Hydrothermal synthesis, structure and magnetic properties of Ru doped La_0.5Sr_0.5MnO₃

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Abstract

Synthesis, structure and magnetic properties of Ru doped perovskite structured manganite La_0.5Sr_0.5MnO₃ were investigated experimentally. A hydrothermal method was used for the preparation of the samples. A high-temperature annealing process was also employed to make a comparison. A slightly enhancement of the unit cell volume was observed with the increase of Ru concentration. Scanning electron microscopy shows that the materials are made up of cube-shaped particles with dimension of several micrometers. Importantly, it is found that both the Curie temperature T_C and saturation moment can be reduced by Ru doping. The value of coercive field is not affected by the introduction of Ru.

Graphical abstract

Keywords

La_0.5Sr_0.5MnO₃ / hydrothermal synthesis / Ru doping / Curie temperature

1 Introduction

Transition metal compounds with the perovskite structure were investigated extensively for several decades, since the discovery of high-temperature superconductivity in cuprates [1]. The manganite (La,Sr)MnO₃ and other analogues, also with the perovskite structure, were studied for many years due to the colossal magnetoresistance (CMR) which has potential applications in magnetic devices [2-7]. Besides the CMR feature, this system also shows a rather complicated magnetic behavior and crystal structure with the variation of the chemical compositions [8-10]. The physical properties can also be tuned by chemical doping on the site of Mn [11-21]. However, the influence of the dopants on the physical properties is rather controversial. For example, the Curie temperature T_C was found to decrease with Ru doping by some researchers [15-17], while an enhancement of T_C was also reported by other groups [18, 19]. So these issues need to be clarified by more reliable experiments.

The solid-state reaction method at the temperature above 1200°C was used widely to synthesize the polycrystalline samples of (La,Sr)MnO₃ [17-19]. Later on, a mild hydrothermal method under low temperature was also developed [22-24]. Besides the simplicity of the synthesis procedures and the low energy consumption, hydrothermal method also shows advantages in improving the homogeneity of the samples, especially controlling the oxygen content [23]. It was found that the chemical composition of the hydrothermal samples is close to the ideal value and the deviation of oxygen is only about 1% for La_0.5Sr_0.5MnO₃ [23]. This is important for the fundamental research and may facilitate the obtaining of intrinsic properties. To our knowledge, there is no reports on the Ru doped La_0.5Sr_0.5MnO₃ by the hydrothermal method.

In this paper, we report the hydrothermal synthesis, structure characterization, and physical investigations of Ru doped La_0.5Sr_0.5MnO₃. With the increase of Ru doping, the c-axis lattice constant increases slightly while the a-axis lattice remains unchanged. Both the Curie temperature T_C and saturation moment are suppressed by Ru doping. A comparative experiment was also conducted by annealing one hydrothermal sample at high temperature.

2 Experiments

La_0.5Sr_0.5Mn_1-xRu_xO₃ (x=0, 0.1, 0.2, 0.3) samples was synthesized by hydrothermal method. In a typical synthesis, 1.75 mmol MnCl₂·4H₂O, 1.25 mmol SrCl₂·6H₂O, 1.25 mmol La(NO₃)₃·6H₂O, 0.75 mmol KMnO₄ were successively added into the 15 mL deionized water by stirring for 30 minutes and then KOH as a mineralizer was slowly added with continued stirring for 60 minutes. Finally, the solution was poured into 25 mL Teflon-lined autoclave and heated to 240°C for 40 hours. The reacted precipitate was cleaned with deionized water and ethanol several times and then dried at 80°C for 24 h. The dried black powder is polycrystalline La_0.5Sr_0.5MnO₃, along with a small amount of La(OH)₃. The Ru doped La_0.5Sr_0.5Mn_1-xRu_xO₃ (x=0.1, 0.2, 0.3) were obtained by partially replacing the reagent KMnO₄ with KRuO₄ appropriately. In order to have a comparison, the hydrothermally synthesized samples were annealed at high temperature: La_0.5Sr_0.5Mn_1-xRu_xO₃ powders with small amounts of La(OH)₃ were cold pressed into pellets and thermally treated at 1000°C for 24 hours in air.

The actual Ru concentrations were checked and determined by the energy dispersive X-ray spectroscopy (EDS) measurements on an Oxford Instruments device. The crystal structure was checked by powder X-ray diffraction (PXRD) measurements at room temperature using a DX-2700 diffractometer with Cu K_{\alpha } radiation. The morphology of the samples was characterized by JSM-6510 scanning electron microscope (SEM). The magnetic properties were measured on a Quantum Design magnetic property measurement system (MPMS).

3 Results and discussion

3.1 Composition, morphology and structure

The actual doping levels of the samples were analyzed and determined by the energy dispersive X-ray spectroscopy (EDS) measurements. The results can be seen in Table 1. The Ru concentration determined from EDS measurements, x_{\text{E}\text{D}\text{S}}, will be used in the following discussions of the present paper. The morphology of the hydrothermal samples with x_{\text{E}\text{D}\text{S}}=0 and 0.066 were evaluated by SEM. As shown in Fig. 1, the material is made up of cube shaped particles with typical dimension 5–10 μm, which is quite similar with that previous reported by Spooren et al. [23].

As shown in Fig. 2(a), both the X-ray and neutron diffraction experiments have revealed a distorted perovskite structure in La_0.5Sr_0.5MnO₃ [8, 23]. Here we examined the crystal structure of our samples by the PXRD measurements. The PXRD patterns of hydrothermal La_0.5Sr_0.5Mn_1-xRu_xO₃ (x_{\text{E}\text{D}\text{S}}=0, 0.066, 0.082, 0.136), along with the annealed sample with x_{\text{E}\text{D}\text{S}}=0, were showed in Fig. 2(b). Obviously all the hydrothermal samples are accompanied by a small amount of La(OH)₃ as indexed by the asterisks, which rises gradually with the increase of Ru concentration. In the annealed sample, La(OH)₃ is absent and small amounts of La₂O₃ can be observed.

La(OH)₃ is the major impurity in the process of hydrothermal synthesis. It has been reported that the hydrothermal temperature and reaction time have significant influences on the products [24]. During the process of heating, the La(OH)₃ nucleus begin to appear and form nanocrystals around 180°C [25]. With the prolongation of reaction time and the increasing of temperature, the La(OH)₃ nanorods release La³⁺ ions, which provide enough elements for the formation of La_0.5Sr_0.5MnO₃ crystals [24]. In this meaning, higher temperature or longer reaction time is beneficial to decrease the number of La(OH)₃ nanorods. We have tried longer reaction time (40 and 60 hours) and no differences can be found compared with 24 h. As for the reaction temperature, Spooren et al. [23] showed that 240°C is enough to prepare La_0.5Sr_0.5MnO₃. Thus, the reaction condition was fixed to 240°C and 24 h in our experiment. In addition, we found the formation of La_0.5Sr_0.5MnO₃ crystals can be affected by the amount of KOH. The samples were not produced when the concentration of KOH was lower than 6 mol/L.

The (004) and (220) peaks overlap in the samples with x_{\text{E}\text{D}\text{S}}=0 and separate gradually when the Ru doping content is increased stage by stage. Similar behaviors can been observed for the (224) and (400) peaks. These features can be understood in terms of the different evolution tendencies of a- and c-axis lattice constants with Ru doping. It is also found that the (114) peak is weakened and disappear finally with increasing Ru content. This change has been reported by other group and ascribed to influence of Ru ionic substitution [18]. In addition, the raw data of PXRD was analyzed by PowderX software [26]. It indicates that all the polycrystalline samples have a tetragonal structure with I4/mcm space group. The obtained lattice constants are showed in Table 1. For the hydrothermal sample without Ru doping, the lattice constants are consistent with other reports on hydrothermal La_0.5Sr_0.5MnO₃ [23], which indicates that the actual Sr content of our samples is also around 0.5. With increasing Ru contents, the c-axis lattice constants increase, while the a-axis lattice remains almost unchanged. The unit cell volume becomes larger for the Ru doped samples. The content of impurity for the sample with x_{\text{E}\text{D}\text{S}}=0.136 is rather high, which prevents obtaining the accurate lattice constants for this sample.

3.2 Magnetic properties

Figure 3 shows the temperature dependence of magnetization (M–T) for the hydrothermal La_0.5Sr_0.5Mn_1-xRu_xO₃ (x_{\text{E}\text{D}\text{S}}=0, 0.066, 0.082, 0.136) and the annealed sample with x_{\text{E}\text{D}\text{S}}=0. Both the zero-field-cooled (ZFC) and field-cooling (FC) magnetization with applied field 100 Oe were measured from 2 K up to 400 K. With the temperature decreasing, all the M–T curves undergo a transition from paramagnetic(PM) to ferromagnetic(FM) states. Note that the presence of impurity should have influences on the magnitude of the magnetization data per unit mass, nevertheless the FM transition is not affected because La(OH)₃ does not show any magnetic transition in the temperature range we measured. The Curie temperature T_C was defined as the intersection point of two trendlines below and above the transition in M–T curve, as shown by the blue arrow in Fig. 3. The obtained values of T_C as a function of Ru concentration were shown in Fig. 5. We will discuss the detailed behaviors later.

The field dependence of magnetization (M–H) measurements were performed for all the four samples at 5 K with the external field from 0 to 6 T and the results were plotted in Fig. 4. The value of magnetization (M) increases rapidly under low field and then inclines to saturation with the increase of field, exhibiting a spontaneous magnetizing process. This is consistent with the results of M–T measurements and indicates a ferromagnetic ground state in low temperature. From the hysteresis loop, the saturation moments can be calculated. We find that for the hydrothermal samples, the magnetic moments were reduced from 1.15 {\mu }_B for the undoped compound to 0.56 {\mu }_B for the compound with x=0.082. The annealed sample has a larger moment of 1.44 {\mu }_B. We note that the accuracy of the estimated magnetic moments was weakened by the presence of La(OH)₃ impurities. As can be seen in an enlarged view of the data in the inset of Fig. 4, the coercive field is about 400 Oe and remains almost unchanged with Ru doping for the hydrothermal samples. The annealed sample shows a smaller coercive field of about 300 Oe.

As shown in Fig. 5, with the increase of Ru concentration, the Curie temperature T_C is suppressed, indicating the destructive effect on the FM order induced by Ru doping. Such a tendency is consistent with the substitution of most other elements for Mn ions [11-14]. For the Ru substitution, just as we have mentioned in the introduction, the existing experimental results are still rather controversial. The results of Sahu et al. [15], Wang et al. [16], and Zaidi et al. [17] revealed the same tendency as our observations. Meanwhile, Ying et al. [18] and Saber et al. [19] reported the opposite behaviors upon Ru doping. Moreover, no significant change in T_C as a function of Ru doping was found in the experiments of Dhiman et al. [20]. These reports along with our result are summarized in Table 2 for clarity. One possible reason for such scattering experimental results is the complex magnetic phase diagram of this system. The type and strength of the magnetic interactions may be different with the variation of rare-earth elements (La, Sm, …), alkaline-earth elements (Ba, Sr, Ca, …), and even the alkaline-earth element concentration. Another noteworthy factor is the difficulty in the precise controlling of the Sr concentration and oxygen content in preparing the samples under the condition of high temperature. The low-temperature hydrothermal synthesis supplied a scheme to achieve a more reliable result of this issue. It was found that the compound with La:Sr = 1:1 is rather stable and La_0.5Sr_0.5MnO₃ was always produced even the amount of the reagent was modified [23]. In addition, the Curie temperature does not change among samples with different amount of impurity La(OH)₃ prepared in different hydrothermal conditions [24]. So the effect of Ru doping in our present work was achieved on a more solid foundation. As can be seen from our comparative experiment, the behavior of the annealed sample deviated from that of the hydrothermal samples in terms of lattice constants, Curie temperature, saturation moment, and coercive field, possibly originating from the variation of Sr and oxygen contents during the high-temperature annealing process. The annealed samples with Ru doping showed some indications of phase separation and the data were not shown in this paper.

4 Conclusion

In conclusion, we have reported the experimental investigations on the hydrothermal synthesis, structural, and magnetic properties of the Ru doped manganite La_0.5Sr_0.5MnO₃ with a distorted perovskite structure. The crystal lattice expanded slightly with the increase of Ru doping. The destructive effect on the FM order by Ru doping was observed: both the Curie temperature T_C and saturation moment are suppressed with the introduction of Ru elements on the site of Mn. The coercive field remains almost unchanged by Ru doping. The behavior of the annealed sample deviated from that of the hydrothermal samples, indicating the subtle different in the detailed compositions due to the high-temperature treatment.

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References

[1] J. G. Bednorz and K. A. Mäuller, Possible high T_C superconductivity in the Ba-La-Cu-O system, Z. Phys. B 64, 189 (1986)

[2] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer, Giant negative magnetoresistance in perovskitelike La_2/3Ba_1/3MnO_x ferromagnetic films, Phys. Rev. Lett. 71(14), 2331 (1993)

[3] K. Chahara, T. Ohno, M. Kasai, and Y. Kozono, Magnetoresistance in magnetic manganese oxide with intrinsic antiferromagnetic spin structure, Appl. Phys. Lett. 63(14), 1990 (1993)

[4] S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh, and L. H. Chen, Thousandfold change in resistivity in magnetoresistive La-Ca-Mn-O Films, Science 264(5157), 413 (1994)

[5] M. McCormack, S. Jin, T. H. Tiefel, R. M. Fleming, J. M. Phillips, and R. Ramesh, Very large magnetoresistance in perovskite-like La-Ca-Mn-O thin films, Appl. Phys. Lett. 64(22), 3045 (1994)

[6] P. Schiffer, A. P. Ramirez, W. Bao, and S. W. Cheong, Low temperature magnetoresistance and the magnetic phase diagram of La_1-xCa_xMnO₃, Phys. Rev. Lett. 75(18), 3336 (1995)

[7] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, and Y. Tokura, Insulator-metal transition and giant magnetoresistance in La_1-xSr_xMnO₃, Phys. Rev. B 51(20), 14103 (1995)

[8] A. Sundaresan, P. L. Paulose, R. Mallik, and E. V. Sampathkumaran, Bandwidth-controlled magnetic and electronic transitions in La_0.5Ca_0.5-xSr_xMnO₃ (0 <~ x <~ 0.5) distorted perovskite, Phys. Rev. B 57(5), 2690 (1998)

[9] O. Chmaissem, B. Dabrowski, S. Kolesnik, J. Mais, J. D. Jorgensen, and S. Short, Structural and magnetic phase diagrams of La_1-xSr_xMnO₃ and Pr_1-ySr_yMnO₃, Phys. Rev. B 67(9), 094431 (2003)

[10] C. Autret-Lambert, M. Gervais, S. Roger, F. Gervais, M. Lethiecq, N. Raimboux, and P. Simon, Inhomogeneous magnetism studied by ESR in La_1-xSr_xMnO₃ (0.45 ≤ x ≤ 0.62), Solid State Sci. 71, 139 (2017)

[11] A. Maignan, C. Martin, and B. Raveau, Substitution of manganese by trivalent and tetravalent elements in the CMR perovskites Pr_1-x(Ca, Sr)_xMnO₃, Z. Phys. B 102, 19 (1996)

[12] K. Ghosh, S. B. Ogale, R. Ramesh, R. L. Greene, T. Venkatesan, K. M. Gapchup, R. Bathe, and S. I. Patil, Transition-element doping effects in La_0.7Ca_0.3MnO₃, Phys. Rev. B 59(1), 533 (1999)

[13] M. Rubinstein, D. J. Gillespie, J. E. Snyder, and T. M. Tritt, Effects of Gd, Co, and Ni doping in La_2/3Ca_1/3MnO₃: Resistivity, thermopower, and paramagnetic resonance, Phys. Rev. B 56(9), 5412 (1997)

[14] Y. Sun, X. Xu, L. Zheng, and Y. Zhang, Effects of Ga doping in the colossal magnetoresistance material La_0.67Ca_0.33MnO₃, Phys. Rev. B 60(17), 12317 (1999)

[15] R. K. Sahu and S. S. Manoharan, A Zener pair effect in lanthanum rutheno manganite, J. Appl. Phys. 91(10), 7517 (2002)

[16] L. M. Wang, J. H. Lai, J. I. Wu, Y. K. Kuo, and C. L. Chang, Effects of Ru substitution for Mn on La_0.7Sr_0.3MnO₃ perovskites, J. Appl. Phys. 102(2), 023915 (2007)

[17] N. Zaidi, S. Mnefgui, J. Dhahri, and E. K. Hlil, Effect of Ru substitution on the physical properties of La_0.6Pr_0.1Sr_0.3Mn_1-xRu_xO₃ (x=0.00, 0.05 and 0.15) perovskites, RSC Adv. 5, 31901 (2015)

[18] Y. Ying, J. Fan, L. Pi, Z. Qu, W. Wang, B. Hong, S. Tan, and Y. Zhang, Effect of Ru doping in La_0.5Sr_0.5MnO₃ and La_0.45Sr_0.55MnO₃, Phys. Rev. B 74(14), 144433 (2006)

[19] M. M. Saber, M. Egilmez, A. I. Mansour, I. Fan, K. H. Chow, and J. Jung, Evolution of Curie-Weiss behavior and cluster formation temperatures in Ru-doped Sm_0.55Sr_0.45MnO₃ manganites, Phys. Rev. B 82(17), 172401 (2010)

[20] I. Dhiman, A. Das, A. K. Nigam, and U. Gasser, Influence of B-site disorder in La_0.5Ca_0.5Mn_1-xB_xO₃ (B=Fe, Ru, Al and Ga) manganites, J. Phys.: Condens. Matter 23(24), 246006 (2011)

[21] Y. Ying, J. Zheng, L. Qiao, W. Li, W. Cai, S. Che, L. Jiang, J. Fan, and M. Lin, Double exchange interaction between Mn³⁺ and Ru⁴⁺ ions in La_1-xSr_xMn_1-xRu_xO₃, J. Superconduct. Novel Magnet. 28(10), 3117 (2015)

[22] D. Zhu, H. Zhu, and Y. Zhang, Microstructure and magnetization of single-crystal perovskite manganites nanowires prepared by hydrothermal method, J. Cryst. Growth 249(1–2), 172 (2003)

[23] J. Spooren, R. I. Walton, and F. Millange, A study of the manganites La_0.5M_0.5MnO₃ (M=Ca, Sr, Ba) prepared by hydrothermal synthesis, J. Mater. Chem. 15(15), 1542 (2005)

[24] Y. Cheng, J. Dai, X. Zhu, D. Wu, and Y. Sun, Preparation, magnetic and microwave absorption properties of La_0.5Sr_0.5MnO₃/La(OH)₃ composites, Mater. Res. Bull. 45(6), 663 (2010)

[25] B. Tang, J. Ge, and L. Zhuo, The fabrication of La(OH)₃ nanospheres by a controllable-hydrothermal method with citric acid as a protective agent, Nanotechnology 15(12), 1749 (2004)

[26] C. Dong, PowderX: Windows-95-based program for powder X-ray diffraction data processing, J. Appl. Cryst. 32(4), 838 (1999)

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