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Structure, transport and magnetic properties in a new layered ferromagnetic perovskite cobalt oxide Sr2−xPrxCoO4
Physica C 460–462 (2007) 481–482 www.elsevier.com/locate/physc
Structure, transport and magnetic properties in a new layered ferromagnetic perovskite cobalt oxide Sr2 xPrxCoO4 Q.W. Yao *, X.L. Wang, D.Q. Shi, S.X. Dou Institute for Superconducting and Electronic Materials, University of Wollongong, Northfield Avenue, NSW 2522, Australia Available online 24 March 2007
Abstract In this work, we report our studies on the structures, transport, and magnetic properties in the two-dimensional layer structured perovskite compound Sr2 xPrxCoO4. Structure refinement revealed that these compounds crystallized in K2NiF4-type structures with space group I4/mmm. The lattice parameters c decreased with Pr doping levels x. The Curie temperatures (Tc) are found to be 200 K for Sr1.5Pr0.5CoO4. The resistivities were found to increase with doping levels x. A large coercive field of about 1 T was found for the sample of x = 0.75. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ferromagnet; Magnetoresistance; Two-dimensional cobalt compound
1. Introduction Compounds with the K2NiF4-type structure are well known to exhibit various interesting properties, such as high-temperature superconductivity in cuprates, spin-triplet superconductivity in ruthenates, spin/charge stripes in nickelates and manganites. Recent discovery of metallic ferromagnetism, superconductivity in NaxCoO3 Æ H2O [1] and Sr2CoO4 have generated a great interests in the study of the two-dimensional structured compounds. The compound Sr2CoO4 consists of CoO2 planes separated by rock-salttype SrO planes. It has been reported that the Sr2CoO4 single-crystalline thin films and polycrytalline samples are metallic ferromagnets with a high Tc of 255 K [2,3] and high coercive field. Investigation of the system Sr2 xPrxCoO4 will help understand this Sr2CoO4 compound. 2. Experimental Polycrystalline samples of Sr2 xPrxCoO4 (0.25 6 x 6 1.75) were synthesized by a conventional solid-state reac*
Corresponding author. Tel.: +61 02 4221 5766; fax: +61 02 4221 5731. E-mail address: [email protected] (Q.W. Yao).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.043
tion. Highly pure powders of Pr6O11, SrCO3 and Co3O4 were mixed according to appropriate atomic ratios. Samples were sintered in air at 1000 °C for 12 h with several intermediate grindings. The phases and structures were studied using powder X-ray diffraction. Structure refinements were carried out by the Rietveld method using the RIETICA program [4]. Magnetic and transport properties were investigated using a commercial Quantum Design MPMS and PPMS system between 5 and 330 K in magnetic fields up to 8 T. 3. Results and discussion The XRD patterns of the samples shows that all the diffraction peaks belong to the Sr2CoO4 phase (Fig. 1). A single 214 phase was found to form for x = 0.5–1.25. The lattice parameters c were found to decrease gradually from ˚ for x = 0.5–1.5, respectively, indicating 12.42 to 12.25 A that Pr doping reduces the c parameter. While, the parameter a remains the same for all the samples. The reduction of the parameters c is in agreement with the fact that the sizes of Pr4+ or Pr3+ ions are smaller than that of Sr2+. Rietveld refinement by Reitica program reveals that the SrPrCoO4 crystallized as the K2NiF4 type structure with
482
Q.W. Yao et al. / Physica C 460–462 (2007) 481–482
(020)
(114)
Sr2-xPrxCoO4
15.0
Magnetization (emu/g)
Intensity (a.u.)
x=1.25
(015)
(112)
(110)
(013)
XRDs for Sr2-xPrxCo O4
x=1
x=0.75
x=0.5
7.5
x=1.25 0.0 -7.5
x=1.5
-15.0 -3.0
35
0.0
1.5
3.0
Magnetic Field (T)
x=0.5
30
-1.5
40
45
2θ (degrees)
50
Fig. 4. Magnetic hysteresis loops for Sr2 xPrxCoO4.
space group I4/mmm. As shown in Fig. 2, the refined pattern agrees well with the sample’s XRD (solid line) pattern. The temperature dependence of the field cooled and zero field cooled dc magnetization for the samples measured at 0.2 T is shown in Fig. 3. The sample Sr1.5Pr0.5CoO4 reveals a ferromagnetic transition with Tc = 200 K. Magnetization decreases with increasing x. It becomes paramagnetic for samples with x P 0.75. Fig. 4 shows the MH loops for three samples. The results further support the typical ferromagnetic behavior seen in Fig. 3 for x = 0.5 whose loop reveals a typical ferromagnetic state with a large coercive field of 1 T. For x = 1.25–1.5 samples, the magnetizations as a function of field follow a trend of paramagnetic state.
Fig. 1. X-ray diffraction patterns for Sr2 xPrxCoO4.
4. Conclusion
Fig. 2. The observed (crosses), calculated (solid line), and difference diffraction (bottom solid line) profiles at 300 K for SrPrCoO4. The top peak markers relate to Sr2CoO4 while the lower markers pertain to the impurity SrO2.
Acknowledgements
Magnetization Vs T for Sr 2-xPrxCoO4
x=0.25 x=0.5
Magnetization (emu/gram)
8
This work is supported by the Australian Research Council. Q.W. Yao thanks the University of Wollongong for providing a UPA scholarship for his Ph.D. study.
6 x=0.75 x=1.25 x=1.5
4
2
References [1] K. Takada, H. Skurai, E. Muramachi, F. Izumi, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53. [2] X.L. Wang, E. Takayama-Muromachi1, Phys. Rev. B 72 (2005) 064401. [3] J. Matsuno, Y. Okimoto, Z. Fang, X.Z. Yu, Y. Matsui, N. Nagaosa, M. Kawasaki, Y. Tokura, Phys. Rev. Lett. 93 (2004) 167202. [4] B.A. Hunter, Commission Powder Diffraction Newsletter 20 (1998) 21.
H = 0.2 T
0 0
50
100
150
200
The lattice parameter c decreases with the Pr doping levels. The Curie temperatures are found to decrease gradually from 200 K to lower temperatures with the increasing levels of Pr doping. A large coercive field of about 1 T was found for the sample of Sr1.5Pr0.5CoO4.
250
300
Temperature (K) Fig. 3. Magnetization vs. temperature for Sr2 xPrxCoO4, field = 0.2 T.
Structure, transport and magnetic properties in a new layered ferromagnetic perovskite cobalt oxide Sr2 xPrxCoO4 Q.W. Yao *, X.L. Wang, D.Q. Shi, S.X. Dou Institute for Superconducting and Electronic Materials, University of Wollongong, Northfield Avenue, NSW 2522, Australia Available online 24 March 2007
Abstract In this work, we report our studies on the structures, transport, and magnetic properties in the two-dimensional layer structured perovskite compound Sr2 xPrxCoO4. Structure refinement revealed that these compounds crystallized in K2NiF4-type structures with space group I4/mmm. The lattice parameters c decreased with Pr doping levels x. The Curie temperatures (Tc) are found to be 200 K for Sr1.5Pr0.5CoO4. The resistivities were found to increase with doping levels x. A large coercive field of about 1 T was found for the sample of x = 0.75. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ferromagnet; Magnetoresistance; Two-dimensional cobalt compound
1. Introduction Compounds with the K2NiF4-type structure are well known to exhibit various interesting properties, such as high-temperature superconductivity in cuprates, spin-triplet superconductivity in ruthenates, spin/charge stripes in nickelates and manganites. Recent discovery of metallic ferromagnetism, superconductivity in NaxCoO3 Æ H2O [1] and Sr2CoO4 have generated a great interests in the study of the two-dimensional structured compounds. The compound Sr2CoO4 consists of CoO2 planes separated by rock-salttype SrO planes. It has been reported that the Sr2CoO4 single-crystalline thin films and polycrytalline samples are metallic ferromagnets with a high Tc of 255 K [2,3] and high coercive field. Investigation of the system Sr2 xPrxCoO4 will help understand this Sr2CoO4 compound. 2. Experimental Polycrystalline samples of Sr2 xPrxCoO4 (0.25 6 x 6 1.75) were synthesized by a conventional solid-state reac*
Corresponding author. Tel.: +61 02 4221 5766; fax: +61 02 4221 5731. E-mail address: [email protected] (Q.W. Yao).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.043
tion. Highly pure powders of Pr6O11, SrCO3 and Co3O4 were mixed according to appropriate atomic ratios. Samples were sintered in air at 1000 °C for 12 h with several intermediate grindings. The phases and structures were studied using powder X-ray diffraction. Structure refinements were carried out by the Rietveld method using the RIETICA program [4]. Magnetic and transport properties were investigated using a commercial Quantum Design MPMS and PPMS system between 5 and 330 K in magnetic fields up to 8 T. 3. Results and discussion The XRD patterns of the samples shows that all the diffraction peaks belong to the Sr2CoO4 phase (Fig. 1). A single 214 phase was found to form for x = 0.5–1.25. The lattice parameters c were found to decrease gradually from ˚ for x = 0.5–1.5, respectively, indicating 12.42 to 12.25 A that Pr doping reduces the c parameter. While, the parameter a remains the same for all the samples. The reduction of the parameters c is in agreement with the fact that the sizes of Pr4+ or Pr3+ ions are smaller than that of Sr2+. Rietveld refinement by Reitica program reveals that the SrPrCoO4 crystallized as the K2NiF4 type structure with
482
Q.W. Yao et al. / Physica C 460–462 (2007) 481–482
(020)
(114)
Sr2-xPrxCoO4
15.0
Magnetization (emu/g)
Intensity (a.u.)
x=1.25
(015)
(112)
(110)
(013)
XRDs for Sr2-xPrxCo O4
x=1
x=0.75
x=0.5
7.5
x=1.25 0.0 -7.5
x=1.5
-15.0 -3.0
35
0.0
1.5
3.0
Magnetic Field (T)
x=0.5
30
-1.5
40
45
2θ (degrees)
50
Fig. 4. Magnetic hysteresis loops for Sr2 xPrxCoO4.
space group I4/mmm. As shown in Fig. 2, the refined pattern agrees well with the sample’s XRD (solid line) pattern. The temperature dependence of the field cooled and zero field cooled dc magnetization for the samples measured at 0.2 T is shown in Fig. 3. The sample Sr1.5Pr0.5CoO4 reveals a ferromagnetic transition with Tc = 200 K. Magnetization decreases with increasing x. It becomes paramagnetic for samples with x P 0.75. Fig. 4 shows the MH loops for three samples. The results further support the typical ferromagnetic behavior seen in Fig. 3 for x = 0.5 whose loop reveals a typical ferromagnetic state with a large coercive field of 1 T. For x = 1.25–1.5 samples, the magnetizations as a function of field follow a trend of paramagnetic state.
Fig. 1. X-ray diffraction patterns for Sr2 xPrxCoO4.
4. Conclusion
Fig. 2. The observed (crosses), calculated (solid line), and difference diffraction (bottom solid line) profiles at 300 K for SrPrCoO4. The top peak markers relate to Sr2CoO4 while the lower markers pertain to the impurity SrO2.
Acknowledgements
Magnetization Vs T for Sr 2-xPrxCoO4
x=0.25 x=0.5
Magnetization (emu/gram)
8
This work is supported by the Australian Research Council. Q.W. Yao thanks the University of Wollongong for providing a UPA scholarship for his Ph.D. study.
6 x=0.75 x=1.25 x=1.5
4
2
References [1] K. Takada, H. Skurai, E. Muramachi, F. Izumi, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53. [2] X.L. Wang, E. Takayama-Muromachi1, Phys. Rev. B 72 (2005) 064401. [3] J. Matsuno, Y. Okimoto, Z. Fang, X.Z. Yu, Y. Matsui, N. Nagaosa, M. Kawasaki, Y. Tokura, Phys. Rev. Lett. 93 (2004) 167202. [4] B.A. Hunter, Commission Powder Diffraction Newsletter 20 (1998) 21.
H = 0.2 T
0 0
50
100
150
200
The lattice parameter c decreases with the Pr doping levels. The Curie temperatures are found to decrease gradually from 200 K to lower temperatures with the increasing levels of Pr doping. A large coercive field of about 1 T was found for the sample of Sr1.5Pr0.5CoO4.
250
300
Temperature (K) Fig. 3. Magnetization vs. temperature for Sr2 xPrxCoO4, field = 0.2 T.