溶胶－凝胶法制备的La_（2/3）Ca_（1/3）MnO_3中Mn位Cu和Zn掺杂的比较研究
摘要
本文以择优掺杂的La2/3Ca1/3MnO3为基础,研究了Mn位Cu和Zn掺杂对其输运性质及磁电阻行为的影响,主要研究内容概括如下:
回顾了磁电阻材料的研究情况;介绍了钙钛矿结构锰基氧化物的晶体结构、电子特征、锰离子之间的交换相互作用以及锰基氧化物的电磁性质;最后重点对掺杂效应对锰基氧化物的影响作了综述。
分别采用固相反应法和溶胶-凝胶法制备了名义组分为La2/3Ca1/3MnO3的系列样品,研究了它们的电输运行为。研究表明:用溶胶-凝胶法制备并经高温1450oC烧结的样品和固相反应法制备的样品有相同的电输运行为,都在～260K发生绝缘体-金属导电行为的转变。对于溶胶-凝胶法制备的样品,随烧结温度的降低,样品的半导体-金属转变温度向低温移动,样品发生绝缘体-金属转变的温度区间展宽,样品的电阻率增大。
用溶胶-凝胶法制备并经不同的温度烧结得到了名义组分为La2/3Ca1/3Mn1-xCuxO3的系列样品。对样品电输运性质的研究表明:随Cu掺杂量的增加和烧结温度的降低,样品的绝缘体-金属转变温度TIM向低温移动,同时电阻率增加。特别当Cu掺杂量x>3.5%以后,样品的电阻率急剧增加,系统表现出反常的物理行为。在Cu掺杂4%的样品中存在着热滞效应,而且热滞现象随样品烧结温度的升高(颗粒尺寸的增大)会逐渐消失。
以用溶胶-凝胶方法制备并经不同温度烧结得到的名义组分为La2/3Ca1/3Mn1-xZnxO3(0x6%)的系列样品为研究对象,对它们的电输运性质的研究表明: La2/3Ca1/3Mn1-xZnxO3系列样品和La2/3Ca1/3Mn1-xCuxO3系列样品在电输运行为上非常相似。Zn掺杂使得样品的绝缘体-金属转变温度TIM向低温方向移动,电阻率增加。掺杂量x在3.5%附近,样品的电输运行为有很大的变化。x>3.5%以后,TIM迅速向低温方向移动,电阻率急剧增加。对样品在高温区导电行为的研究表明:当掺杂量x>3.5%以后,样品在T>TIM的整个高温区的实验数据用Mott变程跳跃机理能得到较好的拟合。
对La2/3Ca1/3Mn1-xCuxO3和La2/3Ca1/3Mn1-xZnxO3(x=4%)多晶样品在加磁场下的电输运行为和磁电阻效应的研究表明:La2/3Ca1/3MnO3中的Cu掺杂能大大的增强样品在绝缘体-金属转变温度附近的磁电阻效应,最大的磁电阻值出现在1100oC烧结的掺Cu样品中,它在0.3T的磁场下可以达到80%。掺Cu样品的颗粒间磁电阻效应也被增强,1000oC烧结的掺Cu样品在0.3T的磁场下的颗粒间磁电阻的最大值可以达到40%。虽然4%的Zn掺杂的样品和4%的Cu掺杂的样品有类似磁电阻行为。但4%的Zn掺杂的样品的磁电阻峰值明显不如4%的Cu掺杂的样品高。在0.3T的磁场下,4%的Zn掺杂样品的磁电阻最大值只有30%。
In this dissertation, the effect of the doping in Mn site with Cu and Zn on the electrical transport and magnetoresistance effect of La2/3Ca1/3MnO3 is carried out. The main investigative results are as follows:
The different magnetoresistance material is briefly reviewed. The advance in the study of perovskite manganites has been summarized. More attentions have been paid to the doping effect.
Polycrystalline samples of nominal composition La2/3Ca1/3MnO3 were prepared with sol-gel method and solid-state reaction method. Experiments indicate that the sample fabricated by a sol-gel method following sintering treatments at 1450oC has the same electronic transport properties with the sample prepared with solid-state reaction method. As the samples prepared with a sol-gel method, the temperature of insulator-metal transition moved to lower temperature and the resistivity increased with the sintering temperature decreased.
Polycrystalline samples of nominal composition La2/3Ca1/3Mn1-xCuxO3 were fabricated by a sol-gel method following sintering treatments at different temperature. Experiments indicate that the temperature of insulator-metal transition moved to lower temperature and the resistivity increased with Cu doping content increased the sintering temperature decreased. Especially when Cu doping content x exceed 3.5%, the samples have abnormal behavior and the resistivity increased dramatically. Unusual thermal hysteresis is observed in the x=4% samples. The thermal hysteresis disappears gradually with increasing of the sintering temperature.
Polycrystalline samples of nominal composition La2/3Ca1/3Mn1-xZnxO3() were fabricated by a sol-gel method following sintering treatments at different temperature. Experiments indicate that the electrical transport of La2/3Ca1/3Mn1-xZnxO3
have a similar feature with the samples of La2/3Ca1/3Mn1-xCuxO3. A significant downward shift of I-M transition and a sharp enhancement in resistivity are present in the sample containing Zn as well as. The study of the conductive mechanism of samples sintered at 1100oC indicated that the Mott’s variable range hopping model can simulate the experiment data very well when the doping content x>3.5%.
A comparative study of electrical transport and magnetoresistance is performed for the samples of La2/3Ca1/3Mn1-xZnxO3 and La2/3Ca1/3Mn1-xZnxO3(x=4%). Experiments indicate that the MR effect near the insulator-metal (I-M) transition can be substantially enhanced through 4% Cu doping in La2/3Ca1/3MnO3. The large MR value can be obtained in a low magnetic field, which is different from the intrinsic CMR usually observed in perovskite manganites. The maximum MR with a value ~80% of that for H=0.3T is obtained in the sample prepared at 1100oC. Cu doping in La2/3Ca1/3MnO3 also causes a clear enhancement of the Intergrain Magnetoresistance (IMR). The largest IMR, with a value ~40%, for the 0.3T field, is observed in the doped sample sintered at 1000oC. Although the same behavior of transport and MR are observed in two samples, the MR peak near the I-M transition of the sample containing Zn is lower than the sample containing Cu. For the sample containing Cu, the maximum MR appearing near the I-M transition for the 0.3T filed have a value as high as ~80%. This is a sharp contrast with a low value ~30% for the maximum MR obtained in the sample containing Zn.
回顾了磁电阻材料的研究情况;介绍了钙钛矿结构锰基氧化物的晶体结构、电子特征、锰离子之间的交换相互作用以及锰基氧化物的电磁性质;最后重点对掺杂效应对锰基氧化物的影响作了综述。
分别采用固相反应法和溶胶-凝胶法制备了名义组分为La2/3Ca1/3MnO3的系列样品,研究了它们的电输运行为。研究表明:用溶胶-凝胶法制备并经高温1450oC烧结的样品和固相反应法制备的样品有相同的电输运行为,都在～260K发生绝缘体-金属导电行为的转变。对于溶胶-凝胶法制备的样品,随烧结温度的降低,样品的半导体-金属转变温度向低温移动,样品发生绝缘体-金属转变的温度区间展宽,样品的电阻率增大。
用溶胶-凝胶法制备并经不同的温度烧结得到了名义组分为La2/3Ca1/3Mn1-xCuxO3的系列样品。对样品电输运性质的研究表明:随Cu掺杂量的增加和烧结温度的降低,样品的绝缘体-金属转变温度TIM向低温移动,同时电阻率增加。特别当Cu掺杂量x>3.5%以后,样品的电阻率急剧增加,系统表现出反常的物理行为。在Cu掺杂4%的样品中存在着热滞效应,而且热滞现象随样品烧结温度的升高(颗粒尺寸的增大)会逐渐消失。
以用溶胶-凝胶方法制备并经不同温度烧结得到的名义组分为La2/3Ca1/3Mn1-xZnxO3(0x6%)的系列样品为研究对象,对它们的电输运性质的研究表明: La2/3Ca1/3Mn1-xZnxO3系列样品和La2/3Ca1/3Mn1-xCuxO3系列样品在电输运行为上非常相似。Zn掺杂使得样品的绝缘体-金属转变温度TIM向低温方向移动,电阻率增加。掺杂量x在3.5%附近,样品的电输运行为有很大的变化。x>3.5%以后,TIM迅速向低温方向移动,电阻率急剧增加。对样品在高温区导电行为的研究表明:当掺杂量x>3.5%以后,样品在T>TIM的整个高温区的实验数据用Mott变程跳跃机理能得到较好的拟合。
对La2/3Ca1/3Mn1-xCuxO3和La2/3Ca1/3Mn1-xZnxO3(x=4%)多晶样品在加磁场下的电输运行为和磁电阻效应的研究表明:La2/3Ca1/3MnO3中的Cu掺杂能大大的增强样品在绝缘体-金属转变温度附近的磁电阻效应,最大的磁电阻值出现在1100oC烧结的掺Cu样品中,它在0.3T的磁场下可以达到80%。掺Cu样品的颗粒间磁电阻效应也被增强,1000oC烧结的掺Cu样品在0.3T的磁场下的颗粒间磁电阻的最大值可以达到40%。虽然4%的Zn掺杂的样品和4%的Cu掺杂的样品有类似磁电阻行为。但4%的Zn掺杂的样品的磁电阻峰值明显不如4%的Cu掺杂的样品高。在0.3T的磁场下,4%的Zn掺杂样品的磁电阻最大值只有30%。
In this dissertation, the effect of the doping in Mn site with Cu and Zn on the electrical transport and magnetoresistance effect of La2/3Ca1/3MnO3 is carried out. The main investigative results are as follows:
The different magnetoresistance material is briefly reviewed. The advance in the study of perovskite manganites has been summarized. More attentions have been paid to the doping effect.
Polycrystalline samples of nominal composition La2/3Ca1/3MnO3 were prepared with sol-gel method and solid-state reaction method. Experiments indicate that the sample fabricated by a sol-gel method following sintering treatments at 1450oC has the same electronic transport properties with the sample prepared with solid-state reaction method. As the samples prepared with a sol-gel method, the temperature of insulator-metal transition moved to lower temperature and the resistivity increased with the sintering temperature decreased.
Polycrystalline samples of nominal composition La2/3Ca1/3Mn1-xCuxO3 were fabricated by a sol-gel method following sintering treatments at different temperature. Experiments indicate that the temperature of insulator-metal transition moved to lower temperature and the resistivity increased with Cu doping content increased the sintering temperature decreased. Especially when Cu doping content x exceed 3.5%, the samples have abnormal behavior and the resistivity increased dramatically. Unusual thermal hysteresis is observed in the x=4% samples. The thermal hysteresis disappears gradually with increasing of the sintering temperature.
Polycrystalline samples of nominal composition La2/3Ca1/3Mn1-xZnxO3() were fabricated by a sol-gel method following sintering treatments at different temperature. Experiments indicate that the electrical transport of La2/3Ca1/3Mn1-xZnxO3
have a similar feature with the samples of La2/3Ca1/3Mn1-xCuxO3. A significant downward shift of I-M transition and a sharp enhancement in resistivity are present in the sample containing Zn as well as. The study of the conductive mechanism of samples sintered at 1100oC indicated that the Mott’s variable range hopping model can simulate the experiment data very well when the doping content x>3.5%.
A comparative study of electrical transport and magnetoresistance is performed for the samples of La2/3Ca1/3Mn1-xZnxO3 and La2/3Ca1/3Mn1-xZnxO3(x=4%). Experiments indicate that the MR effect near the insulator-metal (I-M) transition can be substantially enhanced through 4% Cu doping in La2/3Ca1/3MnO3. The large MR value can be obtained in a low magnetic field, which is different from the intrinsic CMR usually observed in perovskite manganites. The maximum MR with a value ~80% of that for H=0.3T is obtained in the sample prepared at 1100oC. Cu doping in La2/3Ca1/3MnO3 also causes a clear enhancement of the Intergrain Magnetoresistance (IMR). The largest IMR, with a value ~40%, for the 0.3T field, is observed in the doped sample sintered at 1000oC. Although the same behavior of transport and MR are observed in two samples, the MR peak near the I-M transition of the sample containing Zn is lower than the sample containing Cu. For the sample containing Cu, the maximum MR appearing near the I-M transition for the 0.3T filed have a value as high as ~80%. This is a sharp contrast with a low value ~30% for the maximum MR obtained in the sample containing Zn.
引文
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G.. Verbanck, K. Temst, K. Mae et al., Large positive magnetoresistance in Cr/Ag/Cr trilayers, Appl. Phys. Lett. 1997, 70:1477-1479
E. N. Mitchell, H. B. Haukaas, H. D. Bale et al. J. Appl. Phys. 35, 2604(1964)
H. C. Van Elst, Physica 25, 708(1959)
T. R. McGuire and R. I. Potter, Anistropic magnetoresistance in ferromagnetic 3d alloys, IEEE Trans. Magn. 1975, 11:1018-1038
M. N. Baibich, J. M. Broto, A. Fert et al., Giant magnetoresistance of (001)Fe/(001)Cr magnetic superlattices, Phys. Rev. Lett. 1988, 61:2472–2475
S. S. P. Parkin, R. Bhadra, and K. P. Roche, Oscillatory magnetic exchange coupling through thin copper layers, Phys. Rev. Lett. 1991, 66:2152–2155
B. Dieny, V. S. Speriosu, S. S. P. Parkin et al., Giant magnetoresistive in soft ferromagnetic multilayers, Phys.Rev.B,1991, 43 : 1297-1300
M. Julliere, Phys. Lett. A, 54, 225(1975)
T. Miyazaki and N. Tezuka, Giant magnetic tunneling effect in Fe/Al2O3/Fe junction, J.Magn. Magn. Mater. 1995,139:L231-L234
G. H. Jonker, Physica(Amsterdam) 22, 707(1956)
E. O. Wollan, and W. C. Koehler, Phys. Rev. 100,545(1955)
J, B. Goodenough, Phys. Rev. 100,564(1955)
C. W. Searle and S. T. Wang, Can. J. Phys. 48,2023(1970)
R. M. Kusters, J. Singleton, D. A. Keen et al., Physica 155, 362(1989)
R. von Helmolt, J. Wecker, B. Holzapfel et al., Magnetoresistance in perovskite La2/3Ba1/3MnO3 films. Phys.Rev.Lett.,1993, 71 : 2331-2335
S. Jin, T. H. Tiefel, M. McCormack et al., Science 264,413(1994)
Y. Moritomo, H. Kuwahara, and Y. Tokura, Nature 380, 141(1996)
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