- Email: [email protected]
Effect of the calcination temperature on the magnetic and transport properties of rhombohedral LaMnO3+δ compounds
ARTICLE IN PRESS Physica B 405 (2010) 1362–1368
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effect of the calcination temperature on the magnetic and transport properties of rhombohedral LaMnO3 + d compounds X.L. Wang, D. Li , C.X. Shi, B. Li, T.Y. Cui, Z.D. Zhang Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 October 2009 Received in revised form 30 November 2009 Accepted 1 December 2009
Rhombohedral LaMnO3 + d powders were successfully synthesized by annealing gel precursors obtained by a polymerized complex method. The average grain size increases with increasing calcination temperature. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves indicate that LaMnO3 + d compounds obtained at 700 and 800 1C are ferromagnetic, while both ferromagnetic and antiferromagnetic components are present in those calcined at 900 and 1000 1C. This is because the concentration of Mn4 + ions drops with increasing calcination temperature. The temperature dependence of the resistivity of LaMnO3 + d compounds exhibits an insulator–metal transition at about 90 K and also a low-temperature upturn with a resistivity minimum at about 40 K. Upon increasing particle size, the zero-field resistivity of the LaMnO3 + d powders increases due to suppression of the conduction by the insulating antiferromagnetic component. The negative magnetoresistance of a sample pellet prepared at 700 1C ranges from 82% to 88% in the temperature range of 40–100 K due to suppression of the spin-dependent scattering by an applied field. & 2009 Elsevier B.V. All rights reserved.
Keywords: Polymerized synthesis Magnetic properties Magnetoresistance
1. Introduction Doped perovskite manganites with the general formula R1 xAxMnO3 (R is rare-earth metal, A is univalent element Na or divalent element such as Ca, Sr, Ba) have been extensively investigated in the past decades, because of their abundant physical properties such as colossal magnetoresistance (CMR) effect, Jahn–Teller effect, metal–insulator transition and their potential applications in magneto-electronic devices, magnetodata storage [1–9]. These properties are closely related to the interactions among charge, orbital, spin, lattice and magnetic degrees of freedom. Through cationic replacement in the antiferromagnetic orthorhombic parent compound LaMnO3, the La1 xAxMnO3 manganite transforms into a doped mixedvalence Mn3 + –Mn4 + phase that is ferromagnetic and displays a metal–insulator transition in the vicinity of the Curie temperature TC. The spin dynamics and electronic transport are conventionally interpreted in terms of the Zener double-exchange (DE) mechanism [10]. The Mn3 + –Mn4 + mixed-valence state can also be obtained by altering the chemical stoichiometry, as for self- or vacancy-doped powders with the general formula La1 xMn1 yO3 + d. In these compounds, variations of x, y and d can modify the Mn3 + –Mn4 + ratio, which is beneficial for
Corresponding author.
E-mail address: [email protected] (D. Li). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.12.001
activating the DE interaction. This enhances the ferromagnetic coupling and gives rise to the CMR effect as in the case of the substituted compounds [11,12]. Usually, the crystal structures of these compounds, which possess the Mn3 + –Mn4 + mixed-valence state through either element substitution or in a self-doped form, are rhombohedral or cubic [11–18]. In the present work, we successfully synthesized rhombohedral LaMnO3 + d compounds at a relative low temperature by annealing gel precursors obtained by a polymerized complex method. The effect of the calcination temperature on the magnetic and transport properties of LaMnO3 + d compounds was investigated.
2. Experimental procedure LaMnO3 + d compounds were synthesized by the polymerized complex method described elsewhere [19]. All reagents are commercially available in analytical purity and used without further purification. In a typical procedure, a solution was prepared by mixing aqueous solutions of Mn(NO3)2 and La(NO3)3 in 1:1 molar ratio and then citric acid was added under stirring. After the solution was adjusted to pH= 2 by adding dilute ammonial solution under stirring, the Mn2 + and La3 + cations complexed with the anions of citric acid into their citrates. Then, ethylene glycol was added for gelation. The molar amounts of citric acid and ethylene glycol were twice those of the La and Mn cations. More citric acid and ethylene glycol were added than in
ARTICLE IN PRESS X.L. Lang et al. / Physica B 405 (2010) 1362–1368
1363
previous work [12,15], because it is beneficial for the formation of chelate precursors. As a result, LaMnO3 + d powders can be obtained at lower temperature than reported previously [12,15]. The solution was continuously stirred at room temperature for 5 h and then heated at 100 1C under stirring till a gel was formed. This gel with high viscosity was further dried overnight in an oven at 130 1C and ground into fine black powder. Finally, the black precursor powders were calcined in air atmosphere at 600, 700, 800, 900 and 1000 1C for 4 h. The final products corresponding to these calcination temperatures are denoted as LMO600, LMO700, LMO800, LMO900 and LMO1000, respectively. Phase analysis of all products was performed by using powder X-ray diffraction (XRD) on a D/max-2000 diffractometer at a voltage of 56 kV and a current of 182 mA with Cu-Ka (l = 0.154056 nm) radiation. The average crystal–grain size of the powders was estimated from the (0 2 4) reflections, by means of the Scherrer equation, d ¼ kl=b cos y where d is the average diameter of the crystal grains, k is a particle shape factor and taken as 0.9 for spherical nanoparticles, l is the wavelength of Cu-Ka radiation, y is the corresponding Bragg angle and b is the angular half-width of the reflection at 2y. The surface compositions of the powders were determined with an ESCALAB250 X-ray photoelectron spectroscopy (XPS) spectrometer using an Al X-ray source emitting at 1486.6 eV and a monochromator. The morphology was investigated with a supra 35 field-emission scanning electron microscope (FESEM) equipped with energydispersive spectrum (EDS) operated at an acceleration voltage of 20 kV and a high-resolution transmission electron microscopy (HRTEM) using a JEOL 2010 with an emission voltage of 200 kV. The magnetic and electrical properties of the products were measured in a superconducting quantum interference device (SQUID, Quantum Design MPMS-7). The resistivity was recorded in the temperature range of 10 200 K in zero magnetic field and in an applied field of 50 kOe, using the standard four-probe technique on rectangular parallelepiped samples (8.0 2.2 0.5 mm) pressed at a pressure of about 0.67 GPa.
3. Results and discussion Fig. 1 shows the XRD patterns of the precursor and the powders calcined at different temperatures. The main phase in the precursor is in an amorphous state mixed with orthorhombic NH4NO3. When the gel powders are calcined at 600 1C, NH4NO3 decomposes and LaMnO3 + d is formed with small amounts of La2O3 and Mn3O4 impurities. Upon increasing the calcination temperature up to 700 1C, single-phase rhombohedral LaMnO3 + d with ðR3CÞ space group (no. 167) is obtained and this synthesis temperature is much lower than reported in literature [12,15]. When the calcination temperature is continuously increased from 800 to 1000 1C, the XRD reflections of rhombohedral LaMnO3 + d clearly enhance, indicating crystallization and an increase of the grain size with increasing calcination temperature. The average grain size is estimated to be 25, 33, 39 and 49 nm for the samples LMO700, LMO800, LMO900 and LMO1000, respectively. The Mn4 + –Mn3 + ratio is related to the magnetic properties of the manganites and the binding energy of the Mn 2p3/2 peak is usually used to study the Mn valence state in manganites [12]. Peak fitting on the Mn 2p3/2 peak was carried out as shown in Fig. 2, using the XPSPEAK 4.1 software. It is well known that the binding energy of Mn4 + is close to 642.5 eV and that of Mn3 + is close to 641.2 eV. It is clear that the Mn oxidation state is somewhere between Mn3 + and Mn4 + in all the samples. The content of Mn4 + is estimated to be 40%, 38%, 29% and 26% for the
Fig. 1. XRD patterns of the LaMnO3 + d precursor and powders calcined at different temperatures.
LMO700, LMO800, LMO900 and LMO1000 samples, respectively. The samples calcined at relatively low temperatures have a higher Mn4 + content for the LMO700 and LMO800 samples, while the LMO900 and LMO1000 samples have a lower Mn4 + content. It means that the Mn4 + content tends to decrease with increasing calcination temperature, similar to previous results in Ref. [17]. The morphology and particle size of LMO700 powder were examined by SEM and TEM (Fig. 3). In Fig. 3(a), one can clearly see that the LMO700 sample consists of LaMnO3 + d sheets. The highmagnification SEM image (Fig. 3(b)) further reveals that the sheets are comprised of irregular LaMnO3 + d nanoparticles. Many interstices exist between the nanoparticles, which may be caused by the evaporation of organic components and adsorbed water in the precursor gel. The TEM image in Fig. 3(c) shows that the LaMnO3 + d particles are cylindrical with aspect ratio in the range of 1.77–2.67 or spherical with a mean particle size of about 40 nm. The corresponding selected area electron diffraction (SAED) data recorded on a segment of an individual particle (Fig. 3(c)) exhibit sharp diffraction spots, indicating preferred formation of singlephase and well-crystallized rhombohedral structure. In order to further confirm the composition of the compounds, EDS spectra (Fig. 3(d)) taken from the surface of LMO700 powders show the main elements La, Mn, O and that the molar La:Mn ratio is 1.005, very near the nominal concentration. We also display the typical morphologies of LMO800 and LMO900 powders in Figs. 4(a) and (b), respectively. The results indicate that with increasing calcination temperature, the powders aggregate a little accompanied by a slight increase of the particle size. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of LMO700, LMO800, LMO900 and LMO1000 recorded at an applied field of 100 Oe between 5 and 230 K are shown in Fig. 5. The Curie temperature TC of the powders was determined as the minimum of the derivative of the magnetization dM(FC)/dT [12,15]. The Curie temperatures of LMO700, LMO800, LMO900 and LMO1000 are about 140, 165, 180 and 185 K, respectively, a little lower than the Curie temperature of La1 xMnO3 + d nanoparticles [15]. This may be attributed to a relatively broad
ARTICLE IN PRESS 1364
X.L. Lang et al. / Physica B 405 (2010) 1362–1368
Fig. 2. Binding energy of Mn 2p3/2 for (a) LMO700, (b) LMO800, (c) LMO900 and (d) LMO1000 powder.
Fig. 3. (a) Low-magnification and (b) high-magnification SEM, (c) TEM and (d) EDS images of LMO700 powder. The inset in Fig. 3(c) shows the corresponding SAED data recorded from the segment of an individual particle.
ARTICLE IN PRESS X.L. Lang et al. / Physica B 405 (2010) 1362–1368
1365
Fig. 4. High-magnification SEM images of (a) LMO800 and (b) LMO900 powder.
Fig. 5. ZFC (open symbols) and FC (filled symbols) magnetization curves for (a) LMO700, (b) LMO800, (c) LMO900 and (d) LMO1000 powder.
size distribution. At about 105 K, there is a peak in the ZFC curve of LMO700 powder, which corresponds to the blocking temperature TB [12]. The FC curves of LMO700 and LMO800 nanoparticles show a broad paramagnetic to ferromagnetic transition which is due to the distribution of the particle size and shape as well as to intergranular interaction [12]. In contrast, the FC curves of LMO900 and LMO1000 exhibit an increase of magnetization as the temperature increases to about 114 K for LMO900 and 118 K for LMO1000. After showing a maximum, the magnetization gradually decreases, which can be attributed to the
coexistence of antiferromagnetic and ferromagnetic phases [16]. It has been reported that the structure of the orthorhombic parent LaMnO3 will change to rhombohedral or cubic upon increase of the number of Mn4 + ions [14]. Only the rhombohedral and cubic samples with a relatively high Mn4 + content exhibit ferromagnetism [14]. In the present work, the Mn4 + content in LaMnO3 + d powders decreases which weakens the Mn3 + –Mn4 + DE interaction and favors the Mn3 + –Mn3 + superexchange interaction. As a result, LMO700 and LMO800 are rhombohedral and ferromagnetic, while LMO900 and LMO1000 show
ARTICLE IN PRESS 1366
X.L. Lang et al. / Physica B 405 (2010) 1362–1368
Fig. 6. Hysteresis loops for the LaMnO3 + d powders recorded at 5 K.
coexistence of antiferromagnetic and ferromagnetic phases. Fig. 6 shows the hysteresis loops of annealed LaMnO3 + d powders recorded at 5 K after ZFC. The saturation magnetization values MS are 71.8, 76.2, 79.5 and 84.8 emu/g for LMO700, LMO800, LMO900 and LMO1000, respectively. With increasing calcination temperature, MS of the LaMnO3 + d powders enhances. A similar result has also been obtained by other groups and has been discussed in a model based on a core-shell structure, which consists of a core having the same property as the bulk material and a magnetically dead shell [10,15,18]. Fig. 7 presents resistivity versus temperature curves of the four LaMnO3 + d samples. The resistivity of the LMO1000 pellet is much larger than those of other samples, very different from the conventional cases that manganites with a large particle size show a high magnetization and a low resistivity because of the larger surface and the reduced number of conducting channels [17–19]. The resistivity measurements have been performed on samples obtained at high pressure without sintering, which may result in slightly different resistivity values for each sample. It is worth noticing that the manganites are always ferromagnetic although the particle size increases with increasing the calcination temperature [17–19]. In the present system, the insulating antiferromagnetic phase gradually appears as the calcination temperature increases up to 900 1C. It is reasonable that the insulating antiferromagnetic component in the samples
Fig. 7. Temperature dependence of resistivity of the LaMnO3 + d powders at zero field and at an applied field of 50 kOe between 10 and 200 K: (a) LMO700, (b) LMO800, (c) LMO900 and (d) LMO1000.
ARTICLE IN PRESS X.L. Lang et al. / Physica B 405 (2010) 1362–1368
will lead to an enhancement of zero-field resistivity and suppress the size effect. Both the zero-field and the applied-field resistivities of all samples increase as the temperature decreases to about 90 K, showing a maximum, and then a gradual decrease. The negative temperature coefficient of the resistivity (dr/dTo0) above 90 K indicates an insulating nature. On the other hand, the positive temperature coefficient (dr/dT40) below 90 K points at metallic behavior. The metal–insulator transition occurs just below the Curie temperature and therefore seems connected with the paramagnetic to ferromagnetic transition. However, all resistivities exhibit a rapid upturn below a temperature of about 40 K, indicating insulating characteristics at lower temperatures, similar to the previous reports on various polycrystalline manganites [6,20] and nanoparticles of manganites [12,19,21]. The upturn of the low-temperature resistivity may originate from a breakdown of the DE interactions in the disordered interfacial region due to broken Mn–O–Mn bonds at the surface of nanoparticles and breaking of the translational symmetry of the crystal lattice [10,19,20]. According to a theoretical model [22], the temperature dependence of the resistivity of a granular metal obeys the relation r(T)EA exp[(D/T)1/2], where D is proportional to the charging energy EC = e2/(4pe0)F(s/d). EC is the electrostatic energy required to create a positive–negative charged pair of grains, e0 is the vacuum permittivity and the function F(s/d) depends on the shape of a granule. The parameters s and d are the grain size and separation between grains, respectively. Balcells et al. [23] ascribed this behavior to the Coulomb blockade (CB) effect in the granular manganites. The ferromagnetic DE interaction in granular manganites can be further disturbed by the CB effect due to the charge accumulation on the surface of small grains. The ln(r) versus T 1/2 curves shown in the inset of Fig. 8 are linear in the temperature range of 10–40 K, which provides us with the value of D. For the LMO700 sample pellet, the estimated D values are 5.48 K at zero field and 7.75 K at 50 kOe. When a magnetic field is applied, the resistivity of the sample pellets decreases due to the suppression of spin-dependent scattering by the applied magnetic field [24]. The negative
1367
magnetoresistance MR= [r(H) r(0)]/r(0) 100% increases with decreasing temperature and has a maximum near 90 K. The maximal MR E 88% is obtained for the LMO700 pellet in an applied field of 50 kOe (Fig. 8). For polycrystalline samples [25,26], MR can be attributed to spin-dependent tunneling across the boundaries between the grains. It is worth noticing that MR has a high value of 82% to 88% in the temperature range of 40–100 K.
4. Conclusion We have studied the effect of the calcination temperature on the magnetic and transport properties of rhombohedral LaMnO3 + d compounds. The rhombohedral phase can be obtained at a relatively low temperature of 700 1C. All LaMnO3 + d compounds exhibit ferromagnetism with an increase of TC from 140 K for LMO700 to 185 K for LMO1000 and an enhancement of the saturation magnetization at 5 K from 71.8 emu/g for LMO700 to 84.8 emu/g for LMO1000 because of the increase of the average grain size of the LaMnO3 + d powders with increasing calcination temperature. The antiferromagnetic LaMnO3 + d component gradually appears upon reduction of the content of Mn4 + ions, which results in a relatively large zero-field resistivity. All samples show a metal–insulator transition near 90 K and an upturn at about 40 K, which is interpreted in terms of the CB effect and insulating tunneling barriers. The LMO700 pellet has a large negative magnetoresistance ranging from 82% to 88% in the temperature range between 40 and 100 K. This is due to suppression of the spin dependent scattering by the applied magnetic field.
Acknowledgments This work has been supported by the National Natural Science Foundation of China (nos. 50831006 and 50701045) and by the National Major Fundamental Research Program of China (no. 2010CB934603), Ministry of Science and Technology, China. References
Fig. 8. Temperature dependence of the magnetoresistance of an LMO700 pellet in an applied field of 50 kOe. The inset shows ln(r) versus T 1/2 curves.
[1] E. Rozenberg, A.I. Shame, M. Auslender, G. Jung, I. Felner, J. Sinha, S.S. Banerjee, D. Mogilyansky, E. Sominski, A. Gedanken, Y.M. Mukovskii, G. Gorodetsky, Phys. Rev. B 76 (2007) 214429. [2] J.S. Zhou, J.B. Goodenough, Phys. Rev. Lett. 96 (2006) 247202. [3] F. Millange, S. de Brion, G. Chouteau, Phys. Rev. B 62 (2000) 5619. [4] L. Malavasi, M.C. Mozzati, S. Polizzi, C.B. Azzoni, G. Flor, Chem. Mater. 15 (2003) 5036. [5] Y.C. Liou, Y.R. Chen, Ceram. Internat. 34 (2008) 273. [6] Y. Xu, J.C. Zhang, G.X. Cao, C. Jing, S.X. Cao, Phys. Rev. B 73 (2006) 224410. [7] D. Niebieskikwiat, M.B. Salamon, Phys. Rev. B 72 (2005) 174422. [8] E.C. Fonseca, J.A. Souza, R.F. Jardim, R. Muccillo, E.N.S. Muccillo, D. Gouvea, M.H. Jung, A.H. Lacerda, Phys. Stat. Sol. (a) 199 (2003) 255. ˜ a, M. Bahout, K. Ghanimi, P. Duran, D. Gutierrez, C. Moure, J. Mater. [9] O. Pen Chem. 12 (2002) 2480. [10] C. Zener, Phys. Rev. 81 (1951) 440. [11] G. Dezanneau, M. Audier, H. Vincent, C. Meneghini, E. Djurado, Phys. Rev. B 69 (2004) 014412. [12] V. Markovich, G. Jung, I. Fita, D. Mogilyansky, X. Wu, A. Wisniewski, R. Puzniak, N. Froumin, L. Titelman, L. Vradman, M. Herskowitz, G. Gorodetsky, J. Phys. D Appl. Phys. 41 (2008) 185001. [13] M. Imada, A. Fujimori, Y. Tokura, Rev. Modern Phys. 70 (1998) 1039. [14] R. Mahendiran, S.K. Tiway, A.K. Raychaudhuri, T.V. Ramakrishnan, Phys. Rev. B 53 (1995) 3348. [15] V. Markovich, I. Fita, D. Mogilyansky, A. Wisniewski, R. Puzniak, L. Titelman, M. Herskowitz, G. Gorodetsky, J. Phys. Condens. Matter 19 (2007) 346210. [16] T.I. Arbuzova, B.A. Gizhevskii, R.G. Zakharov, S.A. Petrova, N.M. Chebotaev, Phys. Solid State 50 (2008) 1487. [17] M.A. Lo´pez-Quintela, L.E. Hueso, J. Rivas, F. Rivadulla, Nanotechnology 14 (2003) 212. [18] N. Zhang, W.P. Ding, W. Zhong, D.Y. Xing, Y.W. Du, Phys. Rev. B 56 (1997) 8138. [19] B. Roy, A. Poddar, S. Das, J. Appl. Phys. 100 (2006) 104318.
ARTICLE IN PRESS 1368
X.L. Lang et al. / Physica B 405 (2010) 1362–1368
[20] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [21] D. Niebieskikwiat, R.D. Sa´nchez, D.G. Lamas, A. Caneiro, L.E. Hueso, J. Rivas, J. Appl. Phys. 93 (2003) 6305. [22] P. Sheng, B. Abeles, Y. Arie, Phys. Rev. Lett. 31 (1973) 44. [23] L. Balcells, J. Fontcuberta, B. Martı´nez, X. Obradors, Phys. Rev. B 58 (1998) R14697.
[24] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [25] E. Rozenberg, M. Auslender, I. Felner, G. Gorodetsky, J. Appl. Phys. 88 (2000) 2578. [26] M. Auslender, A.E. Karkin, E. Rozenberg, G. Gorodetsky, J. Appl. Phys. 89 (2001) 6639.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effect of the calcination temperature on the magnetic and transport properties of rhombohedral LaMnO3 + d compounds X.L. Wang, D. Li , C.X. Shi, B. Li, T.Y. Cui, Z.D. Zhang Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 October 2009 Received in revised form 30 November 2009 Accepted 1 December 2009
Rhombohedral LaMnO3 + d powders were successfully synthesized by annealing gel precursors obtained by a polymerized complex method. The average grain size increases with increasing calcination temperature. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves indicate that LaMnO3 + d compounds obtained at 700 and 800 1C are ferromagnetic, while both ferromagnetic and antiferromagnetic components are present in those calcined at 900 and 1000 1C. This is because the concentration of Mn4 + ions drops with increasing calcination temperature. The temperature dependence of the resistivity of LaMnO3 + d compounds exhibits an insulator–metal transition at about 90 K and also a low-temperature upturn with a resistivity minimum at about 40 K. Upon increasing particle size, the zero-field resistivity of the LaMnO3 + d powders increases due to suppression of the conduction by the insulating antiferromagnetic component. The negative magnetoresistance of a sample pellet prepared at 700 1C ranges from 82% to 88% in the temperature range of 40–100 K due to suppression of the spin-dependent scattering by an applied field. & 2009 Elsevier B.V. All rights reserved.
Keywords: Polymerized synthesis Magnetic properties Magnetoresistance
1. Introduction Doped perovskite manganites with the general formula R1 xAxMnO3 (R is rare-earth metal, A is univalent element Na or divalent element such as Ca, Sr, Ba) have been extensively investigated in the past decades, because of their abundant physical properties such as colossal magnetoresistance (CMR) effect, Jahn–Teller effect, metal–insulator transition and their potential applications in magneto-electronic devices, magnetodata storage [1–9]. These properties are closely related to the interactions among charge, orbital, spin, lattice and magnetic degrees of freedom. Through cationic replacement in the antiferromagnetic orthorhombic parent compound LaMnO3, the La1 xAxMnO3 manganite transforms into a doped mixedvalence Mn3 + –Mn4 + phase that is ferromagnetic and displays a metal–insulator transition in the vicinity of the Curie temperature TC. The spin dynamics and electronic transport are conventionally interpreted in terms of the Zener double-exchange (DE) mechanism [10]. The Mn3 + –Mn4 + mixed-valence state can also be obtained by altering the chemical stoichiometry, as for self- or vacancy-doped powders with the general formula La1 xMn1 yO3 + d. In these compounds, variations of x, y and d can modify the Mn3 + –Mn4 + ratio, which is beneficial for
Corresponding author.
E-mail address: [email protected] (D. Li). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.12.001
activating the DE interaction. This enhances the ferromagnetic coupling and gives rise to the CMR effect as in the case of the substituted compounds [11,12]. Usually, the crystal structures of these compounds, which possess the Mn3 + –Mn4 + mixed-valence state through either element substitution or in a self-doped form, are rhombohedral or cubic [11–18]. In the present work, we successfully synthesized rhombohedral LaMnO3 + d compounds at a relative low temperature by annealing gel precursors obtained by a polymerized complex method. The effect of the calcination temperature on the magnetic and transport properties of LaMnO3 + d compounds was investigated.
2. Experimental procedure LaMnO3 + d compounds were synthesized by the polymerized complex method described elsewhere [19]. All reagents are commercially available in analytical purity and used without further purification. In a typical procedure, a solution was prepared by mixing aqueous solutions of Mn(NO3)2 and La(NO3)3 in 1:1 molar ratio and then citric acid was added under stirring. After the solution was adjusted to pH= 2 by adding dilute ammonial solution under stirring, the Mn2 + and La3 + cations complexed with the anions of citric acid into their citrates. Then, ethylene glycol was added for gelation. The molar amounts of citric acid and ethylene glycol were twice those of the La and Mn cations. More citric acid and ethylene glycol were added than in
ARTICLE IN PRESS X.L. Lang et al. / Physica B 405 (2010) 1362–1368
1363
previous work [12,15], because it is beneficial for the formation of chelate precursors. As a result, LaMnO3 + d powders can be obtained at lower temperature than reported previously [12,15]. The solution was continuously stirred at room temperature for 5 h and then heated at 100 1C under stirring till a gel was formed. This gel with high viscosity was further dried overnight in an oven at 130 1C and ground into fine black powder. Finally, the black precursor powders were calcined in air atmosphere at 600, 700, 800, 900 and 1000 1C for 4 h. The final products corresponding to these calcination temperatures are denoted as LMO600, LMO700, LMO800, LMO900 and LMO1000, respectively. Phase analysis of all products was performed by using powder X-ray diffraction (XRD) on a D/max-2000 diffractometer at a voltage of 56 kV and a current of 182 mA with Cu-Ka (l = 0.154056 nm) radiation. The average crystal–grain size of the powders was estimated from the (0 2 4) reflections, by means of the Scherrer equation, d ¼ kl=b cos y where d is the average diameter of the crystal grains, k is a particle shape factor and taken as 0.9 for spherical nanoparticles, l is the wavelength of Cu-Ka radiation, y is the corresponding Bragg angle and b is the angular half-width of the reflection at 2y. The surface compositions of the powders were determined with an ESCALAB250 X-ray photoelectron spectroscopy (XPS) spectrometer using an Al X-ray source emitting at 1486.6 eV and a monochromator. The morphology was investigated with a supra 35 field-emission scanning electron microscope (FESEM) equipped with energydispersive spectrum (EDS) operated at an acceleration voltage of 20 kV and a high-resolution transmission electron microscopy (HRTEM) using a JEOL 2010 with an emission voltage of 200 kV. The magnetic and electrical properties of the products were measured in a superconducting quantum interference device (SQUID, Quantum Design MPMS-7). The resistivity was recorded in the temperature range of 10 200 K in zero magnetic field and in an applied field of 50 kOe, using the standard four-probe technique on rectangular parallelepiped samples (8.0 2.2 0.5 mm) pressed at a pressure of about 0.67 GPa.
3. Results and discussion Fig. 1 shows the XRD patterns of the precursor and the powders calcined at different temperatures. The main phase in the precursor is in an amorphous state mixed with orthorhombic NH4NO3. When the gel powders are calcined at 600 1C, NH4NO3 decomposes and LaMnO3 + d is formed with small amounts of La2O3 and Mn3O4 impurities. Upon increasing the calcination temperature up to 700 1C, single-phase rhombohedral LaMnO3 + d with ðR3CÞ space group (no. 167) is obtained and this synthesis temperature is much lower than reported in literature [12,15]. When the calcination temperature is continuously increased from 800 to 1000 1C, the XRD reflections of rhombohedral LaMnO3 + d clearly enhance, indicating crystallization and an increase of the grain size with increasing calcination temperature. The average grain size is estimated to be 25, 33, 39 and 49 nm for the samples LMO700, LMO800, LMO900 and LMO1000, respectively. The Mn4 + –Mn3 + ratio is related to the magnetic properties of the manganites and the binding energy of the Mn 2p3/2 peak is usually used to study the Mn valence state in manganites [12]. Peak fitting on the Mn 2p3/2 peak was carried out as shown in Fig. 2, using the XPSPEAK 4.1 software. It is well known that the binding energy of Mn4 + is close to 642.5 eV and that of Mn3 + is close to 641.2 eV. It is clear that the Mn oxidation state is somewhere between Mn3 + and Mn4 + in all the samples. The content of Mn4 + is estimated to be 40%, 38%, 29% and 26% for the
Fig. 1. XRD patterns of the LaMnO3 + d precursor and powders calcined at different temperatures.
LMO700, LMO800, LMO900 and LMO1000 samples, respectively. The samples calcined at relatively low temperatures have a higher Mn4 + content for the LMO700 and LMO800 samples, while the LMO900 and LMO1000 samples have a lower Mn4 + content. It means that the Mn4 + content tends to decrease with increasing calcination temperature, similar to previous results in Ref. [17]. The morphology and particle size of LMO700 powder were examined by SEM and TEM (Fig. 3). In Fig. 3(a), one can clearly see that the LMO700 sample consists of LaMnO3 + d sheets. The highmagnification SEM image (Fig. 3(b)) further reveals that the sheets are comprised of irregular LaMnO3 + d nanoparticles. Many interstices exist between the nanoparticles, which may be caused by the evaporation of organic components and adsorbed water in the precursor gel. The TEM image in Fig. 3(c) shows that the LaMnO3 + d particles are cylindrical with aspect ratio in the range of 1.77–2.67 or spherical with a mean particle size of about 40 nm. The corresponding selected area electron diffraction (SAED) data recorded on a segment of an individual particle (Fig. 3(c)) exhibit sharp diffraction spots, indicating preferred formation of singlephase and well-crystallized rhombohedral structure. In order to further confirm the composition of the compounds, EDS spectra (Fig. 3(d)) taken from the surface of LMO700 powders show the main elements La, Mn, O and that the molar La:Mn ratio is 1.005, very near the nominal concentration. We also display the typical morphologies of LMO800 and LMO900 powders in Figs. 4(a) and (b), respectively. The results indicate that with increasing calcination temperature, the powders aggregate a little accompanied by a slight increase of the particle size. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of LMO700, LMO800, LMO900 and LMO1000 recorded at an applied field of 100 Oe between 5 and 230 K are shown in Fig. 5. The Curie temperature TC of the powders was determined as the minimum of the derivative of the magnetization dM(FC)/dT [12,15]. The Curie temperatures of LMO700, LMO800, LMO900 and LMO1000 are about 140, 165, 180 and 185 K, respectively, a little lower than the Curie temperature of La1 xMnO3 + d nanoparticles [15]. This may be attributed to a relatively broad
ARTICLE IN PRESS 1364
X.L. Lang et al. / Physica B 405 (2010) 1362–1368
Fig. 2. Binding energy of Mn 2p3/2 for (a) LMO700, (b) LMO800, (c) LMO900 and (d) LMO1000 powder.
Fig. 3. (a) Low-magnification and (b) high-magnification SEM, (c) TEM and (d) EDS images of LMO700 powder. The inset in Fig. 3(c) shows the corresponding SAED data recorded from the segment of an individual particle.
ARTICLE IN PRESS X.L. Lang et al. / Physica B 405 (2010) 1362–1368
1365
Fig. 4. High-magnification SEM images of (a) LMO800 and (b) LMO900 powder.
Fig. 5. ZFC (open symbols) and FC (filled symbols) magnetization curves for (a) LMO700, (b) LMO800, (c) LMO900 and (d) LMO1000 powder.
size distribution. At about 105 K, there is a peak in the ZFC curve of LMO700 powder, which corresponds to the blocking temperature TB [12]. The FC curves of LMO700 and LMO800 nanoparticles show a broad paramagnetic to ferromagnetic transition which is due to the distribution of the particle size and shape as well as to intergranular interaction [12]. In contrast, the FC curves of LMO900 and LMO1000 exhibit an increase of magnetization as the temperature increases to about 114 K for LMO900 and 118 K for LMO1000. After showing a maximum, the magnetization gradually decreases, which can be attributed to the
coexistence of antiferromagnetic and ferromagnetic phases [16]. It has been reported that the structure of the orthorhombic parent LaMnO3 will change to rhombohedral or cubic upon increase of the number of Mn4 + ions [14]. Only the rhombohedral and cubic samples with a relatively high Mn4 + content exhibit ferromagnetism [14]. In the present work, the Mn4 + content in LaMnO3 + d powders decreases which weakens the Mn3 + –Mn4 + DE interaction and favors the Mn3 + –Mn3 + superexchange interaction. As a result, LMO700 and LMO800 are rhombohedral and ferromagnetic, while LMO900 and LMO1000 show
ARTICLE IN PRESS 1366
X.L. Lang et al. / Physica B 405 (2010) 1362–1368
Fig. 6. Hysteresis loops for the LaMnO3 + d powders recorded at 5 K.
coexistence of antiferromagnetic and ferromagnetic phases. Fig. 6 shows the hysteresis loops of annealed LaMnO3 + d powders recorded at 5 K after ZFC. The saturation magnetization values MS are 71.8, 76.2, 79.5 and 84.8 emu/g for LMO700, LMO800, LMO900 and LMO1000, respectively. With increasing calcination temperature, MS of the LaMnO3 + d powders enhances. A similar result has also been obtained by other groups and has been discussed in a model based on a core-shell structure, which consists of a core having the same property as the bulk material and a magnetically dead shell [10,15,18]. Fig. 7 presents resistivity versus temperature curves of the four LaMnO3 + d samples. The resistivity of the LMO1000 pellet is much larger than those of other samples, very different from the conventional cases that manganites with a large particle size show a high magnetization and a low resistivity because of the larger surface and the reduced number of conducting channels [17–19]. The resistivity measurements have been performed on samples obtained at high pressure without sintering, which may result in slightly different resistivity values for each sample. It is worth noticing that the manganites are always ferromagnetic although the particle size increases with increasing the calcination temperature [17–19]. In the present system, the insulating antiferromagnetic phase gradually appears as the calcination temperature increases up to 900 1C. It is reasonable that the insulating antiferromagnetic component in the samples
Fig. 7. Temperature dependence of resistivity of the LaMnO3 + d powders at zero field and at an applied field of 50 kOe between 10 and 200 K: (a) LMO700, (b) LMO800, (c) LMO900 and (d) LMO1000.
ARTICLE IN PRESS X.L. Lang et al. / Physica B 405 (2010) 1362–1368
will lead to an enhancement of zero-field resistivity and suppress the size effect. Both the zero-field and the applied-field resistivities of all samples increase as the temperature decreases to about 90 K, showing a maximum, and then a gradual decrease. The negative temperature coefficient of the resistivity (dr/dTo0) above 90 K indicates an insulating nature. On the other hand, the positive temperature coefficient (dr/dT40) below 90 K points at metallic behavior. The metal–insulator transition occurs just below the Curie temperature and therefore seems connected with the paramagnetic to ferromagnetic transition. However, all resistivities exhibit a rapid upturn below a temperature of about 40 K, indicating insulating characteristics at lower temperatures, similar to the previous reports on various polycrystalline manganites [6,20] and nanoparticles of manganites [12,19,21]. The upturn of the low-temperature resistivity may originate from a breakdown of the DE interactions in the disordered interfacial region due to broken Mn–O–Mn bonds at the surface of nanoparticles and breaking of the translational symmetry of the crystal lattice [10,19,20]. According to a theoretical model [22], the temperature dependence of the resistivity of a granular metal obeys the relation r(T)EA exp[(D/T)1/2], where D is proportional to the charging energy EC = e2/(4pe0)F(s/d). EC is the electrostatic energy required to create a positive–negative charged pair of grains, e0 is the vacuum permittivity and the function F(s/d) depends on the shape of a granule. The parameters s and d are the grain size and separation between grains, respectively. Balcells et al. [23] ascribed this behavior to the Coulomb blockade (CB) effect in the granular manganites. The ferromagnetic DE interaction in granular manganites can be further disturbed by the CB effect due to the charge accumulation on the surface of small grains. The ln(r) versus T 1/2 curves shown in the inset of Fig. 8 are linear in the temperature range of 10–40 K, which provides us with the value of D. For the LMO700 sample pellet, the estimated D values are 5.48 K at zero field and 7.75 K at 50 kOe. When a magnetic field is applied, the resistivity of the sample pellets decreases due to the suppression of spin-dependent scattering by the applied magnetic field [24]. The negative
1367
magnetoresistance MR= [r(H) r(0)]/r(0) 100% increases with decreasing temperature and has a maximum near 90 K. The maximal MR E 88% is obtained for the LMO700 pellet in an applied field of 50 kOe (Fig. 8). For polycrystalline samples [25,26], MR can be attributed to spin-dependent tunneling across the boundaries between the grains. It is worth noticing that MR has a high value of 82% to 88% in the temperature range of 40–100 K.
4. Conclusion We have studied the effect of the calcination temperature on the magnetic and transport properties of rhombohedral LaMnO3 + d compounds. The rhombohedral phase can be obtained at a relatively low temperature of 700 1C. All LaMnO3 + d compounds exhibit ferromagnetism with an increase of TC from 140 K for LMO700 to 185 K for LMO1000 and an enhancement of the saturation magnetization at 5 K from 71.8 emu/g for LMO700 to 84.8 emu/g for LMO1000 because of the increase of the average grain size of the LaMnO3 + d powders with increasing calcination temperature. The antiferromagnetic LaMnO3 + d component gradually appears upon reduction of the content of Mn4 + ions, which results in a relatively large zero-field resistivity. All samples show a metal–insulator transition near 90 K and an upturn at about 40 K, which is interpreted in terms of the CB effect and insulating tunneling barriers. The LMO700 pellet has a large negative magnetoresistance ranging from 82% to 88% in the temperature range between 40 and 100 K. This is due to suppression of the spin dependent scattering by the applied magnetic field.
Acknowledgments This work has been supported by the National Natural Science Foundation of China (nos. 50831006 and 50701045) and by the National Major Fundamental Research Program of China (no. 2010CB934603), Ministry of Science and Technology, China. References
Fig. 8. Temperature dependence of the magnetoresistance of an LMO700 pellet in an applied field of 50 kOe. The inset shows ln(r) versus T 1/2 curves.
[1] E. Rozenberg, A.I. Shame, M. Auslender, G. Jung, I. Felner, J. Sinha, S.S. Banerjee, D. Mogilyansky, E. Sominski, A. Gedanken, Y.M. Mukovskii, G. Gorodetsky, Phys. Rev. B 76 (2007) 214429. [2] J.S. Zhou, J.B. Goodenough, Phys. Rev. Lett. 96 (2006) 247202. [3] F. Millange, S. de Brion, G. Chouteau, Phys. Rev. B 62 (2000) 5619. [4] L. Malavasi, M.C. Mozzati, S. Polizzi, C.B. Azzoni, G. Flor, Chem. Mater. 15 (2003) 5036. [5] Y.C. Liou, Y.R. Chen, Ceram. Internat. 34 (2008) 273. [6] Y. Xu, J.C. Zhang, G.X. Cao, C. Jing, S.X. Cao, Phys. Rev. B 73 (2006) 224410. [7] D. Niebieskikwiat, M.B. Salamon, Phys. Rev. B 72 (2005) 174422. [8] E.C. Fonseca, J.A. Souza, R.F. Jardim, R. Muccillo, E.N.S. Muccillo, D. Gouvea, M.H. Jung, A.H. Lacerda, Phys. Stat. Sol. (a) 199 (2003) 255. ˜ a, M. Bahout, K. Ghanimi, P. Duran, D. Gutierrez, C. Moure, J. Mater. [9] O. Pen Chem. 12 (2002) 2480. [10] C. Zener, Phys. Rev. 81 (1951) 440. [11] G. Dezanneau, M. Audier, H. Vincent, C. Meneghini, E. Djurado, Phys. Rev. B 69 (2004) 014412. [12] V. Markovich, G. Jung, I. Fita, D. Mogilyansky, X. Wu, A. Wisniewski, R. Puzniak, N. Froumin, L. Titelman, L. Vradman, M. Herskowitz, G. Gorodetsky, J. Phys. D Appl. Phys. 41 (2008) 185001. [13] M. Imada, A. Fujimori, Y. Tokura, Rev. Modern Phys. 70 (1998) 1039. [14] R. Mahendiran, S.K. Tiway, A.K. Raychaudhuri, T.V. Ramakrishnan, Phys. Rev. B 53 (1995) 3348. [15] V. Markovich, I. Fita, D. Mogilyansky, A. Wisniewski, R. Puzniak, L. Titelman, M. Herskowitz, G. Gorodetsky, J. Phys. Condens. Matter 19 (2007) 346210. [16] T.I. Arbuzova, B.A. Gizhevskii, R.G. Zakharov, S.A. Petrova, N.M. Chebotaev, Phys. Solid State 50 (2008) 1487. [17] M.A. Lo´pez-Quintela, L.E. Hueso, J. Rivas, F. Rivadulla, Nanotechnology 14 (2003) 212. [18] N. Zhang, W.P. Ding, W. Zhong, D.Y. Xing, Y.W. Du, Phys. Rev. B 56 (1997) 8138. [19] B. Roy, A. Poddar, S. Das, J. Appl. Phys. 100 (2006) 104318.
ARTICLE IN PRESS 1368
X.L. Lang et al. / Physica B 405 (2010) 1362–1368
[20] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [21] D. Niebieskikwiat, R.D. Sa´nchez, D.G. Lamas, A. Caneiro, L.E. Hueso, J. Rivas, J. Appl. Phys. 93 (2003) 6305. [22] P. Sheng, B. Abeles, Y. Arie, Phys. Rev. Lett. 31 (1973) 44. [23] L. Balcells, J. Fontcuberta, B. Martı´nez, X. Obradors, Phys. Rev. B 58 (1998) R14697.
[24] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [25] E. Rozenberg, M. Auslender, I. Felner, G. Gorodetsky, J. Appl. Phys. 88 (2000) 2578. [26] M. Auslender, A.E. Karkin, E. Rozenberg, G. Gorodetsky, J. Appl. Phys. 89 (2001) 6639.