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High-Curie-Temperature Ferromagnetism in (Sc,Fe)F3 Fluorides and its Dependence on Chemical Valence Lei Hu, Jun Chen,* Longlong Fan, Yang Ren, Qingzhen Huang, Andrea Sanson, Zheng Jiang, Mei Zhou, Yangchun Rong, Yong Wang, Jinxia Deng, and Xianran Xing* Magnetic semiconductors with high Curie temperature (TC), which are capable of simultaneously and externally manipulating electron charge and electron spin degrees of freedom, are essential to the development of spin-based multifunctional devices.[1,2] Additionally, their studies are significant for the effective control of the magnetic properties of magnetic semiconductors.[3,4] Remarkable progress has been made so far in the development of high-Tc semiconductors; this includes the introduction of local defects,[5] the utilization of UV photoexcitation,[6] and the application of surface modifications.[7] When it comes to practical applications however, room-temperature spintronic devices require a TC of at least 500 K.[8] As a result, it is the chemical manipulation of high-TC magnetic semiconductors that would be most beneficial for the field.[9] Effective tailoring of the ferromagnetism of high-TC magnetic semiconductors also remains a significant challenge for the field.
L. Hu, Prof. J. Chen, L. Fan, Y. Rong, Prof. J. Deng, Prof. X. Xing Department of Physical Chemistry University of Science and Technology Beijing Beijing 100083, China E-mail: [email protected]; [email protected] Prof. J. Chen Beijing Key Laboratory of Special Melting and Preparation of High-End Metal Materials University of Science and Technology Beijing Beijing 100083, China Dr. Y. Ren Argonne National Laboratory X-Ray Science Division Argonne, IL 60439, USA Dr. Q. Huang NIST Center for Neutron Research National Institute of Standards and Technology Gaithersburg, MD 20899–6102, USA Dr. A. Sanson Department of Physics and Astronomy University of Padova Padova I-35131, Italy Dr. Z. Jiang, Dr. Y. Wang Shanghai Synchrotron Radiation Facility Shanghai Institute of Applied Physics Chinese Academy of Sciences Shanghai 201800, China M. Zhou Physics Department Tsinghua University Beijing 100084, China
DOI: 10.1002/adma.201500868
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Two decades of extensive research efforts in striving for high-TC magnetic semiconductors have focused on nitrides or oxides;[1,10,11] there have not been any reports on efforts involving fluorides, probably because they are overlooked due to their wide bandgaps and insulator characteristics. Here, we screen out scandium fluoride (ScF3) as a host matrix. In ScF3, the solubility limit of 3d transitional metals, such as Ti resulting in (Sc1-xTix)F3 species, is close to 100%.[12] For larger atoms, such as Y, its solubility limit in (Sc1-xYx)F3 species is as high as x = 0.28.[13] Therefore, the amount of injected spins and carriers can be large, making it a potentially viable candidate for the development of spin electronics. Moreover, ScF3 exhibits pronounced isotropic negative thermal expansion,[14] and the incorporation of ferromagnetism in ScF3 could induce various multifunctional phenomena.[15] In this work, we report a high Curie temperature of ≈545 K for magnetic semiconductor fluorides. The ferromagnetism of (Sc0.9Fe0.1)F3 is confirmed to be intrinsic within the limits of detection. Furthermore, the ferromagnetic order can be tailored through the intentional variation of the Fe2+/Fe3+ ratio via a simple but effective chemical approach. Three different compositions of (Sc1-xFex)F3 (x = 0, 0.05, and 0.1) were prepared in the present study. The crystal structures were refined using neutron powder diffraction and high-energy synchrotron X-ray powder diffraction data. All the microstructures exhibit the same phase constitution as the cubic ReO3type structure, which comprises a 3D framework of cornersharing (Sc,Fe)F6 octahedra (Figure 1a). After full-profile refinements (Table S1, Supporting Information (SI)), the structure is shown to have Pm3m symmetry, where the Fe atoms are found to mix with the Sc atoms at the 1a site (0, 0, 0), and the F atoms are located at the 3d site (0.5, 0, 0). The (100) peaks seem to shift to higher 2θ values with increasing Fe content (Figure S1e, SI). This indicates that the lattice contraction is probably ascribed to the incorporation of smaller Fe ions into the Sc sites; the ionic radius of Fe3+ is 0.645 Å, compared to the 0.745-Å radius for Sc3+.[16] Transmission electron microscopy (TEM) and related techniques have been performed to further investigate (Sc0.9Fe0.1)F3, and detailed information is available in the SI (Figure S2). The magnetic response (M) was measured as a function of the magnetic field strength (H = ±2 T) for as-prepared ScF3, (Sc0.95Fe0.05)F3, and (Sc0.9Fe0.1)F3 (Figure S3, SI). ScF3 is clearly diamagnetic because of the 3d0 electron configuration of the Sc3+ ions; however, once iron ions are incorporated into the ScF3 matrix, hysteretic behavior is observed such as in (Sc0.9Fe0.1)F3 at 300 K (Figure 2a). The absorption-mode electron paramagnetic resonance (EPR) spectrum of the as-prepared (Sc0.9Fe0.1)F3
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Figure 1. a) Crystal structure with thermal ellipsolids (Sc/Fe and F are indicated) determined by the structural refinement of b) the neutron powder diffraction pattern of the as-prepared (Sc0.9Fe0.1)F3. In the panel b, final observed (circle) and calculated (black line) neutron powder diffraction profiles are presented. The bottom grey line exhibits the difference profile, and the short vertical marks show the reflection positions.
with an asymmetric line shape (red circles) can be deconvoluted to two overlapping signals. One is an intense and narrow Lorentzian signal (signal A, green dotted curve) with a g factor of ≈1.99, and the other is a weak and broad Gaussian signal (signal B, blue dashed–dotted curve) with a g factor of ≈2.28 (Figure 2a, inset). Signal A can be identified as the uncoupled iron ions,[17] probably contributing to the paramagnetic component and the linear part of the M–H curve, while signal B presumably originates from the ferromagnetic exchange interaction between the iron ions.[18] The temperature-dependence of the magnetization of as-prepared (Sc0.9Fe0.1)F3 at 5 kOe was also plotted (Figure 2b). With increasing temperature, the magnetic moment decreases slowly and demonstrates a broad magnetic transition temperature span,[19] leading to a high TC = ≈545 K. The temperature-dependent magnetization under zero-field-cooled (ZFC) and field-cooled (FC) conditions (H = 5 kOe) indicates a ferromagnetic contribution over the entire low-temperature range (Figure 2b, inset). It is interesting to note that the as-prepared (Sc0.9Fe0.1)F3 exhibits a high
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Figure 2. a) M–H curves of as-prepared (Sc0.9Fe0.1)F3 measured in an external magnetic field of up to 6 T at 300 K, showing the initial magnetic loop and deduction of a large linear paramagnetic contribution. The inset shows the observed (red circles) and calculated (black line) EPR spectra for (Sc0.9Fe0.1)F3, collected at a temperature of T = 300 K. It can be deconvoluted to a Lorentzian signal (signal A, green dotted curve) and another Gaussian signal (signal B, blue dashed–dotted curve). b) M–T curve of as-prepared (Sc0.9Fe0.1)F3 with H = 5 kOe. The inset shows M–T curves under zero-field-cooled and field-cooled conditions with H = 5 kOe. In the main panels, magnetization is given per Fe dopants, in units defined by the Bohr magneton µB.
TC. To our knowledge, such a high TC in metal–halogen compounds has never been reported. Furthermore, the high TC is comparable with that of nitrides and oxides, such as the archetypical Mn-doped GaN (TC = ≈300 K),[20] and Mn-doped ZnO (TC = ≈420 K).[21] A striking bandgap (Eg) narrowing in the (Sc,Fe)F3 system was also observed. UV–vis absorption measurements show that the fundamental absorption of ScF3 deviates far away from the visible-light region (≈400–700 nm). Theoretical calculations show that ScF3 is an insulator with a Eg of ≈5.76 eV (Figure S4a, SI), consistent with a previous study.[22] In sharp contrast, both as-prepared (Sc0.95Fe0.05)F3 and (Sc0.9Fe0.1)F3 exhibit a noticeable visible-light absorption (Figure 3a). The Eg of the as-prepared (Sc0.9Fe0.1)F3 is estimated to be ≈1.87 eV (Figure S4b, SI), much smaller than that of typical semiconductors, such as TiO2 (≈3.2 eV) and ZnO (≈3.4 eV).[23,24] The substitution of Fe for Sc clearly modifies the electronic structure
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Figure 3. a) UV–vis diffuse absorption spectra of (Sc1-xFex)F3 (x = 0, 0.05, and 0.1). The inset shows the color change of the samples. b) The density of states (DOS) of (Sc0.875Fe0.125)F3. Total DOS (black line), projected DOS of Sc 3d (red line), Fe 3d (blue line), and F 2p (green line). Both Fe 3d and F 2p states are scaled by a factor of 10 for clarity. Solid lines indicate spin up (↑) and dotted lines represent spin down (↓).
of ScF3 (Figure 3a). It is not only the Sc 3d states that strongly hybridize with the F 2p states introduced by Fe doping, but there are also some impurity states originating from the Fe that are introduced into the bandgap of ScF3 around the Fermi level,
which noticeably narrows the Eg (Figure 3b). Consequently, these new emerging impurity energy levels, mainly composed of Fe 3d and F 2p states around the Fermi level, are responsible for the great reduction of the bandgap. X-ray absorption fine structure (XAFS) spectra were also carefully collected, and the Fe K-edge X-ray absorption near-edge structure (XANES) spectra of the (Sc,Fe)F3 system are distinctly different from those of reference compounds (Figure 4a), corroborating the absence of these references as secondary phases precipitated in ScF3. Specifically, the weak 1s→3d pre-edge feature (marked by the black box in Figure 4a) reveals that the as-prepared (Sc0.9Fe0.1)F3 has a nearly centrosymmetrically octahedral iron site.[25] Intriguingly, the robust ferromagnetic order of the as-prepared (Sc0.9Fe0.1)F3 could be manipulated via a simple but effective chemical route. Annealing (Sc0.9Fe0.1)F3 in N2 atmosphere intensifies the ferromagnetic order. Subsequently, heating the N2-annealed (Sc0.9Fe0.1)F3 sample embedded in the NH4HF2 powder weakens the magnetism remarkably (Figure S5a, SI). For convenience, we label the as-prepared (Sc0.9Fe0.1)F3, the N2annealed (Sc0.9Fe0.1)F3, and the NH4HF2-treated (Sc0.9Fe0.1)F3 as SFF-1, SFF-2, and SFF-3, respectively (see the experimental section of the SI). It is interesting that the lattice volumes share the same trend with the magnetic ordering in the three samples (Figure S5b, SI). Quantitatively, the lattice volume of SFF-2 is 62.328 Å3, which is 0.21% and 0.36% larger than that of SFF-1 (62.196 Å3) and SFF-3 (62.106 Å3), respectively. It seems that the crystal lattice tries to expand as the magnetic order intensifies.[26] Assuming that the ferromagnetic order originated from impurity phases, the (100) peaks of these three samples should remain the same, as well as the lattice volumes; however, according to the above experimental observation, the lattice volumes exhibit a character of magnetic dependence. Consequently, the weak magneto-volume effect in the (Sc,Fe)F3 system further confirms the intrinsic magnetic ordering. The magnetic behavior of diluted magnetic semiconductors (DMS) as reported by theoretical and experimental studies, is intimately related to the valence state of dopant ions.[27] The energy of the edge jump of SFF-1, SFF-2, and SFF-3 is located between those of FeF2 (Fe2+) and FeF3 (Fe3+), more adjacent to the latter (Figure 4b). This indicates a mixed valence state (+2/+3) of the Fe ions in (Sc0.9Fe0.1)F3. In order
Figure 4. Fe K-edge XANES spectra of a) the as-prepared (Sc0.9Fe0.1)F3 and reference compounds, and of b) SFF-1, SFF-2, SFF-3, FeF2, and FeF3. c) Fourier transform of the Fe K-edge EXAFS k3χ(k) functions for the calculated (dotted) and the three experimental samples of (Sc0.9Fe0.1)F3 (three solid lines). For clarity, the intense calculated data is multiplied by 0.5.
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theoretical aspects can be described with the bound magnetic polaron (BMP) model, where donors are included to generate bound magnetic polarons, interacting with 3d ions within their orbits and resulting in ferromagnetic exchange coupling; the average number of donor electrons interacting with a particular magnetic ion is (4/3)πrH3 n , where rH is the hydrogenic orbit where the donor electrons are located.[8] More defects, as defined by n , means more donor electrons, and more shallow electrons, interacting with the fixed number of magnetic cations, enhance the ferromagnetic coupling. The final picture is that the magnetism of (Sc,Fe)F3 could be mediated by intentionally changing the Fe2+/Fe3+ ratio. Here, a phenomenological interpretation has been simply provided. Further investigation on carrier–dopant exchange mechanism of the (Sc,Fe)F3 system is in progress. In summary, a high-TC (≈545 K) ferromagnetism in ScF3-based fluorides, with a narrow bandgap of ≈1.87 eV, has been demonstrated. The ferromagnetic order intensifies with the increase in the Fe2+/Fe3+ ratio from SFF-1 to SFF-2, and it becomes weak with the decrease in the Fe2+/Fe3+ ratio from SFF-1 to SFF-3. The existence of Fe2+ is intimately related to fluorine vacancies, confirmed by EXAFS analysis and EPR spectra. An unambiguous correlation between the chemical valence of the iron ions and the magnetism in (Sc0.9Fe0.1)F3 has been observed experimentally. The dependence of the magnetism on the chemical valence is consistent with theoretical models in which ferromagnetic exchange interactions originate from shallow donor electrons. The present diluted magnetic fluoride of (Sc,Fe)F3 could broaden the scope of high-TC DMSs and further pave the way for the design of unconventional spin electronics. ⵧ
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to reveal the variance of the (Fe2+/Fe3+) ratio, difference plots are presented by subtracting the SFF-1 and SFF-2 data from the SFF-3 data (Figure S6, SI). Obviously, the low-energy component at ≈7128 eV originating from Fe2+ immediately intensifies, while the high-energy peak at ≈7134 eV from Fe3+ decreases. This evolution of different energy components implies the conversion of Fe3+ to Fe2+ from SFF-3 to SFF-1 and finally to SFF-2. In order to investigate the local structure of the Fe atom in (Sc0.9Fe0.1)F3, the Fourier transforms (FTs) of the extended XAFS (EXAFS) k3χ(k) functions at the Fe K-edge for the calculated (Sc,Fe)F3 (black dotted line), and the experimental SFF-1 (blue line), SFF-2 (red line), and SFF-3 (green line) are presented (Figure 4c). The calculation details are in the SI. Evidently, the main peak positions of the calculated FT signal have a good agreement with that of the experimental FT ones. Additionally, the resemblance between the experimental FT of the Fe K-edge EXAFS signal of the as-prepared (Sc0.9Fe0.1)F3 and the FT of the Sc K-edge EXAFS signal of the pure ScF3 indicates the similar local structures of Fe in (Sc0.9Fe0.1)F3 and Sc in ScF3 (Figure S7a, SI). They both unambiguously validate the substitution of Fe for Sc in (Sc0.9Fe0.1)F3. Since the FEFF (a code for ab initio calculation of XAFS) calculation can be done only at T = 0 K (atoms frozen in their equilibrium position), the calculated FT (black dotted line in Figure 4c) is much more intense than the experimental ones (three lines) measured at room temperature. This approximation also affects the interference between the EXAFS signals of the different scattering paths and, therefore, the final FT, like the structure between about 2 and 4 Å, which is due to at least seven multiple scattering paths. The first peak shown in Figure 4c was also fitted with the effective radii (Reff ) in the range of ≈1.07–1.95 Å, assuming the substitution of Fe for Sc site in (Sc0.9Fe0.1)F3. This structural model gives a satisfactory fitting quality (Figure S7b, SI). The fitting parameters are available in the SI (Table S2). The coordination numbers of fluorine nearest neighbors (FNN) are obviously lower than 6; they are 5.0(7), 4.5(3), and 5.4(8) for the SFF-1, SFF-2, and SFF-3, respectively. This indicates fluorine vacancies in the first coordination shell of the central Fe ions. The local anion vacancies induce extra electron carriers. Consistently, the EPR spectra of SFF-1, SFF-2, and SFF-3 also suggest that fluorine vacancies obviously increase in SFF-2 but decrease in SFF-3, as reflected from the absorption-mode EPR signal at 324.4 mT (inset of Figure S8, SI). To maintain the charge neutrality, the Fe dopants (Fe3+) are likely to capture donor electrons and to be transformed to Fe2+. This leads to the mixed valence state (Fe2+/Fe3+) in (Sc,Fe)F3. Consequently, the increase in Fe2+ is ascribed to the decrease of the FNN. The above experimental observations have suggested the intimate relationship between the Fe2+/Fe3+ ratio and ferromagnetism. Theoretical models have already described that ferromagnetism in diluted magnetic semiconductors has a strong electronic coupling between magnetic ions and charge carriers at the Fermi level;[8,28] however, the substitution of isovalent Fe3+ for Sc3+ in (Sc,Fe)F3 does not itself introduce carriers. Thus carriers in (Sc,Fe)F3 are associated with the presence of Fe2+ ions, which resulted from fluorine vacancies. The
Experimental Section Detailed sample preparation methods for (Sc1-xFex)F3 (x = 0, 0.05, and 0.1), SFF-1, SFF-2, and SFF-3; their structural, magnetic, and optical characterization; and theoretical calculations are available in the SI.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. It includes sample preparation, crystal structure, characterization, and other detailed information.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21322102, 91422301, 21231001), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1207), the Fundamental Research Funds for the Central Universities, China (FRF-TP-14–012C1), and the Program of Introducing Talents of Discipline to Universities (B14003). The use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC02–06CH11357). We thank the staff at beamlines BL14W and BL08U of the Shanghai Synchrotron Radiation Facility (SSRF) for providing beam time to collect the Fe L- and K-edge EXAFS spectra and for assisting with the XAFS measurements. We are also grateful
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to the ELETTRA synchrotron radiation facility and the staff of the XAFS beamline for the Sc K-edge EXAFS spectrum of pure ScF3, which was collected during the project N. 20140214. Received: February 18, 2015 Revised: May 7, 2015 Published online: July 6, 2015
[1] T. Dietl, Nat. Mater. 2010, 9, 965. [2] D. A. Bussian, S. A. Crooker, M. Yin, M. Brynda, A. L. Efros, V. I. Klimov, Nat. Mater. 2009, 8, 35. [3] S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnár, M. L. Roukes, A. Y. Chtchelkanova, D. M. Treger, Science 2001, 294, 1488. [4] Y. Yamada, K. Ueno, T. Fukumura, H. T. Yuan, H. Shimotani, Y. Iwasa, L. Gu, S. Tsukimoto, Y. Ikuhara, M. Kawasaki, Science 2011, 332, 1065. [5] J. B. Yi, C. C. Lim, G. Z. Xing, H. M. Fan, L. H. Van, S. L. Huang, K. S. Yang, X. L. Huang, X. B. Qin, B. Y. Wang, T. Wu, L. Wang, H. T. Zhang, X. Y. Gao, T. Liu, A. T. S. Wee, Y. P. Feng, J. Ding, Phys. Rev. Lett. 2010, 104, 137201. [6] S. T. Ochsenbein, Y. Feng, K. M. Whitaker, E. Badaeva, W. K. Liu, X. Li, D. R. Gamelin, Nat. Nanotechnol. 2009, 4, 681. [7] W. Yan, Q. Liu, C. Wang, X. Yang, T. Yao, J. He, Z. Sun, Z. Pan, F. Hu, Z. Wu, Z. Xie, S. Wei, J. Am. Chem. Soc. 2014, 136, 1150. [8] J. M. D. Coey, M. Venkatesan, C. B. Fitzgerald, Nat. Mater. 2005, 4, 173. [9] K. R. Kittilstved, N. S. Norberg, D. R. Gamelin, Phys. Rev. Lett. 2005, 94, 147209. [10] N. Khare, M. J. Kappers, M. Wei, M. G. Blamire, J. L. MacManusDriscoll, Adv. Mater. 2006, 18, 1449. [11] J. Philip, A. Punnoose, B. I. Kim, K. M. Reddy, S. Layne, J. O. Holmes, B. Satpati, P. R. LeClair, T. S. Santos, J. S. Moodera, Nat. Mater. 2006, 5, 298.
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High-Curie-Temperature Ferromagnetism in (Sc,Fe)F3 Fluorides and its Dependence on Chemical Valence Lei Hu, Jun Chen,* Longlong Fan, Yang Ren, Qingzhen Huang, Andrea Sanson, Zheng Jiang, Mei Zhou, Yangchun Rong, Yong Wang, Jinxia Deng, and Xianran Xing* Magnetic semiconductors with high Curie temperature (TC), which are capable of simultaneously and externally manipulating electron charge and electron spin degrees of freedom, are essential to the development of spin-based multifunctional devices.[1,2] Additionally, their studies are significant for the effective control of the magnetic properties of magnetic semiconductors.[3,4] Remarkable progress has been made so far in the development of high-Tc semiconductors; this includes the introduction of local defects,[5] the utilization of UV photoexcitation,[6] and the application of surface modifications.[7] When it comes to practical applications however, room-temperature spintronic devices require a TC of at least 500 K.[8] As a result, it is the chemical manipulation of high-TC magnetic semiconductors that would be most beneficial for the field.[9] Effective tailoring of the ferromagnetism of high-TC magnetic semiconductors also remains a significant challenge for the field.
L. Hu, Prof. J. Chen, L. Fan, Y. Rong, Prof. J. Deng, Prof. X. Xing Department of Physical Chemistry University of Science and Technology Beijing Beijing 100083, China E-mail: [email protected]; [email protected] Prof. J. Chen Beijing Key Laboratory of Special Melting and Preparation of High-End Metal Materials University of Science and Technology Beijing Beijing 100083, China Dr. Y. Ren Argonne National Laboratory X-Ray Science Division Argonne, IL 60439, USA Dr. Q. Huang NIST Center for Neutron Research National Institute of Standards and Technology Gaithersburg, MD 20899–6102, USA Dr. A. Sanson Department of Physics and Astronomy University of Padova Padova I-35131, Italy Dr. Z. Jiang, Dr. Y. Wang Shanghai Synchrotron Radiation Facility Shanghai Institute of Applied Physics Chinese Academy of Sciences Shanghai 201800, China M. Zhou Physics Department Tsinghua University Beijing 100084, China
DOI: 10.1002/adma.201500868
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Two decades of extensive research efforts in striving for high-TC magnetic semiconductors have focused on nitrides or oxides;[1,10,11] there have not been any reports on efforts involving fluorides, probably because they are overlooked due to their wide bandgaps and insulator characteristics. Here, we screen out scandium fluoride (ScF3) as a host matrix. In ScF3, the solubility limit of 3d transitional metals, such as Ti resulting in (Sc1-xTix)F3 species, is close to 100%.[12] For larger atoms, such as Y, its solubility limit in (Sc1-xYx)F3 species is as high as x = 0.28.[13] Therefore, the amount of injected spins and carriers can be large, making it a potentially viable candidate for the development of spin electronics. Moreover, ScF3 exhibits pronounced isotropic negative thermal expansion,[14] and the incorporation of ferromagnetism in ScF3 could induce various multifunctional phenomena.[15] In this work, we report a high Curie temperature of ≈545 K for magnetic semiconductor fluorides. The ferromagnetism of (Sc0.9Fe0.1)F3 is confirmed to be intrinsic within the limits of detection. Furthermore, the ferromagnetic order can be tailored through the intentional variation of the Fe2+/Fe3+ ratio via a simple but effective chemical approach. Three different compositions of (Sc1-xFex)F3 (x = 0, 0.05, and 0.1) were prepared in the present study. The crystal structures were refined using neutron powder diffraction and high-energy synchrotron X-ray powder diffraction data. All the microstructures exhibit the same phase constitution as the cubic ReO3type structure, which comprises a 3D framework of cornersharing (Sc,Fe)F6 octahedra (Figure 1a). After full-profile refinements (Table S1, Supporting Information (SI)), the structure is shown to have Pm3m symmetry, where the Fe atoms are found to mix with the Sc atoms at the 1a site (0, 0, 0), and the F atoms are located at the 3d site (0.5, 0, 0). The (100) peaks seem to shift to higher 2θ values with increasing Fe content (Figure S1e, SI). This indicates that the lattice contraction is probably ascribed to the incorporation of smaller Fe ions into the Sc sites; the ionic radius of Fe3+ is 0.645 Å, compared to the 0.745-Å radius for Sc3+.[16] Transmission electron microscopy (TEM) and related techniques have been performed to further investigate (Sc0.9Fe0.1)F3, and detailed information is available in the SI (Figure S2). The magnetic response (M) was measured as a function of the magnetic field strength (H = ±2 T) for as-prepared ScF3, (Sc0.95Fe0.05)F3, and (Sc0.9Fe0.1)F3 (Figure S3, SI). ScF3 is clearly diamagnetic because of the 3d0 electron configuration of the Sc3+ ions; however, once iron ions are incorporated into the ScF3 matrix, hysteretic behavior is observed such as in (Sc0.9Fe0.1)F3 at 300 K (Figure 2a). The absorption-mode electron paramagnetic resonance (EPR) spectrum of the as-prepared (Sc0.9Fe0.1)F3
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Figure 1. a) Crystal structure with thermal ellipsolids (Sc/Fe and F are indicated) determined by the structural refinement of b) the neutron powder diffraction pattern of the as-prepared (Sc0.9Fe0.1)F3. In the panel b, final observed (circle) and calculated (black line) neutron powder diffraction profiles are presented. The bottom grey line exhibits the difference profile, and the short vertical marks show the reflection positions.
with an asymmetric line shape (red circles) can be deconvoluted to two overlapping signals. One is an intense and narrow Lorentzian signal (signal A, green dotted curve) with a g factor of ≈1.99, and the other is a weak and broad Gaussian signal (signal B, blue dashed–dotted curve) with a g factor of ≈2.28 (Figure 2a, inset). Signal A can be identified as the uncoupled iron ions,[17] probably contributing to the paramagnetic component and the linear part of the M–H curve, while signal B presumably originates from the ferromagnetic exchange interaction between the iron ions.[18] The temperature-dependence of the magnetization of as-prepared (Sc0.9Fe0.1)F3 at 5 kOe was also plotted (Figure 2b). With increasing temperature, the magnetic moment decreases slowly and demonstrates a broad magnetic transition temperature span,[19] leading to a high TC = ≈545 K. The temperature-dependent magnetization under zero-field-cooled (ZFC) and field-cooled (FC) conditions (H = 5 kOe) indicates a ferromagnetic contribution over the entire low-temperature range (Figure 2b, inset). It is interesting to note that the as-prepared (Sc0.9Fe0.1)F3 exhibits a high
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Figure 2. a) M–H curves of as-prepared (Sc0.9Fe0.1)F3 measured in an external magnetic field of up to 6 T at 300 K, showing the initial magnetic loop and deduction of a large linear paramagnetic contribution. The inset shows the observed (red circles) and calculated (black line) EPR spectra for (Sc0.9Fe0.1)F3, collected at a temperature of T = 300 K. It can be deconvoluted to a Lorentzian signal (signal A, green dotted curve) and another Gaussian signal (signal B, blue dashed–dotted curve). b) M–T curve of as-prepared (Sc0.9Fe0.1)F3 with H = 5 kOe. The inset shows M–T curves under zero-field-cooled and field-cooled conditions with H = 5 kOe. In the main panels, magnetization is given per Fe dopants, in units defined by the Bohr magneton µB.
TC. To our knowledge, such a high TC in metal–halogen compounds has never been reported. Furthermore, the high TC is comparable with that of nitrides and oxides, such as the archetypical Mn-doped GaN (TC = ≈300 K),[20] and Mn-doped ZnO (TC = ≈420 K).[21] A striking bandgap (Eg) narrowing in the (Sc,Fe)F3 system was also observed. UV–vis absorption measurements show that the fundamental absorption of ScF3 deviates far away from the visible-light region (≈400–700 nm). Theoretical calculations show that ScF3 is an insulator with a Eg of ≈5.76 eV (Figure S4a, SI), consistent with a previous study.[22] In sharp contrast, both as-prepared (Sc0.95Fe0.05)F3 and (Sc0.9Fe0.1)F3 exhibit a noticeable visible-light absorption (Figure 3a). The Eg of the as-prepared (Sc0.9Fe0.1)F3 is estimated to be ≈1.87 eV (Figure S4b, SI), much smaller than that of typical semiconductors, such as TiO2 (≈3.2 eV) and ZnO (≈3.4 eV).[23,24] The substitution of Fe for Sc clearly modifies the electronic structure
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Figure 3. a) UV–vis diffuse absorption spectra of (Sc1-xFex)F3 (x = 0, 0.05, and 0.1). The inset shows the color change of the samples. b) The density of states (DOS) of (Sc0.875Fe0.125)F3. Total DOS (black line), projected DOS of Sc 3d (red line), Fe 3d (blue line), and F 2p (green line). Both Fe 3d and F 2p states are scaled by a factor of 10 for clarity. Solid lines indicate spin up (↑) and dotted lines represent spin down (↓).
of ScF3 (Figure 3a). It is not only the Sc 3d states that strongly hybridize with the F 2p states introduced by Fe doping, but there are also some impurity states originating from the Fe that are introduced into the bandgap of ScF3 around the Fermi level,
which noticeably narrows the Eg (Figure 3b). Consequently, these new emerging impurity energy levels, mainly composed of Fe 3d and F 2p states around the Fermi level, are responsible for the great reduction of the bandgap. X-ray absorption fine structure (XAFS) spectra were also carefully collected, and the Fe K-edge X-ray absorption near-edge structure (XANES) spectra of the (Sc,Fe)F3 system are distinctly different from those of reference compounds (Figure 4a), corroborating the absence of these references as secondary phases precipitated in ScF3. Specifically, the weak 1s→3d pre-edge feature (marked by the black box in Figure 4a) reveals that the as-prepared (Sc0.9Fe0.1)F3 has a nearly centrosymmetrically octahedral iron site.[25] Intriguingly, the robust ferromagnetic order of the as-prepared (Sc0.9Fe0.1)F3 could be manipulated via a simple but effective chemical route. Annealing (Sc0.9Fe0.1)F3 in N2 atmosphere intensifies the ferromagnetic order. Subsequently, heating the N2-annealed (Sc0.9Fe0.1)F3 sample embedded in the NH4HF2 powder weakens the magnetism remarkably (Figure S5a, SI). For convenience, we label the as-prepared (Sc0.9Fe0.1)F3, the N2annealed (Sc0.9Fe0.1)F3, and the NH4HF2-treated (Sc0.9Fe0.1)F3 as SFF-1, SFF-2, and SFF-3, respectively (see the experimental section of the SI). It is interesting that the lattice volumes share the same trend with the magnetic ordering in the three samples (Figure S5b, SI). Quantitatively, the lattice volume of SFF-2 is 62.328 Å3, which is 0.21% and 0.36% larger than that of SFF-1 (62.196 Å3) and SFF-3 (62.106 Å3), respectively. It seems that the crystal lattice tries to expand as the magnetic order intensifies.[26] Assuming that the ferromagnetic order originated from impurity phases, the (100) peaks of these three samples should remain the same, as well as the lattice volumes; however, according to the above experimental observation, the lattice volumes exhibit a character of magnetic dependence. Consequently, the weak magneto-volume effect in the (Sc,Fe)F3 system further confirms the intrinsic magnetic ordering. The magnetic behavior of diluted magnetic semiconductors (DMS) as reported by theoretical and experimental studies, is intimately related to the valence state of dopant ions.[27] The energy of the edge jump of SFF-1, SFF-2, and SFF-3 is located between those of FeF2 (Fe2+) and FeF3 (Fe3+), more adjacent to the latter (Figure 4b). This indicates a mixed valence state (+2/+3) of the Fe ions in (Sc0.9Fe0.1)F3. In order
Figure 4. Fe K-edge XANES spectra of a) the as-prepared (Sc0.9Fe0.1)F3 and reference compounds, and of b) SFF-1, SFF-2, SFF-3, FeF2, and FeF3. c) Fourier transform of the Fe K-edge EXAFS k3χ(k) functions for the calculated (dotted) and the three experimental samples of (Sc0.9Fe0.1)F3 (three solid lines). For clarity, the intense calculated data is multiplied by 0.5.
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theoretical aspects can be described with the bound magnetic polaron (BMP) model, where donors are included to generate bound magnetic polarons, interacting with 3d ions within their orbits and resulting in ferromagnetic exchange coupling; the average number of donor electrons interacting with a particular magnetic ion is (4/3)πrH3 n , where rH is the hydrogenic orbit where the donor electrons are located.[8] More defects, as defined by n , means more donor electrons, and more shallow electrons, interacting with the fixed number of magnetic cations, enhance the ferromagnetic coupling. The final picture is that the magnetism of (Sc,Fe)F3 could be mediated by intentionally changing the Fe2+/Fe3+ ratio. Here, a phenomenological interpretation has been simply provided. Further investigation on carrier–dopant exchange mechanism of the (Sc,Fe)F3 system is in progress. In summary, a high-TC (≈545 K) ferromagnetism in ScF3-based fluorides, with a narrow bandgap of ≈1.87 eV, has been demonstrated. The ferromagnetic order intensifies with the increase in the Fe2+/Fe3+ ratio from SFF-1 to SFF-2, and it becomes weak with the decrease in the Fe2+/Fe3+ ratio from SFF-1 to SFF-3. The existence of Fe2+ is intimately related to fluorine vacancies, confirmed by EXAFS analysis and EPR spectra. An unambiguous correlation between the chemical valence of the iron ions and the magnetism in (Sc0.9Fe0.1)F3 has been observed experimentally. The dependence of the magnetism on the chemical valence is consistent with theoretical models in which ferromagnetic exchange interactions originate from shallow donor electrons. The present diluted magnetic fluoride of (Sc,Fe)F3 could broaden the scope of high-TC DMSs and further pave the way for the design of unconventional spin electronics. ⵧ
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to reveal the variance of the (Fe2+/Fe3+) ratio, difference plots are presented by subtracting the SFF-1 and SFF-2 data from the SFF-3 data (Figure S6, SI). Obviously, the low-energy component at ≈7128 eV originating from Fe2+ immediately intensifies, while the high-energy peak at ≈7134 eV from Fe3+ decreases. This evolution of different energy components implies the conversion of Fe3+ to Fe2+ from SFF-3 to SFF-1 and finally to SFF-2. In order to investigate the local structure of the Fe atom in (Sc0.9Fe0.1)F3, the Fourier transforms (FTs) of the extended XAFS (EXAFS) k3χ(k) functions at the Fe K-edge for the calculated (Sc,Fe)F3 (black dotted line), and the experimental SFF-1 (blue line), SFF-2 (red line), and SFF-3 (green line) are presented (Figure 4c). The calculation details are in the SI. Evidently, the main peak positions of the calculated FT signal have a good agreement with that of the experimental FT ones. Additionally, the resemblance between the experimental FT of the Fe K-edge EXAFS signal of the as-prepared (Sc0.9Fe0.1)F3 and the FT of the Sc K-edge EXAFS signal of the pure ScF3 indicates the similar local structures of Fe in (Sc0.9Fe0.1)F3 and Sc in ScF3 (Figure S7a, SI). They both unambiguously validate the substitution of Fe for Sc in (Sc0.9Fe0.1)F3. Since the FEFF (a code for ab initio calculation of XAFS) calculation can be done only at T = 0 K (atoms frozen in their equilibrium position), the calculated FT (black dotted line in Figure 4c) is much more intense than the experimental ones (three lines) measured at room temperature. This approximation also affects the interference between the EXAFS signals of the different scattering paths and, therefore, the final FT, like the structure between about 2 and 4 Å, which is due to at least seven multiple scattering paths. The first peak shown in Figure 4c was also fitted with the effective radii (Reff ) in the range of ≈1.07–1.95 Å, assuming the substitution of Fe for Sc site in (Sc0.9Fe0.1)F3. This structural model gives a satisfactory fitting quality (Figure S7b, SI). The fitting parameters are available in the SI (Table S2). The coordination numbers of fluorine nearest neighbors (FNN) are obviously lower than 6; they are 5.0(7), 4.5(3), and 5.4(8) for the SFF-1, SFF-2, and SFF-3, respectively. This indicates fluorine vacancies in the first coordination shell of the central Fe ions. The local anion vacancies induce extra electron carriers. Consistently, the EPR spectra of SFF-1, SFF-2, and SFF-3 also suggest that fluorine vacancies obviously increase in SFF-2 but decrease in SFF-3, as reflected from the absorption-mode EPR signal at 324.4 mT (inset of Figure S8, SI). To maintain the charge neutrality, the Fe dopants (Fe3+) are likely to capture donor electrons and to be transformed to Fe2+. This leads to the mixed valence state (Fe2+/Fe3+) in (Sc,Fe)F3. Consequently, the increase in Fe2+ is ascribed to the decrease of the FNN. The above experimental observations have suggested the intimate relationship between the Fe2+/Fe3+ ratio and ferromagnetism. Theoretical models have already described that ferromagnetism in diluted magnetic semiconductors has a strong electronic coupling between magnetic ions and charge carriers at the Fermi level;[8,28] however, the substitution of isovalent Fe3+ for Sc3+ in (Sc,Fe)F3 does not itself introduce carriers. Thus carriers in (Sc,Fe)F3 are associated with the presence of Fe2+ ions, which resulted from fluorine vacancies. The
Experimental Section Detailed sample preparation methods for (Sc1-xFex)F3 (x = 0, 0.05, and 0.1), SFF-1, SFF-2, and SFF-3; their structural, magnetic, and optical characterization; and theoretical calculations are available in the SI.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author. It includes sample preparation, crystal structure, characterization, and other detailed information.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21322102, 91422301, 21231001), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1207), the Fundamental Research Funds for the Central Universities, China (FRF-TP-14–012C1), and the Program of Introducing Talents of Discipline to Universities (B14003). The use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-AC02–06CH11357). We thank the staff at beamlines BL14W and BL08U of the Shanghai Synchrotron Radiation Facility (SSRF) for providing beam time to collect the Fe L- and K-edge EXAFS spectra and for assisting with the XAFS measurements. We are also grateful
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to the ELETTRA synchrotron radiation facility and the staff of the XAFS beamline for the Sc K-edge EXAFS spectrum of pure ScF3, which was collected during the project N. 20140214. Received: February 18, 2015 Revised: May 7, 2015 Published online: July 6, 2015
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