Accepted Manuscript Structural, electronic and magnetic properties of Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) system Masroor Ahmad Bhat, kumar Devendra, Aga Shahee, N.K. Gaur PII:
S0925-8388(15)31690-X
DOI:
10.1016/j.jallcom.2015.11.132
Reference:
JALCOM 35987
To appear in:
Journal of Alloys and Compounds
Received Date: 16 July 2015 Revised Date:
16 November 2015
Accepted Date: 19 November 2015
Please cite this article as: M.A. Bhat, k. Devendra, A. Shahee, N.K. Gaur, Structural, electronic and magnetic properties of Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) system, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.11.132. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract The higher MR% of ~99.99% and TCR% of 43% resulted due to silver doping can be used to
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tune the sensing ability of spintronic devices and magnetic sensors.
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Structural, electronic and magnetic properties of Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) system *
Department of Physics, Barkatullah university, Bhopal (M.P.) – 462026 (India)
b
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a
Masroor Ahmad Bhata, Devendra kumarb, Aga Shaheeb and N.K. Gaura.
UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore (M.P.) – 452001(India)
*
Corresponding author’s address: Superconductivity Research Lab., Department of Physics
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Barkatullah University, Bhopal 462026, India.
Tel. No. +91-755-2517117, +91-9630290348; Fax No. +91-755-2491823 Corresponding author’s email:
[email protected]
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Abstract
We have synthesized the single phase polycrystalline sample of silver doped disordered Sm0.55Sr0.45-xAgxMnO3 using the conventional solid state method. They have been investigated by X- ray diffraction for phase evolution and their Rietveld refined x-ray
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diffraction pattern indicates an increase in lattice parameters and unit cell volume with Ag doping. The magnetization analysis shows the presence of a first order paramagnetic to ferromagnetic transition; the transition temperature and amount of thermal hysteresis
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decreases on Ag doping. The transport, magneto-transport results indicates decrease in metal to insulator transition, drastic increase in resistance and temperature coefficient of resistance
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(TCR %) with nearly unaffected colossal magnetoresistance effect which indicates that at least a part of silver is incorporated in the lattice. Scanning electron micrographs indicate that silver has also increased the grain growth sharpness and thus decrease the effect of grain boundary scattering. The significant value of TCR of ~43% of our samples indicates their applied nature for bolometric application. Keywords: Rietveld refinement, Metal - Insulator Transition Temperature, Temperature Coefficient of Resistance, First Order Magnetism
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I.
Introduction The preceding century observed that perovskite manganites have attracted enormous
interest for their colossal magnetoresistance (CMR) effect. Apart from the CMR effect, the
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manganites also exhibit intriguing physical properties such as insulator-metal transition (TIantiferromagnetic-ferromagnetic transition, structural transition, charge/orbital order
(CO/OO), phase separation induced by application of magnetic field, photon radiation,
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temperature change etc [1-5] . The CMR is mostly observed in manganites with maxima (peak) at ferromagnetic transition temperature (TC). The efforts have been made to enhance
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the CMR value for wider temperate range where maximum CMR can be observed (i.e. peak broadening) for its potential applications in magnetic sensor, read head memory, and information storage devices [6, 7].
Sm0.55Sr0.45MnO3 (SSMO) system has unique features among all the manganites because it is very close to charge/orbital order (CO/OO) instability and shows the most abrupt
xSrxMnO3
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dramatic insulator - metal transition on tuning the external parameters [8, 9]. In Sm1the competition among spin, charge, lattice and orbital degrees of freedom leads to
a complex and multi-critical phase diagram [10]. The various phases can be tuned by
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appropriate dopants, application of external magnetic field, pressure, electric field etc [1116]. The charge ordering (CO) occurs for a narrow composition region around x = 0.5 and
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can be easily suppressed by application of magnetic field. Around x ~ 0.45, a sharp first order transition from paramagnetic insulating (PMI) to the ferromagnetic metallic (FMM) state with dramatic CMR is observed. Numerous attempts have been made by various groups to enhance its CMR properties [8, 10, 11, 14, 17-28]. In this system CMR is mainly correlated with phase separation arising due to site disorder and phase inhomogeneity [11]. Srivastava et al [19] has found the large enhancement in TC and metal-insulator transition temperature (TM-I) in Sm0.55Sr0.45MnO3 annealed in oxygen, which can be explained as a
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consequence of correlated quenched disorder caused by the effect of spatially non uniform strain and ordered oxygen vacancies. Egilmez et al [8] has suggested that higher sintering/annealing process can increase the grain size of the Sm0.55Sr0.45MnO3 which causes
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major enhancement of its magnetoresistance, decrease of both the resistivity and the resistive peak width at the TM-I. The first order nature of the transition is generally argued because of the result of the enhanced antiferromagnetic metallic/charge ordered (AFM/CO) fluctuations
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above Tc [29]. Moreover with the increase in particle size, the stability of AFM/CO states enhances which is attributed to the sharpening of the first order transition in the SSMO
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system [30]. So there are several unresolved issues in distorted Sm0.55Sr0.45MnO3 system which needs to be explored in order to make them more promising from application point of view. The purpose of our study is to overcome this necessity by substituting silver in this compound to remove the chaos factor so that new novel mechanisms may arise. In this paper, we present the investigations of the effect of silver doping on the
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structural, transport, magneto-transport and magnetic properties in Sm0.55Sr0.45MnO3 (SSMO). Our results show for the first time, that the intrinsic properties of SSMO can be tailored via monovalent Ag induction. The Ag doping in SSMO enhances the TCR
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mechanism which is an important agent for application in bolometric sensors. Experimental details
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The polycrystalline Sm0.55Sr0.45-xAgxMnO3 samples (x = 0.00, 0.05, and 0.10) were prepared by conventional solid state reaction route. High purity chemicals of Sm2O3 (Merck 99+), SrCO3 (Aldrich 99.999%), AgO (Aldrich 99.9%) and MnO2 (Aldrich 99%) were mixed in stoichiometric ratio and thoroughly grinded for hours and then calcination was done at 950°C for 24 hours. The resulting calcined powder was re-grinded to ensure the homogeneous phase formation and finally pressed into pallets which were sintered at 1200°C for 24 hours. Powder x-ray diffraction (XRD) patterns of the samples were recorded using a Bruker AXS D8
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advance diffractometer, with Cu-Kα (λ=1.56406Ả) radiation operating at 40 kV/100 mA. The data collected from 20o to 80o in 2θ range with steps size 0.02o and a counting time 15s/step. The surface morphology was studied at room temperature on a JSM-6400 apparatus working
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at 20 kV. The temperature dependent transport and magneto-transport measurements were carried out by four probe method in Oxford magnet 8T system using indium soldering contacts over a temperature range 5 K – 300 K. In addition the DC magnetization
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measurements were carried out using a vibrating sample magnetometer (PPMS made by M/S Quantum design) at temperature 2 K ≤ T ≤ 300 K under Zero field cooled (ZFC) and field
at 5 K. 3.
Results and discussion
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cooled (FC) regimes and the field dependence of magnetization curve (up to 8 kOe) recorded
The Rietveld refined X–ray diffraction patterns for the bulk Sm0.55Sr0.45-xAgxMnO3 (x = 0.00, 0.05, 0.10) samples at room temperature are shown in Fig.1. The XRD data were
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refined using the FullProf Suite Rietveld refinement Program. The space-group used for refinement was Pnma. All the observed peaks were very nicely accounted by Pnma phase. We could not observe any un-accounted peak which confirms the formation of single phase
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solid solution of Sm0.55Sr0.45-xAgxMnO3 (x = 0.00, 0.05, 0.10) with orthorhombically distorted perovskite structure having Pnma space-group. The lattice parameters and unit cell volume
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based on the final refinement obtained by us are listed in table 1. The lattice parameters and unit cell volume increases with the increase of silver composition which indicates that the Ag+ has been incorporated in lattice by replacing Sr2+. This is an effect of larger ionic size of Ag+ compared to Sr2+. This is further supported by un-evidence of any impurity peak in the XRD patterns. The Mn - O - Mn bond angle perpendicular to ab- plane (i.e. ferromagnetic plane) first decreases and then increases, while the bond angle parallel to ab- plane increases first and then decreases on enhancing the Ag doping indicating the non-linear variation of
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bandwidth and strength of DE interaction. The tolerance factor (t), average cationic radius and size variance (σ2) increases on enhancing the Ag content [31]. The increase in tolerance factor signals a decrease in global structural distortion while the increase in σ2
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represents the enhancement in local structural distortion. In this case local deformations of the oxygen octahedron around the Mn ion can also be presented as a superposition of the tetragonal mode (Q3) and the orthorhombic mode (Q2) of Jahn – Teller (J-T) distortion. The J-
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T distortion (JδT) have a direct correlation with Mn-O bond distance. The non-monotonically variation of Mn-O bond distance led to non-monotonically variance in Jahn – Teller (J-T)
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distortion with the Ag doping see table 1. The variation in the structural parameters and J-T distortion with Ag doping plays a vital role in changing the electronic and magnetic properties of these systems [32].
The direct evidence of the change in microstructure of Ag substituted samples as revealed through scanning electron microscopy images are shown in Fig. 2. The variation of
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silver composition has more profound effect on the grain size and shape. The pure sample SSMO had spherical like grains while the doped ones show an enhancement in the grain size with flake shaped grains each having size of the order of 5 µm suggesting that Ag act as a
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catalyst in the process of grain growth.
The temperature dependence of resistivity (ρ-T) of the bulk samples Sm0.55Sr0.45(0.00 ≤ x ≤ 0.10) at zero field is displayed in Fig. 3 (a). The resistivity of the
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insulating regime (and the maximum resistivity ρmax) increases with the Ag doping while the resistivity of the metallic regime first decreases (for x = 0.05) and then increases (for x = 0.10) with Ag doping. The TMI varies from 134 K to 123 K for x = 0.00 to x = 0.10. The metal insulator transition of Ag doped sample at x = 0.05 is relatively sharp in comparison to un-doped and Ag doped samples at x = 0.10. It is interesting to note that as we induce the Ag doping level, the resistivity in the insulating state increases which might be due to the 5
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enhancement in Mn+4/Mn3+ ratio and A-site quench disorder. Mn+4:Mn3+ ratio from 45:55 towards
The enhancement in
50:50 and 55:45 due to 5 and 10% Ag+ doping
respectively drives it towards composition which mostly favours an insulation ground state
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[15]. While enhancement in grain size reduces the grain boundary density and defect levels, thus reducing scattering effects due to grain boundary and defect, which lead to reduction in the resistivity of insulating state [33]. In our case an enhancement is resistivity indicates that
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the effect of increase in Mn+4/Mn3+ ratio and A-site quench disorder is overwhelming the grain boundary effects. Similarly in the metallic regime, the non-monotonic variation of
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resistivity with grain size further suggests that the observed change in resistivity is not only due to grain size variation and the other factors, like Mn+4/Mn3+ ratio and A-site quench disorder, J-T distortion, Mn - O2 - Mn bond angle (which determines the strength of ferromagnetic DE interaction) also play a significant role in determining the metallic state resistivity. The Mn - O2 - Mn bond angle and J-T distortion first increases and then decreases
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on enhancing the Ag doping (See table 1)[34].
Fig. 3 (b) shows the temperature dependence of magnetoresistance percentage (MR%) of all the Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) samples obtained using the formula MR% =
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[(ρ0 – ρH)/ρ0]*100, where ρ0 and ρH are the resistivity at zero and 5T magnetic fields respectively. Inset of fig 3 (b) represents the temperature dependent TCR. All three samples
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show a strong CMR effect, with a maximum (around 99.99%) in magneto resistance observed around 130 K. The Ag doping affects the width and the sharpness of the magneto resistance peak. There is smooth spin tunnelling across the grains boundary and suppression of the disordered magnetic fluctuations with increase in field. The variation in sharpness of MR% with Ag content can be correlated with the variation in Mn – O - Mn bond angle and JT distortion.
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The nature of the metal-insulator transition can be clearly visualized by evaluation from temperature coefficient of resistivity (TCR) given by TCR% = (1/ρ)(dρ /dT)*100 depicted in inset of fig. 3 (b). The TCR peak value of pure SSMO is only ~11% while as it rises abruptly
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~ 43% at 5% and ~ 25% at 10% Ag doping respectively. The resistivity peak width becomes sharper and it increases with Ag concentration and peaks broadened with soft relaxation as compared to the pristine sample. At 5% Ag doping, the transition width become narrower
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and increases the polaron binding energy resulting in high TCR value [35]. Thus Ag doping has been able to enhance A-site quench chemical disorder, a key parameter for stabilizing
enhance CMR and TCR effect.
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electronic phase separation, and extrinsic (grain boundary) parameters which are necessary to
The field cooled cooling (FCC) and field cooled warming (FCW) magnetization data (M T) of Ag doped SSMO (0.00 ≤ x ≤ 0.10) at 500Oe and 1T is shown in figure 4 and its inset (a). The paramagnetic to ferromagnetic (PM - FM) transition temperature (Tc) (determined by
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dM/dT) decreases on increasing the Ag doping and thus confirms that Ag+1 ions are incorporated in lattice (See inset of figure 4 (b)). A similar trend was also observed in TM-I. The decreasing in the transition temperature on enhancing the Ag doping is related to the
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enhancement in concentration of Mn4+ ions which moves the system away from the optimum doping (i.e. Mn4+~45%) for double exchange (DE) ferromagnetism [15, 36-37]. The
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paramagnetic to ferromagnetic transition is associated with a thermal hysteresis suggesting the first order nature of the transition. This first order nature of magnetic transition indicates possibility towards a strong spin-lattice coupling in these compositions. The sharpness of the transition decreases on increasing the Ag doping which further can be correlated with increase in A-site quench disorder. The enhancement in disorder has been known to be a well-established parameter for broadening the first order phase transition. This suggests that
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the possible reason of the PM - FM transition broadening is the enhancement of quenched disorder due to difference in effective ionic radii of Ag from Sr and Sm. The magnetization of 0% or 10% Ag doped SSMO in the ferromagnetic state is
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unexpectedly higher than the 5% Ag doped SSMO. This behavior cannot be explained only on the basis of DE mechanism. Here, we note that the 5% Ag doped SSMO has a higher J-T distortion compared to the 0 and 10% Ag doped SSMO. The J-T distortion competes with DE
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and the enhancement of J-T distortion weakens the DE ferromagnetism. This suggests that the decay in magnetization of the Ag doped SSMO in ferromagnetic state is related to J-T
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distortion. At 500 Oe, the magnetization of 10% Ag doped SSMO in the ferromagnetic state lies between 0 and 5% Ag doped SSMO, but at 1T the magnetization of 0 and 10% Ag doped SSMO nearly coincides. These results indicate that the relatively weak J-T distortion in 10% Ag doped SSMO get suppressed on application of high magnetic field. All three samples indicate the presence of some antiferromagnetic like ordering below 50 K which is also
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suppressed at higher field. The hysteresis behaviour of Ag doped Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) measured at 5 K is displayed in Fig. 5. It can be seen from Fig. 5 that the metallic phase of pure sample is showing ferromagnetic behaviour. The magnetization
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decreases as we increase the doping content which results in the decrease of coercivity, remnant magnetization and thus drives the system towards competing ferro-antiferromagnetic
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state. The magnetization of pure SSMO tends to saturate at very low fields. However the saturation can not be reached even at higher fields for x = 0.10 doped samples. This mechanism demonstrates the idea that the ferromagnetic interaction is weakened in the doped samples. This can be well correlated with the phase diagram of Korbakov et al [15], which have shown that ferromagnetic state is favoured only for Mn4+ concentration close to 45% (i.e around Sm0.55Sr0.45MnO3), where an enhancement in Mn4+ will drive it towards a magnetic phase separated or AFM ground state. Thus in our case, Ag doping lead to
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enhancing the Mn4+ % and drives system towards a magnetic phase separated state. On the basis of above, we suggest that the magnetic disorder caused by the addition of silver is responsible for an enhanced magnetotransport and suppression in ferromagnetic phenomena.
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4. Conclusion In summary we have successfully synthesized Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) system via solid state reaction route and studied the effect of silver doping on physical
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properties of Sm0.55Sr0.45MnO3 organization. It has been observed that the Ag doping strongly influence the structural, magnetic and electric properties of the system. The intrinsic Ag agent
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suppresses the structural distortion, enhances the grain size, Mn4+ /Mn3+ ratio, and causes a nonlinear variation in Mn-O-Mn bond angle and Jahn Teller distortion. The system exhibits a first order paramagnetic to ferromagnetic and a ferromagnetic to antiferromagnetic transition on lowering the temperature and both of these transition temperature decreases on silver doping. The insulating state resistivity increases on increasing the silver doping and can
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be correlated with increase in Mn4+/Mn3+ ratio and A-site quench chemical disorder. The resistivity of the metallic state exhibits a non-monotonic variation with Ag doping and can be described as the aggregative induction due to correlated grain size, J-T distortion, Mn-O- Mn
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bond angle and the ratio of Mn3+/Mn4+ ions. The large enhancement in TCR% on silver doping provides guiding information which will help in exploring and tuning of sensible
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devices and bolometric sensors. Acknowledgements
Masroor Ahmad Bhat is thankful to University Grants Commission (UGC), New Delhi for providing the financial support under the project No. F.7-19/2007 (BSR). The authors express sincere thanks and gratefulness to Dr. Mukul Gupta, Dr. Rajeev Rawat, Dr. D. M. Phase and Dr. Alok Banerjee, UGC-DAE CSR Indore for helping us to carry out the characterizations and fruitful discussions.
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Figure Captions:
xAgxMnO3
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Figure 1: Typical Rietveld fittings of the x-ray diffraction patterns of bulk Sm0.55Sr0.45(0.00 ≤ x ≤ 0.10) samples. The black solid circle and the red solid line correspond
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to the observed and calculated patterns, respectively. The difference pattern is given by the line below the experimental patterns. The vertical bars correspond to the Bragg reflections. Figure 2: SEM micrographs of Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) system Figure 3 (a): Resistivity vs Temperature dependent plots under 0T magnetic fields for the bulk Sm0.55Sr0.45-xAgxMnO3(0.00 ≤ x ≤ 0.10) samples and inset shows ρmax/TMI vs Ag%
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respectively.
Figure 3 (b): Temperature dependence of the MR% for Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) at doping level x = 0.00, 0.05 and 0.10 and Inset shows TCR% respectively.
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Figure 4: Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) in FCC and FCW run at 500 Oe. The inset (a) shows the same for 1 T. The inset (b) shows the variation of TC with Ag doping.
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Figure 5: Magnetization versus field at 5 K for Sm0.55Sr0.45-xAgxMnO3 (x = 0.00, 0.05, 0.10).
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Table 1. Structural, electronic and magnetic parameters of Sm0.55Sr0.45-xAgxMnO3 (0.00 ≤ x ≤ 0.10) system x = 0.00
x = 0.05
x = 0.10
a (Å)
5.42669 (9)
5.4298 (0)
5.4311 (7)
b (Å)
7.6887 (5)
7.6920 (3)
c (Å)
5.4266 (4)
5.4302 (7)
V (Å3)
226.4236 (4) 226.8016 (6) 226.9048 (7)
Mn – O1 – Mn (deg)
164.10
144.25
Mn – O2 – Mn (deg)
156.76
165.24
161.64
‹Mn – O1› (Å)
1.9388 (3)
2.02055 (0)
2.00550 (15)
‹Mn – O2›s (Å)
1.7779 (4)
1.75323 (17) 1.80829 (17)
‹Mn – O2›l (Å)
2.1390 (40)
2.1182 (2)
2.08142 (18)
δJT = (l − s)/((l + s)/2) 0.18438
0.188545
0.140437
Rp
33.1
30.3
32.0
Rwp
17.6
17.2
17.2
Re
13.5
13.3
14.0
1.695
1.673
1.503
Tolerance Factor (t)
0.97556
0.98301
0.990502
σ2
0.0099
0.010251
0.010264
TMI
134 K
133 K
123 K
MR%
96.32%
99.45%
98.90%
TCR%
11%
43%
25%
Tc (K)
137 K
133 K
127 K
TN (K)
35 K
31 K
29 K
7.6933 (2)
5.4304 (6)
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147.08
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Highights The lattice parameters and unit cell volume increases with silver concentration. The silver acts as a catalyst and hence enhances the grain growth in the The maximum MR% of 99.99% is obtained.
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system.
The high TCR% of 43% resulted due to silver doping useful for spintronic devices.
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The first order nature of magnetism is prevailing in our study.
