Electrical conductivity and low-field magnetoresistance in. Zhigao Huang b,

Transcription

1 Solid State Phenomena Vols (27) pp Online available since 27/Mar/15 at (27) Trans Tech Publications, Switzerland doi:1.428/ Electrical conductivity and low-field magnetoresistance in La 2/3 (Ca 6 Ba 4 ) 1/3 Mn 1-x V x O 3 prepared by sol-gol Hong Zhang a, Shuiyuan Chen a, Suzhen Tang a, Wenpao Ke a, Heng Lai a, Zhigao Huang b, Department of Physics, Fujian Normal University, Fuzhou 357, China a nano@fjnu.edu.cn, b zghuang@fjnu.edu.cn Keywords: perovskite manganite, magnetoresistance, sol-gel Abstrct. La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 (x=,.3,.5,.7,,.15,.2) nanoparticles were synthesized using sol-gel technology. The experimental results reveal that, (1) the substitution of V for Mn in La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 lowers the Curie temperature T C and the metal insulator transition temperature T MI ; (2) there exists the evident difference between the T C and the T MI for different V substitution ratio; (3) the low-temperature tunneling magnetoresistance and maximum magnetoresistance near T c increase with the enhancement of V-doping content. Based on the tunneling magnetoresistance model and the percolation model near Curie temperature, the experimental results are explained well. Introduction Recently, a lot of theories and experiments have been done to study the physical mechanism of the colossal magnetoresistance (CMR) effect due to its interesting magnetic properties and application potential [1-2]. A prominent feature of perovskite manganite materials is a metallicinsulating(m-i) transition associated with the ferromagnetic-paramagnetic (FM-PM) transition, which was traditionally understood within the framework of Mn 3+ -O-Mn 4+ double-exchange (DE) mechanism In La 1-x A x MnO 3 (A=Ca, Ba, Sr), Mn acts at the heart of DE interaction, the influence of the substitution at Mn sites with other elements has spurred considerable interest in recent years. The influences of Fe, Ni, Co, Cr, Ti, Al, Cu and Zn substitution on the electronic and magnetic properties of La 1-x A x MnO 3 have been reported [3-7]. Mn-side substitution is an interesting subject because Mn-side doped ions will destroy the perfection of the Mn-O plane and the double-exchange interaction. But up to now, the work using V (3d 3 4s 2 ) to substitute Mn-side hasn t been reported yet. Here, we use V to dope Mn-side of La 2/3 (Ba.4 Ca.6 ) 1/3 MnO 3, and a strong effect of the substitution on the magnetic and electrical Properties are expected. Experiment Polycrystalline samples of La 2/3 (Ba.4 Ca.6 ) 1/3 Mn 1-X V X O 3 (x=,.3,.5,.7,,.15,.2) were prepared by the sol-gel and conventional synthetic methods. Appropriate amounts of powder of La 2 O 3 CaCO 3 Ba(C 2 H 4 O 2 ) 2 Mn(C 2 H 4 O 2 ) 2 and V 2 O 5 were dissolved in solution of rare nitric acid sequentially, making the solution transparent by whipping. Then it turned into sol after being kept at 8 C in the water bath for 2 hours. The sol was dried at 25 C for 5 hours, and All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: , Pennsylvania State University, University Park, United States of America-4/6/14,14:44:22)

2 92 Nanoscience and Technology we got the precursor gel. The resulting powder was calcined at 7 C for 5 hours, then pressed to form round tablets (Φ13 mm), and finally these tablets were sintered in the air atmosphere at 11 C for 1 hours. The samples were characterized by XRD. The magnetization for all the samples was measured by VSM at the.1t magnetic field. The resistivity under and.4t magnetic field was measured by standard four-probe method from 77K to 37K. Results and discussion The typical XRD pattern of the La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 (x=.7) are shown in Fig 1. From the figure, it is found that the main phase is the standard XRD patterns of perovskite structure. However, there exists an additional phase, which will be further studied. The temperature dependence of magnetization M and dm/dt for the all samples under the field of.1t from 77K to 35K have been measured, and the curie temperature T c can be defined. The V content dependence of the Curie temperature for La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 (x=,.3,.5,.7,,.15,.2) is shown in Fig.2. From the figure, the value of T c decreases with increasing V content. Especially, it reduces evidently for x 1%. This change trend is similar to that for La.7 Sr.3 Mn 1-x Me x O 3 and La.7 Ca.3 Mn 1-x Me x O 3 (Me=Fe,Cr,Al,Ti) [4,9,1]. The V substitution breaks the Mn-O-Mn networks and weakens the superexchange between Mn ions. As a result, a short range ordered ferromagnetic clusters may be created. The temperature dependences of the resistivity ρ (T) and ρ H (T) with H=. and.4t for La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 (x=,.3,.5,.2) are shown in Fig.3(a)-(b). From the figure, it is found that for the samples with x=%, 3%, 5%, the curves of the resistivity ρ(t) exhibit double-peak feature, and the double peaks for sample with x=3% are more obvious than for sample with x=5%. However, as x>5%, the double peaks disappear. This variation of double peak behavior is mainly associated with an increase both of the height and width of tunnel barriers with increasing V-doping [8, 9]. Moreover, it can be observed that the metal-insulator transition, the site of peak in ρ(t)-t curves (larger site for the double peak), near T c shifts to lower temperature with increasing V-doping contents. Fig. 4 shows the temperature dependence of MR for all samples. Here, we define that MR=(ρ()-ρ(H))/ ρ(), where ρ() and ρ(h) mean the resistivity at zero-field and.4t applied magnetic field, respectively. From the figure, it is found that the maximum magnetoresistance near T c increases with increasing V content except for x=5%. For example, the 2 34 T C Intensity(a.u.) T MI θ x(%) Fig.1 The typical XRD patterns of the La 2/3 (Ca.6 Ba.4 ) 1/3 Mn.93 V.7 O 3 Fig.2 The V content dependence of T c and T MI for La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3

3 Solid State Phenomena Vols x=% (a) 4.4T x=3% (b) 4T.12 4T T x=5% (c).2. x=2% (d) Fig.3 The temperature dependences of the resistivity ρ (T) and ρ H (T) with H=. and.4t for La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 (x=,.3,.5,.2) maximum MR values are 8.5 and 25% under H=.4T for x=5%, 2%, respectively. Meanwhile, it can be found that the V-doping has the low-temperature tunneling magnetoresistance increase. The temperature of the MR peak T MI shifts to a lower temperature. The V-doping content dependence of T IM is shown in Fig.2. From the figure, it is found that the value of T MI decreases with increasing V-doping content. Compared the value of T c with that of T MI, it is found that the value of T c is larger clearly than that of T MI. However, for a-site electron-doped A 1-x A x MnO 3 (x=.3), the value 5 25 MR(%) % 3% 5% 7% 1% 15% 2% MR(%) Percolation Tunneling Total T(K) Fig.4 The temperature dependence of MR for La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 (x=,.3,.5,.7,,.15,.2) Fig.5 The temperature dependence of the simulated percolative, tunneling and total MR for La 2/3 (Ca.6 Ba.4 ) 1/3 MnO 3

4 94 Nanoscience and Technology of T MI coincides with that of T c, and the metal-insulator transition is strongly coupled with the magnetic ordering transition. Now, let us discuss the mechanism of MR. Based on our recent studies on the tunneling magnetoresistance model in the nanostructured materials [11] and percolation model near Curie temperature [12-13], we construct the percolative structure containing ferromagnetic metallic (FMM) clusters and paramagnetic insulating matrix and spin-tunneling model between the near clusters consisting of complex spin configurations [14]. Fig.6 shows the temperature dependence of the simulated percolative, tunneling and total MR for La 2/3 (Ca.6 Ba.4 ) 1/3 MnO 3. Comparing with the results in Fig.4, it is found that the simulated ones are consistent with experimental fact, which means that the spin-tunneling and percolative models are quite successful to explain MR effect of CMR materials. It is considered that the enhancement of the low-temperature tunneling magnetoresistance and maximum magnetoresistance near T c with the increase of V-doping should be attributed to the structure of the short range ordered ferromagnetic clusters and the tunneling layer between the near clusters. The detailed simulation will be studied further in the future. Conclusion La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 (x=,.3,.5,.7,,.15,.2) were obtained using sol-gel technology and conventional synthetic procedures. The substitution of Mn with V strongly changes the properties of the undoped compound. The V-doping not only has the Curie temperature T C and the metal insulator transition temperature T MI reduce, but also let the low-temperature tunneling magnetoresistance and maximum magnetoresistance near T c rise. Meanwhile, it is found that there exists the evident difference between T c and T MI for different V-doping content. We also simulated the magnetoresistance as a function of temperature using the tunneling magnetoresistance model and the percolation model. This work was supported by NSF of Fujian Province (E322) and NSF of China under Grant No References [1] Y. Tokura, Y. Tomioka: J.Magn.Magn.Mater.Vol.2(1999), p.23. [2] A. Gupta, J.Z. Sun: J.Magn.Magn.Mater.Vol.2(1999), p.24. [3] K. Y. Wang, et al.: J.Appl.Phys., Vol.9(21), P [4] I.O.Troyanchuk, et al.:j.magn.magn.mater., Vol.225(21), P.331. [5] J.Dho, W.S.Kim and N.H.Hur: Phys.Rev.Lett., Vol.89(22), P [6] Wei Tong et al.: Phys.Rev.B Vol.68(23), p [7] J.Yang et al., Phys.Rev.B Vol.7(24), p [8] R.Gross et al.: J.Magn.Magn.Mater.Vol.211(1999), p.15. [9] M.S.Kim et al, Phys.Rev.B Vol.71(25), p [1] Xiaming Liu: Xiaojun Xu and Yuheng Zhang, Phys.Rev.B Vol.62(2), p [11] Zhigao Huang et al.: Phys. Rev. B69, 9442 (24). [12] H.Kim, J. Dho, S.Lee: Phys.Rev.B,Vol.62(2), p [13] S.L.Ye et al.: J.Appl.Phys. Vol.9(21), p [14] Rongquan Gai, Zhigao HUANG, et al.: Transations of Nonferrous Metals Society of China (in press).

5 Nanoscience and Technology 1.428/ Electrical Conductivity and Low-Field Magnetoresistance in La 2/3 (Ca.6 Ba.4 ) 1/3 Mn 1-x V x O 3 Prepared by Sol-Gol 1.428/