Supported by National Natural Science Foundation of China(60807036,10774154,11074265,10804060);the "Chenguang" Program of Shanghai Educational Development Foundation of China(2007CG57);the Science and Technology Commission of Shanghai Municipality (10ZR1422300);Shanghai Municipal Education Commission(11YZ83)
Acknowledgements The authors would like to thank the National Science Foundation of China (60807036), the Leading Foundation of Shanghai (08JC1417200), and the Laboratory of Rare Earths Functional Materials of Shanghai (07dz22303) for supporting the research.