- Email: [email protected]
Influence of Sm-doping on the structural, magnetic, and electrical properties of La0.8−xSmxSr0.2MnO3 (0 < x < 0.45) manganites
Journal of Alloys and Compounds 579 (2013) 406–414
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Influence of Sm-doping on the structural, magnetic, and electrical properties of La0.8xSmxSr0.2MnO3 (0 < x < 0.45) manganites M.H. Ehsani a,⇑, P. Kameli b, F.S. Razavi c, M.E. Ghazi d, B. Aslibeiki b a
Department of Physics, Semnan University, Semnan 35195-363, Iran Department of Physics, Isfahan University of Technology, Isfahan 84156-8311, Iran c Department of Physics, Brock University, St. Catharines L2S 3A1, Canada d Department of Physics, Shahrood University of Technology, Shahrood 36155-316, Iran b
a r t i c l e
i n f o
Article history: Received 1 May 2013 Received in revised form 11 June 2013 Accepted 12 June 2013 Available online 22 June 2013 Keywords: Ceramics Manganite Solid-state reaction Doping Glassy state
a b s t r a c t Structural, magnetic, and electrical properties of the La0.8xSmxSr0.2MnO3 (0 6 x 6 0.45) manganites prepared by a solid-state reaction technique was studied systematically. It was found that with increase in the Sm content, the crystal structure transformed from rhombohedral (x < 0.3 samples) to orthorhombic (x > 0.3 samples). The ac magnetic susceptibility measurements show that all samples undergo a transition from paramagnetic (PM) to ferromagnetic (FM) phase at the Curie temperature, TC, which decreases from 296 K down to 165 K with increase in the Sm doping level from x = 0 to x = 0.45. In addition, the glassy state exists in the x = 0.15–0.45 samples, which is stronger in higher doped compounds (x = 0.30 and x = 0.45). This behavior indicates that the substitution of Sm weakens the double exchange (DE) process. The field dependence of magnetization for the samples shows a soft FM nature with a small hysteresis loop and a low coercive field, Hc, for the doped samples. The irreversibility in the magnetization for increasing and decreasing the applied field is due to the glassy behavior observed in highly doped samples. The temperature dependence of resistivity, q(T), measurement indicates that by increasing the Sm doping level, the metal–insulator transition temperature decreases, and the heavily doped samples become insulators. The metallic region of the q(T) curve for the x = 0–0.10 samples was fitted with the model of electron–electron and electron–magnon scattering, while the insulating region was fitted with the small polaron hopping (SPH) at T > hD/2 (hD, Debye temperature) and the variable range hopping (VRH) models at T < hD/2. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past few decades, the hole-doped mangnaites, having the common formula A1xBxMnO3 (A = trivalent rare earth ion and B = divalent alkaline earth ion) have received considerable attention due to their magnetic, colossal magnetorésistance (CMR), and electrical transport properties. These attentions have been exposed not only from academic research but also in their potential applications in technology [1–7]. These compounds are known as mixed-valence manganites because the parent sample (AMnO3) is Mn3+-rich, whereas the A1xBxMnO3 samples are mixtures of Mn3+ and Mn4+ ions that play a major role in the double exchange (DE) ferromagnetic (FM) interaction coupled with the mechanism proposed by Zener [8] immediately after the discovery of manganites. In fact, DE effect is an exchange of electrons from neighboring Mn3+ to Mn4+ ions through oxygen when their core spins are parallel. ⇑ Corresponding author. Tel.: +98 2314432229. E-mail address: [email protected] (M.H. Ehsani). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.067
Although this theory basically explains the simultaneous occurrence of transition from paramagnetic (PM) insulator to FM metallic (FMM) for most hole doped manganites, however, more illustrative mechanisms are needed to explain other observations related to the physics of manganites, including electron–phonon coupling arising from the Jahn–Teller effect [9], phase-separation tendencies [10–12], Griffith’s phase [13,14], glassy behavior [15–17], charge and orbital ordering and their competitions [18–21], small polaron, and magnon correlated transport [22–24]. In order to control and regulate the above-mentioned properties of manganites, there are several approaches. One of the most effective ways is the doping of manganites with various elements in the Mn-site and A-site [25–30]. Mn-site doping with other elements affects the Mn3+AO2AMn4+ networks and DE mechanism remarkably. So far, these kinds of substitutions have been studied extensively for the magnetic and nonmagnetic ion substitutions [28,31–33]. A-site doping can be strongly influenced by the average ionic radius of the A-site (hrAi), which exhibits a close relation between the
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
bending of the MnAOAMn bond angle and the narrowing of the electronic band width [34,35]. Several researchers have reported the effects of the size variance parameter r2 on the physical properties of manganites [27,36,37]. This parameter is related to hrAi, P and is expressed by the formula r2 = xiri2 hrAi2, where xi P ( xi = 1, i = 1, 2, 3, the number of ions in A-site in this case) is the fractional occupancy of the A-site, and ri is the corresponding ionic radii. For example, Zhu et al. have reported that the introduction of large A-site cations such as La3+ or Ba2+ will locally suppress the lattice distortion of the Pr0.5LaxCa0.5MnO3 and Pr0.5Ca0.5xBaxMnO3 compounds. Krishna et al. have presented the magnetic behavior study of Pr0.67A0.33MnO3 (A = Ca, Sr, Pb, and Ba) manganites, and their results have completely been correlated by hrAi. Rodriguez-Martinez and co-worker have found that with increase in the size variance in the La0.7A0.30MnO3 (A = Ca, Sr, Ba) manganites, the Curie temperature (TC) decreases monotonically with a corresponding fall in the CMR effect [38]. In present work, we investigated the effect of gradual A-site substitution by the Sm3+ ion at the La-site in the La1xSrxMnO3 (x = 0.2) compound. The La1xSrxMnO3 manganite is one of the most attractive manganites, having the TC as high as 300–370 K in the range of x 0.17 to x 0.5 [39]. Also these compounds have a large bandwidth, and their transport behavior is like a metal. So far, a few report on La-site substitution by Sm in (La1xSmx)0.66Sr0.33MnO3 (x = 0–1) [40], (La1xSmx)0.66Sr0.33MnO3 (0.40 < x < 0.60) [41], and La0.7Sr0.3MnO3 [42] have been presented. With Sm3+ ions doping in these compounds, the resistivity, magnetoresistivity, magnetostriction, and magnetic properties of doped samples have been influenced. In this work, the structural, magnetic, and electrical properties of the La0.80xSmxSr0.2MnO3 (x = 0, 0.5, 0.10, 0.15, 0.20, 0.30, and 0.45) manganite polycrystalline samples using dc magnetization, ac magnetic susceptibility, and electrical resistivity measurements have been reported. 2. Experimental The polycrystalline La0.8xSmxSr0.2MnO3 (x = 0, 0.5, 0.10, 0.15, 0.20, 0.30, and 0.45) samples were prepared by the solid-state reaction method. High purity powders of La2O3, Sm2O3, SrCO3, and MnO2 were mixed in stoichiometric proportions, and then calcined at 900 °C, 1000 °C, and 1200 °C for 16, 12, and 12 h, respectively. The powder obtained was pelletized and sintered at 1350 °C for 24 h. The expected chemical reaction is as:
1=2xðSm2 O3 Þ þ 0:20 SrCO3 þ MnO2 þ ð0:40 x=2ÞLa2 O3 ! La0:8x Smx Sr0:2 MnO3þd þ 0:2 CO2
407
two samples are single phase with an orthorhombic (Pbnm) perovskite structure. In other words, for 1.2180 < hrAi < 1.2348, the crystal structure is rhombohedral, and for hrAi < 1.2180, the orthorhombic structure is formed. In fact, LaMnO3 itself has a cubic symmetry, and after Sr doping, the La0.8Sr0.2MnO3 cell is distorted into rhombohedral symmetry [43] because the average ionic radius of Sr2+ (ionic radius = 1.31 Å) is larger than that for La3+ (ionic radius = 1.216 Å). After Sm3+ (ionic radius = 1.132 Å) [44] doping at A-site, the structure of the low-doped La0.8xSmxSr0.2MnO3 samples remain rhombohedral but for heavily doped samples (x = 0.30 and x = 0.45), the crystal structure transforms from rhombohedral to orthorhombic symmetry. Studies on the compounds La0.67xSmxSr0.33MnO3 (0.40 < x < 0.60), La0.7xNdxSr0.30MnO3 and La0.7xGdxSr0.30MnO3 (0 < x < 0.60) have shown the same behavior. A typical diagram for Rietveld refinement analysis of the x = 0.30 sample is displayed in Fig. 2. From the data collected in Table 1, one can see that the unit cell volume of the samples decrease with increase in the Sm3+ doping level. This reduction in the unit cell is caused by substitution of the smaller Sm3+ ion instead of the larger La3+ ion in the A-site of the crystal lattice. Also the substitutions change the average ionic radius of A-site hrAi in the AMnO3 perovskite system hrAi was calculated by hrAi = (0.80x) rLa3+ + xrSm3+ + 0.20rSr2+. The typical FE-SEM photographs of the compounds (x = 0, 0.15, x = 0.30, and x = 0.45) are shown in Fig. 3. It is observable that the average particle size of the samples increases slightly by increasing the Sm doping. The same behavior was observed in another literature, where Sm was doped in La0.85Ag0.15MnO3 manganite [47]. 3.2. Magnetic properties The ac magnetic susceptibility, v0 (T) = v0 + iv00 , measurements were performed in a magnetic field of 10 Oe and frequency of 333 Hz. In order to clearly see the magnetic behavior of the samples, the susceptibility data for low level doped (x = 0–0.20) and high level doped (x = 0.30, 0.45) samples were indicated in Figs. 4 and 5 separately. It can be found in Figs. 4 and 5 that the ac magnetic susceptibility measurements are strongly influenced by the Sm doping level in the La0.8xSmxSr0.2MnO3 compounds. All samples were found to show a PM to FM phase transition at TC, which was determined from the peak in the v00 (T) curves, and indicated in Table 2. As one can see, by further increasing the Sm doping level, at the low temperature region, an anomalous decrease in the v0 of the sample
Structural properties of the samples were studied by X-ray diffraction (ADVANCE-D8 model). The microstructures of the samples were analyzed by field emission scanning electron microscope (FE-SEM Hitachi, Model S-4160). The ac susceptibility measurements were performed using a Lake Shore Ac Susceptometer (Model 7000). The resistivity measurements were carried out by the four-probe method using a Leybold closed cycle refrigerator. The dc magnetizations were made using a SQUID magnetometer.
3. Results and discussion 3.1. Structural properties Fig. 1 shows the X-ray diffraction (XRD) patterns for the La0.8x SmxSr0.2MnO3 (x = 0–0.45) samples at room temperature. The XRD data was analyzed with Rietveld refinement using the FULLPROF software and Pseudo-Voigt function, and the results obtained were collected in Table 1. In the XRD pattern of all samples, no trace of secondary phase was detectable, and only by increasing the doping level, a structural transition occurred. The analyzed data shows that all samples have the rhombohedral crystal structure with R-3C space group belonging to the hexagonal setting where a = b – c, except heavily doped samples, namely, x = 0.30 and x = 0.45. These
Fig. 1. XRD patterns for La0.8xSmxSr0.2MnO3 (x = 0, 0.5, 0.10, 0.15, 0.20, 0.30).
408
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
Table 1 Lattice parameters and volumes, obtained from Rietveld refinement. Sample
Structure
a (Å)
b (Å)
c (Å)
V (Å3)
hrAi Å
r2 104 (Å2)
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20 x = 0.30 x = 0.45
Rhombo
5.5184 5.5182 5.4986 5.4926 5.4903 5.4932 5.4705
5.5184 5.5182 5.4986 5.4926 5.4903 7.7494 7.7387
13.4937 13.4940 13.4914 13.4775 13.4695 5.4871 5.4775
355.8568 355.8389 353.2476 352.1139 351.6102 233.5803 231.8875
1.2348 1.2306 1.2264 1.2222 1.2180 1.2096 1.1970
14.1 19.1 23.6 27.9 31.7 38.4 45.8
Ortho
x = 0.15 is evident. There is a same behavior in the v0 of the samples x = 0.20–0.45. At the same time, the imaginary (v00 )part of the ac susceptibility shows a peak that indicates a PM–FM transition for samples. Also by further decreasing the temperature, a peak is observed in the v00 of the samples x = 0.15–0.45, in harmony with the sharp decline of v0 around the same temperature region. This peak is frequency dependent, and shifts towards a higher temperature with increasing frequency (see Fig. 7). This is normally related to the existence of the glassy state in these samples. Large drops in the TC values are observed with increase in the Sm3+ doping content in Fig. 6. Decrease in TC and magnetization values with the replacement of La by smaller rare-earth ions has been reported previously [45,46–50]. It has been interpreted that Sm3+ substitution could not change the Mn3+/Mn4+ ratio but the smaller average A-site ionic radius (hrAi) causes a larger distortion of the MnAOAMn bond and alignment of Mn3+/Mn4+ spins, and consequently weakens the double exchange interaction, and affects the transfer integral of eg electron hopping between Mn sites. In other words, in the pure manganite La0.8Sr0.2MnO3, the coupling between the moments of Mn ions is FM, and the alignment of Mn moments is almost parallel. The impacting of Sm ions in La sites introduces a larger canting angle of Mn moments. To recognize the decrease in TC, it is necessary to consider a competition between FM–DE and anti-ferromagnetic (AFM) superexchange (SE) interactions in the Sm-doped compounds. The exchange integral of DE, Jex,DE, is positive, while the exchange integral of SE, Jex,SE, induced by the canted Mn spins is negative. The total exchange integral, Jex,total, is approximately equal to Jex,DE + Jex,SE, and becomes smaller for Sm-doped compounds [45]. It is well-known that TC is directly proportional to the total exchange integral. Thus the decrease in TC is related to the decrease in the total exchange integral induced by Sm doping with smaller ionic radius.
One can see in Fig. 6 that with the structural phase transition, the TC variation pattern has changed as well. TC decreased linearly by the good linear correlation coefficient R2 = 0.99984 with increase in the doping level for the x = 0–0.20 samples with rhombohedral symmetry. However, a rapid reduction in the TC values is seen for the x = 0.30 and 0.45 samples with orthorhombic symmetry. In addition, the variation of the size variance parameter r2 with hrAi is observed in Fig. 6. It is clear from this figure that r2 increases with increase in the Sm doping level. It is well-known that this parameter signifies magnitude of disorder among the materials. Generally, r2 has a critical value which if a material exhibits a value beyond this critical value, its physical properties such as TC is affected intensively [27]. In this investigation, also for r2 values more than 0.0035 Å2, this behavior happened. In addition, one can say that the magnetic behavior is closely connected with the occupation condition of Sm3+ and its magnetism. In this substitution, Sm3+ is a magnetic ion with an effective magnetic moment of 0.84 lB [48]. In our samples, when x = 0–0.20, the distribution of Sm3+ ions are random, and this interrupts LaAOALa(Sr) bonds and forms LaAOASm bonds in the LaAO layer. Randomly distributed magnetic potential and coulomb potential of Sm cause MnAOAMn bonds not to be able to form homogeneous long-range order but to form cluster glass state. That is why one can see that the PM–FM transition in the parent sample is sharp but with increase in the doping level, the homogeneity of longrange order decreases. In this case, the flat region pattern by smooth sloping below TC, which is seen in the parent sample, would change. However,, when x = 0.30 and 0.45, the SmAOASm bonds start to form as well. With temperature decreasing, SmAOASm will gradually transform from random arrangement to orderly arrangement, and the AFM arrangement between MnAOAMn and SmAOASm is formed at the low temperature
Fig. 2. Rietveld refinement of XRD pattern for sample x = 0.15.
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
409
Fig. 3. FE-SEM photographs of the La0.8xSmxSr0.2MnO3 compounds: (a) x = 0, (b) x = 0.15, (c) x = 0.30, and (d) x = 0.45.
Fig. 4. Temperature dependence of (a) v0 and (b) v00 for x = 0–0.20 samples in a magnetic field of 10 Oe and frequency of 333 Hz.
range. Thus appearance of the AFM phase for the samples depends on the Sm and Sr doping level. For example, Huanyin and co-workers have reported the appearance of AFM phase in La0.67xSmxSr0.33MnO3 at heavy Sm doping (x > 0.60) [41].
Fig. 5. Temperature dependence of of (a) v0 and (b) v00 for x = 0.30 and x = 0.45 samples in a magnetic field of 10 Oe and frequency of 333 Hz.
To distinguish the magnetic structure of the heavily doped samples (x = 0.30 and 0.45), the temperature dependence of the magnetization M(T) was measured in both the zero-field-cooling (ZFC) and field-cooling (FC) modes at the applied magnetic fields 100, 250, and 1000 Oe for the x = 0.30 sample and at 100 Oe for
410
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
Table 2 Magnetic parameters for samples. Sample
Structure
TC (K)
Tf (K)
Hc (Oe)
MS (lB/f.u.)
MSa (lB/f.u.)
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20 x = 0.30 x = 0.45
Rhombo
294 268 238 215 193 176 165
– – – – 80 94 105
10 29 35 37 38 98 149
4.07 3.93 3.57 3.42 2.47 2.37 1.68
4.24 4.11 4.01 3.81 3.69 3.59 3.49
Ortho
Fig. 6. Variation of Curie temperature as a function of hrAi in the La0.8xSmxSr0.2MnO3 compounds.
observed at low temperatures. This behavior exists even at the applied magnetic field 1000 Oe for the x = 0.30 sample. The splittings of FC and ZFC and a slight downward bending at low temperatures in the FC and ZFC curves are signatures of the magnetic cluster formation and a conventional spin glass/cluster (SG/CSG) systems [51]. To further investigate the magnetic state of heavily doped samples at low temperatures, we measured the ac magnetic susceptibility for the x = 0.30 and x = 0.45 samples at different frequencies from 33 Hz to 1000 Hz. Fig. 8 shows the temperature dependence of v00 for the x = 0.30 and x = 0.45 samples at different frequencies. As one can see, the high temperature peak which is corresponding to the PM–FM transition is almost frequency independent. However, the low temperature peak shifts towards higher temperature with the increase of frequencies. Similar behavior is also obtained by doping of Fe, Al, Ti, and other cations in manganites [52–55]. This is normally related to the formation of glassy clusters by decreasing of temperature. In fact with increase in the doping level of Sm ions in the La sites of compounds and consequently weakening of FM state, some frustrated clusters of spins have formed at low temperatures which called cluster glass [56,57]. Formation of these glassy clusters leads to a decrease in magnetization and response of samples to the external magnetic field which is clear in both FC magnetization and ac magnetic susceptibility curves. Although these samples are not in the class of canonical spin glasses, but frequency dependence of magnetic susceptibility and splitting of FC and ZFC magnetization can be a sign of frustrated cluster glass state in these samples at low temperatures [52,54,55]. For more investigate the glassy behavior of these samples, we used critical slowing down model as a useful model for study the behavior of spins in frustrated states such as SG, cluster spin glasses (CSG) and super spin glasses (SSG) [58,59]. This model is given as:
f ¼ f0 ½ðT f T 0 Þ=T 0 zv
Fig. 7. (a) ZFC and FC magnetization as a function of temperature at some applied magnetic fields (100, 250, and 1000 Oe) for sample x = 0.30. (b) Inset: temperature dependence of the ZFC and FC magnetization at an applied magnetic field of 100 Oe for sample x = 0.45.
the x = 0.45 sample. It can be seen in Fig. 7 that the ZFC curves do not coincide with the FC curves at low temperatures for both samples. This splitting of ZFC and FC M(T) curves is a characteristic of magnetic inhomogeneity. Generally, the magnetic inhomogenity is considered as the competition between the FM and AFM phases, discussed before. The splittings of FC and ZFC in the x = 0.45 sample is larger than those in the x = 0.30 sample, implying increase in the magnetic inhomogeneity with increase in the doping level of Sm ions in the La sites. It can be noticed that in the ZFC and FC curves, a considerable decrease with a slight downward bending is
ð1Þ
where f is the applied frequency, f0 is a constant in order of 10+9– 10+13, Tf is the peak temperature in the v00 component of ac susceptibility, T0 corresponds to the dc value of Tf for f ? 0, and the parameter zt as a dynamic critical exponent shows strength of the interactions, and varies between 4 and 12 for spin glasses [58,59]. From best fits of this model (Fig. 9), we estimated T0 as 75 K and 87 K, and also the zt values as 11.2 and 9.8 for the x = 0.30 and x = 0.45 samples, respectively. These values of zt are in the predicted range for the CSG systems. These results prove the presence of competition between the FM–DE and AFM–SE interactions in these samples, which leads to a disordered state at low temperatures. Fig. 10 shows the magnetic field (H) dependent dc magnetization (M) at 5 K for all samples. The samples have a typical soft FM behavior with a small hysteresis loop and a low coercive field (Hc) which is collected in Table 2. The small values for coercive field confirm that the magnetic domains can rotate easily to the direction of the applied magnetic field. Although in heavily doped samples the Hc values are larger than others and the magnetization does not increase rapidly like the parent sample, after applying the magnetic field of higher than 0.6 T, magnetization gradually saturates with increase in H (not shown). When Sm ion doping level is low, magnetism in the La(Sm) sublattice is weak, and coupling between the La(Sm) and Mn sublattices is also weak. Thus the applied magnetic field rotates the magnetic moments easily to the applied magnetic field direction. However, when the Sm ion doping level is high, magnetism in the La(Sm) sublattice is stronger, and AFM coupling between the two sublattices is also stronger. Therefore, a stronger magnetic field is required to rotate the magnetic moments.
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
411
Fig. 9. Ln–Ln plot of the reduced temperature (Tf–T0/T0) versus frequency for x = 0.3 and x = 0.45 samples.
Fig. 8. Temperature dependence of v00 for (a) x = 0.30 and (b) x = 0.45 samples at frequencies 33, 111, 333, 666, and 1000 Hz. The inset shows the evolution of the peak by increase in frequency.
A similar behavior has been reported by Xu and co-workers when they studied the influence of doped Dy on magnetic properties of La0.67xDyxSr0.33MnO3 [60]. In addition, Huanyin and coworkers have reported the same behavior for La0.67xSmxSr0.33MnO3 (x = 0.4, 0.50, and 0.60) [41]. They have also reported that in the x = 0.6 sample, due to the AFM coupling between sublattices, the system exhibits typical anti-ferromagnetism. However, according to M(H) measurement of the x = 0.30 and 0.45 samples, it seems that in these samples, the long range FM order of Mn sublattice with a CSG state still dominates in the system. The saturation magnetization (Msa) was obtained from the linear extrapolation of high fields in the M(H) curves at H = 0 in Fig. 9, and the data was recorded in Table 2. It can be seen that the values of Msa for doped samples become smaller with increase in the doping level x. We suggest that this is due to the existence of AFM and FM competition and consequently the formation of spin disordered glassy state in doped samples. Also the spontaneous magnetization (Ms) was calculated using the well-known Arrott–Belov–Kouvel (ABK) plot (M2 versus H/M), and was shown in Fig. 11. ABK plot shows a strong convex curvature with a finite Ms which is a signature of FM phase of the parent sample. Due to the existence of a glassy state in doped samples, especially in highly doped samples, the FM phase and Ms have been weakened, which is clearly deduced from the data collected in Table 2.
ture dependence of resistivity for all samples in two groups. It is clear that resistivity of the samples increases with increase in the Sm doping, so that in the sample x = 0.45, the resistivity trend dramatically changes. The first group samples (x = 0, 0.05, and 0.10) show a clear metal–insulator transition, and the metal–insulator transition temperature, TMI, shifts to lower temperatures with increase in the Sm doping level. In the second group samples (x = 0.15, 0.20, 0.30, and 0.45), TMI suppresses, and the samples behave like an insulator. The observed behavior may be explained as follows: In manganites, the magnetic and transport properties are strongly coupled with the MnO6 octahedra distortion and the Mn3+AOAMn4+ bond angle and length in compound. Based on DE mechanism, the transfer integral (t) of eg carrier is governed by the relative angle (h) of the local t2g spins between neighboring Mn3+ and Mn4+ ions in a manner that t = t0cos(h/2), where t0 is the normal transfer integral. In parent sample the core spins are parallel and the electron can move from neighboring Mn3+ and Mn4+ through oxygen. Substitution of the Sm ions at La sites reduces the hrAi value, as tabulated in Table 1 and this can lead to changes in the Mn3+AOAMn4+ bond angle and length, which can suppress the DE interactions. Consequently, the tendencies for charge localization increase due to the reduction in the carrier mobility and a broad forbidden band near the Fermi level. On the other hand, as mentioned before, in highly doped samples, the possibility of SmAOASm bond formation increases. For this reason, the FM long-range ordering gradually reduces to short-range ordering in the clusters, and for heavily
3.3. Electrical properties Undoubtedly, electrical transport is one of the most attractive physical properties of the manganites. Fig. 12 shows the tempera-
Fig. 10. Magnetic field dependence of magnetization for all samples.
412
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
doped samples, due to the high degree level of disorder, the FM clusters are separated, and the FM insulating (FMI) behavior is observed. In order to further understand the Sm doping effect as well as finding the electrical transport behaviour, describing the temperature dependence of resistivity of the samples, we focused on the high temperature ranges (T > hD/2, hD is the Debye temperature) in the PM phase region, and the low temperature region namely the FM metallic (FMM) region for the first group separately. Moreover, we focused our attention for the second group on the high temperature ranges (T > hD/2) in the PM phase region. 3.3.1. Resistivity behavior in the first group samples 3.3.1.1. Low temperature resistivity in the FMM region. Generally, electrical conduction in the FM phase region, below the TMI is described according to the DE theory. Based on this model, the Mn3+AOAMn4+ coupling produces conduction from the half-filled to the empty eg orbital. Different equations were used to characterize the low-temperature resistivity of manganites, as follow [39,61–63]:
qðTÞ ¼ q0 þ q2 T 2
ð2Þ
qðTÞ ¼ q0 þ q2:5 T 2:5
ð3Þ
qðTÞ ¼ q0 þ q2 T 2 þ q4:5 T 4:5
ð4Þ
where q0 is the residual resistivity, which represents the resistivity due to grain boundary, charge/spin tunneling transport, domain wall, defects, etc. The q2T2 term in Eqs. (2) and (4) may be due to the electron–electron scattering process generally observed in manganites. The q2.5T2.5 term in Eq. (3) represents the resistivity due to the single-magnon scattering process in common ferromagnets [64] which is suppressed at low temperature for manganites [65]. The q4.5T4.5 term is attributed to the two magnon scattering process [66] in the FM region. The fitting of the experimental data shows that Eq. (4) is the best fit with a good linear correlation coefficient, R2, for the samples x = 0, 0.05, and 0.10. Fig. 13 shows typically the fitting of the experimental data with Eqs. (2) and (3). The values for the fitting parameters are collected in Table 3. From the analysis of the transport data recorded in Table 3, it is found that the q0 values increase continuously with increase in the Sm content, implying that Sm doping might have resulted in the enhancement of domain or grain boundary scattering. Similarly, an increase in the q2 and q4.5 values can be observed in Table 3, due to the reduction of DE mechanism. The values obtained for the fit parameters are close to those reported by Chen and co-
Fig. 12. Temperature dependence of resistivity (a) x = 0–0.10 samples (b) x = 0.15– 0.30 samples.
Fig. 13. Fitting of the experimental data for x = 0.05 sample in FMM region with Eq. (2) (inset graph) and Eq. (3).
workers for the La0.6Sm0.1Sr0.1MnO3 sample [67]. Thus it seems that when the Sm doping level increases in the samples, the MnO6 octahedra distortion created by this substitution can affect the eg bandwidth, the number of carrier and localization effects such as electron–electron and magnon scattering.
Fig. 11. ABK plot for all samples.
3.3.1.2. High temperature resistivity (T > hD/2) in PM region. In the semiconducting-like region above the Curie temperature, the
413
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
Table 3 Parameters obtained from fitting by models.
q(T) = q0 + q2T2
Sample code
R2 x = 0 T < 220 K x = 0.05 T < 200 K x = 0.10 T < 160 K
0.9989 0.9980 0.9471
q(T) = q0 + q2T2 + q4.5T4.5
q0
q2
(O.cm)
(O.cm/K2)
0.0155 0.0455 1.3682
7.7 107 1.3 107 2 104
R2 0.9992 0.9992 0.9987
q2
q4.5
(O.cm)
(O.cm/K2)
(O.cm/K4.5)
0.0159 0.0441 1.23
7.3 107 1.5 106 5 105
5.4 1014 2.8 1013 7.2 1011
resistivity can be described by the adiabatic small polaron hopping mechanism according to the following formula [68,69]:
qðTÞ ¼ q0 T exp Ea =kB T
ð5Þ
where Ea is the activation energy, kB is the Boltzman constant, and
q0 is the residual resistivities given by the following formula:
q0 ¼ ð2kB Þ=ð3ne2 a2p v ph Þ
q(T) = q0T exp Ea/kB T R2
q0
ð6Þ
where kB, e, and n are the Boltzmann constant, electronic charge, and charge carrier density, and ap and mph are the site to site hoppping distance, and the longitudinal optical phonon frequency, respectively. The fiting parameters for the samples in this group are tabulated in Table 3. In this calculation, the Debye temperature for the three samples was estimated from the plot of Ln(q/T) versus (1/T) by choosing the deviation point from linearity behavior, and the optical phonon frequency (mph) values were given using hmph = kBhD which takes into account the same value for all samples in the order of 1013. The hD values and their decreasing trend with increase in the doping level are similar to a previous report [25]. Also it can be seen in Table 3 that the Ea values increase continuously with increase in the Sm content, and the behaviour may be attributed to decrease in the band width and localization of charge carriers.
Fig. 14. Fitting of the experimental data for x = 0.20 sample with SPH model (inset graph) and VRH model.
0.9967 0.9969 0.9978
Ea (meV)
q0 105
hD (k)
mph 1013
(O.cm/K1)
54 103 112
3.08 1.17 0.86
610 562 494
1.27 1.17 1.03
3.3.2. Resistivity behavior in the second group samples As mentioned before, the second group samples (x = 0.15, 0.20, 0.30, and 0.45) show a semiconducting-like behavior. In order to find the resistivity behavior, two distinct phenomena were utilized on the basis of two different models, the variable range hopping (VRH) and the small polaron hoping (SPH) models. The proposed conduction mechanism for manganites at the temperatures T > hD/2 (Eq. (5)), is mainly due to the thermally activated small polarons which in the adiabatic regime, the nearest neighboring hopping of small polarons (Holstein polarons) leads to mobility with thermally activated form, in which charge-carrier motion is faster than lattice vibrations. In the VRH model the carriers are localized by random potential fluctuation, Mott’s VRH expression (Eq. (7)) and it is appropriate to describe the experimental data at the temperatures T < hD/2 [31,70].
qðTÞ ¼ q0 exp ðT 0 =TÞ1=4
ð7Þ
where T0 is the expression for Mott’s activation energy is given as [71]:
kB T 0 ¼ 24=pNðEF Þn3
ð8Þ
where N(EF) is the density of states (DOS) near the Fermi level, and n is the localization length. Fig. 14 shows the fitting of the experimental data of x = 0.20 sample with both models as a typical. The values for R2 in Eqs. (5) and (7) are close to 1, which represents the best fitting quality. The parameters obtained from fitting are in good agreement with the previously reported results about other mangnaites [25,72,73]. The N(EF) values were obtained to be 1.03 1022 to 0.44 1022 eV1cm3 for samples in the second group. By incresing the doping level, the N(EF) decrease, which is consistent with the resistivity data. Also one can easily observe a decrease in Ea for the samples with increase in the doping level from the data tabulated in Table 4. This may be attributed to the localization of charge carriers that hop between the Mn3+ to Mn4+ sites through DE mechanism which occur only if the two moments are parallel. As mentioned before, analysis of the ac susceptibility measurement results prove that the cluster glass behaviour exists in highly doped samples, which is the result of the random freezing of the Mn3+ and Mn4+ ion moments. Therefore, this would lead to a reduction in the conductivity, favoring the system to become an insulator. In this case, more activation energy is needed for carrier hoppings, and consequently, N(EF) decreases as well.
Table 4 Parameters acquired from fitting by models. Sample
x = 0.15 x = 0.20 x = 0.30 x = 0.45
q(T) = q0 exp (T0/T)1/4
q(T) = q0T exp Ea/kBT 2
Temp. range (K)
R
T > 238 T > 233 T > 233 T > 232
0.9987 0.9994 0.9969 0.9987
Ea (meV)
q0 10
128 141 146 155
6.3 5.3 4.4 2.6
6
hD (K)
mph 10
476 466 466 464
0.99 0.97 0.97 0.96
(O.cm/K1)
13
Temp. range (K)
R2
T0 107
N(EF)1022 (eV1cm3)
215 < T < 238 193 < T < 233 175 < T < 233 160 < T < 232
0.9973 0.9942 0.99 0.9924
0.86 1.08 1.76 1.99
1.03 0.82 0.5 0.44
414
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
4. Conclusion The effects of Sm substitution on La0.80xSmxSr0.2MnO3 were investigated. For the samples, by the average ion radius of A-site, 1.2180 < hrAi < 1.2348, the crystal structure is rhombohedral, and for hrAi < 1.2180, the orthorhombic structure was formed. All samples had the FM phase but by increasing the Sm doping at the La sites, the Curie temperature shifted to lower temperatures. Moreover, in medium as well as heavy doped samples, the cluster spin glass state appeared. With increase in the Sm doping level, x, the MI transition temperatures decreased and the system became an insulator. The applied electrical models (SPH and VRH) for doped samples at different temperature ranges confirm the localization of charge carriers due to Sm substitution. References [1] G.H. Jonker, J.H. Santen, Physica 16 (1950) 337. [2] B.P. McCarthy, L.R. Pederson, Y.S. Chou, X.-D. Zhou, W.A. Surdoval, L.C. Wilson, J. Power Sources 180 (2008) 294. [3] L. Millar, H. Taherparvar, N. Filkin, P. Slater, J. Yeomans, Solid State Ionics 179 (2009) 32. [4] V.N. Krivoruchko, M.A. Marchenko, Y. Melikhov, Phys. Rev. B 82 (2010) 064419. [5] M. Pekala, V. Drozd, J. Alloys Comp. 456 (2008) 30. [6] S. Soiron, L. Aymard, A. Rougier, J.-M. Tarascon, Top. Catal. 16/17 (2001) 391. [7] Y. Li, S. Yao, L. Xue, Y. Yan, J. Mater. Sci. 44 (2009) 4455. [8] C. Zener, Phys. Rev. 82 (1951) 403. [9] A.J. Millis, B.I. Shraiman, R. Mueller, Phys. Rev. Lett. 77 (1996) 175. [10] D. Louca, T. Egami, E.L. Brosha, H. Roder, A.R. Bishop, Phys. Rev. B 56 (1997) R8475. [11] A. Moreo, J. Verges, E. Dagotto, J. Magn. Magn. Mater. 226–230 (2001) 773. [12] M. Battabyal, T.K. Dey, Physica B 367 (2005) 40. [13] L. Joshi, V. Dayal, N. Rama, S. Keshri, J. Alloys Comp. 479 (2009) 879. [14] G.N. Rao, J.H. Chen, S. Neeleshwar, Y.Y. Chen, M.K. Wu, J. Phys. D: Appl. Phys. 42 (2009) 095003. [15] J. Lopez, P.N. Lisboa-Filho, W.A.C. Passos, W.A. Ortiz, F.M. Araujo-moreira, O.F. De Lima, D. Schaniel, K. Ghosh, Phys. Rev. B 63 (2001) 224422. [16] A. Chakravarti, R. Ranganathan, S.B. Roy, Phys. Rev. B 46 (1992) 6236. [17] C. Raj Sankar, S. Vijayanand, P.A. Joy, Solid State Sci. 11 (2009) 714. [18] E.O. Wallan, W.C. Koehler, Phys. Rev. 100 (1955) 545. [19] J.B. Goodenough, Phys. Rev. 100 (1955) 564. [20] R. Kajimoto, T. Kakeshita, Y. Ohara, H. Yoshizawa, Y. Tomioka, Y. Tokura, Phys. Rev. B 58 (1998) R11837. [21] S. Yunoki, T. Hotta, E. Dagotto, Phys. Rev. Lett. 84 (2000) 3714. [22] T. Mizokawa, D.I. Khomskii, G.A. Sawatzky, Phys. Rev. B 63 (2001) 024403. [23] B.J. Campbell, S.K. Sinha, R. Osborn, S. Rosenkranz, J.F. Mitchell, D.N. Argyriou, L. Vasiliu-Doloc, O.H. Seeck, J.W. Lynn, Phys. Rev. B 67 (2003) R020409. [24] J.Y. Yu, S.Y. Zhang, G.H. Liu, H.Y. Wu, L. Wang, Solid State Commun. 142 (2007) 333. [25] E. Tka, K. Cherif, J. Dhahri, E. Dhahri, J. Alloys Comp. 509 (2011) 8047. [26] Z. Wang, F. Xu, Y. Du, J. Zhu, H. Zhang, J. Magn. Magn. Mater. 307 (2006) 205. [27] J.A. Collado, J.L. Garcio-Munoz, M.A.G. Aranda, J. Solid State Chem. 183 (2010) 1083. [28] B. Aslibeiki, P. Kameli, H. Salamati, Solid State Commun. 149 (2009) 1274. [29] D. Debasis, R. Rawat, S. Ram, A. Banerjee, S.K. Roy, J. Phys. Condense. Matter 24 (2012) 076001. [30] N. Kallel, S.B. Abdelkhalek, S. Kallel, O. Pena, M. Oumezzine, J. Alloys Comp. 501 (2010) 30.
[31] M.F. Mostafa, S. Ata- Allah, A.A. Youssef, H.S. Refai, J. Magn. Magn. Mater. 320 (2008) 344. [32] S.K. Srivastava, M. Kar, S. Ravi, J. Magn. Magn. Mater. 320 (2008) e107. [33] Y. Ying, T.W. Eom, N.V. Dai, Y.P. Lee, J. Magn. Magn. Mater. 323 (2011) 4. [34] H.Y. Hwang, S.W. Cheong, P.G. Radalli, Phys. Rev. Lett. 75 (1995) 914. [35] Z.B. Guo, N. Zhang, W.P. Ding, W. Yang, J.R. Zhang, Y.W. Du, Solid State Commun. 100 (1996) 769. [36] D.C. Krishna, P.V. Reddy, J. Alloys Comp. 479 (2009) 661. [37] D. Zhu, X.N. Tan, P. Cao, F. Jia, X. Ma, Y. Lu, J. Appl. Phys. 105 (2009) 063914. [38] L.M. Rodriguez-Martinez, J.P. Attfield, Phys. Rev. B 63 (2001) 024424. [39] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [40] Q.Q. Cao, J. Wu, K.M. Gu, S.Y. Zhang, U.W. Du, J. Appl. Phys. 85 (1999) 4494. [41] G. Huanyin, L. Ning, Y. Guoqing, T. Wei, J. Rare Earths 24 (2006) 206. [42] S. Qixiang, W. Guiying, Y. Guoqing, M. Qiang, W. Wenqi, P. Zhensheng, J. Rare Earths 6 (2008) 821. [43] J.M.D. Coey, M. Viret, S. Von Molnar, Adv. Phys. 48 (1999) 167. [44] R.D. Shannon, Acta Cryst. A32 (1976) 51. [45] K. Cherif, J. Dhahri, E. Dhahri, M. Oumezzine, H. Vincent, J. Solid State Chem. 163 (2002) 466. [46] Y. Sun, M.B. Salamon, W. Tong, Y. Zhang, Phys. Rev. B 66 (2002) 094414. [47] N. Ibrahim, A.K. Yahya, S.S. Rajput, S. Keshri, M.K. Talari, J. Magn. Magn. Mater. 323 (2011) 2179. [48] H.Y. Hawang, S.W. Cheong, P.G. Radaelli, M.A. Marezio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. [49] J.P. Zhou, J.T. McDevitt, J.S. Zhou, H.Q. Yin, J.B. Goodenough, Y. Gim, Q.X. Jia, Appl. Phys. Lett. 75 (1999) 1146. [50] Z. Guo, J. Zhang, N. Zhang, W. Ding, H. Huang, Y. Du, Appl. Phys. Lett. 70 (14) (1996) 1897. [51] B. Roy, A. Poddar, S. Das, J. Appl. Phys. 100 (2006) 104318. [52] X.J. Liu, Z.Q. Li, A. Yu, M.L. Liu, W.R. Li, B.L. Li, P. Wu, H.L. Bai, E.Y. Jiang, J. Magn. Magn. Mater. 313 (2007) 354. [53] J. Blasco, J. Garcia, J.M. de Teresa, M.R. Ibarra, J. Perez, P.A. Algarabel, C. Ritter, Phys. Rev. B 55 (1997) 8905. [54] M.M. Xavier Jr, F.A.O. Cabral, J.H. de Araujo, T. Dumelow, A.A. Coelho, J. Mater. Res. 7 (2004) 355. [55] X. Liu, X. Xu, Y. Zhang, Phys. Rev. B 62 (15) (2000) 112. [56] P.S. Antil Kumar, P.A. Joy, S.K. Date, J. Phys: Condense. Matter 10 (1998) L487. [57] K.F. Wang, Y. Wang, L.F. Wang, S. Dong, D. Li, Z.D. Zhang, H. Yu, Q.C. Li, J.-M. Liu, Phys. Rev. B 73 (2006) 134411. [58] K.H. Fischer, H. Hertz, Spin Glasses, Cambridge University Press, 1993. [59] J.A. Mydosh, Spin Glasses, Taylor & Francis, London, 1993. [60] S. Xu, W. Tong, J. Fan, J. Gao, C. Zha, Y. Zhang, J. Magn. Magn. Mater. 288 (2005) 92. [61] P. Shiffer, A.P. Ramirez, W. Bao, S.W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [62] E.S. Vlakhov, R.A. Chakalov, R.I. Chakalov, K.A. Nenkov, K. Dorr, A. Handstein, K.H. Mueller, J. Appl. Phys. 83 (1998) 2152. [63] K. Kubo, N. Ohata, J. Phys. Soc. Jpn. 33 (1972) 21. [64] M. Jaime, M.B. Salamon, M. Rubinstein, R.E. Treece, J.S. Horwitz, D.B. Chrisey, Phys. Rev. B 54 (1996) 11914. [65] M.B. Salmon, M. Jaime, Rev. Mod. Phys. 73 (2001) 583. [66] L. Pi, L. Zhang, Y. Zhang, Phys. Rev. B 61 (2000) 8917. [67] C. Shuiyuan, Y. Qingying, L. Heng, H. Shangho, H. Zhigao, J. Rare Earths 24 (2006) 788. [68] D. Emin, T. Holstein, Ann. Phys. 53 (1969) 439. [69] M. Ziese, C. Srinitiwarawong, Phys. Rev. B 58 (1998) 11519. [70] S. Mollah, Z.A. Khan, D.K. Shukla, M. Arshad, R. Kumar, A. Das, J. Phys. Chem. Solids 69 (2008) 1023. [71] T. Holstein, Ann. Phys. 8 (1959) 343. [72] G. Venkataiah, D.C. Krishna, M. Vithal, S.S. Rao, S.V. Bhat, V. Prasad, S.V. Subramanyam, P. Venugopal Reddy, Physica B 357 (2005) 370. [73] S.P. Altintas, A. Amira, N. Mahamdioua, A. Varilci, C. Terzioglu, J. Alloys Comp. 509 (2011) 4510.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Influence of Sm-doping on the structural, magnetic, and electrical properties of La0.8xSmxSr0.2MnO3 (0 < x < 0.45) manganites M.H. Ehsani a,⇑, P. Kameli b, F.S. Razavi c, M.E. Ghazi d, B. Aslibeiki b a
Department of Physics, Semnan University, Semnan 35195-363, Iran Department of Physics, Isfahan University of Technology, Isfahan 84156-8311, Iran c Department of Physics, Brock University, St. Catharines L2S 3A1, Canada d Department of Physics, Shahrood University of Technology, Shahrood 36155-316, Iran b
a r t i c l e
i n f o
Article history: Received 1 May 2013 Received in revised form 11 June 2013 Accepted 12 June 2013 Available online 22 June 2013 Keywords: Ceramics Manganite Solid-state reaction Doping Glassy state
a b s t r a c t Structural, magnetic, and electrical properties of the La0.8xSmxSr0.2MnO3 (0 6 x 6 0.45) manganites prepared by a solid-state reaction technique was studied systematically. It was found that with increase in the Sm content, the crystal structure transformed from rhombohedral (x < 0.3 samples) to orthorhombic (x > 0.3 samples). The ac magnetic susceptibility measurements show that all samples undergo a transition from paramagnetic (PM) to ferromagnetic (FM) phase at the Curie temperature, TC, which decreases from 296 K down to 165 K with increase in the Sm doping level from x = 0 to x = 0.45. In addition, the glassy state exists in the x = 0.15–0.45 samples, which is stronger in higher doped compounds (x = 0.30 and x = 0.45). This behavior indicates that the substitution of Sm weakens the double exchange (DE) process. The field dependence of magnetization for the samples shows a soft FM nature with a small hysteresis loop and a low coercive field, Hc, for the doped samples. The irreversibility in the magnetization for increasing and decreasing the applied field is due to the glassy behavior observed in highly doped samples. The temperature dependence of resistivity, q(T), measurement indicates that by increasing the Sm doping level, the metal–insulator transition temperature decreases, and the heavily doped samples become insulators. The metallic region of the q(T) curve for the x = 0–0.10 samples was fitted with the model of electron–electron and electron–magnon scattering, while the insulating region was fitted with the small polaron hopping (SPH) at T > hD/2 (hD, Debye temperature) and the variable range hopping (VRH) models at T < hD/2. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past few decades, the hole-doped mangnaites, having the common formula A1xBxMnO3 (A = trivalent rare earth ion and B = divalent alkaline earth ion) have received considerable attention due to their magnetic, colossal magnetorésistance (CMR), and electrical transport properties. These attentions have been exposed not only from academic research but also in their potential applications in technology [1–7]. These compounds are known as mixed-valence manganites because the parent sample (AMnO3) is Mn3+-rich, whereas the A1xBxMnO3 samples are mixtures of Mn3+ and Mn4+ ions that play a major role in the double exchange (DE) ferromagnetic (FM) interaction coupled with the mechanism proposed by Zener [8] immediately after the discovery of manganites. In fact, DE effect is an exchange of electrons from neighboring Mn3+ to Mn4+ ions through oxygen when their core spins are parallel. ⇑ Corresponding author. Tel.: +98 2314432229. E-mail address: [email protected] (M.H. Ehsani). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.067
Although this theory basically explains the simultaneous occurrence of transition from paramagnetic (PM) insulator to FM metallic (FMM) for most hole doped manganites, however, more illustrative mechanisms are needed to explain other observations related to the physics of manganites, including electron–phonon coupling arising from the Jahn–Teller effect [9], phase-separation tendencies [10–12], Griffith’s phase [13,14], glassy behavior [15–17], charge and orbital ordering and their competitions [18–21], small polaron, and magnon correlated transport [22–24]. In order to control and regulate the above-mentioned properties of manganites, there are several approaches. One of the most effective ways is the doping of manganites with various elements in the Mn-site and A-site [25–30]. Mn-site doping with other elements affects the Mn3+AO2AMn4+ networks and DE mechanism remarkably. So far, these kinds of substitutions have been studied extensively for the magnetic and nonmagnetic ion substitutions [28,31–33]. A-site doping can be strongly influenced by the average ionic radius of the A-site (hrAi), which exhibits a close relation between the
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
bending of the MnAOAMn bond angle and the narrowing of the electronic band width [34,35]. Several researchers have reported the effects of the size variance parameter r2 on the physical properties of manganites [27,36,37]. This parameter is related to hrAi, P and is expressed by the formula r2 = xiri2 hrAi2, where xi P ( xi = 1, i = 1, 2, 3, the number of ions in A-site in this case) is the fractional occupancy of the A-site, and ri is the corresponding ionic radii. For example, Zhu et al. have reported that the introduction of large A-site cations such as La3+ or Ba2+ will locally suppress the lattice distortion of the Pr0.5LaxCa0.5MnO3 and Pr0.5Ca0.5xBaxMnO3 compounds. Krishna et al. have presented the magnetic behavior study of Pr0.67A0.33MnO3 (A = Ca, Sr, Pb, and Ba) manganites, and their results have completely been correlated by hrAi. Rodriguez-Martinez and co-worker have found that with increase in the size variance in the La0.7A0.30MnO3 (A = Ca, Sr, Ba) manganites, the Curie temperature (TC) decreases monotonically with a corresponding fall in the CMR effect [38]. In present work, we investigated the effect of gradual A-site substitution by the Sm3+ ion at the La-site in the La1xSrxMnO3 (x = 0.2) compound. The La1xSrxMnO3 manganite is one of the most attractive manganites, having the TC as high as 300–370 K in the range of x 0.17 to x 0.5 [39]. Also these compounds have a large bandwidth, and their transport behavior is like a metal. So far, a few report on La-site substitution by Sm in (La1xSmx)0.66Sr0.33MnO3 (x = 0–1) [40], (La1xSmx)0.66Sr0.33MnO3 (0.40 < x < 0.60) [41], and La0.7Sr0.3MnO3 [42] have been presented. With Sm3+ ions doping in these compounds, the resistivity, magnetoresistivity, magnetostriction, and magnetic properties of doped samples have been influenced. In this work, the structural, magnetic, and electrical properties of the La0.80xSmxSr0.2MnO3 (x = 0, 0.5, 0.10, 0.15, 0.20, 0.30, and 0.45) manganite polycrystalline samples using dc magnetization, ac magnetic susceptibility, and electrical resistivity measurements have been reported. 2. Experimental The polycrystalline La0.8xSmxSr0.2MnO3 (x = 0, 0.5, 0.10, 0.15, 0.20, 0.30, and 0.45) samples were prepared by the solid-state reaction method. High purity powders of La2O3, Sm2O3, SrCO3, and MnO2 were mixed in stoichiometric proportions, and then calcined at 900 °C, 1000 °C, and 1200 °C for 16, 12, and 12 h, respectively. The powder obtained was pelletized and sintered at 1350 °C for 24 h. The expected chemical reaction is as:
1=2xðSm2 O3 Þ þ 0:20 SrCO3 þ MnO2 þ ð0:40 x=2ÞLa2 O3 ! La0:8x Smx Sr0:2 MnO3þd þ 0:2 CO2
407
two samples are single phase with an orthorhombic (Pbnm) perovskite structure. In other words, for 1.2180 < hrAi < 1.2348, the crystal structure is rhombohedral, and for hrAi < 1.2180, the orthorhombic structure is formed. In fact, LaMnO3 itself has a cubic symmetry, and after Sr doping, the La0.8Sr0.2MnO3 cell is distorted into rhombohedral symmetry [43] because the average ionic radius of Sr2+ (ionic radius = 1.31 Å) is larger than that for La3+ (ionic radius = 1.216 Å). After Sm3+ (ionic radius = 1.132 Å) [44] doping at A-site, the structure of the low-doped La0.8xSmxSr0.2MnO3 samples remain rhombohedral but for heavily doped samples (x = 0.30 and x = 0.45), the crystal structure transforms from rhombohedral to orthorhombic symmetry. Studies on the compounds La0.67xSmxSr0.33MnO3 (0.40 < x < 0.60), La0.7xNdxSr0.30MnO3 and La0.7xGdxSr0.30MnO3 (0 < x < 0.60) have shown the same behavior. A typical diagram for Rietveld refinement analysis of the x = 0.30 sample is displayed in Fig. 2. From the data collected in Table 1, one can see that the unit cell volume of the samples decrease with increase in the Sm3+ doping level. This reduction in the unit cell is caused by substitution of the smaller Sm3+ ion instead of the larger La3+ ion in the A-site of the crystal lattice. Also the substitutions change the average ionic radius of A-site hrAi in the AMnO3 perovskite system hrAi was calculated by hrAi = (0.80x) rLa3+ + xrSm3+ + 0.20rSr2+. The typical FE-SEM photographs of the compounds (x = 0, 0.15, x = 0.30, and x = 0.45) are shown in Fig. 3. It is observable that the average particle size of the samples increases slightly by increasing the Sm doping. The same behavior was observed in another literature, where Sm was doped in La0.85Ag0.15MnO3 manganite [47]. 3.2. Magnetic properties The ac magnetic susceptibility, v0 (T) = v0 + iv00 , measurements were performed in a magnetic field of 10 Oe and frequency of 333 Hz. In order to clearly see the magnetic behavior of the samples, the susceptibility data for low level doped (x = 0–0.20) and high level doped (x = 0.30, 0.45) samples were indicated in Figs. 4 and 5 separately. It can be found in Figs. 4 and 5 that the ac magnetic susceptibility measurements are strongly influenced by the Sm doping level in the La0.8xSmxSr0.2MnO3 compounds. All samples were found to show a PM to FM phase transition at TC, which was determined from the peak in the v00 (T) curves, and indicated in Table 2. As one can see, by further increasing the Sm doping level, at the low temperature region, an anomalous decrease in the v0 of the sample
Structural properties of the samples were studied by X-ray diffraction (ADVANCE-D8 model). The microstructures of the samples were analyzed by field emission scanning electron microscope (FE-SEM Hitachi, Model S-4160). The ac susceptibility measurements were performed using a Lake Shore Ac Susceptometer (Model 7000). The resistivity measurements were carried out by the four-probe method using a Leybold closed cycle refrigerator. The dc magnetizations were made using a SQUID magnetometer.
3. Results and discussion 3.1. Structural properties Fig. 1 shows the X-ray diffraction (XRD) patterns for the La0.8x SmxSr0.2MnO3 (x = 0–0.45) samples at room temperature. The XRD data was analyzed with Rietveld refinement using the FULLPROF software and Pseudo-Voigt function, and the results obtained were collected in Table 1. In the XRD pattern of all samples, no trace of secondary phase was detectable, and only by increasing the doping level, a structural transition occurred. The analyzed data shows that all samples have the rhombohedral crystal structure with R-3C space group belonging to the hexagonal setting where a = b – c, except heavily doped samples, namely, x = 0.30 and x = 0.45. These
Fig. 1. XRD patterns for La0.8xSmxSr0.2MnO3 (x = 0, 0.5, 0.10, 0.15, 0.20, 0.30).
408
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
Table 1 Lattice parameters and volumes, obtained from Rietveld refinement. Sample
Structure
a (Å)
b (Å)
c (Å)
V (Å3)
hrAi Å
r2 104 (Å2)
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20 x = 0.30 x = 0.45
Rhombo
5.5184 5.5182 5.4986 5.4926 5.4903 5.4932 5.4705
5.5184 5.5182 5.4986 5.4926 5.4903 7.7494 7.7387
13.4937 13.4940 13.4914 13.4775 13.4695 5.4871 5.4775
355.8568 355.8389 353.2476 352.1139 351.6102 233.5803 231.8875
1.2348 1.2306 1.2264 1.2222 1.2180 1.2096 1.1970
14.1 19.1 23.6 27.9 31.7 38.4 45.8
Ortho
x = 0.15 is evident. There is a same behavior in the v0 of the samples x = 0.20–0.45. At the same time, the imaginary (v00 )part of the ac susceptibility shows a peak that indicates a PM–FM transition for samples. Also by further decreasing the temperature, a peak is observed in the v00 of the samples x = 0.15–0.45, in harmony with the sharp decline of v0 around the same temperature region. This peak is frequency dependent, and shifts towards a higher temperature with increasing frequency (see Fig. 7). This is normally related to the existence of the glassy state in these samples. Large drops in the TC values are observed with increase in the Sm3+ doping content in Fig. 6. Decrease in TC and magnetization values with the replacement of La by smaller rare-earth ions has been reported previously [45,46–50]. It has been interpreted that Sm3+ substitution could not change the Mn3+/Mn4+ ratio but the smaller average A-site ionic radius (hrAi) causes a larger distortion of the MnAOAMn bond and alignment of Mn3+/Mn4+ spins, and consequently weakens the double exchange interaction, and affects the transfer integral of eg electron hopping between Mn sites. In other words, in the pure manganite La0.8Sr0.2MnO3, the coupling between the moments of Mn ions is FM, and the alignment of Mn moments is almost parallel. The impacting of Sm ions in La sites introduces a larger canting angle of Mn moments. To recognize the decrease in TC, it is necessary to consider a competition between FM–DE and anti-ferromagnetic (AFM) superexchange (SE) interactions in the Sm-doped compounds. The exchange integral of DE, Jex,DE, is positive, while the exchange integral of SE, Jex,SE, induced by the canted Mn spins is negative. The total exchange integral, Jex,total, is approximately equal to Jex,DE + Jex,SE, and becomes smaller for Sm-doped compounds [45]. It is well-known that TC is directly proportional to the total exchange integral. Thus the decrease in TC is related to the decrease in the total exchange integral induced by Sm doping with smaller ionic radius.
One can see in Fig. 6 that with the structural phase transition, the TC variation pattern has changed as well. TC decreased linearly by the good linear correlation coefficient R2 = 0.99984 with increase in the doping level for the x = 0–0.20 samples with rhombohedral symmetry. However, a rapid reduction in the TC values is seen for the x = 0.30 and 0.45 samples with orthorhombic symmetry. In addition, the variation of the size variance parameter r2 with hrAi is observed in Fig. 6. It is clear from this figure that r2 increases with increase in the Sm doping level. It is well-known that this parameter signifies magnitude of disorder among the materials. Generally, r2 has a critical value which if a material exhibits a value beyond this critical value, its physical properties such as TC is affected intensively [27]. In this investigation, also for r2 values more than 0.0035 Å2, this behavior happened. In addition, one can say that the magnetic behavior is closely connected with the occupation condition of Sm3+ and its magnetism. In this substitution, Sm3+ is a magnetic ion with an effective magnetic moment of 0.84 lB [48]. In our samples, when x = 0–0.20, the distribution of Sm3+ ions are random, and this interrupts LaAOALa(Sr) bonds and forms LaAOASm bonds in the LaAO layer. Randomly distributed magnetic potential and coulomb potential of Sm cause MnAOAMn bonds not to be able to form homogeneous long-range order but to form cluster glass state. That is why one can see that the PM–FM transition in the parent sample is sharp but with increase in the doping level, the homogeneity of longrange order decreases. In this case, the flat region pattern by smooth sloping below TC, which is seen in the parent sample, would change. However,, when x = 0.30 and 0.45, the SmAOASm bonds start to form as well. With temperature decreasing, SmAOASm will gradually transform from random arrangement to orderly arrangement, and the AFM arrangement between MnAOAMn and SmAOASm is formed at the low temperature
Fig. 2. Rietveld refinement of XRD pattern for sample x = 0.15.
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
409
Fig. 3. FE-SEM photographs of the La0.8xSmxSr0.2MnO3 compounds: (a) x = 0, (b) x = 0.15, (c) x = 0.30, and (d) x = 0.45.
Fig. 4. Temperature dependence of (a) v0 and (b) v00 for x = 0–0.20 samples in a magnetic field of 10 Oe and frequency of 333 Hz.
range. Thus appearance of the AFM phase for the samples depends on the Sm and Sr doping level. For example, Huanyin and co-workers have reported the appearance of AFM phase in La0.67xSmxSr0.33MnO3 at heavy Sm doping (x > 0.60) [41].
Fig. 5. Temperature dependence of of (a) v0 and (b) v00 for x = 0.30 and x = 0.45 samples in a magnetic field of 10 Oe and frequency of 333 Hz.
To distinguish the magnetic structure of the heavily doped samples (x = 0.30 and 0.45), the temperature dependence of the magnetization M(T) was measured in both the zero-field-cooling (ZFC) and field-cooling (FC) modes at the applied magnetic fields 100, 250, and 1000 Oe for the x = 0.30 sample and at 100 Oe for
410
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
Table 2 Magnetic parameters for samples. Sample
Structure
TC (K)
Tf (K)
Hc (Oe)
MS (lB/f.u.)
MSa (lB/f.u.)
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20 x = 0.30 x = 0.45
Rhombo
294 268 238 215 193 176 165
– – – – 80 94 105
10 29 35 37 38 98 149
4.07 3.93 3.57 3.42 2.47 2.37 1.68
4.24 4.11 4.01 3.81 3.69 3.59 3.49
Ortho
Fig. 6. Variation of Curie temperature as a function of hrAi in the La0.8xSmxSr0.2MnO3 compounds.
observed at low temperatures. This behavior exists even at the applied magnetic field 1000 Oe for the x = 0.30 sample. The splittings of FC and ZFC and a slight downward bending at low temperatures in the FC and ZFC curves are signatures of the magnetic cluster formation and a conventional spin glass/cluster (SG/CSG) systems [51]. To further investigate the magnetic state of heavily doped samples at low temperatures, we measured the ac magnetic susceptibility for the x = 0.30 and x = 0.45 samples at different frequencies from 33 Hz to 1000 Hz. Fig. 8 shows the temperature dependence of v00 for the x = 0.30 and x = 0.45 samples at different frequencies. As one can see, the high temperature peak which is corresponding to the PM–FM transition is almost frequency independent. However, the low temperature peak shifts towards higher temperature with the increase of frequencies. Similar behavior is also obtained by doping of Fe, Al, Ti, and other cations in manganites [52–55]. This is normally related to the formation of glassy clusters by decreasing of temperature. In fact with increase in the doping level of Sm ions in the La sites of compounds and consequently weakening of FM state, some frustrated clusters of spins have formed at low temperatures which called cluster glass [56,57]. Formation of these glassy clusters leads to a decrease in magnetization and response of samples to the external magnetic field which is clear in both FC magnetization and ac magnetic susceptibility curves. Although these samples are not in the class of canonical spin glasses, but frequency dependence of magnetic susceptibility and splitting of FC and ZFC magnetization can be a sign of frustrated cluster glass state in these samples at low temperatures [52,54,55]. For more investigate the glassy behavior of these samples, we used critical slowing down model as a useful model for study the behavior of spins in frustrated states such as SG, cluster spin glasses (CSG) and super spin glasses (SSG) [58,59]. This model is given as:
f ¼ f0 ½ðT f T 0 Þ=T 0 zv
Fig. 7. (a) ZFC and FC magnetization as a function of temperature at some applied magnetic fields (100, 250, and 1000 Oe) for sample x = 0.30. (b) Inset: temperature dependence of the ZFC and FC magnetization at an applied magnetic field of 100 Oe for sample x = 0.45.
the x = 0.45 sample. It can be seen in Fig. 7 that the ZFC curves do not coincide with the FC curves at low temperatures for both samples. This splitting of ZFC and FC M(T) curves is a characteristic of magnetic inhomogeneity. Generally, the magnetic inhomogenity is considered as the competition between the FM and AFM phases, discussed before. The splittings of FC and ZFC in the x = 0.45 sample is larger than those in the x = 0.30 sample, implying increase in the magnetic inhomogeneity with increase in the doping level of Sm ions in the La sites. It can be noticed that in the ZFC and FC curves, a considerable decrease with a slight downward bending is
ð1Þ
where f is the applied frequency, f0 is a constant in order of 10+9– 10+13, Tf is the peak temperature in the v00 component of ac susceptibility, T0 corresponds to the dc value of Tf for f ? 0, and the parameter zt as a dynamic critical exponent shows strength of the interactions, and varies between 4 and 12 for spin glasses [58,59]. From best fits of this model (Fig. 9), we estimated T0 as 75 K and 87 K, and also the zt values as 11.2 and 9.8 for the x = 0.30 and x = 0.45 samples, respectively. These values of zt are in the predicted range for the CSG systems. These results prove the presence of competition between the FM–DE and AFM–SE interactions in these samples, which leads to a disordered state at low temperatures. Fig. 10 shows the magnetic field (H) dependent dc magnetization (M) at 5 K for all samples. The samples have a typical soft FM behavior with a small hysteresis loop and a low coercive field (Hc) which is collected in Table 2. The small values for coercive field confirm that the magnetic domains can rotate easily to the direction of the applied magnetic field. Although in heavily doped samples the Hc values are larger than others and the magnetization does not increase rapidly like the parent sample, after applying the magnetic field of higher than 0.6 T, magnetization gradually saturates with increase in H (not shown). When Sm ion doping level is low, magnetism in the La(Sm) sublattice is weak, and coupling between the La(Sm) and Mn sublattices is also weak. Thus the applied magnetic field rotates the magnetic moments easily to the applied magnetic field direction. However, when the Sm ion doping level is high, magnetism in the La(Sm) sublattice is stronger, and AFM coupling between the two sublattices is also stronger. Therefore, a stronger magnetic field is required to rotate the magnetic moments.
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
411
Fig. 9. Ln–Ln plot of the reduced temperature (Tf–T0/T0) versus frequency for x = 0.3 and x = 0.45 samples.
Fig. 8. Temperature dependence of v00 for (a) x = 0.30 and (b) x = 0.45 samples at frequencies 33, 111, 333, 666, and 1000 Hz. The inset shows the evolution of the peak by increase in frequency.
A similar behavior has been reported by Xu and co-workers when they studied the influence of doped Dy on magnetic properties of La0.67xDyxSr0.33MnO3 [60]. In addition, Huanyin and coworkers have reported the same behavior for La0.67xSmxSr0.33MnO3 (x = 0.4, 0.50, and 0.60) [41]. They have also reported that in the x = 0.6 sample, due to the AFM coupling between sublattices, the system exhibits typical anti-ferromagnetism. However, according to M(H) measurement of the x = 0.30 and 0.45 samples, it seems that in these samples, the long range FM order of Mn sublattice with a CSG state still dominates in the system. The saturation magnetization (Msa) was obtained from the linear extrapolation of high fields in the M(H) curves at H = 0 in Fig. 9, and the data was recorded in Table 2. It can be seen that the values of Msa for doped samples become smaller with increase in the doping level x. We suggest that this is due to the existence of AFM and FM competition and consequently the formation of spin disordered glassy state in doped samples. Also the spontaneous magnetization (Ms) was calculated using the well-known Arrott–Belov–Kouvel (ABK) plot (M2 versus H/M), and was shown in Fig. 11. ABK plot shows a strong convex curvature with a finite Ms which is a signature of FM phase of the parent sample. Due to the existence of a glassy state in doped samples, especially in highly doped samples, the FM phase and Ms have been weakened, which is clearly deduced from the data collected in Table 2.
ture dependence of resistivity for all samples in two groups. It is clear that resistivity of the samples increases with increase in the Sm doping, so that in the sample x = 0.45, the resistivity trend dramatically changes. The first group samples (x = 0, 0.05, and 0.10) show a clear metal–insulator transition, and the metal–insulator transition temperature, TMI, shifts to lower temperatures with increase in the Sm doping level. In the second group samples (x = 0.15, 0.20, 0.30, and 0.45), TMI suppresses, and the samples behave like an insulator. The observed behavior may be explained as follows: In manganites, the magnetic and transport properties are strongly coupled with the MnO6 octahedra distortion and the Mn3+AOAMn4+ bond angle and length in compound. Based on DE mechanism, the transfer integral (t) of eg carrier is governed by the relative angle (h) of the local t2g spins between neighboring Mn3+ and Mn4+ ions in a manner that t = t0cos(h/2), where t0 is the normal transfer integral. In parent sample the core spins are parallel and the electron can move from neighboring Mn3+ and Mn4+ through oxygen. Substitution of the Sm ions at La sites reduces the hrAi value, as tabulated in Table 1 and this can lead to changes in the Mn3+AOAMn4+ bond angle and length, which can suppress the DE interactions. Consequently, the tendencies for charge localization increase due to the reduction in the carrier mobility and a broad forbidden band near the Fermi level. On the other hand, as mentioned before, in highly doped samples, the possibility of SmAOASm bond formation increases. For this reason, the FM long-range ordering gradually reduces to short-range ordering in the clusters, and for heavily
3.3. Electrical properties Undoubtedly, electrical transport is one of the most attractive physical properties of the manganites. Fig. 12 shows the tempera-
Fig. 10. Magnetic field dependence of magnetization for all samples.
412
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
doped samples, due to the high degree level of disorder, the FM clusters are separated, and the FM insulating (FMI) behavior is observed. In order to further understand the Sm doping effect as well as finding the electrical transport behaviour, describing the temperature dependence of resistivity of the samples, we focused on the high temperature ranges (T > hD/2, hD is the Debye temperature) in the PM phase region, and the low temperature region namely the FM metallic (FMM) region for the first group separately. Moreover, we focused our attention for the second group on the high temperature ranges (T > hD/2) in the PM phase region. 3.3.1. Resistivity behavior in the first group samples 3.3.1.1. Low temperature resistivity in the FMM region. Generally, electrical conduction in the FM phase region, below the TMI is described according to the DE theory. Based on this model, the Mn3+AOAMn4+ coupling produces conduction from the half-filled to the empty eg orbital. Different equations were used to characterize the low-temperature resistivity of manganites, as follow [39,61–63]:
qðTÞ ¼ q0 þ q2 T 2
ð2Þ
qðTÞ ¼ q0 þ q2:5 T 2:5
ð3Þ
qðTÞ ¼ q0 þ q2 T 2 þ q4:5 T 4:5
ð4Þ
where q0 is the residual resistivity, which represents the resistivity due to grain boundary, charge/spin tunneling transport, domain wall, defects, etc. The q2T2 term in Eqs. (2) and (4) may be due to the electron–electron scattering process generally observed in manganites. The q2.5T2.5 term in Eq. (3) represents the resistivity due to the single-magnon scattering process in common ferromagnets [64] which is suppressed at low temperature for manganites [65]. The q4.5T4.5 term is attributed to the two magnon scattering process [66] in the FM region. The fitting of the experimental data shows that Eq. (4) is the best fit with a good linear correlation coefficient, R2, for the samples x = 0, 0.05, and 0.10. Fig. 13 shows typically the fitting of the experimental data with Eqs. (2) and (3). The values for the fitting parameters are collected in Table 3. From the analysis of the transport data recorded in Table 3, it is found that the q0 values increase continuously with increase in the Sm content, implying that Sm doping might have resulted in the enhancement of domain or grain boundary scattering. Similarly, an increase in the q2 and q4.5 values can be observed in Table 3, due to the reduction of DE mechanism. The values obtained for the fit parameters are close to those reported by Chen and co-
Fig. 12. Temperature dependence of resistivity (a) x = 0–0.10 samples (b) x = 0.15– 0.30 samples.
Fig. 13. Fitting of the experimental data for x = 0.05 sample in FMM region with Eq. (2) (inset graph) and Eq. (3).
workers for the La0.6Sm0.1Sr0.1MnO3 sample [67]. Thus it seems that when the Sm doping level increases in the samples, the MnO6 octahedra distortion created by this substitution can affect the eg bandwidth, the number of carrier and localization effects such as electron–electron and magnon scattering.
Fig. 11. ABK plot for all samples.
3.3.1.2. High temperature resistivity (T > hD/2) in PM region. In the semiconducting-like region above the Curie temperature, the
413
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
Table 3 Parameters obtained from fitting by models.
q(T) = q0 + q2T2
Sample code
R2 x = 0 T < 220 K x = 0.05 T < 200 K x = 0.10 T < 160 K
0.9989 0.9980 0.9471
q(T) = q0 + q2T2 + q4.5T4.5
q0
q2
(O.cm)
(O.cm/K2)
0.0155 0.0455 1.3682
7.7 107 1.3 107 2 104
R2 0.9992 0.9992 0.9987
q2
q4.5
(O.cm)
(O.cm/K2)
(O.cm/K4.5)
0.0159 0.0441 1.23
7.3 107 1.5 106 5 105
5.4 1014 2.8 1013 7.2 1011
resistivity can be described by the adiabatic small polaron hopping mechanism according to the following formula [68,69]:
qðTÞ ¼ q0 T exp Ea =kB T
ð5Þ
where Ea is the activation energy, kB is the Boltzman constant, and
q0 is the residual resistivities given by the following formula:
q0 ¼ ð2kB Þ=ð3ne2 a2p v ph Þ
q(T) = q0T exp Ea/kB T R2
q0
ð6Þ
where kB, e, and n are the Boltzmann constant, electronic charge, and charge carrier density, and ap and mph are the site to site hoppping distance, and the longitudinal optical phonon frequency, respectively. The fiting parameters for the samples in this group are tabulated in Table 3. In this calculation, the Debye temperature for the three samples was estimated from the plot of Ln(q/T) versus (1/T) by choosing the deviation point from linearity behavior, and the optical phonon frequency (mph) values were given using hmph = kBhD which takes into account the same value for all samples in the order of 1013. The hD values and their decreasing trend with increase in the doping level are similar to a previous report [25]. Also it can be seen in Table 3 that the Ea values increase continuously with increase in the Sm content, and the behaviour may be attributed to decrease in the band width and localization of charge carriers.
Fig. 14. Fitting of the experimental data for x = 0.20 sample with SPH model (inset graph) and VRH model.
0.9967 0.9969 0.9978
Ea (meV)
q0 105
hD (k)
mph 1013
(O.cm/K1)
54 103 112
3.08 1.17 0.86
610 562 494
1.27 1.17 1.03
3.3.2. Resistivity behavior in the second group samples As mentioned before, the second group samples (x = 0.15, 0.20, 0.30, and 0.45) show a semiconducting-like behavior. In order to find the resistivity behavior, two distinct phenomena were utilized on the basis of two different models, the variable range hopping (VRH) and the small polaron hoping (SPH) models. The proposed conduction mechanism for manganites at the temperatures T > hD/2 (Eq. (5)), is mainly due to the thermally activated small polarons which in the adiabatic regime, the nearest neighboring hopping of small polarons (Holstein polarons) leads to mobility with thermally activated form, in which charge-carrier motion is faster than lattice vibrations. In the VRH model the carriers are localized by random potential fluctuation, Mott’s VRH expression (Eq. (7)) and it is appropriate to describe the experimental data at the temperatures T < hD/2 [31,70].
qðTÞ ¼ q0 exp ðT 0 =TÞ1=4
ð7Þ
where T0 is the expression for Mott’s activation energy is given as [71]:
kB T 0 ¼ 24=pNðEF Þn3
ð8Þ
where N(EF) is the density of states (DOS) near the Fermi level, and n is the localization length. Fig. 14 shows the fitting of the experimental data of x = 0.20 sample with both models as a typical. The values for R2 in Eqs. (5) and (7) are close to 1, which represents the best fitting quality. The parameters obtained from fitting are in good agreement with the previously reported results about other mangnaites [25,72,73]. The N(EF) values were obtained to be 1.03 1022 to 0.44 1022 eV1cm3 for samples in the second group. By incresing the doping level, the N(EF) decrease, which is consistent with the resistivity data. Also one can easily observe a decrease in Ea for the samples with increase in the doping level from the data tabulated in Table 4. This may be attributed to the localization of charge carriers that hop between the Mn3+ to Mn4+ sites through DE mechanism which occur only if the two moments are parallel. As mentioned before, analysis of the ac susceptibility measurement results prove that the cluster glass behaviour exists in highly doped samples, which is the result of the random freezing of the Mn3+ and Mn4+ ion moments. Therefore, this would lead to a reduction in the conductivity, favoring the system to become an insulator. In this case, more activation energy is needed for carrier hoppings, and consequently, N(EF) decreases as well.
Table 4 Parameters acquired from fitting by models. Sample
x = 0.15 x = 0.20 x = 0.30 x = 0.45
q(T) = q0 exp (T0/T)1/4
q(T) = q0T exp Ea/kBT 2
Temp. range (K)
R
T > 238 T > 233 T > 233 T > 232
0.9987 0.9994 0.9969 0.9987
Ea (meV)
q0 10
128 141 146 155
6.3 5.3 4.4 2.6
6
hD (K)
mph 10
476 466 466 464
0.99 0.97 0.97 0.96
(O.cm/K1)
13
Temp. range (K)
R2
T0 107
N(EF)1022 (eV1cm3)
215 < T < 238 193 < T < 233 175 < T < 233 160 < T < 232
0.9973 0.9942 0.99 0.9924
0.86 1.08 1.76 1.99
1.03 0.82 0.5 0.44
414
M.H. Ehsani et al. / Journal of Alloys and Compounds 579 (2013) 406–414
4. Conclusion The effects of Sm substitution on La0.80xSmxSr0.2MnO3 were investigated. For the samples, by the average ion radius of A-site, 1.2180 < hrAi < 1.2348, the crystal structure is rhombohedral, and for hrAi < 1.2180, the orthorhombic structure was formed. All samples had the FM phase but by increasing the Sm doping at the La sites, the Curie temperature shifted to lower temperatures. Moreover, in medium as well as heavy doped samples, the cluster spin glass state appeared. With increase in the Sm doping level, x, the MI transition temperatures decreased and the system became an insulator. The applied electrical models (SPH and VRH) for doped samples at different temperature ranges confirm the localization of charge carriers due to Sm substitution. References [1] G.H. Jonker, J.H. Santen, Physica 16 (1950) 337. [2] B.P. McCarthy, L.R. Pederson, Y.S. Chou, X.-D. Zhou, W.A. Surdoval, L.C. Wilson, J. Power Sources 180 (2008) 294. [3] L. Millar, H. Taherparvar, N. Filkin, P. Slater, J. Yeomans, Solid State Ionics 179 (2009) 32. [4] V.N. Krivoruchko, M.A. Marchenko, Y. Melikhov, Phys. Rev. B 82 (2010) 064419. [5] M. Pekala, V. Drozd, J. Alloys Comp. 456 (2008) 30. [6] S. Soiron, L. Aymard, A. Rougier, J.-M. Tarascon, Top. Catal. 16/17 (2001) 391. [7] Y. Li, S. Yao, L. Xue, Y. Yan, J. Mater. Sci. 44 (2009) 4455. [8] C. Zener, Phys. Rev. 82 (1951) 403. [9] A.J. Millis, B.I. Shraiman, R. Mueller, Phys. Rev. Lett. 77 (1996) 175. [10] D. Louca, T. Egami, E.L. Brosha, H. Roder, A.R. Bishop, Phys. Rev. B 56 (1997) R8475. [11] A. Moreo, J. Verges, E. Dagotto, J. Magn. Magn. Mater. 226–230 (2001) 773. [12] M. Battabyal, T.K. Dey, Physica B 367 (2005) 40. [13] L. Joshi, V. Dayal, N. Rama, S. Keshri, J. Alloys Comp. 479 (2009) 879. [14] G.N. Rao, J.H. Chen, S. Neeleshwar, Y.Y. Chen, M.K. Wu, J. Phys. D: Appl. Phys. 42 (2009) 095003. [15] J. Lopez, P.N. Lisboa-Filho, W.A.C. Passos, W.A. Ortiz, F.M. Araujo-moreira, O.F. De Lima, D. Schaniel, K. Ghosh, Phys. Rev. B 63 (2001) 224422. [16] A. Chakravarti, R. Ranganathan, S.B. Roy, Phys. Rev. B 46 (1992) 6236. [17] C. Raj Sankar, S. Vijayanand, P.A. Joy, Solid State Sci. 11 (2009) 714. [18] E.O. Wallan, W.C. Koehler, Phys. Rev. 100 (1955) 545. [19] J.B. Goodenough, Phys. Rev. 100 (1955) 564. [20] R. Kajimoto, T. Kakeshita, Y. Ohara, H. Yoshizawa, Y. Tomioka, Y. Tokura, Phys. Rev. B 58 (1998) R11837. [21] S. Yunoki, T. Hotta, E. Dagotto, Phys. Rev. Lett. 84 (2000) 3714. [22] T. Mizokawa, D.I. Khomskii, G.A. Sawatzky, Phys. Rev. B 63 (2001) 024403. [23] B.J. Campbell, S.K. Sinha, R. Osborn, S. Rosenkranz, J.F. Mitchell, D.N. Argyriou, L. Vasiliu-Doloc, O.H. Seeck, J.W. Lynn, Phys. Rev. B 67 (2003) R020409. [24] J.Y. Yu, S.Y. Zhang, G.H. Liu, H.Y. Wu, L. Wang, Solid State Commun. 142 (2007) 333. [25] E. Tka, K. Cherif, J. Dhahri, E. Dhahri, J. Alloys Comp. 509 (2011) 8047. [26] Z. Wang, F. Xu, Y. Du, J. Zhu, H. Zhang, J. Magn. Magn. Mater. 307 (2006) 205. [27] J.A. Collado, J.L. Garcio-Munoz, M.A.G. Aranda, J. Solid State Chem. 183 (2010) 1083. [28] B. Aslibeiki, P. Kameli, H. Salamati, Solid State Commun. 149 (2009) 1274. [29] D. Debasis, R. Rawat, S. Ram, A. Banerjee, S.K. Roy, J. Phys. Condense. Matter 24 (2012) 076001. [30] N. Kallel, S.B. Abdelkhalek, S. Kallel, O. Pena, M. Oumezzine, J. Alloys Comp. 501 (2010) 30.
[31] M.F. Mostafa, S. Ata- Allah, A.A. Youssef, H.S. Refai, J. Magn. Magn. Mater. 320 (2008) 344. [32] S.K. Srivastava, M. Kar, S. Ravi, J. Magn. Magn. Mater. 320 (2008) e107. [33] Y. Ying, T.W. Eom, N.V. Dai, Y.P. Lee, J. Magn. Magn. Mater. 323 (2011) 4. [34] H.Y. Hwang, S.W. Cheong, P.G. Radalli, Phys. Rev. Lett. 75 (1995) 914. [35] Z.B. Guo, N. Zhang, W.P. Ding, W. Yang, J.R. Zhang, Y.W. Du, Solid State Commun. 100 (1996) 769. [36] D.C. Krishna, P.V. Reddy, J. Alloys Comp. 479 (2009) 661. [37] D. Zhu, X.N. Tan, P. Cao, F. Jia, X. Ma, Y. Lu, J. Appl. Phys. 105 (2009) 063914. [38] L.M. Rodriguez-Martinez, J.P. Attfield, Phys. Rev. B 63 (2001) 024424. [39] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [40] Q.Q. Cao, J. Wu, K.M. Gu, S.Y. Zhang, U.W. Du, J. Appl. Phys. 85 (1999) 4494. [41] G. Huanyin, L. Ning, Y. Guoqing, T. Wei, J. Rare Earths 24 (2006) 206. [42] S. Qixiang, W. Guiying, Y. Guoqing, M. Qiang, W. Wenqi, P. Zhensheng, J. Rare Earths 6 (2008) 821. [43] J.M.D. Coey, M. Viret, S. Von Molnar, Adv. Phys. 48 (1999) 167. [44] R.D. Shannon, Acta Cryst. A32 (1976) 51. [45] K. Cherif, J. Dhahri, E. Dhahri, M. Oumezzine, H. Vincent, J. Solid State Chem. 163 (2002) 466. [46] Y. Sun, M.B. Salamon, W. Tong, Y. Zhang, Phys. Rev. B 66 (2002) 094414. [47] N. Ibrahim, A.K. Yahya, S.S. Rajput, S. Keshri, M.K. Talari, J. Magn. Magn. Mater. 323 (2011) 2179. [48] H.Y. Hawang, S.W. Cheong, P.G. Radaelli, M.A. Marezio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. [49] J.P. Zhou, J.T. McDevitt, J.S. Zhou, H.Q. Yin, J.B. Goodenough, Y. Gim, Q.X. Jia, Appl. Phys. Lett. 75 (1999) 1146. [50] Z. Guo, J. Zhang, N. Zhang, W. Ding, H. Huang, Y. Du, Appl. Phys. Lett. 70 (14) (1996) 1897. [51] B. Roy, A. Poddar, S. Das, J. Appl. Phys. 100 (2006) 104318. [52] X.J. Liu, Z.Q. Li, A. Yu, M.L. Liu, W.R. Li, B.L. Li, P. Wu, H.L. Bai, E.Y. Jiang, J. Magn. Magn. Mater. 313 (2007) 354. [53] J. Blasco, J. Garcia, J.M. de Teresa, M.R. Ibarra, J. Perez, P.A. Algarabel, C. Ritter, Phys. Rev. B 55 (1997) 8905. [54] M.M. Xavier Jr, F.A.O. Cabral, J.H. de Araujo, T. Dumelow, A.A. Coelho, J. Mater. Res. 7 (2004) 355. [55] X. Liu, X. Xu, Y. Zhang, Phys. Rev. B 62 (15) (2000) 112. [56] P.S. Antil Kumar, P.A. Joy, S.K. Date, J. Phys: Condense. Matter 10 (1998) L487. [57] K.F. Wang, Y. Wang, L.F. Wang, S. Dong, D. Li, Z.D. Zhang, H. Yu, Q.C. Li, J.-M. Liu, Phys. Rev. B 73 (2006) 134411. [58] K.H. Fischer, H. Hertz, Spin Glasses, Cambridge University Press, 1993. [59] J.A. Mydosh, Spin Glasses, Taylor & Francis, London, 1993. [60] S. Xu, W. Tong, J. Fan, J. Gao, C. Zha, Y. Zhang, J. Magn. Magn. Mater. 288 (2005) 92. [61] P. Shiffer, A.P. Ramirez, W. Bao, S.W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [62] E.S. Vlakhov, R.A. Chakalov, R.I. Chakalov, K.A. Nenkov, K. Dorr, A. Handstein, K.H. Mueller, J. Appl. Phys. 83 (1998) 2152. [63] K. Kubo, N. Ohata, J. Phys. Soc. Jpn. 33 (1972) 21. [64] M. Jaime, M.B. Salamon, M. Rubinstein, R.E. Treece, J.S. Horwitz, D.B. Chrisey, Phys. Rev. B 54 (1996) 11914. [65] M.B. Salmon, M. Jaime, Rev. Mod. Phys. 73 (2001) 583. [66] L. Pi, L. Zhang, Y. Zhang, Phys. Rev. B 61 (2000) 8917. [67] C. Shuiyuan, Y. Qingying, L. Heng, H. Shangho, H. Zhigao, J. Rare Earths 24 (2006) 788. [68] D. Emin, T. Holstein, Ann. Phys. 53 (1969) 439. [69] M. Ziese, C. Srinitiwarawong, Phys. Rev. B 58 (1998) 11519. [70] S. Mollah, Z.A. Khan, D.K. Shukla, M. Arshad, R. Kumar, A. Das, J. Phys. Chem. Solids 69 (2008) 1023. [71] T. Holstein, Ann. Phys. 8 (1959) 343. [72] G. Venkataiah, D.C. Krishna, M. Vithal, S.S. Rao, S.V. Bhat, V. Prasad, S.V. Subramanyam, P. Venugopal Reddy, Physica B 357 (2005) 370. [73] S.P. Altintas, A. Amira, N. Mahamdioua, A. Varilci, C. Terzioglu, J. Alloys Comp. 509 (2011) 4510.