Elsevier

Journal of Alloys and Compounds

Volume 579, 5 December 2013, Pages 406-414
Journal of Alloys and Compounds

Letter
Influence of Sm-doping on the structural, magnetic, and electrical properties of La0.8−xSmxSr0.2MnO3 (0 < x < 0.45) manganites

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Abstract

Structural, magnetic, and electrical properties of the La0.8−xSmxSr0.2MnO3 (0  x  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, ρ(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 ρ(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 > θD/2 (θD, Debye temperature) and the variable range hopping (VRH) models at T < θD/2.

Introduction

In the past few decades, the hole-doped mangnaites, having the common formula A1−xBxMnO3 (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], [2], [3], [4], [5], [6], [7].

These compounds are known as mixed-valence manganites because the parent sample (AMnO3) is Mn3+-rich, whereas the A1−xBxMnO3 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. 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], [11], [12], Griffith’s phase [13], [14], glassy behavior [15], [16], [17], charge and orbital ordering and their competitions [18], [19], [20], [21], small polaron, and magnon correlated transport [22], [23], [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], [26], [27], [28], [29], [30].

Mn-site doping with other elements affects the Mn3+single bondO2−single bondMn4+ networks and DE mechanism remarkably. So far, these kinds of substitutions have been studied extensively for the magnetic and non-magnetic ion substitutions [28], [31], [32], [33].

A-site doping can be strongly influenced by the average ionic radius of the A-site (〈rA〉), which exhibits a close relation between the bending of the Mnsingle bondOsingle bondMn bond angle and the narrowing of the electronic band width [34], [35]. Several researchers have reported the effects of the size variance parameter σ2 on the physical properties of manganites [27], [36], [37]. This parameter is related to 〈rA〉, and is expressed by the formula σ2 = xiri2  rA2, where xi (∑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 Pr0.5Ca0.5−xBaxMnO3 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 〈rA〉. 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 La1−xSrxMnO3 (x = 0.2) compound. The La1−xSrxMnO3 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 (La1−xSmx)0.66Sr0.33MnO3 (x = 0–1) [40], (La1−xSmx)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 La0.80−xSmxSr0.2MnO3 (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.

Section snippets

Experimental

The polycrystalline La0.8−xSmxSr0.2MnO3 (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:1/2x(Sm2O3)+0.20SrCO3+MnO2+(0.40-x/2)La2O3La0.8-xSmxSr0.2MnO3+δ+0.2CO2

Structural properties

Fig. 1 shows the X-ray diffraction (XRD) patterns for the La0.8−xSmxSr0.2MnO3 (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

Conclusion

The effects of Sm substitution on La0.80−xSmxSr0.2MnO3 were investigated. For the samples, by the average ion radius of A-site, 1.2180 < rA < 1.2348, the crystal structure is rhombohedral, and for 〈rA < 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

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