- Email: [email protected]
Structural, electrical, magnetic and thermal properties of Gd1–xSrxMnO3 (0.2≤x≤0.5) manganites
Physica B 479 (2015) 10–20
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural, electrical, magnetic and thermal properties of Gd1–xSrx MnO3 (0.2 r xr 0.5) manganites Nagaraja B.S a, Ashok Rao a,n, P.D Babu b, G.S. Okram c a
Department of Physics, Manipal Institute of Technology, Manipal University, Manipal 576104, India UGC-DAE Consortium for Scientific Ressearch, R5 Shed, Bhabha Atomic Research Centre, Mumbai 400085, India c UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2015 Received in revised form 1 September 2015 Accepted 13 September 2015
A systematic study on structural, electrical, magnetic and thermoelectric properties of bulk samples of Gd1–xSrxMnO3, synthesized by solid state reaction, is carried out in this communication. All the samples are in single phase with no detectable impurities. All the samples exhibit insulating behavior and the results indicate that high temperature electrical resistivity can be explained using small polaron hopping (SPH) model, whereas variable hopping model (VRH) model is valid in the low temperature regime. The magnetization results show that large irreversible magnetization is observed at low temperatures. The thermoelectric power measurement demonstrates that these samples exhibit colossal thermoelectric power. & 2015 Elsevier B.V. All rights reserved.
Keywords: Manganites Solid state reaction Rietveld refinement Magnetization Thermoelectric power
1. Introduction The rare earth mixed valent pervoskite manganites RE1 xAExMnO3 (RE¼ rare earth elements and AE¼a divalent ion) have been extensively studied as they exhibit huge magneto-resistance and magneto-capacitance which is due to the presence of spin, lattice, and charge order [1–10]. These properties provide a wide range of technological applications such as magnetic read head device, IR detector, magnetic storage and bolometric devices [11–15]. The divalent ions generate Mn4 þ ions in the materials, in addition to the Mn3 þ already present in the system. The Mn3 þ /Mn4 þ ratio along with the average ionic radius of the A-site cation and ionic mismatch at the A-site, modulates the physical properties of the manganites. The double exchange mechanism can explain the physical properties of these materials [16–18]. The substitution of AE elements with RE elements are essentially done due to the fact that they induce a transition from ferromagnetic (FM) to charge ordered anti-ferromagnetic (CO–AFM) state [10,19,20]. The presence of different magnetic phases at low temperature and strong magneto-elastic effect lead to such fieldinduced transitions which resemble meta-magnetic transitions. This exhibits strong field-induced effects in the dielectric properties of material [21]. Most of studies have been focused on the electrical and n
Corresponding author. Fax: þ91 820 2571071. E-mail address: [email protected] (A. Rao).
http://dx.doi.org/10.1016/j.physb.2015.09.025 0921-4526/& 2015 Elsevier B.V. All rights reserved.
magnetic properties of manganites of larger rare earth ions such as La, Pr and Nd [4–7]. Less work seems to have done on the electrical and magnetic properties of RE1 xAExMnO3 with smaller RE ions such as Gd, Dy, Tb and Eu. These smaller RE-based manganites are very interesting among the mangnite systems due to the complex and inter-twined effects of their electrical and magnetic properties [10,16–22]. The irreversible and sharp anomalies in the magnetostriction and magneto-resistance take place at low temperatures which have been attributed to the charge order state. The spin glass behavior is observed in low bandwidth materials essentially due to the presence of magnetic cluster [19,23]. In Dy0.7Sr0.3MnO3 system spin reversal [24,25] and negative magnetization have been observed as a result of a negative exchange interaction between RE and Mn sublattices [26]. The reversible sign change of the magnetization with temperature has been observed in the manganite, which is attributed to the anti-parallel coupling of the Gd moments with Mn [26,27]. The charge order is observed when hole doping is increased [28]. The spin glass behavior is observed in the half doped manganite which is attributed to the magnetic cluster present in the low bandwidth manganites [29]. Large value of thermoelectric power of the order of 35 mV/K has been observed in low bandwidth systems [30]. In our previous communication, colossal thermoelectric power was reported in the low band gap Dy1–xSrxMnO3 manganites. In particular the compound, Dy0.7Sr0.3MnO3 exhibited the highest value of thermoelectric power which is order of 224 mV/K [31]. The fractured glass state and AFM spin ordering at low temperatures are responsible for localization of charge carriers which manifests in the form of
N. B.S et al. / Physica B 479 (2015) 10–20
colossal thermoelectric power [32]. This motivated us to perform systematic investigation on another smaller ion system like Gd1 xSrxMnO3. It was thus describe to investigate structural, electrical, magnetic and thermal properties of Gd1 xSrxMnO3 (GSMO) manganites
2. Experimental details Samples with general formula Gd1 xSrxMnO3 (0.2rx r0.5) were prepared using the well-known solid state reaction method. High purity (99.99%) oxides and carbonate (Gd2O3, SrCO2, and MnO2) were mixed well for about 5–6 h which was followed by calcinations at 1100 °C for 24 h which was repeated thrice to ensure homogeneity of the samples. The powder was then pelletized and sintered at 1250 °C for 36 h. To confirm the purity of the samples and to determine the lattice parameters, X-ray diffraction studies were performed on the samples prepared under the
11
present investigation. The surface morphology of the samples was imaged using Scanning Electron Microscopy (SEM) attached with Energy Dispersive X-ray (EDAX) analyzer (EVO MA18 with Oxford EDS (X-act)). EDAX measurement was carried out to ascertain the elemental composition of dopants in the prepared samples. Temperature-dependent electrical resistivity ρ(T ) was measured using four-probe method in a closed cycle refrigerator (CCR). Keithley Current Source (Model 6221) was used to pass constant current through the current terminals, and the voltage across the voltage terminals was measured using Keithley Nanovoltmeter (Model 2182 A). The magnetic properties of the samples were investigated (between 5 and 300 K) using a superconducting quantum interference device (SQUID) magnetometer and 9 T PPMS based vibrating sample magnetometer (VSM) (both Quantum Design) in both zero-field-cooled (ZFC) and field-cooled (FC) conditions. The hysteresis loops (M versus H) of the samples were also recorded at several temperatures between 5 and 300 K. Thermoelectric power (S) measurement was performed in the temperature range of 5–
Fig. 1. XRD pattern for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. The observed intensities are shown as dots and calculated intensities are shown by solid line. Line at the bottom denotes the difference between the experimental and refined patterns.
12
N. B.S et al. / Physica B 479 (2015) 10–20
300 K, using differential DC method, taking oxygen-free highly conducting copper (OFHC) as reference. The details of thermoelectric measurements are described elsewhere [33].
3. Results and discussion 3.1. Structural properties Fig. 1 shows the room temperature XRD patterns of Gd1 xSrxMnO3 (0.2 ox o0.5) samples. The XRD data confirms that all the samples are in single phase and no impurities were observed within the experimental limits. The samples were indexed on the orthorhombic crystal structure with the Pbnm space group. We have analyzed XRD data using Rietveld refinement using Fullprof program. From the refinement, we have calculated the lattice parameter, unit cell volume and goodness of the fit. The observed results are shown in the Table 1. It can be seen that both the lattice parameters a and c are increasing with increasing x, on contrary the lattice parameter b is decreasing with increasing x. This suggests that the GSMO samples have distorted orthorhombic crystal structure. The amount of Sr substitution increases the hole doping of GSMO samples which increases the amount of Mn3 þ ions and this distortion is correlated with Jahn–Teller (JT) distortion which affect the Mn3 þ ion in the manganese octahedral [28,34]. Thus increase in concentration of Sr, increases Mn4 þ ion and hence lattice parameter b decreases. In addition, the unit cell volume is decreasing with increasing the value of x which is consistent with earlier reports [34,35]. This result suggests that the large value of Mn4 þ creates charge compensation in the Sr2 þ ion. The calculated Mn–O–Mn bond angle and Mn–O bond distance are also given in Table 1. It can be clearly seen that both the bond angle and bond distance decrease with increase in x. This results in enhancement of the super exchange interaction and lattice distortion in the GSMO samples. To estimate the average crystalline size of the samples, we have used the Scherer formula which is given by
D = 0. 94λ /β cos θ
(1)
Table 1 Summary of structural parameters and refinement parameters for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples. x System Space Group
0.2 0.3 0.4 0.5 Orthorhombic Orthorhombic Orthorhombic Orthorhombic Pbnm Pbnm Pbnm Pbnm
a (Å)
5.365 (0.001) 5.643 (0.001) 7.541 (0.001) 90° 228.38 (0.08) 2.60 3.64 2.27 1.97
5.3817 (0.0007) 5.5021 (0.0008) 7.615 (0.001) 90° 225.89 (0.05) 1.85 2.41 2.33 1.17
5.3906 (0.0007) 5.4312 (0.0008) 7.6357 (0.0008) 90° 224.38 (0.03) 2.00 2.66 2.28 0.986
5.397 (0.001) 5.402 (0.001) 7.6667 (0.0008) 90° 223.61 (0.03) 1.89 2.47 2.29 1.24
1.992 166.75
1.967 166.56
1.953 166.49
1.947 166.42
57.32
81.69
70.03
92.15
2027.0
1540.7
1163.0
1067.3
b (Å) c (Å) α ¼ β¼ γ Cell Volume (Å3) Rp rwp rexp Goodness of fit. χ2 Mn–O (Å) Mn–O–Mn (deg) Crystalline size (nm) from XRD Average Grain size (nm) from SEM
where λ is the X-ray wavelength, θ is the Bragg angle and β is the corrected full width half maxima of the XRD peaks of the samples. The estimated average crystalline sizes are given in Table 1. It is clearly seen that the crystalline size increases with increase in the concentration of Sr. The SEM images are presented in Fig. 2. From the SEM image, we can identify the grain size of samples, the grain sizes are given in Table 1. One can see that the particle size is decreasing with increasing the concentration of x. The comparison of XRD and SEM results reveals that the particle size determined using from SEM images are much bigger than that from XRD. This is due to fact that the SEM results give the size of secondary particles of the samples and XRD results provide the size of primary particles [36–38]. In order to estimate the composition of the compounds, EDS analysis results have been done for all the GSMO samples. The results are presented in Table 2. It is seen that the experimental composition matches well with the nominal composition. 3.2. Electrical resistivity and conduction mechanisms The temperature dependent resistivity for Gd1 xSrxMnO3 (0.2ox o0.5) samples are shown in Fig. 3. It can be observed that the electrical resistivity for all the samples increases monotonically with decreasing temperature. In order to understand the electrical transport properties of the GSMO system, we assume that electronic structure of the doping element plays a crucial role. It is clearly observed from the insets of Fig. 3 that the electrical resistivity decreases with increase in concentration of strontium. It is evident from Fig. 3 that all the samples exhibit high values of resistivity in semiconducting–insulating region which is attributed to lattice mismatch between Mn3 þ and Mn4 þ which is due to the spin glass behavior of the system [10,19,39] which in is turn related to Jahn–Teller distortion which favors insulating state due to localization of electrons [23,31]. This behavior is a typical trend seen in low bandwidth manganites and they normally require very high magnetic fields to exhibit the metal–insulator transition. The variation of electrical resistivity with Sr doing can be explained using double exchange mechanism. Increase in Sr content produces more Mn4 þ ion and hence Mn3 þ /Mn4 þ ion ratio deviates away from the optimum value of 0.5 and consequently resistivity value decreases. In order to understand the temperature dependence of electrical resistivity, there are three conduction mechanisms which can be used to account for the resistivity behavior viz. (1) thermal activation model (2) small polaron hopping model (SPH) and (3) variable range hoping (VRH) model. Thermal activation model is usually used to explain the conduction mechanism in oxide materials. This model is applicable when the carriers are magnetic polarons. If the thermal energy is sufficient to overcome the band gap the electrons becomes free to conduct. Resistivity can be expressed using the equation [31],
ρ = ρ0 exp (EA/kB T )
(2)
where T is temperature, ρ0 is the value of resistivity at infinite temperature, EA is the activation energy and kB the Boltzmann constant. The present results were analyzed using Eq. (2). One expects that a plot of ln ρ versus 1/T would be straight line. From Fig. 4, it is clear that the present results do not match with Eq. (2). Hence we conclude that the thermal activation model is not applicable for the presently investigated manganites. Similar results are reported in literature [31,40,41]. In order to understand the conduction mechanism in high temperature region (θD/2 oT) we have made an attempt to see the applicability of small polaron hopping (SPH) model. The thermally activated small polaron hopping model is based on the polaronic
N. B.S et al. / Physica B 479 (2015) 10–20
13
Fig. 2. SEM images of (a) Gd0.5Sr0.5MnO3 (b) Gd0.6Sr0.4MnO3 (c) Gd0.7Sr0.3MnO3 and (d) Gd0.8Sr0.2MnO3.
Table 2 EDS results for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. x
Element
At%
Nominal composition
Experimental composition
0.2
Gd Sr Mn Gd Sr Mn Gd Sr Mn Gd Sr Mn
17.93 3.29 17.85 13.55 5.26 24.33 12.65 7.82 22.64 10.58 7.95 20.96
0.80 0.20 1.00 0.70 0.30 1.00 0.60 0.40 1.00 0.50 0.50 1.00
0.84 0.16 0.90 0.72 0.23 0.95 0.61 0.38 1.01 0.57 0.48 1.08
0.3
0.4
0.5
model in high temperature region in mangnites. The SPH model can be described using the following equation:
ρ = AT exp (EA/kB T )
(3)
where T is the temperature, EA is the activation energy, A is a constant and it is given by A ¼2kB/3ne2a2ν. Here kB is Boltzmann's constant, n is number of charge carriers, e is electronic charge, a is
Fig. 3. Electrical resistivity variation with temperature for Gd1 xSrxMnO3 (0.2r x r0.5) samples. Inset shows the resistivity behavior in the temperature 150–300 K.
14
N. B.S et al. / Physica B 479 (2015) 10–20
Fig. 4. Variation of lnρ with 1/T for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. Fig. 6. Variation of lnρ with 1/T1/4 for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. Inset shows the fitting of experimental data using VRH model.
increase the number of effective charge carrier in the Sr doped systems [40–43]. Hence, we can conclude that the SPH model alone cannot explain the conduction mechanism for the entire temperature range. To validate the conduction mechanism in low temperature region (T o θD/2), we have fitted the temperature dependence of electrical resistivity using Mott's VRH model. The VRH model is applicable when the thermal energy is insufficient to excite the electrons to hop to their nearest neighbors, it is more favorable for the electrons to hop further to find a site with a smaller potential difference. The expression for VRH can written as
ρ = ρ0 exp (T0/T )1/4
Fig. 5. : Variation of ln(ρ/T) with 1/T for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. Inset shows the fitting of experimental data using SPH model. Table 3 Fitting parameters obtained from resistivity data for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. x
0.2 0.3 0.4 0.5
SPH model
VRH model
EA (meV)
A*10 5
T0*107
ρ0*10 8
N(Ef)*1025(eV 1 m 3)
152.75 122.29 123.77 125.02
17.63 10.38 5.43 2.75
8.88 5.81 2.85 3.42
1.14 4.86 0.21 0.12
2.57 3.93 8.01 6.67
the site to site hopping distance, and ν is the longitudinal optical phonon frequency. For validity of this model, a plot of ln(ρ/T) versus 1/T is expected to be a straight line. We have fitted the experimental data of the presently investigated samples using Eq. (3) as shown in Fig. 5 and a linear behavior is observed in the temperature range of 200–300 K. From the slope of the linear curve above θD/2, we have calculated the activation energy using Eq. (3) and the activation energy results are presented in Table 3. Here, we can observe that increasing the value of Sr content in the GSMO system, the activation energy gradually decreases due to the
(4)
where ρ0 is the residual resistivity and T0 is the characteristic temperature which is expressed as T0 ¼18α3/kBN(EF), where the N(EF) is the density of state at Fermi level, α is localization length and kB is the Boltzmann constant. In order to validate the VRH model for GSMO system, we have fitted the experimental data of resistivity using Eq. (4). Fig. 6 shows the behavior of lnρ versus T 1/4. The plots show linear trend in the temperature range 100–200 K suggesting that VRH model is applicable for temperatures below θD/2. Finally, we have estimated the value of N(EF) by taking α as constant (α ¼2.22 nm 1) [44,45] and using the T0 from the slope of Fig. 6. The calculated value of T0 and N(EF) are given in Table 3. It can be clearly observed that the T0 values decrease with increasing Sr doping and N(EF) value are increasing with increasing Sr doping. These values are found to be in good arguments with results reported in literature [40,46]. 3.3. Magnetization measurements The variation of magnetization with temperature of GSMO samples were carried out using zero-field-cooled (ZFC) and fieldcooled (FC) conditions at two different fields viz. 50 Oe and 200 Oe. The temperature dependent magnetization at applied field of 50 Oe is shown in Fig. 7. All the M–T curves depicted in Fig.7 demonstrate that at low temperatures, the values of MZFC are always lower than that of MFC. However, at high temperatures, the two curves merge and abruptly decrease around room temperature. From Fig. 7, we observe that the samples show large irreversible thermo-magnetization at low temperatures between the
N. B.S et al. / Physica B 479 (2015) 10–20
15
Fig. 7. Variation of magnetization with temperature with external field of 50 Oe for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples.
ZFC and FC curves. The irreversible temperature (Tirr) is taken as the temperature where these curve start to deviate from the each other. These results suggest that the deviations for all samples are mainly connected to the spin glass and cluster-glass behavior. This has also been observed by other researchers [24–29]. The present results indicate that the spin glass behavior is observed for all the samples due to the larger lattice distortion in the low field region. At low temperatures, the magnetization shows sharp peak and thereafter starts decreasing with further lowering of temperature. The sudden drop in the magnetization indicates that the magnetic interaction between the Gd and Mn sublattices are anti-ferromagnetic, and strong magneto-crystalline anisotropy prevents rotation of Gd moments in the direction of external field [25– 27,47]. These results reveal that all the samples exhibit the paramagnetic to anti-ferromagnetic transition at low temperatures. The Neel temperature (TN) for the GSMO samples was determined by plotting dM/dT versus T. With increase in x, the Neel temperature (TN) shows a shift towards higher temperatures for both the magnetic fields (50 and 200 Oe). For the sample with x¼ 0.2, it clearly seen from Fig. 7 that the magnetization goes to negative values at low temperatures. The characteristic temperature when it becomes negative is called the compensation temperature. The
compensation is due to anti-ferromagnetism of the Gd spins at low temperatures [25–27,47]. The negative magnetization can also arise due to the negative coupling between the sub-lattices of 3d and 4f ions. Similar results have been observed in the literature [25–27,47–49]. The present results indicate the presence of short range spin ordering in the AFM state of these materials. Similar results are observed when field of is 200 Oe applied and the temperature dependent magnetizations are shown in Fig. 8. In order to understand the temperature dependent susceptibility (χ ¼M/H) of Gd1 xSrxMnO3 (0.2ox o0.5) samples in the paramagnetic region, we have used the relation between 1/χ and T given by Curie-Weiss law which is given by the expression,
1/χ = T /(C − Θ)
(5)
where C is Curie constant given by C ¼ μeff2/3kB, Θ is the CurieWeiss temperature, μeff is the effective magnetic moment and kB is the Boltzmann constant. The data was fitted in the temperature range 60–300 K. Fig. 9 shows the variation of inverse susceptibility with temperature for both the fields (50 and 200 Oe) for the presently investigated samples. The values of Curie constant and the effective magnetic moment were calculated from the slope of the linear curve. The calculated values are given in Table 4. The Θ
16
N. B.S et al. / Physica B 479 (2015) 10–20
Fig. 8. Variation of magnetization with temperature with external field of 200 Oe for Gd1 xSrxMnO3 (0.2r x r0.5) samples.
Fig. 9. Variation of inverse susceptibility with temperature with field of 50 Oe and 200 Oe for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples.
N. B.S et al. / Physica B 479 (2015) 10–20
17
Table 4 Neel temperature (TN), Curie constant (C), Curie-Weiss temperature (ΘW) and magnetic moment μeff (μB) (observed and calculated) from magnetization data for Gd1 xSrx MnO3 (0.2r x r0.5) samples. x
0.2 0.3 0.4 0.5
H¼ 50 Oe
H¼ 200 Oe
TN (K)
C (emu K g 1)
ΘW (K)
μeff (μB)
μeff
49 50 55 60
8.21 8.90 10.10 9.66
40.56 67.25 53.12 101.76
8.10 8.43 8.99 8.78
8.52 8.08 7.63 7.14
(cal)
(μB)
TN (K)
C (emu K g 1)
ΘW (K)
μeff (μB)
μeff
50 52 57 66
8.48 9.22 10.37 10.85
41.27 67.17 52.97 83.57
8.24 8.59 9.11 9.31
8.52 8.08 7.63 7.14
(cal)
(μB)
Fig. 10. Variation of magnetization with field at different temperatures of 5, 35 and 300 K for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples.
value is negative for all the samples indicating that the anti-ferromagnetic interactions are present in the system. The observed magnetic moments are increasing with increase in Sr concentration. If one assumes that the paramagnetic regime is simply the superposition of Mn and the RE sublattices, then one can calculate
the effective magnetic moment of the Mn sublattice. The theoretical value of effective magnetic moment (μeff (cal)) for Gd1 xSrxMnO3 (0.2o xo0.5) system can be written using wellknown formula [24–25,50],
18
N. B.S et al. / Physica B 479 (2015) 10–20
Fig. 11. Variation of thermoelectric power with temperature for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples.
2 2 Gd3 + + (1 − x) μeff Mn3 + μeff (cal) = ⎡⎣ (1 − x) μeff
(
) 2 2 + (x) μeff ( Mn4+) ⎤⎦
(
) (6)
For calculation of μeff we have used μeff (Gd3 þ )¼ 7.94 μB and the contribution of the effective magnetic moments of Mn3 þ and Mn4 þ are respectively taken as 4.9 μB and 3.87 μB. The observed and calculated values are presented in the Table 4. It can be seen that the observed values in the paramagnetic regime are significantly much larger than the calculated values which can be attributed to the existence of short range FM correlation in the PM state [26,49,50] and we can observe that the calculated effective magnetic moment increases with the conversion of Mn3 þ ion into Mn4 þ ion, which is due to the presence of magnetic clusters in the system [50–53]. Fig. 10 depicts the M–H curves for GSMO samples at different temperatures such as 5 K, 35 K and 300 K. The M–H curve is obtained by sweeping the magnetic field from þ 9 T to 9 T. It is clearly observed that all the samples show paramagnetic behavior at 300 K. A clear change of M–H loop is observed at 35 K and 5 K. It is found that at low temperatures, the low magnetic field induces the non-linear behavior of M–H loop as we seen in the insets of Fig. 10. It is clearly seen that the hysteresis behavior is observed at 5 K and 35 K. These results demonstrate that the anti-ferromagnetic or ferromagnetic state is present in the system at low temperatures [25,29,50]. 3.4. Thermo-electric power (S) The variation of thermoelectric power (S) with temperature (T) for the samples Gd1 xSrxMnO3 (0.2ox o0.5) measured in the temperature range 5–300 K is shown in Fig. 11. It can be seen three samples viz. Gd0.5Sr0.5MnO3, Gd0.6Sr0.4MnO3 and Gd0.7Sr0.3MnO3 show negative thermoelectric power (S) values at high temperatures. There is a cross over from negative to positive value of S at low temperatures. On other hand, the sample Gd0.8Sr0.2MnO3 shows positive S value in the entire temperature range. Present work suggests that the grain size plays an important role in the sign of S. Normally change of sign of S is usually exhibited for samples with smaller grain size. The SEM data indicates that
Gd0.8Sr0.2MnO3 sample has the largest grain size. The positive value of S observed for Gd0.8Sr0.2MnO3 sample in the entire temperature range can also be attributed to the holes which are excited from the valence band (VB) into the impurity band, while in the case of the remaining three samples, no change of sign is attributed to the orbital degeneracy of the eg band. According to orbital degeneracy model, the peroviskite type of transition metal oxide plays an important role in degree of freedom of eg band [34,54]. From Fig. 11 a sharp increase in the thermoelectric power is observed for the GSMO samples at low temperatures and they exhibit colossal thermoelectric power. It can be seen that the S value is decreasing with increase the Sr dopant. The colossal thermoelectric power is mainly attributed to the phonon drag, magnon drag, or spin glass cluster [30–33,55]. In GSMO system, the charge order is one of the main reasons which contribute to the colossal value of thermoelectric power at low temperature [30,31]. In order to understand the high temperature thermoelectric power behavior of GSMO system, we have analyzed the experimental data using small polaron hoping model. The paramagnetic insulating region in manganites is mainly due to the lattice distortion, so that the charge carries are localized within the Mn4 þ ion and it forms polarons in the manganites. Hence, the transport properties like TEP data are governed by the thermally activated small polaron. The experimental data for GSMO system at high temperatures can be analyzed using the well-known Mott's equation given by
S=
⎞ kB ⎛ Δ + B⎟ ⎜ ⎠ e ⎝ kB T
(7)
where kB is the Boltzmann constant, B is a constant related to entropy of the charge carriers and Δ is the activation energy. The physical significance of activation energy is that it is the energy required to hop across the barrier between two neighboring sites. In addition, B o1, means that the conduction mechanism is due to small polaron and if B 41, it means that the conduction mechanism is due to the large polaron [30,31,55–57]. The plot of S versus 1/T is expected to a linear curve. The plot of S versus 1/T for presently investigated samples is shown in Fig. 12, it clearly seen that experimental data are well fitted using Eq. 7. We have calculated the Δ and B values using least square fitting and the results are given in the Table 5. The results show that the activation energy is decreasing with increasing dopant. The observed B values suggest that the small polaron hopping are present in the GSMO samples.
4. Conclusion We have investigated the structural, electrical, magnetic and thermoelectric power of Gd1 xSrxMnO3 (0.2ox o0.5) samples. All the samples studied in the present work are single phased and have orthorhombic crystal structure with Pbnm space group. The a and c lattice parameters increase with increasing Sr concentration but the lattice parameter b is found to decrease with increasing the concentration of Sr. Thus suggests that the GSMO samples have distorted orthorhombic crystal structure. The temperature dependence of electrical resistivity shows insulating behavior. For low temperatures, the electrical resistivity is too high to be measured. We have analyzed the validity of small polaron and variable-range hopping models for the samples under present investigation. It is demonstrated that the former is valid in the high temperature region, whereas the latter is applicable in low temperature regime. We have estimated activation energy EA using SPH model, and the activation energy gradually decreases with increase in Sr content which is due to the increase the number of
N. B.S et al. / Physica B 479 (2015) 10–20
19
Fig. 12. Variation of S versus 1/T for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples in high temperature regime. Points denote experimental values and lines indicate the fitted curves. Table 5 Fitting parameters obtained from thermo-electric power data for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples. x
Δ (meV)
B
0.2 0.3 0.4 0.5
28.00 22.80 9.40 4.35
0.96 0.44 0.70 0.88
effective charge carrier in the Sr doped systems. From the VRH model, we have calculated T0 and N(EF) and the results show that the T0 values decrease with increasing the Sr content. On the other hand N(EF) values are increasing with increasing Sr doping. Large irreversible thermo-magnetization is observed at low temperatures between the ZFC and FC curves which indicates the spin glass behavior in all the samples. Thermoelectric power measurements show that these low band gap manganites exhibit colossal values of S. Thermoelectric power decreases with increase in Sr content.
Acknowledgment One of the authors (Nagaraja B S) acknowledges Manipal University for providing the financial support to pursue Ph.D.
Reference [1] Ramesh Chandra Bhatt, Shiva Kumar Singh, P.C. Srivastava, S.K. Agarwal, V.P. S. Awana, J. Alloy. Compd. 580 (2013) 377. [2] E.L. Nagaev, Phys. Rep. 346 (2001) 387. [3] Kalipada Das, Tapas Paramanik, I. Das, J. Magn. Magn. Mater. 374 (2015) 707. [4] Shilpi Karmakar, E. Bose, S. Taran, B.K. Chaudhuri, C.P. Sun, H.D. Yang, J. Appl. Phys. 103 (2008) 023901. [5] Z.P. Luo, D.J. Miller, J.F. Mitchell, J. Appl. Phys. 105 (2009) 07D528. [6] N.S. Bingham, M.H. Phan, H. Srikanth, M.A. Torija, C. Leighton, J. Appl. Phys. 106 (2009) 023909. [7] Manh-Huong Phan, Seong-Cho Yu, J. Magn. Magn. Mater. 308 (2007) 325. [8] P.K. Siwach, H.K. Singh, O.N. Srivastava, J. Phys. Condens. Matter 20 (2008) 273201. [9] S.M. Dunaevsky, V.V. Deriglazov, Phys. Solid State 49 (2007) 1514. [10] B. Garcı´A-Landa, J.M. De Teresa, M.R. Ibarra, J. Appl. Phys. 83 (1998) 7664. [11] Yunhui Xu, U. Memmert, U. Hartmann, Sens. Actuators A 91 (2001) 26. [12] M. Quintero, J. Sacanell, L. Ghivelder, A.M. Gomes, A.G. Leyva, F. Parisi, Appl. Phys. Lett. 97 (2010) 121916.
20
N. B.S et al. / Physica B 479 (2015) 10–20
[13] Ll Balcells, E. Calvo, J. Fontcuberta, J. Magn. Magn. Mater. 242–245 (2002) 1166. [14] G.J. Yong, Rajeswari M. Kolagani, S. Adhikari, O.B. Drury, C. Gardner, R. M. Bionta, S. Friedrich, Rev. Sci. Instrum. 81 (2010) 113906. [15] Sudarshan Vadnala, Saket Asthana, Prem Pal, S. Srinath, ISRN Mater. Sci. 2013 (2013) 1. [16] M. Staruch, M. Jain, J. Phys. Condens. Matter 25 (2013) 296005. [17] J. Blasco, C. Ritter, J. Garcı´a, J.M. de Teresa, J. Pe´rez-Cacho, M.R. Ibarra, Phys. Rev. B 62 (2000) 5609. [18] Esa Bose, S. Karmakar, B.K. Chaudhuri, S. Pal, C. Martin, S. Hebert, A. Maignan, J. Phys. D: Appl. Phys. 40 (2007) 3728. [19] T. Terai, T. Sasaki, T. Kakeshita, T. Fukuda, T. Saburi, Phys. Rev. B 61 (2000) 3488. [20] S. Harikrishnan, C.M. Naveen Kumar, H.L. Bhat, Suja Elizabeth, U.K. Roßler, K. Dorr, S. Roßler, S. Wirth, J. Phys.: Condens. Matter 20 (2008) 275234. ̈ ckel, Phys. [21] Harikrishnan S. Nair, C.M.N. Kumar, H.L. Bhat, Suja Elizabeth, Th Bru Rev. B 83 (2011) 104424. [22] Parul Pandey, Nilesh Awari, Rakesh Rana, Abhishek Singh, S.S. Prabhu, D. S. Rana, Appl. Phys. Lett. 100 (2012) 062408. [23] Aditya A. Wagh, P.S. Anil Kumar, H.L. Bhat, Suja Elizabeth, J. Phys. Condens. Matter 22 (2010) 026005. [24] S. Roßler, S. Harikrishnan, U.K. Roßler, Suja Elizabeth, H.L. Bhat, F. Stegkich, S. Wirth, J. Phys. Conf. Ser. 200 (2010) 012168. [25] Octavio Pen~a, Mona Bahout, Karim Ghanimi, Pedro Duran, Dionisio Gutierrez, Carlos Moure, J. Mater. Chem. 12 (2002) 2480. [26] Sanjay Biswas, Momin Hossain Khan, Sudipta Pal, Esa Bose, J. Supercond. Nov. Magn. 27 (2014) 463. [27] Yanwei Ma, M. Guilloux-Viry, P. Barahona, O. Peña, C. Moure, Appl. Phys. Lett. 86 (2005) 062506. [28] T. Elovaara, H. Huhtinen, S. Majumdar, P. Paturi, J. Phys. Condens. Matter 24 (2012) 216002. [29] S. Harikrishnan, C.M. Naveen Kumar, S.S. Rao, H.L. Bhat, S.V. Bhat, Suja Elizabetha, J. Appl. Phys. 104 (2008) 023902. [30] S. Sagar, V. Ganesan, P.A. Joy, Senoy Thomas, A. Liebig, M. Albrecht, M. R. Anantharaman, Europhys. Lett. 91 (2010) 17008. [31] B.S. Nagaraja, Ashok Rao, G.S. Okram, J. Supercond. Nov. Magn. 28 (2015) 223. [32] Lija K. Joy, S. Shanmukharao Samatham, Senoy Thomas, V. Ganesan, Salim AlHarthi, A. Liebig, M. Albrecht, M.R. Anantharaman, J. Appl. Phys. 116 (2014) 213701. [33] Ajay Soni, G.S. Okram, Rev. Sci. Instrum. 79 (2008) 125103.
[34] G. Venkataiah, Y.K. Lakshmi, P.V. Reddy, PMC Phys. B 1 (2008) 1. [35] G. Ch. Kostogloudis, Ch Ftikos, J. Mater. Sci. 34 (1999) 2169. [36] P.A. Yadav, A.V. Deshmukh, K.P. Adhi, B.B. Kale, N. Basavaih, S.I. Patil, J. Magn. Magn. Mater. 328 (2013) 86. [37] L. Li, L. Lin, Z.B. Yan, Q.Y. He, J.-M. Liu, J. Appl. Phys. 112 (2012) 034115. [38] S. Hcini, S. Zemnia, A. Triki, H. Rahmouni, M. Boudard, J. Alloy. Compd. 509 (2011) 1394. [39] A.M. Kadomtseva, Yu. F. Popov, G.P. Vorob’ev, V. Yu Ivanov, A.A. Mukhin, A. M. Balbashov, JETP Lett. 82 (2005) 590. [40] R. Kannan, D. Vanidha, A. ArunKumar, J. Eng. Sci. Technol. 5 (2013) 211. [41] S. Sagar, M.R. Anantharaman, Bull. Mater. Sci. 35 (2012) 41. [42] V. Raja Kumari, G. Venkataiah, P. Venugopal Reddy, Int. J. Mod. Phys. B 19 (2005) 3619. [43] T. Izgi, V.S. Kolat, N. Bayri, A.O. Kaya, H. Gencer, S. Atalay, J. Optoelectron. Adv. Mater. 13 (2011) 648. [44] M. Viret, L. Ranno, J.M.D. Coey, J. Appl. Phys. 81 (1997) 4964. [45] S. Mollaha, H.L. Huang, H.D. Yang, Sudipta Pal, S. Taran, B.K. Chaudhuri, J. Magn. Magn. Mater. 284 (2004) 383. [46] K. Padmavathi, G. Venkataiah, P. Venugopal Reddy, J. Magn. Magn. Mater. 309 (2007) 237. [47] O. Pen~ a, C. Moure, P. Barahona, M. Baibich, G. Martinez, Phys. B: Condens. Matter 384 (2006) 57. [48] J. Hemberger, S. Lobina, H.-A. Krug von Nidda, N. Tristan, V. Yu Ivanov, A. A. Mukhin, A.M. Balbashov, A. Loidl, Phys. Rev. B 70 (2004) 024414. [49] V.A. Khomchenko, I.O. Troyanchuk, R. Szymczak, H. Szymczak, J. Mater Sci. 43 (2008) 5662. [50] Sobhi Hcini, Michel Boudard, Sadok Zemni, Mohamed Oumezzine, Ceram. Int. 40 (2014) 16041. [51] S. Zemni, A. Gasmi, M. Boudard, M. Oumezzin, Mater. Sci. Eng. B 144 (2007) 117. [52] J. Yang, Y.P. Lee, J. Korean Phys. Soc. 51 (2007) 1560. [53] A.A. Zatsiupa, L.A. Bashkirov, I.O. Troyanchuk, G.S. Petrov, A.I. Galyas, L. S. Lobanovsky, S.V. Truhanov, J. Solid State Chem. 212 (2014) 147. [54] Aritra Banerjee, S. Pal, S. Bhattacharya, B.K. Chaudhuri, H.D. Yang, Phys. Rev. B 64 (2001) 104428. [55] G. Venkataiah, P. Venugopal Reddy, J. Appl. Phys. 106 (2009) 033706. [56] Mamatha D. Daivajna, Ashok Rao, G.S. Okram, J. Alloy. Compd. 617 (2014) 345. [57] D. Salazar, D. Arias, O.J. Durá, M.A. López de la Torre, J. Alloy. Compd. 583 (2014) 141.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Structural, electrical, magnetic and thermal properties of Gd1–xSrx MnO3 (0.2 r xr 0.5) manganites Nagaraja B.S a, Ashok Rao a,n, P.D Babu b, G.S. Okram c a
Department of Physics, Manipal Institute of Technology, Manipal University, Manipal 576104, India UGC-DAE Consortium for Scientific Ressearch, R5 Shed, Bhabha Atomic Research Centre, Mumbai 400085, India c UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 3 July 2015 Received in revised form 1 September 2015 Accepted 13 September 2015
A systematic study on structural, electrical, magnetic and thermoelectric properties of bulk samples of Gd1–xSrxMnO3, synthesized by solid state reaction, is carried out in this communication. All the samples are in single phase with no detectable impurities. All the samples exhibit insulating behavior and the results indicate that high temperature electrical resistivity can be explained using small polaron hopping (SPH) model, whereas variable hopping model (VRH) model is valid in the low temperature regime. The magnetization results show that large irreversible magnetization is observed at low temperatures. The thermoelectric power measurement demonstrates that these samples exhibit colossal thermoelectric power. & 2015 Elsevier B.V. All rights reserved.
Keywords: Manganites Solid state reaction Rietveld refinement Magnetization Thermoelectric power
1. Introduction The rare earth mixed valent pervoskite manganites RE1 xAExMnO3 (RE¼ rare earth elements and AE¼a divalent ion) have been extensively studied as they exhibit huge magneto-resistance and magneto-capacitance which is due to the presence of spin, lattice, and charge order [1–10]. These properties provide a wide range of technological applications such as magnetic read head device, IR detector, magnetic storage and bolometric devices [11–15]. The divalent ions generate Mn4 þ ions in the materials, in addition to the Mn3 þ already present in the system. The Mn3 þ /Mn4 þ ratio along with the average ionic radius of the A-site cation and ionic mismatch at the A-site, modulates the physical properties of the manganites. The double exchange mechanism can explain the physical properties of these materials [16–18]. The substitution of AE elements with RE elements are essentially done due to the fact that they induce a transition from ferromagnetic (FM) to charge ordered anti-ferromagnetic (CO–AFM) state [10,19,20]. The presence of different magnetic phases at low temperature and strong magneto-elastic effect lead to such fieldinduced transitions which resemble meta-magnetic transitions. This exhibits strong field-induced effects in the dielectric properties of material [21]. Most of studies have been focused on the electrical and n
Corresponding author. Fax: þ91 820 2571071. E-mail address: [email protected] (A. Rao).
http://dx.doi.org/10.1016/j.physb.2015.09.025 0921-4526/& 2015 Elsevier B.V. All rights reserved.
magnetic properties of manganites of larger rare earth ions such as La, Pr and Nd [4–7]. Less work seems to have done on the electrical and magnetic properties of RE1 xAExMnO3 with smaller RE ions such as Gd, Dy, Tb and Eu. These smaller RE-based manganites are very interesting among the mangnite systems due to the complex and inter-twined effects of their electrical and magnetic properties [10,16–22]. The irreversible and sharp anomalies in the magnetostriction and magneto-resistance take place at low temperatures which have been attributed to the charge order state. The spin glass behavior is observed in low bandwidth materials essentially due to the presence of magnetic cluster [19,23]. In Dy0.7Sr0.3MnO3 system spin reversal [24,25] and negative magnetization have been observed as a result of a negative exchange interaction between RE and Mn sublattices [26]. The reversible sign change of the magnetization with temperature has been observed in the manganite, which is attributed to the anti-parallel coupling of the Gd moments with Mn [26,27]. The charge order is observed when hole doping is increased [28]. The spin glass behavior is observed in the half doped manganite which is attributed to the magnetic cluster present in the low bandwidth manganites [29]. Large value of thermoelectric power of the order of 35 mV/K has been observed in low bandwidth systems [30]. In our previous communication, colossal thermoelectric power was reported in the low band gap Dy1–xSrxMnO3 manganites. In particular the compound, Dy0.7Sr0.3MnO3 exhibited the highest value of thermoelectric power which is order of 224 mV/K [31]. The fractured glass state and AFM spin ordering at low temperatures are responsible for localization of charge carriers which manifests in the form of
N. B.S et al. / Physica B 479 (2015) 10–20
colossal thermoelectric power [32]. This motivated us to perform systematic investigation on another smaller ion system like Gd1 xSrxMnO3. It was thus describe to investigate structural, electrical, magnetic and thermal properties of Gd1 xSrxMnO3 (GSMO) manganites
2. Experimental details Samples with general formula Gd1 xSrxMnO3 (0.2rx r0.5) were prepared using the well-known solid state reaction method. High purity (99.99%) oxides and carbonate (Gd2O3, SrCO2, and MnO2) were mixed well for about 5–6 h which was followed by calcinations at 1100 °C for 24 h which was repeated thrice to ensure homogeneity of the samples. The powder was then pelletized and sintered at 1250 °C for 36 h. To confirm the purity of the samples and to determine the lattice parameters, X-ray diffraction studies were performed on the samples prepared under the
11
present investigation. The surface morphology of the samples was imaged using Scanning Electron Microscopy (SEM) attached with Energy Dispersive X-ray (EDAX) analyzer (EVO MA18 with Oxford EDS (X-act)). EDAX measurement was carried out to ascertain the elemental composition of dopants in the prepared samples. Temperature-dependent electrical resistivity ρ(T ) was measured using four-probe method in a closed cycle refrigerator (CCR). Keithley Current Source (Model 6221) was used to pass constant current through the current terminals, and the voltage across the voltage terminals was measured using Keithley Nanovoltmeter (Model 2182 A). The magnetic properties of the samples were investigated (between 5 and 300 K) using a superconducting quantum interference device (SQUID) magnetometer and 9 T PPMS based vibrating sample magnetometer (VSM) (both Quantum Design) in both zero-field-cooled (ZFC) and field-cooled (FC) conditions. The hysteresis loops (M versus H) of the samples were also recorded at several temperatures between 5 and 300 K. Thermoelectric power (S) measurement was performed in the temperature range of 5–
Fig. 1. XRD pattern for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. The observed intensities are shown as dots and calculated intensities are shown by solid line. Line at the bottom denotes the difference between the experimental and refined patterns.
12
N. B.S et al. / Physica B 479 (2015) 10–20
300 K, using differential DC method, taking oxygen-free highly conducting copper (OFHC) as reference. The details of thermoelectric measurements are described elsewhere [33].
3. Results and discussion 3.1. Structural properties Fig. 1 shows the room temperature XRD patterns of Gd1 xSrxMnO3 (0.2 ox o0.5) samples. The XRD data confirms that all the samples are in single phase and no impurities were observed within the experimental limits. The samples were indexed on the orthorhombic crystal structure with the Pbnm space group. We have analyzed XRD data using Rietveld refinement using Fullprof program. From the refinement, we have calculated the lattice parameter, unit cell volume and goodness of the fit. The observed results are shown in the Table 1. It can be seen that both the lattice parameters a and c are increasing with increasing x, on contrary the lattice parameter b is decreasing with increasing x. This suggests that the GSMO samples have distorted orthorhombic crystal structure. The amount of Sr substitution increases the hole doping of GSMO samples which increases the amount of Mn3 þ ions and this distortion is correlated with Jahn–Teller (JT) distortion which affect the Mn3 þ ion in the manganese octahedral [28,34]. Thus increase in concentration of Sr, increases Mn4 þ ion and hence lattice parameter b decreases. In addition, the unit cell volume is decreasing with increasing the value of x which is consistent with earlier reports [34,35]. This result suggests that the large value of Mn4 þ creates charge compensation in the Sr2 þ ion. The calculated Mn–O–Mn bond angle and Mn–O bond distance are also given in Table 1. It can be clearly seen that both the bond angle and bond distance decrease with increase in x. This results in enhancement of the super exchange interaction and lattice distortion in the GSMO samples. To estimate the average crystalline size of the samples, we have used the Scherer formula which is given by
D = 0. 94λ /β cos θ
(1)
Table 1 Summary of structural parameters and refinement parameters for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples. x System Space Group
0.2 0.3 0.4 0.5 Orthorhombic Orthorhombic Orthorhombic Orthorhombic Pbnm Pbnm Pbnm Pbnm
a (Å)
5.365 (0.001) 5.643 (0.001) 7.541 (0.001) 90° 228.38 (0.08) 2.60 3.64 2.27 1.97
5.3817 (0.0007) 5.5021 (0.0008) 7.615 (0.001) 90° 225.89 (0.05) 1.85 2.41 2.33 1.17
5.3906 (0.0007) 5.4312 (0.0008) 7.6357 (0.0008) 90° 224.38 (0.03) 2.00 2.66 2.28 0.986
5.397 (0.001) 5.402 (0.001) 7.6667 (0.0008) 90° 223.61 (0.03) 1.89 2.47 2.29 1.24
1.992 166.75
1.967 166.56
1.953 166.49
1.947 166.42
57.32
81.69
70.03
92.15
2027.0
1540.7
1163.0
1067.3
b (Å) c (Å) α ¼ β¼ γ Cell Volume (Å3) Rp rwp rexp Goodness of fit. χ2 Mn–O (Å) Mn–O–Mn (deg) Crystalline size (nm) from XRD Average Grain size (nm) from SEM
where λ is the X-ray wavelength, θ is the Bragg angle and β is the corrected full width half maxima of the XRD peaks of the samples. The estimated average crystalline sizes are given in Table 1. It is clearly seen that the crystalline size increases with increase in the concentration of Sr. The SEM images are presented in Fig. 2. From the SEM image, we can identify the grain size of samples, the grain sizes are given in Table 1. One can see that the particle size is decreasing with increasing the concentration of x. The comparison of XRD and SEM results reveals that the particle size determined using from SEM images are much bigger than that from XRD. This is due to fact that the SEM results give the size of secondary particles of the samples and XRD results provide the size of primary particles [36–38]. In order to estimate the composition of the compounds, EDS analysis results have been done for all the GSMO samples. The results are presented in Table 2. It is seen that the experimental composition matches well with the nominal composition. 3.2. Electrical resistivity and conduction mechanisms The temperature dependent resistivity for Gd1 xSrxMnO3 (0.2ox o0.5) samples are shown in Fig. 3. It can be observed that the electrical resistivity for all the samples increases monotonically with decreasing temperature. In order to understand the electrical transport properties of the GSMO system, we assume that electronic structure of the doping element plays a crucial role. It is clearly observed from the insets of Fig. 3 that the electrical resistivity decreases with increase in concentration of strontium. It is evident from Fig. 3 that all the samples exhibit high values of resistivity in semiconducting–insulating region which is attributed to lattice mismatch between Mn3 þ and Mn4 þ which is due to the spin glass behavior of the system [10,19,39] which in is turn related to Jahn–Teller distortion which favors insulating state due to localization of electrons [23,31]. This behavior is a typical trend seen in low bandwidth manganites and they normally require very high magnetic fields to exhibit the metal–insulator transition. The variation of electrical resistivity with Sr doing can be explained using double exchange mechanism. Increase in Sr content produces more Mn4 þ ion and hence Mn3 þ /Mn4 þ ion ratio deviates away from the optimum value of 0.5 and consequently resistivity value decreases. In order to understand the temperature dependence of electrical resistivity, there are three conduction mechanisms which can be used to account for the resistivity behavior viz. (1) thermal activation model (2) small polaron hopping model (SPH) and (3) variable range hoping (VRH) model. Thermal activation model is usually used to explain the conduction mechanism in oxide materials. This model is applicable when the carriers are magnetic polarons. If the thermal energy is sufficient to overcome the band gap the electrons becomes free to conduct. Resistivity can be expressed using the equation [31],
ρ = ρ0 exp (EA/kB T )
(2)
where T is temperature, ρ0 is the value of resistivity at infinite temperature, EA is the activation energy and kB the Boltzmann constant. The present results were analyzed using Eq. (2). One expects that a plot of ln ρ versus 1/T would be straight line. From Fig. 4, it is clear that the present results do not match with Eq. (2). Hence we conclude that the thermal activation model is not applicable for the presently investigated manganites. Similar results are reported in literature [31,40,41]. In order to understand the conduction mechanism in high temperature region (θD/2 oT) we have made an attempt to see the applicability of small polaron hopping (SPH) model. The thermally activated small polaron hopping model is based on the polaronic
N. B.S et al. / Physica B 479 (2015) 10–20
13
Fig. 2. SEM images of (a) Gd0.5Sr0.5MnO3 (b) Gd0.6Sr0.4MnO3 (c) Gd0.7Sr0.3MnO3 and (d) Gd0.8Sr0.2MnO3.
Table 2 EDS results for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. x
Element
At%
Nominal composition
Experimental composition
0.2
Gd Sr Mn Gd Sr Mn Gd Sr Mn Gd Sr Mn
17.93 3.29 17.85 13.55 5.26 24.33 12.65 7.82 22.64 10.58 7.95 20.96
0.80 0.20 1.00 0.70 0.30 1.00 0.60 0.40 1.00 0.50 0.50 1.00
0.84 0.16 0.90 0.72 0.23 0.95 0.61 0.38 1.01 0.57 0.48 1.08
0.3
0.4
0.5
model in high temperature region in mangnites. The SPH model can be described using the following equation:
ρ = AT exp (EA/kB T )
(3)
where T is the temperature, EA is the activation energy, A is a constant and it is given by A ¼2kB/3ne2a2ν. Here kB is Boltzmann's constant, n is number of charge carriers, e is electronic charge, a is
Fig. 3. Electrical resistivity variation with temperature for Gd1 xSrxMnO3 (0.2r x r0.5) samples. Inset shows the resistivity behavior in the temperature 150–300 K.
14
N. B.S et al. / Physica B 479 (2015) 10–20
Fig. 4. Variation of lnρ with 1/T for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. Fig. 6. Variation of lnρ with 1/T1/4 for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. Inset shows the fitting of experimental data using VRH model.
increase the number of effective charge carrier in the Sr doped systems [40–43]. Hence, we can conclude that the SPH model alone cannot explain the conduction mechanism for the entire temperature range. To validate the conduction mechanism in low temperature region (T o θD/2), we have fitted the temperature dependence of electrical resistivity using Mott's VRH model. The VRH model is applicable when the thermal energy is insufficient to excite the electrons to hop to their nearest neighbors, it is more favorable for the electrons to hop further to find a site with a smaller potential difference. The expression for VRH can written as
ρ = ρ0 exp (T0/T )1/4
Fig. 5. : Variation of ln(ρ/T) with 1/T for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. Inset shows the fitting of experimental data using SPH model. Table 3 Fitting parameters obtained from resistivity data for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples. x
0.2 0.3 0.4 0.5
SPH model
VRH model
EA (meV)
A*10 5
T0*107
ρ0*10 8
N(Ef)*1025(eV 1 m 3)
152.75 122.29 123.77 125.02
17.63 10.38 5.43 2.75
8.88 5.81 2.85 3.42
1.14 4.86 0.21 0.12
2.57 3.93 8.01 6.67
the site to site hopping distance, and ν is the longitudinal optical phonon frequency. For validity of this model, a plot of ln(ρ/T) versus 1/T is expected to be a straight line. We have fitted the experimental data of the presently investigated samples using Eq. (3) as shown in Fig. 5 and a linear behavior is observed in the temperature range of 200–300 K. From the slope of the linear curve above θD/2, we have calculated the activation energy using Eq. (3) and the activation energy results are presented in Table 3. Here, we can observe that increasing the value of Sr content in the GSMO system, the activation energy gradually decreases due to the
(4)
where ρ0 is the residual resistivity and T0 is the characteristic temperature which is expressed as T0 ¼18α3/kBN(EF), where the N(EF) is the density of state at Fermi level, α is localization length and kB is the Boltzmann constant. In order to validate the VRH model for GSMO system, we have fitted the experimental data of resistivity using Eq. (4). Fig. 6 shows the behavior of lnρ versus T 1/4. The plots show linear trend in the temperature range 100–200 K suggesting that VRH model is applicable for temperatures below θD/2. Finally, we have estimated the value of N(EF) by taking α as constant (α ¼2.22 nm 1) [44,45] and using the T0 from the slope of Fig. 6. The calculated value of T0 and N(EF) are given in Table 3. It can be clearly observed that the T0 values decrease with increasing Sr doping and N(EF) value are increasing with increasing Sr doping. These values are found to be in good arguments with results reported in literature [40,46]. 3.3. Magnetization measurements The variation of magnetization with temperature of GSMO samples were carried out using zero-field-cooled (ZFC) and fieldcooled (FC) conditions at two different fields viz. 50 Oe and 200 Oe. The temperature dependent magnetization at applied field of 50 Oe is shown in Fig. 7. All the M–T curves depicted in Fig.7 demonstrate that at low temperatures, the values of MZFC are always lower than that of MFC. However, at high temperatures, the two curves merge and abruptly decrease around room temperature. From Fig. 7, we observe that the samples show large irreversible thermo-magnetization at low temperatures between the
N. B.S et al. / Physica B 479 (2015) 10–20
15
Fig. 7. Variation of magnetization with temperature with external field of 50 Oe for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples.
ZFC and FC curves. The irreversible temperature (Tirr) is taken as the temperature where these curve start to deviate from the each other. These results suggest that the deviations for all samples are mainly connected to the spin glass and cluster-glass behavior. This has also been observed by other researchers [24–29]. The present results indicate that the spin glass behavior is observed for all the samples due to the larger lattice distortion in the low field region. At low temperatures, the magnetization shows sharp peak and thereafter starts decreasing with further lowering of temperature. The sudden drop in the magnetization indicates that the magnetic interaction between the Gd and Mn sublattices are anti-ferromagnetic, and strong magneto-crystalline anisotropy prevents rotation of Gd moments in the direction of external field [25– 27,47]. These results reveal that all the samples exhibit the paramagnetic to anti-ferromagnetic transition at low temperatures. The Neel temperature (TN) for the GSMO samples was determined by plotting dM/dT versus T. With increase in x, the Neel temperature (TN) shows a shift towards higher temperatures for both the magnetic fields (50 and 200 Oe). For the sample with x¼ 0.2, it clearly seen from Fig. 7 that the magnetization goes to negative values at low temperatures. The characteristic temperature when it becomes negative is called the compensation temperature. The
compensation is due to anti-ferromagnetism of the Gd spins at low temperatures [25–27,47]. The negative magnetization can also arise due to the negative coupling between the sub-lattices of 3d and 4f ions. Similar results have been observed in the literature [25–27,47–49]. The present results indicate the presence of short range spin ordering in the AFM state of these materials. Similar results are observed when field of is 200 Oe applied and the temperature dependent magnetizations are shown in Fig. 8. In order to understand the temperature dependent susceptibility (χ ¼M/H) of Gd1 xSrxMnO3 (0.2ox o0.5) samples in the paramagnetic region, we have used the relation between 1/χ and T given by Curie-Weiss law which is given by the expression,
1/χ = T /(C − Θ)
(5)
where C is Curie constant given by C ¼ μeff2/3kB, Θ is the CurieWeiss temperature, μeff is the effective magnetic moment and kB is the Boltzmann constant. The data was fitted in the temperature range 60–300 K. Fig. 9 shows the variation of inverse susceptibility with temperature for both the fields (50 and 200 Oe) for the presently investigated samples. The values of Curie constant and the effective magnetic moment were calculated from the slope of the linear curve. The calculated values are given in Table 4. The Θ
16
N. B.S et al. / Physica B 479 (2015) 10–20
Fig. 8. Variation of magnetization with temperature with external field of 200 Oe for Gd1 xSrxMnO3 (0.2r x r0.5) samples.
Fig. 9. Variation of inverse susceptibility with temperature with field of 50 Oe and 200 Oe for Gd1 xSrxMnO3 (0.2 r xr 0.5) samples.
N. B.S et al. / Physica B 479 (2015) 10–20
17
Table 4 Neel temperature (TN), Curie constant (C), Curie-Weiss temperature (ΘW) and magnetic moment μeff (μB) (observed and calculated) from magnetization data for Gd1 xSrx MnO3 (0.2r x r0.5) samples. x
0.2 0.3 0.4 0.5
H¼ 50 Oe
H¼ 200 Oe
TN (K)
C (emu K g 1)
ΘW (K)
μeff (μB)
μeff
49 50 55 60
8.21 8.90 10.10 9.66
40.56 67.25 53.12 101.76
8.10 8.43 8.99 8.78
8.52 8.08 7.63 7.14
(cal)
(μB)
TN (K)
C (emu K g 1)
ΘW (K)
μeff (μB)
μeff
50 52 57 66
8.48 9.22 10.37 10.85
41.27 67.17 52.97 83.57
8.24 8.59 9.11 9.31
8.52 8.08 7.63 7.14
(cal)
(μB)
Fig. 10. Variation of magnetization with field at different temperatures of 5, 35 and 300 K for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples.
value is negative for all the samples indicating that the anti-ferromagnetic interactions are present in the system. The observed magnetic moments are increasing with increase in Sr concentration. If one assumes that the paramagnetic regime is simply the superposition of Mn and the RE sublattices, then one can calculate
the effective magnetic moment of the Mn sublattice. The theoretical value of effective magnetic moment (μeff (cal)) for Gd1 xSrxMnO3 (0.2o xo0.5) system can be written using wellknown formula [24–25,50],
18
N. B.S et al. / Physica B 479 (2015) 10–20
Fig. 11. Variation of thermoelectric power with temperature for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples.
2 2 Gd3 + + (1 − x) μeff Mn3 + μeff (cal) = ⎡⎣ (1 − x) μeff
(
) 2 2 + (x) μeff ( Mn4+) ⎤⎦
(
) (6)
For calculation of μeff we have used μeff (Gd3 þ )¼ 7.94 μB and the contribution of the effective magnetic moments of Mn3 þ and Mn4 þ are respectively taken as 4.9 μB and 3.87 μB. The observed and calculated values are presented in the Table 4. It can be seen that the observed values in the paramagnetic regime are significantly much larger than the calculated values which can be attributed to the existence of short range FM correlation in the PM state [26,49,50] and we can observe that the calculated effective magnetic moment increases with the conversion of Mn3 þ ion into Mn4 þ ion, which is due to the presence of magnetic clusters in the system [50–53]. Fig. 10 depicts the M–H curves for GSMO samples at different temperatures such as 5 K, 35 K and 300 K. The M–H curve is obtained by sweeping the magnetic field from þ 9 T to 9 T. It is clearly observed that all the samples show paramagnetic behavior at 300 K. A clear change of M–H loop is observed at 35 K and 5 K. It is found that at low temperatures, the low magnetic field induces the non-linear behavior of M–H loop as we seen in the insets of Fig. 10. It is clearly seen that the hysteresis behavior is observed at 5 K and 35 K. These results demonstrate that the anti-ferromagnetic or ferromagnetic state is present in the system at low temperatures [25,29,50]. 3.4. Thermo-electric power (S) The variation of thermoelectric power (S) with temperature (T) for the samples Gd1 xSrxMnO3 (0.2ox o0.5) measured in the temperature range 5–300 K is shown in Fig. 11. It can be seen three samples viz. Gd0.5Sr0.5MnO3, Gd0.6Sr0.4MnO3 and Gd0.7Sr0.3MnO3 show negative thermoelectric power (S) values at high temperatures. There is a cross over from negative to positive value of S at low temperatures. On other hand, the sample Gd0.8Sr0.2MnO3 shows positive S value in the entire temperature range. Present work suggests that the grain size plays an important role in the sign of S. Normally change of sign of S is usually exhibited for samples with smaller grain size. The SEM data indicates that
Gd0.8Sr0.2MnO3 sample has the largest grain size. The positive value of S observed for Gd0.8Sr0.2MnO3 sample in the entire temperature range can also be attributed to the holes which are excited from the valence band (VB) into the impurity band, while in the case of the remaining three samples, no change of sign is attributed to the orbital degeneracy of the eg band. According to orbital degeneracy model, the peroviskite type of transition metal oxide plays an important role in degree of freedom of eg band [34,54]. From Fig. 11 a sharp increase in the thermoelectric power is observed for the GSMO samples at low temperatures and they exhibit colossal thermoelectric power. It can be seen that the S value is decreasing with increase the Sr dopant. The colossal thermoelectric power is mainly attributed to the phonon drag, magnon drag, or spin glass cluster [30–33,55]. In GSMO system, the charge order is one of the main reasons which contribute to the colossal value of thermoelectric power at low temperature [30,31]. In order to understand the high temperature thermoelectric power behavior of GSMO system, we have analyzed the experimental data using small polaron hoping model. The paramagnetic insulating region in manganites is mainly due to the lattice distortion, so that the charge carries are localized within the Mn4 þ ion and it forms polarons in the manganites. Hence, the transport properties like TEP data are governed by the thermally activated small polaron. The experimental data for GSMO system at high temperatures can be analyzed using the well-known Mott's equation given by
S=
⎞ kB ⎛ Δ + B⎟ ⎜ ⎠ e ⎝ kB T
(7)
where kB is the Boltzmann constant, B is a constant related to entropy of the charge carriers and Δ is the activation energy. The physical significance of activation energy is that it is the energy required to hop across the barrier between two neighboring sites. In addition, B o1, means that the conduction mechanism is due to small polaron and if B 41, it means that the conduction mechanism is due to the large polaron [30,31,55–57]. The plot of S versus 1/T is expected to a linear curve. The plot of S versus 1/T for presently investigated samples is shown in Fig. 12, it clearly seen that experimental data are well fitted using Eq. 7. We have calculated the Δ and B values using least square fitting and the results are given in the Table 5. The results show that the activation energy is decreasing with increasing dopant. The observed B values suggest that the small polaron hopping are present in the GSMO samples.
4. Conclusion We have investigated the structural, electrical, magnetic and thermoelectric power of Gd1 xSrxMnO3 (0.2ox o0.5) samples. All the samples studied in the present work are single phased and have orthorhombic crystal structure with Pbnm space group. The a and c lattice parameters increase with increasing Sr concentration but the lattice parameter b is found to decrease with increasing the concentration of Sr. Thus suggests that the GSMO samples have distorted orthorhombic crystal structure. The temperature dependence of electrical resistivity shows insulating behavior. For low temperatures, the electrical resistivity is too high to be measured. We have analyzed the validity of small polaron and variable-range hopping models for the samples under present investigation. It is demonstrated that the former is valid in the high temperature region, whereas the latter is applicable in low temperature regime. We have estimated activation energy EA using SPH model, and the activation energy gradually decreases with increase in Sr content which is due to the increase the number of
N. B.S et al. / Physica B 479 (2015) 10–20
19
Fig. 12. Variation of S versus 1/T for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples in high temperature regime. Points denote experimental values and lines indicate the fitted curves. Table 5 Fitting parameters obtained from thermo-electric power data for Gd1 xSrxMnO3 (0.2 rx r 0.5) samples. x
Δ (meV)
B
0.2 0.3 0.4 0.5
28.00 22.80 9.40 4.35
0.96 0.44 0.70 0.88
effective charge carrier in the Sr doped systems. From the VRH model, we have calculated T0 and N(EF) and the results show that the T0 values decrease with increasing the Sr content. On the other hand N(EF) values are increasing with increasing Sr doping. Large irreversible thermo-magnetization is observed at low temperatures between the ZFC and FC curves which indicates the spin glass behavior in all the samples. Thermoelectric power measurements show that these low band gap manganites exhibit colossal values of S. Thermoelectric power decreases with increase in Sr content.
Acknowledgment One of the authors (Nagaraja B S) acknowledges Manipal University for providing the financial support to pursue Ph.D.
Reference [1] Ramesh Chandra Bhatt, Shiva Kumar Singh, P.C. Srivastava, S.K. Agarwal, V.P. S. Awana, J. Alloy. Compd. 580 (2013) 377. [2] E.L. Nagaev, Phys. Rep. 346 (2001) 387. [3] Kalipada Das, Tapas Paramanik, I. Das, J. Magn. Magn. Mater. 374 (2015) 707. [4] Shilpi Karmakar, E. Bose, S. Taran, B.K. Chaudhuri, C.P. Sun, H.D. Yang, J. Appl. Phys. 103 (2008) 023901. [5] Z.P. Luo, D.J. Miller, J.F. Mitchell, J. Appl. Phys. 105 (2009) 07D528. [6] N.S. Bingham, M.H. Phan, H. Srikanth, M.A. Torija, C. Leighton, J. Appl. Phys. 106 (2009) 023909. [7] Manh-Huong Phan, Seong-Cho Yu, J. Magn. Magn. Mater. 308 (2007) 325. [8] P.K. Siwach, H.K. Singh, O.N. Srivastava, J. Phys. Condens. Matter 20 (2008) 273201. [9] S.M. Dunaevsky, V.V. Deriglazov, Phys. Solid State 49 (2007) 1514. [10] B. Garcı´A-Landa, J.M. De Teresa, M.R. Ibarra, J. Appl. Phys. 83 (1998) 7664. [11] Yunhui Xu, U. Memmert, U. Hartmann, Sens. Actuators A 91 (2001) 26. [12] M. Quintero, J. Sacanell, L. Ghivelder, A.M. Gomes, A.G. Leyva, F. Parisi, Appl. Phys. Lett. 97 (2010) 121916.
20
N. B.S et al. / Physica B 479 (2015) 10–20
[13] Ll Balcells, E. Calvo, J. Fontcuberta, J. Magn. Magn. Mater. 242–245 (2002) 1166. [14] G.J. Yong, Rajeswari M. Kolagani, S. Adhikari, O.B. Drury, C. Gardner, R. M. Bionta, S. Friedrich, Rev. Sci. Instrum. 81 (2010) 113906. [15] Sudarshan Vadnala, Saket Asthana, Prem Pal, S. Srinath, ISRN Mater. Sci. 2013 (2013) 1. [16] M. Staruch, M. Jain, J. Phys. Condens. Matter 25 (2013) 296005. [17] J. Blasco, C. Ritter, J. Garcı´a, J.M. de Teresa, J. Pe´rez-Cacho, M.R. Ibarra, Phys. Rev. B 62 (2000) 5609. [18] Esa Bose, S. Karmakar, B.K. Chaudhuri, S. Pal, C. Martin, S. Hebert, A. Maignan, J. Phys. D: Appl. Phys. 40 (2007) 3728. [19] T. Terai, T. Sasaki, T. Kakeshita, T. Fukuda, T. Saburi, Phys. Rev. B 61 (2000) 3488. [20] S. Harikrishnan, C.M. Naveen Kumar, H.L. Bhat, Suja Elizabeth, U.K. Roßler, K. Dorr, S. Roßler, S. Wirth, J. Phys.: Condens. Matter 20 (2008) 275234. ̈ ckel, Phys. [21] Harikrishnan S. Nair, C.M.N. Kumar, H.L. Bhat, Suja Elizabeth, Th Bru Rev. B 83 (2011) 104424. [22] Parul Pandey, Nilesh Awari, Rakesh Rana, Abhishek Singh, S.S. Prabhu, D. S. Rana, Appl. Phys. Lett. 100 (2012) 062408. [23] Aditya A. Wagh, P.S. Anil Kumar, H.L. Bhat, Suja Elizabeth, J. Phys. Condens. Matter 22 (2010) 026005. [24] S. Roßler, S. Harikrishnan, U.K. Roßler, Suja Elizabeth, H.L. Bhat, F. Stegkich, S. Wirth, J. Phys. Conf. Ser. 200 (2010) 012168. [25] Octavio Pen~a, Mona Bahout, Karim Ghanimi, Pedro Duran, Dionisio Gutierrez, Carlos Moure, J. Mater. Chem. 12 (2002) 2480. [26] Sanjay Biswas, Momin Hossain Khan, Sudipta Pal, Esa Bose, J. Supercond. Nov. Magn. 27 (2014) 463. [27] Yanwei Ma, M. Guilloux-Viry, P. Barahona, O. Peña, C. Moure, Appl. Phys. Lett. 86 (2005) 062506. [28] T. Elovaara, H. Huhtinen, S. Majumdar, P. Paturi, J. Phys. Condens. Matter 24 (2012) 216002. [29] S. Harikrishnan, C.M. Naveen Kumar, S.S. Rao, H.L. Bhat, S.V. Bhat, Suja Elizabetha, J. Appl. Phys. 104 (2008) 023902. [30] S. Sagar, V. Ganesan, P.A. Joy, Senoy Thomas, A. Liebig, M. Albrecht, M. R. Anantharaman, Europhys. Lett. 91 (2010) 17008. [31] B.S. Nagaraja, Ashok Rao, G.S. Okram, J. Supercond. Nov. Magn. 28 (2015) 223. [32] Lija K. Joy, S. Shanmukharao Samatham, Senoy Thomas, V. Ganesan, Salim AlHarthi, A. Liebig, M. Albrecht, M.R. Anantharaman, J. Appl. Phys. 116 (2014) 213701. [33] Ajay Soni, G.S. Okram, Rev. Sci. Instrum. 79 (2008) 125103.
[34] G. Venkataiah, Y.K. Lakshmi, P.V. Reddy, PMC Phys. B 1 (2008) 1. [35] G. Ch. Kostogloudis, Ch Ftikos, J. Mater. Sci. 34 (1999) 2169. [36] P.A. Yadav, A.V. Deshmukh, K.P. Adhi, B.B. Kale, N. Basavaih, S.I. Patil, J. Magn. Magn. Mater. 328 (2013) 86. [37] L. Li, L. Lin, Z.B. Yan, Q.Y. He, J.-M. Liu, J. Appl. Phys. 112 (2012) 034115. [38] S. Hcini, S. Zemnia, A. Triki, H. Rahmouni, M. Boudard, J. Alloy. Compd. 509 (2011) 1394. [39] A.M. Kadomtseva, Yu. F. Popov, G.P. Vorob’ev, V. Yu Ivanov, A.A. Mukhin, A. M. Balbashov, JETP Lett. 82 (2005) 590. [40] R. Kannan, D. Vanidha, A. ArunKumar, J. Eng. Sci. Technol. 5 (2013) 211. [41] S. Sagar, M.R. Anantharaman, Bull. Mater. Sci. 35 (2012) 41. [42] V. Raja Kumari, G. Venkataiah, P. Venugopal Reddy, Int. J. Mod. Phys. B 19 (2005) 3619. [43] T. Izgi, V.S. Kolat, N. Bayri, A.O. Kaya, H. Gencer, S. Atalay, J. Optoelectron. Adv. Mater. 13 (2011) 648. [44] M. Viret, L. Ranno, J.M.D. Coey, J. Appl. Phys. 81 (1997) 4964. [45] S. Mollaha, H.L. Huang, H.D. Yang, Sudipta Pal, S. Taran, B.K. Chaudhuri, J. Magn. Magn. Mater. 284 (2004) 383. [46] K. Padmavathi, G. Venkataiah, P. Venugopal Reddy, J. Magn. Magn. Mater. 309 (2007) 237. [47] O. Pen~ a, C. Moure, P. Barahona, M. Baibich, G. Martinez, Phys. B: Condens. Matter 384 (2006) 57. [48] J. Hemberger, S. Lobina, H.-A. Krug von Nidda, N. Tristan, V. Yu Ivanov, A. A. Mukhin, A.M. Balbashov, A. Loidl, Phys. Rev. B 70 (2004) 024414. [49] V.A. Khomchenko, I.O. Troyanchuk, R. Szymczak, H. Szymczak, J. Mater Sci. 43 (2008) 5662. [50] Sobhi Hcini, Michel Boudard, Sadok Zemni, Mohamed Oumezzine, Ceram. Int. 40 (2014) 16041. [51] S. Zemni, A. Gasmi, M. Boudard, M. Oumezzin, Mater. Sci. Eng. B 144 (2007) 117. [52] J. Yang, Y.P. Lee, J. Korean Phys. Soc. 51 (2007) 1560. [53] A.A. Zatsiupa, L.A. Bashkirov, I.O. Troyanchuk, G.S. Petrov, A.I. Galyas, L. S. Lobanovsky, S.V. Truhanov, J. Solid State Chem. 212 (2014) 147. [54] Aritra Banerjee, S. Pal, S. Bhattacharya, B.K. Chaudhuri, H.D. Yang, Phys. Rev. B 64 (2001) 104428. [55] G. Venkataiah, P. Venugopal Reddy, J. Appl. Phys. 106 (2009) 033706. [56] Mamatha D. Daivajna, Ashok Rao, G.S. Okram, J. Alloy. Compd. 617 (2014) 345. [57] D. Salazar, D. Arias, O.J. Durá, M.A. López de la Torre, J. Alloy. Compd. 583 (2014) 141.