Journal of Magnetism and Magnetic Materials 310 (2007) e720–e722 Melting of the charge-ordering state by ruthenium doping in Ca 0.6 Pr 0.4 Mn 1y Ru y O 3 (y ¼ 0, 0.03, 0.05, 0.07) perovskites P.Q. Thanh, B.T. Cong à , C.T.A. Xuan, N.H. Luong Solid State Physics Department, Faculty of Physics, Hanoi University of Science. 334 Nguyen Trai, Hanoi, Vietnam Available online 11 December 2006 Abstract Single-phase perovskite compounds Ca 0.6 Pr 0.4 Mn 1y Ru y O 3 (y ¼ 0, 0.03, 0.05, 0.07) were prepared by ceramic-technology method. Thermomagnetic plots, temperature dependence of resistivities, ac-susceptibilities of the produced samples were determined. The non- ruthenium doping (y ¼ 0) sample presents charge-ordering phenomenon at T CO ¼ 291 K. The experiment results show that the ruthenium doping destroys the charge ordering state, enhances ferromagnetism tendency and conductivity. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Pa; 75.50.Pp Keywords: Perovskite; Charge ordering The calcium-praseodymium manganites Ca 1x Pr x MnO 3 systems exhibit charge-ordering (CO) phenomenon and complicate magnetic-phase diagram [1]. The good thermo- magnetic properties of these compounds were examined in [2]. In this contribution, we investigate the destroying of CO state by doping of the ruthenium for manganese in antiferromagnetic Ca 0.6 Pr 0.4 MnO 3 perovskites. The Ca 0.6 Pr 0.4 Mn 1y Ru y O 3 (y ¼ 0, 0.03, 0.05, 0.07) perovskites compounds were synthesized by solid-state reaction method described in [2]. Room temperature X-ray diffraction patterns recorded by a D5005 diffractometer showed that all samples were of single phase and had Pnma crystal structure. The lattice parameters of samples were given in the Table 1. The FC thermomagnetic measurements (Fig. 1) of the samples demonstrate that small Ru doping from y ¼ 0 to 0.07 increases the maximum value of magnetization up to 100 times. Figs. 2 and 3 show the temperature dependence of real (w 0 ) and imaginary (w 00 ) ac- susceptibilities for y ¼ 0 and 0.07 samples. There were three anomalies in the susceptibilities curves for y ¼ 0 sample: the first occurred at the charge-ordering tempera- ture T CO ¼ 291 K and two other ones at the Neel temperatures. The strong Ru doping y ¼ 0.07 sample had only one paramagnetic–ferromagnetic phase transition at T C ¼ 210 K (see Fig. 3). Characteristic temperatures for all samples were listed in Table 1. DC resistivity of the samples was measured and results were plotted in Fig. 4. One can see that Ru doping for Mn reduces resistivity in the region T4100 K strongly. CO exhibits anomaly at the same temperature 291 K for y ¼ 0 sample (see inset in Fig. 4) like that observed in Fig. 2. Ru ion may have valences varied from +4 to +8. Mn ions in our perovskites have valences +3 and +4. We suggest that strong damping of resistivity in doping samples may concern with pentavalent state of Ru ion differing from what of Mn ions. Raveau et al. [3] also supports this point of view. According to the Mott–Viret’s variable range hopping (VRH) model [4], electrical carriers in magnetic perovskites are hopping between localized states due to random magnetic potential. Using the VRH law and the cell-volume value V given in Table 1, we obtained the characteristic temperature T o , the localization length l of the sample’s carriers in the temperature range ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.786 à Corresponding author. Tel.: +84 4 8584069; fax: +84 4 8584069. E-mail address: bach_thanh_cong@yahoo.com (B.T. Cong).