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The structural and magnetic properties of one-step mechanochemical route synthesized La0.8Pb0.2MnO3 manganites
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 2533–2536
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
The structural and magnetic properties of one-step mechanochemical route synthesized La0.8Pb0.2MnO3 manganites H. Bahrami, P. Kameli , H. Salamati Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e in fo
abstract
Article history: Received 15 October 2008 Received in revised form 19 February 2009 Available online 24 March 2009
Mechanochemical route has been used to produce La0.8Pb0.2MnO3 (LPMO) nanocrystalline samples from oxide precursors. The samples were characterized using X-ray diffraction, scanning electron microscope and AC susceptibility measurements. The results showed that it is possible to produce LPMO perovskite powders after 10 h of ball milling. The crystallite size and microstrain were estimated using Williamson–Hall equation. The results showed that the crystallite size and microstrain increase initially and then decrease by the increase of milling time. By decreasing particle size the dislocation density (strain) increases and reaches to a saturation point at a particular particle size, further particle size reduction takes place through gliding motion along grain boundaries, which leads to a reduction of the strain. The dynamic properties of 15 h ball-milled sample were investigated by AC susceptibility using the Neel–Brown and Vogel–Fulcher law for superparamagnetism. The frequency dependence of blocking temperature is well described by the Vogel–Fulcher law, and fitting the experimental data with Neel–Brown law gives unphysical value for relaxation time. & 2009 Elsevier B.V. All rights reserved.
Keywords: Perovskites Mechanochemical X-ray techniques Superparamagnetism
1. Introduction Perovskite manganites with the general composition La1xAxMnO3 (A ¼ Sr, Ca and Pb) have been intensively studied in the last decade since the discovery of the colossal magnetoresistance (CMR) phenomenon [1–4]. Potential applications of this type of materials as magnetic sensors, permanent magnets, catalysts, pigment and electrode materials for solid oxide fuel cell maintain intense research for these systems. Several synthetic methods of producing these materials such as: solid state reaction [5], sol–gel [6], coprecipitation methods [7] and high-energy ball milling (mechanochemical route) [8] have been developed. The mechanochemical route has been recognized as a powerful method for the production of novel, high performance and lowcost materials such as ferrites, intermetallics and etc. [9,10]. The mechanochemical synthesis can deliver the designed phases and structures by a single step of the high-energy milling conducted in an enclosed activation chamber at room temperature. In recent years, some authors have used high-energy ball milling to the mechanosynthesis of stoichiometric manganites [8,11]. In these cases, complete formation of perovskite manganites was obtained only after milling followed by sintering, i.e. by employing two processing steps. In this paper, we will report on the single-step synthesis of nanocrystalline La0.8Pb0.2MnO3 (LPMO) powders via
Corresponding author. Tel.: +98 3113912375; fax: +98 3113912376.
E-mail address: [email protected] (P. Kameli). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.03.035
high-energy milling of oxide precursors and investigation of their structural and magnetic properties.
2. Experimental PbO, La2O3 and MnO2 powders with purity 99.99% from Merck were used as the starting materials. These materials were mixed in the stoichiometric ratio and were milled in a PM100 planetary ball mill (Retsch, Germany) in an air atmosphere using hardened steel balls of 20 mm in diameter and stainless steel containers of 500 ml. The milling speed was 300 rpm and the ball-to-powder mass ratio was 15:1. The milling was interrupted at milling time and a small amount of powder was taken out of the vial. In order to prevent the excessive overheating of the containers, the experiments were carried out by altering 120 min of milling with 30 min of rest. The AC susceptibility measurements were performed using a Lake Shore AC Susceptometer, Model 7000. X-ray diffraction (XRD) patterns were taken on Philips XPERT MPD (Cu Ka radiation: l ¼ 0.154 nm) diffractometer. The microstructural observations of samples were analyzed by Philips XL 30 scanning electron microscope (SEM).
3. Results and discussion Fig. 1 shows the XRD patterns at room temperature for LPMO powder compound with different milling times. As expected the
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Fig. 1. X-ray powder diffraction patterns of unmilled and ball-milled powders. (*) La2O3, (]) PbO and (+) MnO2.
10
15
20
25
30
35
40
milling time(h)
B cos y ¼
Kl þ 2ðÞ sin y D
(1)
where B is the full-width at half-maximum (FWHM) of the XRD peaks, K is the Scherer constant, D is the particle size, l is the wavelength of the X-ray, e is the lattice strain and y is the Bragg angle. In this method, B cos y is plotted against sin y. Using a linear extrapolation to this plot, the intercept gives a quantity proportional to the reciprocal of the particle size (Kl/D) and the slope gives two times the strain (e). Fig. 2 (a) shows the effects of milling time on particle size. As one can see, the particle size increases from 10 to 15 h milled sample and then decreases by increasing the milling time. It seems that the 15 h milling time is the optimum time for phase and structural formation of LPMO perovskite manganites in this experiment. Fig. 2 (b) shows the variation of microstrain as a function of milling time. The microstrain increases initially and reaches the maximum value after a 20 h milling time. Further milling leads to a decrease in microstrain. Such a behavior has also been observed for ball-milled ferrite samples [13]. It is clear that
0.018
0.016
Strain
powder pattern of unmilled mixture contains only the individual reflections of PbO, La2O3 and MnO phases. The intensity ratio of individual reflections is in accordance with the stoichiometric composition of the mixture. As one can see after 2.5 h of milling, intensities of all reflections reduce substantially followed by peak broadening and peak overlapping. Also appearance of new peak at 2y ¼ 32.71, after 2.5 h milling, clearly establishes the formation of nanocrystalline LPMO phase. In the course of milling, growth of LPMO phase increases continuously in the expense of starting phases. It seems that the XRD pattern of 10 h ball-milled powder as that of a LPMO rhombohedral phase without any noticeable trace of starting phases. From the XRD spectrum, it appears that a stoichiometric LPMO manganite in nanocrystalline form may be obtainable by ball milling the stoichiometric mixture of PbO, La2O3 and MnO for about 10 h. By continuing the milling process the diffraction pattern of LPMO powder gradually broadens out; however, the crystalline structure is maintained even for 40 h milled sample. In almost all cases, line broadening occurs due to simultaneous size and strain effects. Since the high-energy ball milling introduces considerable strain in the compound, the contribution of strain to the diffraction line broadening is not negligible. One way to separate these two effects has been developed by Williamson and Hall and is now known as the Williamson–Hall plot. The Williamson–Hall equation is expressed as follows [12]:
0.014
0.012
0.010
10
15
20
25
30
35
40
milling time(h) Fig. 2. Variation of crystallite size (a) and strain (b) versus milling time for ballmilled powders.
the ball milling process induces some strain in the specimens, and increasing the milling time, increases the amount of internal microstrain. Also, increasing the milling time leads to a rapid decrease of the crystallite size. On the other hand, the decrease of the particle size is associated with an increase of the dislocation densities. Since the microstrain generally follows the features of dislocation density, it increases as the particle size decreases. In addition to dislocation, particle size reduction also takes place at a lower stress by a gliding motion along the grain boundaries [14]. When the particles are big, the grain boundary contribution to the microstrain is very small. However, as the particle size decreases the grain boundary increases relatively and act as a barrier to the dislocation motion. As a result, the dislocation density reaches to a saturation point at a particular grain size. Further, particle size reduction takes place through gliding motion along the grain boundaries. The overall effect is then a reduction of the microstrain as shown in Fig. 2(b). Fig. 3 shows typical SEM images for unmilled and 15 h milled samples. The sample that has not been subjected to milling consists mainly of large grains (1–2 mm). The 15 h milled sample shows a narrow particle size distribution, with average particle
ARTICLE IN PRESS H. Bahrami et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2533–2536
size of less than 500 nm, but we can see some particle agglomeration as a result of size reduction. Fig. 4 shows the real (x0 ) part of AC susceptibility for LPMO powder compound with different milling times, which were measured in an AC field of 10 Oe and frequency of 333 Hz. As expected, the susceptibility x0 of unmilled mixture is almost zero without any signature of ferromagnetic transition. By incarcerating milling time, the susceptibility increases and samples show paramagnetic–ferromagnetic transition. It is clear that the increasing of susceptibility and occurrence of
2535
paramagnetic–ferromagnetic transition in ball-milled powders are due to the perovskite phase formation. Also, as one can see the magnitude of susceptibility decreases by further increasing the milling time. An increase of the width of transition and a reduction of the transition temperature are observed for longtime milled powders (410 h). These behaviors can be explored by the model proposed by Zhang [15]. According to this model, in granular perovskite system a grain of the perovskite can be divided into a body and surface phase. The body phase would have the same properties as the bulk compound (structural, magnetic and transport properties), but the surface phase would have low transition temperature and magnetization. This is because of the magnetically disordered state in the surface. In the case of nanometer-size particles, defects are expected to occur to a higher extent and enhanced grain surface due to the oxygen vacancies and microstrain. Since the surface layer increases as the particle size decreases, therefore, the magnitude of susceptibility and transition temperature decrease by increasing the milling time. Surface contribution is larger for smaller particles and therefore the magnetization is diminished in a proportional way. To further study these nanocrystalline powders, we have performed AC susceptibility measurements as functions of frequency and AC field amplitude on 15 h milled sample. Fig. 5 shows the variation of AC susceptibility as function of
-4
3.5x10
-4
3.0x10
-4
X'(emu/gOe)
2.5x10
-4
2.0x10
-4
1.5x10
-4
1.0x10
f=111 Hz f=333 Hz f=666 Hz f=1000 Hz
-5
5.0x10
Fig. 3. SEM micrographs of unmilled (a) and 15 h milled (b) powders.
14
0.0 80
100
120
140
160
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200
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T(K)
3.0x10-4
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X'(emu/gOe)
2.5x10-4
20h 25h
-4
2.0x10
30h -4
1.5x10
5h 40h
1.0x10-4 5.0x10-5
0h
0.0
50
100
150
200
250
300
T(K) Fig. 4. Temperature dependence of the real part of AC susceptibility for ball-milled powders.
Fig. 5. Temperature dependence of the (a) real and (b) imaginary part of AC susceptibility for 15 h milled powder at different frequencies.
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In Fig. 6 we tried to fit the experimental data of Fig. 5, using Eq. (3). For T0 ¼ 130 K the value of t0 ¼ 5 109 s and Ea/k ¼ 326 K have been obtained. A good agreement of experimental data and Vogel–Fulcher law is the evidence that the phenomenon taking place at TB is related to blocking of an ensemble of interacting nanoparticles.
4. Conclusions
Fig. 6. The best fit of ln(f) versus 1/(TBT0) for 15 h milled powder.
temperature in an AC field of 10 Oe and frequencies of 111, 333, 666 and 1000 Hz. The frequency-dependent features highlighting the peaks in both real, x0 and imaginary, x00 parts. It is clearly evident from Fig. 5 that the data for both x0 and x00 exhibit the expected behavior of blocking/freezing process, i.e. the occurrence of a maximum at a temperature TB for both x0 and x00 components that shifts towards higher temperature. For isolated nanoparticles or weak interaction between them, the frequency dependence of blocking temperature, TB has been given by Neel–Brown model [16], E (2) t ¼ t0 exp a kB T where t0 for superparamagnetic systems is in the range of 109–1013 s [17,18] and t is related to measuring frequency as t ¼ 1/f. In the absence of an external magnetic field, the energy barrier, Ea, can be assumed to be proportional to particle volume V, Ea ¼ keffV, where keff is an effective magnetic anisotropy constant. The x00 peak temperature was taken as blocking temperature, TB. By fitting the experimental data from Fig. 5 with Eq. (2), we have found an unphysical small t01 1038 s in comparison of 1013 s for superparamagnetic systems (the fit is not shown here). This result simply indicates that there exists strong interaction between nanocrystalline powders. Magnetic interactions modify the energy barrier coming from the anisotropy contributions of each particle. It is worth noticing that in contrast with the static energy barrier distributions, the reversal of one particle moment may change the energy barriers of the assembly, even in the weak interaction limit. The first attempt to introduce interactions in the Neel–Brown model was made by Shtrikman and Wohlfarth [19,20] by introducing T0 as an effective temperature which accounts for the interaction effects. Therefore, for interacting magnetic nanoparticles the frequency dependence of TB is given by the Vogel–Fulcher law [16], Ea (3) t ¼ t0 exp kB ðT B T 0 Þ
High-energy ball milling of PbO, La2O3 and MnO2 powders leads to the synthesis of nanocrystalline La0.8Pb0.2MnO3 (LPMO) perovskite phase. It is found that the particle size and microstrain increase initially and then decrease by increasing the milling time. By decreasing particle size the dislocation density (strain) increases and reaches saturation point at a particular particle size. Further, particle size reduction takes place through gliding motion along grain boundaries, which leads to a reduction of the microstrain. The dynamic properties of 15 h ball-milled sample were investigated by AC susceptibility using the Neel–Brown and Vogel–Fulcher law for superparamagnetism. The frequency dependence of blocking temperature is well described by the Vogel–Fulcher law, and fitting the experimental data with Neel–Brown law gives unphysical value for relaxation time.
Acknowledgement The authors would like to thank Isfahan University of Technology for supporting this project. References [1] S. Jin, T.H. Tiefel, M. MacCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [2] A. Asamitsu, Y. Moritomo, Y. Tomika, T. Arima, Y. Tokura, Nature 373 (3) (1995) 407. [3] Y. Tokura, Colossal Magnetoresistive Oxides, Gordon and Breach Science, Singapore, 2000. [4] H.Y. Hwang, S.-W. Cheong, P.G. Radaelli, M. Marezio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. [5] P. Kameli, H. Salamati, A. Aezami, J. Appl. Phys. 100 (2006) 10053917. [6] T. Wang, X. Fang, W. Dong, R. Tao, Z. Deng, D. Li, Y. Zhao, G. Meng, S. Zhou, X. Zhu, J. Alloy Compd. 458 (2008) 248. [7] P. Dey, T.K. Nath, Appl. Phys. Lett. 87 (2005) 162501. [8] Y.H. Xiong, W. Xu, Y.T. Mai, H.L. Pi, C.L. Sun, X.C. Bao, et al., J. Magn. Magn. Mater. 320 (2008) 257. [9] Z.G. Zheng, X.C. Zhong, Y.H. Zhang, H.Y. Yu, D.C. Zeng, J. Alloy Compd. 466 (2008) 377. [10] F.C. Gennari, M.R. Esquivel, J. Alloy Compd. 459 (2008) 425. [11] M. Muroi, P.G. MacCormick, R. Street, Rev. Adv. Mater. Sci. 5 (2003) 76. [12] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed, Prentice Hall, Upper Saddle River, NJ, 2002. [13] B.H. Liu, J. Ding, Z.L. Dong, C.B. Boothroyd, J.H. Yin, J.B. Yi, Phys. Rev. B 74 (2006) 184427. [14] J. Karch, R. Berringer, H. Gleiter, Nature 330 (1987) 556. [15] N. Zhang, Phys. Rev. B 56 (1997) 8138. [16] J.L. Dorman, L. Bessais, D. Fiorani, J. Phys. C: Solid State Phys. 21 (1988) 2015. [17] S.K. Sharma, R. Kumar, S. Kumar, V.V.K. Siva, M. Knobel, V.R. Reddy, A. Banerjee, Solid State Commun. 141 (2007) 203. [18] G.F. Goya, T.S. Berquo, F.C. Fonseca, M.P. Morales, J. Appl. Phys. 94 (2003) 3520. [19] S. Shtrikman, E.P. Wohlfarth, Phys. Lett. A. 85 (1981) 467. [20] X. Batlle, A. Labarta, J. Phys. D: Appl. Phys. 35 (2002) R15.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
The structural and magnetic properties of one-step mechanochemical route synthesized La0.8Pb0.2MnO3 manganites H. Bahrami, P. Kameli , H. Salamati Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran
a r t i c l e in fo
abstract
Article history: Received 15 October 2008 Received in revised form 19 February 2009 Available online 24 March 2009
Mechanochemical route has been used to produce La0.8Pb0.2MnO3 (LPMO) nanocrystalline samples from oxide precursors. The samples were characterized using X-ray diffraction, scanning electron microscope and AC susceptibility measurements. The results showed that it is possible to produce LPMO perovskite powders after 10 h of ball milling. The crystallite size and microstrain were estimated using Williamson–Hall equation. The results showed that the crystallite size and microstrain increase initially and then decrease by the increase of milling time. By decreasing particle size the dislocation density (strain) increases and reaches to a saturation point at a particular particle size, further particle size reduction takes place through gliding motion along grain boundaries, which leads to a reduction of the strain. The dynamic properties of 15 h ball-milled sample were investigated by AC susceptibility using the Neel–Brown and Vogel–Fulcher law for superparamagnetism. The frequency dependence of blocking temperature is well described by the Vogel–Fulcher law, and fitting the experimental data with Neel–Brown law gives unphysical value for relaxation time. & 2009 Elsevier B.V. All rights reserved.
Keywords: Perovskites Mechanochemical X-ray techniques Superparamagnetism
1. Introduction Perovskite manganites with the general composition La1xAxMnO3 (A ¼ Sr, Ca and Pb) have been intensively studied in the last decade since the discovery of the colossal magnetoresistance (CMR) phenomenon [1–4]. Potential applications of this type of materials as magnetic sensors, permanent magnets, catalysts, pigment and electrode materials for solid oxide fuel cell maintain intense research for these systems. Several synthetic methods of producing these materials such as: solid state reaction [5], sol–gel [6], coprecipitation methods [7] and high-energy ball milling (mechanochemical route) [8] have been developed. The mechanochemical route has been recognized as a powerful method for the production of novel, high performance and lowcost materials such as ferrites, intermetallics and etc. [9,10]. The mechanochemical synthesis can deliver the designed phases and structures by a single step of the high-energy milling conducted in an enclosed activation chamber at room temperature. In recent years, some authors have used high-energy ball milling to the mechanosynthesis of stoichiometric manganites [8,11]. In these cases, complete formation of perovskite manganites was obtained only after milling followed by sintering, i.e. by employing two processing steps. In this paper, we will report on the single-step synthesis of nanocrystalline La0.8Pb0.2MnO3 (LPMO) powders via
Corresponding author. Tel.: +98 3113912375; fax: +98 3113912376.
E-mail address: [email protected] (P. Kameli). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.03.035
high-energy milling of oxide precursors and investigation of their structural and magnetic properties.
2. Experimental PbO, La2O3 and MnO2 powders with purity 99.99% from Merck were used as the starting materials. These materials were mixed in the stoichiometric ratio and were milled in a PM100 planetary ball mill (Retsch, Germany) in an air atmosphere using hardened steel balls of 20 mm in diameter and stainless steel containers of 500 ml. The milling speed was 300 rpm and the ball-to-powder mass ratio was 15:1. The milling was interrupted at milling time and a small amount of powder was taken out of the vial. In order to prevent the excessive overheating of the containers, the experiments were carried out by altering 120 min of milling with 30 min of rest. The AC susceptibility measurements were performed using a Lake Shore AC Susceptometer, Model 7000. X-ray diffraction (XRD) patterns were taken on Philips XPERT MPD (Cu Ka radiation: l ¼ 0.154 nm) diffractometer. The microstructural observations of samples were analyzed by Philips XL 30 scanning electron microscope (SEM).
3. Results and discussion Fig. 1 shows the XRD patterns at room temperature for LPMO powder compound with different milling times. As expected the
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(012)
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(110) (104)
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Intensity (a.u)
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50 40
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* *# #
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*
40
#
#
*
50 2Theta
+#
*
60
*
+
70
*
2.5h + * 0h
30
80
20 10
Fig. 1. X-ray powder diffraction patterns of unmilled and ball-milled powders. (*) La2O3, (]) PbO and (+) MnO2.
10
15
20
25
30
35
40
milling time(h)
B cos y ¼
Kl þ 2ðÞ sin y D
(1)
where B is the full-width at half-maximum (FWHM) of the XRD peaks, K is the Scherer constant, D is the particle size, l is the wavelength of the X-ray, e is the lattice strain and y is the Bragg angle. In this method, B cos y is plotted against sin y. Using a linear extrapolation to this plot, the intercept gives a quantity proportional to the reciprocal of the particle size (Kl/D) and the slope gives two times the strain (e). Fig. 2 (a) shows the effects of milling time on particle size. As one can see, the particle size increases from 10 to 15 h milled sample and then decreases by increasing the milling time. It seems that the 15 h milling time is the optimum time for phase and structural formation of LPMO perovskite manganites in this experiment. Fig. 2 (b) shows the variation of microstrain as a function of milling time. The microstrain increases initially and reaches the maximum value after a 20 h milling time. Further milling leads to a decrease in microstrain. Such a behavior has also been observed for ball-milled ferrite samples [13]. It is clear that
0.018
0.016
Strain
powder pattern of unmilled mixture contains only the individual reflections of PbO, La2O3 and MnO phases. The intensity ratio of individual reflections is in accordance with the stoichiometric composition of the mixture. As one can see after 2.5 h of milling, intensities of all reflections reduce substantially followed by peak broadening and peak overlapping. Also appearance of new peak at 2y ¼ 32.71, after 2.5 h milling, clearly establishes the formation of nanocrystalline LPMO phase. In the course of milling, growth of LPMO phase increases continuously in the expense of starting phases. It seems that the XRD pattern of 10 h ball-milled powder as that of a LPMO rhombohedral phase without any noticeable trace of starting phases. From the XRD spectrum, it appears that a stoichiometric LPMO manganite in nanocrystalline form may be obtainable by ball milling the stoichiometric mixture of PbO, La2O3 and MnO for about 10 h. By continuing the milling process the diffraction pattern of LPMO powder gradually broadens out; however, the crystalline structure is maintained even for 40 h milled sample. In almost all cases, line broadening occurs due to simultaneous size and strain effects. Since the high-energy ball milling introduces considerable strain in the compound, the contribution of strain to the diffraction line broadening is not negligible. One way to separate these two effects has been developed by Williamson and Hall and is now known as the Williamson–Hall plot. The Williamson–Hall equation is expressed as follows [12]:
0.014
0.012
0.010
10
15
20
25
30
35
40
milling time(h) Fig. 2. Variation of crystallite size (a) and strain (b) versus milling time for ballmilled powders.
the ball milling process induces some strain in the specimens, and increasing the milling time, increases the amount of internal microstrain. Also, increasing the milling time leads to a rapid decrease of the crystallite size. On the other hand, the decrease of the particle size is associated with an increase of the dislocation densities. Since the microstrain generally follows the features of dislocation density, it increases as the particle size decreases. In addition to dislocation, particle size reduction also takes place at a lower stress by a gliding motion along the grain boundaries [14]. When the particles are big, the grain boundary contribution to the microstrain is very small. However, as the particle size decreases the grain boundary increases relatively and act as a barrier to the dislocation motion. As a result, the dislocation density reaches to a saturation point at a particular grain size. Further, particle size reduction takes place through gliding motion along the grain boundaries. The overall effect is then a reduction of the microstrain as shown in Fig. 2(b). Fig. 3 shows typical SEM images for unmilled and 15 h milled samples. The sample that has not been subjected to milling consists mainly of large grains (1–2 mm). The 15 h milled sample shows a narrow particle size distribution, with average particle
ARTICLE IN PRESS H. Bahrami et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 2533–2536
size of less than 500 nm, but we can see some particle agglomeration as a result of size reduction. Fig. 4 shows the real (x0 ) part of AC susceptibility for LPMO powder compound with different milling times, which were measured in an AC field of 10 Oe and frequency of 333 Hz. As expected, the susceptibility x0 of unmilled mixture is almost zero without any signature of ferromagnetic transition. By incarcerating milling time, the susceptibility increases and samples show paramagnetic–ferromagnetic transition. It is clear that the increasing of susceptibility and occurrence of
2535
paramagnetic–ferromagnetic transition in ball-milled powders are due to the perovskite phase formation. Also, as one can see the magnitude of susceptibility decreases by further increasing the milling time. An increase of the width of transition and a reduction of the transition temperature are observed for longtime milled powders (410 h). These behaviors can be explored by the model proposed by Zhang [15]. According to this model, in granular perovskite system a grain of the perovskite can be divided into a body and surface phase. The body phase would have the same properties as the bulk compound (structural, magnetic and transport properties), but the surface phase would have low transition temperature and magnetization. This is because of the magnetically disordered state in the surface. In the case of nanometer-size particles, defects are expected to occur to a higher extent and enhanced grain surface due to the oxygen vacancies and microstrain. Since the surface layer increases as the particle size decreases, therefore, the magnitude of susceptibility and transition temperature decrease by increasing the milling time. Surface contribution is larger for smaller particles and therefore the magnetization is diminished in a proportional way. To further study these nanocrystalline powders, we have performed AC susceptibility measurements as functions of frequency and AC field amplitude on 15 h milled sample. Fig. 5 shows the variation of AC susceptibility as function of
-4
3.5x10
-4
3.0x10
-4
X'(emu/gOe)
2.5x10
-4
2.0x10
-4
1.5x10
-4
1.0x10
f=111 Hz f=333 Hz f=666 Hz f=1000 Hz
-5
5.0x10
Fig. 3. SEM micrographs of unmilled (a) and 15 h milled (b) powders.
14
0.0 80
100
120
140
160
180
200
220
240
T(K)
3.0x10-4
10h 15h
X'(emu/gOe)
2.5x10-4
20h 25h
-4
2.0x10
30h -4
1.5x10
5h 40h
1.0x10-4 5.0x10-5
0h
0.0
50
100
150
200
250
300
T(K) Fig. 4. Temperature dependence of the real part of AC susceptibility for ball-milled powders.
Fig. 5. Temperature dependence of the (a) real and (b) imaginary part of AC susceptibility for 15 h milled powder at different frequencies.
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In Fig. 6 we tried to fit the experimental data of Fig. 5, using Eq. (3). For T0 ¼ 130 K the value of t0 ¼ 5 109 s and Ea/k ¼ 326 K have been obtained. A good agreement of experimental data and Vogel–Fulcher law is the evidence that the phenomenon taking place at TB is related to blocking of an ensemble of interacting nanoparticles.
4. Conclusions
Fig. 6. The best fit of ln(f) versus 1/(TBT0) for 15 h milled powder.
temperature in an AC field of 10 Oe and frequencies of 111, 333, 666 and 1000 Hz. The frequency-dependent features highlighting the peaks in both real, x0 and imaginary, x00 parts. It is clearly evident from Fig. 5 that the data for both x0 and x00 exhibit the expected behavior of blocking/freezing process, i.e. the occurrence of a maximum at a temperature TB for both x0 and x00 components that shifts towards higher temperature. For isolated nanoparticles or weak interaction between them, the frequency dependence of blocking temperature, TB has been given by Neel–Brown model [16], E (2) t ¼ t0 exp a kB T where t0 for superparamagnetic systems is in the range of 109–1013 s [17,18] and t is related to measuring frequency as t ¼ 1/f. In the absence of an external magnetic field, the energy barrier, Ea, can be assumed to be proportional to particle volume V, Ea ¼ keffV, where keff is an effective magnetic anisotropy constant. The x00 peak temperature was taken as blocking temperature, TB. By fitting the experimental data from Fig. 5 with Eq. (2), we have found an unphysical small t01 1038 s in comparison of 1013 s for superparamagnetic systems (the fit is not shown here). This result simply indicates that there exists strong interaction between nanocrystalline powders. Magnetic interactions modify the energy barrier coming from the anisotropy contributions of each particle. It is worth noticing that in contrast with the static energy barrier distributions, the reversal of one particle moment may change the energy barriers of the assembly, even in the weak interaction limit. The first attempt to introduce interactions in the Neel–Brown model was made by Shtrikman and Wohlfarth [19,20] by introducing T0 as an effective temperature which accounts for the interaction effects. Therefore, for interacting magnetic nanoparticles the frequency dependence of TB is given by the Vogel–Fulcher law [16], Ea (3) t ¼ t0 exp kB ðT B T 0 Þ
High-energy ball milling of PbO, La2O3 and MnO2 powders leads to the synthesis of nanocrystalline La0.8Pb0.2MnO3 (LPMO) perovskite phase. It is found that the particle size and microstrain increase initially and then decrease by increasing the milling time. By decreasing particle size the dislocation density (strain) increases and reaches saturation point at a particular particle size. Further, particle size reduction takes place through gliding motion along grain boundaries, which leads to a reduction of the microstrain. The dynamic properties of 15 h ball-milled sample were investigated by AC susceptibility using the Neel–Brown and Vogel–Fulcher law for superparamagnetism. The frequency dependence of blocking temperature is well described by the Vogel–Fulcher law, and fitting the experimental data with Neel–Brown law gives unphysical value for relaxation time.
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