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Observation of a near 100% magnetoresistance in (La0.8 Bi0.2 )0.67 Ca0.33 MnO3
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 292 (2005) 260–265 www.elsevier.com/locate/jmmm
Observation of a near 100% magnetoresistance in (La0:8 Bi0:2)0:67Ca0:33MnO3 Z.C. Xia, G. Liu, B. Dong, L. Chen, D.W. Liu, C.H. Fang, D. Doyananda, L. Liu, S. Liu, C.Q. Tang, S.L. Yuan Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 26 July 2004; received in revised form 5 November 2004 Available online 30 November 2004
Abstract The electrical transport properties of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 was studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T, respectively. An obvious difference in temperature and magnetic field dependence of resistivity between the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 and pure La0:67 Ca0:33 MnO3 was observed. Especially, an enhanced magnetoresistance effect was obtained in the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; such as, a near 100% magnetoresistance plateau within a temperature ranging from 10 to 180 K was observed in a 3 T field. The unusual large magnetoresistance value is encouraging as regard to the potential application of CMR materials. On the other hand, a significant hysteresis both in temperature and magnetic field dependence resistivity is simultaneously observed in the Bi-doped sample. r 2004 Elsevier B.V. All rights reserved. PACS: 74.43.Qt; 75.60.d; 72.90.þy Keywords: Magnetoresistance; Hysteresis; Transport
1. Introduction In the recent years, there has been extensive research activity in the manganites of R1x Bx MnO3 (R ¼ La; Nd, Y; B ¼ Ca; Ba, Sr, Bi) since the discovery of colossal magnetoresisCorresponding authors. Tel.: 86 278 7556580; fax: 86 278 7544525. E-mail addresses: [email protected] (Z.C. Xia), [email protected] (S.L. Yuan).
tance (CMR) properties in the manganites perovskite in 1993 [1]. Meanwhile, there is fine interplay of magnetic exchange, structure properties and electronic transport in these materials which gives rise to several novel properties [2]. Generally, the research activity on CMR materials brings out underlying fundamental aspects which are of great interest for physics of highly correlated electron system and application, such as, sensors etc. However, the so-called CMR is found on a magnetic field scale of several teslas (T) and a
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.140
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
(La0:8 Bi0:2 )0:67 Ca0:33 MnO3 powders were sintered at 1200 C for 6 h, the X-ray diffraction (XRD) shows that the obtained La0:67 Ca0:33 MnO3 and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 powders have a uniform perovskite structure. Then, the sintered La0:67 Ca0:33 MnO3 and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 powders were pelletized at a pressure of 10 MPa and sintered at 1300 C for 10 h. The electrical transport properties were measured with a commercial physical property measurement system (Quantum design PPMS) in a magnetic field range 0 TpHp3 T and temperature range 10 KpTp300 K:
3. Results and discussion For all studied samples, the phase structure is characterized by XRD. Indicating the XRD pattern of pure La0:67 Ca0:33 MnO3 in Fig. 1(b) exhibits an orthorhombic structure. As shown in Fig. 1(a), the XRD pattern of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 phase is similar to that of pure La0:67 Ca0:33 MnO3 : The similar crystal structure results mainly from the ion radii of Bi3þ and La3þ being very close to each other. Meanwhile, the XRD patterns also confirm that all the studied samples have a uniform crystal structure and no impurity phases. The electrical transport properties were measured with a standard four probe method. As shown in Fig. 2(a), the resistivity (r) versus 140 120
(b):La0.67Ca0.33MnO3 (a): (La0.8Bi0.2)0.67Ca0.33MnO3
100
CPS
narrow temperature range near Curie temperature (T C ), which is not very appealing for applications [3,4]. Accordingly, many research groups focused on the investigation of extrinsic magnetoresistance (MR) effects found in various magnetic oxides, since these promised a large MR ratio in a low magnetic field. Extrinsic MR in ferromagnetic oxides usually falls into three broad classes, namely grain-boundary MR, spin-polarized transport in ferromagnetic tunneling junctions MR and domain-wall MR [5]. Gupta et al. [6] investigated the effects of magnetic domain boundary pinning by polycrystalline grain boundaries and an enhanced MR was observed in a low-temperature region. These extrinsic effects usually rely on the existence of an insulating tunneling barrier separating the ferromagnetic grains. Hwang et al. [7] was the first to propose a model to explain the low field magnetoresistance (LFMR) in both high- and low-temperature regions. They assign the LFMR to spin-polarized tunneling from one grain to another through highly resistive grain boundaries. On the other hand, the properties of these compounds are mainly influenced by the Mn3þ / Mn4þ ratio and the Mn–O–Mn bond angle, which affects the orbital overlapping between neighboring ions [8,9]. In this work, we try to investigate the effect of Bi3þ ion on the transport properties of La0:67 Ca0:33 MnO3 ; since the ion radii of trivalent bismuth (Bi3þ 1:300 nm) and trivalent lanthanum (La3þ 1:302 nm) are very close to each other [10], one may expect similar physical properties between them. However, an obvious different transport behavior and an enhanced MR were observed in the Bi3þ ion-doped La0:67 Ca0:33 MnO3 : Especially, a near 100% MR value with a wide temperature window was observed in the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 compound, which is encouraging as regards the potential application of CMR materials.
261
80 60 40
2. Experimental procedure
(b)
20
(a)
The polycrystalline samples of pure La0:67 Ca0:33 MnO3 and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 were prepared using a standard sol–gel method. The obtained La0:67 Ca0:33 MnO3 and
0 30
40
50
60
70
80
2θ Fig. 1. XRD patterns of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 (a) and pure La0:67 Ca0:33 MnO3 (b).
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
262 0.0075
ρ(Ω.cm)
0.0060
0.5T
temperature with decreasing resistivity r; which results in a MR effect. Defining the MR as
(a)
0
1T
MR ¼
0.0045 3T
0.0030 0.0015 0.0000 28 0
(b)
24 20
ρ(Ω.cm)
RðT; H ¼ 0Þ RðT; HÞ 100%; RðT; H ¼ 0Þ
16 12
0.5T
8
3T
4
1T
0 0
50
100
150
200
250
300
T(K)
Fig. 2. Temperature dependence of resistivity r of pure La0:67 Ca0:33 MnO3 (a) and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 (b), measured in magnetic fields of 0; 0:5; 1 and 3 T.
where the RðT; H ¼ 0Þ and RðT; HÞ are the resistivity measured in a zero-field and a magnetic field H, respectively. We obtain MR as a function of temperature in a field range 0pHp3 T as shown in Fig. 3. It can be seen that the pure La0:67 Ca0:33 MnO3 sample shows typical features of intergrain MR and intrinsic CMR in the low-temperature regions and near T P ; respectively. The low-temperature MR monotonically increases on cooling below T P : Meanwhile, with increasing the applied magnetic field, both the increasing low-temperature MR and CMR are observed. In an applied magnetic field of 3 T, a near 50% MR and CMR are observed. For the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; an obvious enhanced low-temperature MR effect and CMR are 55
(a)
50 40
MR(%)
35 30 1T
25 20 15
0.5T
10 5 0
3T
100
(b)
1T
80
MR(%)
temperature (T) curve was measured in pure La0:67 Ca0:33 MnO3 : In zero magnetic field, decreasing temperature (cooling mode), the sample shows an insulating behavior in a high-temperature region and metallic behavior in a low-temperature region. The insulator–metal (I–M) transition is characterized by a peak at T P 180 K: The zerofield r–T curve with a warming mode was also measured (not shown), and no difference was found in both the modes, which indicates no thermal hysteresis in the pure La0:67 Ca0:33 MnO3 : For the Bi3þ ion-doped (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; the temperature dependence resistivity is measured in the magnetic field range 0pHp3 T and temperature range 10 KpTp300 K; and the r–T curves are shown in Fig. 2(b). Compared to Fig. 2(a), the I–M transition temperature of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 is shifted to a more lower temperature T P 107 K: On the other hand, the resistivity of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 is much higher than that of pure La0:67 Ca0:33 MnO3 : When a field is applied, as indicated in Fig. 2, the T P shifts to a higher
3T
45
0.5T
60 40
0.3T
20 0.1T 0
0
50
100
150
200
250
300
T(K) Fig. 3. Temperature dependence of magnetoresistance of pure La0:67 Ca0:33 MnO3 (a) and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 (b) measured in magnetic field range of 0pHp3 T:
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
obtained as shown in Fig. 3(b). Compared with the temperature dependence MR of pure La0:67 Ca0:33 MnO3 ; the low field MR of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 shows a plateau in a low-temperature range. Meanwhile, with an increase in the applied magnetic field, the width of the temperature window of the plateau increases. In a magnetic field of 3 T, a near 100% MR value is observed in a temperature window 10 KpTp180 K; which is rarely reported in the system. On the other hand, as shown in Fig. 3(b), in a lower magnetic field, such as 0.3 T, the MR value ( 35%) of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 is much larger than that of pure La0:67 Ca0:33 MnO3 measured in a higher magnetic field, such as 1 T. The large sensitivity of the Bi3þ ion-doped (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 compound to relatively small external field opens up fresh opportunities for application. The temperature dependence resistivity of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 measured in a magnetic field 0.005 T is presented in Fig. 4 (plotted both cooling and heating). With the decrease in temperature, the resistivity gradually grows attaining the maximum value in a temperature 110 K: After that, a steep resistivity drop is observed. Then, the resistivity almost maintains a constant in a temperature region of 50 to 10 K. On a heating mode, first, the sample is cooled to 10 K in zero field, then the sample is warmed in a magnetic field
25 FC ZFC
ρ(Ω.cm)
20 15 10 5 0 0
50
100
150
200
250
300
T(K) Fig. 4. Temperature dependence of the resistivity of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 measured at 0.005 T. The directions of the temperature change (zero-field-cooled and field-cooled) are indicated by arrows.
263
of 0.005 T. As shown in Fig. 4, the resistivity curve first follows the cooling curve in the temperature region 10pTp50 K: Second, it undergoes a very slow increase from 50 to 100 K. After that, the resistivity grows rapidly and passes through a peak in a temperature 135 K: Then, the resistivity decreases and coincides with the cooling curve in temperature above 160 K: The r–T behavior is characterized by the pronounced hysteresis which is not observed in pure La0:67 Ca0:33 MnO3 : In Fig. 4, three distinguishable regions can be found in the curves: (1) high-temperature region for T4160 K (HTR), (2) middle temperature region for 50 KpTp160 K (MTR), and (3) lowtemperature region for To50 K (LTR). At HTR and LTR, no thermal hysteresis is observed, both the zero-field-cooled (ZFC) and field-cooled (FC) data are coincided, but a large deviation between FC and ZFC data appears in the MTR, where the FC value is larger than the ZFC one. This observation is similar to that commonly observed in spin glasses. The results assume that there are an inhomogeneous and a metastable phase in the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; in which a transition is carried out during heating and cooling and leads to the hysteresis behavior. On the other hand, an essential fact should be noted in Fig. 3(b), namely, an unusually large MR effect is observed in a wide temperature region. Meanwhile, a thermal hysteresis becomes the most remarkable as shown in Fig. 4, suggesting the same underlying physical origin for them. In order to gain more information for this understanding, here we perform measurements of the magnetic field dependence of the resistivity r in a low temperature 10 K and I–M transition temperature of T P 107 K; respectively. Before each measurement, the sample was always heated to room temperature and then cooled to the desired temperature in zero field. Keeping this temperature, the measurement was carried out, the results are shown in Fig. 5. Insight into the effect of Bi3þ ion on MR effect of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; we first discuss the effect of Bi3þ on electrical transport of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 : The shift of T P to a lower temperature in (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 suggests that the I–M transition should be related
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
264
(a)
ρ(Ω.cm)
10
1
0.1
0.01
ρ(Ω.cm)
1
(b)
0.1
0.01
-5
-4
-3
-2
-1
0
1
2
3
4
5
H(T) Fig. 5. Resistivity versus magnetic field of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 measured at temperatures of 107 K(a) and 10 K(b) respectively. The field sweep directions are indicated by arrows.
to the Bi3þ ion. Although the ion radius of Bi3þ is close to that of La3þ ; the effect of the Bi3þ ion on its neighboring ions is different to that of La3þ ion due to the 6s2 lone pair electron of the Bi3þ ion. The 6s2 lone pair electron of the Bi3þ can be oriented to a surrounding anion (O2 ), which can produce a local distortion or even a hybridization between 6s-orbitals of Bi3þ and 2p-orbitals of O2 [11]. This hybridization would avoid the movement of eg electrons from Mn to Mn through the Mn–O–Mn bridge and favor charge order (CO). Thus, the resistivity measurements have revealed that (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 presents a very large resistivity than the pure La0:67 Ca0:33 MnO3 one, as shown in Fig. 2. The hysteresis behavior of the Bi-doped La0:67 Ca0:33 MnO3 system due to the CO state was investigated by Sun et al. [12–14]. The magnetic-field dependence of resistivity at two fixed temperatures 107 K and 10 K is illustrated in Figs. 5(a) and (b), respectively. At 107 K
temperature, as shown in Fig. 5(a), we see that magnetic field induces the I–M transition with the resistivity drop by several orders of magnitude. The r–H curve shows hysteretic in different magnetic field regions: as the field is decreased after the magnetic field 5 T; the sample cannot even return to the original high-resistivity state. With increase in the negative magnetic field, the resistivity decreases again, similar to the positive loop, as the field is decreased after the magnetic field 5 T; the sample return to the starting point of the negative field with a different curve, in which the resistivity measured in return run is smaller than that in starting run. At the temperature of 10 K, a more strong hysteretic behavior is observed. After a magnetic field process, the resistivity of the sample is decreased to a low resistivity and retains the low resistivity even as the magnetic field returns to zero. At the reverse run, the resistivity of the sample still maintains a small value. Especially, in the negative magnetic loop, no hysteresis is observed. This result suggests that the transition observed at 107 K and near a magnetic field of 2 T is metastable and the phases is inhomogeneous at the temperature of 107 K. Meanwhile, at 10 K, a transition is also observed near the magnetic field 2 T as shown in Fig. 5(b), however, the kink near 2 T is not observed in the return run. On the other hand, at the transition temperature 107 K; the zero-field resistivity is partly reproduced after the field cycling but its value turns out to be much lower than that for the origin curve. Such lowering of the zero-field resistivity can be attributed to a certain remanent spin ordering similar to that discussed for Pr0:7 Ca0:3 MnO3 [15]. The physical original of the pronounced MR and strong hysteresis will be discussed in our future papers.
4. Conclusions In this work, we have shown that the MR with a value of MR 100% for the 3 T field can be realized upon Bi3þ -doped (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 : Such an enhanced MR effect is found to appear in a wide temperature range. The experimental result is encouraging as regards to the potential
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
application of CMR materials. Meanwhile, results show that the Bi3þ doping La0:67 Ca0:33 MnO3 has a direct effect on the thermal and magnetic hysteresis effects, which may result from the inhomogeneous of phases and the CO state due to the hybridization between 6s-orbitals of Bi3þ and 2porbitals of O2 in (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 :
Acknowledgements This work was partly supported by the National Science Foundation of China (Grant nos. 10174022 and 10374032), Postdoctoral Foundation of China and Huazhong University of Science and Technology, School of Science Foundation of Huazhong University of Science and Technology. References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331.
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[2] C.N.R. Rao, R. Mahesh, A.K. Raychaudhuri, R. Mahendiran, J. Phys. Chem. Solids 59 (1998) 487. [3] M. McCormack, S. Jin, T.H. Tiefer, R.M. Fleming, Julia M. Philips, R. Ramesh, Appl. Phys. Lett. 64 (1994) 3045. [4] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [5] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [6] A. Gupta, G.Q. Gong, G. Xiao, P.R. Duncombe, P. Truilloud, P. Lecocur, Y.Y. Wang, V.P. Dravid, J.Z. Sun, Phys. Rev. B 54 (1996) R15629. [7] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [8] C. Zener, Phys. Rev. 82 (1951) 403. [9] C.N.R. Rao, A.K. Cheetham, R. Manesh, Chem. Mater. 8 (1996) 2421. [10] L. Righi, J. Gutierrez, J.M. Barandiaran, J. Phys.: Condens. Matter 11 (1999) 2831. [11] N.A. Hill, K.M. Rabe, Phys. Rev. B 59 (1999) 8759. [12] J.R. Sun, J. Gao, Y. Fei, R.W. Li, B.G. Shen, Phys. Rev. B 67 (2003) 144414. [13] J.R. Sun, J. Gao, L. Kang, Appl. Phys. Lett. 81 (2002) 508. [14] J.R. Sun, B.G. Shen, J. Gao, Y. Fei, Y.P. Nie, Europhys. Lett. 62 (2003) 732. [15] H. Yoshizawa, H. Kawano, Y. Tomioka, Y. Tokura, J. Phys. Soc. Jpn. 65 (1996) 1043.
Journal of Magnetism and Magnetic Materials 292 (2005) 260–265 www.elsevier.com/locate/jmmm
Observation of a near 100% magnetoresistance in (La0:8 Bi0:2)0:67Ca0:33MnO3 Z.C. Xia, G. Liu, B. Dong, L. Chen, D.W. Liu, C.H. Fang, D. Doyananda, L. Liu, S. Liu, C.Q. Tang, S.L. Yuan Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 26 July 2004; received in revised form 5 November 2004 Available online 30 November 2004
Abstract The electrical transport properties of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 was studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T, respectively. An obvious difference in temperature and magnetic field dependence of resistivity between the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 and pure La0:67 Ca0:33 MnO3 was observed. Especially, an enhanced magnetoresistance effect was obtained in the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; such as, a near 100% magnetoresistance plateau within a temperature ranging from 10 to 180 K was observed in a 3 T field. The unusual large magnetoresistance value is encouraging as regard to the potential application of CMR materials. On the other hand, a significant hysteresis both in temperature and magnetic field dependence resistivity is simultaneously observed in the Bi-doped sample. r 2004 Elsevier B.V. All rights reserved. PACS: 74.43.Qt; 75.60.d; 72.90.þy Keywords: Magnetoresistance; Hysteresis; Transport
1. Introduction In the recent years, there has been extensive research activity in the manganites of R1x Bx MnO3 (R ¼ La; Nd, Y; B ¼ Ca; Ba, Sr, Bi) since the discovery of colossal magnetoresisCorresponding authors. Tel.: 86 278 7556580; fax: 86 278 7544525. E-mail addresses: [email protected] (Z.C. Xia), [email protected] (S.L. Yuan).
tance (CMR) properties in the manganites perovskite in 1993 [1]. Meanwhile, there is fine interplay of magnetic exchange, structure properties and electronic transport in these materials which gives rise to several novel properties [2]. Generally, the research activity on CMR materials brings out underlying fundamental aspects which are of great interest for physics of highly correlated electron system and application, such as, sensors etc. However, the so-called CMR is found on a magnetic field scale of several teslas (T) and a
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.140
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
(La0:8 Bi0:2 )0:67 Ca0:33 MnO3 powders were sintered at 1200 C for 6 h, the X-ray diffraction (XRD) shows that the obtained La0:67 Ca0:33 MnO3 and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 powders have a uniform perovskite structure. Then, the sintered La0:67 Ca0:33 MnO3 and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 powders were pelletized at a pressure of 10 MPa and sintered at 1300 C for 10 h. The electrical transport properties were measured with a commercial physical property measurement system (Quantum design PPMS) in a magnetic field range 0 TpHp3 T and temperature range 10 KpTp300 K:
3. Results and discussion For all studied samples, the phase structure is characterized by XRD. Indicating the XRD pattern of pure La0:67 Ca0:33 MnO3 in Fig. 1(b) exhibits an orthorhombic structure. As shown in Fig. 1(a), the XRD pattern of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 phase is similar to that of pure La0:67 Ca0:33 MnO3 : The similar crystal structure results mainly from the ion radii of Bi3þ and La3þ being very close to each other. Meanwhile, the XRD patterns also confirm that all the studied samples have a uniform crystal structure and no impurity phases. The electrical transport properties were measured with a standard four probe method. As shown in Fig. 2(a), the resistivity (r) versus 140 120
(b):La0.67Ca0.33MnO3 (a): (La0.8Bi0.2)0.67Ca0.33MnO3
100
CPS
narrow temperature range near Curie temperature (T C ), which is not very appealing for applications [3,4]. Accordingly, many research groups focused on the investigation of extrinsic magnetoresistance (MR) effects found in various magnetic oxides, since these promised a large MR ratio in a low magnetic field. Extrinsic MR in ferromagnetic oxides usually falls into three broad classes, namely grain-boundary MR, spin-polarized transport in ferromagnetic tunneling junctions MR and domain-wall MR [5]. Gupta et al. [6] investigated the effects of magnetic domain boundary pinning by polycrystalline grain boundaries and an enhanced MR was observed in a low-temperature region. These extrinsic effects usually rely on the existence of an insulating tunneling barrier separating the ferromagnetic grains. Hwang et al. [7] was the first to propose a model to explain the low field magnetoresistance (LFMR) in both high- and low-temperature regions. They assign the LFMR to spin-polarized tunneling from one grain to another through highly resistive grain boundaries. On the other hand, the properties of these compounds are mainly influenced by the Mn3þ / Mn4þ ratio and the Mn–O–Mn bond angle, which affects the orbital overlapping between neighboring ions [8,9]. In this work, we try to investigate the effect of Bi3þ ion on the transport properties of La0:67 Ca0:33 MnO3 ; since the ion radii of trivalent bismuth (Bi3þ 1:300 nm) and trivalent lanthanum (La3þ 1:302 nm) are very close to each other [10], one may expect similar physical properties between them. However, an obvious different transport behavior and an enhanced MR were observed in the Bi3þ ion-doped La0:67 Ca0:33 MnO3 : Especially, a near 100% MR value with a wide temperature window was observed in the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 compound, which is encouraging as regards the potential application of CMR materials.
261
80 60 40
2. Experimental procedure
(b)
20
(a)
The polycrystalline samples of pure La0:67 Ca0:33 MnO3 and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 were prepared using a standard sol–gel method. The obtained La0:67 Ca0:33 MnO3 and
0 30
40
50
60
70
80
2θ Fig. 1. XRD patterns of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 (a) and pure La0:67 Ca0:33 MnO3 (b).
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
262 0.0075
ρ(Ω.cm)
0.0060
0.5T
temperature with decreasing resistivity r; which results in a MR effect. Defining the MR as
(a)
0
1T
MR ¼
0.0045 3T
0.0030 0.0015 0.0000 28 0
(b)
24 20
ρ(Ω.cm)
RðT; H ¼ 0Þ RðT; HÞ 100%; RðT; H ¼ 0Þ
16 12
0.5T
8
3T
4
1T
0 0
50
100
150
200
250
300
T(K)
Fig. 2. Temperature dependence of resistivity r of pure La0:67 Ca0:33 MnO3 (a) and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 (b), measured in magnetic fields of 0; 0:5; 1 and 3 T.
where the RðT; H ¼ 0Þ and RðT; HÞ are the resistivity measured in a zero-field and a magnetic field H, respectively. We obtain MR as a function of temperature in a field range 0pHp3 T as shown in Fig. 3. It can be seen that the pure La0:67 Ca0:33 MnO3 sample shows typical features of intergrain MR and intrinsic CMR in the low-temperature regions and near T P ; respectively. The low-temperature MR monotonically increases on cooling below T P : Meanwhile, with increasing the applied magnetic field, both the increasing low-temperature MR and CMR are observed. In an applied magnetic field of 3 T, a near 50% MR and CMR are observed. For the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; an obvious enhanced low-temperature MR effect and CMR are 55
(a)
50 40
MR(%)
35 30 1T
25 20 15
0.5T
10 5 0
3T
100
(b)
1T
80
MR(%)
temperature (T) curve was measured in pure La0:67 Ca0:33 MnO3 : In zero magnetic field, decreasing temperature (cooling mode), the sample shows an insulating behavior in a high-temperature region and metallic behavior in a low-temperature region. The insulator–metal (I–M) transition is characterized by a peak at T P 180 K: The zerofield r–T curve with a warming mode was also measured (not shown), and no difference was found in both the modes, which indicates no thermal hysteresis in the pure La0:67 Ca0:33 MnO3 : For the Bi3þ ion-doped (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; the temperature dependence resistivity is measured in the magnetic field range 0pHp3 T and temperature range 10 KpTp300 K; and the r–T curves are shown in Fig. 2(b). Compared to Fig. 2(a), the I–M transition temperature of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 is shifted to a more lower temperature T P 107 K: On the other hand, the resistivity of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 is much higher than that of pure La0:67 Ca0:33 MnO3 : When a field is applied, as indicated in Fig. 2, the T P shifts to a higher
3T
45
0.5T
60 40
0.3T
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T(K) Fig. 3. Temperature dependence of magnetoresistance of pure La0:67 Ca0:33 MnO3 (a) and (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 (b) measured in magnetic field range of 0pHp3 T:
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obtained as shown in Fig. 3(b). Compared with the temperature dependence MR of pure La0:67 Ca0:33 MnO3 ; the low field MR of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 shows a plateau in a low-temperature range. Meanwhile, with an increase in the applied magnetic field, the width of the temperature window of the plateau increases. In a magnetic field of 3 T, a near 100% MR value is observed in a temperature window 10 KpTp180 K; which is rarely reported in the system. On the other hand, as shown in Fig. 3(b), in a lower magnetic field, such as 0.3 T, the MR value ( 35%) of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 is much larger than that of pure La0:67 Ca0:33 MnO3 measured in a higher magnetic field, such as 1 T. The large sensitivity of the Bi3þ ion-doped (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 compound to relatively small external field opens up fresh opportunities for application. The temperature dependence resistivity of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 measured in a magnetic field 0.005 T is presented in Fig. 4 (plotted both cooling and heating). With the decrease in temperature, the resistivity gradually grows attaining the maximum value in a temperature 110 K: After that, a steep resistivity drop is observed. Then, the resistivity almost maintains a constant in a temperature region of 50 to 10 K. On a heating mode, first, the sample is cooled to 10 K in zero field, then the sample is warmed in a magnetic field
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T(K) Fig. 4. Temperature dependence of the resistivity of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 measured at 0.005 T. The directions of the temperature change (zero-field-cooled and field-cooled) are indicated by arrows.
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of 0.005 T. As shown in Fig. 4, the resistivity curve first follows the cooling curve in the temperature region 10pTp50 K: Second, it undergoes a very slow increase from 50 to 100 K. After that, the resistivity grows rapidly and passes through a peak in a temperature 135 K: Then, the resistivity decreases and coincides with the cooling curve in temperature above 160 K: The r–T behavior is characterized by the pronounced hysteresis which is not observed in pure La0:67 Ca0:33 MnO3 : In Fig. 4, three distinguishable regions can be found in the curves: (1) high-temperature region for T4160 K (HTR), (2) middle temperature region for 50 KpTp160 K (MTR), and (3) lowtemperature region for To50 K (LTR). At HTR and LTR, no thermal hysteresis is observed, both the zero-field-cooled (ZFC) and field-cooled (FC) data are coincided, but a large deviation between FC and ZFC data appears in the MTR, where the FC value is larger than the ZFC one. This observation is similar to that commonly observed in spin glasses. The results assume that there are an inhomogeneous and a metastable phase in the (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; in which a transition is carried out during heating and cooling and leads to the hysteresis behavior. On the other hand, an essential fact should be noted in Fig. 3(b), namely, an unusually large MR effect is observed in a wide temperature region. Meanwhile, a thermal hysteresis becomes the most remarkable as shown in Fig. 4, suggesting the same underlying physical origin for them. In order to gain more information for this understanding, here we perform measurements of the magnetic field dependence of the resistivity r in a low temperature 10 K and I–M transition temperature of T P 107 K; respectively. Before each measurement, the sample was always heated to room temperature and then cooled to the desired temperature in zero field. Keeping this temperature, the measurement was carried out, the results are shown in Fig. 5. Insight into the effect of Bi3þ ion on MR effect of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 ; we first discuss the effect of Bi3þ on electrical transport of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 : The shift of T P to a lower temperature in (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 suggests that the I–M transition should be related
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
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(a)
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H(T) Fig. 5. Resistivity versus magnetic field of (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 measured at temperatures of 107 K(a) and 10 K(b) respectively. The field sweep directions are indicated by arrows.
to the Bi3þ ion. Although the ion radius of Bi3þ is close to that of La3þ ; the effect of the Bi3þ ion on its neighboring ions is different to that of La3þ ion due to the 6s2 lone pair electron of the Bi3þ ion. The 6s2 lone pair electron of the Bi3þ can be oriented to a surrounding anion (O2 ), which can produce a local distortion or even a hybridization between 6s-orbitals of Bi3þ and 2p-orbitals of O2 [11]. This hybridization would avoid the movement of eg electrons from Mn to Mn through the Mn–O–Mn bridge and favor charge order (CO). Thus, the resistivity measurements have revealed that (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 presents a very large resistivity than the pure La0:67 Ca0:33 MnO3 one, as shown in Fig. 2. The hysteresis behavior of the Bi-doped La0:67 Ca0:33 MnO3 system due to the CO state was investigated by Sun et al. [12–14]. The magnetic-field dependence of resistivity at two fixed temperatures 107 K and 10 K is illustrated in Figs. 5(a) and (b), respectively. At 107 K
temperature, as shown in Fig. 5(a), we see that magnetic field induces the I–M transition with the resistivity drop by several orders of magnitude. The r–H curve shows hysteretic in different magnetic field regions: as the field is decreased after the magnetic field 5 T; the sample cannot even return to the original high-resistivity state. With increase in the negative magnetic field, the resistivity decreases again, similar to the positive loop, as the field is decreased after the magnetic field 5 T; the sample return to the starting point of the negative field with a different curve, in which the resistivity measured in return run is smaller than that in starting run. At the temperature of 10 K, a more strong hysteretic behavior is observed. After a magnetic field process, the resistivity of the sample is decreased to a low resistivity and retains the low resistivity even as the magnetic field returns to zero. At the reverse run, the resistivity of the sample still maintains a small value. Especially, in the negative magnetic loop, no hysteresis is observed. This result suggests that the transition observed at 107 K and near a magnetic field of 2 T is metastable and the phases is inhomogeneous at the temperature of 107 K. Meanwhile, at 10 K, a transition is also observed near the magnetic field 2 T as shown in Fig. 5(b), however, the kink near 2 T is not observed in the return run. On the other hand, at the transition temperature 107 K; the zero-field resistivity is partly reproduced after the field cycling but its value turns out to be much lower than that for the origin curve. Such lowering of the zero-field resistivity can be attributed to a certain remanent spin ordering similar to that discussed for Pr0:7 Ca0:3 MnO3 [15]. The physical original of the pronounced MR and strong hysteresis will be discussed in our future papers.
4. Conclusions In this work, we have shown that the MR with a value of MR 100% for the 3 T field can be realized upon Bi3þ -doped (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 : Such an enhanced MR effect is found to appear in a wide temperature range. The experimental result is encouraging as regards to the potential
ARTICLE IN PRESS Z.C. Xia et al. / Journal of Magnetism and Magnetic Materials 292 (2005) 260–265
application of CMR materials. Meanwhile, results show that the Bi3þ doping La0:67 Ca0:33 MnO3 has a direct effect on the thermal and magnetic hysteresis effects, which may result from the inhomogeneous of phases and the CO state due to the hybridization between 6s-orbitals of Bi3þ and 2porbitals of O2 in (La0:8 Bi0:2 )0:67 Ca0:33 MnO3 :
Acknowledgements This work was partly supported by the National Science Foundation of China (Grant nos. 10174022 and 10374032), Postdoctoral Foundation of China and Huazhong University of Science and Technology, School of Science Foundation of Huazhong University of Science and Technology. References [1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331.
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