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Magnetic field dependence of electrical resistivity in fine grain La0.75Ca0.25MnO3
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932 www.elsevier.com/locate/jmmm
Magnetic field dependence of electrical resistivity in fine grain La0.75Ca0.25MnO3 M. P˛eka"aa,, V. Drozdb, J. Muchac a
Department of Chemistry, Warsaw University, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland b Department of Chemistry, Kiev State University, 60 Volodymyrska, 01033 Kiev, Ukraine c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroc!aw, Poland Available online 9 December 2004
Abstract Structural and magneto-transport studies show that a shift of the electrical resistivity peak to lower temperature is due to a fine grain structure of the La0.75Ca0.25MnO3 manganite. An influence of external magnetic field on transport processes is analysed. r 2004 Elsevier B.V. All rights reserved. PACS: 75.47.Gk; 75.47.Lx; 81.20.Fw; 75.50.Tt Keywords: Magnetoresistance; Perovskite structure; Magnetization; Thermal conductivity
1. Introduction
2. Experimental
Colossal magnetoresistance effect in manganites attracts scientific interest both for the basic research and promising applications. A coupling between the spin, charge and lattice systems leads to a rich-phase diagram and involves high sensitivity to external magnetic field and lattice distortion [1]. Magnetic interaction and conduction processes are known to strongly depend on deviation of the Mn–O–Mn bond angles from 1801. This paper reports an experimental study of interplay between microstructure and magnetotransport properties for the fine grain polycrystalline La0.75Ca0.25MnO3, performed by electrical resistivity, magnetization and thermal conductivity measurements in magnetic field up to 8 T. Applicability of various transport models is analysed.
The La0.75Ca0.25MnO3 samples were synthesized using carbonate precursor method similar to that reported by Aselage et al. [2]. Ammonium carbonate, (NH4)2CO3, was used as the precipitator. Aqueous solutions (with concentrations of about 0.2 M) of La(NO3)3, Ca(NO3)2 and Mn(NO3)2 were chosen as starting reagents. Their concentrations were determined by trilonometric titration. Ammonium carbonate with 15% excess (in the molar ratio to the sum of the moles of cations in the solution) was slowly added to the appropriately mixed solutions of the metal nitrates. The precipitates were held for 24 h in the mother solution, then filtrated and rinsed sequentially by a distilled water and ethanol. The powders obtained were dried in air at 80 1C. Carbonate precursor thermal decomposition was studied by IR-spectroscopy and XRD analysis on the samples of precursor annealed for 2 h at different temperatures from 500 to 1000 1C with the step of 100 1C (Fig. 1). It was found that the decomposition of
Corresponding author. Tel.: +48 22 822 0211; fax: +48 22 822 5996/8230123. E-mail address: [email protected] (M. P˛eka"a).
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.292
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932
1000°C
900°C 800°C
700°C 600°C 500°C carbonate precursor
50
45
40
Relative transmittance (arb. units)
(a)
25
20
15
1000°C × 2h 900°C × 2h 800°C × 2h 700°C × 2h 600°C × 2h 500°C × 2h
M-O
precursor 2-
2CO 3
2000 (b)
35 30 2
CO 3
1500 1000 Wavenumber, cm-1
929
titration assuming that the sample has initial cation stoichiometry. Electrical resistivity was measured by a four-probe method in magnetic fields up to 8 T. Thermal conductivity was measured on the same sample using the steady heat flow method with the temperature gradient about 1 K/cm created by a milliWatt heater [3]. The DC magnetization and AC susceptibility measurements were carried out with an accuracy better than 0.1%.
3. Results and discussion The crystal lattice parameters determined from XRD were found to be a ¼ 5.471(4) A˚, b ¼ 5.475(3) A˚, c ¼ 7.738(4) A˚ for the samples studied. These values are very close to those reported by Aselage et al. [2]. Oxygen stoichiometry index was found to be 2.997. Grain size evaluated by XRD measurements was less than 1 mm. It is lower than in the samples prepared by the traditional ceramic method. Absolute values of electrical resistivity at room temperature are about 1 mO m and weakly depend on magnetic field. They are close to values reported for polycrystalline samples of the same composition [4]. Temperature variation of electrical resistivity (Fig. 2) is characteristic for perovskite manganites. The resistivity peak shifts from 136 to 162 K and gradually diminishes while a magnetic field varies from zero to 8 T. The resistivity peak is located at relatively lower temperatures TP as compared to temperatures found for polycrystalline manganite with the same composition [4]. The even greater reduction of TP was also reported
500 130.0 B=0 0.2 T 0.5 T 1T 2T 4T 6T 8T
120.0
Fig. 1. X-ray diffraction patterns (a) and IR-spectra (b) of carbonate precursor evolution with temperature.
110.0
carbonates finishes at 800 1C. At the same temperature, formation of perovskite phase is observed. Pure perovskite phase can be obtained after several hours annealing of the precursor at 900 1C. Carbonate precipitates were converted to La0.75 Ca0.25MnO3 by slowly heating in the air atmosphere up to 810 1C with the subsequent annealing at this temperature for several hours. Finally, the sample was ground, pressed into pellets and annealed at 1200 1C for 24 h in air and then furnace cooled to room temperature. According to XRD carried out with DRON-3 diffractometer using CuKa radiation the sample was single phase. Its XRD pattern was indexed in orthorhombically distorted perovskite-type unit cell. The oxygen content was determined by the iodometric
RESISTANCE (OHM)
100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0
30
60
90
120 150 180 210 240 270 300 T (K)
Fig. 2. Temperature variation of electrical resistance at various magnetic fields.
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932
2.0 1.8
MAGNETIZATION (A m2 / kg)
for La0.75Ca0.25MnO3, where the transition temperature decreases from 221 to 123 K, when the oxygen content is reduced from y ¼ 2.989 to 2.85 [2]. Metal-insulator transition temperature, TP, in the manganites is known to be governed by several factors. Mn3+/Mn4+ ratio is responsible for the hole carrier density. This ratio is controlled by the concentration of divalent constituent M (Ca, Sr, Ba, Pb) in A1xMxMnO3 (where A stands for rare earth element) as well as by oxygen content. Numerous publications were devoted to the elucidation of the influence of average ionic radius of cations at A site in AMnO3, /rAS, on the transport and magnetic properties of CMR manganites. Average ionic radius /rAS determining the Mn–O–Mn bond length and angle, affects strongly the overlapping of manganese and oxygen orbitals. The transition temperature is found to decrease with a decreasing radius /rAS [5]. For the samples studied the chemical stoichiometry was carefully checked in order to exclude the large oxygen deficit. Moreover, a possible influence of structural deformation plays a negligible role since the distortion coefficient D, determined by deviations of the Mn–O bond lengths [6], is as small as 0.03%. Among other factors affecting Tc in manganites, microstructural inhomogeneities [7] and substitution for Mn with transition and p-metals that suppress Tc [8,9], are worth being mentioned. The most plausible explanation for the reduced transition temperature may involve a small grain size of the sample studied and microstructural inhomogeneities of oxygen distribution between and/or inside the grains. A reduction of grain size enhances the surface-tovolume ratio and leads to higher structural disorder within a surface layer, which in turn suppresses the magnetic exchange interaction. Thus the nanocrystalline manganites usually exhibit lower Tc. It was also reported for nanocrystalline manganites that a decrease in grain size causes a drop in Curie temperature Tc [10]. For the manganite with composition La0.7Ca0.3MnO3 the resistivity peak was shifted from 250 to 150 K, when the grains sizes were suppressed to 25 nm [11,12]. This conclusion is additionally supported by the reduced values of Curie temperature, as determined from magnetic measurements. The temperature variations of the DC magnetization are shown in Fig. 3. There is a small difference below TC between the field-cooled (FC) and zero-field-cooled (ZFC) magnetization at magnetic field of 100 Oe. The difference becomes remarkable for 300 Oe. The temperature dependence of the in-phase AC magnetic susceptibility measured at 10 Oe is shown in Fig. 4 along with the out-of-phase component. The Curie temperatures TC defined by the maximum temperature derivative prove that the material is ferromagnetic below 17772 K.
300 Oe
1.6 1.4 1.2 1.0
100 Oe 0.8 0.6
ZFC
0.4 0.2 0.0 0
30
60
90 120 150 180 210 240 270 300 330
T (K)
Fig. 3. Zero-field cooled and field cooled magnetization versus temperature.
0.06
MAGNETIC SUSCEPTIBILITY (a.u.)
930
χ' 0.05
0.04
0.03
0.02
10 x χ" 0.01
0.00 0
30
60
90 120 150 180 210 240 270 300 330 T (K)
Fig. 4. Magnetic susceptibility versus temperature.
The negative magneto-resistance effect becomes more pronounced in higher magnetic fields and reaches 95% at 8 T field, as shown in Fig. 5. The maximum of magneto-resistance shifts from about 90 K at zero field to 135 K for 8 T. It is remarkable that at low temperatures the magneto-resistance does not tend to zero, which additionally confirms that the grain sizes are small [10]. For the temperature range above the resistivity peak, where electrical resistivity behaves as in semiconductors,
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932 0.0
931
0.15
-10.0
0.12
-30.0
ACTIVATION ENERGY (eV)
MAGNETO - RESISTANCE (%)
-20.0
-40.0 -50.0 -60.0 0.2 T 0.5 T 1T 2T 4T 6T 8T
-70.0 -80.0 -90.0
30
60
90
120 150 180 210 240 270 300 T (K)
Fig. 5. Temperature variation of magneto-resistance at various magnetic fields.
the activation energy EA was determined from resistivity variation. One may notice that in zero field values of EA rise gradually up to 150 meV at room temperature. When applying a magnetic field, plots of EA shift towards higher temperatures and slightly lower values, as shown for B ¼ 8 T (for clarity) in Fig. 6. In order to get more insight into the transport processes the electrical resistivity data were fitted to various models applied for magneto-resistive materials, including rAC ¼ r0 exp ðE A =kTÞ;
(1)
rA ¼ AT exp ðE A =kTÞ;
(2)
rB ¼ AT1=2 exp ðE A =kTÞ;
(3)
rM ¼ r0 exp ðT 0 =TÞ1=4 ;
(4)
expressions for thermal activation, adiabatic polaron, bipolaron and variable range hopping models, respectively. Results of the fitting are summarized in Table 1 and show that for zero field the best correlation is found for the variable range hopping model. However, when a magnetic field up to 8 T is applied the correlation coefficient R gradually diminishes for all the models studied. One may notice that a reduction in R is the lowest for the adiabatic polaron case. The electrical resistivity in the metallic range was fitted to the expression rL ðTÞ ¼ rL0 þ AT N ;
8T 0.06
0.03
-100.0 0
0T 0.09
(5)
where rL0 corresponds to electrical resistivity at 10 K. In the magnetic field rising from zero to 8 T, the exponent
0.00 140
160
180
200
220 T (K)
240
260
280
300
Fig. 6. Activation energy determined from electrical resistivity at zero and 8 T magnetic fields.
N varies monotonically from 3.66 down to 2.74, as listed in Table 1. A mutual relation between the magnetic and transport processes was also studied by thermal conductivity plotted in Fig. 7. In zero magnetic the low-temperature field thermal conductivity varies proportionally to T0.46 and achieves a maximum at 30 K. At magnetic field of 8 T the maximum is 10% lower and located at 40 K. When passing to higher temperatures thermal conductivity maximum at 60 and 70 K, for zero and 8 T, respectively are followed by the monotonic increase observed up to room temperature. The difference between thermal conductivities at zero and 8 T field reveals the additional thermal resistivity caused by the magnetic field affecting heat carriers. The slope of thermal conductivity variation changes at the temperature, which may be determined from temperature derivatives of thermal conductivity as 140–150 K. This temperature corresponds to the resistivity peak at metal—insulator transition. The total thermal conductivity of manganites may be expressed as a sum ltot ¼ lph þ le þ lp þ lm
(6)
of the phonon, electron, polaron and magnon contributions, respectively. A sum of the electron and polaron conductivities calculated according to the Wiedemann– Franz relation [13] is plotted in Fig. 7 for the zero and 8 T cases. One may see that the phonon conductivity is overwhelming in the manganite studied, which is typically observed in manganites and cobaltites [3,14]. It is worth noticing that the electron/polaron contribution considerably increases especially below 140 K, when
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932
932
Table 1 Parameters determined from electrical resistivity data at various magnetic fields Magnetic field
N exponent
T 0 0.2 0.5 1 2 4 6 8
3.66 3.63 3.39 3.27 3.14 3.09 2.80 2.74
Correlation coefficient R for Thermal activation
Adiabatic polaron
Bipolaron
Variable range hopping
107 K
.9962387 .9960746 .99562538 .99556585 .9932788 .99218768 .98656536 .98129464
.99825262 .99786835 .99779321 .9977168 .99637811 .99586181 .9946739 .99328722
.99637996 .99648692 .99602914 .99609922 .99413642 .99321045 .98920543 .98582276
.99928283 .99929908 .99908124 .99897305 .99809219 .99707078 .99220643 .98741347
8.0 8.0 7.3 6.4 4.8 3.9 1.6 1.0
Fund—Foundation for the Promotion of Science (Poland) for the support during his stay in Warsaw.
4.0
THERMAL CONDUCTIVITY (W / Km)
T0 (VRH)
3.0
References 0T 2.0
8T
1.0
Ke - 8 T Ke - 0 T
0.0 0
30
60
90
120 150 180 210 240 270 300 T (K)
Fig. 7. Temperature variation of thermal conductivity.
passing from zero to 8 T magnetic field. This effect is due to the ordering of magnetic moments by external field, which in turn reduces the electron scattering. An influence of magnetic field on electron/polaron conductivity is relatively weaker above 140 K, where polarons play a dominant role.
Acknowledgements Work supported in parts by NATO Grants (PST.CLG.979446 & PST.MEM.CLG.980654), SPUB145 and Warsaw University Scientific Exchange Program. One of the authors (V.D.) thanks the Mianowski
[1] A.P. Ramirez, J. Phys.: Condens. Matter 9 (1997) 8171. [2] T.L. Aselage, D. Emin, S.S. McCready, E.L. Venturini, M.A. Rodriguez, J.A. Voigt, T.J. Headley, Phys. Rev. B 68 (2003) 134448. [3] J. Mucha, S. Dorbolo, H. Bougrine, K. Durczewski, M. Ausloos, Cryogenics 44 (2004) 145. [4] P. Schiffer, A.P. Ramirez, W. Bao, S.-W. Cheong, Phys. Rev. Lett. 75 (1995) 3336. [5] B. Raveau, A. Maignan, C. Martin, M. Hervieu, in: C.N.R. Rao, B. Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering, and Related Properties of Manganese Oxides, World Scientific, Singapore, 1998. [6] J.L. Cohn, J.J. Neumeier, C.P. Popoviciu, K.J. McClellan, Th. Leventouri, Phys. Rev. B 56 (1997) 8495. [7] O.A. Shlyakhtin, Y. Oh, Yu.D. Tretyakov, Solid State Commun. 111 (1999) 711. [8] R. Bathe, S.R. Shinde, K.M. Gapchup, K.P. Adhi, S.I. Patil, J. Magn. Magn. Mater. 256 (2003) 425. [9] K. Ghosh, S.B. Ogale, R. Ramesh, R.L. Greene, T. Venkatesan, Phys. Rev. B 59 (1999) 533. [10] T. Yi, S. Gao, X. Qi, Y. Zhu, F. Cheng, B. Ma, Y. Huang, C. Liao, C. Yan, J. Phys. Chem. Solids 61 (2000) 1407. [11] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, Appl. Phys. Lett. 68 (1996) 2291. [12] R. Mahendiran, R. Mahesh, A.K. Raychaudhuri, C.N.R. Rao, Solid State Commun. 99 (1996) 149. [13] F.J. Blatt, Physics of Electronic Conduction in Solids, McGraw-Hill, New York, 1968. [14] D.D. Khalyavin, M. Pekala, G.L. Bychkov, S.V. Shiryaev, S.N. Barilo, I.O. Troyanchuk, J. Mucha, H. Misiorek, R. Szymczak, M. Baran, H. Szymczak, J. Phys.: Condens. Matter 15 (2003) 925.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932 www.elsevier.com/locate/jmmm
Magnetic field dependence of electrical resistivity in fine grain La0.75Ca0.25MnO3 M. P˛eka"aa,, V. Drozdb, J. Muchac a
Department of Chemistry, Warsaw University, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland b Department of Chemistry, Kiev State University, 60 Volodymyrska, 01033 Kiev, Ukraine c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroc!aw, Poland Available online 9 December 2004
Abstract Structural and magneto-transport studies show that a shift of the electrical resistivity peak to lower temperature is due to a fine grain structure of the La0.75Ca0.25MnO3 manganite. An influence of external magnetic field on transport processes is analysed. r 2004 Elsevier B.V. All rights reserved. PACS: 75.47.Gk; 75.47.Lx; 81.20.Fw; 75.50.Tt Keywords: Magnetoresistance; Perovskite structure; Magnetization; Thermal conductivity
1. Introduction
2. Experimental
Colossal magnetoresistance effect in manganites attracts scientific interest both for the basic research and promising applications. A coupling between the spin, charge and lattice systems leads to a rich-phase diagram and involves high sensitivity to external magnetic field and lattice distortion [1]. Magnetic interaction and conduction processes are known to strongly depend on deviation of the Mn–O–Mn bond angles from 1801. This paper reports an experimental study of interplay between microstructure and magnetotransport properties for the fine grain polycrystalline La0.75Ca0.25MnO3, performed by electrical resistivity, magnetization and thermal conductivity measurements in magnetic field up to 8 T. Applicability of various transport models is analysed.
The La0.75Ca0.25MnO3 samples were synthesized using carbonate precursor method similar to that reported by Aselage et al. [2]. Ammonium carbonate, (NH4)2CO3, was used as the precipitator. Aqueous solutions (with concentrations of about 0.2 M) of La(NO3)3, Ca(NO3)2 and Mn(NO3)2 were chosen as starting reagents. Their concentrations were determined by trilonometric titration. Ammonium carbonate with 15% excess (in the molar ratio to the sum of the moles of cations in the solution) was slowly added to the appropriately mixed solutions of the metal nitrates. The precipitates were held for 24 h in the mother solution, then filtrated and rinsed sequentially by a distilled water and ethanol. The powders obtained were dried in air at 80 1C. Carbonate precursor thermal decomposition was studied by IR-spectroscopy and XRD analysis on the samples of precursor annealed for 2 h at different temperatures from 500 to 1000 1C with the step of 100 1C (Fig. 1). It was found that the decomposition of
Corresponding author. Tel.: +48 22 822 0211; fax: +48 22 822 5996/8230123. E-mail address: [email protected] (M. P˛eka"a).
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.292
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932
1000°C
900°C 800°C
700°C 600°C 500°C carbonate precursor
50
45
40
Relative transmittance (arb. units)
(a)
25
20
15
1000°C × 2h 900°C × 2h 800°C × 2h 700°C × 2h 600°C × 2h 500°C × 2h
M-O
precursor 2-
2CO 3
2000 (b)
35 30 2
CO 3
1500 1000 Wavenumber, cm-1
929
titration assuming that the sample has initial cation stoichiometry. Electrical resistivity was measured by a four-probe method in magnetic fields up to 8 T. Thermal conductivity was measured on the same sample using the steady heat flow method with the temperature gradient about 1 K/cm created by a milliWatt heater [3]. The DC magnetization and AC susceptibility measurements were carried out with an accuracy better than 0.1%.
3. Results and discussion The crystal lattice parameters determined from XRD were found to be a ¼ 5.471(4) A˚, b ¼ 5.475(3) A˚, c ¼ 7.738(4) A˚ for the samples studied. These values are very close to those reported by Aselage et al. [2]. Oxygen stoichiometry index was found to be 2.997. Grain size evaluated by XRD measurements was less than 1 mm. It is lower than in the samples prepared by the traditional ceramic method. Absolute values of electrical resistivity at room temperature are about 1 mO m and weakly depend on magnetic field. They are close to values reported for polycrystalline samples of the same composition [4]. Temperature variation of electrical resistivity (Fig. 2) is characteristic for perovskite manganites. The resistivity peak shifts from 136 to 162 K and gradually diminishes while a magnetic field varies from zero to 8 T. The resistivity peak is located at relatively lower temperatures TP as compared to temperatures found for polycrystalline manganite with the same composition [4]. The even greater reduction of TP was also reported
500 130.0 B=0 0.2 T 0.5 T 1T 2T 4T 6T 8T
120.0
Fig. 1. X-ray diffraction patterns (a) and IR-spectra (b) of carbonate precursor evolution with temperature.
110.0
carbonates finishes at 800 1C. At the same temperature, formation of perovskite phase is observed. Pure perovskite phase can be obtained after several hours annealing of the precursor at 900 1C. Carbonate precipitates were converted to La0.75 Ca0.25MnO3 by slowly heating in the air atmosphere up to 810 1C with the subsequent annealing at this temperature for several hours. Finally, the sample was ground, pressed into pellets and annealed at 1200 1C for 24 h in air and then furnace cooled to room temperature. According to XRD carried out with DRON-3 diffractometer using CuKa radiation the sample was single phase. Its XRD pattern was indexed in orthorhombically distorted perovskite-type unit cell. The oxygen content was determined by the iodometric
RESISTANCE (OHM)
100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0
30
60
90
120 150 180 210 240 270 300 T (K)
Fig. 2. Temperature variation of electrical resistance at various magnetic fields.
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932
2.0 1.8
MAGNETIZATION (A m2 / kg)
for La0.75Ca0.25MnO3, where the transition temperature decreases from 221 to 123 K, when the oxygen content is reduced from y ¼ 2.989 to 2.85 [2]. Metal-insulator transition temperature, TP, in the manganites is known to be governed by several factors. Mn3+/Mn4+ ratio is responsible for the hole carrier density. This ratio is controlled by the concentration of divalent constituent M (Ca, Sr, Ba, Pb) in A1xMxMnO3 (where A stands for rare earth element) as well as by oxygen content. Numerous publications were devoted to the elucidation of the influence of average ionic radius of cations at A site in AMnO3, /rAS, on the transport and magnetic properties of CMR manganites. Average ionic radius /rAS determining the Mn–O–Mn bond length and angle, affects strongly the overlapping of manganese and oxygen orbitals. The transition temperature is found to decrease with a decreasing radius /rAS [5]. For the samples studied the chemical stoichiometry was carefully checked in order to exclude the large oxygen deficit. Moreover, a possible influence of structural deformation plays a negligible role since the distortion coefficient D, determined by deviations of the Mn–O bond lengths [6], is as small as 0.03%. Among other factors affecting Tc in manganites, microstructural inhomogeneities [7] and substitution for Mn with transition and p-metals that suppress Tc [8,9], are worth being mentioned. The most plausible explanation for the reduced transition temperature may involve a small grain size of the sample studied and microstructural inhomogeneities of oxygen distribution between and/or inside the grains. A reduction of grain size enhances the surface-tovolume ratio and leads to higher structural disorder within a surface layer, which in turn suppresses the magnetic exchange interaction. Thus the nanocrystalline manganites usually exhibit lower Tc. It was also reported for nanocrystalline manganites that a decrease in grain size causes a drop in Curie temperature Tc [10]. For the manganite with composition La0.7Ca0.3MnO3 the resistivity peak was shifted from 250 to 150 K, when the grains sizes were suppressed to 25 nm [11,12]. This conclusion is additionally supported by the reduced values of Curie temperature, as determined from magnetic measurements. The temperature variations of the DC magnetization are shown in Fig. 3. There is a small difference below TC between the field-cooled (FC) and zero-field-cooled (ZFC) magnetization at magnetic field of 100 Oe. The difference becomes remarkable for 300 Oe. The temperature dependence of the in-phase AC magnetic susceptibility measured at 10 Oe is shown in Fig. 4 along with the out-of-phase component. The Curie temperatures TC defined by the maximum temperature derivative prove that the material is ferromagnetic below 17772 K.
300 Oe
1.6 1.4 1.2 1.0
100 Oe 0.8 0.6
ZFC
0.4 0.2 0.0 0
30
60
90 120 150 180 210 240 270 300 330
T (K)
Fig. 3. Zero-field cooled and field cooled magnetization versus temperature.
0.06
MAGNETIC SUSCEPTIBILITY (a.u.)
930
χ' 0.05
0.04
0.03
0.02
10 x χ" 0.01
0.00 0
30
60
90 120 150 180 210 240 270 300 330 T (K)
Fig. 4. Magnetic susceptibility versus temperature.
The negative magneto-resistance effect becomes more pronounced in higher magnetic fields and reaches 95% at 8 T field, as shown in Fig. 5. The maximum of magneto-resistance shifts from about 90 K at zero field to 135 K for 8 T. It is remarkable that at low temperatures the magneto-resistance does not tend to zero, which additionally confirms that the grain sizes are small [10]. For the temperature range above the resistivity peak, where electrical resistivity behaves as in semiconductors,
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932 0.0
931
0.15
-10.0
0.12
-30.0
ACTIVATION ENERGY (eV)
MAGNETO - RESISTANCE (%)
-20.0
-40.0 -50.0 -60.0 0.2 T 0.5 T 1T 2T 4T 6T 8T
-70.0 -80.0 -90.0
30
60
90
120 150 180 210 240 270 300 T (K)
Fig. 5. Temperature variation of magneto-resistance at various magnetic fields.
the activation energy EA was determined from resistivity variation. One may notice that in zero field values of EA rise gradually up to 150 meV at room temperature. When applying a magnetic field, plots of EA shift towards higher temperatures and slightly lower values, as shown for B ¼ 8 T (for clarity) in Fig. 6. In order to get more insight into the transport processes the electrical resistivity data were fitted to various models applied for magneto-resistive materials, including rAC ¼ r0 exp ðE A =kTÞ;
(1)
rA ¼ AT exp ðE A =kTÞ;
(2)
rB ¼ AT1=2 exp ðE A =kTÞ;
(3)
rM ¼ r0 exp ðT 0 =TÞ1=4 ;
(4)
expressions for thermal activation, adiabatic polaron, bipolaron and variable range hopping models, respectively. Results of the fitting are summarized in Table 1 and show that for zero field the best correlation is found for the variable range hopping model. However, when a magnetic field up to 8 T is applied the correlation coefficient R gradually diminishes for all the models studied. One may notice that a reduction in R is the lowest for the adiabatic polaron case. The electrical resistivity in the metallic range was fitted to the expression rL ðTÞ ¼ rL0 þ AT N ;
8T 0.06
0.03
-100.0 0
0T 0.09
(5)
where rL0 corresponds to electrical resistivity at 10 K. In the magnetic field rising from zero to 8 T, the exponent
0.00 140
160
180
200
220 T (K)
240
260
280
300
Fig. 6. Activation energy determined from electrical resistivity at zero and 8 T magnetic fields.
N varies monotonically from 3.66 down to 2.74, as listed in Table 1. A mutual relation between the magnetic and transport processes was also studied by thermal conductivity plotted in Fig. 7. In zero magnetic the low-temperature field thermal conductivity varies proportionally to T0.46 and achieves a maximum at 30 K. At magnetic field of 8 T the maximum is 10% lower and located at 40 K. When passing to higher temperatures thermal conductivity maximum at 60 and 70 K, for zero and 8 T, respectively are followed by the monotonic increase observed up to room temperature. The difference between thermal conductivities at zero and 8 T field reveals the additional thermal resistivity caused by the magnetic field affecting heat carriers. The slope of thermal conductivity variation changes at the temperature, which may be determined from temperature derivatives of thermal conductivity as 140–150 K. This temperature corresponds to the resistivity peak at metal—insulator transition. The total thermal conductivity of manganites may be expressed as a sum ltot ¼ lph þ le þ lp þ lm
(6)
of the phonon, electron, polaron and magnon contributions, respectively. A sum of the electron and polaron conductivities calculated according to the Wiedemann– Franz relation [13] is plotted in Fig. 7 for the zero and 8 T cases. One may see that the phonon conductivity is overwhelming in the manganite studied, which is typically observed in manganites and cobaltites [3,14]. It is worth noticing that the electron/polaron contribution considerably increases especially below 140 K, when
ARTICLE IN PRESS M. P˛eka!a et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 928–932
932
Table 1 Parameters determined from electrical resistivity data at various magnetic fields Magnetic field
N exponent
T 0 0.2 0.5 1 2 4 6 8
3.66 3.63 3.39 3.27 3.14 3.09 2.80 2.74
Correlation coefficient R for Thermal activation
Adiabatic polaron
Bipolaron
Variable range hopping
107 K
.9962387 .9960746 .99562538 .99556585 .9932788 .99218768 .98656536 .98129464
.99825262 .99786835 .99779321 .9977168 .99637811 .99586181 .9946739 .99328722
.99637996 .99648692 .99602914 .99609922 .99413642 .99321045 .98920543 .98582276
.99928283 .99929908 .99908124 .99897305 .99809219 .99707078 .99220643 .98741347
8.0 8.0 7.3 6.4 4.8 3.9 1.6 1.0
Fund—Foundation for the Promotion of Science (Poland) for the support during his stay in Warsaw.
4.0
THERMAL CONDUCTIVITY (W / Km)
T0 (VRH)
3.0
References 0T 2.0
8T
1.0
Ke - 8 T Ke - 0 T
0.0 0
30
60
90
120 150 180 210 240 270 300 T (K)
Fig. 7. Temperature variation of thermal conductivity.
passing from zero to 8 T magnetic field. This effect is due to the ordering of magnetic moments by external field, which in turn reduces the electron scattering. An influence of magnetic field on electron/polaron conductivity is relatively weaker above 140 K, where polarons play a dominant role.
Acknowledgements Work supported in parts by NATO Grants (PST.CLG.979446 & PST.MEM.CLG.980654), SPUB145 and Warsaw University Scientific Exchange Program. One of the authors (V.D.) thanks the Mianowski
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