Frequency effects on charge ordering in Y0.5Ca0.5MnO3 by impedance spectroscopy

Journal of Magnetism and Magnetic Materials 375 (2015) 227–233

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Frequency effects on charge ordering in Y0.5Ca0.5MnO3 by impedance spectroscopy Tuba Sarwar a,b,n, Afzaal Qamar c, Muhammad Nadeem a a

EMMG, Physics Division, PINSTECH, P. O. Nilore, Islamabad, Pakistan DPAM, PIEAS, P. O. Nilore, Islamabad, Pakistan c Queensland Micro-Nanotechnology Centre, Griffith University, Nathan, QLD 4111, Australia b

art ic l e i nf o

a b s t r a c t

Article history: Received 21 May 2014 Received in revised form 18 August 2014 Accepted 30 September 2014 Available online 5 October 2014

In this work, structural and electrical properties of Y0.5Ca0.5MnO3 are investigated by employing X-ray diffraction and impedance spectroscopy, respectively. Applied ac electric field showed the charge ordering transition temperature around 265 K and below this temperature the heteromorphic behavior of the sample is discussed in the proximity of TCO. With frequency effects the volume of robust charge orbital ordering (COO) domains diminishes due to different competing phases along with Jahn Teller distortions. Comprehensive melting and collapse of charge orbital ordering occurs below TN(125 K), where a colossal drop in the value of impedance is observed. The change in profile of modulus plane plots determines the spreading of relaxation time of intermingled phases. Hopping mechanism is elaborated in terms of strong electron phonon coupling. Variable range hopping model and Arrhenius model are used to discuss the short and long range hopping between Mn3 þ and Mn4 þ channels assessing the activation energy Ea. & 2014 Elsevier B.V. All rights reserved.

Keywords: Manganites Impedance spectroscopy Hopping mechanism Relaxation time Electron–phonon coupling Jahn–Teller distortion

1. Introduction The perovskite-type manganites are the cynosure of intense research in view of their extraordinary physical, electronic and magnetic properties as well as their potential uses for technological applications. The presence of colossal magnetoresistance (CMR) effect has made these manganese based oxides of immense curiosity with unwoven hope that these may replace metal oxide devices. The mixed-valance manganites Mn3 þ /Mn4 þ show a multitude of fascinating phase transitions such as paramagnetic state (PM), ferromagnetic metallic state (FM), antiferromagnetic state (AFM), canted antiferromagnetic state (CAF), orbital ordering (OO) and charge ordering (CO) etc., depending upon the composition, temperature, magnetic field, applied voltage as well as the doping level. The most peculiar property is charge ordering (CO), where free charge carriers are localized causing an alternation of magnetic ion Mn3 þ /Mn4 þ . For a doping level of x ¼½, charge ordering becomes more facile and a cooperatively ordered pattern of electronic degrees of freedom i.e. spin, charge, and orbital is formed [1,2]. The performance of Y1  xCaxMnO3 under the application of ac electric field is a subject of great importance for research.

n

Corresponding author. E-mail address: [email protected] (T. Sarwar).

http://dx.doi.org/10.1016/j.jmmm.2014.09.066 0304-8853/& 2014 Elsevier B.V. All rights reserved.

The long range robust CO state is unaffected even in high perturbations which make this complex perovskite oxide distinguish from others. Hence, it is interesting to emphasize on the electrical properties of Y0.5Ca0.5MnO3, to describe the nature of charge and orbital ordering states. The physics behind these phase transitions is also associated with the conduction mechanism of the system where it is believed that the electrical properties are controlled by the motion of eg electron from Mn3 þ to Mn4 þ via intervening oxygen ion [3]. The itinerant electrons are trapped within the pairs of Mn sites forming the Zener polarons [4] supplementing to double exchange (DE) interactions and lattice distortions. These polarons describe the magnetic and electrical transport behavior in manganites. The electrical transport in Y1  xCaxMnO3 is dominated by the hopping or tunneling of these polarons from lower to higher valence states. Study of Y0.5Ca0.5MnO3 samples would offer a complementary understanding of the structural and electrical properties of perovskite system, which is of significance for achieving a complete perceptive of physical properties. Intense research on the manganites has emerged many interesting and salient aspects and the present work contains the description of these distinct features in detail using impedance spectroscopy. Impedance spectroscopy is a useful and effective tool for evaluation of many electrical properties and is broadly used for the investigation of bound or mobile charge in the bulk or interfacial regions of any kind of solid or material such as ceramics [5], semiconductors [6], polymer [7],

Polycrystalline sample Y0.5Ca0.5MnO3 was synthesized by conventional solid state reaction method using the appropriate stoichiometric amount of commercially available components of oxides and carbonates with 99.9% purity. The equimolar amounts of powder oxides and carbonates were calcined in box furnace first at 1000 °C, 1100 °C with 3 °C/min heating rate for 18 h and then at 1200 °C with 4 °C/min heating rate for 17 h. After periodical betwixt grindings, the sintered blackish powder was pressed under 8 t pressure into circular pellets having 1.3 mm thickness and 12 mm diameter. Then these pellets were annealed to 1300 °C and finally to 1350 °C with 4 °C/min heating rate for 17 h. A longer annealing time enhances chemical homogeneity, compaction, grain size and also improves the density. The phase purity of samples was established by X-ray diffraction patterns using Rigaku butterfly type diffractometer. The intensities were recorded from 20° r 2θ r70° with a step scan of 0.02° for 3 s per step. AC impedance measurements were performed on sintered pellets using two probe method at room temperature on Alpha-N analyzer (Novocontrol, Germany). Fully automated Windeta software was used for data acquisition. The impedance measurements were carried out in a temperature range of 77–300 K and in the frequency range 1 rf r5 MHz with 0.2 V ac signal amplitude. The surfaces of both sides of pellet were properly cleaned and contacts were made by silver paint on opposite sides of the pellet. The pellet was arranged in a homemade sample holder and was fixed inside a liquid nitrogen dewar. DC power supply was connected to sample holder to stabilize the temperature. Before starting the impedance experiments, the dispersive behaviors of the leads were carefully checked and ensured the absence of any extraneous inductive or capacitive coupling in the whole experimental frequency range. An interactive commercially available computer software ZView was used to interpret the several simultaneous representations of the impedance data.

3. Results and discussions The X-ray diffraction pattern of Y0.5Ca0.5MnO3 is shown in Fig. 1. All the peaks are successfully indexed in (hkl) values and unit cell parameters are determined on the basis of orthorhombic structure by using the software utility program LUSCRI. Using the derived lattice parameters a¼ 5.306 Å, b ¼5.443 Å and c¼ 7.487 Å, mean steric distortion factor 3 D% [3] is calculated as D = ⎡⎣1/3 ∑i = 1 ( (ai − ā) /ai ) ⎤⎦ × 100, where

(

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ā = abc / 2 , a1 = a, a2 = b , a3 = c / 2 that comes to be 1.18% showing distorted orthorhombic O/ structure of Y0.5Ca0.5MnO3. The average radius of cations orA 4 has a predominant influence on the properties and is responsible for the degree of stabilization of charge ordered state on the application of external perturbations. The Mn–O–Mn bond angle and Mn–O distance also depend

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polycrystalline materials [8], insulators (dielectrics) [7] and superconductors [9] etc. Properties of selected Y1  xCaxMnO3 compounds have been investigated by different means [10–12]. But a detailed research and investigation over the electrical and structural properties of Y0.5Ca0.5MnO3 is missing especially about its behavior across the charge ordering (CO) transition, although comparing to Pr0.6Ca0.4MnO3 (PCMO) [13] and La0.5Ca0.5MnO3 (LCM) [14] that has been reported. We have successfully employed impedance spectroscopy to obtain information regarding the magnetic transitions which are masked in the dc electrical transport by passing ac current from the sample.

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Intensity[arb. units]

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2θ [degree] Fig. 1. X-ray diffraction pattern of Y0.5Ca0.5MnO3 sintered at 1350 °C.

upon orA 4. It is calculated as = ∑i x iri , where xi is the fractional substitution level of the ith rare-earth site species with ionic radius ri [15]. For Y0.5Ca0.5MnO3 having r Y3 + (1.075 Å) and rCa2 + (1.18 Å), it is 1.128 Å. The smaller leads to the reduction of Mn–O–Mn bond angle that clasps the MnO6 octahedral influencing the electron hopping and bandwidth. Due to this the bandwidth becomes very narrow and electrons get trapped, leading to the formation of rugged charge and orbital ordering insulating state. From orA 4, the cation size variance s2 is calculated with the equation s2 ¼ ∑ixiri2  orA 4 2 that comes as 1.63  10  3 Å2 describing the effects of disorder due to the disparity or mismatch of average cation radii [15]. Tolerance factor tells the size of mismatch that comes into being when A-site cations are too small to fill the space in the three dimensional network of MnO6 octahedral. An ideal cubic perovskite has t¼1. For this value of tolerance factor there is no deviation of Mn–O–Mn bond angle from 180°. Stable perovskite materials have a tolerance factor lies in the range 0.8 oto 1 [16]. Such materials have symmetry but Mn–O–Mn bond distance shrinks and the octahedral tilt and deviate from 180° by an angle θ and hence leads to φ ¼180°-θ and this θ increases with the decrease in orA 4 . Tolerance factor for Y0.5Ca0.5MnO3 is calculated from < rA > +ro , where r0 and rMn are the ionic radii of orA 4 as t = 2 (rMn + ro)

oxygen O2-(1.40 Å) and manganese Mn3 þ (0.65 Å), Mn4 þ (0.53 Å) sites respectively and is given as 0.909 defining the stable perovskite structure of polycrystalline sample [3]. Fig. 2(a) shows the variation in real part of impedance Z/ against temperature at different frequencies ranging from 1 Hz to 5  106 Hz. At applied lower ac field (i.e. 10 Hz and 100 Hz); an exponentially increasing trend with decrease of temperature is obvious. However, when ac field intensifies an anomaly is observed that shifts towards high temperature side. A second anomaly arises at 100 kHz that also shifts towards high temperature zone with increasing frequency. At lower temperature the trend of Z/ at different frequencies is dispersive whereas at higher temperatures (around 265 K) it shows very small dispersion. This temperature is attributed to charge ordering temperature TCO where a complete ordered pattern charge localization of Mn3 þ and Mn4 þ ions is observed. Above TCO there exist paramagnetic phase. Y0.5Ca0.5MnO3 has a distorted orthorhombic structure (O/-phase), which gives rise to Jahn–Teller distortions that weakens the orbital ordering. The first anomaly at higher temperature side corresponds to these Jahn–Teller distortions whereas the

T. Sarwar et al. / Journal of Magnetism and Magnetic Materials 375 (2015) 227–233

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Fig. 2. Variation in (a) real part of impedance Z/ versus temperature and (b) imaginary part of impedance Z// versus frequency.

second anomaly towards lower temperature side is due to the canted type CE-antiferromagnetic interactions which become dominant around 125 K known as TN. The region between TN oT oTCO is very interesting which is a pronounced representation of coexisting phases. Coulomb interactions play a prominent role on the charge ordering of Y0.5Ca0.5MnO3, that exist among charge carriers [17]. The ionic radius of yttrium is rY3 þ ¼ 1.075 Å. Coulomb potential is inversely proportional to the radius so for a smaller value of distance there is a larger value of Coulomb potential. Due to this large value it attracts oxygen ion, leading to a decrease in the overlapping of orbital of manganese and oxygen. In this way the Mn–O bond is weekend and thus Jahn Teller distortions of the crystal lattice become prominent [18]. Due to Coulomb interactions, spin orbital coupling, anisotropy arise which enhance the strong co-operative Jahn–Teller effect that badly frustrating the spins of manganese. In Y0.5Ca0.5MnO3 sample the localization of eg electrons between Mn3 þ and Mn4 þ introduces a pronounced modulation of charge ordered antiferromagnetic state. These antiferromagnetic interactions are predominant at higher frequencies and are becoming less significant at low frequencies. Also when applied ac field intensifies an obvious decrease in the value of resistance (from 1  107 ohm to 10 ohm) at low temperature region can be seen which is known as colossal drop. The presence of this drastic resistance drop indicate the presence of conducting channels

which reduce the spin barrier and facilitate the nearest neighbors to tunnel by breaking the non-conducting links between two parallel spin clusters. Fig. 2(b) shows frequency dependent profile of imaginary part of impedance Z// at different temperatures. The plot shows the magnitude of Z// goes on decreasing with decreasing temperature defining a peak at a frequency fR. The defined peaks not only broaden with increase in temperature but a significant shift in fR is obvious that finally merges towards the higher frequency zone. Relaxation frequency fR is a measure of ease with which carriers follow the change in applied ac electric field and can be directly correlated with the mobility of the charge carriers [19]. The loss spectrum for Y0.5Ca0.5MnO3 is explained on the basis of distribution of relaxation times τ, where τ = 1/2πfR and it is possibly due to different surroundings of various ions. With increase in temperature, the broadening in the defined peaks with increasing relaxation frequency and thus decreasing relaxation time indicate the spreading of relaxation process in Y0.5Ca0.5MnO3. This is more elaborated in further discussions. The Nyquist plot between the real part of impedance Z/ and the imaginary part of impedance Z// is shown in Fig.3 and the arrow indicates the direction of increasing frequency. On right hand side of the plot the point where semicircle intersects the Z/ give the resistance corresponding to the dc values Rdc as (f-0). The plot shows only a single semicircle in all temperature range from which it is inferred that the relaxation time τ = R × C, where R and C are the corresponding resistance and capacitance, respectively) of different competing phases is not well resolved and the time constants of the two phases are not very different from each other. In order to provide a complete description of the electrical properties of Y0.5Ca0.5MnO3 pellets to a realistic extent, impedance plane plots are successfully fitted by employing equivalent circuit models. Two model circuits are fitted depending on the goodness of fit with 2–3% fitting error in whole temperature range. The equivalent circuit that is used for data fitting in 77–150 K temperature range is (R1C1)(R2Q2)(R3C3), where Q2 is the constant phase element, (CPE). In order to attain a real and better fit of impedance system, the capacitor C is replaced by CPE such that C = R(1 − n)/ nQ1/ n, where n defines the nature of CPE. A CPE exhibits a frequency dependent non-ideal capacitive behavior [20], where n ¼1, represents a pure capacitor and n¼ 0 represents a pure resistor. The equivalent circuit to fit the experimental data in a temperature range of 150–300 K consists of a series association of three (RC) terms, (R1C1)(R2C2)(R3C3). In this model the CPE is replaced by the capacitor itself due to exceeding the value of n from unity for 153 K. Two different fitting models suggest the presence of two relaxation processes in the system with sufficient

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Fig. 4. Variation of (a) resistances of grain (red), intrinsic grain boundary (blue) and extrinsic grain boundary (black) as a function of temperature and (b) n (nature of constant phase element) with respect to temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

different relaxation times. Experimental impedance plane plots fail to recognize different relaxation time corresponding to different relaxation processes but the theoretical models clearly distinguish it by fitting two different models with respect to temperature. Using the equivalent circuit models, R1, R2 and R3 are calculated and plotted against temperature as illustrated in Fig. 4(a). The decrease of resistance with increasing temperature is obvious indicating that the mobility of charge is increasing and hence enhancing the conduction mechanism. The charge and spin dynamics of Y0.5Ca0.5MnO3 cannot be fully understood without taking into account electron–phonon coupling [21]. This strong electron–phonon coupling forms polarons which may be described as the association of the localized charge carriers with lattice distortions. The electrons are trapped in Mn3 þ –O–Mn3 þ and Mn4 þ –O–Mn4 þ channels where they get localized. From the analyses of the values of resistances it is yielded that R2 and R3 may be ascribed as the resistances of grain boundaries because of the enormously large values as grain boundary impedance is dominant impedance in the sample. In polycrystalline colossal magnetoresistive (CMR) compounds grain boundaries play a significant role in contributing towards the low field effects. The

grain boundaries behaving as canted spin regions of high resistance along with the microstructure have a profound effect on the transport properties of these materials. In polycrystalline oxides with ionically or electronically conducting grains, grain boundaries are generally more resistive than grain interiors [22] that provide a conducting channel to the transport of the charge carriers. Hence, the resistance of grain R1 is attributed to small values. Moreover, R3 is ascribed to the resistance of extrinsic grain boundary that is influenced by the external factors such as temperature and applied ac field that have profound effects on lattice parameters. R2 is attributed to the resistance of intrinsic grain boundary that is due to the canted type nature of spins or the random freezing of spins defining the charge ordering, Jahn–Teller distortions and antiferromagnetic interactions. These intrinsic factors may increase the resistance by introducing an electrostatic barrier or a domain wall to reduce the spin polarized tunneling. Certain domains or sites introduced by the intrinsic effects are interconnected through weak channels. These weak links have been ascribed as intrinsic grain boundaries behaving as canted spin region of high resistances. Below 150 K electron–phonon interaction is so robust that these become immobile. With increase in temperature, the density of polarons decreases with breaking the weak links and increasing the mobility that ultimately enhance conduction mechanism. When the charge carriers are activated with the rise of temperature around 150 K, the CPE converts to a capacitor changing the strength of relaxation process. Fig. 4(b) represents the variation of n with temperature. The value of n is set such that it varies freely. It is obvious that when temperature is increased from 77 K the value of n decreases but at a certain temperature i.e. 125 K it start increasing till 150 K where n approaches to 1 showing insulating behavior such that CPE is converting to a capacitor. Fig.5(a), (b) and (c) shows the typical plane plots of the real M/ against the imaginary M// parts of the measured modulus in 300– 153 K, 150–125 K and 125–77 K temperature ranges, respectively where the arrow indicates the direction of decreasing frequency. One can clearly see only single semicircle within 300–153 K that gradually transforms into two in low frequency arc upon decreasing temperature beyond 153 K. For the first time this second contribution appears at 150 K that becomes significant at TN ¼125 K. Further decreasing temperature enhances the second contribution. These clearly show the presence of two relaxation processes with sufficient different relaxation time τ which is in good agreement to earlier discussion. In modulus plots, for the first time the contribution of grain and grain boundary is distinguishable showing the relaxation time of both phases is now well resolved. Due to less thickness as compared to grain interiors, grain boundaries offer large capacitance resulting larger RC, that lead to the greater value of relaxation time [23]. Therefore, the low and high frequency arcs are attributed as the relaxation of charge carriers at grain boundaries and grain interiors, respectively. Hence modulus plane plots can be used to extract more information about the sample as it reveals the contribution of bulk itself whereas the impedance plane plot fails to give details about the bulk and is unable to separate the grain boundary and grain interior. Fig. 6(a) shows the effect of frequency on real part of modulus M/ at different temperatures. It is interesting to note that M/ is strongly frequency dependant at the application of lower ac field but after a certain frequency fR, it becomes frequency independent. Relaxation frequency fR increases and shifts towards high frequency side with the increase in temperature. This shifting is an indication of the accumulation of space charge in the material. These results justify the impedance results and reinforce the previous discussions. However, at higher frequencies M/ seems to merge for all temperatures, which is an indication of possible

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release of space trapped charges. Hence the perturbation of ac field defines the conduction mechanism in the material. Fig. 6(b) illustrates the variation of imaginary part of modulus M// with frequency at different temperatures. Two peaks are obvious in this plot with an obvious shifting in the peaks that occur towards the high frequency zone with the increase of

temperature. These results correlate with the impedance results where the same shifting is observed in the plot of imaginary part of impedance against frequency fR. The high frequency peak can be ascribed to the grain and a peak at relatively low frequencies is attributed to grain boundary. The role of grain boundaries and grains is obvious only for low temperatures whereas at high temperature only the contribution of grain boundary can be viewed because grain is out from the frame of experimental data. It is worth to mention that due to limitations of high ac field facility the contribution of grain is not obvious for high temperature region. Hence the shift in the peak values of both real and imaginary part of modulus is a signature of accumulation of space charge in the material with the spread of relaxation time.Due to the lattice distortions that give rise to Jahn Teller effect there is large degree of heterogeneity in the sample. This heterogeneity is the reason of the dispersive behavior of the curves which separate a single broad peak into two peaks. These results are in good agreement with the published results of Sinclair et al. [24]. Depression angle of the experimentally measured impedance data is determined with the help of ZView software at different temperatures and is shown in Fig.7. Depression angle is defined as the degree of inhomogeneity or heterogeneity that occurs in the material. It is the degree of distortion that occurs when the impedance plane plot is displaced below the real axis [25]. The

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Fig. 7. Variation of depression angle against temperature.

reason of this displacement may be due to the presence of distributed elements in the system. Magnetic transition temperatures are determined from the plots of depression angle against temperature. At 265 K a broad magnetic transition anomaly is observed due to charge ordering, the inhomogeneity is small with minimum width of relaxation time distribution. At TN where antiferromagnetic interactions are active, spins of Mn3 þ tend to align the neighboring spins of Mn4 þ antiparallel to each other and because of this ordered pattern a lower degree of heterogeneity is observed. In TN o ToTCO region, a maxima is observed that predicts a high degree of heterogeneity which is due to canted spin nature of the Y0.5Ca0.5MnO3 material. The distortions of MnO6 octahedral are developed along with the domains of different phases having dissimilar orientations and it results in the development of an in-commensurate structural modulation [26]. To discuss the conduction mechanism in Y0.5Ca0.5MnO3, Arrhenius model and variable range hopping (VRH) model are employed with determination of activation energy Ea (energy required to achieve a transition state).The kinetics of an activated process in Arrhenius model is given by σ = σo exp [ − Ea/(kB T )], where s, sο and kB are the conductivity of the phase, preexponential term and Boltzman's constant respectively [27]. Using Arrhenius model the activation energy for 100 kHz is found to be 93.18 meV. With enhancement of applied ac field, activation energy decreases that reinforce the previous statement less energy is required for transition, availability of free charge carriers increases with increase in frequency, mobility increases. Space charge seems to be melt. Similarly, activation energy is evaluated by employing variable range hopping (VRH) model described by σ = σo exp ⎡⎣ − B /(T 0.25) ⎤⎦, where B = 4Ea/ kB T 0.75 [28]. The calculated

(

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activation energy for 100 kHz and 153 K is 86 meV. The value of Ea calculated using Arrhenius model is comparable to the Ea calculated from VRH model. The difference of 7 meV can be expected due to the fact that activation energy for Arrhenius model is the hopping energy of polarons, whereas VRH model contain the activation energy for hopping as well as disorder and binding energy of the polarons [29].

4. Conclusion In summary, freshly synthesized polycrystalline sample Y0.5Ca0.5MnO3 is characterized structurally and electrically by employing X-ray diffraction and impedance spectroscopy. X-ray

diffraction analysis shows that the sample is single phase with distorted orthorhombic structure. Impedance studies illustrate the existence of charge ordering phase around 265 K. Comprehensive melting and collapse of robust charge orbital ordering occurs below TN (125 K), where canted type CE-antiferromagnetic interactions become dominant and a colossal drop in the value of impedance is observed. In the region TN oT oTCO, Coulomb interactions give rise to strong co-operative Jahn–Teller distortions that badly frustrate the spins of manganese and this kind of heterogeneity is also explained by the depression angle plot. Further impedance studies elaborate that the increase in temperature lead to the increase in relaxation frequency and thus decreasing relaxation time indicate the spreading of relaxation process in the sample. Two different theoretical models fitting with respect to temperature are discussed on the basis of two relaxation processes having different relaxation times. Modulus plane plots are a good representation of the presence of two well resolved phases indicating the contribution of grain boundary and the grain interior. Hopping mechanism is elaborated in terms of strong electron–phonon coupling. The density of strong electron–phonon couples reduces with increase in temperature and the deep traps of charge carriers in the Mn3 þ /Mn4 þ channels intervening oxygen become shallow around 150 K defining the conduction mechanism in the sample.

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