Study of band structure, transport and magnetic properties of BiFeO3–TbMnO3 composite
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Charge transfer across the interface of two materials in a composite can create reconstruction of bands near the interface which in turn brings multiple changes in physical properties of the materials. Thus, investigation of band structure experimentally is of immense importance in studying composite materials to understand their physical properties. Here, we have studied magnetoelectric multiferroic composite of two types of multiferroic (types I and II) consisting of BiFeO3 and TbMnO3 for enhanced magnetic and transport properties. The band structure was investigated with the help of UV–visible absorption spectrum, the valence band X-ray photoemission spectra (XPS), and ultraviolet photoemission spectra. The band structure thus obtained can successfully explain the magnetic and transport properties of the composite. The insulating behavior of the system is understood from the reconstruction of the energy bands at the interface and subsequent decrease in the band gap which happens due to lattice mismatch of the two materials. The large coercivity and the increase in the magnetization value are understood to be due to superexchange interaction between different Mn ions (Mn2+, Mn3+, and Mn4+). From the composition study of EDXA and core-level XPS, oxygen vacancy was found which in turn creates the mixed valence state of Mn to maintain the charge neutrality.
KeywordsBiFeO3 TbMnO3 Band structure Magnetoelectric multiferroic
Multiferroic materials have been attracting researchers recently for their interesting fundamentals as well as for the possibility of application of these materials in different spintronic devices [1, 2, 3, 4]. In magnetoelectric multiferroic materials, ferroic (or anti-ferroic) magnetic and electric ordering coexist in a single phase giving rise to the possibility of controlling the magnetization (intrinsic polarization) with the application of electric field (magnetic field) . Due to these coupling between the two properties, magnetoelectric multiferroic materials have become one of the most important materials of today . The reason for the limited number of multiferroic material is the mutual exclusive origin of the two ordering (empty d shell for ferroelectricity and partially filled d shell for magnetic ordering) [1, 2, 3, 4].
BiFeO3 is one of the most interesting and well-studied multiferroic as it is the only multiferroic material to show both the ordering (magnetic and ferroelectric) above the room temperature (ferroelectric Curie temperature TN ~ 1103 K and Neel temperature TN ~ 643 K) [5, 6, 7, 8]. It exhibits G-type canted antiferromagnetic ordering with a cycloid frequency of ~ 62 nm . BiFeO3 shows large spontaneous polarization of order 10–100 µc/cm2 because of polar displacement of cations and anions relative to each other pointing along one of the eight pseudo-cubic  axes [5, 6, 7]. The lone pair electron at the 6 s shell of Bi is considered to be the main reason behind the observed ferroelectricity on BiFeO3 while the partially filled 3d shell of Fe is responsible for the canted antiferromagnetic ordering [5, 6, 7]. BiFeO3 has been found to be useful in different modern-day technologies including microwave synthesis [9, 10]. In spite of having these features, BiFeO3 is not considered suitable for many applications due to many reasons. Structural instability, difficulty in synthesizing single phase, high leakage current, and low resistivity of the material due to the presence of Fe3+ and oxygen vacancies are some of them [11, 12, 13, 14]. To overcome the shortcomings, there have been numerous attempts involving doping of different transition metal at Fe site and rare earth metals at Bi sites, inducing chemical pressure or strain in the system [15, 16, 17, 18, 19, 20, 21, 22]. An alternate option is to prepare composite of different multiferroic materials with a similar structure involving BiFeO3. There are reports on composite structures and superlattice structures of BiFeO3–BaTiO3, BiFeO3–PbTiO3, and BiFeO3–BiMnO3, showing improvement in multiferroic properties [23, 24, 25]. Yu et al.  have reported exchange bias and other enhanced magnetic properties in BiFeO3–La0.7Sr0.3MnO3 heterostructure due to charge transfer-assisted band reconstruction near the interface which in turn creates additional ferromagnetic and antiferromagnetic exchange interactions.
In this context, we have prepared a composite of BiFeO3 and TbMnO3, belonging to a different type of multiferroic. TbMnO3 is type II multiferroic material in which the ferroelectric ordering (TC ~ 28 K) arises as a result of magnetic ordering (TN ~ 42 K), and the material is known to possess strong magnetoelectric coupling [27, 28]. It crystallizes in orthorhombic perovskite (Pbnm) close to that of BiFeO3 which has rhombohedral perovskite (R3c) structure [27, 29]. The lattice mismatch between them is expected to trigger band reconstruction near the interface . Band reconstruction can influence many physical properties of the composite including transport and magnetic properties. In one of our earlier reports, it was reported that the composite of BiFeO3 and TbMnO3 (in 7:3 ratio) 0.7BFO–0.3TMO shows interesting properties like spin–phonon coupling, magnetodielectric coupling, high dielectric constant, etc. . It was also seen that charge accumulation and charge transfer at the interface of two materials of the composite in determining different properties of the composite. Thus, it would be significant to study the band structure of the composite and band reconstruction if any due to the charge transfer. It is also interesting to study the changes in transport and magnetic properties due to band reconstruction. In this report, we have studied the band structure through valence band X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) and UV–visible absorption spectroscopy. The effect of band reconstruction on the transport and magnetic properties has also been studied.
2 Materials and methods
The compounds BiFeO3, TbMnO3, and the composite of BiFeO3 and TbMnO3 (in 7:3 ratio), i.e., 0.7BFO–0.3TMO, were prepared following a conventional solid-state reaction method by taking the precursors for all the constituent elements i.e., Bi2O3, Fe2O3, Mn2O3, and Tb4O7 (Alfa Aesar, USA) in proper stoichiometric ratio. At first, powders of TbMnO3 were prepared by solid-state reaction of Tb4O7 and Mn2O3 in proper stoichiometric ratio. The mixture was calcined 1200 °C for 12 h. The calcined powders were pelletized and sintered at 1300 and 1400 °C for 24 h. The powder was ground thoroughly in between. Then the TbMnO3 powder was mixed with Bi2O3 and Fe2O3 in stoichiometric ratio, and the mixture was ground with mortar pestle for 4 h. The mixture was calcined at 900 °C for 6 h after which it was pelletized and sintered for 10 h at 1000 °C to get the final sample. The surface morphology and grain growth are studied from field emission scanning electron microscope (FESEM, Nova Nano SEM 450). The composition of the samples was also studied from energy-dispersive X-ray analysis (TEAM EDS SYSTEM with Octane Plus SDD Detector) integrated with the SEM. X-ray photoemission spectroscopy (XPS) experiments were performed using Omicron Multi-probe® Surface Science System, GmBH, equipped with a dual-anode non-monochromatic Mg/Al X-ray source (DAR400), a monochromatic source (XM 1000), and a hemispherical electron energy analyzer (EA 125). All the XPS measurements were performed inside the analysis chamber under base vacuum of ~ 1.8 × 10−10 Torr using monochoromatized AlKα with a power of 300 W. The total energy resolution for monochromatic AlKα line with photon energy 1486.70 eV, estimated from the width of the Fermi edge, was about 0.25 eV. The pass energy for Survey Scan Spectra and core level spectra was kept at 50 eV and 30 eV, respectively. Ultraviolet photoemission spectroscopy (UPS) and UV–visible absorption spectroscopy were employed to study valence band structure and electronic properties. To investigate the fine changes near Fermi level, Fermi edge ultraviolet photoemission spectra were collected using the non-monochromatic He I (21.2 eV) line at an average base pressure of 2.8 × 10−8 Torr. The energy resolution of the analyzer and the step size were set at 0.03 eV and 0.05 eV, respectively. The magnetic measurements were carried out in Magnetic Property Measurement System (SQUID-MPMS, Quantum Design, USA). The temperature-dependent transport properties were measured in Close Cycle He cryostat (Advanced Research Systems, Inc.) using a Keithley 6517B Electrometer.
3 Result and discussion
Figure 3c shows the O1s XPS spectrum of the composite which is split into two parts revealing two kinds of chemical state of oxygen present in the composite. The peaks at binding energy position ~ 529.3 eV correspond to the oxygen situated at lattice denoted as OL and the peak positioned at ~ 531.3 eV corresponds to the surface chemisorbed oxygen denoted by OV. The surface chemisorptions of the oxygen ions occur due to the presence of oxygen vacancy in the system. In BiFeO3-based compounds, it is common to find the oxygen vacancies which are created at the surface due to lattice defects [34, 40]. Moreover, our XPS results conform with EDX analysis which indicated the presence of oxygen vacancy in the system.
In Fig. 3e, core level XPS spectra of Tb3d element is presented. The spectra show spin-orbit coupling peaks of Tb 3d positioned at ~ 1276.7 eV and ~1241.8 eV for 3d3/2 and 3d5/2, respectively. The peak positions and the doublet separation clearly suggest that Tb is in 3+ oxidation state [NIST]. The core-level spectra of Bi4f are also studied and are shown in Fig. 3f. The spin–orbit split peaks of Bi4f5/2 and Bi4f7/2 are observed to be present at ~ 163.7 eV and ~ 158.3 eV, which corroborate well with earlier reports of trivalent valence states of Bi [43, 44].
Furthermore, three main features of XPS VB spectra below the EF can be observed at ~ 2.5 eV, 7 eV, and 10.5 eV, which are denoted as VB1, VB2, and VB3 respectively. The complete occupied VB spectra below EF (ranging from 0 eV to ~ 14 eV) is mainly composed of hybridized states of Tb4f, extended Mn3d, Fe3d, and O2p orbitals. Since, for the present system, the Mn and Fe are in octahedral co-ordination with ligand oxygen ions, the crystal field effect causes the Mn/Fe3d states to split into eg and t2g levels. Moreover, for such kind of co-ordination, the t2g states always lie below the eg states. Therefore, the spectral weight (in the range of 0 eV to ~ 1 eV) immediately below EF can be mainly attributed to the contributions from partially occupied Mn3d_eg and Fe3d_eg orbitals . However, the first shoulder like feature VB1 positioned at ~2.5 eV has appeared mainly due to the hybridization of Tb4f, extended Mn3d_t2g, and Fe3d_t2g with O 2p orbitals. However, a significant contribution to this feature VB1 is expected to be from Tb4f states as the other states (Mn3d_t2g, Fe3d_t2g) are extended over a long range . The most intense feature of the VB spectra at ~ 7 eV (VB2) can be attributed mainly to the hybridization of the Mn3d_t2g and Fe3d_t2g orbitals with the O2p states. On the other hand, the feature VB3 at the lowest energy ~10.5 eV emanates from the hybridized states of O2p - Mn3d_t2g/Fe3d_t2g and other oxygen bonding states O2p–Mn/Fe4sp and O2p–Tb5sd, etc., with significant contribution being from O2p states .
Moreover, to probe how the VB spectral features get modified with higher resolution and also to augment with our previous results of a electronic structure near Fermi level, we have carried out the ultraviolet photoemission spectroscopy (UPS) measurement on the same system (Fig. 5c). It is interesting to note here that the spectral features in UPS VB spectra immediately below the EF are of relatively low intensity as compared to those observed in XPS VB spectra. This in turn predicts the fact that the region immediately below EF has significant contribution from the Mn/Fe3d states since the photo-ionization cross-section for the Mn/Fe3d photoemission is considerably higher than that for the O2p photoemission in the XPS process. As a consequence, the low-energy UPS is less sensitive to the heavier atoms and highly sensitive to the lighter atoms such as oxygen. Hence, in the whole UPS spectra, the predominant contribution comes from the O2p states, while the contributions from the Tb4f, Mn3d, and Fe3d states get suppressed. Irrespective of the above facts, the UPS VB spectra agreed mostly with the XPS VB spectra. The absence of the electronic states near the Fermi level in the UPS VB spectra supports our previous results, thus suggesting its insulating nature. Moreover, a feature (VB1) near ~ 2.5 eV (associated with the hybridized states of O2p–Mn/Fe3d and O2p–Tb4f orbitals) can be visible in the UPS VB spectra which corroborate with the XPS VB spectra. Similarly, the second feature (VB2) which is the intense broad peak near ~ 7 eV matches well with the most intense peak of the XPS VB spectra. Another weak feature (VB3) near ~ 11.5 eV can also be visible which is seemingly associated with the O2p–Mn/Fe3d hybridized states with significant contribution coming from O2p states. The position of the last feature differs by ~ 1 eV with that of the VB3 feature observed in XPS VB spectra. This can presumably be attributed to the possible charging effect due to the high resistivity of the system.
To summarize, we have prepared the BiFeO3–TbMnO3 composite (7:3) via conventional solid-state reaction and studied the interface through different characterizations. Morphological detail and the grain growth were studied from the SEM images. Chemical state and the composition of the material were studied from XPS and EDXA which revealed that oxygen vacancy is there which in turn creates mix valence state of Mn ions to maintain the charge neutrality. A remarkable decrease in band gap was observed from the UV–visible absorption spectra from that of BiFeO3. Based on the band gap (~ 1.45 eV) and the results from XPS valence band spectra and UPS spectra, the band structure of the material was drawn in which the conduction band edge was found ~ 0.45 eV. In the valence band, three main features were observed at binding energy positions ~2.5 eV, 7 eV, and 10.5 eV (VB1, VB2, VB3, and VB4) which were composed of hybridized states of Tb4f, extended Mn3d, Fe3d, and O2p orbitals. The most intense feature (VB2) was attributed to the hybridization of the Mn3d_t2g and Fe3d_t2g orbitals with the O2p states, while the weak shoulder-like feature VB1 which close to the Fermi energy is attributed to the Tb4f states. Such band diagram and the reduction in the band graph are due to the reconstruction of the bands due to interfacial strain. Moreover, the transport property was found to be dominated by variable-range hopping mechanism, and the high resistivity of the material was also found to be consistent with the band diagram. Antiferromagnetic like non-saturating M–H loop was observed with weak ferromagnetic features at low fields which was attributed to the superexchange interaction between different Mn ions.
Authors would like to acknowledge the central instrument facility center of IIT (BHU) for SEM, EDXA, and magnetic measurements.
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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