overcoating. Solid lines are a guide for the eyes.

order path from the ordered phase in the case of Fe3O4−3CdS and Fe3O4−4CdS, along with the Fe−S path, supports the formation of a thin layer of Fe−S surrounding the Fe3O4 core which is acting as the origin of the AFM center.

CONCLUSIONS Thus, based on the structure−property correlation obtained from local internal structure through XAFS and magnetic property measurements, we observe the in situ formation of an FeS AFM interface layer in an otherwise magnetic core/ nonmagnetic shell heterostructure. While this report is an attempt toward this end, it is important to perform more specific investigations like interface-sensitive magnetometry and theoretical calculations predicting the thickness of the intermediate layer to extensively understand the nature of this interface layer. However, the implications of the presence of this AFM layer is significant. Specifically, substantial exchange bias observed in this work at the magnetic/ nonmagnetic interface has so far not been observed in bulk measurements and can be used in hyperthermia or magnetic storage-like applications. In addition, we show that even though growth of magnetic/nonmagnetic semiconductor core/shell interfaces in QDs has so far been largely ignored except for few reports,20,43,44,54 these interfaces, unlike in bulk, can be responsible for a large number of interesting properties that have not been noticed and/or understood. We believe that the fundamental understanding of the internal structure and magnetic property in nanocrystals reported in this work will provide a stepping stone to understand a wide range of properties of magnetic/nonmagnetic heterostructure nanocrystals.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b00711. TEM images, XRD, room temperature M vs H hysteresis, linear combination fit of XANES spectra, XAFS spectra of Fe-doped CdS, and XAFS fitting parameters (PDF)

METHODS Synthesis. Fe3O4 nanocrystals of 4.5 (±0.8) nm size were synthesized as a magnetic core by following the literature method.18 SILAR technique48 was used to overcoat CdS at high temperature (240−260 °C), which allowed us to control both the growth of the semiconducting layer as well as the diffusion of the magnetic core inside the semiconductor matrix precisely. The self-purification

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 3352

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Avijit Saha: 0000-0003-1945-9076 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS A.S. and R.V. thank JNCASR, Sheikh Saqr Laboratory and Department of Science and Technology, Government of India. for financial support. A.S. thanks CSIR for a research fellowship. R.V. is grateful for the Sheikh Saqr Career Award Fellowship. We thank Somnath Ghara for magnetic measurement, and Dr. Syed Khalid for his help with experimental setup during XAFS. XAFS was carried out at Stanford Synchrotron Radiation Lightsource, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by SLAC National Accelerator Laboratory under Contract No. DE-AC02-76SF00515. Use of SSRL BL2-2 was partially supported by the National Synchrotron Light Source II, Brookhaven National Laboratory, under DOE Contract No. DE-SC0012704. We thank the Department of Science and Technology, India (SR/NM/Z-07/2015), for the financial support and JNCASR for managing the project. REFERENCES (1) Ohtomo, A.; Hwang, H. Y. A High-Mobility Electron Gas at the LaAlO3/SrTiO3 Heterointerface. Nature 2004, 427, 423−426. (2) Kalisky, B.; Bert, J. A.; Klopfer, B. B.; Bell, C.; Sato, H. K.; Hosoda, M.; Hikita, Y.; Hwang, H. Y.; Moler, K. A. Critical Thickness for Ferromagnetism in LaAlO3/SrTiO3 Heterostructures. Nat. Commun. 2012, 3, 922. (3) Brown, K. A.; He, S.; Eichelsdoerfer, D. J.; Huang, M.; Levy, I.; Lee, H.; Ryu, S.; Irvin, P.; Mendez-Arroyo, J.; Eom, C.-B.; Mirkin, C. A.; Levy, J. Giant Conductivity Switching of LaAlO3 /SrTiO 3 Heterointerfaces Governed by Surface Protonation. Nat. Commun. 2016, 7, 10681. (4) Brinkman, A.; Huijben, M.; van Zalk, M.; Huijben, J.; Zeitler, U.; Maan, J. C.; van der Wiel, W. G.; Rijnders, G.; Blank, D. H. A.; Hilgenkamp, H. Magnetic Effects at the Interface Between NonMagnetic Oxides. Nat. Mater. 2007, 6, 493−496. (5) Chan, N. Y.; Zhao, M.; Wang, N.; Au, K.; Wang, J.; Chan, L. W. H.; Dai, J. Palladium Nanoparticle Enhanced Giant Photoresponse at LaAlO3/SrTiO3 Two-Dimensional Electron Gas Heterostructures. ACS Nano 2013, 7, 8673−8679. (6) Mathew, S.; Annadi, A.; Chan, T. K.; Asmara, T. C.; Zhan, D.; Wang, X. R.; Azimi, S.; Shen, Z.; Rusydi, A.; Ariando; Breese, M. B. H.; Venkatesan, T. Tuning the Interface Conductivity of LaAlO3/SrTiO3 Using Ion Beams: Implications for Patterning. ACS Nano 2013, 7, 10572−10581. (7) Fan, Y.; Smith, K. J.; Lupke, G.; Hanbicki, A. T.; Goswami, R.; Li, C. H.; Zhao, H. B.; Jonker, B. T. Exchange Bias of the Interface Spin System at the Fe/MgO Interface. Nat. Nanotechnol. 2013, 8, 438−444. (8) Sa de Melo, C. A. R. Magnetic Exchange Coupling in Ferromagnet/Superconductor/Ferromagnet Multilayers. Phys. Rev. Lett. 1997, 79, 1933−1936. (9) Zanolli, Z. Graphene-Multiferroic Interfaces for Spintronics Applications. Sci. Rep. 2016, 6, 31346. (10) Sarma, S. D. Ferromagnetic Semiconductors: A Giant Appears in Spintronics. Nat. Mater. 2003, 2, 292−294. (11) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Spintronics: A Spin-Based Electronics Vision for the Future. Science 2001, 294, 1488−1495. (12) Si, P. Z.; Li, D.; Choi, C. J.; Li, Y. B.; Geng, D. Y.; Zhang, Z. D. Large Coercivity and Small Exchange Bias in Mn3O4/MnO Nanoparticles. Solid State Commun. 2007, 142, 723−726. 3353

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