ELSEVIER Journal of Magnetism and Magnetic Materials 152 (1996) 219-225 J H Journal of magnetism ~ H and magnetic ~ H materials Magnetic and electrical properties of the R(Co, Si)2 compounds (R -- Gd, Tb, Dy) with invariable crystal unit cell parameters Nguyen Huu Duc 1 Cryogenic Laboratory, Physics Department, Universityof Hanoi, 90-Nguyen Trai, Dongda, Hanoi, Viet Nam Received 6 February 1995; revised 23 May 1995 Abstract The invariable crystal unit cell parameter compounds R(Co, Si) 2 (R = Gd, Tb and Dy) have been studied by mean of the magnetization, ac susceptibility and resistivity measurements. By partial substitution Co by Si, the ordering temperature is almost constant for all of the considered compounds, whereas the reduction of the 3d magnetic moment is observed. In the condition of fixed volume, these findings suggest the important effects of the hybridization between the 3d states of Co and the p states of Si. The anomalies of magnetization, susceptibility, resistivity and the character of the magnetic phase transition at T c are also discussed for the compounds with Dy. 1. Introduction The appearance of the weak itinerant ferromag- netism in the Laves phase compounds RCo 2 (R = Y, Lu, Sc) as AI is substituted for Co has generated a lot of interest in the last decade [1,2]. The role of the A1 atom is to change the density of the states at the Fermi level. However, there is still confusion as to whether it is due to the decrease of the 3d-electron concentration or the lattice expansion. There have been many works dedicated to isolating these two contributions. In a study of the invariable crystal unit cell parameter compounds (Y1- tLu/XC°l-xAlx)2, Dubenko et al. [3] have shown that the magnetic behaviours obtained for this system can be compared well with those observed for the Lu(COl_xAlx) 2 system. By neglecting the difference between the 4d(Y)-3d and 5d(Lu)-3d hybridizations, the impor- t Research address: Laboratoire de Magnetisme Louis Nrel, CNRS, BP 166, 38042 Grenoble Cedex 9, France. tance of the concentration of the 3d electrons has been stressed. The analysis of the ordering tempera- tures in the R(Col_xAlx) 2 compounds (with R = magnetic rare earths), however, suggested strongly that the volume has a major influence [2]. The role of the volume effect can be isolated by high-pressure experiments [4,5]. It indicates, fortunately, that for the Y(COt_xAlx) 2 compounds the change in the chemical pressure between two concentrations of x = 0.145 and x = 0.16 is roughly equal to the change in the critical pressure required to destroy the mag- netic ordering in these compounds. In addition, the studies of the itinerant electron metamagnetism in the invariable 3d-electron concentration Y(C°xAlyCUz)2 compounds [6] highlight the impor- tance of the magneto-elastic coupling to this picture. Furthermore, the band-structure calculation [7] has shown that the width of the 3d band is certainly narrowed by the lattice expansion, however, it is not this effect but the effect of the strong hybridization between the 3d states of Co and the 3p states of A! that plays an essential role to the onset of the mag- 0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00425-4