Journal of Magnetism and Magnetic Materials 242–245 (2002) 844–846 Structure and magnetic properties of RCo 7x Mn x alloys (R=Sm, Gd; x=0.1–1.4) M. Gjoka a,b, *, O. Kalogirou c , C. Sarafidis c , D. Niarchos b , G.C. Hadjipanayis d a Department of Physics, University of Ioannina, 45110 Ioannina, Greece b Institute for Materials Science, NCSR ‘‘Demokritos’’, 153 10 Ag. Paraskevi, Attiki, Greece c Department of Physics, Aristotle University of Thessaloniki, 54 006 Thessaloniki, Greece d Department of Physics and Astronomy, University of Delaware, Newark, DE 19716-2570, USA Abstract Alloys with starting stoichiometry RCo 7x Mn x (R ¼ Sm; Gd; x ¼ 0:121:4) were prepared by arc melting. The effect of Mn doping on the formation of the TbCu 7 -type structure and magnetic properties are studied. The SmCo 7x Mn x as cast alloys with xo0:4 present uniaxial anisotropy along the c-axis as determined by X-ray diffraction on magnetically oriented samples. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare earth transition metal compounds; Structure; Magnetic properties; Magnetocrystalline anisotropy In order to stabilise the metastable SmCo 7 phase with the TbCu 7 -type structure (1:7) phase and increase its anisotropy a third doping element is needed. Huang et al. have stabilised the TbCu 7 -type structure with a small amount of Zr in the series RCo 7x Zr x (R ¼ Sm; Pr, Er, Y, Gd, Nd, Ho) [1–3]. In a previous work, we have reported the stabilisation of the TbCu 7 -type structure by melt spinning SmCo 7x Mn x (x ¼ 0:7; 1.4) alloys [4]. Here, we extend the study of the effect of Mn doping on the formation of 1:7 phases in the series RCo 7x Mn x (R ¼ Sm; Gd; x ¼ 0:121:4). Alloys with the starting stoichiometry RCo 7x Mn x (x ¼ 0:121:4) were prepared by arc melting high purity elemental constituents under Ar. The samples were annealed in evacuated sealed quartz tubes for 72 h at 873, 973 and 1073 K followed by water quenching. X- ray diffraction measurements, Cu K a ; were performed on randomly and magnetically aligned powder samples to determine the phase components and the easy magnetisation direction. The samples where charac- terised by Rietveld analysis, scanning electron micro- scopy (SEM) and electron microprobe probe (EDAX). The Curie temperature was derived from MðT Þ curves measured by a VSM magnetometer in a field of 0.1 T. MðHÞ curves were obtained by a SQUID magnetometer. The phase composition of the as cast RCo 7x Mn x alloys varies with Mn content. Qualitative analysis of the XRD spectra (Fig. 1) suggested that in the SmCo 7x Mn x system two main phases, of the CaCu 5 - (1:5) and TbCu 7 -type (1:7), were present. Although both structures crystallise in the hexagonal P6/mmm space group, they have distinct site populations, which could lead, at least in some cases, to detectable differences in the XRD pattern. The Rietveld analysis of the XRD pattern for x ¼ 0:2 gave R ¼ 11:63% and relatively low R Bragg coefficients 11.63% and 9.34% for the 1:5 and 1:7 phases, respectively. For 1:7, we used the starting model described by Buschow et al. [5] but one reflection at 2y=38.51 remained unidentified. Saito et al. [6] studying the system Sm 2 (Co,Mn) 17 have indexed such a reflection as a superlattice reflection of the rhombohedral Th 2 Zn 17 -type structure (2:17R) which is a superstructure of the disordered TbCu 7 -type structure. Adding a 2:17R phase in the refinement it was impossible to identify this reflection. Thus, we decided to use the model suggested in Ref. [7] for the refinement of a disordered Gd 2 (Fe,V) 17 phase. In that model, the Sm atom was positioned at a new site: 3a (0 0 0) in the middle of the dumb-bell of Co atoms. Thus, we obtained R ¼ 10:0% and R Bragg ¼ 9:4%; 10.5% and 10.4% for the 1:5, 1:7 *Corresponding author. Tel.: +30-301-6503321; fax: +30- 301-6519430. E-mail address: gjoka@ims.demokritos.gr (M. Gjoka). 0304-8853/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0304-8853(01)01339-7