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Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3
Solid State Communications 142 (2007) 634–638 www.elsevier.com/locate/ssc
Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3 Shiming Zhou, Lei Shi ∗ , Jiyin Zhao, Haipeng Yang, Lin Chen Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received 7 February 2007; received in revised form 15 March 2007; accepted 13 April 2007 by E.V. Sampathkumaran Available online 4 May 2007
Abstract The structural, transport and electron spin resonance properties of bulk and nanosized La0.875 Sr0.125 MnO3 prepared by a sol–gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below TC . In addition to a sharp peak in the vicinity of TC , another sharp peak close to TOO is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle. c 2007 Elsevier Ltd. All rights reserved.
PACS: 75.47.Lx; 76.30.-v; 64.75.+g Keywords: A. Manganites; D. Phase separation; E. Electron spin resonance
1. Introduction Perovskite manganites exhibiting colossal magnetroresistance (CMR) such as the prototype La1−x Srx MnO3 system have recently attracted much interest due to their rich structural, magnetic and electronic properties. The ferromagnetism and mechanism of CMR have been traditionally interpreted with the double-exchange (DE) model [1]. However, there is increasing evidence that it is difficult to understand the physics of manganites when only using this model. For instance, it cannot account for the existence of the ferromagnetic insulating (FMI) phase in lightly doped manganites La1−x Srx MnO3 of x close to 1/8. The interplay of lattice, spin, charge and orbital degrees of freedom must certainly play an important role in the system [2,3]. The phase diagram of lightly doped single crystalline and bulk polycrystalline samples of La1−x Srx MnO3 is well established [4–6]. For x = 1/8, as the temperature lowers, a transition from high temperature rhombohedral (R) to ∗ Corresponding address: Structure Research Laboratory, University of Science and Technology of China, Jinzhai Road 96, 230026 Hefei, Anhui, People’s Republic of China. Tel.: +86 551 3607924; fax: +86 551 3602803. E-mail address: [email protected] (L. Shi).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.04.027
orthorhombic (O) structure occurs at ∼450 K. Upon cooling, the O structure distorts to a cooperative Jahn–Teller (JT) distorted orthorhombic (O 0 ) phase at TJT ∼ 270 K, followed by a transition from a paramagnetic insulating (PMI) to a ferromagnetic metallic (FMM) state at TC ∼ 180 K. With the temperature further lowering, the FMM state gives way to a FMI state at T O O ∼ 150 K, which coincides with a structural transition from O 0 to another orthorhombic (O 00 ) phase. A d3x 2 −r 2 /d3y 2 2 orbital ordering which is similar to −r that of LaMnO3 is present between TJT and TC [5,6], and strongly suppressed between TC and T O O . In the FMI phase, charge ordering [2] and a different type of orbital ordering [6] compared with the LaMnO3 -type is supposed to coexist. Electron spin resonance (ESR) is recognized to be very sensitive to magnetic inhomogeneities and anisotropy and has been extensively utilized to investigate the magnetic properties of single crystals and polycrystalline samples of various manganites [7–12]. ESR studies on single crystals of La1−x Srx MnO3 (x ≤ 0.2) revealed that the temperature dependence of the ESR linewidth marks all significant transitions between both orthorhombic (O, O 0 ) and the R structural phases [7]. Alejandro et al. [8] gave a detailed
S. Zhou et al. / Solid State Communications 142 (2007) 634–638
ESR study on the temperature evolution of crystal field interactions across the JT transition in La1−x Srx MnO3 (x = 1/8) single crystal and explained the ESR linewidth and resonance field anisotropy in the term of a Kubo–Tomita perturbational formalism. A recent ESR study on single crystal La1−x Srx MnO3 disclosed the coexistence of FM entities within the globally paramagnetic phase above TC for x ∼ 0.07–0.16, which is related to the presence of the Griffiths phase [9]. It is noticeable that most of those ESR studies on FMI La1−x Srx MnO3 were performed above TC , whereas extremely few publications are devoted to the studies below TC [10]. Meanwhile, the recent ESR studies on other manganites below TC revealed some complex magnetic states [11–13]. For example, the ESR measurements below TC revealed the coexistence of the FMI and FMM phases in La1−x Cax MnO3 single crystals with x = 0.18–0.22 [11]. Therefore, it is necessary to perform a detailed ESR study on La1−x Srx MnO3 with a FMI ground state at a broad temperature range. In this work, we carry out this issue on the polycrystalline La0.875 Sr0.125 MnO3 . On the other hand, a number of studies of the doped manganites showed that the structural, magnetic and transport properties are strongly dependent on the preparation method and the grain size of the samples [14,15]. However, at present ESR studies on manganite nanoparticles [16,17] are rather insufficient. In the present work, we also synthesize a nanosized La0.875 Sr0.125 MnO3 and give a comparison with the bulk concerning the structural, electronic transport and ESR properties. 2. Experimental details The polycrystalline bulk and nanosized samples of La0.875 Sr0.125 MnO3 were prepared by a sol–gel method. The stoichiometric amounts of high-purity La2 O3 , SrCO3 and metal Mn were used as starting materials. They were first converted into nitrates by adding dilute nitric acid. Then an amount of citric acid and ethylene diamine were added to the solution, which was slowly evaporated to get a gel and decomposed at about 400 ◦ C for 4 h to result in a dark brown powder. A part of precursor powders was annealed at 700 ◦ C for 6 h to produce the nanosized sample. Another part was first annealed at 1000 ◦ C for 12 h, then pelleted and sintered at 1400 ◦ C for 24 h with a rapid cooling to produce the polycrystalline bulk sample. The phase purity and crystal structure of the samples were determined by an 18 kW rotating anode x-ray diffractometer (XRD, Type MXP18AHF, MAC Science). The temperature dependence of the resistance of the samples was measured by a standard four-probe technique. ESR measurements were performed with a Bruker ER200D spectrometer at the X-band (9.06 GHz). 3. Results and discussion Room temperature x-ray diffraction patterns of the bulk and nanosized La0.875 Sr0.125 MnO3 are shown in Fig. 1. The bulk sample shows a single phase with an O crystal structure (Pbnm symmetry) and the lattice parameters are a = 5.540, b = 5.515
635
Fig. 1. (Color online) (a) Room temperature XRD pattern of the bulk La0.875 Sr0.125 MnO3 . (b) Room temperature and 70 K XRD patterns of the nanosized La0.875 Sr0.125 MnO3 . The insets show the temperature dependence of resistivity of the bulk and nanosized sample, respectively.
˚ which are in good agreement with the and c = 7.795 A, earlier reports [4,18]. However, the nanosized sample at room temperature gives an R crystal structure (space group R-3C) ˚ α = 60.30◦ . The with the lattice parameters a = 5.152 A, average particle size of the nanosized sample, determined by the Scherrer formula D = 0.89λ/B cos θ , where λ is the x-ray wavelength and B is full width at half maximum of XRD peaks after an instrumental broadening correction, is about 23 nm. It is well known that the lattice structure of the grain surface is often distorted, which may cause a structural relaxation from the surface to the core of the grain. When the grain size decreases, the internal structure of the grain may be more influenced by the surface. In fact, a structural transition with the particle size reducing to nanoscale was found in many doped manganites [15,19]. An R structure in La0.875 Sr0.125 MnO3 nanoparticles had been also reported by Dutta et al. [15]. As mentioned in Section 1, the La0.875 Sr0.125 MnO3 single crystals and polycrystalline samples undergo several structural transitions with the decreasing of the temperature. Does the nanosized sample exhibit any of those transitions? To verify it, the low-temperature XRD measurements for the nanosized sample were carried out. However, the nanosized sample displays a significantly different scenario. No structural transition was observed down to 70 K, the lowest experimental temperature in the present work, as shown in Fig. 1(b). The electric resistivity of the bulk La0.875 Sr0.125 MnO3 is shown in the inset of Fig. 1(a), which agrees well with the earlier literatures [4,6]. Upon cooling an abrupt increase of the resistivity is observed at ∼260 K, which coincides with TJT and corresponds with the structural transition from O to
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Fig. 3. (Color online) Temperature dependence of the intensity of ESR spectra for the bulk La0.875 Sr0.125 MnO3 . The up and down panel of the inset show the Arrhenius plots of the intensity and temperature dependence of the peak-topeak linewidth (∆HPP ) of the spectra above 220 K, respectively.
Fig. 2. (Color online) ESR spectra of the bulk La0.875 Sr0.125 MnO3 between 110 K and 370 K. A three-line spectrum at 195 K is marked by triangles.
O 0 phase, and then an insulating-metallic transition occurs at ∼ 200 K, which may coincide with TC . As the temperature further decreases, the resistivity shows a sharp upturn at ∼170 K, corresponding to T O O , and a FMI ground state appears. However, for the nanosized sample, it is found that only a transition from insulating to metallic state occurs at ∼190 K as shown in the inset of Fig. 1(b). In contrast to the bulk sample, the nanosized sample has a FMM ground state. Fig. 2 shows the ESR spectra of the bulk La0.875 Sr0.125 MnO3 between 110 and 370 K. These measurements are performed on small amounts (∼5 mg) of loose-packed fine powdered sample. Although under a applied magnetic field the partial crystalline’s orientation may be present for loose-packed powders, and to avoid the orientation and any texture in FM states fine powders fixed in paraffin or high-vacuum grease are usually used [13], the ESR lines for loose-packed powders are narrower than those obtained for powders fixed in paraffin and give a better resolution in our case. Moreover, the general structure of the spectrum for both powders are about the same, thus the loose-packed powders are preferred in the present work. Above 220 K the spectrum shows a single line centered at g ∼ 2.0. However, below the temperature the spectra display complex ferromagnetic resonance multilines. One can see that a three-line spectrum is observed between 200 and 180 K. As a representation, it is marked by triangles in the spectrum at 195 K as shown in Fig. 2. A three-line spectrum was also reported in La1−x Cax MnO3 single crystals with x = 0.18 and 0.20 [11], where the intense low-field line and two weak high-field lines were attributed to the FMM and FMI phases, respectively. We suggest that the FMM and FMI phases also coexist in our bulk sample. However, unlike those single crystals with a relatively low intensity of two high-field lines [11], a relatively intense high-field line is observed in our polycrystalline sample. It is noticeable that in uniaxial polycrystalline ferromagnets the ESR spectrum
below TC is characterized by an asymmetrical ferromagnetic resonance pattern comprising a low-field peak and a high-field derivative-like line [13], which is found in our nanosized sample as shown below. Therefore, it is suggested that in our case the relatively intense high-field line should also include an attribution from the FMM phase, whose high-field derivative-like feature appears around the field. The relatively intense low-field signal in the spectra indicates that the FMM phase dominates in this temperature range, which is confirmed by the transport property where a metallic behavior is observed. As the temperature decreases, the low-field signal weakens and shifts to lower field. Below 180 K it shifts to zero field and appears as a nonresonance microwave absorption, and the resonance lines are attributed to the FMI phase characterized by a strong nonuniaxal magnetocrystalline anisotropy, which is a characteristic feature of orbital ordering FMI systems [11]. With the temperature further lowering, the resonance field remarkably shifts to a high field, implying that the anisotropy magnetic field increases with the temperature decreasing [11–13]. The temperature dependence of the ESR spectra reveals the existence of the strongly magnetic anisotropy in this system and the coexistence of the FMI and FMM signals indicates the phenomena of phase separation. These results are consistent with a previous study of ac susceptibility on single crystal La0.875 Sr0.125 MnO3 where the observed anomalous ac susceptibility implied the existence of the strongly magnetic anisotropy and the nonhomogeneous magnetic state below TC and a phase separation had been also proposed [20]. Fig. 3 displays the temperature dependence of the intensity of the ESR spectra I (T ), derived by double integration of the first derivative spectra and normalized to the value of 300 K. As the temperature reduces, the intensity rises rapidly and shows a sharp peak at about 200 K. Below this temperature it sharply drops again. The intensity of ESR spectra is proportional to the relative amount of the corresponding magnetic phase. The temperature dependence of the intensity reaching a maximum value in the vicinity of TC was reported in many doped manganites [11–13]. The sharp drop is usually attributed to
S. Zhou et al. / Solid State Communications 142 (2007) 634–638
the shift of the ESR signal to zero field, i.e. the appearance of nonresonance microwave absorption. In our case, one can see that the FMM signal shifts to zero field with the temperature decreasing, which may be responsible for the sharp drop. On the other hand, the intensity is also proportional to the frequencydependent magnetic susceptibility. As the appearance of the glasslike state causes some magnetic moments freezing at high frequency, it is also considered as a factor responsible for the sharp drop in intensity [11–13]. Recently, a spin glass state was also observed at TO O < T < TC in La1−x Srx MnO3 single crystals with x close to 1/8 [6]. Therefore, it is suggested that the existence of the spin glass state in this system may be also responsible to the sharp drop of the intensity. With the temperature further decreasing, it is surprising that the intensity rises again and gives a second sharp peak at about 155 K. A local maximum of the intensity of ESR spectra, in addition to the sharp peak close to TC , was also reported recently in polycrystalline La1−x Cax MnO3 (x = 0.125–0.19) at 60–70 K [13]. It was proposed that the variation of the intensity is traced with the structural transformation between two monoclinic phases, implicating concurrent changes of the magnetic anisotropy or the domain structure. It is noteworthy that in our case the second peak occurs close to TO O . Furthermore, it had been reported that in La1−x Srx MnO3 single crystals with x close to 1/8 the orbital ordering strengthens the superexchange interaction and causes a magnetization jump at TO O [21]. Ac susceptibility measurements also revealed a maximum at TO O in the system [20]. Thus, we suggest that the second peak in the intensity may be correlated with the structural transition and orbital ordering at this temperature. It is well known that the intensity I (T ) usually obeys the Arrhenius law, I (T ) = I0 exp(E a /K B T ), within the PM region [11–13]. The up-panel of the inset of Fig. 3 shows the Arrhenius plots of the intensity. It is seen that the intensity well obeys the law with the activation energy of E a ∼ 93 meV above 260 K, close to TJT . The down-panel displays the temperature dependence of the peak-to-peak linewidth (∆HPP ) above 220 K. It is also observed that the linewidth has a minimum value at about 260 K and above the temperature it exhibits a linear temperature dependence with a slope of about 3 Oe/K. Furthermore, the ESR spectrum only above TJT can be well fitted by a single Lorentzian line. These results strongly indicate that the bulk sample is in a pure PM phase only above TJT [7,8]. At the temperature range between TC and TJT , as mentioned in Section 1, a coexistence of FM entities within the globally PM phase had been reported [9]. Although no visible FM resonance signal is found between TC and TJT for our polycrystalline sample, the temperature dependence of the intensity and linewidth deviating from a pure PM phase implies the possible existence of the FM entities at this temperature range. Fig. 4 shows the temperature dependence of ESR spectra of the nanosized La0.875 Sr0.125 MnO3 . Above 330 K the spectrum shows a single narrow line centered at g ∼ 2.0, which is attributed to a PM phase. As the temperature decreases, besides the narrow signal, a broad FM resonance signal appears, which is characterized by an asymmetrical ferromagnetic resonance
637
Fig. 4. (Color online) ESR spectra of the nanosized La0.875 Sr0.125 MnO3 between 110 and 340 K. The inset shows the temperature dependence of the intensity of ESR spectra.
pattern as mentioned above. The coexistence of the PM and FM signals is observed down to 110 K, and no FMI signal like the bulk is found at the temperature range, which is consistent with the transport property. When the temperature decreases, the FM resonance signal broadens and shows a growing asymmetry, whereas the narrow PM signal exhibits a weak temperature dependence, which implies that the signal derives from the disorder spins at the surface of the grains [22]. Recently, similar coexistence of PM and FM signals was also reported in nanosized La0.9 Ca0.1 MnO3 [16] and La0.7 Sr0.3 MnO3 [17]. The temperature dependence of the intensity of the ESR spectra I (T ) for the nanosized La0.875 Sr0.125 MnO3 is displayed in the inset of Fig. 4. Upon cooling, the intensity gradually rises and exhibits a broad maximum around 200 K. Moreover, with the temperature further lowering, the intensity only shows a slight decrease at low temperature. These features are drastically distinct from the bulk sample. Recently, the magnetic properties of La0.875 Sr0.125 MnO3 nanoparticles were investigated by Dutta et al. [15]. The temperature-dependent intensity of the ESR spectra for our nanosized sample is consistent with the zero-field cooled magnetization of their nanoparticles. According to their studies, the behavior of the intensity of the ESR spectra is correlated with the magnetic domain status in the nanoparticles. The broad peak in I (T ) is attributed to the wide FM transition, which is due to a distribution of the strength of exchange coupling from surface to core of the nanoparticles. 4. Conclusion In summary, we have investigated the structural, electronic transport and ESR properties of the polycrystalline bulk and nanosized La0.875 Sr0.125 MnO3 prepared by a sol–gel method. The bulk sample has an orthorhombic lattice structure and
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exhibits a FMI ground state, which agrees well with those reports in earlier literature. The ESR spectra reveal that the FMM and FMI phases coexist below TC . In addition to the sharp peak in the vicinity of TC , another sharp peak is clearly observed close to TO O , which may be correlated with the structural transition and orbital ordering at this temperature. On the other hand, the nanosized sample shows a drastically different behavior from the bulk. It gives a rhombohedral structure and has a FMM ground state. The coexistence of the PM and FM signals is observed in the ESR spectra. The PM signal shows a weak temperature dependence and is attributed to the disorder spins at the surface of the nanoparticles. Only a broad peak is found in the temperature dependence of the intensity of the ESR spectra, which may be due to a wide ferromagnetic transition in the nanoparticles. Acknowledgments This project was financially supported by the Ministry of Science and Technology of China (NKBRSF-G1999064603), and the National Science Foundation of China, Grant No. 10174071. References [1] C. Zener, Phys. Rev. 82 (1951) 403. [2] S. Uhlenbruck, R. Teipen, R. Klingeler, B. B¨uchner, O. Friedt, M. H¨ucker, H. Kierspel, T. Niem¨oller, L. Pinsard, A. Revcolevschi, R. Gross, Phys. Rev. Lett. 82 (1999) 185. [3] T. Koide, H. Miyauchi, J. Okamoto, T. Shidara, T. Sekine, T. Saitoh, A. Fujimori, H. Fukutani, M. Takano, Y. Takeda, Phys. Rev. Lett. 87 (2001) 246404.
[4] B. Dabrowski, X. Xiong, Z. Bukowski, R. Dybzinski, P.W. Klamut, J.E. Siewenie, O. Chmaissem, J. Shaffer, C.W. Kimball, Phys. Rev. B 60 (1999) 7006. [5] J. Geck, P. Wochner, D. Bruns, B. B¨uchner, U. Gebhardt, S. Kiele, P. Reutler, A. Revcolevschi, Phys. Rev. B 69 (2004) 104413. [6] G.-L. Liu, J.-S. Zhou, J.B. Goodenough, Phys. Rev. B 64 (2001) 144414. [7] V.A. Ivanshin, J. Deisenhofer, H.-A. Krug von Nidda, A. Loidl, A.A. Mukhin, A.M. Balbashov, M.V. Eremin, Phys. Rev. B 61 (2000) 6213. [8] G. Alejandro, M.C.G. Passeggi, D. Vega, C.A. Ramos, M.T. Causa, M. Tovar, R. Senis, Phys. Rev. B 68 (2003) 214429. [9] J. Deisenhofer, D. Braak, H.-A. Krug von Nidda, J. Hemberger, R.M. Eremina, V.A. Ivanshin, A.M. Balbashov, G. Jug, A. Loidl, T. Kimura, Y. Tokura, Phys. Rev. Lett. 95 (2005) 257202. [10] N.A. Viglin, S.V. Naumov, Ya.M. Mukovski˘ı, Phys. Solid State 43 (2001) 1855. [11] A.I. Shames, E. Rozenberg, G. Gorodetsky, Ya.M. Mukovskii, Phys. Rev. B 68 (2003) 174402. [12] V. Markovich, I. Fita, A.I. Shames, R. Puzniak, E. Rozenberg, C. Martin, A. Wisniewski, Y. Yuzhelevskii, A. Wahl, G. Gorodetsky, Phys. Rev. B 68 (2003) 094428. [13] V. Likodimos, M. Pissas, Phys. Rev. B 73 (2006) 214417. [14] J.F. Mitchell, D.N. Argyriou, C.D. Potter, D.G. Hinks, J.D. Jorgensen, S.D. Bader, Phys. Rev. B 54 (1996) 6172. [15] A. Dutta, N. Gayathri, R. Ranganathan, Phys. Rev. B 68 (2003) 054432. [16] A.I. Shames, M. Auslender, E. Rozenberg, G. Gorodetsky, E. Sominski, A. Gedanken, Ya.M. Mukovskii, J. Magn. Magn. Mater. 300 (2006) 12. [17] J.-H. Wu, J.G. Lin, J. Appl. Phys. 99 (2006) 08Q316. [18] L. Pinsard, J. Rodr´ıguez-Carvajal, A. Revcolevschi, J. Alloys Compd. 262 (1997) 152. [19] Y.W. Duan, X.L. Kou, J.G. Li, Phys. B 355 (2005) 250. [20] V. Skumryev, J. Nogu´es, J.S. Mu˜noz, B. Mart´ınez, R. Senis, J. Fontcuberta, L. Pinsard, A. Revcolevschi, Y.M. Mukovskii, Phys. Rev. B 62 (2000) 3879. [21] H. Nojiri, K. Kaneko, M. Motokawa, K. Hirota, Y. Endoh, K. Takahashi, Phys. Rev. B 60 (1999) 4142. [22] G. W¨ubbeler, O.F. Schirmer, S. K¨ohne, Phys. Rev. B 54 (1996) 9054.
Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3 Shiming Zhou, Lei Shi ∗ , Jiyin Zhao, Haipeng Yang, Lin Chen Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received 7 February 2007; received in revised form 15 March 2007; accepted 13 April 2007 by E.V. Sampathkumaran Available online 4 May 2007
Abstract The structural, transport and electron spin resonance properties of bulk and nanosized La0.875 Sr0.125 MnO3 prepared by a sol–gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below TC . In addition to a sharp peak in the vicinity of TC , another sharp peak close to TOO is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle. c 2007 Elsevier Ltd. All rights reserved.
PACS: 75.47.Lx; 76.30.-v; 64.75.+g Keywords: A. Manganites; D. Phase separation; E. Electron spin resonance
1. Introduction Perovskite manganites exhibiting colossal magnetroresistance (CMR) such as the prototype La1−x Srx MnO3 system have recently attracted much interest due to their rich structural, magnetic and electronic properties. The ferromagnetism and mechanism of CMR have been traditionally interpreted with the double-exchange (DE) model [1]. However, there is increasing evidence that it is difficult to understand the physics of manganites when only using this model. For instance, it cannot account for the existence of the ferromagnetic insulating (FMI) phase in lightly doped manganites La1−x Srx MnO3 of x close to 1/8. The interplay of lattice, spin, charge and orbital degrees of freedom must certainly play an important role in the system [2,3]. The phase diagram of lightly doped single crystalline and bulk polycrystalline samples of La1−x Srx MnO3 is well established [4–6]. For x = 1/8, as the temperature lowers, a transition from high temperature rhombohedral (R) to ∗ Corresponding address: Structure Research Laboratory, University of Science and Technology of China, Jinzhai Road 96, 230026 Hefei, Anhui, People’s Republic of China. Tel.: +86 551 3607924; fax: +86 551 3602803. E-mail address: [email protected] (L. Shi).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.04.027
orthorhombic (O) structure occurs at ∼450 K. Upon cooling, the O structure distorts to a cooperative Jahn–Teller (JT) distorted orthorhombic (O 0 ) phase at TJT ∼ 270 K, followed by a transition from a paramagnetic insulating (PMI) to a ferromagnetic metallic (FMM) state at TC ∼ 180 K. With the temperature further lowering, the FMM state gives way to a FMI state at T O O ∼ 150 K, which coincides with a structural transition from O 0 to another orthorhombic (O 00 ) phase. A d3x 2 −r 2 /d3y 2 2 orbital ordering which is similar to −r that of LaMnO3 is present between TJT and TC [5,6], and strongly suppressed between TC and T O O . In the FMI phase, charge ordering [2] and a different type of orbital ordering [6] compared with the LaMnO3 -type is supposed to coexist. Electron spin resonance (ESR) is recognized to be very sensitive to magnetic inhomogeneities and anisotropy and has been extensively utilized to investigate the magnetic properties of single crystals and polycrystalline samples of various manganites [7–12]. ESR studies on single crystals of La1−x Srx MnO3 (x ≤ 0.2) revealed that the temperature dependence of the ESR linewidth marks all significant transitions between both orthorhombic (O, O 0 ) and the R structural phases [7]. Alejandro et al. [8] gave a detailed
S. Zhou et al. / Solid State Communications 142 (2007) 634–638
ESR study on the temperature evolution of crystal field interactions across the JT transition in La1−x Srx MnO3 (x = 1/8) single crystal and explained the ESR linewidth and resonance field anisotropy in the term of a Kubo–Tomita perturbational formalism. A recent ESR study on single crystal La1−x Srx MnO3 disclosed the coexistence of FM entities within the globally paramagnetic phase above TC for x ∼ 0.07–0.16, which is related to the presence of the Griffiths phase [9]. It is noticeable that most of those ESR studies on FMI La1−x Srx MnO3 were performed above TC , whereas extremely few publications are devoted to the studies below TC [10]. Meanwhile, the recent ESR studies on other manganites below TC revealed some complex magnetic states [11–13]. For example, the ESR measurements below TC revealed the coexistence of the FMI and FMM phases in La1−x Cax MnO3 single crystals with x = 0.18–0.22 [11]. Therefore, it is necessary to perform a detailed ESR study on La1−x Srx MnO3 with a FMI ground state at a broad temperature range. In this work, we carry out this issue on the polycrystalline La0.875 Sr0.125 MnO3 . On the other hand, a number of studies of the doped manganites showed that the structural, magnetic and transport properties are strongly dependent on the preparation method and the grain size of the samples [14,15]. However, at present ESR studies on manganite nanoparticles [16,17] are rather insufficient. In the present work, we also synthesize a nanosized La0.875 Sr0.125 MnO3 and give a comparison with the bulk concerning the structural, electronic transport and ESR properties. 2. Experimental details The polycrystalline bulk and nanosized samples of La0.875 Sr0.125 MnO3 were prepared by a sol–gel method. The stoichiometric amounts of high-purity La2 O3 , SrCO3 and metal Mn were used as starting materials. They were first converted into nitrates by adding dilute nitric acid. Then an amount of citric acid and ethylene diamine were added to the solution, which was slowly evaporated to get a gel and decomposed at about 400 ◦ C for 4 h to result in a dark brown powder. A part of precursor powders was annealed at 700 ◦ C for 6 h to produce the nanosized sample. Another part was first annealed at 1000 ◦ C for 12 h, then pelleted and sintered at 1400 ◦ C for 24 h with a rapid cooling to produce the polycrystalline bulk sample. The phase purity and crystal structure of the samples were determined by an 18 kW rotating anode x-ray diffractometer (XRD, Type MXP18AHF, MAC Science). The temperature dependence of the resistance of the samples was measured by a standard four-probe technique. ESR measurements were performed with a Bruker ER200D spectrometer at the X-band (9.06 GHz). 3. Results and discussion Room temperature x-ray diffraction patterns of the bulk and nanosized La0.875 Sr0.125 MnO3 are shown in Fig. 1. The bulk sample shows a single phase with an O crystal structure (Pbnm symmetry) and the lattice parameters are a = 5.540, b = 5.515
635
Fig. 1. (Color online) (a) Room temperature XRD pattern of the bulk La0.875 Sr0.125 MnO3 . (b) Room temperature and 70 K XRD patterns of the nanosized La0.875 Sr0.125 MnO3 . The insets show the temperature dependence of resistivity of the bulk and nanosized sample, respectively.
˚ which are in good agreement with the and c = 7.795 A, earlier reports [4,18]. However, the nanosized sample at room temperature gives an R crystal structure (space group R-3C) ˚ α = 60.30◦ . The with the lattice parameters a = 5.152 A, average particle size of the nanosized sample, determined by the Scherrer formula D = 0.89λ/B cos θ , where λ is the x-ray wavelength and B is full width at half maximum of XRD peaks after an instrumental broadening correction, is about 23 nm. It is well known that the lattice structure of the grain surface is often distorted, which may cause a structural relaxation from the surface to the core of the grain. When the grain size decreases, the internal structure of the grain may be more influenced by the surface. In fact, a structural transition with the particle size reducing to nanoscale was found in many doped manganites [15,19]. An R structure in La0.875 Sr0.125 MnO3 nanoparticles had been also reported by Dutta et al. [15]. As mentioned in Section 1, the La0.875 Sr0.125 MnO3 single crystals and polycrystalline samples undergo several structural transitions with the decreasing of the temperature. Does the nanosized sample exhibit any of those transitions? To verify it, the low-temperature XRD measurements for the nanosized sample were carried out. However, the nanosized sample displays a significantly different scenario. No structural transition was observed down to 70 K, the lowest experimental temperature in the present work, as shown in Fig. 1(b). The electric resistivity of the bulk La0.875 Sr0.125 MnO3 is shown in the inset of Fig. 1(a), which agrees well with the earlier literatures [4,6]. Upon cooling an abrupt increase of the resistivity is observed at ∼260 K, which coincides with TJT and corresponds with the structural transition from O to
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Fig. 3. (Color online) Temperature dependence of the intensity of ESR spectra for the bulk La0.875 Sr0.125 MnO3 . The up and down panel of the inset show the Arrhenius plots of the intensity and temperature dependence of the peak-topeak linewidth (∆HPP ) of the spectra above 220 K, respectively.
Fig. 2. (Color online) ESR spectra of the bulk La0.875 Sr0.125 MnO3 between 110 K and 370 K. A three-line spectrum at 195 K is marked by triangles.
O 0 phase, and then an insulating-metallic transition occurs at ∼ 200 K, which may coincide with TC . As the temperature further decreases, the resistivity shows a sharp upturn at ∼170 K, corresponding to T O O , and a FMI ground state appears. However, for the nanosized sample, it is found that only a transition from insulating to metallic state occurs at ∼190 K as shown in the inset of Fig. 1(b). In contrast to the bulk sample, the nanosized sample has a FMM ground state. Fig. 2 shows the ESR spectra of the bulk La0.875 Sr0.125 MnO3 between 110 and 370 K. These measurements are performed on small amounts (∼5 mg) of loose-packed fine powdered sample. Although under a applied magnetic field the partial crystalline’s orientation may be present for loose-packed powders, and to avoid the orientation and any texture in FM states fine powders fixed in paraffin or high-vacuum grease are usually used [13], the ESR lines for loose-packed powders are narrower than those obtained for powders fixed in paraffin and give a better resolution in our case. Moreover, the general structure of the spectrum for both powders are about the same, thus the loose-packed powders are preferred in the present work. Above 220 K the spectrum shows a single line centered at g ∼ 2.0. However, below the temperature the spectra display complex ferromagnetic resonance multilines. One can see that a three-line spectrum is observed between 200 and 180 K. As a representation, it is marked by triangles in the spectrum at 195 K as shown in Fig. 2. A three-line spectrum was also reported in La1−x Cax MnO3 single crystals with x = 0.18 and 0.20 [11], where the intense low-field line and two weak high-field lines were attributed to the FMM and FMI phases, respectively. We suggest that the FMM and FMI phases also coexist in our bulk sample. However, unlike those single crystals with a relatively low intensity of two high-field lines [11], a relatively intense high-field line is observed in our polycrystalline sample. It is noticeable that in uniaxial polycrystalline ferromagnets the ESR spectrum
below TC is characterized by an asymmetrical ferromagnetic resonance pattern comprising a low-field peak and a high-field derivative-like line [13], which is found in our nanosized sample as shown below. Therefore, it is suggested that in our case the relatively intense high-field line should also include an attribution from the FMM phase, whose high-field derivative-like feature appears around the field. The relatively intense low-field signal in the spectra indicates that the FMM phase dominates in this temperature range, which is confirmed by the transport property where a metallic behavior is observed. As the temperature decreases, the low-field signal weakens and shifts to lower field. Below 180 K it shifts to zero field and appears as a nonresonance microwave absorption, and the resonance lines are attributed to the FMI phase characterized by a strong nonuniaxal magnetocrystalline anisotropy, which is a characteristic feature of orbital ordering FMI systems [11]. With the temperature further lowering, the resonance field remarkably shifts to a high field, implying that the anisotropy magnetic field increases with the temperature decreasing [11–13]. The temperature dependence of the ESR spectra reveals the existence of the strongly magnetic anisotropy in this system and the coexistence of the FMI and FMM signals indicates the phenomena of phase separation. These results are consistent with a previous study of ac susceptibility on single crystal La0.875 Sr0.125 MnO3 where the observed anomalous ac susceptibility implied the existence of the strongly magnetic anisotropy and the nonhomogeneous magnetic state below TC and a phase separation had been also proposed [20]. Fig. 3 displays the temperature dependence of the intensity of the ESR spectra I (T ), derived by double integration of the first derivative spectra and normalized to the value of 300 K. As the temperature reduces, the intensity rises rapidly and shows a sharp peak at about 200 K. Below this temperature it sharply drops again. The intensity of ESR spectra is proportional to the relative amount of the corresponding magnetic phase. The temperature dependence of the intensity reaching a maximum value in the vicinity of TC was reported in many doped manganites [11–13]. The sharp drop is usually attributed to
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the shift of the ESR signal to zero field, i.e. the appearance of nonresonance microwave absorption. In our case, one can see that the FMM signal shifts to zero field with the temperature decreasing, which may be responsible for the sharp drop. On the other hand, the intensity is also proportional to the frequencydependent magnetic susceptibility. As the appearance of the glasslike state causes some magnetic moments freezing at high frequency, it is also considered as a factor responsible for the sharp drop in intensity [11–13]. Recently, a spin glass state was also observed at TO O < T < TC in La1−x Srx MnO3 single crystals with x close to 1/8 [6]. Therefore, it is suggested that the existence of the spin glass state in this system may be also responsible to the sharp drop of the intensity. With the temperature further decreasing, it is surprising that the intensity rises again and gives a second sharp peak at about 155 K. A local maximum of the intensity of ESR spectra, in addition to the sharp peak close to TC , was also reported recently in polycrystalline La1−x Cax MnO3 (x = 0.125–0.19) at 60–70 K [13]. It was proposed that the variation of the intensity is traced with the structural transformation between two monoclinic phases, implicating concurrent changes of the magnetic anisotropy or the domain structure. It is noteworthy that in our case the second peak occurs close to TO O . Furthermore, it had been reported that in La1−x Srx MnO3 single crystals with x close to 1/8 the orbital ordering strengthens the superexchange interaction and causes a magnetization jump at TO O [21]. Ac susceptibility measurements also revealed a maximum at TO O in the system [20]. Thus, we suggest that the second peak in the intensity may be correlated with the structural transition and orbital ordering at this temperature. It is well known that the intensity I (T ) usually obeys the Arrhenius law, I (T ) = I0 exp(E a /K B T ), within the PM region [11–13]. The up-panel of the inset of Fig. 3 shows the Arrhenius plots of the intensity. It is seen that the intensity well obeys the law with the activation energy of E a ∼ 93 meV above 260 K, close to TJT . The down-panel displays the temperature dependence of the peak-to-peak linewidth (∆HPP ) above 220 K. It is also observed that the linewidth has a minimum value at about 260 K and above the temperature it exhibits a linear temperature dependence with a slope of about 3 Oe/K. Furthermore, the ESR spectrum only above TJT can be well fitted by a single Lorentzian line. These results strongly indicate that the bulk sample is in a pure PM phase only above TJT [7,8]. At the temperature range between TC and TJT , as mentioned in Section 1, a coexistence of FM entities within the globally PM phase had been reported [9]. Although no visible FM resonance signal is found between TC and TJT for our polycrystalline sample, the temperature dependence of the intensity and linewidth deviating from a pure PM phase implies the possible existence of the FM entities at this temperature range. Fig. 4 shows the temperature dependence of ESR spectra of the nanosized La0.875 Sr0.125 MnO3 . Above 330 K the spectrum shows a single narrow line centered at g ∼ 2.0, which is attributed to a PM phase. As the temperature decreases, besides the narrow signal, a broad FM resonance signal appears, which is characterized by an asymmetrical ferromagnetic resonance
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Fig. 4. (Color online) ESR spectra of the nanosized La0.875 Sr0.125 MnO3 between 110 and 340 K. The inset shows the temperature dependence of the intensity of ESR spectra.
pattern as mentioned above. The coexistence of the PM and FM signals is observed down to 110 K, and no FMI signal like the bulk is found at the temperature range, which is consistent with the transport property. When the temperature decreases, the FM resonance signal broadens and shows a growing asymmetry, whereas the narrow PM signal exhibits a weak temperature dependence, which implies that the signal derives from the disorder spins at the surface of the grains [22]. Recently, similar coexistence of PM and FM signals was also reported in nanosized La0.9 Ca0.1 MnO3 [16] and La0.7 Sr0.3 MnO3 [17]. The temperature dependence of the intensity of the ESR spectra I (T ) for the nanosized La0.875 Sr0.125 MnO3 is displayed in the inset of Fig. 4. Upon cooling, the intensity gradually rises and exhibits a broad maximum around 200 K. Moreover, with the temperature further lowering, the intensity only shows a slight decrease at low temperature. These features are drastically distinct from the bulk sample. Recently, the magnetic properties of La0.875 Sr0.125 MnO3 nanoparticles were investigated by Dutta et al. [15]. The temperature-dependent intensity of the ESR spectra for our nanosized sample is consistent with the zero-field cooled magnetization of their nanoparticles. According to their studies, the behavior of the intensity of the ESR spectra is correlated with the magnetic domain status in the nanoparticles. The broad peak in I (T ) is attributed to the wide FM transition, which is due to a distribution of the strength of exchange coupling from surface to core of the nanoparticles. 4. Conclusion In summary, we have investigated the structural, electronic transport and ESR properties of the polycrystalline bulk and nanosized La0.875 Sr0.125 MnO3 prepared by a sol–gel method. The bulk sample has an orthorhombic lattice structure and
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exhibits a FMI ground state, which agrees well with those reports in earlier literature. The ESR spectra reveal that the FMM and FMI phases coexist below TC . In addition to the sharp peak in the vicinity of TC , another sharp peak is clearly observed close to TO O , which may be correlated with the structural transition and orbital ordering at this temperature. On the other hand, the nanosized sample shows a drastically different behavior from the bulk. It gives a rhombohedral structure and has a FMM ground state. The coexistence of the PM and FM signals is observed in the ESR spectra. The PM signal shows a weak temperature dependence and is attributed to the disorder spins at the surface of the nanoparticles. Only a broad peak is found in the temperature dependence of the intensity of the ESR spectra, which may be due to a wide ferromagnetic transition in the nanoparticles. Acknowledgments This project was financially supported by the Ministry of Science and Technology of China (NKBRSF-G1999064603), and the National Science Foundation of China, Grant No. 10174071. References [1] C. Zener, Phys. Rev. 82 (1951) 403. [2] S. Uhlenbruck, R. Teipen, R. Klingeler, B. B¨uchner, O. Friedt, M. H¨ucker, H. Kierspel, T. Niem¨oller, L. Pinsard, A. Revcolevschi, R. Gross, Phys. Rev. Lett. 82 (1999) 185. [3] T. Koide, H. Miyauchi, J. Okamoto, T. Shidara, T. Sekine, T. Saitoh, A. Fujimori, H. Fukutani, M. Takano, Y. Takeda, Phys. Rev. Lett. 87 (2001) 246404.
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