Article pubs.acs.org/JPCC
Cobalt−Carbon Complexes Induced Ferromagnetism in Chemically Modified Perovskite Dilute Magnetic Complex Oxides Xuefeng Wang,*,† Bo Wan,‡ Kang Zhang,† Bo Zhao,‡ Zhaoguo Li,‡ Xiangang Wan,‡ Fengqi Song,‡ Bin Liu,† Xiangqian Xiu,† Yongbing Xu,† Yi Shi,† and Rong Zhang*,† †
National Laboratory of Solid State Microstructures and School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China ‡ National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *
ABSTRACT: The clarification of ferromagnetism in dilute magnetic oxides (DMOs) is of vital importance to realize future spintronic devices. However, the mixed control of carriers, defects, and spin in most cases is inconvenient to disclose the origin of ferromagnetism in DMOs. The perovskite complex compound of (La,Sr)TiO3 is apt to the independent control of carriers, defects, or spin through the codoping method. Here we demonstrate the combined experimental and theoretical studies on the cobalt−carbon complexes that can lead to the room-temperature ferromagnetism in perovskite chemically modified La0.4Sr0.6Ti1−xCoxO3 (LSTCO) samples. Such complexes are investigated by studying the dependence of magnetization on the concentrations of Co dopants and the in situ produced carbon that substitutes for some of the lattice oxygen atoms. The origin of ferromagnetism is attributed to the long-rangemediated magnetization among Co2+−C complexes through percolation-bound magnetic polarons, as strongly supported by the controlled experiments of thermal treatment and the structural characterization. Our density functional theoretical calculations further corroborate the important role of Co2+−C complexes on the induced robust ferromagnetism in LSTCO, in agreement with our experimental observations. The manipulation scheme of Co2+−C complexes not only provides a new understanding of the origin of ferromagnetism in DMOs but also evidences the feasibility and significance in design of other magnetic materials and spintronic devices. Moreover, the demonstrated solvothermal synthesis and postannealing method offer a facile and highyield approach to produce high-quality transition-metal (cation) and carbon (anion) codoped DMOs to explore spintronic properties.
■
structural defects,12−15 mixed valence state,16 and the presence of secondary phases.17−19 Although the field has suffered from the intense controversy on the origin of FM, recent considerable efforts have also been devoted to the control of FM in DMOs, for instance, by systematically studying dopant−defect complexes,20,21 tuning dopant valence states,22 as well as exerting an electric field.23,24 Only through such ways will significant advances in both understanding of the origin of FM and fabrication of the practical spin-based devices be realized. However, the mixed control of carriers, defects, and spin in most cases is inconvenient to disclose the origin of FM in DMOs. In this regard, the LSTO-based complex compound, in our opinion, is apt to the independent control of carriers, defects, or spin through the codoping method. Different from monodoping, the codoping technique possesses synergistic effects, which have
INTRODUCTION Dilute magnetic oxides (DMOs), a class of magnetic semiconductors, are promising supporting materials for future spintronic devices, utilizing both spin and charge degrees of freedom.1−4 Since the first observation of high-temperature ferromagnetism (FM) in Co-doped TiO2 thin films by Matsumoto et al.,5 intensive research efforts have been devoted to exploring various properties and the origin of FM in DMOs with/without dilute doping of transition metals (TMs) and non-TMs, which have been discussed in detail in recent reviews by Ogale and Dietl.2−4 Complex compounds based on perovskite (La,Sr)TiO3 (LSTO) are especially appealing in DMOs because, in contrast to TiO2 and ZnO, they belong to a family of strongly correlated conductors. In such compounds, the La3+ substitution for Sr2+ introduces itinerant electrons into the Ti 3d conduction band,6 allowing independent controls of both the magnetic and carrier (bound or free) doping.7−9 However, there are very few reports on the magnetism investigations in the LSTO-based DMOs. 10−19 Several irreconcilable origins of FM include intrinsic carriers,10,11 © XXXX American Chemical Society
Received: July 12, 2013 Revised: August 9, 2013
A
dx.doi.org/10.1021/jp4068922 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
formed using a Gatan Image Filter system attached to the TEM in a scanning mode. The electronic structures were detected by X-ray photoelectron spectroscopy (XPS) (ULVAC PHI-5000 using an Al Kα source). Before measurement the atmospheric degradation was eliminated by a surface-cleaning procedure with an argon-ion laser. Co K-edge XANES spectra were measured in fluorescence mode at the U7C beamline of the National Synchrotron Radiation Laboratory, China. The Raman spectra were acquired from a micro-Raman spectrometer (NT-MDT nanofinder-30) in the backscattering geometry with a 488 nm Ar+ laser as an excitation source. The magnetic properties were measured by a vibrating sample magnetometer (VSM) (ADE EV7) and a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS XL-7). First-Principles Calculations. The first-principles calculations were performed based on the local density approximation (LDA) method, which is contained in a WIEN2k package.32 To consider the Co-doping effect, calculations were performed for a 2 × 2 × 2 supercell of LSTO with k-mesh 4 × 4 × 4, in which replacement of one Ti by one Co and one O by one C occurred, while the effect of the La atom in the compound was calculated using virtual crystal approximation (VCA). To provide the magnetic properties of Co and C in LSTCO, we calculated the system with ferromagnetic/ antiferromagnetic configuration between Co and C, respectively, using the LSDA method. Moreover, the role that oxygen vacancy played in magnetism was also calculated for comparison.
been considered a prescription for tunable properties and enhanced magnetism in dilute magnetic semiconductors.25,26 Such a codoping strategy (i.e., cation−anion codoping in this work) is deliberately designed and has not been reported yet in the complex DMOs. In other areas, such as photocatalytic materials, the codoping technique has also been proved to be extremely important. For example, it greatly enhances the photoelectrochemical water-spilitting performance in TiO2 with cation−anion codoping from theoretical design27 and experimental realization.28 Moreover, in the previous work regarding dopant−defect complexes induced FM in DMOs,20,29 it is still challenging to identify and control defects because defects (e.g., oxygen vacancies20,29,30 and cation vacancies21,22) are complicated and/or usually formed at the fusion interfaces. In this article, we demonstrate the independent control of spin (by cobalt doping) and defects (by carbon doping) on engineering magnetic properties in chemically modified (CM) La0.4Sr0.6Ti1−xCoxO3 (LSTCO) samples. Herein, we refer to carbon-doped LSTCO as CM-LSTCO. We show our combined experimental and theoretical studies on the cobalt−carbon (Co2+−C) complexes that can directly induce room-temperature FM in multicomponent LSTCO nanocrystalline powders. We clearly evidence the intrinsic FM in CM-LSTCO samples through the independent control of defects and spin, making Co2+−C complexes induced FM feasible in design of future spintronic devices.
■
EXPERIMENTAL SECTION Here we present a postannealing process in high-pure argon atmosphere by which carbon is in situ incorporated into the LSTCO solids during crystallization, as implemented in a previous study on the synthesis of CM-TiO2 (C-doped TiO2) by flame pyrolysis, resulting in a lower band gap energy and thus a maximum photoconversion efficiency.31 Synthesis. A series of CM-LSTCO powdered samples (0.001 ≤ x ≤ 0.03) with nominal compositions of x = 0.001, 0.002, 0.005, 0.008, 0.01, 0.02, and 0.03 were prepared by a simple solvothemal technique combined with the postannealing process. In brief, samples were directly prepared using highpurity La(NO3)3·6H2O, Sr(NO3)2, C16H36O4Ti, Co(NO3)2· 6H2O, citric acid (C6H8O7·H2O), and ethylene glycol (HOCH 2 CH 2 OH) as starting materials with designed stoichiometry. After vigorous magnetic stirring for 0.5 h and ultrasonication for 0.5 h, the resulting mixtures were transferred into 100 mL Teflon-lined stainless-steel autoclaves. The autoclaves were sealed and heated at ca. 180 °C for 12 h in a conventional furnace and then cooled to room temperature. The precipitates were collected from solution, rinsed with deionized water and absolute ethanol several times, and dried in air, yielding the amorphous light-yellow powdery products. The powders were subsequently annealed in a pumped tube furnace under Ar flow of ∼10−3 Torr at 800 °C for 2 h, yielding the crystalline dark-gray LSTCO nanocrystals. Care was taken to prevent samples from directly contacting any ferrous tools during preparation. The pure LSTO (x = 0) and heavily doped LSTCO (x = 0.15) samples were also prepared under the identical conditions for comparison. Characterization. The crystallography and nanostructure were examined by powder X-ray diffraction (XRD) (Rigaku D/ Max-2000PC using a Cu Kα line) and transmission electron microscopy (TEM)/high-resolution TEM (HRTEM) (FEI Tecnai F20 with FEG operated at 200 kV). The electron energy-loss spectroscopy (EELS) measurements were per-
■
RESULTS AND DISCUSSION We successfully synthesized a series of crystallized LSTCO samples with the annealing duration of 2 h in Ar atmosphere. While annealed for less than 2 h, the samples showed a weaker diffraction intensity, suggesting the incomplete crystallization. Figure 1 shows the XRD patterns of a series of CM-LSTCO samples, indicating the perovskite structure. The standard peak positions for the perovskite LSTO phase are also displayed at
Figure 1. Powder XRD patterns of the CM-LSTCO (0 < x ≤ 0.03). The reference samples of pure LSTO (x = 0) and heavily doped LSTCO (x = 0.15) are also included. The dominant diffraction peaks are labeled. B
dx.doi.org/10.1021/jp4068922 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 2. Microstructures of x = 0.02 sample. (a) The typical TEM and (b,c) HRTEM images. (d) The schematic crystal structure of LSTO, where O−Ti−O/O−Co−O species locate in (200) planes. (e) The high-contrast HRTEM image shown in (c). (f) The HAADF-scanning TEM image where EELS mappings are taken. (g−j) The EELS mapping results of Sr, Ti, O, and Co elements at Sr−L, Ti−L, O−K, and Co−L edges, respectively. (k) The EDX spectrum collected from the sample.
EELS mapping in La0.37Sr0.63Ti0.98Co0.02O3 films.11 It is known that for low-concentration (usually
