- Email: [email protected]
Low-field magneto-transport property of La0.5Sr0.5MnO3 thin films deposited at low temperature by laser ablation
August 2001
Materials Letters 50 Ž2001. 97–102 www.elsevier.comrlocatermatlet
Low-field magneto-transport property of La 0.5 Sr0.5 MnO 3 thin films deposited at low temperature by laser ablation J.-M. Liu a,) , Q. Huang b, J. Li b, C.K. Ong b, X.Y. Chen a , Z.G. Liu a , Y.W. Du a a
Laboratory of Solid State Microstructures, Faculty of Sciencer Department of Physics, Nanjing UniÕersity, Hankou Road No. 22, Microstructures, Nanjing 210093, China b Department of Physics, National UniÕersity of Singapore, Singapore 119260, China Received 6 October 2000; accepted 14 November 2000
Abstract The low-field magneto-transport property of polycrystalline La 0.5 Sr0.5 MnO 3 ŽLSMO. thin films deposited on quartz wafers by pulsed laser ablation at various temperatures is investigated. The film is of mixed amorphous and polycrystalline microstructure, depending on the depositing temperature. The low-field magneto-transport property of the thin films can be explained in terms of the two-channel model where the insulating channels and metallic channels coexist. Enhanced low-field magnetoresistance for the samples deposited from 5708C to 6008C in which nano-crystal phase is embedded in the amorphous matrix is demonstrated. q 2001 Elsevier Science B.V. All rights reserved. PACS: 75.50.Cn; 72.60.Tg; 73.50.Jt Keywords: Doped manganite oxides; Low-field magnetoresistance; Laser ablation
The problem of enhanced low-field magnetoresistance ŽLFMR. in manganese perovskite oxides La 1 y x A x MnO 3 y y ŽA s divalent cation such as Ca, Sr, Ba. has recently attracted special attention w1,2x. Although there has been for thin film samples recorded very high MR ratio ŽMR s w r Ž0. y r Ž H .xrr Ž0., where r is the sample resistivity and H is the applied magnetic field. at high field, the LFMR ratio remains yet small. The microstructure exploitation w3–6x for La 1y x Ca x MnO 3 ŽLCMO. or La 1y x Sr x MnO 3 ŽLSMO. where microscopic defects or spin-disordered media are introduced does show
) Corresponding author. Tel.: q86-25-359-5979; fax: q86-25359-5535. E-mail address: [email protected] ŽJ.-M. Liu..
enhanced LFMR w1x and reveals a number of new effects in due course. The intrinsic MR effect in these materials is believed to result from the doubleexchanging sequence w7x where the ferromagnetic transition and insulating-metal transition occur almost concurrently at Tc and Tm , respectively. The enhanced LFMR in polycrystalline samples is, however, extrinsic and attributed to the existence of interfaces, grain boundaries or insulating second phases w8x, so that spin-polarized tunneling ŽSPT. w2x or spin-dependent scattering ŽSDS. w1x across these interfaces or boundaries produces MR under external field. In this letter, we investigate the microstructural and magneto-transport property of LSMO thin films deposited at various temperatures Ts . In our work, LSMO Ž x s 0.5., not far from the ferromagneticrcharge-ordering ŽCO. boundary w9–11x, is
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 2 3 - 7
98
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
Fig. 1. XRD u –2u spectra for a series of LSMO thin films deposited at different Ts as numerically indicated. The spectrum of ceramic LSMO target is inserted too.
chosen for study. It is our scheme to introduce amorphousrcrystalline interface, grain boundaries and if possible, charge-ordering phase coexisting in the sample, which may be beneficial to the LFMR enhancement, although no CO region in our samples is observed at temperature T ) 270 K. The LSMO Ž x s 0.5. thin films were deposited using pulsed laser deposition ŽPLD. on quartz wafers so that a mixture of amorphous and polycrystalline phases was obtained. The PLD experiment was performed by utilizing KrF excimer laser of wavelength of 248 nm and pulse width of 30 ns, detail of which was reported earlier w11,12x. The optimized laser fluency density of 1.2 Jrcm2 and reprate of 5 Hz were used during the ablation. The oxygen ambient pressure of 0.25 mbar was kept. The films are of 800 nm in thickness and 10 = 1.5 mm2 in in-plane dimension. The crystallization and grain size of the films were controlled by varying Ts from 4008C to 7108C, and checked by X-ray diffraction ŽXRD.. The microstructures were investigated by the plane-view high-resolution transmission electron microscopy ŽTEM. performed at 300 kV on Philips CM 300. The
Oxford superconducting Vibrating Sample Magnetometer ŽVSM. was used to characterize the magnetic property of the samples. The resistivity and magneto-transport properties were measured by standard four-pad probing method. A magnetic field of ; 4 kOe is viewed as the low field here. Fig. 1 presents the XRD spectra of a series of samples deposited at different Ts and the target, too. The samples for Ts - 5308C show no identifiable reflection, thus, are amorphous. At Ts s 5308C, only weak Ž110. reflection is recorded. The FWHM Žfullwidth at half maximum. is ; 0.658, three times that of the target. At Ts s 550–6008C the Ž111., Ž200. and Ž211. reflections besides preferential Ž110. are recorded. These peaks are quite weak in comparison with those from the well-crystallined films. We may argue that these samples contain incomplete crystalline or amorphous phase. The TEM observation confirms above argument. Fig. 2 presents the TEM images of the sample for Ts s 6008C. The white and dim regions in Fig. 2 image, respectively, the amorphous and crystalline phases, confirmed with corresponding electron diffraction patterns. The intercrystal separation is ; 10 nm or more. For this sample, a relative volume proportion of 20–40% for the crystalline phase is estimated. The two-phased microstructure shown in Fig. 2 is quite similar to the
Fig. 2. TEM image of LSMO thin film deposited at 6008C.
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
two-phase percolation pattern as reported by Uehara et al. w8x and here, the insulating phase is amorphous LSMO w12x. Our TEM observation does not show any CO region at T ) 270 K, although appearance of CO at very low T cannot be excluded. As Ts s 6808C, the thin film shows the XRD spectrum quite similar to that of the target, thus, consisting of polycrystalline phase. The VSM evaluation of the magnetic property indicates that the samples deposited at T ) 6408C are ferromagnetic overall T-range until T s Tc ; 320 K, the Curie point. With decreasing of Ts , the measured magnetization falls down and the evaluated Curie point shifts downward the low-T side. The completely amorphous samples deposited at low Ts are, however, non-ferromagnetic. The magneto-transport response of the deposited samples is measured. As an example, the MR y H
99
hysteresis ŽMR s 1 y rrr 0 with r 0 the zero-field resistivity. of three samples for Ts s 5508C, 6008C and 6808C, respectively, is presented in Fig. 3. Within low-field range where
Fig. 3. Ža. Magnetic field H Žsolid line., recorded MR ratio Ž rrr 0 . Žopen circle spot. and temperature T Ždot line. as a function of time for the LSMO film deposited at 6008C. Žb. rrr 0 y H hysteresis loops at T s 77 K for three LSMO films deposited at 5508C, 6008C and 6808C.
100
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
Fig. 4. Zero field resistivity r 0 Žtop row. and MR ratio D rrr 0 at H s 4 kOe Žbottom row. as a function of temperature for a series of samples deposited at different Ts as indicated.
and GBs. As Ts - 5308C, the resistivity is too high to be measurable. As Ts increases from 5308C to 6008C, r 0 ŽT s 77 K. shows a rapid decreasing over three orders of magnitude. While the sample for Ts s 5308C shows a linear log r 0 ; T dependence, the low T part of the log r 0 ; T relation for the samples deposited at higher Ts begins to deviate negatively from the linear dependence. The LFMR ratio at low T depends largely on Ts . It is evidenced that the single-phased polycrystalline films show much lower LFMR effect than the multi-phase films. The microstructures controlled by Ts are optimized in terms of the MR ratio, says, at Ts ; 570–6008C.
The enhanced LFMR can be explained by the two-channel model w4x. One is the metallic conduction channel ŽMCC. and the other is the insulator conduction channel ŽICC.. Here, the former covers the crystalline grains that are supposed to contact intimately through GBs if neighboring. The ICC is composed of the amorphous phase chain. The resistivity for ICC and MCC is denoted by r i and rm , respectively. The sample resistivity r is expressed as ry1 s ry1 q g ry1 m , where g is a geometric parameter that characterizes the proportion of MCC in relative to ICC. r i can be formulated in terms of the variable-range-hopping model ŽVRH. w2,6x, yielding
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
r 1 s r i0 expŽT0rT .1r4 , where r i0 is the pre-factor and T0 is the characteristic temperature for VRH. rm for MCC can be written as rm s a q bT, where a and b are two constants. Therefore, r can be rewritten as:
rs
½
1
r i 0 exp Ž T0rT .
gra 1r4
q 1 q Ž bra . T
y1
5
Ž 1.
The four parameters, r i0 , T0 , gra and bra, are evaluated by best fitting using Eq. Ž1.. It is revealed that parameter bra is nearly Ts-independent, predicting little change of the transport property of crystalline LSMO phase. We then fix bra s 8.4 = 10 -4 Ky1 and evaluate the other parameters in order to improve the fitting reliability. For ICC, it is clearly identified that r i0 at both zero-field and H s 4 kOe drops rapidly with increasing Ts at low Ts range. As Ts ) 6408C, the dropping is slowed down. The change of T0 as a function of Ts is similar. When Ts increases, the ground state conduction for ICC is significantly improved. On the other hand, for a fixed Ts , the difference either between r i0 Ž H s 0. and r i0 Ž H s 4 kOe. Žnote r i0 Ž0. ) r i0 Ž4 kOe.., or between T0 Ž0. and T0 Ž4 kOe. Žnote T0 Ž0. - T0 Ž4 kOe.., is not much. The biggest relative difference for either r i0 or T0 appears at Ts s 5708C; 6008C. The carrier density at the Fermi surface, N Ž E F ., can be estimated according to the VRH model. Taking T0 s 7 = 10 6 K and the localization length j ; 0.4 nm ŽMn–Mn separation in LSMO., N Ž E F . ; 2.30 = 10 26 eVy1 my3 yields via w13x kT0 s 24rp N Ž E F . j 3 , where k is Boltzmann constant. This value is two orders of magnitude lower than that for single crystal LSMO w14x, predicting that existence of the amorphous phase results in localization of part of the carriers. Parameter gra at H s 0 and 4 kOe as a function of Ts starts to fast grow as Ts ) 5508C until 6008C. Afterwards, the growth slows down. Because gra characterizes the relative amount of MCC, the amount of ICC decreases rapidly with increasing Ts at low Ts range but not so much as Ts G 6408C. This prediction is supported with the microstructural evolution. Comparing with the tiny change of r i0 and T0 , the change of gra induced by applying magnetic field is very remarkable once Ts ) 5508C.
101
For convenience, we look at s s s i q g sm , where s represents the total electrical conductivity, s i and sm for ICC and MCC, respectively. Since s i has been proven to change little against magnetic field and s i < g sm , the MR ratio can be roughly written as: MR f D sm rsm q D Ž gra . r Ž gra .
Ž 2.
where sm Ž 0 . ; sm Ž H . is taken for simplifying the second term on the right side, noting that D s is anyway much smaller than s . We can prove that the first term on right side of Eq. Ž2. takes accounts of the MR ratio at very low field Ž H ; Hc . and the second term is mainly the contribution at high field Ž H 4 Hc .. The resistivity reduction at low field Žlower than field for saturated magnetization Ms . is attributed to movement of the domain walls and spin-rotation if any at the walls, i.e. w1x MR s D rrr Ž 0 . s D sm rsm Ž H . A Ž MrM s .
2
Ž 3.
A comparison of Eq. Ž3. with Eq. Ž2. gives D g s 0. As H is much higher than that required for magnetization M s Ms , the MR ratio becomes dependent of gra. Taking the zero-order approximation, we have MR f ŽŽ DŽ gra..rŽŽ gra.Ž H .... Plot-
Fig. 5. Measured MR ratio Žsquare dots with error bars. and calculated MR ratio Žcircle dots. from the two-channel model as a function of Ts .
102
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
ting of the measured MR ratio and calculated MR ratio yields very good coincidence between them, as shown in Fig. 5. The two-channel model seems a good description of the magnetotransport behavior in our systems. In conclusion, we have investigated in detail the microstructural and low-field magneto-transport property of LSMO thin films deposited on quartz wafers at various Ts . It has been confirmed that coexistence of insulating amorphous and metallic polycrystalline grains in the films is beneficial to the LFMR enhancement. The two-channel model in which the insulator channels and metallic channels align in parallel has been used to explain the conductivity and magneto-transport behaviors of the thin films.
Acknowledgements The authors acknowledge support from NSFC through normal and special projects, the National Key Project for Basic Research of China and LSSMS of Nanjing University.
References w1x X.W. Li, A. Gupta, G. Xiao, G.Q. Gong, Appl. Phys. Lett. 71 Ž1997. 1124. w2x S. Lee, H.Y. Hwang, B.I. Shraiman, W.D. Ratcliff, S.W. Cheong, Phys. Rev. Lett. 82 Ž1999. 4508. w3x L. Balcells, J. Fontcuberta, B. Martinez, X. Oberadors, Phys. Rev. B 58 Ž1998. R14679. w4x A. de Andres, M. Garcia-Hernandez, J.L. Martinez, C. Prieto, Appl. Phys. Lett. 74 Ž1999. 3884. w5x H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 Ž1996. 2041. w6x H.B. Peng, B.R. Zhao, Z. Xie, Y. Lin, B.Y. Zhu, Z. Hao, Y.M. Ni, H.J. Tao, X.L. Dong, B. Xu, Appl. Phys. Lett. 74 Ž1999. 1606. w7x C. Zener, Phys. Rev. 82 Ž1951. 403. w8x M. Uehara, S. Mori, C.H. Chen, S.-W. Cheong, Nature 399 Ž1999. 560. w9x H.Y. Hwang, S.W. Cheong, P.G. Radaelli, M. Marezio, B. Batlogg, Phys. Rev. Lett. 75 Ž1995. 914. w10x G.H. Jonker, Physica 22 Ž1956. 707. w11x J. Li, C.K. Ong, J.-M. Liu, Q. Huang, S.J. Wang, Appl. Phys. Lett. 76 Ž2000. 1051. w12x J.-M. Liu, J. Li, Q. Huang, L.P. You, S.J. Wang, C.K. Ong, Z.G. Liu, Y.W. Du, Appl. Phys. Lett. 76 Ž2000. 2286. w13x M. Ziese, C. Srinitiwarawong, Phys. Rev. B 58 Ž1998. 11519. w14x B.F. Woodfield, M.L. Wilson, J.M. Byers, Phys. Rev. Lett. 78 Ž1997. 3201.
Materials Letters 50 Ž2001. 97–102 www.elsevier.comrlocatermatlet
Low-field magneto-transport property of La 0.5 Sr0.5 MnO 3 thin films deposited at low temperature by laser ablation J.-M. Liu a,) , Q. Huang b, J. Li b, C.K. Ong b, X.Y. Chen a , Z.G. Liu a , Y.W. Du a a
Laboratory of Solid State Microstructures, Faculty of Sciencer Department of Physics, Nanjing UniÕersity, Hankou Road No. 22, Microstructures, Nanjing 210093, China b Department of Physics, National UniÕersity of Singapore, Singapore 119260, China Received 6 October 2000; accepted 14 November 2000
Abstract The low-field magneto-transport property of polycrystalline La 0.5 Sr0.5 MnO 3 ŽLSMO. thin films deposited on quartz wafers by pulsed laser ablation at various temperatures is investigated. The film is of mixed amorphous and polycrystalline microstructure, depending on the depositing temperature. The low-field magneto-transport property of the thin films can be explained in terms of the two-channel model where the insulating channels and metallic channels coexist. Enhanced low-field magnetoresistance for the samples deposited from 5708C to 6008C in which nano-crystal phase is embedded in the amorphous matrix is demonstrated. q 2001 Elsevier Science B.V. All rights reserved. PACS: 75.50.Cn; 72.60.Tg; 73.50.Jt Keywords: Doped manganite oxides; Low-field magnetoresistance; Laser ablation
The problem of enhanced low-field magnetoresistance ŽLFMR. in manganese perovskite oxides La 1 y x A x MnO 3 y y ŽA s divalent cation such as Ca, Sr, Ba. has recently attracted special attention w1,2x. Although there has been for thin film samples recorded very high MR ratio ŽMR s w r Ž0. y r Ž H .xrr Ž0., where r is the sample resistivity and H is the applied magnetic field. at high field, the LFMR ratio remains yet small. The microstructure exploitation w3–6x for La 1y x Ca x MnO 3 ŽLCMO. or La 1y x Sr x MnO 3 ŽLSMO. where microscopic defects or spin-disordered media are introduced does show
) Corresponding author. Tel.: q86-25-359-5979; fax: q86-25359-5535. E-mail address: [email protected] ŽJ.-M. Liu..
enhanced LFMR w1x and reveals a number of new effects in due course. The intrinsic MR effect in these materials is believed to result from the doubleexchanging sequence w7x where the ferromagnetic transition and insulating-metal transition occur almost concurrently at Tc and Tm , respectively. The enhanced LFMR in polycrystalline samples is, however, extrinsic and attributed to the existence of interfaces, grain boundaries or insulating second phases w8x, so that spin-polarized tunneling ŽSPT. w2x or spin-dependent scattering ŽSDS. w1x across these interfaces or boundaries produces MR under external field. In this letter, we investigate the microstructural and magneto-transport property of LSMO thin films deposited at various temperatures Ts . In our work, LSMO Ž x s 0.5., not far from the ferromagneticrcharge-ordering ŽCO. boundary w9–11x, is
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 2 3 - 7
98
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
Fig. 1. XRD u –2u spectra for a series of LSMO thin films deposited at different Ts as numerically indicated. The spectrum of ceramic LSMO target is inserted too.
chosen for study. It is our scheme to introduce amorphousrcrystalline interface, grain boundaries and if possible, charge-ordering phase coexisting in the sample, which may be beneficial to the LFMR enhancement, although no CO region in our samples is observed at temperature T ) 270 K. The LSMO Ž x s 0.5. thin films were deposited using pulsed laser deposition ŽPLD. on quartz wafers so that a mixture of amorphous and polycrystalline phases was obtained. The PLD experiment was performed by utilizing KrF excimer laser of wavelength of 248 nm and pulse width of 30 ns, detail of which was reported earlier w11,12x. The optimized laser fluency density of 1.2 Jrcm2 and reprate of 5 Hz were used during the ablation. The oxygen ambient pressure of 0.25 mbar was kept. The films are of 800 nm in thickness and 10 = 1.5 mm2 in in-plane dimension. The crystallization and grain size of the films were controlled by varying Ts from 4008C to 7108C, and checked by X-ray diffraction ŽXRD.. The microstructures were investigated by the plane-view high-resolution transmission electron microscopy ŽTEM. performed at 300 kV on Philips CM 300. The
Oxford superconducting Vibrating Sample Magnetometer ŽVSM. was used to characterize the magnetic property of the samples. The resistivity and magneto-transport properties were measured by standard four-pad probing method. A magnetic field of ; 4 kOe is viewed as the low field here. Fig. 1 presents the XRD spectra of a series of samples deposited at different Ts and the target, too. The samples for Ts - 5308C show no identifiable reflection, thus, are amorphous. At Ts s 5308C, only weak Ž110. reflection is recorded. The FWHM Žfullwidth at half maximum. is ; 0.658, three times that of the target. At Ts s 550–6008C the Ž111., Ž200. and Ž211. reflections besides preferential Ž110. are recorded. These peaks are quite weak in comparison with those from the well-crystallined films. We may argue that these samples contain incomplete crystalline or amorphous phase. The TEM observation confirms above argument. Fig. 2 presents the TEM images of the sample for Ts s 6008C. The white and dim regions in Fig. 2 image, respectively, the amorphous and crystalline phases, confirmed with corresponding electron diffraction patterns. The intercrystal separation is ; 10 nm or more. For this sample, a relative volume proportion of 20–40% for the crystalline phase is estimated. The two-phased microstructure shown in Fig. 2 is quite similar to the
Fig. 2. TEM image of LSMO thin film deposited at 6008C.
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
two-phase percolation pattern as reported by Uehara et al. w8x and here, the insulating phase is amorphous LSMO w12x. Our TEM observation does not show any CO region at T ) 270 K, although appearance of CO at very low T cannot be excluded. As Ts s 6808C, the thin film shows the XRD spectrum quite similar to that of the target, thus, consisting of polycrystalline phase. The VSM evaluation of the magnetic property indicates that the samples deposited at T ) 6408C are ferromagnetic overall T-range until T s Tc ; 320 K, the Curie point. With decreasing of Ts , the measured magnetization falls down and the evaluated Curie point shifts downward the low-T side. The completely amorphous samples deposited at low Ts are, however, non-ferromagnetic. The magneto-transport response of the deposited samples is measured. As an example, the MR y H
99
hysteresis ŽMR s 1 y rrr 0 with r 0 the zero-field resistivity. of three samples for Ts s 5508C, 6008C and 6808C, respectively, is presented in Fig. 3. Within low-field range where
Fig. 3. Ža. Magnetic field H Žsolid line., recorded MR ratio Ž rrr 0 . Žopen circle spot. and temperature T Ždot line. as a function of time for the LSMO film deposited at 6008C. Žb. rrr 0 y H hysteresis loops at T s 77 K for three LSMO films deposited at 5508C, 6008C and 6808C.
100
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
Fig. 4. Zero field resistivity r 0 Žtop row. and MR ratio D rrr 0 at H s 4 kOe Žbottom row. as a function of temperature for a series of samples deposited at different Ts as indicated.
and GBs. As Ts - 5308C, the resistivity is too high to be measurable. As Ts increases from 5308C to 6008C, r 0 ŽT s 77 K. shows a rapid decreasing over three orders of magnitude. While the sample for Ts s 5308C shows a linear log r 0 ; T dependence, the low T part of the log r 0 ; T relation for the samples deposited at higher Ts begins to deviate negatively from the linear dependence. The LFMR ratio at low T depends largely on Ts . It is evidenced that the single-phased polycrystalline films show much lower LFMR effect than the multi-phase films. The microstructures controlled by Ts are optimized in terms of the MR ratio, says, at Ts ; 570–6008C.
The enhanced LFMR can be explained by the two-channel model w4x. One is the metallic conduction channel ŽMCC. and the other is the insulator conduction channel ŽICC.. Here, the former covers the crystalline grains that are supposed to contact intimately through GBs if neighboring. The ICC is composed of the amorphous phase chain. The resistivity for ICC and MCC is denoted by r i and rm , respectively. The sample resistivity r is expressed as ry1 s ry1 q g ry1 m , where g is a geometric parameter that characterizes the proportion of MCC in relative to ICC. r i can be formulated in terms of the variable-range-hopping model ŽVRH. w2,6x, yielding
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
r 1 s r i0 expŽT0rT .1r4 , where r i0 is the pre-factor and T0 is the characteristic temperature for VRH. rm for MCC can be written as rm s a q bT, where a and b are two constants. Therefore, r can be rewritten as:
rs
½
1
r i 0 exp Ž T0rT .
gra 1r4
q 1 q Ž bra . T
y1
5
Ž 1.
The four parameters, r i0 , T0 , gra and bra, are evaluated by best fitting using Eq. Ž1.. It is revealed that parameter bra is nearly Ts-independent, predicting little change of the transport property of crystalline LSMO phase. We then fix bra s 8.4 = 10 -4 Ky1 and evaluate the other parameters in order to improve the fitting reliability. For ICC, it is clearly identified that r i0 at both zero-field and H s 4 kOe drops rapidly with increasing Ts at low Ts range. As Ts ) 6408C, the dropping is slowed down. The change of T0 as a function of Ts is similar. When Ts increases, the ground state conduction for ICC is significantly improved. On the other hand, for a fixed Ts , the difference either between r i0 Ž H s 0. and r i0 Ž H s 4 kOe. Žnote r i0 Ž0. ) r i0 Ž4 kOe.., or between T0 Ž0. and T0 Ž4 kOe. Žnote T0 Ž0. - T0 Ž4 kOe.., is not much. The biggest relative difference for either r i0 or T0 appears at Ts s 5708C; 6008C. The carrier density at the Fermi surface, N Ž E F ., can be estimated according to the VRH model. Taking T0 s 7 = 10 6 K and the localization length j ; 0.4 nm ŽMn–Mn separation in LSMO., N Ž E F . ; 2.30 = 10 26 eVy1 my3 yields via w13x kT0 s 24rp N Ž E F . j 3 , where k is Boltzmann constant. This value is two orders of magnitude lower than that for single crystal LSMO w14x, predicting that existence of the amorphous phase results in localization of part of the carriers. Parameter gra at H s 0 and 4 kOe as a function of Ts starts to fast grow as Ts ) 5508C until 6008C. Afterwards, the growth slows down. Because gra characterizes the relative amount of MCC, the amount of ICC decreases rapidly with increasing Ts at low Ts range but not so much as Ts G 6408C. This prediction is supported with the microstructural evolution. Comparing with the tiny change of r i0 and T0 , the change of gra induced by applying magnetic field is very remarkable once Ts ) 5508C.
101
For convenience, we look at s s s i q g sm , where s represents the total electrical conductivity, s i and sm for ICC and MCC, respectively. Since s i has been proven to change little against magnetic field and s i < g sm , the MR ratio can be roughly written as: MR f D sm rsm q D Ž gra . r Ž gra .
Ž 2.
where sm Ž 0 . ; sm Ž H . is taken for simplifying the second term on the right side, noting that D s is anyway much smaller than s . We can prove that the first term on right side of Eq. Ž2. takes accounts of the MR ratio at very low field Ž H ; Hc . and the second term is mainly the contribution at high field Ž H 4 Hc .. The resistivity reduction at low field Žlower than field for saturated magnetization Ms . is attributed to movement of the domain walls and spin-rotation if any at the walls, i.e. w1x MR s D rrr Ž 0 . s D sm rsm Ž H . A Ž MrM s .
2
Ž 3.
A comparison of Eq. Ž3. with Eq. Ž2. gives D g s 0. As H is much higher than that required for magnetization M s Ms , the MR ratio becomes dependent of gra. Taking the zero-order approximation, we have MR f ŽŽ DŽ gra..rŽŽ gra.Ž H .... Plot-
Fig. 5. Measured MR ratio Žsquare dots with error bars. and calculated MR ratio Žcircle dots. from the two-channel model as a function of Ts .
102
J.-M. Liu et al.r Materials Letters 50 (2001) 97–102
ting of the measured MR ratio and calculated MR ratio yields very good coincidence between them, as shown in Fig. 5. The two-channel model seems a good description of the magnetotransport behavior in our systems. In conclusion, we have investigated in detail the microstructural and low-field magneto-transport property of LSMO thin films deposited on quartz wafers at various Ts . It has been confirmed that coexistence of insulating amorphous and metallic polycrystalline grains in the films is beneficial to the LFMR enhancement. The two-channel model in which the insulator channels and metallic channels align in parallel has been used to explain the conductivity and magneto-transport behaviors of the thin films.
Acknowledgements The authors acknowledge support from NSFC through normal and special projects, the National Key Project for Basic Research of China and LSSMS of Nanjing University.
References w1x X.W. Li, A. Gupta, G. Xiao, G.Q. Gong, Appl. Phys. Lett. 71 Ž1997. 1124. w2x S. Lee, H.Y. Hwang, B.I. Shraiman, W.D. Ratcliff, S.W. Cheong, Phys. Rev. Lett. 82 Ž1999. 4508. w3x L. Balcells, J. Fontcuberta, B. Martinez, X. Oberadors, Phys. Rev. B 58 Ž1998. R14679. w4x A. de Andres, M. Garcia-Hernandez, J.L. Martinez, C. Prieto, Appl. Phys. Lett. 74 Ž1999. 3884. w5x H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 Ž1996. 2041. w6x H.B. Peng, B.R. Zhao, Z. Xie, Y. Lin, B.Y. Zhu, Z. Hao, Y.M. Ni, H.J. Tao, X.L. Dong, B. Xu, Appl. Phys. Lett. 74 Ž1999. 1606. w7x C. Zener, Phys. Rev. 82 Ž1951. 403. w8x M. Uehara, S. Mori, C.H. Chen, S.-W. Cheong, Nature 399 Ž1999. 560. w9x H.Y. Hwang, S.W. Cheong, P.G. Radaelli, M. Marezio, B. Batlogg, Phys. Rev. Lett. 75 Ž1995. 914. w10x G.H. Jonker, Physica 22 Ž1956. 707. w11x J. Li, C.K. Ong, J.-M. Liu, Q. Huang, S.J. Wang, Appl. Phys. Lett. 76 Ž2000. 1051. w12x J.-M. Liu, J. Li, Q. Huang, L.P. You, S.J. Wang, C.K. Ong, Z.G. Liu, Y.W. Du, Appl. Phys. Lett. 76 Ž2000. 2286. w13x M. Ziese, C. Srinitiwarawong, Phys. Rev. B 58 Ž1998. 11519. w14x B.F. Woodfield, M.L. Wilson, J.M. Byers, Phys. Rev. Lett. 78 Ž1997. 3201.