Structure and transport properties of (La_1-x-yY_y)_2/3Ca_1/3MnO₃ with La-site vacancies

LUO Guang-sheng(罗广圣)^{1, 2}, LIU Guang-hua(刘光华)², LI Xiao-yi(李小怡)²,

XIONG Wei-hao(熊惟皓)¹, WU Xiao-shan(吴小山)³

1. School of Materials Science and Engineering, Huazhong University of Science and Technology,Wuhan 430074, China;

2. School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China;

3. Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

Received 17 January 2007; accepted 21 May 2007

Abstract: Several samples of manganese oxides La_(1-x)2/3Ca_1/3MnO₃ (V-LCMO) and (La_0.7-xY_0.3)_2/3Ca_1/3MnO₃ (x＜0.15) (V-LYCMO) with vacancies at La-site (La-vacancy) were prepared by solid-state reaction method. The X-ray diffraction(XRD) patterns refined by Rietveld confirm that these compounds exhibit single phase structure with orthorhombic symmetry (Pnma). The lattice parameters, Mn—O bond length and Mn—O—Mn bond angle vary with La-vacancy concentration, as an indication of the occurrence of the local Jahn-Teller effect. The measurement result of V-LCMO compounds shows that the maximum magnetoresistance(MR) is about 220% at T_IM=268 K and La-vacancy content x=0.04. For V-LYCMO compounds, there exists metal-insulator transition at about 50 K, and a very large MR (over 10⁶%) is observed at the temperature ranging from 40 K to 50 K.

Key words: La_(1-x)2/3Ca_1/3MnO₃; (La_0.7-xY_0.3)_2/3Ca_1/3MnO₃; La-vacancy; orthorhombic symmetry; Jahn-Teller effect

1 Introduction

After the giant magnetoresistance effect(GMR) was firstly observed by HELMOLT et al[1] in 1993, many research groups have tried to understand its mechanism and find new materials with better properties. Recently much attention has been paid to the perovskite manganese oxides La_1-xCa_xMnO₃ and Ln_1-xA_xMnO₃ (Ln=La, Pr, Nd etc and A=Ca, Ba, Pb, Sr etc) with doped-vacancies[2-3]. For La_1-xCa_xMnO₃(LCMO) compounds, ion-doped concentration will bring about different effect. Firstly, it is deemed, generally, that the Curie temperature T_C is higher at doped content x=1/3 than that at other doped contents. In addition, below T_C there exists a better ferromagnetic state and GMR effect. For Ln_1-xA_xMnO₃ compounds, T_C is very sensitive to chemical pressure of rare earth cation[4-6]. It is discussed that e_g electron hopping between Mn³⁺ and Mn⁴⁺ ions relates to the lattice parameters and gives rise to Jahn-Teller effect. Although doped trivalent cation doesn’t change the proportion between Mn³⁺ and Mn⁴⁺ ions, it changes the lattice parameters and leads to the occurrence of Jahn-Teller effect. Lately there were also some studies on the influence of vacancies at A-site on structural and electromagnetic properties in perovskite manganese oxide[7-10]. Now it is necessary to analyze the influence of La-vacancies and Y-doped concentration on structures and transport properties of La_2/3Ca_1/3MnO₃ compounds.

2 Experimental

2.1 Samples preparation

The samples La_(1-x)2/3Ca_1/3MnO₃ and (La_0.7-xY_0.3)_2/3- Ca_1/3MnO₃ (x=0, 0.02, 0.04, 0.08, 0.10) compounds were prepared by solid-state reaction method. The stoichiometric mixtures of La₂O₃, Y₂O₃, CaCO₃ and MnO₂ powders with over 99.9% purity were ground for about 40 min, and pre-sintered at 1 000 ℃ for 12 h. The powders were ground again, and sintered at 1 350 ℃ for 24 h after pressed into pellets with a diameter of 10 mm under 20 MPa pressure. Finally, the pellets were cooled to room temperature slowly in the furnace.

2.2 Structure measurement and refinement

The crystal structures of the samples were studied by XRD (Brucker D8, Cu K_α) at room temperature (300 K) and lower temperatures. The XRD spectrometer was operated at 40 kV and 20 mA, and the scanning step 2θ ranges from 20? to 90?. The XRD patterns were refined by Rietveld refinement method, and the method of Rietveld refinement is similar to that of testifying super- conduct structure of rare earth materials before[11-12].

2.3 Magnetoresistance measurement

The transport properties of samples were measured by Normal Four Contacts method. The dependence of electric resistivity on the temperature was measured at zero and 8 T magnetic field, respectively. The MR is defined as:

MR=(ρ₀-ρ_H)/ρ_H×100%                        (1)

where  ρ₀, ρ_H are resistance values at zero and external magnetic field, respectively[12].

3 Result and discussion

3.1 Structure parameters

The XRD patterns show that polycrystalline compounds V-LCMO and V-LYCMO (x=0, 0.02, 0.04, 0.08, 0.10) all exhibit perovskite type structure with single phase. In Fig.1, there exhibit experimental spectrum, refinement spectrum and different spectrum with their Bragg diffraction peaks. The structure parameters of the samples V-LCMO and V-LYCMO are refined by Rietveld refinement, and the refinement results of bond length and the bond angle are shown in Fig.2. It is displayed that the structures of the samples with different La-vacancy contents keep orthorhombic (a≠b≠c, α=β=γ), space group Pnma. The lattice parameters, the Mn—O bond length and Mn—O—Mn bond angle decrease much drastically in V-LYCMO compounds than that of V-LCMO compounds. This is responsible for different mean ion radius of the La-sites with partial substitution of Y³⁺ for La³⁺. The Y³⁺ radius (about 0.107 nm) is smaller than that of La³⁺ (about 0.122 nm) and the mean radius of La³⁺ and Ca³⁺ ions decrease with doping Y³⁺, hence the atomic radius between A—O layer and Mn—O layer is mismatched and changes the value of tolerance factor t:

Fig.1 Typical XRD pattern, together with refined curve, different curve, and calculated Bragg positions of V-LCMO and V-LYCMO compounds

t=(r_A+r_O)/(r_n+r_O)                             (2)

When t＜1, the force of pressing Mn—O bond and stretching A—O bond results in Mn—O—Mn bond torsion from 180? to 180?-φ. The Mn—O—Mn bond torsion gives rise to Jahn-Teller distortion in MnO₆ octahedron[13], and the violent coupling interaction between electron and phonon makes e_g electrons of Mn³⁺ localization due to Jahn-Teller distortion. For V-LYCMO system, it is discussed in Fig.2 that the doped vacancies change the structure parameters and affect transport properties.

Fig.2 Dependence of lattice parameters (a), Mn—O—Mn bond angle (b) and Mn—O bond length (c) on vacancy ratio x

3.2 Dependence of MR of V-LCMO compounds on temperature and magnetic field

For V-LCMO system, the dependence of electric resistivity on temperature at zero and 8 T magnetic field is shown in Fig.3. As shown in Fig.3(a), for the V-LCMO compounds, insulator-metal transition occurs at about 267 K, and a little of La-vacancy keeps insulator-metal transition temperature T_IM unchanged. Additionally, at magnetic field 8 T T_IM moves towards higher temperature (＞300 K) and the MR is quite sensitive to external magnetic field near T_IM, because external magnetic field induces magnetic moments ordering to increase T_IM [13-15]. In Fig.3(b), for the samples of V-LCMO compounds the largest MR about 220% is observed at La-vacancy content x=0.04 and insulator-metal transition temperature T_IM= 268.6 K, which is almost equal to MR about 218% at 267 K without La-vacancies. Additionally, the MR value decreases dramatically and the electric resistance increases when vacancy content x＞0.04, because La-vacancies may enhance magnetic scattering and lead to the electric resistance increasing.

Fig.3 Dependence of electric resistivity (a) and MR (b) on temperature for V-LCMO compounds

3.3 Dependence of MR of V-YLCMO compounds on temperature and magnetic field

For the samples V-LYCMO, in Fig.4 there exhibits the dependence of electric resistivity on temperature at zero and magnetic field 8 T, respectively. As shown in Fig.4(a), the electric resistivities of the V-LYCMO compounds are up to several decuples larger than those of V-LCMO compounds. The MR value is over 10⁶ in Fig.4(b), but the insulator-metal transition temperature decreases to about 50 K dramatically.

Fig.4 Dependence of electric resistivity (a) and MR (b) on temperature for V-LYCMO compounds

3.4 Physical mechanism of MR effect

It is well known that electric conductance of perovskite-type compounds can be understood by the double-exchange mechanism under low temperature, but the double-exchange mechanism can not interpret MR behavior effectively. Millis[16] and many research groups agree that the Jahn-Teller effect with electroacoustic coupling interaction is responsible for the MR behavior.

Firstly, the Jahn-Teller effect makes e_g electrons of Mn³⁺ ion localize and results in electric resistance increasing dramatically at T_IM. In addition, intense electroacoustic coupling interaction changes metal into insulator above T_IM.

The results show that the MR effect is related to not only external magnetic field but also the doped contents. One reason may be that magnetic moments are arrayed more orderly in the high magnetic field than in the low one. The other may be that the substitution of Ca²⁺ ions for La³⁺ ions, and the proportion of Mn³⁺/Mn⁴⁺ arosed by La-vacancies promote the competition between double-exchange and super-exchange interaction of Mn³⁺ and Mn⁴⁺.

Additionally, the MR of V-LYCMO compounds is up to several decuples larger than that of V-LCMO compounds. On one hand, this may be that t＜1 leads to Jahn-Teller distortion in the MnO₆ octahedron. On the other hand, ferromagnetic interaction is suppressed and antiferromagnetic interaction is enhanced with the increase of partial substitution Y³⁺ for La³⁺, therefore, T_c moves towards lower temperature. The competition between ferromagnetism and antiferromagnetism results in spin-glass states developing, and it destroys double- exchange interaction to increase the MR values.

4 Conclusions

1) For the manganese oxides LCMO, partial substitution of Y³⁺ for La³⁺ leads to the lattice parameters, and the Mn—O bond length as well as Mn—O—Mn bond angle decreasing apparently due to the Jahn-Teller effect of MnO₆ octahedrons.

2) For the V-LCMO compounds, the electric resistance increases dramatically and the MR decreases when La-vacancy content x＞0.04, because the vacancy concentration changes the Mn³⁺/Mn⁴⁺ proportion and accelerates magnetic scattering.

3) For the V-LYCMO compounds, partial substitution of Y³⁺ for La³⁺ results in electric resistivity and the MR increasing and T_c decreasing due to the Jahn-Teller effect.

References

[1] HELMOLT R H, WECKER J, HOLZAPFEL B, SCHULTZ L, SAMWER K. Giant negative magnetoelectric resistance in perovskite like La_2/3Ca_1/3MnO_x ferromagnetic film [J]. Phys Rev Lett, 1993, 71: 2331-2333.

[2] MATHUR N, LITTLEWOOD P. Mesoscopic texture in manganites [J]. Phys Today, 2003, 56(1): 25-30.

[3] Guo G Q, Chadwick C, Gang X. Colossal magnetoelectric resistance of 1000000-fold magnitude achieved in the antiferromagnetic phase of La_1-xCa_xMnO₃ [J]. Appl Phys Lett, 1995, 67: 1783-1785.

[4] HWANG H Y, CHEONG S W, ONG N P, BATLOGG B. Spin-polarized intergrain tunneling in La_2/3Sr_1/3MnO₃ [J]. Phys Rev Lett, 1996, 77: 2041-2044.

[5] SIRENA M, HABERKORN N, STEREN L B, GUIMPEL. Magnetic coupling and magnetoresistance in La_0.55Sr_0.45MnO₃/La_0.67Ca_0.33- MnO₃ multilayers [J]. J Appl Phys, 2003, 93(10): 6177-6181.

[6] LOUDON J C, COX S, WILLIAMS A J, ATTFIELD J P, LITTLEWOOD P B, MIDGLEY P A, MATHUR N D. Weak charge-lattice coupling requires.reinterpretation of stripes of charge order in La_1-xCa_xMn0₃ [J]. Phys Rev Lett, 2005, 94: 197-202.

[7] XU Q Y, GU K M, LIANG X L, NI G, WANG Z M, SANG H, DU Y W. Magnetic entropy change in La_0.54Ca_0.32MnO_3-δ[J]. J Appl Phys, 2001, 90(1): 524-526.

[8] CHEN W, ZHONG W, HOU D L, GAO R W, FENG W C, ZHU M G, DU Y W. Preparation and magnetocalric effect of self-doped La_0.8-xNa_0.24□_xMnO_3+δ（□=vacanvies） polycrystal [J]. J Phys: Condens Matter, 2002, 14: 11889-11896.

[9] PHAN M H, YU S C, HUR N H. Magnetic and magnetocaloric properties of La_{(1-x)(0. 8)}Ca_{0. 2}MnO₃ (x=0.05, 0.20) single crystals [J]. J Magn Magn Mater, 2003, 262(3): 407-411.

[10] SINH N H, THUY N P. Some properties of La-deficient La_0.54Ca_0.32MnO_3-delta[J]. J Magn Magn Mater, 2003, 262(3): 502-507.

[11] NIE Shu, WU Xiao-shan, JIANG Shu-sheng. Microstructural effects on zinc doped LSCO cuprates [J]. Journal of the Chinese Rare Earth Society, 2001, 19(3): 213-216.

[12] SHA H, WU X S, XU Y M. X-ray diffraction studies on yttrium- doped La_0.67Ca_0.33MnO₃ [J]. J Supercond, 2004, 17: 247-251.

[13] KOZLENKO D P, VORONIN V I, GLAZKOV V P, MEDVEDEVA I V, SAVENKO B N. Magnetic phase transitions in the iron-doped Pr_0.7Ca_0.3Mn_1-yFe_yO₃ manganites at high pressures [J]. Physics of the Solid State, 2004, 46(3): 484-490.

[14] OKUNEV V D, SAMOILENKO Z A, D'YACHENKO T A. Changes in the electronic, optical, and magnetic properties of LaSrMnO films upon transition from the rhombohedral phase to the orthorhombic phase [J]. Physics of the Solid State, 2004, 46(10): 1895-1905.

[15] OKUNEV V D, SAMOILENKO Z A, PAFOMOV N N, D’YACHENKO T A, PLEHOV A L, SZYMCZAK R, BARAN M, SZYMCZAK H, LEWANDOWSKI S J. Onset of variable-range hopping at ferromagnetic phase nucleation in amorphous paramagnetic LaSrMnO films [J]. Phys Lett A, 2005, 346: 232-242.

[16] MILLIS A J, LETTLEWOOD P B, SHRAINMAN B I. Double exchange alone does not explain the resistivity of La_1-xSr_xMnO₃ [J]. Phys Rev Lett, 1995, 74: 5144-5147.

Foundation item: Project(0150014) supported by the Natural Science Foundation of Jiangxi Province, China

Corresponding author: LUO Guang-sheng; Tel: +86-791-3969559; E-mail: gsluo566@yahoo.com.cn

(Edited by YUAN Sai-qian)