Physica B 321 (2002) 163–166 M . ossbauer spectroscopy study of GdFe 6x Co x Ge 6 alloy Abdel-Fatah D. Lehlooh a, *, Sami H. Mahmood a , K.H.J. Buschow b a Physics Department, Yarmouk University, Irbid 211-63, Jordan b Van der Waals-Zeeman Institute, Valckenierstraat 65, Amsterdam, Netherlands Abstract Room temperature M . ossbauer spectroscopy and X-ray diffractometry have been used to study GdFe 6x Co x Ge 6 alloys (with x ¼ 0; 1, 2 and 4) and the effect of partially substituting the cobalt for iron in the alloy. The X-ray diffraction study of the alloy indicates that its crystal structure is a single phase and does not change with this substitution, apart from a reduction in the lattice constant. The M . ossbauer study shows that substituting cobalt for iron in the system significantly changes its magnetic properties. As cobalt is introduced, it occupies the iron sites randomly in accordance with the binomial distribution, leading to new magnetic components of the spectra with appreciably lower magnetic fields. The reduction in the hyperfine field with cobalt concentration is due to the fact that Co in this structural phase has zero moment. At higher cobalt concentrations the hyperfine field vanishes as the material becomes magnetically disordered at room temperature. r 2002 Elsevier Science B.V. All rights reserved. Keywords: M. ossbauer spectroscopy; Alloy; Intermetallic; X-ray diffraction; Hyperfine field 1. Introduction The structural and magnetic properties of the rare-earth intermetallic compounds RMn 6 Sn 6 have been studied by X-ray diffraction, magneti- zation measurements and M . ossbauer spectroscopy [1–3]. The structural studies have shown that the system crystallizes in HfFe 6 Ge 6 -type structure (P6/ mmm) and contains one Mn site and three inequivalent Sn sites with equal population [1,2]. The magnetic measurements shows that RMn 6 Sn 6 orders magnetically at temperatures above 400 K [1]. The system with R=Y was characterized by an antiferromagnetic ordering of the manganese sublattice at 333 K, while others (with R=Gd, Tb, Dy and Ho) show ferrimagnetic behavior, due to an antiferromagnetic coupling of the ferromag- netic R and Mn sublattices (T C > 376 K) [3]. The alloy systems RFe 6 Ge 6 with R=Y, Gd–Lu, have been investigated using X-ray diffraction, magnetic measurements and M . ossbauer spectro- scopy [2,4,5]. The magnetization measurements have shown that the iron sublattice orders anti- ferromagnetically below about 480 K and T N is insensitive to the rare-earth element present in the alloy [5]. Furthermore, the rare-earth sublattice orders ferromagnetically at much lower tempera- tures, ranging from 30 K for Gd alloy to 3 K in case of Er, without affecting the order in the iron sublattice [4]. The great difference in ordering temperature between the iron and the rare-earth indicates that the two magnetic species order independently and the R–Fe exchange is essentially zero. M . ossbauer spectra and magneti- zation measurements on RFe 6 Ge 6 alloys with *Corresponding author. Fax: +962-2-724-7983. E-mail address: aflehlooh@yu.edu.jo (A.-F.D. Lehlooh). 0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(02)00844-X