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xCoFe2O4 composites
Journal of Alloys and Compounds 474 (2009) 316–320
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrical properties and magnetoelectric effect measurement in La0.7 Ca0.2 Sr0.1 MnO3 /xCoFe2 O4 composites C.S. Xiong ∗ , F.F. Wei, Y.H. Xiong, L.J. Li, Z.M. Ren, X.C. Bao, Y. Zeng, Y.B. Pi, Y.P. Zhou, X. Wu, C.F. Zheng Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 27 April 2008 Received in revised form 15 June 2008 Accepted 17 June 2008 Available online 26 July 2008 PACS: 77.84.Lf 82.33.Pt 75.30.Kz 75.30.Vn 71.30.+h Keywords: Composite materials Solid state reaction Grain boundary Electrical transport Low-field magnetoresistance
a b s t r a c t Composites with compositions La0.7 Ca0.2 Sr0.1 MnO3 (LCSMO)/xCoFe2 O4 (CFO) were prepared by a standard ceramic technique. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and CFO grains, and that CFO segregates mostly at the grain boundaries of LCSMO. The variation in resistivity with temperature has been studied and shows a semiconducting behavior, furthermore the composites exhibit metallic percolation threshold at x = xP = 8%. The magnetic moment ( S ) changed with the increase of CFO. Curie temperature (TC ) is a dividing point; T < TC , S decreases with CFO content increasing, while T >TC , S changes contrarily with the addition of CFO. The magnetoresistance (MR) effect is enhanced at a wide temperature range in an applied magnetic field (3 kOe). The spin-polarized tunneling and the spin-dependent scattering may be attributed to the enhanced low-field magnetoresistance (LFMR) effect. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, colossal magnetoresistance (CMR) has been a hot topic in condense matter physics and material sciences. Besides a fundamental understanding of the mechanism of the CMR effect, the studies have been attracted by potential applications of these materials, too. In the La1−x Ax MnO3 (A = Sr, Ca, Ba) system, there have been extensive research activities on these doped perovskites [1–4]. There are two kinds of CMR effects found in these manganites: intrinsic CMR and extrinsic CMR. The intrinsic CMR effect, which is mostly observed near TC , can be explained by the double exchange (DE) mechanism proposed by Zener [5]. However, the extrinsic CMR is usually thought to be caused by natural and artificial grain boundaries [6,7]. In particular that it is sensitive to low magnetic field and can be largely enhanced in a wide temperature range below TC , the effect is called low-field magnetoresistance (LFMR) effect. The LFMR is due to spin-polarized tunneling [8] or spin-dependent
∗ Corresponding author. Tel.: +86 27 87556914; fax: +86 27 87556914. E-mail address: [email protected] (C.S. Xiong). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.071
scattering [6] between neighboring grains and has potential applications. In order to enhance the LFMR, a lot of efforts have been done, such as making composites of these CMR materials with secondary phases including LCMO/Fe3 O4 [9], LCMO/CuMn2 O4 [10], LSMO/CoFe2 O4 [11], LSMO/TiO2 [12], LBMO/YSZ [13], LSMO/NiO [14] and so on. In this paper, we choose LCSMO and CoFe2 O4 (CFO) as the matrix material and insulator barrier, respectively. There are three reasons: (1) LCSMO is a kind of CMR materials. The TP (near TC ) of pure LCSMO is about 310 K, which is close to the room temperature. (2) CFO belongs to spinel ferrites material. Spinel ferrite is one of the most significant magnetic materials, which are extensively used in modern electronic technologies. Nanoparticles of spinel ferrites are of practical interest in a wide range of applications like high-density magnetic information storage, magnetic resonance imaging, targeted drug deliver, etc. [15,16]. Spinel ferrites own high-electrical resistivities, low eddy current and dielectric losses. (3) We have synthesized a homogeneous composite and introduced CFO as the second phase of the composites, which makes an enhanced magnetoresistance at a wide temperature range.
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2. Experimental procedure Composites with compositions La0.7 Ca0.2 Sr0.1 MnO3 /xCoFe2 O4 , in which x varies as 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, and 20 wt%, were prepared by a standard ceramic technique. The matrix phase, i.e. La0.7 Ca0.2 Sr0.1 MnO3 , was prepared through a standard solid state reaction, using La2 O3 , CaCO3 , SrCO3 , and MnO2 powders in weight proportions as starting materials. Similarly, the insulator barrier phase, CoFe2 O4 was prepared by solid reaction method, using Co2 O3 and Fe3 O4 as original materials. The mixtures were all presintered at 800 ◦ C for 10 h, and then, the resulting mixtures were sintered at 1400 ◦ C for 10 h and at 1200 ◦ C for 10 h, respectively. After sintering, the constituent phases were ground to fine power. Composites LCSMO/x CFO were prepared by properly mixing with as-prepared LCSMO and CFO powder, and palletized at a pressure of 10 MPa/cm2 ; then the samples in the forms of pellets were sintered at 1000 ◦ C for 3 h in a programmable furnace and slowly cooled to room temperature. In order to exclude possibility of chemical reaction between LCSMO and CFO, the final sintering temperature of the composite samples was kept lower than those of LCSMO by 1400 ◦ C and CFO by 1200 ◦ C. The structural characterization was examined by X-ray diffraction (XRD) using Cu K␣ radiation. Surface morphology was investigated by scanning electronic microscopy (SEM). The resistivity measurement was carried out by a standard four-probe technique in the temperature range 100–340 K without or with magnetic fields (3 kOe). The DC magnetization measurements were done by using a vibrating sample magnetometer (VSM) in an applied magnetic field of 8 kOe in the temperature range 120–360 K.
3. Results and discussion Fig. 1 shows the XRD of composites with x = 0, 5, 10, 15 and 20%, respectively. All the peaks present in the diffraction patterns can be identified. They reveal the presence of both LCSMO and CFO phases without structural change in its constituent phases. The CFO shows cubic spinel structure and the LCSMO shows cubic perovskite structure. From Fig. 1, we can observe that, the number of CFO peaks and their intensities increase with the content of CFO increase. In order to illuminate the phenomena better, we have calculated the lattice parameters carefully, the value of lattice parameters are 6.47811 Å, 6.48253 Å, 6.48266 Å, 6.48288 Å and 6.48319 Å corresponding to x = 0, 5, 10, 15 and 20%. We can find that the value of lattice parameters nearly unchanged with the increase of CFO content, which suggests that no reaction occurs between LCSMO and CFO
Fig. 1. Room temperature XRD patterns of LCSMO/x CFO composites. The inset shows XRD patterns for CoFe2 O4 .
grains, and that CFO segregates mostly at the grain boundaries of LCSMO. The SEM micrographs of LCSMO/x CFO composites with x = 0 and 15% are shown in Fig. 2(a) and (b), respectively. A clear grain boundary is observed in pure LCSMO as shown in Fig. 2(a). However, the grain boundaries of LCSMO/x CFO composites become ambiguous by the addition of CFO as shown in Fig. 2(b). From the SEM image (Fig. 2(b)) and EDX, the doped CFO segregated mainly at the grain boundaries and the surfaces of LCSMO grains. These results also demonstrate that LCSMO/x CFO composites are the mixture of two phases. In Fig. 2(b), LCSMO and CFO grains, are
Fig. 2. Scanning electron micrographs of LCSMO/x CFO composites: (a) x = 0; (b) x = 15%. LCSMO and CFO grains are indicated by white and black arrows, respectively, according to the results EDX of the right two plots.
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Fig. 4. M/I transition TP2 as a function of the CFO content for composites of LCSMO/x CFO; the inset shows resistivity at TP2 as a function of the CFO content increasing.
Fig. 3. The resistivity as a function of temperature for the composites of LCSMO/x CFO, measured under a zero field.
indicated by white and black arrows respectively, to distinguish them. The variation of resistivity as a function of temperature (–T) in zero fields for all composites in the temperature range 100–340 K is shown in Fig. 3. From the plot we can see that there are two M/I transition temperatures in the samples with x ≤ 6%. One is an intrinsic M/I transition at a higher temperature (TP1 ), corresponding to the transition temperature of LCSMO, and the other is an extrinsic M/I transition at a lower temperature TP2 (TP2 < TP1 ), corresponding to the transition temperature of the second phase CFO. However, when x > 6%, there is a “shoulder” corresponding to TP1 . Compared with pure LCSMO, a different electrical transport behavior has been observed in the composites. In order to explain the dual transition in resistivity, we consult the brick layer model [17], the total resistivity can be written as: (T) = b + 2(ı/d)gb , where d is the grain size, ı is the grain boundary thickness, gb is the grain boundary resistivity and b is the bulk grain resistivity. Comparing CFO doped samples with pure LCSMO samples, we can find that gb is much larger than b . According to quantum mechan√ 2 ics, gb = 0 e2ı 2U/h , 0 is a constant which is determined by material, U is the barrier height, ı is the thickness of the grain boundary and will augment with the increase of CFO content, and is the electronic mass. The CFO grains, which are mainly segregated at the interspaces of the LCSMO grains, show an energy barrier; the conductive electrons can get across the energy barrier through spin-polarized tunneling which is related to the magnetic disorder at the grain boundaries. The magnetization states of the grains and grain boundaries have direct correlation with the degree of spin-polarized tunneling, which determined the value of gb . According to the DE interaction, we assume mb and mgb as the normalized magnetizations of the grain phase and the grain boundary phase, respectively. m is defined as m = mb − mgb . TC1 and TC2 are regarded as the Curie temperature of grain phase and grain boundary phase, respectively; while TP1 is close to TC1 and TP2 is close to TC2 , the variation trend of them is consistent. Firstly, when T > TP1 , all the composites show an intrinsic M/I transition which result from the DE mechanism in the LCSMO. The high-temperature resistivity peak TP1 is determined by LCSMO grains phase, the grains and the
grain boundaries are in paramagnetic (PM) state. Secondly, when TC2 < T < TC1 , in the interior of LCSMO grains, the grains phase is in ferromagnetic (FM) state, the spins of Mn3+ and Mn4+ were parallel, so the DE can be easily processed [18]. But the grain boundary phase is in PM state, which is going with deformation and randomly oriented magnetic moments. In pure LCSMO, the spins of Mn3+ and Mn4+ were not parallel at the grain boundary region. The addition of ferrimagnetic CFO induces the magnetic disorder on the grain boundary, so there is a difference of DE energy between the intragrain and the intergrain. The extra energy can be considered as U to the spin-polarized tunneling process [17]. We can conclude that U ∝ m and U is influenced by temperature and the thickness of the grain boundary. When TC2 < T < TC1 , the grain phase is in FM state and the grain boundary phase is in PM state. We can found that when T is close to TC2 , m will have a maximum, so the tunneling resistivity gb has a maximum. Thus, we can observe a resistivity peak at a relative low temperature TP2 (near TC2 ) in the composites with a suitable grain size and thickness of the grain boundary. Thirdly, when T < TC2 , the grains and the grain boundaries are all in FM state. In a word, we can suggest that spin-polarized tunneling and intrinsic transport properties play a key role in the composites. In Fig. 4, we can see that the extrinsic transition temperature TP2 decreases rapidly with the increase of CFO content for lowdoped (x ≤ 6%) composites, whereas relatively decrease slowly at high (x > 6%) doped. This phenomenon can be explained by considering CFO effect on the grain boundaries of LCSMO. We regard x = 8% as the metallic percolation threshold, the reason is: the resistivity of the sample with x = 8% is four times higher than that with x = 6%, which is similar to another group report [19]. From the inset we can see that the resistivity rises abruptly and the sample becomes an insulator at x = 15%, which indicates that there is no metallic conducting path in the samples with x ≥ 15%. The high resistivity of the composites can be explained by two reasons: on the one hand, the doped CFO has an effect on the electron transport channel. In pure LCSMO, the electrical transport is achieved through a direct contact among LCSMO grains. However, there are two parallel conduction channels in LCSMO/x CFO composites. One is related to the LCSMO grains, which is the main transport properties of the composites and achieved through direct contact between LCSMO grains. The other is related to CFO, which is mainly segregated at the grain boundaries and the surfaces of LCSMO grains, showing energy barriers and polarization to electrical transport process at all temperatures. On the other hand, it is related to the increase of
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Fig. 5. Temperature dependence of the magnetization under applied magnetic field of 8 kOe for the composites LCSMO/x CFO.
Fig. 6. Temperature dependence of MR for the selected composites of LCSMO/x CFO measured under applied magnetic field of 3 kOe.
the electron scattering with CFO grains embedded in the LCSMO matrix. In addition, the disorder and contamination at the grain boundaries of the LCSMO should also be considered which make inhibition to the metallic conduction. The temperature dependence of the magnetic moment ( S ) (measured at 8 kOe) for the composites is shown in Fig. 5. All the composites have almost the same behavior of the magnetization as a function of temperature. There is a shift in the paramagnetic to ferromagnetic-phase transition temperature (TC ), while TC is unchanged with increasing CFO concentration x. Under TC , S monotonously decreases, which is due to the ferromagnetic LCSMO phase and the doping of ferrimagnetic CFO; but above TC , S begins to increase, which is attributed to CFO, and CFO is a kind of material of stronger magnetism. The mechanism can be explained as below: (1) under TC , the grains of LCSMO and CFO are all in FM state, while introducing of ferrimagnetic CFO diluted ferromagnetic LCSMO, so the S of the composites begins to decrease with the addition of CFO; (2) above TC , the TC of pure LCSMO is about 310 K, the LCSMO becomes paramagnetic material above 310 K, while CFO belongs to ferrimagnetic and the TC of CFO is 790 K, so CFO takes on ferrimagnetic and the S of the composites begins to increase with the addition of CFO. In addition, CFO is an interesting magnetic material due to its high coercivity (5.4 kOe) and moderate saturation magnetization (about 80 emu/g) [20]. Moreover, the external magnetic field is 8 kOe, which is larger than the coercivity of CFO (5.4 kOe), under the effect of external magnetic field, the magnetic moments tend to consistent and this is propitious to the increase of the S . The temperature dependence of MR of the selected samples in 3 kOe applied magnetic field is shown in Fig. 6. The MR ratio is defined as: MR(%) = [(0, T) − (H, T)]/(0, T) × 100, where (0, T) and (H, T) are the resistivity values for zero and applied fields, respectively. Fig. 6 shows the changes of MR in the composites. Under TC , with the increase of CFO content, the values of MR begin to increase firstly and then decrease, the maximum value of MR appeared at x = 5%. For x = 5%, the value of MR at 250 K is 6% and at 110 K is 12%, but the value of MR of pure LCSMO sample at 250 K is only 2% and at 110 K is 6%. The enhancement in the value of MR at low temperature can be explained by two reasons: firstly, it is due to spin-polarized tunneling, the spin disorder gives birth to the tunneling process at
the grain boundaries; the spin disorder is suppressed in an applied magnetic, which induce to a high value of MR [14]. As reported by Hwang et al., the spin-polarized tunneling between neighboring grains of manganites induced to MR effect in polycrystalline samples. This tunneling takes place across the grain boundaries and surfaces. Secondly, at the surfaces of the composites grains, the electron scattering of polarized charge carriers may be responsible for enhanced MR of the composites [11]. The spin-polarized tunneling is controlled by the thickness of the insulating CFO layer, the reduction in the value of MR for the composite sample with x = 10% at low field decreases because of grain boundaries becoming too thick for electron tunneling. In addition, it is well known that the MR magnitude increases with decreasing temperature for manganate samples [21]. Like other polycrystalline CMR materials [22], the pure LCSMO sample undergoes a characteristic transition peak corresponding to the intrinsic CMR effect around TMI ∼ 310 K. As for LCSMO/x CFO composites, above TC , with the increase of CFO content, the values of MR begin to decrease, at about 310 K, the phenomena are very clear. The phenomena can refer to Hwang, who proposed that tunneling phenomena occurs only at T < TC , so in high temperature (room temperature), the decrease of MR is due to vanish of the tunneling phenomena. The high-temperature peak, called as intrinsic CMR, can be explained that doped CFO dilutes the concentration of FM phase of LCSMO and the intensity of intrinsic MR decreases with the increase of x content. 4. Conclusions We have investigated the influence of CFO phase on electrical properties and magnetoelectric effect of composites consisting of LCSMO and CFO. On the basis of XRD and SEM, there is no reaction between LCSMO and CFO, and CFO distributed at the grain boundaries and surfaces of LCSMO grains. The change from one peak to two peaks in –T curves may be caused by both grain resistivity and grain boundary resistivity, the spin-polarized tunneling and intrinsic transport properties make an influence on the samples. The varieties in the magnetic moment are attributed to the effect of magnetic disorder and the effect of high-external magnetic fields on CFO. At x = 8%, there is a percolation threshold. Compared with pure LCSMO, the MR effect of the composites is enhanced in low
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magnetic fields of 3 kOe at a wide temperature range below TC . The enhanced LFMR is related to the enhancement of spin-dependent tunneling of electrons and spin-dependent scattering at the surfaces between LCSMO grains. Acknowledgements The authors thank Analytical and Testing Center of Huazhong University of Science and Technology, which supplied us the facilities to fulfill the measurement. This work was supported by the National Science Foundation of China (Grant No. 10274022). References [1] K. Chahara, T. Ohno, M. Kasai, Y. Kosono, Appl. Phys. Lett. 63 (1993) 1990– 1992. [2] Y. Okimoto, Y. Tomioka, Y. Onose, Y. Otsuka, Y. Tokura, Phys. Rev. B 57 (1998) 9377–9380. [3] J.B. Goodenough, Phys. Rev. B 100 (1955) 564–573. [4] T. Terai, T. Kakeshita, T. Fukuda, T. Saburi, N. Takamoto, K. Kindo, M. Honda, Phys. Rev. B 58 (1998) 14908–14912. [5] C. Zener, Phys. Rev. 82 (1951) 403–405. [6] A. Gupta, J.Z. Sun, J. Magn. Magn. Mater. 200 (1999) 24–43.
[7] S.P. Issac, N.D. Mathur, J.E. Evetts, M.G. Blamire, Appl. Phys. Lett. 72 (1998) 2038–2040. [8] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041–2044. [9] Z.C. Xia, S.L. Yuan, F. Wang, et al., Solid State Commun. 126 (2003) 567–571. [10] J.H. Miao, S.L. Yuan, G.M. Ren, X. Xiao, G.Q. Yu, Y.Q. Wang, S.Y. Yin, J. Phys. D: Appl. Phys. 40 (2007) 707–711. [11] C.H. Yan, Z.G. Xu, T. Zhu, Z.M. Wang, F.X. Cheng, et al., J. Appl. Phys. 87 (2000) 5588–5590. [12] P. Kamelia, H. Salamati, M. Eshraghi, M.R. Mohammadizadeh, J. Appl. Phys. 98 (2005), 043908(1)–043908(4). [13] Z.C. Xia, S.L. Yuan, F. Wang, L.J. Zhang, G.H. Zhang, J. Tang, Solid State Commun. 128 (2003) 291–294. [14] G. Anurag, G.D. Varma, Solid State Commun. 139 (2006) 310–314. [15] A.S. Lübbe, C. Bergemann, J. Brock, D.G. McClure, J. Magn. Magn. Mater. 194 (1999) 149–155. [16] J.W.M. Bulte, M. de Cuyper, D. Despres, J.A. Frank, J. Magn. Magn. Mater. 194 (1999) 204–209. [17] N. Zhang, W.P. Ding, W. Zhong, et al., Phys. Rev. B 56 (1997) 8138–8142. [18] S.L. Yuan, et al., Phys. Rev. B 68 (2003), 172408(1)–172408(4). [19] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri, C.N.R. Rao, Appl. Phys. Lett. 68 (1996) 2291–2293. [20] D. Zhao, X. Wu, H. Guan, E. Han, J. Supercrit. Fluids 42 (2007) 226–233. [21] R.D. Sanchez, J. Rivas, C. Vazquez-Vazquez, A. Lopez-Quintela, M.T. Causa, M. Tovar, S. Oseroff, Appl. Phys. Lett. 68 (1996) 134–136. [22] P. Schiffer, A.P. Ramirez, W. Bao, S.W. Cheong, Phys. Rev. Lett. 75 (1995) 3336–3339.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Electrical properties and magnetoelectric effect measurement in La0.7 Ca0.2 Sr0.1 MnO3 /xCoFe2 O4 composites C.S. Xiong ∗ , F.F. Wei, Y.H. Xiong, L.J. Li, Z.M. Ren, X.C. Bao, Y. Zeng, Y.B. Pi, Y.P. Zhou, X. Wu, C.F. Zheng Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 27 April 2008 Received in revised form 15 June 2008 Accepted 17 June 2008 Available online 26 July 2008 PACS: 77.84.Lf 82.33.Pt 75.30.Kz 75.30.Vn 71.30.+h Keywords: Composite materials Solid state reaction Grain boundary Electrical transport Low-field magnetoresistance
a b s t r a c t Composites with compositions La0.7 Ca0.2 Sr0.1 MnO3 (LCSMO)/xCoFe2 O4 (CFO) were prepared by a standard ceramic technique. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and CFO grains, and that CFO segregates mostly at the grain boundaries of LCSMO. The variation in resistivity with temperature has been studied and shows a semiconducting behavior, furthermore the composites exhibit metallic percolation threshold at x = xP = 8%. The magnetic moment ( S ) changed with the increase of CFO. Curie temperature (TC ) is a dividing point; T < TC , S decreases with CFO content increasing, while T >TC , S changes contrarily with the addition of CFO. The magnetoresistance (MR) effect is enhanced at a wide temperature range in an applied magnetic field (3 kOe). The spin-polarized tunneling and the spin-dependent scattering may be attributed to the enhanced low-field magnetoresistance (LFMR) effect. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Recently, colossal magnetoresistance (CMR) has been a hot topic in condense matter physics and material sciences. Besides a fundamental understanding of the mechanism of the CMR effect, the studies have been attracted by potential applications of these materials, too. In the La1−x Ax MnO3 (A = Sr, Ca, Ba) system, there have been extensive research activities on these doped perovskites [1–4]. There are two kinds of CMR effects found in these manganites: intrinsic CMR and extrinsic CMR. The intrinsic CMR effect, which is mostly observed near TC , can be explained by the double exchange (DE) mechanism proposed by Zener [5]. However, the extrinsic CMR is usually thought to be caused by natural and artificial grain boundaries [6,7]. In particular that it is sensitive to low magnetic field and can be largely enhanced in a wide temperature range below TC , the effect is called low-field magnetoresistance (LFMR) effect. The LFMR is due to spin-polarized tunneling [8] or spin-dependent
∗ Corresponding author. Tel.: +86 27 87556914; fax: +86 27 87556914. E-mail address: [email protected] (C.S. Xiong). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.071
scattering [6] between neighboring grains and has potential applications. In order to enhance the LFMR, a lot of efforts have been done, such as making composites of these CMR materials with secondary phases including LCMO/Fe3 O4 [9], LCMO/CuMn2 O4 [10], LSMO/CoFe2 O4 [11], LSMO/TiO2 [12], LBMO/YSZ [13], LSMO/NiO [14] and so on. In this paper, we choose LCSMO and CoFe2 O4 (CFO) as the matrix material and insulator barrier, respectively. There are three reasons: (1) LCSMO is a kind of CMR materials. The TP (near TC ) of pure LCSMO is about 310 K, which is close to the room temperature. (2) CFO belongs to spinel ferrites material. Spinel ferrite is one of the most significant magnetic materials, which are extensively used in modern electronic technologies. Nanoparticles of spinel ferrites are of practical interest in a wide range of applications like high-density magnetic information storage, magnetic resonance imaging, targeted drug deliver, etc. [15,16]. Spinel ferrites own high-electrical resistivities, low eddy current and dielectric losses. (3) We have synthesized a homogeneous composite and introduced CFO as the second phase of the composites, which makes an enhanced magnetoresistance at a wide temperature range.
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2. Experimental procedure Composites with compositions La0.7 Ca0.2 Sr0.1 MnO3 /xCoFe2 O4 , in which x varies as 0, 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, and 20 wt%, were prepared by a standard ceramic technique. The matrix phase, i.e. La0.7 Ca0.2 Sr0.1 MnO3 , was prepared through a standard solid state reaction, using La2 O3 , CaCO3 , SrCO3 , and MnO2 powders in weight proportions as starting materials. Similarly, the insulator barrier phase, CoFe2 O4 was prepared by solid reaction method, using Co2 O3 and Fe3 O4 as original materials. The mixtures were all presintered at 800 ◦ C for 10 h, and then, the resulting mixtures were sintered at 1400 ◦ C for 10 h and at 1200 ◦ C for 10 h, respectively. After sintering, the constituent phases were ground to fine power. Composites LCSMO/x CFO were prepared by properly mixing with as-prepared LCSMO and CFO powder, and palletized at a pressure of 10 MPa/cm2 ; then the samples in the forms of pellets were sintered at 1000 ◦ C for 3 h in a programmable furnace and slowly cooled to room temperature. In order to exclude possibility of chemical reaction between LCSMO and CFO, the final sintering temperature of the composite samples was kept lower than those of LCSMO by 1400 ◦ C and CFO by 1200 ◦ C. The structural characterization was examined by X-ray diffraction (XRD) using Cu K␣ radiation. Surface morphology was investigated by scanning electronic microscopy (SEM). The resistivity measurement was carried out by a standard four-probe technique in the temperature range 100–340 K without or with magnetic fields (3 kOe). The DC magnetization measurements were done by using a vibrating sample magnetometer (VSM) in an applied magnetic field of 8 kOe in the temperature range 120–360 K.
3. Results and discussion Fig. 1 shows the XRD of composites with x = 0, 5, 10, 15 and 20%, respectively. All the peaks present in the diffraction patterns can be identified. They reveal the presence of both LCSMO and CFO phases without structural change in its constituent phases. The CFO shows cubic spinel structure and the LCSMO shows cubic perovskite structure. From Fig. 1, we can observe that, the number of CFO peaks and their intensities increase with the content of CFO increase. In order to illuminate the phenomena better, we have calculated the lattice parameters carefully, the value of lattice parameters are 6.47811 Å, 6.48253 Å, 6.48266 Å, 6.48288 Å and 6.48319 Å corresponding to x = 0, 5, 10, 15 and 20%. We can find that the value of lattice parameters nearly unchanged with the increase of CFO content, which suggests that no reaction occurs between LCSMO and CFO
Fig. 1. Room temperature XRD patterns of LCSMO/x CFO composites. The inset shows XRD patterns for CoFe2 O4 .
grains, and that CFO segregates mostly at the grain boundaries of LCSMO. The SEM micrographs of LCSMO/x CFO composites with x = 0 and 15% are shown in Fig. 2(a) and (b), respectively. A clear grain boundary is observed in pure LCSMO as shown in Fig. 2(a). However, the grain boundaries of LCSMO/x CFO composites become ambiguous by the addition of CFO as shown in Fig. 2(b). From the SEM image (Fig. 2(b)) and EDX, the doped CFO segregated mainly at the grain boundaries and the surfaces of LCSMO grains. These results also demonstrate that LCSMO/x CFO composites are the mixture of two phases. In Fig. 2(b), LCSMO and CFO grains, are
Fig. 2. Scanning electron micrographs of LCSMO/x CFO composites: (a) x = 0; (b) x = 15%. LCSMO and CFO grains are indicated by white and black arrows, respectively, according to the results EDX of the right two plots.
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Fig. 4. M/I transition TP2 as a function of the CFO content for composites of LCSMO/x CFO; the inset shows resistivity at TP2 as a function of the CFO content increasing.
Fig. 3. The resistivity as a function of temperature for the composites of LCSMO/x CFO, measured under a zero field.
indicated by white and black arrows respectively, to distinguish them. The variation of resistivity as a function of temperature (–T) in zero fields for all composites in the temperature range 100–340 K is shown in Fig. 3. From the plot we can see that there are two M/I transition temperatures in the samples with x ≤ 6%. One is an intrinsic M/I transition at a higher temperature (TP1 ), corresponding to the transition temperature of LCSMO, and the other is an extrinsic M/I transition at a lower temperature TP2 (TP2 < TP1 ), corresponding to the transition temperature of the second phase CFO. However, when x > 6%, there is a “shoulder” corresponding to TP1 . Compared with pure LCSMO, a different electrical transport behavior has been observed in the composites. In order to explain the dual transition in resistivity, we consult the brick layer model [17], the total resistivity can be written as: (T) = b + 2(ı/d)gb , where d is the grain size, ı is the grain boundary thickness, gb is the grain boundary resistivity and b is the bulk grain resistivity. Comparing CFO doped samples with pure LCSMO samples, we can find that gb is much larger than b . According to quantum mechan√ 2 ics, gb = 0 e2ı 2U/h , 0 is a constant which is determined by material, U is the barrier height, ı is the thickness of the grain boundary and will augment with the increase of CFO content, and is the electronic mass. The CFO grains, which are mainly segregated at the interspaces of the LCSMO grains, show an energy barrier; the conductive electrons can get across the energy barrier through spin-polarized tunneling which is related to the magnetic disorder at the grain boundaries. The magnetization states of the grains and grain boundaries have direct correlation with the degree of spin-polarized tunneling, which determined the value of gb . According to the DE interaction, we assume mb and mgb as the normalized magnetizations of the grain phase and the grain boundary phase, respectively. m is defined as m = mb − mgb . TC1 and TC2 are regarded as the Curie temperature of grain phase and grain boundary phase, respectively; while TP1 is close to TC1 and TP2 is close to TC2 , the variation trend of them is consistent. Firstly, when T > TP1 , all the composites show an intrinsic M/I transition which result from the DE mechanism in the LCSMO. The high-temperature resistivity peak TP1 is determined by LCSMO grains phase, the grains and the
grain boundaries are in paramagnetic (PM) state. Secondly, when TC2 < T < TC1 , in the interior of LCSMO grains, the grains phase is in ferromagnetic (FM) state, the spins of Mn3+ and Mn4+ were parallel, so the DE can be easily processed [18]. But the grain boundary phase is in PM state, which is going with deformation and randomly oriented magnetic moments. In pure LCSMO, the spins of Mn3+ and Mn4+ were not parallel at the grain boundary region. The addition of ferrimagnetic CFO induces the magnetic disorder on the grain boundary, so there is a difference of DE energy between the intragrain and the intergrain. The extra energy can be considered as U to the spin-polarized tunneling process [17]. We can conclude that U ∝ m and U is influenced by temperature and the thickness of the grain boundary. When TC2 < T < TC1 , the grain phase is in FM state and the grain boundary phase is in PM state. We can found that when T is close to TC2 , m will have a maximum, so the tunneling resistivity gb has a maximum. Thus, we can observe a resistivity peak at a relative low temperature TP2 (near TC2 ) in the composites with a suitable grain size and thickness of the grain boundary. Thirdly, when T < TC2 , the grains and the grain boundaries are all in FM state. In a word, we can suggest that spin-polarized tunneling and intrinsic transport properties play a key role in the composites. In Fig. 4, we can see that the extrinsic transition temperature TP2 decreases rapidly with the increase of CFO content for lowdoped (x ≤ 6%) composites, whereas relatively decrease slowly at high (x > 6%) doped. This phenomenon can be explained by considering CFO effect on the grain boundaries of LCSMO. We regard x = 8% as the metallic percolation threshold, the reason is: the resistivity of the sample with x = 8% is four times higher than that with x = 6%, which is similar to another group report [19]. From the inset we can see that the resistivity rises abruptly and the sample becomes an insulator at x = 15%, which indicates that there is no metallic conducting path in the samples with x ≥ 15%. The high resistivity of the composites can be explained by two reasons: on the one hand, the doped CFO has an effect on the electron transport channel. In pure LCSMO, the electrical transport is achieved through a direct contact among LCSMO grains. However, there are two parallel conduction channels in LCSMO/x CFO composites. One is related to the LCSMO grains, which is the main transport properties of the composites and achieved through direct contact between LCSMO grains. The other is related to CFO, which is mainly segregated at the grain boundaries and the surfaces of LCSMO grains, showing energy barriers and polarization to electrical transport process at all temperatures. On the other hand, it is related to the increase of
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Fig. 5. Temperature dependence of the magnetization under applied magnetic field of 8 kOe for the composites LCSMO/x CFO.
Fig. 6. Temperature dependence of MR for the selected composites of LCSMO/x CFO measured under applied magnetic field of 3 kOe.
the electron scattering with CFO grains embedded in the LCSMO matrix. In addition, the disorder and contamination at the grain boundaries of the LCSMO should also be considered which make inhibition to the metallic conduction. The temperature dependence of the magnetic moment ( S ) (measured at 8 kOe) for the composites is shown in Fig. 5. All the composites have almost the same behavior of the magnetization as a function of temperature. There is a shift in the paramagnetic to ferromagnetic-phase transition temperature (TC ), while TC is unchanged with increasing CFO concentration x. Under TC , S monotonously decreases, which is due to the ferromagnetic LCSMO phase and the doping of ferrimagnetic CFO; but above TC , S begins to increase, which is attributed to CFO, and CFO is a kind of material of stronger magnetism. The mechanism can be explained as below: (1) under TC , the grains of LCSMO and CFO are all in FM state, while introducing of ferrimagnetic CFO diluted ferromagnetic LCSMO, so the S of the composites begins to decrease with the addition of CFO; (2) above TC , the TC of pure LCSMO is about 310 K, the LCSMO becomes paramagnetic material above 310 K, while CFO belongs to ferrimagnetic and the TC of CFO is 790 K, so CFO takes on ferrimagnetic and the S of the composites begins to increase with the addition of CFO. In addition, CFO is an interesting magnetic material due to its high coercivity (5.4 kOe) and moderate saturation magnetization (about 80 emu/g) [20]. Moreover, the external magnetic field is 8 kOe, which is larger than the coercivity of CFO (5.4 kOe), under the effect of external magnetic field, the magnetic moments tend to consistent and this is propitious to the increase of the S . The temperature dependence of MR of the selected samples in 3 kOe applied magnetic field is shown in Fig. 6. The MR ratio is defined as: MR(%) = [(0, T) − (H, T)]/(0, T) × 100, where (0, T) and (H, T) are the resistivity values for zero and applied fields, respectively. Fig. 6 shows the changes of MR in the composites. Under TC , with the increase of CFO content, the values of MR begin to increase firstly and then decrease, the maximum value of MR appeared at x = 5%. For x = 5%, the value of MR at 250 K is 6% and at 110 K is 12%, but the value of MR of pure LCSMO sample at 250 K is only 2% and at 110 K is 6%. The enhancement in the value of MR at low temperature can be explained by two reasons: firstly, it is due to spin-polarized tunneling, the spin disorder gives birth to the tunneling process at
the grain boundaries; the spin disorder is suppressed in an applied magnetic, which induce to a high value of MR [14]. As reported by Hwang et al., the spin-polarized tunneling between neighboring grains of manganites induced to MR effect in polycrystalline samples. This tunneling takes place across the grain boundaries and surfaces. Secondly, at the surfaces of the composites grains, the electron scattering of polarized charge carriers may be responsible for enhanced MR of the composites [11]. The spin-polarized tunneling is controlled by the thickness of the insulating CFO layer, the reduction in the value of MR for the composite sample with x = 10% at low field decreases because of grain boundaries becoming too thick for electron tunneling. In addition, it is well known that the MR magnitude increases with decreasing temperature for manganate samples [21]. Like other polycrystalline CMR materials [22], the pure LCSMO sample undergoes a characteristic transition peak corresponding to the intrinsic CMR effect around TMI ∼ 310 K. As for LCSMO/x CFO composites, above TC , with the increase of CFO content, the values of MR begin to decrease, at about 310 K, the phenomena are very clear. The phenomena can refer to Hwang, who proposed that tunneling phenomena occurs only at T < TC , so in high temperature (room temperature), the decrease of MR is due to vanish of the tunneling phenomena. The high-temperature peak, called as intrinsic CMR, can be explained that doped CFO dilutes the concentration of FM phase of LCSMO and the intensity of intrinsic MR decreases with the increase of x content. 4. Conclusions We have investigated the influence of CFO phase on electrical properties and magnetoelectric effect of composites consisting of LCSMO and CFO. On the basis of XRD and SEM, there is no reaction between LCSMO and CFO, and CFO distributed at the grain boundaries and surfaces of LCSMO grains. The change from one peak to two peaks in –T curves may be caused by both grain resistivity and grain boundary resistivity, the spin-polarized tunneling and intrinsic transport properties make an influence on the samples. The varieties in the magnetic moment are attributed to the effect of magnetic disorder and the effect of high-external magnetic fields on CFO. At x = 8%, there is a percolation threshold. Compared with pure LCSMO, the MR effect of the composites is enhanced in low
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