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Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite
Solid State Communications 127 (2003) 567–572 www.elsevier.com/locate/ssc
Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite Z.C. Xia, S.L. Yuan*, W. Feng, L.J. Zhang, G.H. Zhang, J. Tang, L. Liu, D.W. Liu, Q.H. Zheng, L. Chen, Z.H. Fang, S. Liu, C.Q. Tang Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 2 May 2002; accepted 5 June 2003 by C.N.R. Rao
Abstract An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe3O4 is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La0.67Ca0.33MnO3 and the impurity doped La0.67Ca0.33MnO3 composites. Compared with pure La0.67Ca0.33MnO3, a possible interpretation is presented by considering the influences of YSZ and Fe3O4 on the structure of grain boundaries and/or surfaces of La0.67Ca0.33MnO3grains. q 2003 Elsevier Ltd. All rights reserved. PACS: 71.30. þ h; 75.30.Vn Keywords: D. Insulator–metal transitions and other electronic transitions; D. Colossal magnetoresistance
1. Introduction The discovery of colossal magnetoresistance (CMR) in doped manganites and their unusual properties and potential applications in magnetoresistive transducers and sensors triggered an increasing attention [1,2]. Meanwhile, their nearly 100% spin polarization [3] may boost low field magnetoresistance (LFMR), which associates with a spinmemory contribution to change transport across the interfaces (or grain boundaries) by modifying the microstructure of manganites. Generally, extrinsic magnetoresistance (MR) effects in ferromagnetic oxides fall into three broad classes, namely grain boundary MR, spin polarized transport in ferromagnetic tunnelling junctions and domain wall MR [4]. These MR effects were reported at various samples, saying, Balcells et al. [5] and Petrov et al. [6] reported the MR effects of granular LSMO/CeO2 and * Corresponding author. Fax: þ 86-27-87544525. E-mail addresses: [email protected] (S.L. Yuan), xia9020@ public.wh.hb.cn (Z.C. Xia).
LCMO/SrTiO3 composites. Meanwhile, an enhanced LFMR value was also found in LSMO/glass composites [7], LSMO/Pr0.5Sr0.5MnO3 composites [8] and a LCMO/ Al2O3 system [9]. Spin-polarized tunneling or spin-dependent scattering between neighboring grains seems to be responsible for this kind of MR effects [3,10,11]. In MR materials, intrinsic effects to a material are distinguished from extrinsic effects which depend on the direction of magnetization in adjacent ferromagnetic regions (or domains). Examples of the former include the anisotropic MR of permalloy or the CMR of EuO and mixed-valuence manganites. Examples of the latter are the giant MR of multilayers and granular metals or the behavior of spin-dependent tunnel junctions [12]. Although the effects of high resistive phases on electrical transport properties of manganites have been investigated in some of groups, much attention generally focus on percolation effects, the extrinsic effects coming from doped high resistance phases or modified grain boundaries on magnetotransport behavior was not well clear. This present paper is to investigate the different effects of
0038-1098/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00506-4
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nonmagnetic YSZ and magnetic Fe3O4 dopant on transport properties and magnetoresistance of La0.67Ca0.33MnO3 composites. Using a special sample preparation procedure, the doped YSZ(Fe3O4) as a second phase is mainly segregated the grain boundaries and/or the surfaces of La0.67Ca0.33MnO3 grains. Experimental results show that the impurity dopant can obviously enhance low temperature MR and affect the electrical and magnetic transport behavior of La0.67Ca0.33MnO3 composites. Compared with pure La0.67Ca0.33MnO3, a possible interpretation was given on the basis of the analysis of structure and the spatial distribution of doped impurity in La0.67Ca0.33MnO3 matrix.
2. Experimental procedure In order to introduce an impurity dopant as a second phase and segregate at grain boundaries and/or surfaces of La0.67Ca0.33MnO3 phases, a special experimental procedure was desigened. Frist, a pure La0.67Ca0.33MnO3 raw powder was prepared using a standard sol-gel method and the raw powder was pre-sintered at 1200 8C for 20 h, after this process a uniform La0.67Ca0.33MnO3 powder with perovskite structure was formed. Then, the obtained La0.67Ca0.33MnO3 powder was mixed with YSZ and Fe3 O4, respectively. The homogeneous composites of nominal (1 2 x)La0.67Ca0.33MnO3 þ x YSZ (La0.67Ca0.33MnO3/YSZ) and (1 2 x)La0.67Ca0.33MnO3 þ x Fe3O4 (La0.67Ca0.33MnO3/Fe3O4) were obtained (x ¼1 mol%). Finally, the pure La0.67Ca0.33MnO3 and two composites, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4, were sintered at 1350 8C for 6 h with the same procedure. The distribution of the dopants in the matrix was analyzed with scanning electron microscopy (SEM) and electron probe analyzer (EPA), the crystal structure of the resulting samples was checked by X-ray diffraction (XRD). The XRD analysis confirms that the homogeneous composite is closed to the nominal composition. The resistivity r, susceptibility x, and magnetization M were measured on a commercial Physical Property Measured System (PPMS, Quantum Design) in magnetic field range of 0 – 6 T and temperature range of 10 – 300 K.
3. Results and discussion The raw fracture cross-sections of all samples were examined with SEM. In pure La0.67Ca0.33MnO3, a uniform grain and clear grain boundary are observed as shown in Fig. 1(a). In the impurity doped composites, the spatial distribution of doped Fe3O4(YSZ) in composites were analyzed with EPA. As show in Fig. 1(b) and (c), the doped YSZ and Fe3O4 present mainly at the grain boundaries and/or surfaces of La0.67Ca0.33MnO3. The segregation behavior of the dopants can be understood by
considering the special experimental procedure, in which the La0.67Ca0.33MnO3 grain has been formed before mixed with the impurities of YSZ and Fe3O4, thus, the dopant is mainly distributed at grain boundaries and/or surfaces of La0.67Ca0.33MnO3 phases. In YSZ doped composite, as shown in Fig. 1(b), a clear segregation is observed at the grain boundaries. EPA shows the segregation consists mainly of YSZ. In the Fe3O4 doped composite, as shown in Fig. 1(c), partial diffuse and reaction between La0.67Ca0.33MnO3 and Fe3O4 as well as a further oxide of Fe3O4 may be taken due to the chemical activity of Fe3O4, thus, the grain boundaries of the matrix become ambiguity. The crystal structure of all the samples was characterized with XRD. As shown in Fig. 2, the pure La0.67Ca0.33MnO3 is a single phase with a perovskite structure. Compared with the pure La0.67Ca0.33MnO3, two composites exhibit a superposition of La0.67Ca0.33MnO3 and an impurity phase. In the XRD patterns of the composites, the added XRD patterns could be indexed with that of the Fe3O4(Fe2O3) and YSZ, respectively. Together with the SEM images, the superposition behavior of XRD patterns suggest, on one hand, the doped Fe3O4(YSZ) did not change the crystal structure of La0.67Ca0.33MnO3 phase, on the other hand, the La0.67Ca0.33MnO3 and doped impurities were coexistence in the composites. In the YSZ doped composite, it is conventional assume that the doped YSZ is randomly distributed at the grain boundaries and/or surfaces of La0.67Ca0.33MnO3. Together with the SEM image, the composite clearly contain chemically well-separated crystalline of La0.67Ca0.33MnO3 and YSZ grains due to the high stability of YSZ at high temperature. The similar segregation behavior was reported in our other paper [13]. However, in the Fe3O4 doped composite, a new phase of Fe2O3 is observed in the XRD, the added new phase of Fe2O3 may result from the oxidizing of Fe3O4 in the high temperature sintering procedure. As shown in Fig. 1(c), the grain boundaries of matrix become ambiguity due to the segregation of impurity and reaction between La0.67Ca0.33MnO3 and Fe3O4. The electrical transport behavior of pure La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4 were experimentally studied by measuring resistivity r as a function of temperature, the results are shown in Fig. 3. All the resistivity was measured by means of a standard four-probe method in various magnetic fields using PPMS. In pure La0.67Ca0.33MnO3, an insulator-metal (I-M) transition behavior was observed at a temperature ,261 K as shown in Fig. 3(a), which is an intrinsic behavior of La0.67Ca0.33MnO3 resulting mainly from a paramagnetic – ferromagnetic (PM – FM) transition. Compared to pure La0.67Ca0.33MnO3, a similar I-M transition peak was observed at the composite of La0.67Ca0.33MnO3/YSZ, but the I-M transition temperature TP was shifted to a lower temperature , 206 K. In the composite of La0.67Ca0.33MnO3/Fe3O4, it is interesting to notice that two I-M transition peaks were observed at temperatures 205 and
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Fig. 1. SEM images of (a) La0.67Ca0.33MnO3, (b) La0.67Ca 0.33MnO3/YSZ and (c) La0.67Ca0.33MnO3/Fe3O4, respectively. The magnification is 3000.
184 K, respectively. Compared with pure La0.67Ca0.33MnO3, (1) the I-M transition temperatures of the impurity doped composites are obvious lower than that of pure La0.67Ca0.33MnO3, and (2) the resistivity of doped composites, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4, is larger than that of pure La0.67Ca0.33MnO3. Analysis of the stoichiometry of the three samples, the new electrical transport behavior and higher resistivity of the composites should be related to the dopants of YSZ and Fe3O4. That is, the different electrical transport properties should result from the effects of the dopant on the electron transport channels in the composites. In pure La0.67Ca0.33MnO3, the electrical transport is achieved through a direct contact between La0.67Ca0.33MnO3 grains. However, in the doped composites, together with literature reports [14,15], we suggest that there are existence of two kinds of conduction channels parallel connected in the doped composites. One is related to the La0.67Ca0.33MnO3 grains, which determines the transport properties of the system and is achieved through the direct contact between La0.67Ca0.33MnO3 grains. The other is related to the dopants of YSZ and Fe3O4, respectively, since the dopant was mainly distributed at the grain boundaries and/or surfaces of La0.67Ca0.33MnO3 grains showing energy barriers to electrical transport process, an obvious higher resistivity was observed at the doped composites as shown in Fig. 3(b) and(c). Meanwhile,
Fig. 2. XRD patterns of (a) La0.67Ca0.33MnO3, (b) La0.67Ca0.33MnO3/Fe3O4 and (c) La0.67Ca0.33MnO3/YSZ, respectively.
Fig. 3. Temperature dependence of resistivity of (a) La0.67Ca0.33MnO3, (b) La0.67Ca0.33MnO3/YSZ and (c) La0.67Ca0.33MnO3/Fe3O4 measured in magnetic fields 0, 0.3, 0.5 and 1 T, respectively.
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considering the higher resistivity of YSZ, it is easy to understand that the resistivity of La0.67Ca0.33MnO3/YSZ is higher than that of La0.67Ca0.33MnO3/Fe3O4 and pure La0.67Ca0.33MnO3. The formation of two I-M transition in the La0.67Ca0.33MnO3/Fe3O4 composite may result from the different phases of La0.67Ca0.33MnO3 and La0.67Ca0.33 (MnFe)O3, the detail will be discussed in other paper. Compared to pure La0.67Ca0.33MnO3, an enhanced magnetoresistance MR0(MR0 ¼ (r(0) 2 r(H)/r(0), where r(0) and r(H) are the resistivity measured at zero- and applied magnetic(3 T), respectively) was observed in both the composites. The temperature dependence of MR0of pure La0.67Ca0.33MnO3 and composites are shown in Fig. 4. Usually, the lower temperature MR results mainly from a grain boundary effect [3]. Thus, in this paper, the enhanced low temperature MR suggests further that the doped impurities have an obviously modifying effect on the microstructure of the grain boundaries and/or the surfaces of La0.67Ca0.33MnO3. The influence of dopant on the magnetization behavior was investigated by measuring a magnetization and susceptibility. The temperature and magnetic field dependence of magnetizations of pure La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4 are shown in Fig. 5(a), (b) and (c), respectively. It is well known that, at zero-field, the magnetization directions of granular ferromagnet grains are randomly oriented due to the random orientation of their magnetocrystalline anisotropy axes. Thus, the spontaneous magnetization is directly related to the microstructure of the magnetocrystalline. In pure La0.67Ca0.33MnO3, with the increasing in applied field, the low temperature magnetization increases, in a higher magnetic fields ,1 T, the magnetization (,96.5 emu/g) is nearly saturated and of equal to the theory value of 98 (emu/ g). However, in the doped composites, a double-fold effect resulting from the impurity was observed. In the YSZ doped composite, at lower magnetic fields, such as 0.3 T as shown Fig. 5. Temperature dependence of magnetization of (a) La0.67Ca0.33MnO3, (b) La0.67Ca0.33MnO3/YSZ and (c) La0.67Ca0.33MnO3/Fe3O4. Measured in magnetic fields of 0.1, 0.3, 0.5 and 1 T, respectively.
Fig. 4. Temperature dependence of magnetoresistance MR0 of La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/ Fe3O4, measured in magnetic field ,3 T.
in Fig. 5(b), the magnetization (,40 emu/g) is lower that of pure La0.67Ca0.33MnO3 (, 60 emu/g) due to the pinning effects of YSZ segregated at grain boundaries or surfaces of La0.67Ca0.33MnO3 FM domains, with the increasing in magnetic field, such as 0.5 T, the applied field could made the La0.67Ca0.33MnO3 FM domains overcome the pinning effects and rotate and results in magnetization rapid increasing. On the other hand, since the segregation of nonmagnetic YSZ at grain boundaries or surfaces of La0.67Ca0.33MnO3 FM domains, the magnetic interaction between neighboring La0.67Ca0.33MnO3 FM domains become weaker, the YSZ doped composite is much easier to attain saturation, as shown in Fig. 5(b), the magnetization
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was attained to near-saturation at a lower magnetic field , 0.5 T (the magnetization , 91.5 emu/g). In the Fe3O4 doped composite, an unusual enhanced magnetization were observed at a lower magnetic field, as shown in Fig. 5(c), the magnetization is 87 emu/g at 0.3 T, which is much higher than that of pure La0.67Ca0.33MnO3 (60 emu/g) and YSZ (45 emu/g). The magnetic field dependence of magnetization measured at 10 K also exhibits the similar result (as shown in Fig. 6). A saturation magnetization was observed at , 1 T for pure La0.67Ca0.33MnO3, 0.5 T for La0.67Ca0.33MnO3/YSZ and 0.3 T for La0.67Ca0.33MnO3/Fe3O4, respectively. From the analysis of low field magnetization behavior, the different magnetization could be assumed that the dopant have modified the microstructure, especially the structure of boundaries and surfaces of La0.67Ca0.33MnO3 grains or domains. Usually, a magnetization of impurity doped composite should be smaller than that of pure La0.67Ca0.33MnO3 due to a volume dilution, in high fields, the saturation magnetizations of the three samples are well consistent with the dilution behavior. For the Fe3O4 doped composite, the high field magnetization is smaller than that of YSZ doped composite may result from the antiferromagnetic Fe2O3. However, in lower magnetic fields, the magnetization behavior is total deviated to the results. According to the results, we could assume that the ability of the magnetic impurity Fe3O4 to induce the FM– PM transition is greater than that of the nonmagnetic impurity YSZ. Thus, a maximum magnetization ,60 emu/g measured at 0.1 T at magnetic impurity Fe3O4 doped composite, whereas the magnetization is reached only for 25 emu/g at the YSZ doped composite. This shows that a magnetic impurity Fe3O4 is able to participate in the formation of FM domains, and a nonmagnetic impurity YSZ could act as obstacles for the formation of FM domains. Usually, TP (near Curie temperature TC ) is an intrinsic property of manganites, which is mainly determined by the double-exchange interaction between Mn ions of La0.67Ca0.33MnO3 grains. However, the extrinsic influences (such as, grain size in polycrystalline samples) dramatically
modify this interaction [16]. For the magnetic transports, the disorder of structure, especially at the domain walls of La0.67Ca0.33MnO3 FM domains, has a crucial role on the PM– FM transition. In this present sample preparation procedure, the doped YSZ (or Fe3O4) is mainly segregated at boundaries and/or surfaces of La0.67Ca0.33MnO3 grains (or domains), thus, the dopant has directly effects on the transport properties across the boundaries or surfaces of La0.67Ca0.33MnO3 FM domains. At lower temperatures T , TC , in pure La0.67Ca0.33MnO3, at zero-field and low temperature, there is a strong spin coupling between the contacting La0.67Ca0.33MnO3 FM domains, by which the spin inside La0.67Ca0.33MnO3 FM domains near the contacting boundaries prefer antiparallel alignment (AFM type), thus, the existence of grain boundaries will decrease the magnetization. With increase of applied magnetic fields, the applied magnetic fields suppressed the antiparallel alignment and attain to a saturation magnetization. In the lower Fe3O4 doping level , 1 mol%, the magnetic impurity, one hand, can induce the FM coupling between neighboring La0.67Ca0.33MnO3 FM domains, thus, an enhanced lower field magnetization was observed. However, in the nonmagnetic impurity YSZ, the segregation of YSZ at grain boundaries or surfaces of La0.67Ca0.33MnO3 FM domains, which lead to the interaction become weak and make the La0.67Ca0.33MnO3 FM domains rotation become easy and a saturation magnetization was easy obtained at a lower field of 0.5 T. On the other hand, the segregated YSZ has a pinning effect on the rotation of La0.67Ca0.33MnO3 FM domains, thus, at a much lower magnetic field , 0.3 T, the magnetization become much low ,45 emu/g as shown in Fig. 5(b). On the other hand, the doped impurities have pinning effects on the PM – FM transition, especially at the temperature TC . Thus, a hysteresis at TC was observed at both the impurities doped La0.67Ca0.33MnO3 composites as shown in Fig. 7. Which shows the temperature dependence of the real (x0 ) part of the ac susceptibility x, where the susceptibility was measured at magnetic field 10 Oe and
Fig. 6. Magnetic field dependence of magnetization of La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4, measured in temperature ,10 K and magnetic field range of 0–6 T.
Fig. 7. Temperature dependence of ac susceptibility (zfc and fc) of La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4 measured in magnetic field 10 Oe and frequency 113 Hz.
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frequency 113 Hz. On cooling (field cooling), the PM –FM transition temperatures of both the composites shift to lower temperature, however, on warming (zero-field cooling), the PM– FM transition temperatures of both the composites shift to higher temperature. For pure La0.67Ca0.33MnO3, no obvious hysteresis was observed at the same measured conditions.
4. Summary Both the nonmagnetic YSZ and magnetic Fe3O4 impurities have a two-fold effect on the transport properties of La0.67Ca0.33MnO3. At lower magnetic fields (much lower than saturation fields), the doped impurities have directly an enhancement effect on the magnetization, which leads to an enhanced low temperature magnetoresistance effect on the impurity doped La0.67Ca0.33MnO3 composites. However, at higher magnetic, both the impurities have an obvious suppression effect on the magnetization,
Acknowledgements This work was supported by the National Science Foundation of China (Grand No. 10174022) and Post-D octial Foundation and School of Science Foundation of Huazhong University of Science and Technology.
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Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite Z.C. Xia, S.L. Yuan*, W. Feng, L.J. Zhang, G.H. Zhang, J. Tang, L. Liu, D.W. Liu, Q.H. Zheng, L. Chen, Z.H. Fang, S. Liu, C.Q. Tang Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China Received 2 May 2002; accepted 5 June 2003 by C.N.R. Rao
Abstract An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe3O4 is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La0.67Ca0.33MnO3 and the impurity doped La0.67Ca0.33MnO3 composites. Compared with pure La0.67Ca0.33MnO3, a possible interpretation is presented by considering the influences of YSZ and Fe3O4 on the structure of grain boundaries and/or surfaces of La0.67Ca0.33MnO3grains. q 2003 Elsevier Ltd. All rights reserved. PACS: 71.30. þ h; 75.30.Vn Keywords: D. Insulator–metal transitions and other electronic transitions; D. Colossal magnetoresistance
1. Introduction The discovery of colossal magnetoresistance (CMR) in doped manganites and their unusual properties and potential applications in magnetoresistive transducers and sensors triggered an increasing attention [1,2]. Meanwhile, their nearly 100% spin polarization [3] may boost low field magnetoresistance (LFMR), which associates with a spinmemory contribution to change transport across the interfaces (or grain boundaries) by modifying the microstructure of manganites. Generally, extrinsic magnetoresistance (MR) effects in ferromagnetic oxides fall into three broad classes, namely grain boundary MR, spin polarized transport in ferromagnetic tunnelling junctions and domain wall MR [4]. These MR effects were reported at various samples, saying, Balcells et al. [5] and Petrov et al. [6] reported the MR effects of granular LSMO/CeO2 and * Corresponding author. Fax: þ 86-27-87544525. E-mail addresses: [email protected] (S.L. Yuan), xia9020@ public.wh.hb.cn (Z.C. Xia).
LCMO/SrTiO3 composites. Meanwhile, an enhanced LFMR value was also found in LSMO/glass composites [7], LSMO/Pr0.5Sr0.5MnO3 composites [8] and a LCMO/ Al2O3 system [9]. Spin-polarized tunneling or spin-dependent scattering between neighboring grains seems to be responsible for this kind of MR effects [3,10,11]. In MR materials, intrinsic effects to a material are distinguished from extrinsic effects which depend on the direction of magnetization in adjacent ferromagnetic regions (or domains). Examples of the former include the anisotropic MR of permalloy or the CMR of EuO and mixed-valuence manganites. Examples of the latter are the giant MR of multilayers and granular metals or the behavior of spin-dependent tunnel junctions [12]. Although the effects of high resistive phases on electrical transport properties of manganites have been investigated in some of groups, much attention generally focus on percolation effects, the extrinsic effects coming from doped high resistance phases or modified grain boundaries on magnetotransport behavior was not well clear. This present paper is to investigate the different effects of
0038-1098/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00506-4
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nonmagnetic YSZ and magnetic Fe3O4 dopant on transport properties and magnetoresistance of La0.67Ca0.33MnO3 composites. Using a special sample preparation procedure, the doped YSZ(Fe3O4) as a second phase is mainly segregated the grain boundaries and/or the surfaces of La0.67Ca0.33MnO3 grains. Experimental results show that the impurity dopant can obviously enhance low temperature MR and affect the electrical and magnetic transport behavior of La0.67Ca0.33MnO3 composites. Compared with pure La0.67Ca0.33MnO3, a possible interpretation was given on the basis of the analysis of structure and the spatial distribution of doped impurity in La0.67Ca0.33MnO3 matrix.
2. Experimental procedure In order to introduce an impurity dopant as a second phase and segregate at grain boundaries and/or surfaces of La0.67Ca0.33MnO3 phases, a special experimental procedure was desigened. Frist, a pure La0.67Ca0.33MnO3 raw powder was prepared using a standard sol-gel method and the raw powder was pre-sintered at 1200 8C for 20 h, after this process a uniform La0.67Ca0.33MnO3 powder with perovskite structure was formed. Then, the obtained La0.67Ca0.33MnO3 powder was mixed with YSZ and Fe3 O4, respectively. The homogeneous composites of nominal (1 2 x)La0.67Ca0.33MnO3 þ x YSZ (La0.67Ca0.33MnO3/YSZ) and (1 2 x)La0.67Ca0.33MnO3 þ x Fe3O4 (La0.67Ca0.33MnO3/Fe3O4) were obtained (x ¼1 mol%). Finally, the pure La0.67Ca0.33MnO3 and two composites, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4, were sintered at 1350 8C for 6 h with the same procedure. The distribution of the dopants in the matrix was analyzed with scanning electron microscopy (SEM) and electron probe analyzer (EPA), the crystal structure of the resulting samples was checked by X-ray diffraction (XRD). The XRD analysis confirms that the homogeneous composite is closed to the nominal composition. The resistivity r, susceptibility x, and magnetization M were measured on a commercial Physical Property Measured System (PPMS, Quantum Design) in magnetic field range of 0 – 6 T and temperature range of 10 – 300 K.
3. Results and discussion The raw fracture cross-sections of all samples were examined with SEM. In pure La0.67Ca0.33MnO3, a uniform grain and clear grain boundary are observed as shown in Fig. 1(a). In the impurity doped composites, the spatial distribution of doped Fe3O4(YSZ) in composites were analyzed with EPA. As show in Fig. 1(b) and (c), the doped YSZ and Fe3O4 present mainly at the grain boundaries and/or surfaces of La0.67Ca0.33MnO3. The segregation behavior of the dopants can be understood by
considering the special experimental procedure, in which the La0.67Ca0.33MnO3 grain has been formed before mixed with the impurities of YSZ and Fe3O4, thus, the dopant is mainly distributed at grain boundaries and/or surfaces of La0.67Ca0.33MnO3 phases. In YSZ doped composite, as shown in Fig. 1(b), a clear segregation is observed at the grain boundaries. EPA shows the segregation consists mainly of YSZ. In the Fe3O4 doped composite, as shown in Fig. 1(c), partial diffuse and reaction between La0.67Ca0.33MnO3 and Fe3O4 as well as a further oxide of Fe3O4 may be taken due to the chemical activity of Fe3O4, thus, the grain boundaries of the matrix become ambiguity. The crystal structure of all the samples was characterized with XRD. As shown in Fig. 2, the pure La0.67Ca0.33MnO3 is a single phase with a perovskite structure. Compared with the pure La0.67Ca0.33MnO3, two composites exhibit a superposition of La0.67Ca0.33MnO3 and an impurity phase. In the XRD patterns of the composites, the added XRD patterns could be indexed with that of the Fe3O4(Fe2O3) and YSZ, respectively. Together with the SEM images, the superposition behavior of XRD patterns suggest, on one hand, the doped Fe3O4(YSZ) did not change the crystal structure of La0.67Ca0.33MnO3 phase, on the other hand, the La0.67Ca0.33MnO3 and doped impurities were coexistence in the composites. In the YSZ doped composite, it is conventional assume that the doped YSZ is randomly distributed at the grain boundaries and/or surfaces of La0.67Ca0.33MnO3. Together with the SEM image, the composite clearly contain chemically well-separated crystalline of La0.67Ca0.33MnO3 and YSZ grains due to the high stability of YSZ at high temperature. The similar segregation behavior was reported in our other paper [13]. However, in the Fe3O4 doped composite, a new phase of Fe2O3 is observed in the XRD, the added new phase of Fe2O3 may result from the oxidizing of Fe3O4 in the high temperature sintering procedure. As shown in Fig. 1(c), the grain boundaries of matrix become ambiguity due to the segregation of impurity and reaction between La0.67Ca0.33MnO3 and Fe3O4. The electrical transport behavior of pure La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4 were experimentally studied by measuring resistivity r as a function of temperature, the results are shown in Fig. 3. All the resistivity was measured by means of a standard four-probe method in various magnetic fields using PPMS. In pure La0.67Ca0.33MnO3, an insulator-metal (I-M) transition behavior was observed at a temperature ,261 K as shown in Fig. 3(a), which is an intrinsic behavior of La0.67Ca0.33MnO3 resulting mainly from a paramagnetic – ferromagnetic (PM – FM) transition. Compared to pure La0.67Ca0.33MnO3, a similar I-M transition peak was observed at the composite of La0.67Ca0.33MnO3/YSZ, but the I-M transition temperature TP was shifted to a lower temperature , 206 K. In the composite of La0.67Ca0.33MnO3/Fe3O4, it is interesting to notice that two I-M transition peaks were observed at temperatures 205 and
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Fig. 1. SEM images of (a) La0.67Ca0.33MnO3, (b) La0.67Ca 0.33MnO3/YSZ and (c) La0.67Ca0.33MnO3/Fe3O4, respectively. The magnification is 3000.
184 K, respectively. Compared with pure La0.67Ca0.33MnO3, (1) the I-M transition temperatures of the impurity doped composites are obvious lower than that of pure La0.67Ca0.33MnO3, and (2) the resistivity of doped composites, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4, is larger than that of pure La0.67Ca0.33MnO3. Analysis of the stoichiometry of the three samples, the new electrical transport behavior and higher resistivity of the composites should be related to the dopants of YSZ and Fe3O4. That is, the different electrical transport properties should result from the effects of the dopant on the electron transport channels in the composites. In pure La0.67Ca0.33MnO3, the electrical transport is achieved through a direct contact between La0.67Ca0.33MnO3 grains. However, in the doped composites, together with literature reports [14,15], we suggest that there are existence of two kinds of conduction channels parallel connected in the doped composites. One is related to the La0.67Ca0.33MnO3 grains, which determines the transport properties of the system and is achieved through the direct contact between La0.67Ca0.33MnO3 grains. The other is related to the dopants of YSZ and Fe3O4, respectively, since the dopant was mainly distributed at the grain boundaries and/or surfaces of La0.67Ca0.33MnO3 grains showing energy barriers to electrical transport process, an obvious higher resistivity was observed at the doped composites as shown in Fig. 3(b) and(c). Meanwhile,
Fig. 2. XRD patterns of (a) La0.67Ca0.33MnO3, (b) La0.67Ca0.33MnO3/Fe3O4 and (c) La0.67Ca0.33MnO3/YSZ, respectively.
Fig. 3. Temperature dependence of resistivity of (a) La0.67Ca0.33MnO3, (b) La0.67Ca0.33MnO3/YSZ and (c) La0.67Ca0.33MnO3/Fe3O4 measured in magnetic fields 0, 0.3, 0.5 and 1 T, respectively.
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considering the higher resistivity of YSZ, it is easy to understand that the resistivity of La0.67Ca0.33MnO3/YSZ is higher than that of La0.67Ca0.33MnO3/Fe3O4 and pure La0.67Ca0.33MnO3. The formation of two I-M transition in the La0.67Ca0.33MnO3/Fe3O4 composite may result from the different phases of La0.67Ca0.33MnO3 and La0.67Ca0.33 (MnFe)O3, the detail will be discussed in other paper. Compared to pure La0.67Ca0.33MnO3, an enhanced magnetoresistance MR0(MR0 ¼ (r(0) 2 r(H)/r(0), where r(0) and r(H) are the resistivity measured at zero- and applied magnetic(3 T), respectively) was observed in both the composites. The temperature dependence of MR0of pure La0.67Ca0.33MnO3 and composites are shown in Fig. 4. Usually, the lower temperature MR results mainly from a grain boundary effect [3]. Thus, in this paper, the enhanced low temperature MR suggests further that the doped impurities have an obviously modifying effect on the microstructure of the grain boundaries and/or the surfaces of La0.67Ca0.33MnO3. The influence of dopant on the magnetization behavior was investigated by measuring a magnetization and susceptibility. The temperature and magnetic field dependence of magnetizations of pure La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4 are shown in Fig. 5(a), (b) and (c), respectively. It is well known that, at zero-field, the magnetization directions of granular ferromagnet grains are randomly oriented due to the random orientation of their magnetocrystalline anisotropy axes. Thus, the spontaneous magnetization is directly related to the microstructure of the magnetocrystalline. In pure La0.67Ca0.33MnO3, with the increasing in applied field, the low temperature magnetization increases, in a higher magnetic fields ,1 T, the magnetization (,96.5 emu/g) is nearly saturated and of equal to the theory value of 98 (emu/ g). However, in the doped composites, a double-fold effect resulting from the impurity was observed. In the YSZ doped composite, at lower magnetic fields, such as 0.3 T as shown Fig. 5. Temperature dependence of magnetization of (a) La0.67Ca0.33MnO3, (b) La0.67Ca0.33MnO3/YSZ and (c) La0.67Ca0.33MnO3/Fe3O4. Measured in magnetic fields of 0.1, 0.3, 0.5 and 1 T, respectively.
Fig. 4. Temperature dependence of magnetoresistance MR0 of La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/ Fe3O4, measured in magnetic field ,3 T.
in Fig. 5(b), the magnetization (,40 emu/g) is lower that of pure La0.67Ca0.33MnO3 (, 60 emu/g) due to the pinning effects of YSZ segregated at grain boundaries or surfaces of La0.67Ca0.33MnO3 FM domains, with the increasing in magnetic field, such as 0.5 T, the applied field could made the La0.67Ca0.33MnO3 FM domains overcome the pinning effects and rotate and results in magnetization rapid increasing. On the other hand, since the segregation of nonmagnetic YSZ at grain boundaries or surfaces of La0.67Ca0.33MnO3 FM domains, the magnetic interaction between neighboring La0.67Ca0.33MnO3 FM domains become weaker, the YSZ doped composite is much easier to attain saturation, as shown in Fig. 5(b), the magnetization
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was attained to near-saturation at a lower magnetic field , 0.5 T (the magnetization , 91.5 emu/g). In the Fe3O4 doped composite, an unusual enhanced magnetization were observed at a lower magnetic field, as shown in Fig. 5(c), the magnetization is 87 emu/g at 0.3 T, which is much higher than that of pure La0.67Ca0.33MnO3 (60 emu/g) and YSZ (45 emu/g). The magnetic field dependence of magnetization measured at 10 K also exhibits the similar result (as shown in Fig. 6). A saturation magnetization was observed at , 1 T for pure La0.67Ca0.33MnO3, 0.5 T for La0.67Ca0.33MnO3/YSZ and 0.3 T for La0.67Ca0.33MnO3/Fe3O4, respectively. From the analysis of low field magnetization behavior, the different magnetization could be assumed that the dopant have modified the microstructure, especially the structure of boundaries and surfaces of La0.67Ca0.33MnO3 grains or domains. Usually, a magnetization of impurity doped composite should be smaller than that of pure La0.67Ca0.33MnO3 due to a volume dilution, in high fields, the saturation magnetizations of the three samples are well consistent with the dilution behavior. For the Fe3O4 doped composite, the high field magnetization is smaller than that of YSZ doped composite may result from the antiferromagnetic Fe2O3. However, in lower magnetic fields, the magnetization behavior is total deviated to the results. According to the results, we could assume that the ability of the magnetic impurity Fe3O4 to induce the FM– PM transition is greater than that of the nonmagnetic impurity YSZ. Thus, a maximum magnetization ,60 emu/g measured at 0.1 T at magnetic impurity Fe3O4 doped composite, whereas the magnetization is reached only for 25 emu/g at the YSZ doped composite. This shows that a magnetic impurity Fe3O4 is able to participate in the formation of FM domains, and a nonmagnetic impurity YSZ could act as obstacles for the formation of FM domains. Usually, TP (near Curie temperature TC ) is an intrinsic property of manganites, which is mainly determined by the double-exchange interaction between Mn ions of La0.67Ca0.33MnO3 grains. However, the extrinsic influences (such as, grain size in polycrystalline samples) dramatically
modify this interaction [16]. For the magnetic transports, the disorder of structure, especially at the domain walls of La0.67Ca0.33MnO3 FM domains, has a crucial role on the PM– FM transition. In this present sample preparation procedure, the doped YSZ (or Fe3O4) is mainly segregated at boundaries and/or surfaces of La0.67Ca0.33MnO3 grains (or domains), thus, the dopant has directly effects on the transport properties across the boundaries or surfaces of La0.67Ca0.33MnO3 FM domains. At lower temperatures T , TC , in pure La0.67Ca0.33MnO3, at zero-field and low temperature, there is a strong spin coupling between the contacting La0.67Ca0.33MnO3 FM domains, by which the spin inside La0.67Ca0.33MnO3 FM domains near the contacting boundaries prefer antiparallel alignment (AFM type), thus, the existence of grain boundaries will decrease the magnetization. With increase of applied magnetic fields, the applied magnetic fields suppressed the antiparallel alignment and attain to a saturation magnetization. In the lower Fe3O4 doping level , 1 mol%, the magnetic impurity, one hand, can induce the FM coupling between neighboring La0.67Ca0.33MnO3 FM domains, thus, an enhanced lower field magnetization was observed. However, in the nonmagnetic impurity YSZ, the segregation of YSZ at grain boundaries or surfaces of La0.67Ca0.33MnO3 FM domains, which lead to the interaction become weak and make the La0.67Ca0.33MnO3 FM domains rotation become easy and a saturation magnetization was easy obtained at a lower field of 0.5 T. On the other hand, the segregated YSZ has a pinning effect on the rotation of La0.67Ca0.33MnO3 FM domains, thus, at a much lower magnetic field , 0.3 T, the magnetization become much low ,45 emu/g as shown in Fig. 5(b). On the other hand, the doped impurities have pinning effects on the PM – FM transition, especially at the temperature TC . Thus, a hysteresis at TC was observed at both the impurities doped La0.67Ca0.33MnO3 composites as shown in Fig. 7. Which shows the temperature dependence of the real (x0 ) part of the ac susceptibility x, where the susceptibility was measured at magnetic field 10 Oe and
Fig. 6. Magnetic field dependence of magnetization of La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4, measured in temperature ,10 K and magnetic field range of 0–6 T.
Fig. 7. Temperature dependence of ac susceptibility (zfc and fc) of La0.67Ca0.33MnO3, La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4 measured in magnetic field 10 Oe and frequency 113 Hz.
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frequency 113 Hz. On cooling (field cooling), the PM –FM transition temperatures of both the composites shift to lower temperature, however, on warming (zero-field cooling), the PM– FM transition temperatures of both the composites shift to higher temperature. For pure La0.67Ca0.33MnO3, no obvious hysteresis was observed at the same measured conditions.
4. Summary Both the nonmagnetic YSZ and magnetic Fe3O4 impurities have a two-fold effect on the transport properties of La0.67Ca0.33MnO3. At lower magnetic fields (much lower than saturation fields), the doped impurities have directly an enhancement effect on the magnetization, which leads to an enhanced low temperature magnetoresistance effect on the impurity doped La0.67Ca0.33MnO3 composites. However, at higher magnetic, both the impurities have an obvious suppression effect on the magnetization,
Acknowledgements This work was supported by the National Science Foundation of China (Grand No. 10174022) and Post-D octial Foundation and School of Science Foundation of Huazhong University of Science and Technology.
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