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Transport phenomena in La0.8Ba0.2MnO3 single crystal: evidence for activation to mobility edge
Journal of Magnetism and Magnetic Materials 226}230 (2001) 911}913
Transport phenomena in La Ba MnO single crystal: evidence for activation to mobility edge V.V. Ustinov *, N.G. Bebenin , R.I. Zainullina , V.V. Mashkautsan , V.S. Gaviko , Ya.M. Mukovskii, D.A. Shulyatev, V.G. Vassiliev Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, 18, Soxa Kovalevskaya St., 620219 Ekaterinburg, Russia Moscow State Steel & Alloys Institute, Moscow 117936, Russia Institute of Solid State Chemistry, Ural Division of RAS, Ekaterinburg 620219, Russia
Abstract We have studied the magnetic and transport properties of La Ba MnO single crystal over temperature range 77}400 K in magnetic "elds up to 15 kOe. Near the Curie temperature, both the resistivity, the thermopower, and the normal Hall coe$cient R reach the maximal value, R '0 and the Hall mobility is constant. The data indicate that the holes activated to mobility edge dominate in conduction. The singlesite polaron picture is inconsistent with our data on magnetoresistance and magnetothermopower. 2001 Elsevier Science B.V. All rights reserved. Keywords: Phase transitions*metal}insulator; Resistivity; Seebeck e!ect; Hall e!ect
The interest in lanthanum manganites La D MnO (D"Ca, Sr, Ba) is spured by the colos\V V sal magnetoresistance (CMR) e!ect [1,2]. The CMR is observed near the Curie temperature ¹ when the resis! tivity is `metallica i.e. d/d¹'0 in ferromagnetic state and insulatorlike with d/d¹(0 in paramagnetic phase. Well above ¹ the resistivity usually follows thermal! activated behavior: " exp(EM/¹). Far below ¹ , ! a CMR manganite can be a `bada metal or behave as an insulator depending on whether x is greater or less than the critical concentration x . For La Sr MnO single \V V crystals, x is known to be about 0.17 [3]. In the metallic state, the transport is believed to be due to charge carriers of band type with rather complicated Fermi surface [4,5]. In the insulator state, the nature of the transport is not clear. This work is devoted to the magnetic and transport properties of the La Ba MnO single crystal because there is little information on the properties of the La Ba MnO family. The sample has been grown by \V V * Corresponding author. Tel.: #7-3432-444471; fax: #73432-745244. E-mail address: [email protected] (V.V. Ustinov).
#oating zone technique. At +190 K, a transition from low temperature orthorhombic (Pbnm) to rhombohedral (R3 c) phase occurs. The peculiarities of physical properties connected with the structural phase transition will be published elsewhere [6]; in this paper we focus on the vicinity of the Curie point where the CMR is observed. Magnetization, resistivity , Hall e!ect, and thermopower S were measured from 77 to 400 K in magnetic "elds up to 15 kOe. The measurements were performed on as-grown sample and after the annealing of the sample in #owing oxygen for 40 h at 9503C. The analysis of the magnetization curves has led us to conclusion that the crystal is inhomogeneous and about 10% of its volume is not ferromagnetic. The Curie temperature, determined through Arrott-Belov curves, was 251 K and has been increased by the annealing only by 1 K. The temperature dependence of is shown in Fig. 1. The resistivity decreases with increasing ¹ from 80 to 150 K, then behaves as in a metal (d/d¹'0) and reaches maximum at ¹ '¹ . The annealing slightly 0 ! increases the peak resistivity and shifts the peak to a higher temperature. Well above ¹ , follows thermal! activated behavior with "1.5 m cm before and "1.7 m cm after the annealing. The activation en ergy depends on the relative magnetization m as
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 3 9 5 - 0
912
V.V. Ustinov et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 911}913
Fig. 1. The resistivity of the La Ba MnO single crystal. The inset: the temperature dependence of relative magnetization m(H"10 kOe).
Fig. 2. Thermopower of the as-grown sample.
EM"EM ! m, that agrees with the activation to mo bility-edge theory [7]. The parameters are EM "1120 K and "1100 K and do not depend on the annealing. We calculated (¹, H) using the experimental data for m. An example for H"10 kOe is shown in Fig. 1. We take into account the 10% inhomogeneity of our sample [8] and multiply the experimental m by 1.1. In other words, we assume the charge carriers feel the magnetization of ferromagnetic part of the sample rather than the averaged magnetization. The calculated curve agrees well with the experimental not only in paramagnetic state but also in ferromagnetic one above 150 K. Since EM ' , the activation energy is positive even in the ferromagnetic state far below ¹ . This implies that the crystal is an ! insulator, both above and below ¹ , and hence x '0.2. ! The rapid increase of near ¹ is caused by the decrease ! of EM, and is not necessarily related to the metallic conductivity.
Fig. 3. Normal Hall coe$cient of the as-grown sample.
Fig. 2 shows the S}¹ curves. The thermopower as well as the Hall e!ect were practically una!ected by the annealing, so we show the data for the as-grown sample only. At low temperatures S is negative, then changes the sign around 210 K. Far above ¹ , it follows the relation ! S"S #(k /e)(E1/¹). We suppose that thermopower activation energy can be expressed as E1"E1 !E1 m, "nd E1 , E1 , and S in the paramagnetic region (E1 "190 K, E1 "180 K, S "!38 V/K) and then calculated S(¹, H) in the same manner as in the previous paragraph. Near ¹ , the calculated curve reproduces the ! main features of the experimental one. Since OE1 , the di!erence EM!E1 strongly depends on magnetization, in what follows that the wave functions of nearestneighbor Mn ions overlap and the singlesite polaron approach is inadequate. In Fig. 3 we present the temperature dependence of normal Hall coe$cient R over the range where we can separate normal and anomalous Hall e!ects. At low temperatures, R is negative and small in value. When ¹'200 K, R is positive and takes the maximum at the same temperature as . The inset in Fig. 3 shows that near ¹ , the Hall mobility, , is approximately ! & constant and about 0.07 cm/V/s. According to Ref. [9], these facts indicate that in the vicinity of T the conduct! ivity is due to activation of holes to mobility edge. In low temperature region, however, other mechanisms, perhaps of hopping type, is likely to play the key role. In summary, we have shown that the activation to mobility edge dominates near ¹ and explain the main ! features of the temperature dependence of the resistivity, the thermopower, and normal Hall coe$cient in La Ba MnO ; the critical concentration x exceeds 0.2 for La Ba MnO family. \V V The work was supported by RFBR grants 00-02-17544 and 00-15-96745.
V.V. Ustinov et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 911}913
References [1] J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys. 48 (1999) 167. [2] Y. Tokura, Y. Tomioka, J. Magn. Magn. Mater. 200 (1999) 1. [3] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [4] W.E. Pickett, D.J. Singh, J. Magn. Magn. Mater. 172 (1997) 237. [5] E.A. Livesay, R.N. West, S.B. Dugdale, G. Santi, T. Jarlborg, J. Phys.: Condens. Matter 11 (1999) L279.
913
[6] V.E. Arkhipov, N.G. Bebenin, V.P. Dyakina, V.S. Gaviko, A.V. Korolev, V.V. Mashkautsan, E.A. Neifeld, R.I. Zainullina, Ya.M. Mukovskii, D.A. Shulyatev, Phys. Rev. B 61 (2000) 11229. [7] N.G. Bebenin, V.V. Ustinov, J. Phys.: Condens. Matter 10 (1998) 6301. [8] N.G. Bebenin, R.I. Zainullina, V.V. Mashkautsan, V.S. Gaviko, V.V. Ustinov, Ya.M. Mukovskii, D.A. Shulyatev, JETP 90 (2000) 1027. [9] N.F. Mott, E.A. Davis, Electronic Processes in Noncrystalline Materials, 2nd Edition, Clarendon Press, Oxford, 1979.
Transport phenomena in La Ba MnO single crystal: evidence for activation to mobility edge V.V. Ustinov *, N.G. Bebenin , R.I. Zainullina , V.V. Mashkautsan , V.S. Gaviko , Ya.M. Mukovskii, D.A. Shulyatev, V.G. Vassiliev Institute of Metal Physics, Ural Division of the Russian Academy of Sciences, 18, Soxa Kovalevskaya St., 620219 Ekaterinburg, Russia Moscow State Steel & Alloys Institute, Moscow 117936, Russia Institute of Solid State Chemistry, Ural Division of RAS, Ekaterinburg 620219, Russia
Abstract We have studied the magnetic and transport properties of La Ba MnO single crystal over temperature range 77}400 K in magnetic "elds up to 15 kOe. Near the Curie temperature, both the resistivity, the thermopower, and the normal Hall coe$cient R reach the maximal value, R '0 and the Hall mobility is constant. The data indicate that the holes activated to mobility edge dominate in conduction. The singlesite polaron picture is inconsistent with our data on magnetoresistance and magnetothermopower. 2001 Elsevier Science B.V. All rights reserved. Keywords: Phase transitions*metal}insulator; Resistivity; Seebeck e!ect; Hall e!ect
The interest in lanthanum manganites La D MnO (D"Ca, Sr, Ba) is spured by the colos\V V sal magnetoresistance (CMR) e!ect [1,2]. The CMR is observed near the Curie temperature ¹ when the resis! tivity is `metallica i.e. d/d¹'0 in ferromagnetic state and insulatorlike with d/d¹(0 in paramagnetic phase. Well above ¹ the resistivity usually follows thermal! activated behavior: " exp(EM/¹). Far below ¹ , ! a CMR manganite can be a `bada metal or behave as an insulator depending on whether x is greater or less than the critical concentration x . For La Sr MnO single \V V crystals, x is known to be about 0.17 [3]. In the metallic state, the transport is believed to be due to charge carriers of band type with rather complicated Fermi surface [4,5]. In the insulator state, the nature of the transport is not clear. This work is devoted to the magnetic and transport properties of the La Ba MnO single crystal because there is little information on the properties of the La Ba MnO family. The sample has been grown by \V V * Corresponding author. Tel.: #7-3432-444471; fax: #73432-745244. E-mail address: [email protected] (V.V. Ustinov).
#oating zone technique. At +190 K, a transition from low temperature orthorhombic (Pbnm) to rhombohedral (R3 c) phase occurs. The peculiarities of physical properties connected with the structural phase transition will be published elsewhere [6]; in this paper we focus on the vicinity of the Curie point where the CMR is observed. Magnetization, resistivity , Hall e!ect, and thermopower S were measured from 77 to 400 K in magnetic "elds up to 15 kOe. The measurements were performed on as-grown sample and after the annealing of the sample in #owing oxygen for 40 h at 9503C. The analysis of the magnetization curves has led us to conclusion that the crystal is inhomogeneous and about 10% of its volume is not ferromagnetic. The Curie temperature, determined through Arrott-Belov curves, was 251 K and has been increased by the annealing only by 1 K. The temperature dependence of is shown in Fig. 1. The resistivity decreases with increasing ¹ from 80 to 150 K, then behaves as in a metal (d/d¹'0) and reaches maximum at ¹ '¹ . The annealing slightly 0 ! increases the peak resistivity and shifts the peak to a higher temperature. Well above ¹ , follows thermal! activated behavior with "1.5 m cm before and "1.7 m cm after the annealing. The activation en ergy depends on the relative magnetization m as
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 1 3 9 5 - 0
912
V.V. Ustinov et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 911}913
Fig. 1. The resistivity of the La Ba MnO single crystal. The inset: the temperature dependence of relative magnetization m(H"10 kOe).
Fig. 2. Thermopower of the as-grown sample.
EM"EM ! m, that agrees with the activation to mo bility-edge theory [7]. The parameters are EM "1120 K and "1100 K and do not depend on the annealing. We calculated (¹, H) using the experimental data for m. An example for H"10 kOe is shown in Fig. 1. We take into account the 10% inhomogeneity of our sample [8] and multiply the experimental m by 1.1. In other words, we assume the charge carriers feel the magnetization of ferromagnetic part of the sample rather than the averaged magnetization. The calculated curve agrees well with the experimental not only in paramagnetic state but also in ferromagnetic one above 150 K. Since EM ' , the activation energy is positive even in the ferromagnetic state far below ¹ . This implies that the crystal is an ! insulator, both above and below ¹ , and hence x '0.2. ! The rapid increase of near ¹ is caused by the decrease ! of EM, and is not necessarily related to the metallic conductivity.
Fig. 3. Normal Hall coe$cient of the as-grown sample.
Fig. 2 shows the S}¹ curves. The thermopower as well as the Hall e!ect were practically una!ected by the annealing, so we show the data for the as-grown sample only. At low temperatures S is negative, then changes the sign around 210 K. Far above ¹ , it follows the relation ! S"S #(k /e)(E1/¹). We suppose that thermopower activation energy can be expressed as E1"E1 !E1 m, "nd E1 , E1 , and S in the paramagnetic region (E1 "190 K, E1 "180 K, S "!38 V/K) and then calculated S(¹, H) in the same manner as in the previous paragraph. Near ¹ , the calculated curve reproduces the ! main features of the experimental one. Since OE1 , the di!erence EM!E1 strongly depends on magnetization, in what follows that the wave functions of nearestneighbor Mn ions overlap and the singlesite polaron approach is inadequate. In Fig. 3 we present the temperature dependence of normal Hall coe$cient R over the range where we can separate normal and anomalous Hall e!ects. At low temperatures, R is negative and small in value. When ¹'200 K, R is positive and takes the maximum at the same temperature as . The inset in Fig. 3 shows that near ¹ , the Hall mobility, , is approximately ! & constant and about 0.07 cm/V/s. According to Ref. [9], these facts indicate that in the vicinity of T the conduct! ivity is due to activation of holes to mobility edge. In low temperature region, however, other mechanisms, perhaps of hopping type, is likely to play the key role. In summary, we have shown that the activation to mobility edge dominates near ¹ and explain the main ! features of the temperature dependence of the resistivity, the thermopower, and normal Hall coe$cient in La Ba MnO ; the critical concentration x exceeds 0.2 for La Ba MnO family. \V V The work was supported by RFBR grants 00-02-17544 and 00-15-96745.
V.V. Ustinov et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 911}913
References [1] J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys. 48 (1999) 167. [2] Y. Tokura, Y. Tomioka, J. Magn. Magn. Mater. 200 (1999) 1. [3] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [4] W.E. Pickett, D.J. Singh, J. Magn. Magn. Mater. 172 (1997) 237. [5] E.A. Livesay, R.N. West, S.B. Dugdale, G. Santi, T. Jarlborg, J. Phys.: Condens. Matter 11 (1999) L279.
913
[6] V.E. Arkhipov, N.G. Bebenin, V.P. Dyakina, V.S. Gaviko, A.V. Korolev, V.V. Mashkautsan, E.A. Neifeld, R.I. Zainullina, Ya.M. Mukovskii, D.A. Shulyatev, Phys. Rev. B 61 (2000) 11229. [7] N.G. Bebenin, V.V. Ustinov, J. Phys.: Condens. Matter 10 (1998) 6301. [8] N.G. Bebenin, R.I. Zainullina, V.V. Mashkautsan, V.S. Gaviko, V.V. Ustinov, Ya.M. Mukovskii, D.A. Shulyatev, JETP 90 (2000) 1027. [9] N.F. Mott, E.A. Davis, Electronic Processes in Noncrystalline Materials, 2nd Edition, Clarendon Press, Oxford, 1979.