Delocalized Electrons Mediated Magnetic Coupling in Mn–Sn

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Delocalized Electrons Mediated Magnetic Coupling in Mn−Sn Codoped In2O3 Nanocrystals: Plasmonics Shows the Way Bharat Tandon,‡ Anur Yadav,‡ and Angshuman Nag*,†,‡ ‡

Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Pune 411008, India

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S Supporting Information *

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CB e−Mn2+ ferromagnetic exchange interaction in the photoexcited Mn-doped ZnO NCs;5 and (iii) we reported the possibility of CB e−Fe3+ exchange interaction in Fe−Sn codoped In2O3 NCs. However, a fraction of Fe3+ reduces to Fe2+ ions after Sn4+ codoping, which in turn has the opposing effect of decreasing magnetic moment.14,15 Here we report Mn−Sn codoped In2O3 NCs (schematically shown in Figure 1a), with stable oxidation state of Mn2+ ion, and exhibiting interesting CB e−Mn2+ ferromagnetic exchange interaction in the ground state.

ilute magnetic semiconductor oxides (DMSO) are metal oxides doped with magnetic ions, where interaction between a delocalized charge carrier such as conduction band (CB) electron (e) and localized magnetic spin (for example, dopant Mn2+ ion) decides its potential as a future spintronic material.1,2 The possibility of manipulating this carrier−Mn2+ exchange coupling strength via quantum confinement of charge carriers in a semiconductor nanocrystal (NC) has already been reported. 3,4 Gamelin et al. demonstrated CB e−Mn 2+ ferromagnetic exchange interactions in Mn-doped ZnO NCs, where CB e was generated by photoexcitation in a strict anaerobic condition.5 Here we introduce CB e in the air-stable ground state by Sn4+ doping in Mn−Sn codoped In2O3 NCs that exhibit CB e−Mn2+ ferromagnetic exchange interactions, yielding a nearly ideal (∼4.8 μB/Mn2+ ion) magnetic moment at 2 K and 70 kOe. Importantly, the plasmonic band of our NCs provides a way to distinguish delocalized CB e from defect-bound localized e, unlike electrical characterization that suffers from grain boundary related problems. Sn-doped In2O3, popularly known as ITO, is a benchmark transparent conducting oxide (TCO) that is both transparent to visible light and conducts electricity. The fact that TCOs possess CB e in the ground state has motivated researchers to dope magnetic ions in TCOs, and study the CB e−magnetic ion interactions.1,6,7 Careful studies show that good quality single crystals of magnetically doped TCOs do not exhibit ferromagnetism; instead, polycrystalline films exhibit ferromagnetism.1,8 Electrical conductivity of such polycrystalline magnetically doped TCO films typically exhibit nonmetallic behavior, which may arise from extensive contribution from insulating grain boundaries in spite of carriers are metal-like delocalized within a grain.9 Therefore, to characterize the presence of delocalized CB e unambiguously, we used optical (plasmonic) response of Sn−Mn codoped In2O3 NCs and such characterization allows us to study CB e−Mn2+ magnetic interactions with more confidence. One of the major problem hindering ferromagnetic interactions in a doped semiconductor NC is the intrinsic antiferromagnetic superexchange coupling between next-tonearest neighbor magnetic cations such as Mn2+−O2−− Mn2+.5,10−13 On the other hand, the CB e−Mn2+ interaction is ferromagnetic in nature, and has the potential to overcome this antiferromagnetic superexchange interaction. For example, (i) Strouse and co-worker suggested carrier mediated ferromagnetic interaction in Mn-doped CdSe NCs, but the carriers were not CB e, and instead came from surface defect states of ultrasmall (3.5% Mn (Figure 3 and Figure S5a in the SI). Because the size and shape of our NCs along with refractive index of the medium remains unaltered, the observed decrease in both absorbance and peak energy of SPR is attributed to a decrease in delocalized CB e density with increasing Mn content. Also, half width at half maxima (HWHM) of the SPR band increases with Mn codoping (Figure S5b of the SI), suggesting increased scattering of the electron by Mn2+ dopant ions. The SPR behavior discussed above is for colloidal Sn−Mn doped In2O3 NCs dispersed in tetrachloroethylene. Also, spincoated thin films of these NCs on a quartz substrate exhibit similar SPR behavior (Figure S6 of the SI) with minor changes in the SPR peak position because of the change in refractive index around NCs.27 Importantly, 1.3% Mn−10% Sn codoped

Figure 2. (a) Temperature dependent resistivity plots for 10% Sn doped In2O3 NCs. ρT and ρ373 K are resistivities at temperature T and at 373 K, respectively. Open circles are data and solid line is a guide to eye. Inset shows ohmic behavior through current vs voltage plot. (b) Nyqvist plot of AC impedance measurements in the frequency (ω) range of 100 mHz−2 MHz for 10% Sn doped In2O3 NCs.

doped In2O3 NCs decreases with increasing temperature (Figure 2a), exhibiting a nonmetallic behavior in the temperature range of 298−373 K. Figure S4 in the SI shows that “1.3% Mn−10% Sn” and “3.5% Mn−10% Sn” codoped In2O3 NCs also exhibit similar nonmetallic behavior, and resistivity increases with increasing Mn content. Such nonmetallic behavior can be attributed to extensive grain boundary effect 3621

DOI: 10.1021/acs.chemmater.6b01465 Chem. Mater. 2016, 28, 3620−3624

Communication

Chemistry of Materials

13 and ref 31 suggested that in addition to Mn2+ ions, a reasonable amount of Mn3+ ion was also present in their Mndoped In2O3 NCs. Overall, prior literature suggest Mn2+ as the major dopant in In2O3; however, quantitative differences in the amount of Mn3+ ion in different reports might arise from the difference in sample preparation, yielding a difference in local structure around a Mn ion, which in turn can influence the formation energies of different oxidation states. To study the CB e−Mn2+ exchange interaction, in other words, carrier mediated magnetic coupling, we start by comparing the EPR spectra in Figure 4a. Broadening of the EPR signal of Mn2+ ion, after codoping with Sn4+ ion, provides an indication of magnetic coupling among distant Mn2+ ions mediated by delocalized CB e. Influence of this carrier mediated magnetic coupling is more evident in the magnetic data (Figure 4b−d). Figure 4b shows a clear increase in magnetization value for Sn−Mn codoped NCs compared to Mn-doped In2O3 NCs. Near saturation (H = 70 kOe) magnetization, Ms, of 1.3% Mn− 10% Sn codoped NCs increases to 4.8 μB/Mn2+ ion (2.52 emu/ g) compared to 3.5 μB/Mn2+ ion (1.86 emu/g) for 1.3% Mn doped In2O3 NCs. This ∼36% increase in Ms after Sn codoping is substantial, and was found to be beyond any possible experimental uncertainty when multiple samples from different batches were measured. The difference in Ms values of two samples is 0.66 emu/g, which is more than 1 order of magnitude larger compared with the Ms value of 10% Sn doped In2O3 NCs (0.045 emu/g, Figure S8 of the SI). Therefore, the increase in magnetization for the codoped samples cannot be explained by additional magnetization arising from either Sn4+ or CB e. Furthermore, though no evidence of Mn3+ has been observed, we consider an unlikely hypothesis of a significant Mn3+ concentration in Mn-doped In2O3 NCs, which converts to Mn2+ in the Mn−Sn codoped NCs. Even this unrealistic hypothesis cannot account for the observed increase in Ms by 1.3 μB/Mn2+ ion after codoping. Instead, the Ms values in both samples arise from Mn2+ ions, and codoping with nonmagnetic Sn4+ results into ferromagnetic exchange interaction among Mn2+ ions mediated by delocalized CB e. Similar enhancement in Ms after Sn4+ codoping was also obtained for different Mn concentrations, and showing SPR band (Figures S9 and S10 of the SI). Temperature dependent magnetic susceptibility plots in Figure 4c,d do not show significant divergence between field cooled (FC, 100 Oe) and zero field cooled (ZFC) data, apart from a small divergence in the range of 10−40 K. We fitted the temperature dependent magnetization data using the Curie− Weiss law, i.e., 1/ χ = (T − θ)/C, where χ is the magnetic susceptibility, T is the absolute temperature, θ stands for Curie−Weiss temperature, and C is the Curie constant. Fitting the linear part of the magnetization data at higher temperatures yielded θ = −54 K for 1.3% Mn doped In2O3 NCs. This negative value of θ signifies antiferromagnetic superexchange interactions between neighboring Mn2+ ions. Interestingly, θ = +16 K for 1.3% Mn−10% Sn codoped In2O3 NCs, signifying a ferromagnetic interaction in this sample. Curie−Weiss analysis for samples with same compositions, but prepared in different batches, shows that the signs of θ are reproducible, however, with variations in the magnitude of θ, probably because of larger experimental errors at higher temperatures. In any case, the positive θ value in Curie−Weiss analysis, along with the broadening of EPR signal for Mn−Sn codoped NCs, suggest ferromagnetic interactions. This observed ferromagnetic interaction after Sn4+ codoping agrees with the proposed

In2O3 NCs exhibit SPR band in both solution and solid states, signifying the presence of delocalized CB e in both states, whereas 1.3% Mn doped In2O3 NCs do not exhibit SPR because of the absence of sufficient CB e. Therefore, a comparison of magnetic behavior of these two samples will allow us to elucidate CB e−Mn2+ ferromagnetic exchange interactions. The Q-band electron paramagnetic resonance (EPR) spectra in Figure 4a, showing sextet hyperfine splitting with g = 2.003,

Figure 4. Comparison between 1.3% Mn doped In2O3 NCs and 1.3% Mn−10% Sn codoped In2O3 NCs to establish carrier mediated magnetic coupling in the codoped sample. (a) Q-band EPR spectra, the spectrum for codoped sample was vertically shifted for better presentation. (b) Magnetization vs magnetic field strength curves. (c,d) Magnetic susceptibility vs temperature, along with Curie−Weiss susceptibility plots. Thermogravimetric analysis (TGA) analysis (Figure S7 in the SI) showed 84% of the total weight of our NCs is contributed by inorganic part, and the remaining 16% is contributed by organic capping ligand. Our magnetic data were normalized by the weight of 84% inorganic part only.

confirm that the Mn is in 2+ oxidation state for both 1.3% Mn doped and 1.3% Mn−10% Sn codoped In2O3 NCs. The hyperfine coupling constant has been found to be 7.1 mT for both samples, and reported here for the first time for Mn2+ doped In2O3 NCs. This hyperfine splitting of 7.1 mT (66 × 10−4 cm−1) is either smaller or similar to that in other Mn2+ doped in bulk oxides, suggesting the significant covalent character of Mn−O bonds, and therefore reiterating lattice doping of Mn2+ ion in our NCs.28,29 Mn2+ on the surface of a NC is expected to have more ionic character, and typically exhibits hyperfine splitting ∼9 mT for oxide NCs.29 Our results are in agreement with prior literature on local structure of bulk Mn-doped In2O3, where X-ray absorption fine structure (XAFS) has established that Mn remains in 2+ oxidation state, and in order to maintain the charge neutrality after replacing In3+ with Mn2+ ion, oxygen vacancies are created in the vicinity of Mn2+ ions.20 When Mn2+ and Sn4+ are codoped, the charge neutrality can be further maintained by this additional compensation Mn2+ + Sn4+ = 2In3+.30 This equation suggests that an increase in Mn2+ doping will reduce effective CB e density in Sn−Mn codoped In2O3, and our SPR (Figure 3) and electrical resistivity (Figure S4 in the SI) data indeed agrees with it. We note here that the absence of Mn3+ even at high frequency Q-band EPR suggests insignificant amounts of Mn3+ ions in our NCs, similar to ref 20. On the other hand, ref 3622

DOI: 10.1021/acs.chemmater.6b01465 Chem. Mater. 2016, 28, 3620−3624

Communication

Chemistry of Materials

Ms = 3.5 μB/Mn2+ ion at 2 K, which is significantly less compared to the ideal value of 5 μB/Mn2+. Interestingly, after codoping with Sn4+ ions, 1.3% Mn−10% Sn codoped In2O3 NCs exhibit ferromagnetic interaction (θ = +16 K) with nearly ideal Ms of 4.8 μB/Mn2+ at 2 K. These results confirm CB e− Mn2+ ferromagnetic exchange interaction, which can lead to magneto-electric and magneto-plasmonic properties in the future.

carrier mediated ferromagnetic exchange interaction between Mn2+ ions, resulting into an increase in Ms for the codoped NCs. Gamelin et al. provided an insight into the mechanism of this CB e−Mn2+ ferromagnetic exchange interactions, as schematically shown in Figure 5. Their theoretical calculations showed

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01465. Experimental methodology, EDAX analysis, TEM images, UV−visible absorption spectra, XRD, TGA, and electrical conductivity data (PDF).

Figure 5. Schematic for electron mediated magnetic coupling in Mn− Sn codoped In2O3 NCs.

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that the strength of Mn−O−Mn antiferromagnetic superexchange interaction is larger than that of a given ferromagnetic CB e−Mn2+ s−d exchange interaction. However, the presence of multiple Mn2+ ions leads to multiple ferromagnetic interactions among distant Mn2+ ions via the CB e, and the total strength of ferromagnetic interactions can become large enough to overcome the antiferromagnetic Mn−O−Mn superexchange interaction.5 Therefore, with addition of CB e by Sn4+ codoping, 1.3% Mn−10% Sn codoped NCs exhibit strong enough total ferromagnetic exchange coupling through CB e−Mn2+ interactions that overcomes the antiferromagnetic Mn−O−Mn superexchange interaction, finally yielding a nearideal MS = 4.8 μB/Mn2+ (ideal value 5.0 μB/Mn2+) at 2 K. It has to be noted that the delocalization is within the dimension (∼7 nm) of NCs. So, CB e here are much more spread out compared to defect bound electrons, which typically spread over ∼1 nm diameter as discussed in the model of bound magnetic polarons (BMP);1 however, compared to bulk systems, carriers are confined by the boundary of NCs. Our delocalized electron mediated ferromagnetic coupling is different than the charge transfer ferromagnetism13 exhibited by annealed films of Mn-doped In2O3 NCs with mixed valent Mn2+/Mn3+ ions. More, detailed discussions about various possible ferromagnetic exchange interactions are presented in the SI. Further theoretical studies are required to quantify the strength of different ferromagnetic and antiferromagnetic exchange interactions. We also note that the magnetization values of our Sn−Mn codoped NCs are about 2 orders magnitude higher (Figure 4b vs Figure S8 in the SI) than defect related magnetization reported for oxide NCs without magnetic doping;32 but our NCs do not exhibit appreciable hysteresis in M vs H plot. In conclusion, colloidal Sn−Mn codoped NCs were synthesized. Traditional AC and DC electrical measurements get dominated by grain boundaries of NC films, and therefore unambiguous detection of metal-like delocalized electron within a NC grain becomes difficult. To overcome this problem, we used the SPR band of these NCs to distinguish between delocalized CB e from defect-bound localized electrons. This unambiguous detection of delocalized CB e allowed us to study CB e−Mn2+ magnetic interactions. The Curie−Weiss plot of 1.3% Mn doped In2O3 NCs shows antiferromagnetic (θ = −54 K) superexchange interaction between neighboring Mn2+ ions (Mn−O−Mn) resulting into

AUTHOR INFORMATION

Corresponding Author

*A. Nag. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Sunil Nair for magnetic measurements, Dr. G. V. Pavan Kumar for Raman spectroscopy, Dr. Shouvik Datta for Impedence measurements and SAIF, IIT-Bombay for EPR and ICP-OES measurements. A.N. acknowledges Science and Engineering Research Board (SERB) for Ramanujan Fellowship (SR/S2/RJN-61/2012) and DST-Nano Mission (SR/ NM/NS-1474/2014), Gov’t. of India. B.T. and A.Y. thank IISER Pune and DST INSPIRE program, respectively, for fellowships.

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DOI: 10.1021/acs.chemmater.6b01465 Chem. Mater. 2016, 28, 3620−3624