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Structural, magnetic and transport properties of La0.70Sr0.21K0.09MnO3
Accepted Manuscript Structural, Magnetic and Transport Properties of La0.70Sr0.21K0.09MnO3 E. Taşarkuyu, A.E. Irmak, A. Coskuna, S. Aktürk PII: DOI: Reference:
S0925-8388(13)02765-5 http://dx.doi.org/10.1016/j.jallcom.2013.11.035 JALCOM 29864
To appear in: Please cite this article as: E. Taşarkuyu, A.E. Irmak, A. Coskuna, S. Aktürk, Structural, Magnetic and Transport Properties of La0.70Sr0.21K0.09MnO3, (2013), doi: http://dx.doi.org/10.1016/j.jallcom.2013.11.035
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Structural, Magnetic and Transport Properties of La 0.70Sr0.21K0.09MnO3
E. Taşarkuyu, A.E. Irmak, A. Coskun and S. Aktürk Department of Physics, Faculty of Sciences, Mugla Sitki Kocman University, 48000, Mugla, Turkey Magnetic Materials Laboratory, Research Laboratories Center, Mugla Sitki Kocman University, 48000, Mugla, Turkey
Abstract In this work we investigated structural, magnetic and transport properties of La0.70Sr0.21K0.09MnO3 compound produced by the standard sol-gel route. The crystal structure was determined by analyzing XRD data and found to be an orthorhombic perovskite belonging to Pnma space group. The Curie temperature, TC, of the compound was determined to be 295 K which is near room temperature. The calculated magnetic entropy change was 0.77 J/kgK which is not high enough for magnetic cooling application. Insulator-Metal transition temperature, TIM, was determined to be 294 K, which coincides with TC. The low temperature peak near TIM reveals the coexistence of ferromagnetic and paramagnetic phases in the ferromagnetic region. The temperature dependence of resistivity was well fitted (with R2 =0.9999) to ρ(T)= ρ0 + ρ0.5Τ0.5 + ρ2Τ2 + ρ4.5Τ4.5 in a wide temperature interval including low temperature resistivity upturn. 1. Introduction. Lanthanum manganites (LaMnO3), in the form of a perovskite structure ABO3, are antiferromagnetic and insulator in a wide temperature range. When trivalent lanthanum is replaced by a divalent or a monovalent element, compound exhibit a paramagnetic–ferromagnetic (PF) transition at a certain temperature, known as the Curie temperature, TC. Generally, this magnetic phase transition is accompanied by an insulator to metallic transition at a temperature defined as TIM, which occurs near TC [1,3]. Doping of monovalent (such as, Na+, K+, Ag+) or divalent (such as, Ca+2, Sr+2, Ba+2) ions in place of La3+ ions, some of Mn3+ become Mn4+ ions proportional to the substituted ion’s valance state and concentration. A divalent ion oxidizes one Mn3+ to one Mn4+ and a monovalent ion oxidizes two Mn3+into two Mn4+ ions. The co-existence of manganese in two valance states (Mn3+ and Mn4+) plays a crucial role on the physical properties of these materials, such as TC, TIM, magnetization, resistivity and magnetic entropy change. The ferromagnetic nature of doped manganites is explained by the Double Exchange (DE) theory proposed by Zener [4]. DE occurs between Mn3+ and Mn4+ ions by exchanging their electron
and hole via oxygen’s 2p orbitals. Mn-O bond length and Mn-O-Mn bond angle are the key parameters that determine the strength of DE. On the other hand these parameters sensitive to the average A-site and B-site ionic radii. In an ideal perovskite structure, Mn ions are surrounded with a six equidistant oxygen ions constituting an octahedra. In doped lanthanum manganites however, doping ion may have different ionic radius than that of lanthanum ion, the perovskite structure gets distorted, or a total structural transition may occur depending on the concentration and ionic radius of the dopant. Occurrence of any structural change affects the strength of DE, and hence, the TC of the material [2,5,6]. Therefore, the role of A-site and/or B-site average ionic radii, the Goldsmith tolerance factor, t=(rA+rO)/(sqrt(2)(rB+rO)) and cation size mismatch (σ2) [7] of the material are crucial. The change in these parameters are related with Mn-O bond length and Mn-O-Mn angle, thereby the bandwidth [2,8]. Additionally, not only the percentage of Mn4+ ion, but also the structure of the surrounding of Mn4+ ion has an influence on the transition temperatures. In addition to DE mechanism, Jahn–Teller effect and electron–phonon coupling [9-11] arising from the deformation of MnO6 octahedra play an important role. One of the lanthanum manganites, doped with Sr2+, La1-xSrxMnO3, gain ferromagnetic and metallic character up to elevated temperatures which are higher than room temperature [12-14]. However, in order for a material to be a good candidate for magnetic applications, its TC and TIM temperatures should be around room temperature. Several researchers have tried to reduce PF and IM transition temperatures by increasing Sr concentration on La-site or doping small amount of transition metal elements on Mn-site. Lalitha and Venugopal Reddy [15] have managed to reduce TIM from 254 K to 180 K through doping the parent compound, La0.67Sr0.33MnO3, by10 % Bi. The solid-state synthesized La0.7Sr0.3MnO3 compound with TIM=360 K, doping with 5 % Fe to Mn-site leads to 30 degrees decrease in T IM [12]. In another study, the TC of La0.70Sr0.30MnO3 compound was increased by applying pressure up to 5.8 GPa [13]. In that study, TC changed linearly with pressure from 370 K to 395 K, due to the structural changes induced by pressure. Moreover, Urushibara et al studied floating zone produced crystal of La1-xSrxMnO3 in a wide doping range and obtained the material having TC of 309 K for x=0.20 [14]. Dhahri et al [16] studied the La0.67Sr0.33MnO3 compound by keeping Sr content unchanged but replacing some La with Eu. They determined the TC of the parent compound as 350 K and TIM as 310 K. Substituting La by 10 % Eu leads TC to reduce to 296 K and TIM to 285 K. Further increase in Eu decreased TC to 224 K and TIM to 224 K [16]. It has been found by Mnefgui et al that the substitution of Mn with other metals drastically lowers TC and TIM [17]. Itoh et al has shown a close relationship between TC and Mn-O average bond distance [18]. In general, magnetic and transport properties of lanthanum manganites can be modified by doping La-site with divalent/monovalent ions or Mn-site with transition metal ions, leading to change Mn4+/Mn3+ ratio and/or disorder on La-site or Mn-site, which is expected to
reduce the tendency towards ferromagnetic long range order and thus changing magnetic and electrical properties [7,19-22]. In addition, the transport and magnetic properties of manganites are shown to be sensitive to oxygen content, lowering oxygen content results in high resistivity, low magnetization and low Curie temperature [23]. In the present work, we explored the effect of 9 % K doping in place of Sr in La0.70Sr0.30MnO3 compound on TC, TIM. And the produced compound, La0.70Sr0.21K0.09MnO3, investigated in detail by structurally, magnetically and electrically.
2. Experimental Procedure In this study, the La0.70Sr0.21K0.09MnO3 manganite compound has been prepared by sol-gel method. Appropriate amounts of La2O3, Sr(NO3)2, K(NO3)2 and Mn(NO3)2 with desired stoichiometry were dissolved in dilute HNO3 solution at 150 oC. Then citric acid and ethylene glycol were added to the mixture. Viscous residual was formed by slowly boiling this solution at 200 oC. The obtained residual was dried slowly at 300 oC until dry-gel was formed. Finally, the residual precursor was burned in air at 600 oC. The material obtained from this process was ground by using an agate mortar to have fine powder. A pellet was produced from the powder by pressing into 13 mm radius and 2 mm thickness under the pressure of 3 tons. The pellet was sintered at 1100 o
C for 24 hours in air and cooled down to room temperature in the furnace. SEM investigations were performed using a JEOL SEM 7700F. XRD pattern of the samples
were obtained within 20 o≤2θ≤70 o with a step size of 0.01 o waiting 5.4s between the steps using a Bruker D8 Advance X-Ray Diffractometer with a CuKα1 radiation. The TEM images of the sample were taken using JEOL JEM 2100F HRTEM. The magnetic properties of the sample were investigated using Quantum Design PPMS with a closed cycle helium cryostat from 5 K to 320 K with magnetic fields up to 5 Tesla. TC value of the compound was determined from measurements of temperature dependence of magnetization at an applied field of 100 Oe. The measurement sequence is as follows: the sample was first cooled down to 5 K under zero field, and magnetization was measured while heating up the sample up to 320 K with an applied field of 100 Oe (ZFC). Then, under the same magnetic field, the magnetization was measured again while cooling down to 5 K (FC). In order to determine the magnetic entropy change exhibited by the sample, field dependent magnetization M(H) measurements were performed by changing H from 0 to 5 T around TC with constant temperature intervals of 4 K between 260 K-320 K. The resistivity of the sample is measured from 320 K down to 10 K with a standard four probe technique using a closed cycle helium cryostat.
3. Results and Discussion 3.1 Structural Properties The crystal structure of the La0.70Sr0.21K0.09MnO3 compound was investigated by XRD method with the CuKα1 radiation. As stated above, smaller steps with longer period between the steps were chosen in order to have a well resolved diffraction pattern. The obtained XRD pattern of the sample is given in Figure 1. Firstly, one could recognize by inspecting the available data from the literature [24], that the pattern is very likely to belong to a typical orthorhombic structure. With this assumption, some of the peaks were identified accordingly. Then, the calculations were carried out on a computer using both Bragg’s law of reflection and the equation for the plane separations for the orthorhombic structure, namely, h2/a2+ k2 /b 2+ l2/c2=1/d2hkl. The unit cell parameters, a, b, and c, were initially estimated and then refined iteratively until all the calculated reflection angles fitted to the observed ones. The calculated unit cell parameters and the unit cell volume are a=5.525 Å, b=7.787 Å, c=5.475 Å and V=235.6 Å3 respectively for an orthorhombic perovskite with Pnma space group. In fact, changing the orders of the parameters as a=5.475 Å, b=5.525 Å, c=7.787 Å, it is possible to obtain a good fit with Pbnm structure, as discussed by I.S. Smirnova [25 and references there in]. Of course, this will result a different indexing of the peaks. The indices according to Pnma structure were used labeling the reflection peaks as presented in the figure. In our analysis, we also observed the existence of an additional weak reflection at 2θ=30.70o belonging to La2O3 impurity phase, probably due to residual unreacted precursor from sol-gel process. In a paper published by Kallel et al [26] structure of La0.70Sr0.3MnO3 compound was
investigated and it was reported that it had a structure with a simple rhombohedral Bravais lattice. The unit cell parameters of La0.70Sr0.3MnO3 was reported to be a=5.5023 Å, c=13.3569 Å. The discrepancy between their result and our result about the crystal structure would be attributed to K+ doping. In Our La0.70Sr0.21K0.09MnO3 sample as a result of partial replacement of divalent Sr2+ ion by monovalent K+ ion a structural transformation might have taken place due to the ionic radii differences between K+ and Sr2+ ions. The ionic radius of K+ (1.64 Å) is greater than Sr2+ (1.44 Å). This will lead to an increase in average ionic radius of A-site. But, K+ doping causes not only an increase in A-site average ionic radius but also a decrease in B-site average ionic radius. It is known that ionic radius of Mn3+ion is larger than that of Mn4+ ion. Substitution of a monovalent ion, such as K+, results in an increase in the number of Mn4+ ions and a decrease in the number of Mn3+ions twice as much that of a divalent ion substitution may result. Considering both increasing A-site and decreasing B-site average radii with almost the same amount, one would mistakenly think no net
effect in the structure of the compound would takes place. However, an occurrence of a structural change would be expected. The sample was also put under TEM investigation for the determination of micro-structure. Figure 2 shows the images with increasing magnifications from a small portion of a crystallite up to atomic resolution. Actually, Figure 3 was acquired by two successive Fourier Transforms of the real image in order to filter out unwanted noise on the images. An important feature of the sample that was revealed by TEM is that the interplanar distances. Average atomic distances were calculated by taking profiles along different directions, seemingly making right angle. But a closer look shows an obliquity between these directions. The average distances between the most apparent atoms are found to be 3.61 and 3.82 Å. The obliqueness and the difference between the distances are clear evidences indicating that the structure is not cubic (but distorted cubic). It was also found that there are planes (indicated by lines in 2D image) which make right angle to each other with separations of 5.17±0.02 and 5.32±0.02 Å (line profiles along these two lines are given as an inset at Figure 3 for a clear inspection) supporting orthorhombic structure. However, for some reason these parameters do not agree with parameters calculated from XRD data. The surface morphology of La0.70Sr0.21K0.09MnO3 was investigated using a SEM and a typical image is given in Figure 4. From the image, the particle size distribution was investigated and the particle sizes were found to vary from 1 µm to 5 µm, as can be seen on the surface of the sample. A closer look at the image reveals that some grains were actually formed by clusters of several small grains. The sample surface also exhibits porosity between loosely connected grains.
3.2 Magnetic Properties In Figure 5, the temperature dependence of both ZFC and FC (100 Oe) magnetization for the sample is given. The PF transition temperature TC was calculated from FC curve, where
∂M is ∂T
minimum, and found to be about 295 K. The Curie temperature of the sample being coincided with room temperature, due to the K doping, is important for technological applications at room temperature. Another feature, that the magnetization versus temperature curve of the sample displays, is a bifurcation between FC and ZFC processes. The origin of this behavior may be attributed to the anisotropical nature of the compound [27]. The broadness of PF transition observed on the magnetization curve might be an indication of paramagnetic phases besides ferromagnetic phases that the sample possesses [28]. This interpretation would be consistent with our ρ(T) analysis. On the other hand, the decrease of TC due to K doping when compared with the literature on La0.70Sr0.30MnO3 compound, especially with the work of Kallel et al [26] can be understood by two possible explanations. One would be the
crystallographic distortion induced by K substitution. The A-site average ionic radius of the undoped compound increased from 1.2442 to 1.2658 Å doping with K+ ion having a larger ionic radius than Sr2+. In addition, the substitution of K+ for Sr2+ can lead to a decrease in the average Bsite radius, since the ionic radii of Mn+4 is smaller than that of Mn+3 ion, and every K+ creates two Mn4+annihilating two Mn3+, the B-site average radius decreased from 0.6105 to 0.6002 Å. Therefore, K doping results in a local distortion of the MnO6 octahedra because of these ionic radii mismatch. A-site ionic size mismatch, σ, increased from 0.0436 Å, doping A-site with some K+ ion instead of Sr+2, to 0.0970 Å. Increase of A-site average ionic radius, decrease of B-average ionic radius and an increase in size mismatch parameter give rise to distortion of the MnO6 octahedra. As a result of distortion of MnO6 octahedra the Mn−O bond length and Mn−O−Mn bond angle changes [29]. The changes on these parameters has an influence on the hoping of an eg electron of Mn3+ to Mn4+ through O 2p orbital giving rise to changes on the TC and TIM temperature of the compounds. The structure distortions including JT effect, play a less but not negligible role on the eg bandwidth and the Curie temperature TC [2,8]. In our study the Curie temperature of the parent compound decreased to room temperature with a small amount of K+ doping. The data of ionic radii in this paper are obtained from Ref [30]. Another explanation for the decrease of TC would be that the substitution of K+ ions for Sr2+ ions can lead to an increase in the Mn4+/Mn3+ ratio above a critical level favoring super-exchange mechanism. In fact, being a monovalent ion, K+ ion creates two neighboring Mn4+ ions nearby to itself, yielding an additional increase in the possibility of super-exchange with respect to the same Mn4+/Mn3+ ratio one could get with substituting divalent ions. Even though with an appropriate amount of divalent ion substitution, the Mn4+/Mn3+ ratio would be maintained, the possibility of super-exchange lowers since all Mn ions (whether Mn4+ or Mn3+) are more likely to be distributed randomly, having chance to do DE. Figure 6 shows the isothermal magnetization curves taken from 260 to 320 K with intervals of 4 K and from 0 to 5 T applied field. The curves were used for calculation of magnetic entropy changes under the field changes of 1, 3 and 5 T. The magnetization curves exhibit no significant change above and below TC. This is an indication of low magnetic entropy change, because, the considerable change of magnetization near TC yields a larger magnetic entropy change. The temperature dependence of magnetic entropy changes, |∆Sm|, are plotted in Figure 7. The maximum entropy change corresponding to a magnetic field change of 1 T is 0.75 J/kgK. The weak maximum magnetic entropy change in the sample could be caused by a reduction of the double-exchange interactions between Mn3+ and Mn4+ ions and the poor spin-lattice coupling arising from variation in Mn–O bond length and Mn–O–Mn bond angle [31].
3.3 Electrical Transport Properties The temperature dependence of resistivity is plotted in Figure 8. The resistivity increases as temperature is decreased reaching to a maximum value of 1.28 Ωcm at 294 K. This is the turning point for resistivity, where it starts decreasing with temperature showing a metallic conductivity behavior down to lower temperatures. At T=33 K, ρ reaches its minimum (0.46 Ωcm) and further decrease in temperature leads the material to exhibit insulator character. Normally, the electrical resistivity of manganites in the ferromagnetic region decreases with decreasing temperature (dρ/dT>0). In contrast, the electrical resistivity of some manganites in ferromagnetic region is found to increase, exhibiting an insulating behavior. This type of behavior was reported earlier in magnetic materials containing small amounts of magnetic impurities and the behavior was explained using several models based on Kondo effect [32], electron–electron interactions, weak localization effect of electrons, [33] intergranular tunneling of the polarized charge carriers, inelastic scattering effects and uncontrolled magnetic impurities [15,34-40]. The derivative of the ρ-T curve with respect to temperature was calculated numerically to determine paramagnetic-insulator to ferromagnetic-metal phase transition of the compound. The derivative gives two extremum points. The one at high temperature region is paramagnetic-insulator to ferromagnetic-metal transition temperature TMI (294 K). The other is the temperature where low temperature minima occurs TLmin (33 K). In addition, another transition shown in the inset of Figure 8 is the emergence of low temperature peak (LT-peak, TLT=288 K) near TC. It might be due to phase coexistence which is consistent with the broadening of the M(T) curve around TC. Besides, grain size and intergrain connectivity strongly affect electric conductivity, leading in some cases to the formation of LT-peak [28]. To understand the temperature dependence of resistivity, it is useful to examine ρ(T) in more detail. The temperature dependence of ρ is fitted to the following equation within a large temperature rage,10 K
(1)
Here ρ0 is residual resistivity arising from domain boundaries and other temperature-independent scattering mechanisms, ρ0.5 term takes care of contributions from correlated electron-electron interactions, ρ2 indicates the electron-electron scattering, while ρ4.5 is attributed to two magnon scattering process. The experimental data were fitted to Eq. (1) and the best fit curve (colored line) is given in Figure 8 and the calculated parameters are as follows: ρ0=(0.5231±0.0013) Ωcm,
ρ0.5=(0.0147±0.0002) Ωcm/K0.5,
ρ2=(2.0477±0.006)10 −5 Ωcm/K2
and
ρ4.5=(−6.2699±0.0311)10−12 Ωcm/K4.5. 4. Conclusion In this study, we have introduced 9 % potassium into La0.70Sr0.30MnO3 compound material in place of strontium, thereby having a compound with La0.70Sr0.21K0.09MnO3 chemical formula. By doing such a modification we have succeeded lowering both TC and TIM near room temperature with a price of lowering also the magnetic entropy change. Unfortunately, this happened most probably due to large distortion of the Mn−O octahedra caused by the large K+ ions introduced into the crystal structure. Being a monovalent ion, K+ might also have affected the local Mn4+ and Mn3+ distribution in the compound material resulting a pronounced SE interaction favoring antiferromagnetism, in turn, reducing magnetic entropy change, which is necessary for some applications. Acknowledgment:We gratefully acknowledge The Scientific and Technological Research Council of Turkey (TUBITAK) for the support of this study (Grant No:110T637).
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Figure 1 XRD pattern of La0.70Sr0.21K0.09MnO3 sample. The miller indices of the reflections were given on top of corresponding peaks and weak reflection is assigned to La2O3 impurity phase. Figure 2 A magnified TEM image on a crystallite, and the inset shows a portion of crystallite. Scales are shown with bars on the left bottom of the images. Figure 3 Reconstructed image by two successive Fourier Transforms of the real image. The inset shows a typical line profiles along two perpendicular straight lines. Figure 4 SEM image of the sample. Figure 5 Temperature dependence of Zero Field Cooled and Field Cooled magnetizations. Figure 6 Field dependent magnetization isotherms. Figure 7 Temperature dependence of magnetic entropy changes calculated for 1T, 3T and 5T magnetic field changes. Figure 8 Temperature dependence of resistivity. The inset is given here to show the Low Temperature peak.
•
La0.70Sr0.21K0.09MnO3 compound was produced using standard sol-gel route.
•
The produced compound possesses an orthorhombic perovskite structure belonging to Pnma space group.
•
The compound La0.70Sr0.30MnO3 has high TC and TIM. Reducing 9% Sr and adding that amount of K decreased TC and TIM to room temperature.
•
TC of the produced compound was determined to be 295K is almost equivalent to TIM (=294K) which was calculated from temperature dependence of resistivity.
•
The broadening of M(T) curve near TC and the low temperature peak near TIM were revealed the coexistence of ferromagnetic and paramagnetic phases in the ferromagnetic region. These results were consistent with the results of magnetic entropy change.
•
The temperature dependence of resistivity obeyed ρ(Τ)=ρ0 + ρ0.5Τ0.5 + ρ2Τ2 + ρ4.5Τ4.5 equation up to 285K.
S0925-8388(13)02765-5 http://dx.doi.org/10.1016/j.jallcom.2013.11.035 JALCOM 29864
To appear in: Please cite this article as: E. Taşarkuyu, A.E. Irmak, A. Coskuna, S. Aktürk, Structural, Magnetic and Transport Properties of La0.70Sr0.21K0.09MnO3, (2013), doi: http://dx.doi.org/10.1016/j.jallcom.2013.11.035
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural, Magnetic and Transport Properties of La 0.70Sr0.21K0.09MnO3
E. Taşarkuyu, A.E. Irmak, A. Coskun and S. Aktürk Department of Physics, Faculty of Sciences, Mugla Sitki Kocman University, 48000, Mugla, Turkey Magnetic Materials Laboratory, Research Laboratories Center, Mugla Sitki Kocman University, 48000, Mugla, Turkey
Abstract In this work we investigated structural, magnetic and transport properties of La0.70Sr0.21K0.09MnO3 compound produced by the standard sol-gel route. The crystal structure was determined by analyzing XRD data and found to be an orthorhombic perovskite belonging to Pnma space group. The Curie temperature, TC, of the compound was determined to be 295 K which is near room temperature. The calculated magnetic entropy change was 0.77 J/kgK which is not high enough for magnetic cooling application. Insulator-Metal transition temperature, TIM, was determined to be 294 K, which coincides with TC. The low temperature peak near TIM reveals the coexistence of ferromagnetic and paramagnetic phases in the ferromagnetic region. The temperature dependence of resistivity was well fitted (with R2 =0.9999) to ρ(T)= ρ0 + ρ0.5Τ0.5 + ρ2Τ2 + ρ4.5Τ4.5 in a wide temperature interval including low temperature resistivity upturn. 1. Introduction. Lanthanum manganites (LaMnO3), in the form of a perovskite structure ABO3, are antiferromagnetic and insulator in a wide temperature range. When trivalent lanthanum is replaced by a divalent or a monovalent element, compound exhibit a paramagnetic–ferromagnetic (PF) transition at a certain temperature, known as the Curie temperature, TC. Generally, this magnetic phase transition is accompanied by an insulator to metallic transition at a temperature defined as TIM, which occurs near TC [1,3]. Doping of monovalent (such as, Na+, K+, Ag+) or divalent (such as, Ca+2, Sr+2, Ba+2) ions in place of La3+ ions, some of Mn3+ become Mn4+ ions proportional to the substituted ion’s valance state and concentration. A divalent ion oxidizes one Mn3+ to one Mn4+ and a monovalent ion oxidizes two Mn3+into two Mn4+ ions. The co-existence of manganese in two valance states (Mn3+ and Mn4+) plays a crucial role on the physical properties of these materials, such as TC, TIM, magnetization, resistivity and magnetic entropy change. The ferromagnetic nature of doped manganites is explained by the Double Exchange (DE) theory proposed by Zener [4]. DE occurs between Mn3+ and Mn4+ ions by exchanging their electron
and hole via oxygen’s 2p orbitals. Mn-O bond length and Mn-O-Mn bond angle are the key parameters that determine the strength of DE. On the other hand these parameters sensitive to the average A-site and B-site ionic radii. In an ideal perovskite structure, Mn ions are surrounded with a six equidistant oxygen ions constituting an octahedra. In doped lanthanum manganites however, doping ion may have different ionic radius than that of lanthanum ion, the perovskite structure gets distorted, or a total structural transition may occur depending on the concentration and ionic radius of the dopant. Occurrence of any structural change affects the strength of DE, and hence, the TC of the material [2,5,6]. Therefore, the role of A-site and/or B-site average ionic radii, the Goldsmith tolerance factor, t=(rA+rO)/(sqrt(2)(rB+rO)) and cation size mismatch (σ2) [7] of the material are crucial. The change in these parameters are related with Mn-O bond length and Mn-O-Mn angle, thereby the bandwidth [2,8]. Additionally, not only the percentage of Mn4+ ion, but also the structure of the surrounding of Mn4+ ion has an influence on the transition temperatures. In addition to DE mechanism, Jahn–Teller effect and electron–phonon coupling [9-11] arising from the deformation of MnO6 octahedra play an important role. One of the lanthanum manganites, doped with Sr2+, La1-xSrxMnO3, gain ferromagnetic and metallic character up to elevated temperatures which are higher than room temperature [12-14]. However, in order for a material to be a good candidate for magnetic applications, its TC and TIM temperatures should be around room temperature. Several researchers have tried to reduce PF and IM transition temperatures by increasing Sr concentration on La-site or doping small amount of transition metal elements on Mn-site. Lalitha and Venugopal Reddy [15] have managed to reduce TIM from 254 K to 180 K through doping the parent compound, La0.67Sr0.33MnO3, by10 % Bi. The solid-state synthesized La0.7Sr0.3MnO3 compound with TIM=360 K, doping with 5 % Fe to Mn-site leads to 30 degrees decrease in T IM [12]. In another study, the TC of La0.70Sr0.30MnO3 compound was increased by applying pressure up to 5.8 GPa [13]. In that study, TC changed linearly with pressure from 370 K to 395 K, due to the structural changes induced by pressure. Moreover, Urushibara et al studied floating zone produced crystal of La1-xSrxMnO3 in a wide doping range and obtained the material having TC of 309 K for x=0.20 [14]. Dhahri et al [16] studied the La0.67Sr0.33MnO3 compound by keeping Sr content unchanged but replacing some La with Eu. They determined the TC of the parent compound as 350 K and TIM as 310 K. Substituting La by 10 % Eu leads TC to reduce to 296 K and TIM to 285 K. Further increase in Eu decreased TC to 224 K and TIM to 224 K [16]. It has been found by Mnefgui et al that the substitution of Mn with other metals drastically lowers TC and TIM [17]. Itoh et al has shown a close relationship between TC and Mn-O average bond distance [18]. In general, magnetic and transport properties of lanthanum manganites can be modified by doping La-site with divalent/monovalent ions or Mn-site with transition metal ions, leading to change Mn4+/Mn3+ ratio and/or disorder on La-site or Mn-site, which is expected to
reduce the tendency towards ferromagnetic long range order and thus changing magnetic and electrical properties [7,19-22]. In addition, the transport and magnetic properties of manganites are shown to be sensitive to oxygen content, lowering oxygen content results in high resistivity, low magnetization and low Curie temperature [23]. In the present work, we explored the effect of 9 % K doping in place of Sr in La0.70Sr0.30MnO3 compound on TC, TIM. And the produced compound, La0.70Sr0.21K0.09MnO3, investigated in detail by structurally, magnetically and electrically.
2. Experimental Procedure In this study, the La0.70Sr0.21K0.09MnO3 manganite compound has been prepared by sol-gel method. Appropriate amounts of La2O3, Sr(NO3)2, K(NO3)2 and Mn(NO3)2 with desired stoichiometry were dissolved in dilute HNO3 solution at 150 oC. Then citric acid and ethylene glycol were added to the mixture. Viscous residual was formed by slowly boiling this solution at 200 oC. The obtained residual was dried slowly at 300 oC until dry-gel was formed. Finally, the residual precursor was burned in air at 600 oC. The material obtained from this process was ground by using an agate mortar to have fine powder. A pellet was produced from the powder by pressing into 13 mm radius and 2 mm thickness under the pressure of 3 tons. The pellet was sintered at 1100 o
C for 24 hours in air and cooled down to room temperature in the furnace. SEM investigations were performed using a JEOL SEM 7700F. XRD pattern of the samples
were obtained within 20 o≤2θ≤70 o with a step size of 0.01 o waiting 5.4s between the steps using a Bruker D8 Advance X-Ray Diffractometer with a CuKα1 radiation. The TEM images of the sample were taken using JEOL JEM 2100F HRTEM. The magnetic properties of the sample were investigated using Quantum Design PPMS with a closed cycle helium cryostat from 5 K to 320 K with magnetic fields up to 5 Tesla. TC value of the compound was determined from measurements of temperature dependence of magnetization at an applied field of 100 Oe. The measurement sequence is as follows: the sample was first cooled down to 5 K under zero field, and magnetization was measured while heating up the sample up to 320 K with an applied field of 100 Oe (ZFC). Then, under the same magnetic field, the magnetization was measured again while cooling down to 5 K (FC). In order to determine the magnetic entropy change exhibited by the sample, field dependent magnetization M(H) measurements were performed by changing H from 0 to 5 T around TC with constant temperature intervals of 4 K between 260 K-320 K. The resistivity of the sample is measured from 320 K down to 10 K with a standard four probe technique using a closed cycle helium cryostat.
3. Results and Discussion 3.1 Structural Properties The crystal structure of the La0.70Sr0.21K0.09MnO3 compound was investigated by XRD method with the CuKα1 radiation. As stated above, smaller steps with longer period between the steps were chosen in order to have a well resolved diffraction pattern. The obtained XRD pattern of the sample is given in Figure 1. Firstly, one could recognize by inspecting the available data from the literature [24], that the pattern is very likely to belong to a typical orthorhombic structure. With this assumption, some of the peaks were identified accordingly. Then, the calculations were carried out on a computer using both Bragg’s law of reflection and the equation for the plane separations for the orthorhombic structure, namely, h2/a2+ k2 /b 2+ l2/c2=1/d2hkl. The unit cell parameters, a, b, and c, were initially estimated and then refined iteratively until all the calculated reflection angles fitted to the observed ones. The calculated unit cell parameters and the unit cell volume are a=5.525 Å, b=7.787 Å, c=5.475 Å and V=235.6 Å3 respectively for an orthorhombic perovskite with Pnma space group. In fact, changing the orders of the parameters as a=5.475 Å, b=5.525 Å, c=7.787 Å, it is possible to obtain a good fit with Pbnm structure, as discussed by I.S. Smirnova [25 and references there in]. Of course, this will result a different indexing of the peaks. The indices according to Pnma structure were used labeling the reflection peaks as presented in the figure. In our analysis, we also observed the existence of an additional weak reflection at 2θ=30.70o belonging to La2O3 impurity phase, probably due to residual unreacted precursor from sol-gel process. In a paper published by Kallel et al [26] structure of La0.70Sr0.3MnO3 compound was
investigated and it was reported that it had a structure with a simple rhombohedral Bravais lattice. The unit cell parameters of La0.70Sr0.3MnO3 was reported to be a=5.5023 Å, c=13.3569 Å. The discrepancy between their result and our result about the crystal structure would be attributed to K+ doping. In Our La0.70Sr0.21K0.09MnO3 sample as a result of partial replacement of divalent Sr2+ ion by monovalent K+ ion a structural transformation might have taken place due to the ionic radii differences between K+ and Sr2+ ions. The ionic radius of K+ (1.64 Å) is greater than Sr2+ (1.44 Å). This will lead to an increase in average ionic radius of A-site. But, K+ doping causes not only an increase in A-site average ionic radius but also a decrease in B-site average ionic radius. It is known that ionic radius of Mn3+ion is larger than that of Mn4+ ion. Substitution of a monovalent ion, such as K+, results in an increase in the number of Mn4+ ions and a decrease in the number of Mn3+ions twice as much that of a divalent ion substitution may result. Considering both increasing A-site and decreasing B-site average radii with almost the same amount, one would mistakenly think no net
effect in the structure of the compound would takes place. However, an occurrence of a structural change would be expected. The sample was also put under TEM investigation for the determination of micro-structure. Figure 2 shows the images with increasing magnifications from a small portion of a crystallite up to atomic resolution. Actually, Figure 3 was acquired by two successive Fourier Transforms of the real image in order to filter out unwanted noise on the images. An important feature of the sample that was revealed by TEM is that the interplanar distances. Average atomic distances were calculated by taking profiles along different directions, seemingly making right angle. But a closer look shows an obliquity between these directions. The average distances between the most apparent atoms are found to be 3.61 and 3.82 Å. The obliqueness and the difference between the distances are clear evidences indicating that the structure is not cubic (but distorted cubic). It was also found that there are planes (indicated by lines in 2D image) which make right angle to each other with separations of 5.17±0.02 and 5.32±0.02 Å (line profiles along these two lines are given as an inset at Figure 3 for a clear inspection) supporting orthorhombic structure. However, for some reason these parameters do not agree with parameters calculated from XRD data. The surface morphology of La0.70Sr0.21K0.09MnO3 was investigated using a SEM and a typical image is given in Figure 4. From the image, the particle size distribution was investigated and the particle sizes were found to vary from 1 µm to 5 µm, as can be seen on the surface of the sample. A closer look at the image reveals that some grains were actually formed by clusters of several small grains. The sample surface also exhibits porosity between loosely connected grains.
3.2 Magnetic Properties In Figure 5, the temperature dependence of both ZFC and FC (100 Oe) magnetization for the sample is given. The PF transition temperature TC was calculated from FC curve, where
∂M is ∂T
minimum, and found to be about 295 K. The Curie temperature of the sample being coincided with room temperature, due to the K doping, is important for technological applications at room temperature. Another feature, that the magnetization versus temperature curve of the sample displays, is a bifurcation between FC and ZFC processes. The origin of this behavior may be attributed to the anisotropical nature of the compound [27]. The broadness of PF transition observed on the magnetization curve might be an indication of paramagnetic phases besides ferromagnetic phases that the sample possesses [28]. This interpretation would be consistent with our ρ(T) analysis. On the other hand, the decrease of TC due to K doping when compared with the literature on La0.70Sr0.30MnO3 compound, especially with the work of Kallel et al [26] can be understood by two possible explanations. One would be the
crystallographic distortion induced by K substitution. The A-site average ionic radius of the undoped compound increased from 1.2442 to 1.2658 Å doping with K+ ion having a larger ionic radius than Sr2+. In addition, the substitution of K+ for Sr2+ can lead to a decrease in the average Bsite radius, since the ionic radii of Mn+4 is smaller than that of Mn+3 ion, and every K+ creates two Mn4+annihilating two Mn3+, the B-site average radius decreased from 0.6105 to 0.6002 Å. Therefore, K doping results in a local distortion of the MnO6 octahedra because of these ionic radii mismatch. A-site ionic size mismatch, σ, increased from 0.0436 Å, doping A-site with some K+ ion instead of Sr+2, to 0.0970 Å. Increase of A-site average ionic radius, decrease of B-average ionic radius and an increase in size mismatch parameter give rise to distortion of the MnO6 octahedra. As a result of distortion of MnO6 octahedra the Mn−O bond length and Mn−O−Mn bond angle changes [29]. The changes on these parameters has an influence on the hoping of an eg electron of Mn3+ to Mn4+ through O 2p orbital giving rise to changes on the TC and TIM temperature of the compounds. The structure distortions including JT effect, play a less but not negligible role on the eg bandwidth and the Curie temperature TC [2,8]. In our study the Curie temperature of the parent compound decreased to room temperature with a small amount of K+ doping. The data of ionic radii in this paper are obtained from Ref [30]. Another explanation for the decrease of TC would be that the substitution of K+ ions for Sr2+ ions can lead to an increase in the Mn4+/Mn3+ ratio above a critical level favoring super-exchange mechanism. In fact, being a monovalent ion, K+ ion creates two neighboring Mn4+ ions nearby to itself, yielding an additional increase in the possibility of super-exchange with respect to the same Mn4+/Mn3+ ratio one could get with substituting divalent ions. Even though with an appropriate amount of divalent ion substitution, the Mn4+/Mn3+ ratio would be maintained, the possibility of super-exchange lowers since all Mn ions (whether Mn4+ or Mn3+) are more likely to be distributed randomly, having chance to do DE. Figure 6 shows the isothermal magnetization curves taken from 260 to 320 K with intervals of 4 K and from 0 to 5 T applied field. The curves were used for calculation of magnetic entropy changes under the field changes of 1, 3 and 5 T. The magnetization curves exhibit no significant change above and below TC. This is an indication of low magnetic entropy change, because, the considerable change of magnetization near TC yields a larger magnetic entropy change. The temperature dependence of magnetic entropy changes, |∆Sm|, are plotted in Figure 7. The maximum entropy change corresponding to a magnetic field change of 1 T is 0.75 J/kgK. The weak maximum magnetic entropy change in the sample could be caused by a reduction of the double-exchange interactions between Mn3+ and Mn4+ ions and the poor spin-lattice coupling arising from variation in Mn–O bond length and Mn–O–Mn bond angle [31].
3.3 Electrical Transport Properties The temperature dependence of resistivity is plotted in Figure 8. The resistivity increases as temperature is decreased reaching to a maximum value of 1.28 Ωcm at 294 K. This is the turning point for resistivity, where it starts decreasing with temperature showing a metallic conductivity behavior down to lower temperatures. At T=33 K, ρ reaches its minimum (0.46 Ωcm) and further decrease in temperature leads the material to exhibit insulator character. Normally, the electrical resistivity of manganites in the ferromagnetic region decreases with decreasing temperature (dρ/dT>0). In contrast, the electrical resistivity of some manganites in ferromagnetic region is found to increase, exhibiting an insulating behavior. This type of behavior was reported earlier in magnetic materials containing small amounts of magnetic impurities and the behavior was explained using several models based on Kondo effect [32], electron–electron interactions, weak localization effect of electrons, [33] intergranular tunneling of the polarized charge carriers, inelastic scattering effects and uncontrolled magnetic impurities [15,34-40]. The derivative of the ρ-T curve with respect to temperature was calculated numerically to determine paramagnetic-insulator to ferromagnetic-metal phase transition of the compound. The derivative gives two extremum points. The one at high temperature region is paramagnetic-insulator to ferromagnetic-metal transition temperature TMI (294 K). The other is the temperature where low temperature minima occurs TLmin (33 K). In addition, another transition shown in the inset of Figure 8 is the emergence of low temperature peak (LT-peak, TLT=288 K) near TC. It might be due to phase coexistence which is consistent with the broadening of the M(T) curve around TC. Besides, grain size and intergrain connectivity strongly affect electric conductivity, leading in some cases to the formation of LT-peak [28]. To understand the temperature dependence of resistivity, it is useful to examine ρ(T) in more detail. The temperature dependence of ρ is fitted to the following equation within a large temperature rage,10 K
(1)
Here ρ0 is residual resistivity arising from domain boundaries and other temperature-independent scattering mechanisms, ρ0.5 term takes care of contributions from correlated electron-electron interactions, ρ2 indicates the electron-electron scattering, while ρ4.5 is attributed to two magnon scattering process. The experimental data were fitted to Eq. (1) and the best fit curve (colored line) is given in Figure 8 and the calculated parameters are as follows: ρ0=(0.5231±0.0013) Ωcm,
ρ0.5=(0.0147±0.0002) Ωcm/K0.5,
ρ2=(2.0477±0.006)10 −5 Ωcm/K2
and
ρ4.5=(−6.2699±0.0311)10−12 Ωcm/K4.5. 4. Conclusion In this study, we have introduced 9 % potassium into La0.70Sr0.30MnO3 compound material in place of strontium, thereby having a compound with La0.70Sr0.21K0.09MnO3 chemical formula. By doing such a modification we have succeeded lowering both TC and TIM near room temperature with a price of lowering also the magnetic entropy change. Unfortunately, this happened most probably due to large distortion of the Mn−O octahedra caused by the large K+ ions introduced into the crystal structure. Being a monovalent ion, K+ might also have affected the local Mn4+ and Mn3+ distribution in the compound material resulting a pronounced SE interaction favoring antiferromagnetism, in turn, reducing magnetic entropy change, which is necessary for some applications. Acknowledgment:We gratefully acknowledge The Scientific and Technological Research Council of Turkey (TUBITAK) for the support of this study (Grant No:110T637).
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Figure 1 XRD pattern of La0.70Sr0.21K0.09MnO3 sample. The miller indices of the reflections were given on top of corresponding peaks and weak reflection is assigned to La2O3 impurity phase. Figure 2 A magnified TEM image on a crystallite, and the inset shows a portion of crystallite. Scales are shown with bars on the left bottom of the images. Figure 3 Reconstructed image by two successive Fourier Transforms of the real image. The inset shows a typical line profiles along two perpendicular straight lines. Figure 4 SEM image of the sample. Figure 5 Temperature dependence of Zero Field Cooled and Field Cooled magnetizations. Figure 6 Field dependent magnetization isotherms. Figure 7 Temperature dependence of magnetic entropy changes calculated for 1T, 3T and 5T magnetic field changes. Figure 8 Temperature dependence of resistivity. The inset is given here to show the Low Temperature peak.
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La0.70Sr0.21K0.09MnO3 compound was produced using standard sol-gel route.
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The produced compound possesses an orthorhombic perovskite structure belonging to Pnma space group.
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The compound La0.70Sr0.30MnO3 has high TC and TIM. Reducing 9% Sr and adding that amount of K decreased TC and TIM to room temperature.
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TC of the produced compound was determined to be 295K is almost equivalent to TIM (=294K) which was calculated from temperature dependence of resistivity.
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The broadening of M(T) curve near TC and the low temperature peak near TIM were revealed the coexistence of ferromagnetic and paramagnetic phases in the ferromagnetic region. These results were consistent with the results of magnetic entropy change.
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The temperature dependence of resistivity obeyed ρ(Τ)=ρ0 + ρ0.5Τ0.5 + ρ2Τ2 + ρ4.5Τ4.5 equation up to 285K.