- Email: [email protected]
EPR line intensity in La0.7Ca0.3−xBaxMnO3 manganites
Solid State Communications 123 (2002) 383–386 www.elsevier.com/locate/ssc
EPR line intensity in La0.7Ca0.32xBaxMnO3 manganites A.N. Ulyanova,b, G.G. Levchenkob, Seong-Cho Yua,* b
a Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea Donetsk Physico-Technical Institute of National Academy of Sciences, 83114 Donetsk, Ukraine
Received 18 April 2002; accepted 16 July 2002 by H. Takayama
Abstract The La0:7 Ca0:32x Bax MnO3 (x ¼ 0, 0.15, 0.3) manganites were studied by electron paramagnetic resonance (EPR). Temperature dependencies of EPR line intensity can be fitted by the deduced exponential decay IðTÞ ¼ I0 expðEa =kB TÞ at more wide temperature range as compared with the Curie– Weiss law (Ea and kB are the activation energy and Boltzmann constant, respectively). A deviation of the EPR line intensity from the Curie– Weiss law near the Curie point indicates for the spin– lattice interaction in lanthanum manganites near that temperature. This interaction decreases at high temperatures. The exponential decay is valid in the conditions when the temperature dependence of EPR line width is proportional to the conductivity one. q 2002 Published by Elsevier Science Ltd. PACS: 71.38. þ i; 75.70.Pa; 76.30. 2 v; 77.80.Bh Keywords: A. Magnetically ordered materials; D. Spin dynamics; E. Electron paramagnetic resonance
(1) Lanthanum manganites La12x Mx MnO3 reveal the interesting magnetic and transport properties, and most noteworthy among them is the colossal magnetoresistance (CMR) effect observed near the Curie temperature, Tc [1]. However, a nature of CMR is not yet clear though there are a large number of publications on this problem. The difficulties consist in a close connection between the structure, electronic and magnetic properties of perovskitetype manganites. Double-exchange model [2] revealing the connections between the electrical and magnetic subsystems of lanthanum manganites was complemented by the electron – phonon interaction caused by the Jahn– Teller splitting of external d-level of manganese [3]. Resonance methods can give a useful information about the internal dynamics of the CMR materials, especially near the Curie point. The electron paramagnetic resonance (EPR or ESR above Tc ) has been studied in a variety of lanthanum manganites. The EPR linewidth, DH; increases usually very sharp near the Tc with decreasing temperature and it was * Corresponding author. Tel.: þ82-431-261-2269; fax: þ 82-431275-6416. E-mail address: [email protected] (S.C. Yu).
attributed to the demagnetization factor of samples, probably to the damaged surfaces [4,5]. The DH; at first, (from Tmin < 1:1Tc ) increased linearly [6] with increasing temperature and then, at higher temperatures, a small tendency for saturation was found [7,8] for T < 600 K; but full saturation was not achieved up to 1000 K [5]. The temperature dependence of ESR line intensity, IðTÞ; is not so clean enough. According to Refs. [1,9,10] the IðTÞ decreased exponentially with increasing temperature and was inversely proportional to the temperature [5,11]. In present work the ESR measurement of La0:7 Ca0:32x Bax MnO3 compositions were carried out to study systematically the IðTÞ dependencies at x ¼ 0, 0.15, 0.3 and to add the EPR study for the FMR [12,13] and NMR [14,15] ones of Bacontaining lanthanum manganites. (2) X-ray analysis showed single phase samples synthesized by ceramic technique. The EPR measurements were performed at 9.2 GHz (X band) with a Jeol JES-TE300 ESR Spectrometer. Due to the dependence of the resonance linewidth on the mass of samples [5] the EPR measurements were carried out on samples with approximately equal mass (< 2 mg). Curie temperature was deduced by the extrapolation of the magnetization to zero point at its temperature
0038-1098/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 3 9 3 - 9
384
A.N. Ulyanov et al. / Solid State Communications 123 (2002) 383–386
The ESR line intensity, IðTÞ; was determined by double integration of experimental derivative absorption curve. Temperature dependencies of line intensity IðTÞ and inverse value, 1=IðTÞ; for La0:7 Ca0:3 MnO3 sample are shown in Fig. 1. Some expressions describing the CMR materials parameters were considered to fit the obtained IðTÞ dependence. According to the spin– spin interaction model [5,16] the EPR linewidth, DH; follows the law DH / 1=T xdc ;
ð1Þ
where xdc is a static susceptibility. At the same time, Shengelaya et al. [17] observed that DHðTÞ / sðTÞ;
Fig. 1. Temperature dependencies of ESR line intensity for La0:7 Ca0:3 MnO3 sample. IðTÞ: closed squares; inverse line intensity, 1=IðTÞ: open circles; and fitting curve for line intensity, IðTÞ ¼ I0 expðEa =kB TÞ : solid line.
dependence and equals to 267 ^ 1, 305 ^ 1 and 345 ^ 1 K for samples with x ¼ 0, 0.15 and 0.3, respectively. (3) The ESR spectrums showed a single Lorentsian line in the temperature range presented with g < 2 and that value was temperature independent. The g value was obtained using the ESR Mn2þ in MgO marker. The EPR linewidth showed a minimum at Tmin < 1:1Tc and increased linearly from some temperature TL * Tmin with increasing temperature for all compositions studied.
Fig. 2. Natural logarithm for the ESR line intensity as a function of reciprocal temperature for La0:7 Ca0:32x Bax MnO3 compositions (main figure) and the data replotted from Refs. [5,8] (inset, left scale). The natural logarithm for the EPR linewidth DHT dependence on reciprocal temperature from Ref. [21] is shown in the inset (right scale). Solid lines represent the linear fit to the experimental data.
ð2Þ
where s is the conductivity and, according to adiabatic polaron hopping model [18]
sðTÞ / ð1=TÞexpð2Ea =kB TÞ;
ð3Þ
where Ea and kB are the activation energy and Boltzmann constant, respectively. Also taking into account that EPR line intensity [5,11] IðTÞ / xdc ;
ð4Þ
the last can be presented as IðTÞ ¼ I0 expðEa =kB TÞ:
ð5Þ
This expression gives the good fit for the IðTÞ data obtained for La0:7 Ca0:3 MnO3 composition. By the extrapolation of linear (high temperature part) of 1=IðTÞ dependence to zero value it is possible to obtain the Curie – Weiss temperature, Q: As one can see from Fig. 1 the Curie – Weiss law is not valid to describe the temperature dependence of ESR line intensity in wide temperature range near the Tc : An increasing deviation from the Curie– Weiss law has also been observed in ðNd12y Smy Þ0:5 Sr0:5 MnO3 range as the Tc temperature is lowered with y [19]. The temperature dependence of ESR line intensity for La0:7 Ca0:15 Ba0:15 MnO3 and La0:7 Ba0:3 MnO3 compositions as well as for La0:7 Ca0:3 MnO3 sample cannot be fitted by the Curie– Weiss law in a wide temperature range near the Curie temperature and are best described by Eq. (5). The natural logarithm for the ESR line intensity as a function of 1=T for all compositions studied is presented (see Fig. 2). One can see those dependencies are almost linear ones. It permitted to deduce the values of activation energy Ea ¼ 0.15, 0.19 and 0.22 eV for samples with x ¼ 0, 0.15 and 0.3, respectively. These values are consistent, for examples, with the conductivity measurement of La0:67 Ca0:33 MnO3 thin films (Ea ¼ 0.083– 0.136 eV [20] versus annealing treatment). As it was noted above, the exponential decay IðTÞ ¼ I0 expðEa =kB TÞ for ESR line intensity was also obtained at the study of La12x Cax MnO3 samples [9]. But, according to Lofland et al. [11] the spin complex responsible for the ESR must be some combination of Mn3þ and Mn4þ ions and intensity of line IðTÞ should be proportional to the static
A.N. Ulyanov et al. / Solid State Communications 123 (2002) 383–386
susceptibility, xdc : It causes the IðTÞ / 1=ðT 2 QÞ dependence of line intensity. Moreover, the authors [11] replotted the experimental data for temperature dependence of line intensity obtained in Ref. [9] and claimed that it followed to the hyperbolic law. We replotted also the data of work [9] and obtained the hyperbolic law for line intensity, but in more short temperature interval (for more high temperature part only), than that data can be fitted by the exponential decay. Besides, Shengelaya et al. [8] in the EPR study of La0:8 Ca0:2 MnO3 sample noted that “the integral intensity IðTÞ of the EPR signal decreased with temperature much faster than would be expected according to the Curie law”. We replotted data of Shengelaya [8] and obtained again the exponential decay of line intensity at the temperature interval from Tmin to 600 K (where IðTÞ was studied) (see inset in Fig. 2). We also replotted the IðTÞ data from Ref. [5] where the La0:67 Ca0:33 MnO3 composition was studied and the exponential law was obtained again (see inset in Fig. 2). It confirmed the validity of the exponential law for the IðTÞ dependence. An activation energy, Ea ; deduced from the data of Refs. [5,8] equals to 0.12 and 0.11 eV, respectively, which is consistent with the our results. Authors of work [9,10] to explain the deviation of temperature dependence of EPR line intensity from the Curie–Weiss law near the Tc ; assumed the formation of spin clusters, when Tc is approached from above. Authors of Refs. [5,21] explained the deviation of the static susceptibility from the Curie–Weiss law at the frame of the constant coupling approximation model. On the other hand, the polaron formation across the ferromagnetic–paramagnetic phase transition was observed, which was reported by the study on La12x Ax MnO3 (A ¼ Ca, Pb) perovskite system by the extended X-ray absorption fine structure (EXAFS) spectroscopy [22]. It seems, that deviation of experimental IðTÞ data from Curie–Weiss law observed can be caused by the spin– lattice interaction in the vicinity of Tc and are also in agreement with the prediction of the adiabatic polaron hopping model [18]. From this point of view, the exponential law of EPR line intensity on temperature reflects the formation of polarons near the Curie point. Those polarons centered by the Mn3þ and Mn4þ ions and mediated by the double (through the O22 ion) activated hopping of the electrons. The IðTÞ fitting curves obtained using the exponential law (5) and Curie– Weiss one coincide at high temperatures. It indicates the decrease in the spin– phonon interaction at noted region. The validity of Eq. (5) is needed in some comments. The assumptions (2) and (3) were made to deduce it, at first, about Eq. (3). The conductive mechanism in perovskitelike manganites was carefully studied (see, for example, Refs. [20,23– 26] and references therein) and it is known that the conductivity in the paramagnetic region can be described by the nearest-neighbor hopping of small (Holstein) polarons and is given by sðTÞ / ð1=TÞ expð2Ea =kB TÞ;
385
the Mott’s variable-range hopping expression sðTÞ / expð2T0 =TÞ1=4 is appropriate to describe the experimental data if the carriers are localized by the random potential fluctuations (T0 is related to the localized length). The temperature dependence of conductivity (3) in mixed-valence manganites was observed, for examples, in Refs. [20,24– 26]. At second, about the validity of Eq. (2). Except Ref. [17] where the authors observed the proportionality between EPR linewidth and conductivity sðTÞ (/ð1=TÞexp ð2Ea =kB TÞ) token from Ref. [20] we know the only works [5,21] where the EPR linewidth in La0:7 Ca0:3 MnO3 sample was measured in wide temperature range (up to 1000 K). We replotted that data in the scale lnðDHTÞ on 1=T (see inset in Fig. 2) and, as one can see, the obtained dependence is very close to the linear one that confirms the validity of Eq. (2) again. So, it can be concluded that Eqs. (2) and (3) as well as the deduced Eq. (5) are valid at the predicted determined conditions. In summary, the ESR measurements of La0:7 Ca0:32x Bax MnO3 (x ¼ 0, 0.15, 0.3) lanthanum manganites were carried out. The exponential decreasing for ESR line intensity was deduced. The deduced law describes the measured temperature dependencies of ESR line intensity in more wide temperature range then those described by the Curie– Weiss one. The boundary of exponential decay applicability for EPR line intensity is determined. The deviation of the EPR line intensity from the Curie– Weiss law near the Curie point indicates for the spin – lattice interaction in lanthanum manganites near that temperature. The exponential law and Curie– Weiss one coincide at high temperatures indicating the decrease in the spin –phonon interaction at noted region.
Acknowledgements Research at Chungbuk National University was supported by the Korean Research Foundation Grant (KRF2001-005-D20010). The authors are indebted to N.E. Pismenova and S.I. Khokhlova for the help when preparing the samples and to Seong-Gi Min for the assistance in EPR measurements.
References [1] J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys. 48 (1999) 167. [2] R.N. Zener, Phys. Rev. 82 (1951) 403. [3] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [4] C.A. Ramos, M.T. Causa, M. Tovar, X. Obradors, S. Pin˜ol, JMMM 177–181 (1998) 867. [5] M.T. Causa, M. Tovar, A. Caneiro, F. Prado, G. Iban˜ez, C.A.l
386
[6] [7]
[8] [9] [10] [11]
[12] [13]
[14]
A.N. Ulyanov et al. / Solid State Communications 123 (2002) 383–386 Ramos, A. Butera, B. Alascio, X. Obradors, S. Pin˜ol, F. Rivadulla, C. Va´zques-Va´zques, A. Lo´pez-Quintela, J. Rivas, Y. Tokura, S.B. Oseroff, Phys. Rev. B 58 (1998) 3233. M.S. Seehrat, M.M. Ibrahim, V. Suresh Babu, G. Srinivasan, J.Phys.: Condens. Matter 8 (1996) 11283. C. Rettori, D. Rao, J. Singley, D. Kidwell, S.B. Oseroff, M.T. Causa, J.J. Neumeier, K.J. McClellan, S-W. Cheong, S. Schultz, Phys. Rev. B 55 (1997) 3083. A. Shengelaya, G-m. Zhao, H. Keller, K.A. Muller, Phys. Rev. Lett. 77 (1996) 5296. S.B. Oseroff, M. Torikachvili, J. Singley, S. Ali, S-W. Cheong, S. Schultz, Phys. Rev. B 53 (1996) 6521. A.I. Shames, E. Rosenberg, W.H. McCarroll, M. Greenblatt, G. Gorodetsky, Phys. Rev. B 64 (2001) 172401. S.E. Lofland, P. Kim, P. Dahiroc, S.M. Bhabat, S.D. Tyagi, S.G. Karabashev, D.A. Shulyatev, A.A. Arsenov, Y. Mukovskii, Phys. Lett. A 233 (1997) 476. S.E. Lofland, S.M. Bhabat, H.L. Ju, G.C. Xiong, T. Venkatesan, R.L. Greene, Phys. Rev. B 52 (1995) 15058. S.E. Lofland, S.M. Bhabat, H.L. Ju, G.C. Xiong, T. Venkatesan, R.L. Greene, S. Tuagi, J. Appl. Phys. 79 (1996) 5166. P. Novak, M. Marysko, M.M. Savosta, A.N. Ulyanov, Phys. Rev. B 60 (1999) 6655.
[15] M.M. Savosta, A.N. Ulyanov, N.Yu. Starostyuk, M. Marysko, P. Novak, Eur. Phys. J. B 12 (1999) 393. [16] D.L. Huber, G. Alejandro, A. Caneiro, M.T. Causa, F. Prado, M. Tovar, S.B. Oseroff, Phys. Rev. B 60 (1999) 12155. [17] A. Shengelaya, G-m. Zhao, H. Keller, K.A. Muller, Phys. Rev. B 61 (2000) 5888. [18] D. Emin, T. Holstein, Ann. Phys. 53 (1969) 439. [19] H. Kuwahara, Y. Moritomo, Y. Tomioka, A. Asamitsu, M. Kasai, J. Appl. Phys. 81 (1997) 4954. [20] D.C. Worledge, G. Jeffrey Snyder, M.R. Beasley, T.H. Geballe, R. Hiskes, S. DiCarolis, J. Appl. Phys. 80 (1996) 5158. [21] M. Tovar, M.T. Causa, G. Iban˜ez, C.A. Ramos, A. Butera, F. Rivadulla, B. Alascio, S.B. Oseroff, S-W. Cheong, X. Obradors, S. Pin˜ol, J. Appl. Phys. 83 (1998) 7201. [22] C.H. Booth, F. Bridges, G.J. Snyder, T.H. Geballe, Phys. Rev. B 54 (1996) R15606. [23] J.M.D. Coey, M. Viret, L. Ranno, K. Ounadjela, Phys. Rev. Lett. 75 (1995) 3910. [24] M. Ziese, C. Srinitiwarawong, Phys. Rev. B 58 (1998) 11519. [25] L. Pi, L. Zheng, Y. Zhang, Phys. Rev. B 61 (2000) 8917. [26] X. Liu, H. Zhu, Y. Zhang, Phys. Rev. B 65 (2002) 024412.
EPR line intensity in La0.7Ca0.32xBaxMnO3 manganites A.N. Ulyanova,b, G.G. Levchenkob, Seong-Cho Yua,* b
a Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea Donetsk Physico-Technical Institute of National Academy of Sciences, 83114 Donetsk, Ukraine
Received 18 April 2002; accepted 16 July 2002 by H. Takayama
Abstract The La0:7 Ca0:32x Bax MnO3 (x ¼ 0, 0.15, 0.3) manganites were studied by electron paramagnetic resonance (EPR). Temperature dependencies of EPR line intensity can be fitted by the deduced exponential decay IðTÞ ¼ I0 expðEa =kB TÞ at more wide temperature range as compared with the Curie– Weiss law (Ea and kB are the activation energy and Boltzmann constant, respectively). A deviation of the EPR line intensity from the Curie– Weiss law near the Curie point indicates for the spin– lattice interaction in lanthanum manganites near that temperature. This interaction decreases at high temperatures. The exponential decay is valid in the conditions when the temperature dependence of EPR line width is proportional to the conductivity one. q 2002 Published by Elsevier Science Ltd. PACS: 71.38. þ i; 75.70.Pa; 76.30. 2 v; 77.80.Bh Keywords: A. Magnetically ordered materials; D. Spin dynamics; E. Electron paramagnetic resonance
(1) Lanthanum manganites La12x Mx MnO3 reveal the interesting magnetic and transport properties, and most noteworthy among them is the colossal magnetoresistance (CMR) effect observed near the Curie temperature, Tc [1]. However, a nature of CMR is not yet clear though there are a large number of publications on this problem. The difficulties consist in a close connection between the structure, electronic and magnetic properties of perovskitetype manganites. Double-exchange model [2] revealing the connections between the electrical and magnetic subsystems of lanthanum manganites was complemented by the electron – phonon interaction caused by the Jahn– Teller splitting of external d-level of manganese [3]. Resonance methods can give a useful information about the internal dynamics of the CMR materials, especially near the Curie point. The electron paramagnetic resonance (EPR or ESR above Tc ) has been studied in a variety of lanthanum manganites. The EPR linewidth, DH; increases usually very sharp near the Tc with decreasing temperature and it was * Corresponding author. Tel.: þ82-431-261-2269; fax: þ 82-431275-6416. E-mail address: [email protected] (S.C. Yu).
attributed to the demagnetization factor of samples, probably to the damaged surfaces [4,5]. The DH; at first, (from Tmin < 1:1Tc ) increased linearly [6] with increasing temperature and then, at higher temperatures, a small tendency for saturation was found [7,8] for T < 600 K; but full saturation was not achieved up to 1000 K [5]. The temperature dependence of ESR line intensity, IðTÞ; is not so clean enough. According to Refs. [1,9,10] the IðTÞ decreased exponentially with increasing temperature and was inversely proportional to the temperature [5,11]. In present work the ESR measurement of La0:7 Ca0:32x Bax MnO3 compositions were carried out to study systematically the IðTÞ dependencies at x ¼ 0, 0.15, 0.3 and to add the EPR study for the FMR [12,13] and NMR [14,15] ones of Bacontaining lanthanum manganites. (2) X-ray analysis showed single phase samples synthesized by ceramic technique. The EPR measurements were performed at 9.2 GHz (X band) with a Jeol JES-TE300 ESR Spectrometer. Due to the dependence of the resonance linewidth on the mass of samples [5] the EPR measurements were carried out on samples with approximately equal mass (< 2 mg). Curie temperature was deduced by the extrapolation of the magnetization to zero point at its temperature
0038-1098/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 3 9 3 - 9
384
A.N. Ulyanov et al. / Solid State Communications 123 (2002) 383–386
The ESR line intensity, IðTÞ; was determined by double integration of experimental derivative absorption curve. Temperature dependencies of line intensity IðTÞ and inverse value, 1=IðTÞ; for La0:7 Ca0:3 MnO3 sample are shown in Fig. 1. Some expressions describing the CMR materials parameters were considered to fit the obtained IðTÞ dependence. According to the spin– spin interaction model [5,16] the EPR linewidth, DH; follows the law DH / 1=T xdc ;
ð1Þ
where xdc is a static susceptibility. At the same time, Shengelaya et al. [17] observed that DHðTÞ / sðTÞ;
Fig. 1. Temperature dependencies of ESR line intensity for La0:7 Ca0:3 MnO3 sample. IðTÞ: closed squares; inverse line intensity, 1=IðTÞ: open circles; and fitting curve for line intensity, IðTÞ ¼ I0 expðEa =kB TÞ : solid line.
dependence and equals to 267 ^ 1, 305 ^ 1 and 345 ^ 1 K for samples with x ¼ 0, 0.15 and 0.3, respectively. (3) The ESR spectrums showed a single Lorentsian line in the temperature range presented with g < 2 and that value was temperature independent. The g value was obtained using the ESR Mn2þ in MgO marker. The EPR linewidth showed a minimum at Tmin < 1:1Tc and increased linearly from some temperature TL * Tmin with increasing temperature for all compositions studied.
Fig. 2. Natural logarithm for the ESR line intensity as a function of reciprocal temperature for La0:7 Ca0:32x Bax MnO3 compositions (main figure) and the data replotted from Refs. [5,8] (inset, left scale). The natural logarithm for the EPR linewidth DHT dependence on reciprocal temperature from Ref. [21] is shown in the inset (right scale). Solid lines represent the linear fit to the experimental data.
ð2Þ
where s is the conductivity and, according to adiabatic polaron hopping model [18]
sðTÞ / ð1=TÞexpð2Ea =kB TÞ;
ð3Þ
where Ea and kB are the activation energy and Boltzmann constant, respectively. Also taking into account that EPR line intensity [5,11] IðTÞ / xdc ;
ð4Þ
the last can be presented as IðTÞ ¼ I0 expðEa =kB TÞ:
ð5Þ
This expression gives the good fit for the IðTÞ data obtained for La0:7 Ca0:3 MnO3 composition. By the extrapolation of linear (high temperature part) of 1=IðTÞ dependence to zero value it is possible to obtain the Curie – Weiss temperature, Q: As one can see from Fig. 1 the Curie – Weiss law is not valid to describe the temperature dependence of ESR line intensity in wide temperature range near the Tc : An increasing deviation from the Curie– Weiss law has also been observed in ðNd12y Smy Þ0:5 Sr0:5 MnO3 range as the Tc temperature is lowered with y [19]. The temperature dependence of ESR line intensity for La0:7 Ca0:15 Ba0:15 MnO3 and La0:7 Ba0:3 MnO3 compositions as well as for La0:7 Ca0:3 MnO3 sample cannot be fitted by the Curie– Weiss law in a wide temperature range near the Curie temperature and are best described by Eq. (5). The natural logarithm for the ESR line intensity as a function of 1=T for all compositions studied is presented (see Fig. 2). One can see those dependencies are almost linear ones. It permitted to deduce the values of activation energy Ea ¼ 0.15, 0.19 and 0.22 eV for samples with x ¼ 0, 0.15 and 0.3, respectively. These values are consistent, for examples, with the conductivity measurement of La0:67 Ca0:33 MnO3 thin films (Ea ¼ 0.083– 0.136 eV [20] versus annealing treatment). As it was noted above, the exponential decay IðTÞ ¼ I0 expðEa =kB TÞ for ESR line intensity was also obtained at the study of La12x Cax MnO3 samples [9]. But, according to Lofland et al. [11] the spin complex responsible for the ESR must be some combination of Mn3þ and Mn4þ ions and intensity of line IðTÞ should be proportional to the static
A.N. Ulyanov et al. / Solid State Communications 123 (2002) 383–386
susceptibility, xdc : It causes the IðTÞ / 1=ðT 2 QÞ dependence of line intensity. Moreover, the authors [11] replotted the experimental data for temperature dependence of line intensity obtained in Ref. [9] and claimed that it followed to the hyperbolic law. We replotted also the data of work [9] and obtained the hyperbolic law for line intensity, but in more short temperature interval (for more high temperature part only), than that data can be fitted by the exponential decay. Besides, Shengelaya et al. [8] in the EPR study of La0:8 Ca0:2 MnO3 sample noted that “the integral intensity IðTÞ of the EPR signal decreased with temperature much faster than would be expected according to the Curie law”. We replotted data of Shengelaya [8] and obtained again the exponential decay of line intensity at the temperature interval from Tmin to 600 K (where IðTÞ was studied) (see inset in Fig. 2). We also replotted the IðTÞ data from Ref. [5] where the La0:67 Ca0:33 MnO3 composition was studied and the exponential law was obtained again (see inset in Fig. 2). It confirmed the validity of the exponential law for the IðTÞ dependence. An activation energy, Ea ; deduced from the data of Refs. [5,8] equals to 0.12 and 0.11 eV, respectively, which is consistent with the our results. Authors of work [9,10] to explain the deviation of temperature dependence of EPR line intensity from the Curie–Weiss law near the Tc ; assumed the formation of spin clusters, when Tc is approached from above. Authors of Refs. [5,21] explained the deviation of the static susceptibility from the Curie–Weiss law at the frame of the constant coupling approximation model. On the other hand, the polaron formation across the ferromagnetic–paramagnetic phase transition was observed, which was reported by the study on La12x Ax MnO3 (A ¼ Ca, Pb) perovskite system by the extended X-ray absorption fine structure (EXAFS) spectroscopy [22]. It seems, that deviation of experimental IðTÞ data from Curie–Weiss law observed can be caused by the spin– lattice interaction in the vicinity of Tc and are also in agreement with the prediction of the adiabatic polaron hopping model [18]. From this point of view, the exponential law of EPR line intensity on temperature reflects the formation of polarons near the Curie point. Those polarons centered by the Mn3þ and Mn4þ ions and mediated by the double (through the O22 ion) activated hopping of the electrons. The IðTÞ fitting curves obtained using the exponential law (5) and Curie– Weiss one coincide at high temperatures. It indicates the decrease in the spin– phonon interaction at noted region. The validity of Eq. (5) is needed in some comments. The assumptions (2) and (3) were made to deduce it, at first, about Eq. (3). The conductive mechanism in perovskitelike manganites was carefully studied (see, for example, Refs. [20,23– 26] and references therein) and it is known that the conductivity in the paramagnetic region can be described by the nearest-neighbor hopping of small (Holstein) polarons and is given by sðTÞ / ð1=TÞ expð2Ea =kB TÞ;
385
the Mott’s variable-range hopping expression sðTÞ / expð2T0 =TÞ1=4 is appropriate to describe the experimental data if the carriers are localized by the random potential fluctuations (T0 is related to the localized length). The temperature dependence of conductivity (3) in mixed-valence manganites was observed, for examples, in Refs. [20,24– 26]. At second, about the validity of Eq. (2). Except Ref. [17] where the authors observed the proportionality between EPR linewidth and conductivity sðTÞ (/ð1=TÞexp ð2Ea =kB TÞ) token from Ref. [20] we know the only works [5,21] where the EPR linewidth in La0:7 Ca0:3 MnO3 sample was measured in wide temperature range (up to 1000 K). We replotted that data in the scale lnðDHTÞ on 1=T (see inset in Fig. 2) and, as one can see, the obtained dependence is very close to the linear one that confirms the validity of Eq. (2) again. So, it can be concluded that Eqs. (2) and (3) as well as the deduced Eq. (5) are valid at the predicted determined conditions. In summary, the ESR measurements of La0:7 Ca0:32x Bax MnO3 (x ¼ 0, 0.15, 0.3) lanthanum manganites were carried out. The exponential decreasing for ESR line intensity was deduced. The deduced law describes the measured temperature dependencies of ESR line intensity in more wide temperature range then those described by the Curie– Weiss one. The boundary of exponential decay applicability for EPR line intensity is determined. The deviation of the EPR line intensity from the Curie– Weiss law near the Curie point indicates for the spin – lattice interaction in lanthanum manganites near that temperature. The exponential law and Curie– Weiss one coincide at high temperatures indicating the decrease in the spin –phonon interaction at noted region.
Acknowledgements Research at Chungbuk National University was supported by the Korean Research Foundation Grant (KRF2001-005-D20010). The authors are indebted to N.E. Pismenova and S.I. Khokhlova for the help when preparing the samples and to Seong-Gi Min for the assistance in EPR measurements.
References [1] J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys. 48 (1999) 167. [2] R.N. Zener, Phys. Rev. 82 (1951) 403. [3] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [4] C.A. Ramos, M.T. Causa, M. Tovar, X. Obradors, S. Pin˜ol, JMMM 177–181 (1998) 867. [5] M.T. Causa, M. Tovar, A. Caneiro, F. Prado, G. Iban˜ez, C.A.l
386
[6] [7]
[8] [9] [10] [11]
[12] [13]
[14]
A.N. Ulyanov et al. / Solid State Communications 123 (2002) 383–386 Ramos, A. Butera, B. Alascio, X. Obradors, S. Pin˜ol, F. Rivadulla, C. Va´zques-Va´zques, A. Lo´pez-Quintela, J. Rivas, Y. Tokura, S.B. Oseroff, Phys. Rev. B 58 (1998) 3233. M.S. Seehrat, M.M. Ibrahim, V. Suresh Babu, G. Srinivasan, J.Phys.: Condens. Matter 8 (1996) 11283. C. Rettori, D. Rao, J. Singley, D. Kidwell, S.B. Oseroff, M.T. Causa, J.J. Neumeier, K.J. McClellan, S-W. Cheong, S. Schultz, Phys. Rev. B 55 (1997) 3083. A. Shengelaya, G-m. Zhao, H. Keller, K.A. Muller, Phys. Rev. Lett. 77 (1996) 5296. S.B. Oseroff, M. Torikachvili, J. Singley, S. Ali, S-W. Cheong, S. Schultz, Phys. Rev. B 53 (1996) 6521. A.I. Shames, E. Rosenberg, W.H. McCarroll, M. Greenblatt, G. Gorodetsky, Phys. Rev. B 64 (2001) 172401. S.E. Lofland, P. Kim, P. Dahiroc, S.M. Bhabat, S.D. Tyagi, S.G. Karabashev, D.A. Shulyatev, A.A. Arsenov, Y. Mukovskii, Phys. Lett. A 233 (1997) 476. S.E. Lofland, S.M. Bhabat, H.L. Ju, G.C. Xiong, T. Venkatesan, R.L. Greene, Phys. Rev. B 52 (1995) 15058. S.E. Lofland, S.M. Bhabat, H.L. Ju, G.C. Xiong, T. Venkatesan, R.L. Greene, S. Tuagi, J. Appl. Phys. 79 (1996) 5166. P. Novak, M. Marysko, M.M. Savosta, A.N. Ulyanov, Phys. Rev. B 60 (1999) 6655.
[15] M.M. Savosta, A.N. Ulyanov, N.Yu. Starostyuk, M. Marysko, P. Novak, Eur. Phys. J. B 12 (1999) 393. [16] D.L. Huber, G. Alejandro, A. Caneiro, M.T. Causa, F. Prado, M. Tovar, S.B. Oseroff, Phys. Rev. B 60 (1999) 12155. [17] A. Shengelaya, G-m. Zhao, H. Keller, K.A. Muller, Phys. Rev. B 61 (2000) 5888. [18] D. Emin, T. Holstein, Ann. Phys. 53 (1969) 439. [19] H. Kuwahara, Y. Moritomo, Y. Tomioka, A. Asamitsu, M. Kasai, J. Appl. Phys. 81 (1997) 4954. [20] D.C. Worledge, G. Jeffrey Snyder, M.R. Beasley, T.H. Geballe, R. Hiskes, S. DiCarolis, J. Appl. Phys. 80 (1996) 5158. [21] M. Tovar, M.T. Causa, G. Iban˜ez, C.A. Ramos, A. Butera, F. Rivadulla, B. Alascio, S.B. Oseroff, S-W. Cheong, X. Obradors, S. Pin˜ol, J. Appl. Phys. 83 (1998) 7201. [22] C.H. Booth, F. Bridges, G.J. Snyder, T.H. Geballe, Phys. Rev. B 54 (1996) R15606. [23] J.M.D. Coey, M. Viret, L. Ranno, K. Ounadjela, Phys. Rev. Lett. 75 (1995) 3910. [24] M. Ziese, C. Srinitiwarawong, Phys. Rev. B 58 (1998) 11519. [25] L. Pi, L. Zheng, Y. Zhang, Phys. Rev. B 61 (2000) 8917. [26] X. Liu, H. Zhu, Y. Zhang, Phys. Rev. B 65 (2002) 024412.