Interfacial effect on Mn-doped TiO₂ nanoparticles: from paramagnetism to ferromagnetism

Abstract

Manganese-doped TiO₂ nanoparticles with different crystalline structures, namely, anatase, rutile, mixed-phase of anatase and rutile, have been synthesized by a template method. We found that different structures lead to different defect concentrations and, as a result, strongly influence the magnetic properties. The pure anatase and rutile Mn-doped TiO₂ are paramagnetic while the mixed-phase exhibits robust room temperature ferromagnetism. Extended X-ray absorption fine structure data reveal increasing oxygen vacancies when the mixed-phase formed. The key factor for activating ferromagnetism is found to be the interfacial defects created during phase-transformation, which is proposed to be oxygen vacancies. The defect effective Bohr radii are sufficiently large to overlap dopant ions, causing a net alignment of the dopant spins and then leading to a long-range ferromagnetic order. This discovery demonstrates that controlling the interfaces is an effective route to regulate magnetic properties and establish a prominent example of emergent phenomena at oxide interfaces.

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1. Introduction

The appearance and manipulation of magnetism in traditional dilute magnetic semiconductors (DMSs) are nowadays some of the most active branches of material science. Over the past decade, research has been mainly focused on transition metal ions (Mn-, Co-, Cu-) doped oxides because of their potential application in spintronic devices.¹ Recent experimental results indicate that the observed ferromagnetism of ZnO/TiO₂ is attributed to the presence of impurities, defects, grain boundaries and surface/interface effects, but the origin is still a matter of controversy.² Ever since the formation of a metallic layer was observed at the interface between the two insulators LaAlO₃ and SrTiO₃ around a decade ago, the oxide interface has been suggested to be a critical component of interesting phenomena, including magnetism and superconductivity.³ Prompted by the eagerness to understand the abnormal magnetic properties, considerable work has been carried out. García et al. experimentally showed that the interface formed at Zn diffusion front into Mn oxide resulted in room temperature ferromagnetism (RTFM) in Mn–Zn–O system.⁴ Kopnov et al. found an evidential relation between the surface roughness of silicon/silicon oxide interface and the measured magnetism by chemical etching.⁵ Serrano et al. confirmed a surface reduction of octahedral Co³⁺ to Co²⁺ in the mixtures of anatase-TiO₂ and Co₃O₄ was related to the observed ferromagnetism.⁶ Recently, interface-based ferromagnetism was observed in CuO/Cu₂O microspheres, and this observation confirmed the interface effect on engineering architecture with enhanced magnetic property.⁷ What's more, the electronic and magnetic properties of transition metal perovskites based interfaces have been widely studied due to manipulation of magnetism.⁸ Tunnelling experiments revealed that ultrathin BaTiO₃ films simultaneously possessed a magnetization and a polarization with Fe or Co at the interface.⁹ Using LaAlO₃/SrTiO₃ as model, Lee et al. demonstrated an unexpected property with no bulk analogue in the constituent materials.¹⁰ Most recently, high-quality devices have been demonstrated by simply evaporating patterned top gates on the surface of a LaAlO₃/SrTiO₃ interface.¹¹

These observations of interface-related ferromagnetism and recent technical advances in atomic-scale characterization have pioneered a new avenue for explorations of magnetic oxides-based interface systems, accelerating the discovery of new magnetic materials for spintronics application. Transition-metal (TM) doped TiO₂ is a fantastic material with numerous applications in room temperature semiconductor spintronics,¹² photocatalysis,¹³ sensitized solar cells,¹⁴ etc. Following the discovery of RTFM in Co-doped TiO₂ by Matsumoto et al.,¹⁵ the research on the peculiar magnetic properties of these materials was focused on both anatase and rutile TM-doped TiO₂ (e.g. Co²⁺, Mn²⁺, Ni²⁺) thin films systems.¹⁶ The most investigated DMS systems are characterized by super-exchange between magnetic ions via electrons trapped in oxygen vacancies, indicating that the doped TM ions and defects play a crucial role in these systems.^17,18 The d–d double exchange of multiple-valence transition metal d states can also induce local ferromagnetic-like behavior, reduced rutile TiO_2−δ nanoparticles being one possible example.¹⁹ Although particular attention was devoted to Mn-, Co-doped anatase/rutile TiO₂, recent work released enhanced ferromagnetism in Mn-doped TiO₂ films during phase transition from anatase to rutile.²⁰ Yıldırım et al. found different TM-implanted TiO₂ structures strongly influence the magnetic properties.^21,22 In this work, Mn-doped TiO₂ nanoparticles with different magnetic properties have been prepared by adjusting their crystalline phase. Only in mixed-phase robust RTFM can be observed; however, pure anatase or rutile shows paramagnetism. This discrepancy is attributed to an increasing number of active magnetic Mn²⁺ ions and interfacial defects created during the phase transformation. These results demonstrate new possibility for effectively manipulating the ferromagnetic behavior in oxide semiconductors.

2. Experiment

The carbon spheres were firstly synthesized by hydrothermal method. A moderate amount of tetrabutyl titanate and manganese nitrate were dissolved in 100.0 mL anhydrous ethanol, followed by adding 1.0 g carbon spheres for templating. After ultrasonication 30 minutes, the as-obtained solution was vigorously stirred for 4 hours to get Ti⁴⁺, Mn²⁺ ions absorbed homogeneously. The obtained products were collected by centrifugation and rinsed with ethanol. The crystalline Mn-doped TiO₂ powder was obtained by annealing the as-prepared products in air at 400 °C, 500 °C, 600 °C for 6 hours, respectively. XRD patterns were recorded by using a Panalytical X'PERT PRO MPD diffractometer with Cu Kα monochromatic radiation (λ = 1.54056 Å). SEM (SU-70) and HR-TEM (FEI TECNAI G² F20) and corresponding energy-dispersive spectroscopy (EDS) mapping analyses were carried out to identify the morphology and nanostructure. Raman spectra were performed at room temperature by using a Thermo Fisher DXR SmartRaman spectrograph with 532 nm laser. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB 250Xi with 0.45 eV resolution. Magnetic properties were measured on a Quantum Design magnetic property measurement system (MPMS3). Mn concentration was determined by inductively coupled optical emission spectrometry (ICP-OES, Agilent Technologies model 710). Electronic paramagnetic resonance (EPR) measurement was performed using a Bruker ESRA-300 spectrometer at 9.87 GHz (X-band). The Mn K-edge extended X-ray absorption fine structure (EXAFS) spectra were measured at the beamline 20-ID of the Advanced Photon Source (APS), Argonne National Laboratory.

3. Results and discussion

Determined by ICP-OES, Mn concentration is 1.85 at%, which is close to the nominal one (2.0 at%). Fig. 1 shows the XRD patterns of Mn-doped TiO₂ annealing at different temperature. When the sample was annealed at 400 °C, intensive peaks at 2θ = 25.24°, 38.01°, 48.07°, 54.58° and 62.95° are observed, which correspond to the indices of (101), (004), (200), (211) and (213) planes of anatase phase, respectively.²³ When the sample was annealed at 500 °C, besides typical anatase-like peaks, peaks at 2θ = 27.39° and 36.08° occur. These peaks represent the indices of (110) and (101) planes of rutile phase, respectively, indicating that a fraction of anatase phase has transformed to rutile phase after annealed at 500 °C. The rutile phase component is estimated to be 17% by using the following equation:²⁴where W_R is rutile weight fraction, A_ana and A_rut are the X-ray integrated intensities for anatase (101) and rutile (110) diffraction peaks, respectively. When the sample was annealed at 600 °C, the diffraction peaks of anatase phase completely vanish. All the X-ray diffraction peak corresponds to the rutile phase, indicating the completeness of phase transformation. It is noted that no secondary phases of crystalline Mn₂O₃ or Mn₃O₄ are detected by XRD.

Fig. 1 XRD pattern of Mn-doped TiO₂ after annealing at 400 °C, 500 °C, 600 °C.

The template method maintains the hollow spheres morphology as observed by scanning electron microscopy (SEM) with a diameter from 150 to 200 nm (Fig. 2a). A transmission electron microscopy (TEM) image (Fig. 2b) demonstrates a thin shell with a thickness of about 10 nm. Scanning transmission electron microscopes (STEM) mode EDS mapping confirms the homogeneous distribution of Ti, O, Mn elements. High resolution TEM (HRTEM) analysis (Fig. 3) was applied to investigate the microstructure changes during temperature-variable annealing. It's clear to observe that lattice spacing corresponding to anatase (d = 3.54 Å, d = 2.42 Å) changes to that of rutile (d = 3.25 Å, d = 2.51 Å). Combined with TEM measurement, Penn et al.²⁵ proposed that structural elements common to rutile can be produced at a subset of anatase interface, and these might serve as rutile nucleation sites. Lee et al.²⁶ found that the nucleation of rutile occur at the amorphous interface of anatase particles due to strain and the disordered structure. On the basis of UV Raman, visible Raman and XRD results, Zhang et al.²⁷ suggested that the phase transformation actually starts from the interfaces between the agglomerated anatase grains. Here, we propose that interfaces formed during phase transformation are likely to give rise to defects such as oxygen vacancy.^28,29

Fig. 2 (a) SEM image. (b) Magnified TEM image. (c) STEM image and corresponding elemental mapping of (d) Ti, (e) O and (f) Mn elements.

Fig. 3 HRTEM images of Mn-doped TiO₂. (a) Pure anatase. (b) Mixed-phase. (c) Pure rutile.

The Raman spectra collected from out-phase Mn-doped TiO₂ in the region of 100–900 cm⁻¹ are shown in Fig. 4a, and for convenience of comparison, undoped anatase TiO₂ Raman spectra are also shown. According to previous report, anatase TiO₂ (space group D_4h, Z = 2) shows strong Raman peaks at 143 (E_g), 399 (B_1g), 516 (A_1g) and 638 cm⁻¹ (E_g).³⁰ When the sample was annealed at 400 °C, E_g Raman scattering mode shift to higher wavenumbers due to the distortion of Ti–O band by Mn incorporation.³¹ When the sample was annealed at 500 °C, the Raman scattering peaks become broader due to the increase of grain size.³² When the sample was annealed at 600 °C, Raman scattering mode at 143 cm⁻¹ (E_g) diminishes completely. At the same time, new bands at 425 and 618 cm⁻¹ appear, which are assigned to E_g and A_1g modes of rutile phase respectively.²⁴ Meanwhile, the intensity of the bands decrease due to the disorder of oxygen lattice induced by Mn incorporation.³¹ The phase transformation revealed by Raman spectra is consistent with the results of XRD and HRTEM well.

Fig. 4 (a) Raman spectrum. (b) Ti 2p, (c) O 1s, (d) Mn 2p core level spectra of Mn-doped TiO₂ with various annealing temperature.

To get a better insight into the local chemical environments, all these samples were further investigated by means of XPS. All the spectra were corrected for the C 1s peak at 284.6 eV. The X-ray photoelectron spectrum of Ti 2p core electron (Ti 2p_3/2, binding energy 458.8 eV; Ti 2p_1/2, binding energy 464.5 eV) for phase-pure anatase clearly matches previous reports, which are typical BE values of Ti⁴⁺ in anatase TiO₂.³³ For phase-pure rutile, these peak positions of Ti 2p_3/2 and Ti 2p_1/2 are at 458.5 eV and 464.3 eV, respectively, shifting slightly toward lower BE values. The Ti 2p_3/2 spectrum from mixed-phase sample is found to be at 458.6 eV binding energy and fitted through a Gaussian–Lorentzian function according to phase composite.³⁴ The peak area ratio of anatase to rutile is 4 : 1, which is consistent with the observed compositional ratio of anatase to rutile phase by XRD.

Further, the symmetric shape of Ti 2p lines suggest that any magnetic species of titanium such as Ti³⁺ can be ruled out.¹⁹ The binding energy value of the O 1s is found to be around 529.5 eV which is attributed to O–Ti⁴⁺ bond.³¹ And the O 1s spectrum shows an identical trend to Ti 2p spectrum, indicating the phase transformation. The O 1s peaks situated at 531.5 and 533.3 eV correspond to chemisorbed oxygen in hydroxyl group, with less than 10% area of the whole O 1s peaks.³⁵ Hence the Ti 2p peak is supposed to contain only a small contribution from hydroxylated Ti environment.

In order to identify chemical state of Mn in TiO₂ lattice, a slow scan of Mn 2p core level spectrum is recorded. The 12.2 eV-spaced Mn 2p_3/2 and 2p_1/2 peaks located at 641.1 eV and 653.3 eV indicate the presence of Mn²⁺ or Mn³⁺. Since the binding energy of Mn²⁺ and Mn³⁺ are both around ∼641 eV, it's difficult to distinguish Mn²⁺ from Mn³⁺ completely. While Mn²⁺ is discernible and Mn³⁺ is usually invisible in X-band EPR,³⁶ on the basis of EPR result (discuss later), thus we can conclude that Mn ions are present as divalent state substituting Ti⁴⁺ in TiO₂ crystal lattice curves of out-phase Mn-doped TiO₂ nanoparticles. The pure anatase and rutile ones are paramagnetic due to the presence of magnetic Mn²⁺ ions. The mixed-phase sample shows unsaturated magnetization. The actual saturation magnetization possibly occurs at low magnetic field and the linear magnetization occurs at a higher magnetic field. The ferromagnetism may be due to ferromagnetic exchange between the Mn²⁺ ions and oxygen vacancies. Therefore, we can attribute the total magnetization of the sample to the sum of ferromagnetic and paramagnetic part.³⁷ For comparison, the M–H curve of undoped TiO₂ at 300 K is also shown, which exhibits diamagnetism. The large H_c proves a strong ferromagnetic coupling rather than any contamination.³⁸ Fig. 5b shows zero field cooling and field cooling (ZFC–FC) curves for the mixed-phase sample, obtained with a 500 Oe applied field. There is a peak closed to 60 K, which may be attributed to antiferromagnetic oxygen species.³⁸ This particular peak has been widely reported in various O₂-contaminated samples.³⁹

Fig. 5 (a) M–H curves of Mn-doped TiO₂ at room temperature. The inset shows M–H curve of undoped TiO₂. (b) Temperature dependence of ZFC and FC magnetization (M–T curves) of mixed-phase sample at a fixed field (500 Oe). The inset shows enlarged view of coercivity.

In order to explore the origin of magnetism of these out-phase Mn-doped TiO₂ nanoparticles, we measured their EPR spectra. Mn³⁺ (S = 2) is usually silent in X-band EPR due to a large zero-field splitting,^36,40 here we can exclude the presence of Mn³⁺ ions. The Mn²⁺–Mn²⁺ antiferromagnetic super exchange interaction is known to be silent in X-band EPR⁴¹ and the isolated Mn²⁺ (S = 5/2) ions exhibit hyperfine splitting sextet lines in EPR spectrum due to allowed (Δm_I = 0) and forbidden (Δm_I = ±1) hyperfine transitions between the Zeeman sublevels m_S = ±1/2, where m_I and m_S are nuclear spin and electron spin quantum numbers, respectively.⁴² The broad mainly symmetrical lines with a line width of 92 mT is probably due to higher concentration Mn²⁺ ions with enhanced dipolar interaction.⁴³ The g factor in the Zeeman interaction is found centered at ∼2, which is close to previous report, indicating that the incorporated Mn²⁺ ions on Ti⁴⁺ lattice sites.⁴⁴ The EPR integrated intensity demonstrates higher paramagnetic Mn²⁺ concentration in mixed-phase (Fig. 6).

Fig. 6 (a) EPR spectra. (b) Mn K-edge Fourier transform EXAFS spectra.

To locally probe the fine structure, the Mn K-edge EXAFS spectra were taken for all samples. Curve-fitting was performed by ab initio FEFF calculation, to extract quantitative structural parameters.⁴⁵ The structure model used was a anatase/rutile structure with the core Ti atom being replaced with Mn.⁴⁶ This structural model provides a good fit to the experimental spectrum, and the Mn–O bond lengths are observed to be 1.94, 1.98 and 2.04 Å, respectively, excluding the presence the manganese-based oxides (Mn–O bond length is 2.20 Å of manganese-based oxides). It can be also confirm that Mn²⁺ ions substitute Ti⁴⁺ lattice sites, consistent with the EPR result. When Ti⁴⁺ is replaced with Mn²⁺, electroneutrality should decrease the O neighbors, namely, creating some oxygen vacancies. The coordination number is observed to be 4, 3.3 and 4, respectively, indicating more oxygen vacancies formed during phase transformation.

Finally, we briefly discuss the physical origin of the observed ferromagnetism of out-phase Mn-doped TiO₂ nanostructure. Donor type defects, e.g. oxygen vacancies, have been proposed to be vital elements necessary for RTFM in TM-doped oxides.^17,18 According to previous report,¹⁷ hydrogenic orbital radius of TiO₂ is 0.48 nm, which is far smaller than grain size. So we can exclude the possibility of carriers localization stem from the spatial confinement effect.^41,47 Pure anatase and rutile Mn : TiO₂ nanoparticles are paramagnetic, lacking sufficient oxygen vacancies to couple with the magnetic Mn²⁺ ions to form bound magnetic polaron (BMP).^37,48,49 The phase-transformation process creates interfacial defects, whose effective Bohr radii are large enough to overlap the concentrated Mn²⁺ ions. Following the BMP mechanism,¹⁷ a net alignment of the dopant spins occurs, leading to a long-range ferromagnetic order. These results additionally provide new microscopic information of interfacial defects. Finally, this finding suggests approaches to manipulating DMS ferromagnetism by tailoring the interfaces that will help guide experiments aimed at introducing appropriate defects into DMS and can be further applied to thin film-based spintronics application.

4. Conclusions

In summary, we have experimentally demonstrated the feasibility of achieving ferromagnetic interaction in Mn-doped TiO₂ nanostructure, by phase-transformation strategy. Combined a detailed structural analysis, we confirm the phase-transformation process. The pure anatase and rutile Mn-doped TiO₂ are paramagnetic while mixed-phase exhibits robust room temperature ferromagnetism, with saturation magnetization of 0.28 emu g⁻¹. Based on the EPR and EXAFS experiment, we consider that an increasing concentration of interfacial defects forms BMP with the dopant magnetic ions, making the magnetic moment of mixed-phase observable. This finding provides a versatile system to induce and manipulate magnetic moments in non-magnetic materials, which have potential applications in spintronics.

Acknowledgements

This work was supported by National Natural Science Foundation of China 51372224, 51072181 and 51572239, Program for Innovative Research Team in University of Ministry of Education of China (IRT13037) and National Science and Technology Support Program (2012BAC08B08). We would especially like to thank Dr Wenge Yang and Dr Bin Chen at Center for High Pressure Science & Technology Advanced Research (HPSTAR) for arranging EXAFS measurement at Advanced Photon Source (APS), Argonne National Laboratory.

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