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Magnetic, electrical properties and magnetoresistance of Cu substituted La0.7Sr0.3CoO3−δ crystals (0.35≤δ≤0.45)
Materials Science and Engineering B64 (1999) 180 – 186 www.elsevier.com/locate/mseb
Magnetic, electrical properties and magnetoresistance of Cu substituted La0.7Sr0.3CoO3 − d crystals (0.355d50.45) H.W. Hsu a, Y.H. Chang a,*, G.J. Chen a, K.J. Lin b a
Department of Materials Science and Engineering, National Cheng Kung Uni6ersity, Tainan, Taiwan, ROC b Department of Physics, National Cheng Kung Uni6ersity, Tainan, Taiwan, ROC Received 13 October 1998; received in revised form 26 April 1999; accepted 28 April 1999
Abstract The magnetic susceptibility, resistivity, and magnetoresistance of La0.7Sr0.3Co1 − x Cux O (0 5x 5 0.4 and 0.3 5d50.4) compounds have been studied for applied field up to 50 kOe. Cuire temperature and saturation magnetization of the ferromagnetic perovskite La0.7Sr0..3Co1 − x Cux O decreases, when cobalt is substituted by copper. In the doping, all coballtites exhibited ferromagnetic long-range ordered behaviors. If the content of copper exceeds 10%, the electronic properties of the material show the behavior of transformation from metallic to semiconductor. The semiconducting behavior is connected with the magnetic disorder, which show variable range hopping, log rT − 1/4. The magnetoresistance, which can be expressed as MR8 H q when H5 10 kOe, is negative and exceeds 32%. The results are interpreted in terms of a double-exchange and super-exchange model. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Magnetoresistance; Magnetic susceptibility; Resistivity; Double-exchange; Variable range hopping
1. Introduction In recent years, high-temperature superconductivity in hole doped copper oxide compound has been found in the study of electron conduction in closely related class of compounds [1,2]. It has excited renewed and extended interest in the correlated stimulated dynamics of spin and charges near the metal-insulator (M–I) transition in 3d transition-metal oxides. The similarity in the metallic conducting ferromagnetic compounds on hole-doped oxides basis and with perovskite structure, Ln1 − x Ax MO3 (Ln, rare-earth ions, e.g. La, Pr, and Nd; A, divalent ions, e.g. Sr, Ca, Ba, and Pb; M, transition-metal ions, e.g. Co and Mn) has been revisited and the phenomenon of giant magnetoresistance (GMR) have been observed [3 – 13]. The magnitude of magnetoresistance is generally maximum around metalinsulator (M–I) transition, the temperature Tp that is close to the ferromagnetic transition temperature Tc [14] (generally a few temperatures lower). Recently, the considerable attention has been focused on the Ln1 − * Corresponding author. Fax: +886-6-2382800. E-mail address: [email protected] (Y.H. Chang)
xAx MO3, similar properties have been found in material of La1 − x Srx CoO3. Jonker et al. [15] first studied the magnetic and transport properties of La1 − xSrx CoO3 which is a good conductor (rB10 − 1 mVcm) and has a metallic temperature coefficient (a\ 0) for x=0.5. The double-exchange mechanism of Zener [16], which interpreted the magnitude of spontaneous magnetization, and paramagnetic Curie temperature is not considered relevant. Goodenough [17] that assumes the 3d electrons to be localized interpreted the ferromagnetism by the superexchange model of Anderson [18]. The magnetic phase diagram of La1 − xSrx CoO3 from magnetization measurements by Itoh et al. [19] was made. The electronic property of the La1 − xSrx CoO3 near metal–insulator transition was studied by Chainani et al. [20] using electron microscopy. Raccah et al. [21] have described the coexistance of itinerant and localized electrons. Mo¨ssbauer measurement by Bhide et al. [22] and other studies in La1 − x Srx CoO3 have pointed out that ferromagnetic Sr2 + -rich cluster coexist with paramagnetic La3 + -rich regions in the same crystallographic phase. The electrons in the sbonding orbital are delocalized at the highest temperature was conjectured from studies on La1 − x Srx CoO3;
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and at low temperature, some of the electrons get trapped as localized eg. Previous investigations have observed the similarity in composition and crystal structure for high Tc temperature superconductivity in hole doped Cu oxides’ compounds. Therefore, in light of above development, this work investigates the changes of magnetization, magnetoresistance behavior and electrical conduction by doping Cu in La0.7Sr0.3Co1 − x Cux O3 − d systems. 2. Experimental Samples of La0.7Sr0.3Co1 − x Cux O3 − d compounds with various x values were prepared by the standard ceramic technique. The starting materials were prepared by mixing the reagent grade powders of SrCO3, CoO, La2O3, and Cu2O. The mixture was milled for 24 h with alcohol. After drying the mixed powders at 100°C, they were prefired in air at 900°C for 24 h. The obtained products were reground and milled. For measuring the electrical conductivity and magnetic properties, the powders of each compound were compressed into a rectangular shape (15× 6 ×0.8 mm3) under a pressure of about 4350 kg cm − 2 and then the pressed samples were sintered at 970 – 1100°C for 48 h in air. The oxygen-deficient samples were obtained in this manner as well. The X-ray diffraction patterns with filtered Cu–Ka radiation for the various compositions were obtained using a diffractometer at room temperature. The sample was ground to very fine powders in alcohol and deposited on support carbon grids. Electron microscopic observations were carried out using a Jeol-300 electron microscope with electron microscope with a side-entry 40°/30°-tilt goniometer and a Z-translation stage. The wavelength of electrons was 0.0196 A, and camera length, 120 cm. Analyses of the chemical compositions were performed using an electron probe microanalyzer (EPMA). The oxygen content was determined using a standard iodometric titration technique [23]. Magnetization measurements in fields up to 50 kOe were taken with a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 5 –300 K. Next, the electrical resistivities were measured as a function of temperature and magnetic field in a superconducting magnet with the maximum applied field of H =50 kOe. The four-probe leads were formed with silver paste along the long axis of specimen. The outside leads were used to supply constant current (I//H). The magnitude of the magnetoresistance is then defined as MR = (r(H)−r(0))/r(0), where r(H) and r(0) are resistivities at magnetic field H and at zero field, respectively.
Fig. 1. X-ray diffraction spectra of La0.7Sr0.3Co1 − x Cux O3 − d (x = 0, 0.1, 0.2, 0.3 and 0.4) powder samples.
3. Results and discussion According to the results of X-ray powder diffraction pattern at room-temperature, the La0.7Sr0..3Co1 − x Cux O samples (05 x 50.4) were single phase materials as shown in Fig. 1. For samples with a higher amount of Cu, the crystal structure is rhombohedral (Hex) with a small distortion. This is because of the introducing a larger size Cu cation progressively reduces the rhombohedral (Hex) distortion.
Fig. 2. Electron diffraction patterns of La0.7Sr0.3Co0.7Cu0.3O3 − d with the bean in [001] direction.
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domain walls that appear in strong anisotropic ferromagnetic compounds. The magnitude of MZFC at low temperature is small, and starts to increase at temperatures at which the applied magnetic field and/or thermal activation is large enough to produce the domain wall motion overcoming the anisotropy energy responsible for the quenching of these magnetic domains. Such domain wall behavior is reported in ferromagnetic intermetallic and perovskite oxide compounds [26–28]. Whereas, the characteristic feature is similar to spinglass-like behavior [19,29]. In the La1 − y Sry Co1 − x Cux O3 − d samples system, Co3 + /Co4 + valence mixture is induced owing to the La3 + /Sr2 + chemical mixture. Herein, assuring fixed valences for La3 + , Sr2 + , Cu2 + , and O2 − content. By ion replacing a Co3 + ion with Cu2 + , the other Co3 + would convert to Co4 + . According to charge neutrality, we obtain as average Co-valence: Fig. 3. Temperature dependence of the magnetic susceptibility at 200 Oe of La0.7Sr0.3Co1 − x Cux O3 − d (x= 0.2, 0.3 and 0.4). Dotted and solid lines represent zero-field-cooled (ZFC) and field-cooled (FC) magnetizations, respectively.
The electron diffraction pattern of La0.7Sr0.3Co0.7Cu0.3O3 − d sample represent with the beam in [001] orientation are shown in Fig. 2. The pattern represents the rhombohedral (Hex) phase corresponding to the X-ray diffraction pattern of Fig. 1. The temperature dependence of the magnetization of samples, MFC (T), was measured in a field of H= 200 Oe on heating from the lowest temperature after cooling in zero magnetic field (ZFC), or in a field of 200 Oe (FC). Fig. 3 shows MFC (T) curves similar to these observed [19]. According to our results, the doping of Cu introduces some change in magnetic and transport properties. The Curie temperature Tc 220 K for x= 0 decreases to Tc 175K for x = 0.4. Correspondingly, MFC (T) rapidly decreases with x below Tc. MFC increases monotonically down to 5K. A noticeable feature is that the magnetization continuously increases as the temperature is lowered from room temperature. The MFC is typical of a ferromagnet. The FC and ZFC curves significantly differ below TB as shows in Fig. 3. These phenomena maybe produce owing to the effect of local magnetic anisotropy for individual polycrystalline grains. This is because of these ceramic samples are known to be inhomogenous on a microscopic scale. For an applied field of 200 and 1000 Oe on the x= 0.4 samples, thermo-magnetic irreversibility (TMI) occurs shifts from 190 to 170 K [24]. The TMI can arise from pinning of domains. It is worth mentioning here that this composition contains a Co-ion which causes high uni-axial anisotropy [25]. The asymmetrical peak centered can be thought of as arising due to the motion of
Ave=
(3− d+ y−2x) 1− x
(1)
According to Eq. (1), the spin-only moment single ion can be calculated, but only for a uniform spin state per valence. For y=0.3, x= 0.4, and d=0.35, no Co3 + /Co4 + pairs exist anymore. Therefore, the ferromagnetic double exchange coupling and the metallic conduction should disappear. In fact, the ferromagnetic spin-order is observed for x=0.4. Herein, we assume that the effect of oxygen non-stoichometry increases the different spin-states. We have to measure M–H curves and long-time behavior of thermoremanent magnetization (TMR). According to these results, we can exact to decide the magnetic properties. Fig. 4. Shows of curves the M
Fig. 4. ZFC magnetization curves of La0.7Sr0.3Co1 − x Cux O3 − d (x = 0.2, 0.3 and 0.4).
H.W. Hsu et al. / Materials Science and Engineering B64 (1999) 180–186
Fig. 5. Relaxation of the thermoremanent magnetization (TRM) for La0.7Sr0.3Co0.6Cu0.4O3 − d at 5 K.
versus H at 5 K for three samples (0.25x 5 0.4). Samples with higher Cu doping show a smaller magnetization below Tc. The Curie temperature and the magnetization decrease when Cu is added. This is because of the magnetization is mostly contributed to come from Co ions. The hysteresis loops approach to a saturation of M at 50 kOe. Due to lack of saturation in high fields is a characteristic feature of spin glasses [30]. Therefore, we suggest that these compounds are not of spin-glass. The coercive fields indicate the existence of
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magnetic anisotropy that agree with the ZFC and FC curves of Fig. 3. Our results also observed a relaxation behavior of the TMR for x= 0.4. The experimental procedure is as follows. After the field cooling (FC) process from room temperature to 5 K, the sample was maintained for different time intervals when the field is turned off at 5 K as shown in Fig. 5. We measured the magnetization MFC (t) as a function of time t. The plot reveals that the magnetic moment does not relax. Therefore, the MFC (t) showing the long-time relaxation behavior was not observed. Indicating the magnetic properties differ from that of the spin-glass properties. On the basis of magnetization measurements mentioned above, we can prove a ferromagnetic long-range order in La0.7Sr0.3Co1 − x Cux O3 − d compounds. In Figs. 6(a, b) the curves represent the DC resistivity of the La0.7Sr0..3Co1 − x Cux O3 − d (x=0, 0.1, 0.2, 0.3 and 0.4) compounds. The resistivity with increasing Cu contents, because the number of carrier decreases. Samples with x5 0.1 show metallic conductivity, but at higher Cu-concentration, a semiconducting behavior (i.e. (r/(T B0) appears. A cusp in resistivity is observed near the ferromagnetic Curie temperature (Tc). It indicates that the magnon scattering is dominant. Above 100 K the electrical resistivity r(T) in metallic behavior samples (i.e. (r/(T \ 0) show a T 2 dependence (r(T)= r0 + AT 2). A T 2 term in resistivity comes from spin fluctuations [31] or electron-magnon scattering [32] in ferromagnetic metal ions at low temperature is observed. The similar material [33] at low temperature is observed as well. The curves have a small rise in r(T). This rise in r(T) shows a T − 1/2
Fig. 6. Resistivity versus temperature curves r(T) for La0.8Sr0.2Co1 − x Cux O3 − d compounds (a) x= 0, 0.1, and (b) x =0.2, 0.3, 0.4.
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The semiconducting behavior could be caused by magnetic disorder. According to our results, r(T) can be fitted well by Mott-plots than by Arrhenius-plots. It was suggested by Mott [36] that a random magnetic structure could lead to localize electric states. The transport properties are dominated by a hopping process. Because of a frozen-in spin-disorder, variable range hopping could lead to the observed log rT − 1/4 behaviors found. Fig. 8 shows some of the magnetoresistance curves at different Cu content (x=0, 0.1, 0.2, 0.3 and 0.4). At 5 K, the resistivity decreases with increasing field, which demonstrates the typical negative magnetoresistance effect. Therefore, MR increases with increasing H. Due to the applied magnetic field tends to align with the local spin and reduced spin scattering of the conduction electron. Although the exact nature of this change in magnetoresistance behavior is unclear. However, we can expect that the Cu content increase not only with a decreasing Co content but also with a decreasing charge carriers [24]. The numbers of Co4 + –O–Co3 + interactions decrease. Therefore, the Co/Cu mixture ions play prominent role in controlling the resistivity, magnetization, and negative magnetoresistivity effect of La0.7Sr0.3Co1 − x Cux O3 − d compounds. The competition between positive and negative couplings causes large negative magnetoresistance as observed for La1 − xSrx CoO3 − d. At 5 K, the negative magnetoresistance exceed 32% at 50 kOe. In addition, log MR versus log H curves have been plotted in Fig. 9 for different Cu contents. We find that there exists a good linear relationship between log MR and log H when H is lower than 10 kOe. The magnetoresistance is directly
Fig. 7. Arrhenius and Mott plots of r(T) for the La0.8 Sr0.2Co1 − xCux O3 − d compounds (a) x =0.2; (b) x= 0.3, and (c) x= 0.4.
behavior (Fig. 6(a), inset). The T 1/2 dependence of s(T) arises from the electron-electron interaction in disordered systems [33,34]. We expressed this rise in r(T) as a correction to the s(T) described by the equation s(T)=s0 +aT 1/2
(2)
Where s0 is a finite zero-temperature conductivity. Samples with x] 0.2 show semiconductors (i.e. (r/ (T B0). There are two energy gaps in the temperature semiconducting as shown in Figs. 7(a, b): at higher temperature (T \30 K) the conductivity is a higher gap; at lower temperature (T B30 K) a lower energy gap is observed. This is a common behavior for doped semiconductors [35].
Fig. 8. The variation of MR curves with applied field H of La0.7Sr0.3Co1 − x Cux O3 − d (x =0, 0.1, 0.2, 0.3 and 0.4) at 5 K.
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dependent on the magnetization directly proportional to the H.
Acknowledgements This research was supported by the National Science Council of the Republic of China under the grant of NSC 86-2212-E006-001. The authors thank S.T. Lin for his assistance in making the hysteresis measurement. References
Fig. 9. A plot of log (MR) vs. log (H) for La0.8 Sr0.2Co1 − x Cux O3 − d compounds (x =0, 0.1, 0.2, 0.3 and 0.4) at 5 K.
proportional to the H: MR8 H q, where q is a constant. In metallic multilayer or grain-type GMR thin film, the feature is often observed. When H is lower than 10 kOe, magnetoresistance is weakly field-dependent, which is in agreement with the result mentioned in Fig. 3.
4. Conclusions The electronic and magnetic property of polycrystalline La0.7Sr0.3Co1 − x Cux O compounds was studied. Magnetization measurements have been done to reveal the detail magnetic properties. Spontaneous magnetization with the ferromagnetic long-range ordered exist below Tc. But we have not observed the long-time relaxation behavior of MFC having the aging behavior. According to the absolute values of the resistivity, the ferromagnetic samples (x 5 0.1) can be considered as metallic; (a \ 0). Electron-electron interaction in these metallic oxides gives rise to a T 1/2 term in the conductivity at lower temperature. The electrical resistivity at higher temperature (T BTc) also consist with r(T)= r0 + AT 2. The semiconducting behavior, (a B0) which occurs for x ]0.2 could also be caused by magnetic disorder, lead to localization of charge carriers. The transport properties are dominated by a hopping process. The electric resistivity behavior obeys Mott’s law: log r T − 1/4. The large negative magnetoresistance of over 32% occurs for x = 0 at 5 K. When H is lower than 10 KOe, the magnetoresistance was observed to
[1] J.G. Bednorz, K.A. Mu¨ller, Phys. B 64 (1989) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang, C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [3] Y. Tokura, A. Urushiara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, N. Furukawa, J. Phys. Soc. Jpn. 63 (1994) 3931. [4] S. Jin, T.H. Tiefel, M. McCoemack, R.A. Fastnacht, L.H. Chen, Science 264 (1994) 413. [5] K. Chanara, T. Ohno, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993) 1990. [6] M. McCormack, S. Jin, T.H. Tiefel, R.M. Fleming, J.M. Phillips, R. Ramesh, Appl. Phys. Lett. 64 (1994) 3045. [7] R. Von Helmot, J. Weaker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [8] R. Mahendiran, S.K. Tiwari, A.K. Raychaudhuri, T.V. Ramakrishnam, R. Mahesh, N. Rangavittal, C.N.R. Rao, Phys. Rev. B 53 (1996) 3348. [9] R. Mahendiran, R. Mahesh, A.K. Raychaudhuri, C.N.R. Rao, Solid State Commun. 94 (1995) 515. [10] R. Mahendiran, R. Mahesh, A.K. Raychaudhuri, C.N.R. Rao, Solid State Commun. 99 (1996) 149. [11] R. Mahendiran, R. Mahesh, A.K. Raychaudhuri, C.N.R. Rao, J. Solid State Chem. 120 (1995) 204. [12] A. Urvsshibura, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [13] A. Maignan, V. Caignaert, Ch. Simon, M. Hervieu, B. Raveau, J. Mater. Chem. 5 (1995) 1089. [14] H.L. Ju, J. Gopalakrishnan, J.L. Peng, Q. Li, G.C. Xiong, T. Venkatesan, R.L. Greene, Phys. Rev. B 51 (1995) 6143. [15] G.H. Jonker, J.H. Van Santen, Physica 19 (1953) 120. [16] C. Zener, Phys. Rev. 118 (1951) 403. [17] J.B. Goodenough, J. Phys. Chem. Solids 6 (1958) 287. [18] P.W. Anderson, Phys. Rev. 79 (1950) 350; 79 (1950) 705. [19] M. Itoh, I. Natori, S. Kubota, K. Motoya, J. Phys. Soc. Jpn. 63 (1994) 1486. [20] A. Chainani, M. Mathew, D.D. Sarma, Phys. Rev. B 46 (1992) 9976. [21] P.M. Raccah, J.B. Goodenough, J. Appl. Phys. 39 (1968) 1209. [22] V.G. Bhide, D.S. Rajoria, C.N.R. Rao, G.R. Rao, V.G. Jadhao, Phys. Rev. B 12 (1975) 2832. [23] T.A. Vanderah, Chemistry of Superconductor Materials, Noyes, New Jersey, 1991. [24] H.W. Hsu, Y.M. Chang, G.J. Chen, J. Magn. Mater., in press. [25] J.B. Goodenough, Magnetism and Chemical Bond, Wesley, New York, 1963. [26] J. Grasia Soldevilla, J.I. Espeso, J. Rodriguez Fernandez, J.C. Gomez Sal, J.A. Bianco, P. Galez, D. Paccard, J. Magn. Magn. Mater. 157 – 158 (1961) 685.
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Magnetic, electrical properties and magnetoresistance of Cu substituted La0.7Sr0.3CoO3 − d crystals (0.355d50.45) H.W. Hsu a, Y.H. Chang a,*, G.J. Chen a, K.J. Lin b a
Department of Materials Science and Engineering, National Cheng Kung Uni6ersity, Tainan, Taiwan, ROC b Department of Physics, National Cheng Kung Uni6ersity, Tainan, Taiwan, ROC Received 13 October 1998; received in revised form 26 April 1999; accepted 28 April 1999
Abstract The magnetic susceptibility, resistivity, and magnetoresistance of La0.7Sr0.3Co1 − x Cux O (0 5x 5 0.4 and 0.3 5d50.4) compounds have been studied for applied field up to 50 kOe. Cuire temperature and saturation magnetization of the ferromagnetic perovskite La0.7Sr0..3Co1 − x Cux O decreases, when cobalt is substituted by copper. In the doping, all coballtites exhibited ferromagnetic long-range ordered behaviors. If the content of copper exceeds 10%, the electronic properties of the material show the behavior of transformation from metallic to semiconductor. The semiconducting behavior is connected with the magnetic disorder, which show variable range hopping, log rT − 1/4. The magnetoresistance, which can be expressed as MR8 H q when H5 10 kOe, is negative and exceeds 32%. The results are interpreted in terms of a double-exchange and super-exchange model. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Magnetoresistance; Magnetic susceptibility; Resistivity; Double-exchange; Variable range hopping
1. Introduction In recent years, high-temperature superconductivity in hole doped copper oxide compound has been found in the study of electron conduction in closely related class of compounds [1,2]. It has excited renewed and extended interest in the correlated stimulated dynamics of spin and charges near the metal-insulator (M–I) transition in 3d transition-metal oxides. The similarity in the metallic conducting ferromagnetic compounds on hole-doped oxides basis and with perovskite structure, Ln1 − x Ax MO3 (Ln, rare-earth ions, e.g. La, Pr, and Nd; A, divalent ions, e.g. Sr, Ca, Ba, and Pb; M, transition-metal ions, e.g. Co and Mn) has been revisited and the phenomenon of giant magnetoresistance (GMR) have been observed [3 – 13]. The magnitude of magnetoresistance is generally maximum around metalinsulator (M–I) transition, the temperature Tp that is close to the ferromagnetic transition temperature Tc [14] (generally a few temperatures lower). Recently, the considerable attention has been focused on the Ln1 − * Corresponding author. Fax: +886-6-2382800. E-mail address: [email protected] (Y.H. Chang)
xAx MO3, similar properties have been found in material of La1 − x Srx CoO3. Jonker et al. [15] first studied the magnetic and transport properties of La1 − xSrx CoO3 which is a good conductor (rB10 − 1 mVcm) and has a metallic temperature coefficient (a\ 0) for x=0.5. The double-exchange mechanism of Zener [16], which interpreted the magnitude of spontaneous magnetization, and paramagnetic Curie temperature is not considered relevant. Goodenough [17] that assumes the 3d electrons to be localized interpreted the ferromagnetism by the superexchange model of Anderson [18]. The magnetic phase diagram of La1 − xSrx CoO3 from magnetization measurements by Itoh et al. [19] was made. The electronic property of the La1 − xSrx CoO3 near metal–insulator transition was studied by Chainani et al. [20] using electron microscopy. Raccah et al. [21] have described the coexistance of itinerant and localized electrons. Mo¨ssbauer measurement by Bhide et al. [22] and other studies in La1 − x Srx CoO3 have pointed out that ferromagnetic Sr2 + -rich cluster coexist with paramagnetic La3 + -rich regions in the same crystallographic phase. The electrons in the sbonding orbital are delocalized at the highest temperature was conjectured from studies on La1 − x Srx CoO3;
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 1 4 6 - 4
H.W. Hsu et al. / Materials Science and Engineering B64 (1999) 180–186
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and at low temperature, some of the electrons get trapped as localized eg. Previous investigations have observed the similarity in composition and crystal structure for high Tc temperature superconductivity in hole doped Cu oxides’ compounds. Therefore, in light of above development, this work investigates the changes of magnetization, magnetoresistance behavior and electrical conduction by doping Cu in La0.7Sr0.3Co1 − x Cux O3 − d systems. 2. Experimental Samples of La0.7Sr0.3Co1 − x Cux O3 − d compounds with various x values were prepared by the standard ceramic technique. The starting materials were prepared by mixing the reagent grade powders of SrCO3, CoO, La2O3, and Cu2O. The mixture was milled for 24 h with alcohol. After drying the mixed powders at 100°C, they were prefired in air at 900°C for 24 h. The obtained products were reground and milled. For measuring the electrical conductivity and magnetic properties, the powders of each compound were compressed into a rectangular shape (15× 6 ×0.8 mm3) under a pressure of about 4350 kg cm − 2 and then the pressed samples were sintered at 970 – 1100°C for 48 h in air. The oxygen-deficient samples were obtained in this manner as well. The X-ray diffraction patterns with filtered Cu–Ka radiation for the various compositions were obtained using a diffractometer at room temperature. The sample was ground to very fine powders in alcohol and deposited on support carbon grids. Electron microscopic observations were carried out using a Jeol-300 electron microscope with electron microscope with a side-entry 40°/30°-tilt goniometer and a Z-translation stage. The wavelength of electrons was 0.0196 A, and camera length, 120 cm. Analyses of the chemical compositions were performed using an electron probe microanalyzer (EPMA). The oxygen content was determined using a standard iodometric titration technique [23]. Magnetization measurements in fields up to 50 kOe were taken with a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 5 –300 K. Next, the electrical resistivities were measured as a function of temperature and magnetic field in a superconducting magnet with the maximum applied field of H =50 kOe. The four-probe leads were formed with silver paste along the long axis of specimen. The outside leads were used to supply constant current (I//H). The magnitude of the magnetoresistance is then defined as MR = (r(H)−r(0))/r(0), where r(H) and r(0) are resistivities at magnetic field H and at zero field, respectively.
Fig. 1. X-ray diffraction spectra of La0.7Sr0.3Co1 − x Cux O3 − d (x = 0, 0.1, 0.2, 0.3 and 0.4) powder samples.
3. Results and discussion According to the results of X-ray powder diffraction pattern at room-temperature, the La0.7Sr0..3Co1 − x Cux O samples (05 x 50.4) were single phase materials as shown in Fig. 1. For samples with a higher amount of Cu, the crystal structure is rhombohedral (Hex) with a small distortion. This is because of the introducing a larger size Cu cation progressively reduces the rhombohedral (Hex) distortion.
Fig. 2. Electron diffraction patterns of La0.7Sr0.3Co0.7Cu0.3O3 − d with the bean in [001] direction.
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domain walls that appear in strong anisotropic ferromagnetic compounds. The magnitude of MZFC at low temperature is small, and starts to increase at temperatures at which the applied magnetic field and/or thermal activation is large enough to produce the domain wall motion overcoming the anisotropy energy responsible for the quenching of these magnetic domains. Such domain wall behavior is reported in ferromagnetic intermetallic and perovskite oxide compounds [26–28]. Whereas, the characteristic feature is similar to spinglass-like behavior [19,29]. In the La1 − y Sry Co1 − x Cux O3 − d samples system, Co3 + /Co4 + valence mixture is induced owing to the La3 + /Sr2 + chemical mixture. Herein, assuring fixed valences for La3 + , Sr2 + , Cu2 + , and O2 − content. By ion replacing a Co3 + ion with Cu2 + , the other Co3 + would convert to Co4 + . According to charge neutrality, we obtain as average Co-valence: Fig. 3. Temperature dependence of the magnetic susceptibility at 200 Oe of La0.7Sr0.3Co1 − x Cux O3 − d (x= 0.2, 0.3 and 0.4). Dotted and solid lines represent zero-field-cooled (ZFC) and field-cooled (FC) magnetizations, respectively.
The electron diffraction pattern of La0.7Sr0.3Co0.7Cu0.3O3 − d sample represent with the beam in [001] orientation are shown in Fig. 2. The pattern represents the rhombohedral (Hex) phase corresponding to the X-ray diffraction pattern of Fig. 1. The temperature dependence of the magnetization of samples, MFC (T), was measured in a field of H= 200 Oe on heating from the lowest temperature after cooling in zero magnetic field (ZFC), or in a field of 200 Oe (FC). Fig. 3 shows MFC (T) curves similar to these observed [19]. According to our results, the doping of Cu introduces some change in magnetic and transport properties. The Curie temperature Tc 220 K for x= 0 decreases to Tc 175K for x = 0.4. Correspondingly, MFC (T) rapidly decreases with x below Tc. MFC increases monotonically down to 5K. A noticeable feature is that the magnetization continuously increases as the temperature is lowered from room temperature. The MFC is typical of a ferromagnet. The FC and ZFC curves significantly differ below TB as shows in Fig. 3. These phenomena maybe produce owing to the effect of local magnetic anisotropy for individual polycrystalline grains. This is because of these ceramic samples are known to be inhomogenous on a microscopic scale. For an applied field of 200 and 1000 Oe on the x= 0.4 samples, thermo-magnetic irreversibility (TMI) occurs shifts from 190 to 170 K [24]. The TMI can arise from pinning of domains. It is worth mentioning here that this composition contains a Co-ion which causes high uni-axial anisotropy [25]. The asymmetrical peak centered can be thought of as arising due to the motion of
Ave=
(3− d+ y−2x) 1− x
(1)
According to Eq. (1), the spin-only moment single ion can be calculated, but only for a uniform spin state per valence. For y=0.3, x= 0.4, and d=0.35, no Co3 + /Co4 + pairs exist anymore. Therefore, the ferromagnetic double exchange coupling and the metallic conduction should disappear. In fact, the ferromagnetic spin-order is observed for x=0.4. Herein, we assume that the effect of oxygen non-stoichometry increases the different spin-states. We have to measure M–H curves and long-time behavior of thermoremanent magnetization (TMR). According to these results, we can exact to decide the magnetic properties. Fig. 4. Shows of curves the M
Fig. 4. ZFC magnetization curves of La0.7Sr0.3Co1 − x Cux O3 − d (x = 0.2, 0.3 and 0.4).
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Fig. 5. Relaxation of the thermoremanent magnetization (TRM) for La0.7Sr0.3Co0.6Cu0.4O3 − d at 5 K.
versus H at 5 K for three samples (0.25x 5 0.4). Samples with higher Cu doping show a smaller magnetization below Tc. The Curie temperature and the magnetization decrease when Cu is added. This is because of the magnetization is mostly contributed to come from Co ions. The hysteresis loops approach to a saturation of M at 50 kOe. Due to lack of saturation in high fields is a characteristic feature of spin glasses [30]. Therefore, we suggest that these compounds are not of spin-glass. The coercive fields indicate the existence of
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magnetic anisotropy that agree with the ZFC and FC curves of Fig. 3. Our results also observed a relaxation behavior of the TMR for x= 0.4. The experimental procedure is as follows. After the field cooling (FC) process from room temperature to 5 K, the sample was maintained for different time intervals when the field is turned off at 5 K as shown in Fig. 5. We measured the magnetization MFC (t) as a function of time t. The plot reveals that the magnetic moment does not relax. Therefore, the MFC (t) showing the long-time relaxation behavior was not observed. Indicating the magnetic properties differ from that of the spin-glass properties. On the basis of magnetization measurements mentioned above, we can prove a ferromagnetic long-range order in La0.7Sr0.3Co1 − x Cux O3 − d compounds. In Figs. 6(a, b) the curves represent the DC resistivity of the La0.7Sr0..3Co1 − x Cux O3 − d (x=0, 0.1, 0.2, 0.3 and 0.4) compounds. The resistivity with increasing Cu contents, because the number of carrier decreases. Samples with x5 0.1 show metallic conductivity, but at higher Cu-concentration, a semiconducting behavior (i.e. (r/(T B0) appears. A cusp in resistivity is observed near the ferromagnetic Curie temperature (Tc). It indicates that the magnon scattering is dominant. Above 100 K the electrical resistivity r(T) in metallic behavior samples (i.e. (r/(T \ 0) show a T 2 dependence (r(T)= r0 + AT 2). A T 2 term in resistivity comes from spin fluctuations [31] or electron-magnon scattering [32] in ferromagnetic metal ions at low temperature is observed. The similar material [33] at low temperature is observed as well. The curves have a small rise in r(T). This rise in r(T) shows a T − 1/2
Fig. 6. Resistivity versus temperature curves r(T) for La0.8Sr0.2Co1 − x Cux O3 − d compounds (a) x= 0, 0.1, and (b) x =0.2, 0.3, 0.4.
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The semiconducting behavior could be caused by magnetic disorder. According to our results, r(T) can be fitted well by Mott-plots than by Arrhenius-plots. It was suggested by Mott [36] that a random magnetic structure could lead to localize electric states. The transport properties are dominated by a hopping process. Because of a frozen-in spin-disorder, variable range hopping could lead to the observed log rT − 1/4 behaviors found. Fig. 8 shows some of the magnetoresistance curves at different Cu content (x=0, 0.1, 0.2, 0.3 and 0.4). At 5 K, the resistivity decreases with increasing field, which demonstrates the typical negative magnetoresistance effect. Therefore, MR increases with increasing H. Due to the applied magnetic field tends to align with the local spin and reduced spin scattering of the conduction electron. Although the exact nature of this change in magnetoresistance behavior is unclear. However, we can expect that the Cu content increase not only with a decreasing Co content but also with a decreasing charge carriers [24]. The numbers of Co4 + –O–Co3 + interactions decrease. Therefore, the Co/Cu mixture ions play prominent role in controlling the resistivity, magnetization, and negative magnetoresistivity effect of La0.7Sr0.3Co1 − x Cux O3 − d compounds. The competition between positive and negative couplings causes large negative magnetoresistance as observed for La1 − xSrx CoO3 − d. At 5 K, the negative magnetoresistance exceed 32% at 50 kOe. In addition, log MR versus log H curves have been plotted in Fig. 9 for different Cu contents. We find that there exists a good linear relationship between log MR and log H when H is lower than 10 kOe. The magnetoresistance is directly
Fig. 7. Arrhenius and Mott plots of r(T) for the La0.8 Sr0.2Co1 − xCux O3 − d compounds (a) x =0.2; (b) x= 0.3, and (c) x= 0.4.
behavior (Fig. 6(a), inset). The T 1/2 dependence of s(T) arises from the electron-electron interaction in disordered systems [33,34]. We expressed this rise in r(T) as a correction to the s(T) described by the equation s(T)=s0 +aT 1/2
(2)
Where s0 is a finite zero-temperature conductivity. Samples with x] 0.2 show semiconductors (i.e. (r/ (T B0). There are two energy gaps in the temperature semiconducting as shown in Figs. 7(a, b): at higher temperature (T \30 K) the conductivity is a higher gap; at lower temperature (T B30 K) a lower energy gap is observed. This is a common behavior for doped semiconductors [35].
Fig. 8. The variation of MR curves with applied field H of La0.7Sr0.3Co1 − x Cux O3 − d (x =0, 0.1, 0.2, 0.3 and 0.4) at 5 K.
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dependent on the magnetization directly proportional to the H.
Acknowledgements This research was supported by the National Science Council of the Republic of China under the grant of NSC 86-2212-E006-001. The authors thank S.T. Lin for his assistance in making the hysteresis measurement. References
Fig. 9. A plot of log (MR) vs. log (H) for La0.8 Sr0.2Co1 − x Cux O3 − d compounds (x =0, 0.1, 0.2, 0.3 and 0.4) at 5 K.
proportional to the H: MR8 H q, where q is a constant. In metallic multilayer or grain-type GMR thin film, the feature is often observed. When H is lower than 10 kOe, magnetoresistance is weakly field-dependent, which is in agreement with the result mentioned in Fig. 3.
4. Conclusions The electronic and magnetic property of polycrystalline La0.7Sr0.3Co1 − x Cux O compounds was studied. Magnetization measurements have been done to reveal the detail magnetic properties. Spontaneous magnetization with the ferromagnetic long-range ordered exist below Tc. But we have not observed the long-time relaxation behavior of MFC having the aging behavior. According to the absolute values of the resistivity, the ferromagnetic samples (x 5 0.1) can be considered as metallic; (a \ 0). Electron-electron interaction in these metallic oxides gives rise to a T 1/2 term in the conductivity at lower temperature. The electrical resistivity at higher temperature (T BTc) also consist with r(T)= r0 + AT 2. The semiconducting behavior, (a B0) which occurs for x ]0.2 could also be caused by magnetic disorder, lead to localization of charge carriers. The transport properties are dominated by a hopping process. The electric resistivity behavior obeys Mott’s law: log r T − 1/4. The large negative magnetoresistance of over 32% occurs for x = 0 at 5 K. When H is lower than 10 KOe, the magnetoresistance was observed to
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