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Evidence of Enhanced Oxygen Vacancies Defects Inducing Ferromagnetism in Multiferroic CaMnO (CMO) Manganite with Sintering Time 7

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Shashikala Jaiswar, and Kam Deo Mandal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05415 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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The Journal of Physical Chemistry

Evidence of Enhanced Oxygen Vacancies Defects Inducing Ferromagnetism in Multiferroic CaMn7O12 (CMO) Manganite with Sintering Time

Shashikala Jaiswar,* and K. D. Mandal

Department of Chemistry, Indian Institute of Technology (BHU), Varanasi-221005, India

*Corresponding author. Tel.: +91-542-6702868 (off.); Fax: +91-542-2368428 E-mail address: [email protected] (shashikala Jaiswar).

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ABSTRACT: We provide the first experimental evidence of oxygen vacancy defects induced ferromagnetism in undoped multiferroic CaMn7O12 (CMO) manganite synthesized from facile chemical combustion method. The obtained nano-crystalline characterizes by various techniques like TGA, FTIR, XRD, SEM-EDX, AFM, UV- Visible, XPS, and SQUID, etc. to confirm the phase purity and crystallinity of CMO. Surface roughness increases with sintering time attribute to increase of surface oxygen vacancies defects. X-ray photoelectron spectroscopy was carried out to confirm the oxidation state of constituent elements and also, provides direct evidence of enhanced oxygen vacancies. UV-Vis optical absorption used to infer band gap shift from 1.68 to 1.38 eV respectively also attribute to increases in oxygen vacancies defects. Multiple magnetic phase transition temperatures of 90 K, 50 K, and 42 K respectively obtained from the derivative of magnetization. A systematic decrease of full widths at half maxima (FWHM) of dM/dT vs. T curves with sintering time indicates strengthening of ferromagnetism (FM). Transition temperature does not change significantly with sintering time indicate the extrinsic origin of FM. The results of the UV-Vis, XPS and AFM and strengthening of ferromagnetism all are corroborated with each other’s result and also attribute to enhanced oxygen vacancy concentration with sintering time. The origin of FM in undoped CMO manganite with sintering time resultant from bound magnetic polarons (BMP) of enhanced iterant and localized electron of oxygen vacancies trapped centre at the surface or interfaces. Our finding also opens a new perspective for exploiting oxygen vacancies defect engineering at surfaces or interfaces in the design of exotic magnetic and spintronics based devices.


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INTRODUCTION In the last few years, many efforts have been devoted to the search for novel single phase multiferroic compounds that are simultaneously showing two or more ferroic orders i.e. ferroelectricity and ferromagnetism couple to give raise cross phenomenon known as MagnetoElectric (ME) effects. In this magnetoelectric multiferroic the magnetic properties are easily controlled by applying electric field and vice versa and hence potentially applicable to spintronics and information storage devices etc.1 Multiferroic are predominantly desired but their synthesis is a great challenge in solid state science and have attracted consideration of theoreticians and experimentalist with almost identical authority2-6 because conventional ferroelectric requires closed shell d0 cations, while ferromagnetic requires open shell dn cations.78

An example of such multiferroic is CaMn7O12 (CMO) calcium manganite has been considered

in a huge number of works very recently for its magnetoelectric9 and multiferroic properties.10 The magnetic and transports properties of this complex multiferroic manganite have studied with great enthusiasm by scientist and will be continued to be studied in the future because these systems possess as probably all degree of freedom that interplay among spin, orbital, charge, and lattice degree of freedom which makes its desirable from an application point of view. Spins rearranged either in Antiferromagnetic (AFM) by super-exchange (SE) interaction or Ferromagnetic (FM) by double exchange (DE) interaction which leads to either reduced or enhanced magnetization properties respectively. A point of special interest in CMO manganite arises due to the simultaneous presence of Mn3+ and Mn4+ ions in the B site of crystal lattice imply some unusual magnetic and electrical properties of the material.11 The structural phase transition from cubic to trigonal12 and origin of double exchange interaction arising from JahnTeller (J.T.) distortion and hopping of eg electrons from Mn3+ to Mn4+ which leads to origin of



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ferromagnetism (FM) and transition from insulating to conducting material.13 This controls spin degree of freedom of electrons has brought a new revolution in spin-based electronics, with the potential of data storage and information processing which remain mysterious since from the last few decades. The physicochemical properties of the multiferroic metal oxide mostly depend on the concentration of intrinsic or extrinsic defects and impurities.14-15 The multiferroic are desirable for spintronic devices since intrinsic remnant magnetization allow spin-polarized currents to propagate in such materials without the need for a continuous magnetic field.15-16 The ferromagnetism (FM) observed in such system has been attributed to a variety of mechanism arising from transition metal ions, point defects, change in the carrier concentrations but such, a weak FM cannot meet the actual need of spintronic devices. Hence FM with higher magnetization is required to achieve spintronic devices, but without understanding the origin of FM, it is very difficult and challenging task. Several approaches such as tuning oxygen defects and doping with transition metals containing unpaired d electrons etc. has been adopted to improve ferromagnetic and magnetoelectric performance of material.17-20 But neither the true nature of these materials nor the physics behind this magnetism has been adequately determined. In most of the work reported that the double exchange interaction21, 22 between Mn3+-O2--Mn4+ and Jahn-Teller distortion23 are considerably responsible for the origin of their ferromagnetism. Interestingly, more and more theoretical and experimental24, 25 evidence shows that the magnetic ordering is strongly related to oxygen vacancy (VO) and thus it was thought to be the source of FM. Some of the exceptional reports on Sr2FeMoO6 (SFMO) revealed that oxygen vacancy (VO) leads a substantial weakening of magnetic moments.26 In certain reports suggests the segregation and formation of secondary phases or metallic clusters as the origin of the FM27 whereas most


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recent consequences strongly support the intrinsic nature of FM mediated by carriers or defects.28, 29 The Porous structure provides a high surface to volume ratio renders enriching of surface properties in the system. High concentration of surface defects, such as oxygen vacancies (VO) which are rather common defects expected that could ultimately lead to enhanced FM ordering since the fabrication of any system is very sensitive to applied atmosphere. The comprehensive understandings of the individual and collective role of VO are still missing and challenging. These controversial results among research groups suggest that the magnetic properties of materials are critically dependent on the fabrication, growth procedures, doping agents and processing conditions of various synthesis methods results in different defect concentrations, structure, and surface morphology affecting magnetic and electrical properties to different extents. The understand the origin of FM becomes a challenge since both the transition metal (TM) defect/vacancies, and oxygen defects both have been proposed as a potential source for generating FM.30 Some the doping of Transition Metal (TM) also change the number oxygen vacancy (defects) into crystal lattice due to charge compensation mechanism. Thus, it is currently debated whether the observe FM is connected with TM doping or whether it might be solely related to Oxygen defects. Thus it becomes important to understand the presence of specific type defects present in the system and its role towards the FM as it strongly depends on the presence of defects.31 We observed enhanced FM in CMO with of annealing time which suggests that the oxygen defects (VO) were the source of FM.32 CMO inherently exhibits n-type conductivity due to the existence of VO. Recently O-vacancy (VO) in oxides has been reported to be increased with the narrowing (decrement) in the band gap. In this case, the electrons can easily be thermally excited into the conduction band. The XPS and UV-Vis. absorption spectrum



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demonstrates the enhanced concentration of oxygen vacancies (VO) in the system with increased sintering time. In the present system, the multiferroic CMO manganite was synthesized in air atmosphere so that the effects of oxygen deficiency (VO) or defects on the magnetic properties could be examined. In addition, up to till now, the precise mechanism behind the reported ferromagnetism is still not well understood and is an active subject of open debate. Our present attempt in this study is to enhance the magnetic moments via oxygen defects and gain a better insight of the origin of enhanced FM and conductivity. Therefore, to clarify the mechanism underlying the FM and to study systematically the contributions of defects found that CMO samples were annealed at two different sintering times in air for 12 and 15 h respectively and the results was discussed in this paper. The FM and hence the magnetization was changed by annealing the samples at two different sintering time which also leads to changes in the total number of oxygen vacancies/defects. Based on the above experimental observation it has been shown that there is a strong correlation between FM and oxygen vacancies/defects and transport properties and therefore it is believed that oxygen vacancies are the origin of FM here. The results also provide information on Double Exchange (DE) interaction and the formation of Bound Magnetic polarons (BMPs) between Mn3+ and Mn4+ via Oxygen vacancies is the main cause of increased FM. In the present manuscript, we have emphasized on the possible defects present and their role in transport and magnetic properties. The study has been carried out by employing mainly superconducting quantum interface devices (SQUID) magnetometer and impedance spectra and modeling different configuration of surface defects by UV-Vis. Absorption and XPS spectra. The development of strong intrinsic FM constitutes an important step towards the development of an


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improved application of this novel multiferroic material for spintronic devices which can be operated at higher temperatures in nanoscale.

EXPERIMENTAL SECTION Material Synthesis The nano-crystalline multiferroic CaMn7O12 (CMO) manganite was synthesized using chemical combustion route. For the preparation of CMO the analytical grade chemicals, Ca(NO3)2.4H2O (99.2%, Merck), Mn(NO3)2.4H2O (99.6%, Merck) and glycine (99%, Merck) were used as starting materials. Standard aqueous solutions of these metal ions i.e. Ca2+, Mn2+, and glycine were prepared in doubly distilled water. The stoichiometric amounts of standard aqueous solutions of metal ions were mixed in a beaker with aqueous glycine in 1:1 ratio for the synthesis of CMO manganite. The resulting solutions were heated on a hot plate with a magnetic stirrer at 70–80 °C to evaporate the water, and until self-ignition took place. The process of ignition occurred in air at room temperature and burnt under self-propagating combustion, which exhausted a large amount of gas and produced the fluffy mass of CMO ceramic powder. The obtained powder sample was denoted as the AS-Prepared (ASP) sample. In this method, the mixing process was performed in solution as a nitrate solution. The technique involves the mixing of solutions of a metal precursor and an organic polyfunctional acid possessing at least one hydroxyl and one carboxylic acid group, as glycine which results in complexation of the metal by the polycarboxylic acid. The glycine, as a chelating agent, can form a complex with Metal ions at both the amino group end and the carboxylic end and also provide the fuel for the ignition step. The ignition step auto-increased the temperature to form a very fine crystalline



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precursor powder at a very low temperature.33 The resultant ASP powder product were ground and calcined by using an electrical furnace in air. Thereafter the calcined powder was pressed into pellets using a hydraulic press proving a pressure of 4 to 5 Ton with polyvinyl alcohol (3 Wt. %) as the binder. This binder was burnt out at 500 °C for 2 h. Finally, the pellets of CMO were sintered in air at 970 °C for 12 h and 15 h respectively. Material Characterization

Thermo-gravimetric analysis (TGA) of ASP powder of CMO was performed in the air at a heating rate of 10 °C/min from room temperature to 1000 °C using thermal analyzer (PerkinElmer, USA).The crystallinity of the sintered powder was characterized by X-ray diffraction (XRD) using Miniflex X-ray diffractometer (Rigaku Japan) employing Cu ∝ radiation and a Ni filter. Rietveld refinement of the XRD pattern was carried out using fullprof software (ver. 3.00). Pictorial representation of crystal structure made with the help of VESTA Ver. 3.3.8 (Visualization for Electronic Structural Analysis) software using VESTA input file generated from Fullprof Suite software while refinement of XRD data. The relative density of the sintered pellets was determined by using Archimedes principle. The elemental composition and microstructure of the polished surface were examined using a Field Emission Scanning Electron Microscopy (FE-SEM) with Energy Dispersive X-ray (EDX) analyzer (model NOVA NANOSEM 450). Topography and surface roughness and distribution of grain size are estimated from AFM (Model NT- MDT). The oxidation states of the corresponding constituents element were identified via X-ray Photoelectron Spectroscopy technique (XPS) model AMICUS, Kratos Analytical (Shimadzu group company) using a monochromatic source of Mg Kα (1253.6 eV). UV-Vis. absorption spectra of the pallet have been taken by using JASCO (V-650)


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spectrophotometer operating in the spectral range of 200–800 nm. Fourier transforms infrared (FTIR) spectroscopy measurements were done with an FTIR Spectrometer (Spectrum, Perkin Elmer Instrument, USA) in the range of 4000–400 cm−1 with a resolution of 1 cm−1. DC Magnetic measurements were done using a superconducting quantum interference device [Magnetic Property Measurement System (MPMS) XL-7, Quantum Design Inc.]. RESULT AND DISCUSION TG-DTA Study Thermal decomposition of AS Prepared (ASP) CaMn7O12 (CMO) powder in the air been studied by thermogravimetric analysis (TGA) shown in Figure 1. TGA characterization was carried out on ASP of CaMn7O12 (CMO) manganite with a heating rate of 10 °C min-1 from room temperature (RT) to 1000 °C in static air. The TG curve was showing three distinct stages of weight loss. The first weight loss occurred from RT to 220 °C temperature from while the second one occurred from 250 to 600 °C. A third stage occurred from 600 to 900 °C, with no further weight loss were observed between 900 to 1000 °C temperature ranges. The first weight loss (∼2%) observed between RT–250 °C, which attributed to the removal of residual water molecules absorbed on the sample surface. The sharp weight loss observed in second step (~7%) in the temperature range 250–600 °C, which may be accompanied due to the further combustion of organic matrices such as hydrocarbons, nitrates and carbonates of the gel, and excess glycine with the formation of intermediate compounds. It can be seen from the TGA curve that there is very slight weight loss (∼1 %) between 600 - 900 °C in the third step. A very small step up curve was indicating an increase in weight between 800 and 900 °C on the TG curve due to the addition reaction because the final product CMO formed by the combination of the CaMn3O4 intermediate with MnO2 to give the final product at a higher temperature after 900 °C. Moreover,



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weight loss becomes constant, and thereafter no significant loss has been observed above 900 °C. This demonstrates that the decomposition and combustion of all organic constituent precursors in this particular samples have been completed below 900 °C confirming the final product formation. The expected chemical reactions involved in the formation of the CaMn7O12 (CMO) ceramic are given in three steps as shown in the following equations: ( ) ( . ) + 2 ( . ) + → () + 2 () + 4 () + 5 () (ℎ !"# $%) (1) + 3 → ( () !*" ) + (2) ( + 4 → + , (-**"" $%)

(3)

FTIR Spectroscopy To further confirms the phase purity and the formation of CMO compounds by investigated the molecular and functional species present in the sample and its lattice vibration, we used Fourier Transform Infrared (FTIR) spectroscopy of CMO ASP and sintered sample for 12 h and 15 h as shown in Figure 2. Several absorption bands were absorbed at 3450, 2925, 2847, 1795.4, 1636, 1424.4, 1383, 1192, 880, 610, and 480 cm-1. The FTIR spectra of the samples demonstrate sharp absorption bands in the range of 3410-3450 cm-1 and 1636 cm-1 corresponding to stretching and bending vibrational modes of absorbed (O-H) water molecules on the surface of material. The characteristics FTIR band at 2925 cm-1 and 2847 cm-1 for C-H stretching and bending vibration mode of CH2 group of glycine. The small peak at 874 cm−1 is due to glycine precursor, which may be present on the CMO surfaces. The peak position at 1795.4 cm-1 corresponding to the


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symmetric stretching of the C=O carboxylic group, and 2343 cm-1 assigned for CO2 molecules in the air and 1160 cm-1 were assigned to C–N stretch, 1424.4, for C=O, OH (bend deformation) group of glycine precursor. It should be pointed out here that the presence of all such bands has not been considered as the contamination of the nanoparticles, which rather suggests the presence of surface (surface modification) absorbed species on of Nano- crystalline (NC), as confirmed from SEM. A band arising at 1383 cm-1 demonstrates the presence of N-O modes of vibration of HNO3 used in the sample preparation.34 The band around 490-750 cm-1 in the figure print region, corresponds to the characteristics Mn-O bond (stretching and bending).35 The band that appears at 480 – 600 cm-1 and 600-750 cm-1 confirmed that samples strongly comprise MnO bending and stretching mode of vibration respectively. Stretching vibration is responsible for the change in Mn-O-Mn length while bending vibration involves the change of Mn-O-Mn bond angle. The absorption peaks at 3277, 1636 and 1382 cm-1 wave numbers completely vanished in the sintered sample for 12 h and 15 h. The comparative study of samples series as shown in Figure 2 suggests either higher or longer calcination temperatures or times are required to increase the crystallinity of sample phase respectively. Thus it is supposed to be stiffening of the network with structural rearrangement lead to the perovskite phase formation.

X-Ray Diffraction analysis The phase evolution of the CMO has studied through the powder X-ray diffraction (XRD) pattern as shown in Figure 3. The heating of the as prepared (ASP) powder at different temperature whose upper limit (Tmax: 975 °C) conditioned by the melting temperature of the compound. The sample is calcined either at 800 °C or 900 °C for 10 h, contains appropriate amounts of impurity phases like Mn2O3, Mn3O4, and CaMnO3 along with the CMO phase



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(Figure not shown). This possibly due to the metastable or thermodynamically stable conditions of secondary phases. To avoid these impurities phases formed the precursor were ground and then the product was calcined consecutively at 800 °C, 925 °C, and 950 °C respectively in air for 10 h with intermediate grindings. Finally, the resultant powder split into two batches each was pelletized with PVA binder and sintered at 970 °C for 12 h and 15 h respectively to obtain the pure phase. In the present discussion, the system CaMn7O12 designated as CMO and the sintered with 12 h and 15 h of the system are assigned as 12 h and 15 h respectively. Figure 3a and b also shows that the XRD pattern of CMO sintered system contains almost no impurity phases. The diffractions patterns were indexed on the basis of the trigonal structure with space group R-3 of CMO (JCPDF-84-0191). The methods applied before that require a very high calcined and sintering time to obtain the pure phase but such a long duration still contains secondary phases. Often a literature reported earlier are summarized in Table S1 (SI) where the power synthesized via Pechini method, sol-gel method and solid-state method, contain 4-5 Wt. % of Mn2O3, Mn3O4 and CaMnO3 secondary phases even after consecutively calcined at 800 °C, 925 °C, 950 °C each were either for 48 h, 50 h or 64 h respectively. Further, it sintered at 970 °C for 64 h. Therefore, the applied chemical combustion route could be a useful method for the synthesis of CMO, as this technique avoid impurities and eliminates the requirement of larger calcination and sintering time. Moreover, the crystallinity of the sample increases with increasing sintering time. The average crystallite size was calculated by using Scherrer formula, . = 01 /3456 #7 (4) where D average crystallite size, λ is wavelength, k = 0.89, θ is the Bragg’s diffraction angle of the planes and βhkl is the corrected full width at half maximum (FWHM). The above X-ray


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powder diffraction pattern are performed at the wavelength of 1.54 A° with the scan rate of 3°/min and a step size of 0.02° from 20° to 90° of 2θ range. For βhkl correction, βinst (FWHM due to the instrument) is removed using a Si standard. The FWHM values are 0.1748° (0.0030508 rad), and 0.161° (0.00281 rad) at the 27 position of 34.26° for 12 h and 15 h sintered samples respectively. The average crystallite size obtained from Scherrer formula are found to be ∼48 and 61 nm for the CMO annealed at 970 °C for 12 h and 15 h samples, respectively. The average crystallite size of the samples increases with sintering time.

In this study, the powder diffraction pattern was indexed as per the trigonal crystal structure space group R-3 Figure 3a-b. Further detailed Rietveld refinement performed on the XRD data after excluding instrumental broadening using fullprof software shown in Figure 3c-d on sintered CMO powder for 12 and 15 h at 970 °C. The peak profile was modeled using pseudo-voigt function and background was described in term of six coefficient polynomial. The Rwp (weightpattern factor) and goodness–of–fit are used as a numerical criterion of the quality of fit of experimental diffraction data to calculated one. The obtained results from refinement is summarizing in Table 1. It found that the cell volume slightly increases for 15 h sintered samples in comparison to the samples sintered at 12 h. Figure 3e is polyhedral crystal structure representation of CMO compound. CaMn7O12 is a derivative of AC3B3B′O12 (A = Ca+2 and C = Mn+3, B=Mn3+, B′=Mn4+) with trigonal crystal structure. Its crystal structure comprises mainly of four building block units namely, CaO12 polyhedra, Mn+3O4 square planar, Mn+3O6 octahedra and Mn+4O6 octahedra respectively. It the Mn+3O6 octahedra of B site which play a significant role in structure distortion via Jahn-Teller Distortion.



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Microstructure characteristics SEM Study Morphological aspects of the CaMn7O12 (CMO) nano-crystalline are investigated by Scanning Electron Microscope (SEM) and Atomic Force Microscopy (AFM). The typical SEM microstructure of CMO 12 h and 15 h at 970 °C sintered samples shown in Figure 4. As shown in Figure 4a−d, a typical bimodal grain size distribution, with small grains of several micrometers distributed among the larger grains of several tens of micrometers and moreover a dense microstructure was also developed significantly with sintering time. All the micrograph clearly indicate that there are agglomerated particles, in which one particle contains many small crystallites, while the particles are pseudo-spherical in shape. Histogram of particle size distribution are shown in AFM microstructure panel Figure 5c and 5f. The porosity of the samples is evaluated from experimental (bulk) density using Archimedes principle. The resultant experimental densities are 4.47 and 4.59 g/cm3 which are 87.4 and 89.7 % of theoretical density 5.119 g/cm3 and 5.120 g/cm3 and the respective % porosity are 12.6 and 10. 3 of sintered sample at 12 and 15 h (for more detail see SI ref. 4 & 5). To investigate the chemical homogeneity of the samples quantitatively the Energy dispersive Xray (EDX) analysis of 12 and 15 h CMO sintered ceramic are carried out as shown in Figure 4ef. The gold coating was performed for increasing the conductivity by using ion beam sputtering which was necessary to avoid charging of the samples. The normalized atomic percentage from SEM –EDX analysis is found to be very much close to that the expected (theoretical) ratio are Ca 5.07, Mn 36.43 for 12 h and O 58.50 and Ca 5.90, Mn 42.54 and O 51.56 % for 15 h sintered


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respectively. The stoichiometric ratio of Ca: Mn was found to be 1:7.18 and 1:7.21 comes out from these percentage atomic ratios of two different sample respectively which are quite close to the expected stoichiometric ratio 1:7. Moreover, no additional impurities were noticeable, indicating that prepared CMO manganite is chemically pure in the composition prepared via chemical combustion route. Quantitative analysis, which gave the atomic percentages of the various elements, is shown in tabular form in the inset of Figure 4 e-f. The elemental colour mapping of corresponding elements of CMO shows uniform distribution (where bright red colour spot used for O, green for Ca and yellow for Mn respectively, which again confirm the successful synthesis of the sample. AFM Study Atomic Force Microscopy investigated the surface morphology and topography of the CMO pellet. Figure 5 panel a, b, d and f shows 2D and 3D AFM images of the sample sintered at different annealing time. The AFM image show that pellet sintered at two different sintering time are non-uniform distributed crystallite as already depicted from SEM morphology with porous microstructure. However, the measured average and root mean square surface roughness which was abbreviated as Rav. and Rsq. values increases with the increase of sintering time from 54.8 nm and 68.7 nm to 85.6 nm and 109.7 nm for 12 h and 15 h sintered CMO respectively. This change in surface roughness with annealing time may be due to grain growth during thermal annealing. It stated in the literature that oxygen vacancies mostly cause surface roughness of oxide. In this study, surface roughness increases attributed to recrystallization process and oxygen vacancies as well as surface defects. The metal or oxygen defects at the grain boundary favor the merging process at a higher temperature by stimulating the coalescences of more and more grains during sintering.36 Further Average Particle Size (APS) distribution is depicted from



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AFM images and fitted with a Gaussian function. The results shown in Figure 5c and f represent the histogram plot for the particle size distribution of 12 and 15 h sintered CMO respectively. From the Figure, it observed that the average particle size of the agglomerated particles increased from 0.8 and 1.6 µm with sintering time for 12 h to 15 h CMO samples, respectively. While comparing the resulting average particle sizes from AFM and XRD again confirms the agglomerated and bimodal microstructure which are already consistent with SEM micrograph. UV-Vis Spectra Various optical methods such as X-ray photoelectron spectroscopy and UV-Visible absorption spectra used for confirming the increased concentration of oxygen vacancies. The optical properties and band gaps of CMO sample sintered at 970 °C for 12 and 15 h are obtained from ultraviolet-visible (UV-Vis.) absorbance spectra Figure 6. The band gap of the prepared sample can be calculated from the optical absorption edge using Tauc’s equation35 as follows: 8ℎ9 = -(ℎ9 − ; )< (5)

where A, α, ν and Eg are constant of proportionality, absorption coefficient, light frequency, and band gap respectively. Whereas the value of exponent n depend on the nature of the sample transition, where n=1/2 for a direct and n=2 indirect allowed transition respectively and all other values of n are forbidden transition. While absorption coefficients (α) calculated from the following equation: 35

8 = 2.303(-/*) (6)


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A is the optical absorbance and d is the thickness of the sample. In our case, the Band gap Eg of the sample is calculated by plotting (αhν)2 as a function of incident light (hν) and by extrapolating the linear portion of (αhν)2 to zero of the energy hν suggesting direct transition. The intercept of the plots on the energy axis give the band gaps (Eg). The obtained band gaps Eg from these plots are 1.68 and 1.38 respectively with increasing sintering time from 12 h to 15 h. This decrease (narrowing) of band gap between valence band and conduction band (red shift) with sintering time attributed to possess increased concentration of surface oxygen vacancies. Thus in this way we can conclude that with the increase of annealing time, the content of surface oxygen vacancies increases and that of the band gap can efficiently reduce by the introduction of oxygen vacancy between the valence band and the conduction band.36 Figure 6 shows the change in band gap value and corresponding resultant schematic band structure. However, these decrements of the band gap (Eg) i.e. red shift are energetically favouring Since electron transition from O2- to Mn3+ is a low energy change compared with that from O2- to Mn4+. Thus the conversion from Mn4+ to Mn3+ takes place via charge compensation mechanisms which accompanied with formation of oxygen vacancies.

XPS Spectra X-ray Photoelectron Spectroscopy (XPS) is a very sensitive technique for determining the oxidation states of the constituent elements present in the sample represented in Figure 7. All binding energies (B.E.) of the samples referenced to the neutral carbon C 1s peak, which is assigned the value of 284.6 eV to compensate the surface charge effects. A full survey scan of CMO confirms the presence of Ca, Mn and O elements on the surface of the samples in Figure 7.1a. The carbon peak attributed to the residual carbon from the sample or adventitious hydrocarbon from the XPS instrument itself. High-resolution XPS core level B.E. spectra of Ca



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2P, O 1s and Mn 2p regions have been shown in Figure 7.1b-d and 7.2b-d for CMO 12 h and 15 h sintered sample respectively obtained in the range of 0 – 1000 eV. The XPS peak fitting and background subtraction have been performed using the software XPS-PEAK 4.1. The experimental results clearly demonstrate that there exist core level shifts of binding energy for CMO with sintering time. Figure 7.1b represents the deconvoluted XPS corresponding to Ca 2p with a core B.E. of 347.21 and 350.67 eV for 2p3/2 and 2p1/2 while it will be 345.96 and 349.74 eV for 15 h sintered CMO respectively. The full widths at half maxima (FWHM) of these peaks are 1.9 and 2.2 eV respectively for 2p3/2 and 2p1/2. With increasing sintering time from 12 to 15 h, there are slight shifts in binding energy (B.E.) values towards the lower side. But the intensity ratio which is (represented by Ica = B.E.2p3/2/B.E.2p1/2) and also the FWHM (full width at half maxima) value for Ca 2p3/2 and Ca 2p1/2 peaks in both the samples do not change significantly. Based on the above results it is inferred that the calcium ions stabilizes in +2 oxidation states only in both the samples.37 Figure 7.1c and Figure 7.2c depicts the XPS spectrum in Mn 2p core levels for sample sintered for 12 h and 15 h respectively. The main peaks at 642 and 654 corresponds to Mn2p1/2 and Mn2p3/2 respectively are seen. These splitting of Mn 2p peak into 2p3/2 and 2p1/2 occur due to spin orbit coupling. The value of spin orbit splitting absorbed to increase with the increase of sintering time. Besides, a shoulder located at higher energy is observed along the broadness of peak for Mn2p3/2 and Mn2p1/2 as indicated by arrows which mean that there are contributions from the mixed valent state of Mn ions. Due to this, the peaks of Mn2p3/2 and Mn2p1/2 can further deconvolute into two peaks each. The deconvoluted peaks of Mn 2p regions for CMO 12 h sintered at 970 °C have shown in Figure 7.1c. The deconvoluted peaks of Mn 2p3/2 at 640.88 and 642.85 eV (and Mn 2p1/2 at 652.48 and 654.54 eV) represent Mn3+ and Mn4+ respectively.38


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This confirms that Mn exists in dual oxidation states (+3 and +4). Figure 7.2c shows that the Mn 2p3/2 shifts towards lower binding energy with sintering time. The energy difference between Mn2p3/2 and O 1s used to determine the mean Mn oxidation states which decrease with sintering time, i.e., ∆E~112.10 and 111.75 eV for 12 and 15 h respectively. The lower value of ∆E2p3/2-O 1s attributed to the larger number of Mn

3+

ions for 15 h sample. So the number of Mn3+ ions

increases with the increase of sintering time. Figure 7.1d and 7.2d depicts the O 1s core level XPS peaks of the CMO samples sintered for 12 h and 15 h respectively. The O peaks as shown are slightly asymmetric at 530 eV, and the peak deconvoluted into two symmetric Gaussian curves represented as Oa and Ob peaks in Figure 7.3a-b. These two O 1s signal clearly shows two different surface oxygen species. The lower binding energy peaks Oa located at 529-530.7 eV attributed to lattice oxygen (Oa) at normal lattice sites in the structure. This O 1s spectrum provides information about the hybridization of O 2p with transition metal ions, whereas the high binding peaks (Ob 531-532.8 eV) is assigned surface adsorbed oxygen or hydroxyl species (Os + OHs) or other radical, such as CO or CO2, in the oxygen deficient regions. However, the asymmetric nature of the high energy (Ob) in the O 1s XPS spectrum is typical of the presence of oxygen vacancies. The relative peak area ratio of Ob/Oa is used roughly to assess the number of oxygen vacancies39 in Figure 7.3a-b; from which it apparently found that the oxygen vacancies increased with increasing sintering time at higher temperature. These oxygen vacancies are related to the reduction of Mn4+ to Mn3+ ions during sintering in air at higher temperature and are formed to maintain the charge neutrality according to the following reactions mechanism ,

?@ = A?•• + + 2 C

(7)



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,

(J J C C 6EFFGHI + 46EFFGHI = A?•• + + 26EFFGHI + 36EFFGHI

(8) This increments accompanied by the increase in the Mn3+ ions mostly attributed to oxygen vacancies; one of the principal difference between the XPS results and the UV-Vis. absorption spectra with respect to the reduction in Mn4+ ions and the formation of oxygen vacancies on account of increasing sintering time is that the band shifts in the absorption spectra in response to the generation of Mn3+ originates from changes in the bulk and on the surface, whereas the XPS results are mainly concerned with the surface changes only. Thus the oxygen vacancies are tuned between Mn3+/Mn4+ ions as observed in XPS analysis. Magnetic Study In order to investigate the magnetic properties of CMO multiferroic, the temperature dependent DC magnetization was performed in both zero field cooled (ZFC) and Field Cooled (FC) in a temperature range of 2 - 300 K with 0.1 Tesla (T) as an applied field at the time of measurement shown in Figure 8. For the ZFC measurement, the sample was first cooled from room temperature down to 2 K in the absence of magnetic field. Thereafter applying the field at 2 K, the magnetization was measured in the warming cycle. In a similar way for the FC measurement, the sample was cooled from room temperature down to 2 K but in the presence of magnetic field and magnetization was measured in the warming cycle under the same applied field of 0.1 T. Upon cooling first, a small kink in FC at TN1~90 K indicating appearance of antiferromagnetic (AFM) phase transition shown in inset of Figure 8.2a.

40-41

The ZFC and FC curves grows

rapidly when the sample is cooled further below TN2 ~ 50 K as seen from M-T plot in Figure 8.1a and 8.2a. The M -T curve displays the characteristic coexistence of both Antiferromagnetic + ferromagnetic behaviour below TN1 90 K called Neel temperature. It has been absorbed from


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Figure 8.1a and b that bifurcation of ZFC and FC curve takes place at a certain temperature so called irreversibility temperature. The FC curve shows saturation at about 5 K; with the typical shape of ferro (or ferri) magnetic behaviour whereas the ZFC curves display broad maxima (cusp-like shape) at certain maximum temperature Tmax so called freezing or spin glass (Tg) or blocking temperature (TB) reflecting the existence of strong irreversibility below a certain temperature Tirr

Evidence of Enhanced Oxygen Vacancy Defects ... - ACS Publications

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