Huo Guoyan,Yang Qing, Dong Fuying, Song Dayong
(School of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)

 

ABSTRACTThe structure, electrical transport and magnetic properties of the series of La_1-xSrFe_1-xMn (0.3≤≤0.7) compounds have been studied. The lattice parameter, , decreases firstly and following increase with Sr²⁺ and Mn⁴⁺ doped. The cell parameters, and , decrease slightly with coupled substitution of Sr²⁺ for La³⁺ and Mn⁴⁺ for Fe³⁺. All the magnetic transition temperatures of the samples are below 80.0 K. The electrical behaviors of all specimens demonstrate insulator and the electrical resistivity increases with Mn⁴⁺ and Sr²⁺ ions doped. The detailed analysis of electrical transport shows that the electrical process of some samples are controlled by variable range hopping model at low temperature and that the electrical transport of the others are described by two steps transport of small polaron model at low temperature and that electrical transport of all specimens is described by bipolaron model at high temperature.

1. INTRODUCTION
With the discovery of colossal magnetoresistance (CMR) effect in manganites, hole-doping perovskite manganites with unusual electronic transport and magnetic properties have attracted considerable attention. These properties result from an intrinsic interaction among charge, spin, orbit and lattice degrees of freedom that are strongly coupled to each other^[1-2]. Double exchange model combined with John-Teller effect was used to explain these properties partly^[3]
A great deal of work has been carried out on the manganites Ln_1-xMnO since the discovery of colossal magnetoresistance properties in these compounds^[4]. Also, many studies have been carried out on the Mn site doping effect (formula A’_1-xA”B’_1-yB”^[5-6]. The research work reported by Takeuchi et al.^[7] suggests that Fe substitution for Mn dilutes the DE mechanism and shows typical spin glass and insulating behaviors.The results for several authors to studythe Ln_1-xMn_1-yFe Fe-doped manganites with y≤ x indicate that Fe and Mn host lattice is antiferromagnetic interaction^[8-10]
To our knowledge, there is little data which are available on coupled A-B-site substitutions in ABO^[11-12]. On the basis of previously published experimental data^[11-12], we may expect that a coupled substitution involving Sr and Mn would improve the properties of LaFeO. So if Sr is doped at La-site and Mn doped at Fe-site, useful information for understanding could be obtained.
The purpose of the present study is to clarify the transport mechanism in the mixture of manganites La_1-xSrFe_1-xMn (0.3≤≤0.7) and to show how the properties change with the replacement of La by Sr and Fe by Mn.

2. EXPERIMENTAL
Polycrystalline samples of La_1-xSrFe_1-xMn (0.3≤≤0.7) were prepared by standard solid-state reaction from stoichiometric amounts of La, SrCO, Fe and MnO, the purities of which are, at least, 99.9%. A purity of 99.99% La powders was dried for 5 hours at 800C before used. The mixtures were ground by hand in an agate mortar for at least 40 min and pressed into pellets, then preheating the pellets for 8 hours at 800C. Following then the mixtures was ground extensively once again. Then the powders were pressed into pellets and annealed at 1100C in air for 24 h with intermittent grindingsand finally cooled the samples down to 500C at 2C/min, then cooled to room temperature in furnace.
Phase analysis and characterization were carried out by X-ray diffraction (XRD) using CuKa radiation with a graphite monochromator on Rigaku model D/max-2400 X-ray diffractometer at room temperature.
Temperature dependence of magnetization curve was measured by a vibrating sample magnetometer (VSM) in a field of 0.5 T over the temperature range 80-300 K. Electrical resistances were determined by the standard four-probe DC method in the range 80-300 K.

3. RESULTS AND DISCUSSION
3.1 Crystal structure
The products of synthesis by standard solid state technique yielded single-phase materials, which were confirmed by powder XRD (see Fig.1). The XRD patterns of all samples can be indexed in the orthorhombic system with space group Pnma. The lattice parameters of La_1-xSrFe_1-xMn compounds were determined by XRD. This type of structure is common for rare earth perovskite-type oxides. Each unit cell consists of four ABO units, and has the approximate dimensions, where is the lattice parameter of the ideal cubic unit cell. The determined lattice parameters of the orthorhombic cell are shown in Table 1.

Fig. 1 X-ray patterns of the series La_1-xSrFe_1-xMn (0.3≤≤0.7)

Table 1. Lattice parameters, unit cell volume, and_CW (K) of the system La_1-xSrFe_1-xMn (0.3≤≤0.7)

	a(nm)	b(nm)	c(nm)	v(nm	a, b, c/relations	θCW (K)
0.3 0.4 0.5 0.6 0.7	0.55578 0.55231 0.55075 0.55201 0.55308	0.77824 0.77736 0.77728 0.7764 0.77544	0.55006 0.54903 0.54817 0.54826 0.54791	0.23792 0.23572 0.23466 0.23497 0.23499	c<b/<a c<b/<a c<b/<a c<b/<a c<b/<a	-263K -175K -56K -183K -263K

It can be seen from Table 1 that the variations of the orthorhombic lattice parameter, , decreases with Sr content from Sr = 0.3 to 0.5, following then increases with coupled substitution of La³⁺ by Sr²⁺ and Fe³⁺ by Mn⁴⁺ and that the change of lattice parameters, and , reduce slightly with the replacement of La³⁺ by Sr²⁺ and Fe³⁺ by Mn⁴⁺. This is due to the tilting scheme of BO octahedral in Pnma perovskite, of the type b in Glazer’s nomenclature^[13], in which the distortion driven by the increase of average A size and by the reduction of average B size leaves and to decrease slightly. Whilst, the variance of lattice parameters confirm that the compounds form solid solutions but not two compounds simply mixed. The observed size in unit cell volume decreases with the coupled substitution of the La³⁺ by Sr²⁺with that of the Fe by Mn from Sr = 0.3 to 0.5, following then increases slightly with the coupled substitution from Sr = 0.5 to 0.7 (see Table 1). The reason for decrease of this parameter from Sr = 0.30 to 0.50 is that on A site substitution of small radius La³⁺ by big radius Sr²⁺ and on B site replacement of big radius Fe³⁺ by small radius Mn⁴⁺, the reduction out of phase tiltingcaused by distortions of the O-B-O angles of the octahedral for fitting the size of A average size plays an important role^[14-15]

Fig. 2 The temperature dependence of magnetization (M) of La_1-xSrFe_1-xMn samples measured under H = 0.5T

3.2 Magnetic properties
Fig. 2 presents the temperature-dependent magnetization (M-T) of the La_1-xSrFe_1-xMn system under a magnetic field of 0.5T. It can be seen from Fig. 2 that the magnetic transition temperatures of the samples are below 80.0 K. The susceptibility presents the Curie-Weiss-type behavior [= C/(T-)] for the temperatures above 80.0 K. Here, C andare Curie constant and Curie-Weiss temperature, respectively. The values ofof all samples are negative (see Fig. 3 and Table 1), indicating that the local magnetization below magnetic transition temperatures are associated with the antiferromagnetic coupling. Curie-Weiss temperature is related to the strength of antiferromagnetic interaction. With the increase Sr and Mn contents from 0.30 to 0.50 the Curie-Weiss temperatures increase to high temperatures. This indicates that antiferromagnetic interactions become weak. With increasing Sr and Mn content from 0.50 to 0.70 the Curie-Weiss temperatures decrease to low temperatures again. This suggests that antiferromagnetic interactionis strengthened. From the variation of Curie-Weiss temperature with Sr content (x) (see Table 1) it can be inferred that the double exchange interactions between neighboring Fe³⁺ and Mn⁴⁺ ions is relatively strong but short range interactions which are responsible for the formation of the ferromagnetic correlation as overlapping of _g­ for Fe³⁺ and _g­ for Mn^4+[16] and that the exchange interactions between Fe³⁺/Mn⁴⁺ and Fe³⁺/Mn⁴⁺ are strong and long range antiferromagnetic.

Fig. 3 The reciprocal magnetization (1/M) for La_1-xSrFe_1-xMn samples, the solid lines are fitted to the Curie-Weiss law

Fig. 4 The temperature dependence of electrical resistivity for polycrystalline samples of the solid solution La_1-xSr Fe_1-x Mn (0.3≤≤0.7) as a function of temperature

3.3 Electrical resistivity
The temperature dependence of the electrical resistivity of all samples is shown in Fig. 4. Insulating behaviors are observed for all samples in the studied temperature range. As some samples resistances are too high to be measured on our apparatus at low temperatures we can not obtain the values of resistances below certain temperature. It can be seen from Fig. 4 that the resistivity of La_0.6Sr_0.4Fe_0.6Mn_0.4 compound is the lowest among them and of all other samples increase with Sr²⁺ and Mn⁴⁺ doping from x = 0.3 to 0.7 (except x = 0.4). One hand, as well known, the amount of Mn³⁺/Mn⁴⁺ = 7/3 ~ 6/4 the double exchange effect is the strongest in La_1-yCaMnO compounds and La_1-xSrFe_1-xMn compounds are similar to La_1-yCaMnO compounds, therefore, the resistivity of La_0.7Sr_0.3Fe_0.7Mn_0.3 and La_0.6Sr_0.4Fe_0.6Mn_0.4 compounds is low, and the resistivity of the others obviously increases with deviation from 7/3 ~ 6/4 due to double exchange weakening and to Fe³⁺-O-Mn⁴⁺ angle decreased as lattice deformation increasing. On the other hand, with Sr²⁺ and Mn⁴⁺ further substitution for La³⁺ and Fe³⁺ antiferromagnetic interaction is promoted by increasing the proportion of Mn⁴⁺-O-Mn⁴⁺. Therefore, the resistivity evidently is enhanced.


Fig. 5 (a) log r vs. T^-1/4, inset log(r /T^1/2 ) vs. 1/T; (b) log r / T vs. T^-1/4, inset log(r /T^1/2 ) vs. 1/T plots for La1-xSr xFe1-x MnxO3 (0.3≤≤0.7) samples

In order to get more insight into the transport processes, the electrical resistivity data were fitted to various model applied for materials. Fig. 5 shows the electrical transport behaviors for x = 0.3, 0.4, 0.5, 0.6 and 0.7 samples. It can be seen from Fig. 5 that variable range hopping between localized states is demonstrated for x = 0.3, 0.5 and 0.7 at low temperature (see Fig. 5a) and that two steps transport of small polarons model is confirmed for x = 0.4, 0.6 at low temperature (see Fig. 5b). At high temperature the electrical transport behaviors of all specimens show bipolaron model (see inset of Fig. 5).

4. CONCLUSIONS
We have investigated the structure, electrical transport and magnetic properties of the series La_1-xSrFe_1-xMn (0.3≤≤0.7) compounds. The lattice parameter, , decreases firstly with 0.3≤≤0.5 and following then increases with 0.5≤≤0.7. The lattice parameters and reduce slightly with coupled substitution of Sr²⁺ and Mn⁴⁺ for La³⁺ and Fe³⁺. The detailed analysis of magnetic properties demonstrates that local magnetic interaction between Fe³⁺/Mn⁴⁺ and Fe³⁺/Mn⁴⁺ at below magnetic transition temperature is antiferromagnetic. The electrical behaviors of all specimens demonstrate insulator and the electrical resistivity increases with Mn⁴⁺ and Sr²⁺ ions doped. The detailed analysis of electrical transport shows that the electrical process of all samples are controlled by variable range hopping between localized states or two steps transport of small polaron model at low temperature and that the electrical transport are described by bipolaron model at high temperature.