~) Solid State Communications, Vol. 82, No. 4, pp. 211-214, 1992. Printed in Great Britain. 0038-1098/9255.00 +. 00 Pergamon Press Ltd ON THE MAGNETIC PROPERTIES OF THE YI.xGdxCO4B COMPOUNDS Nguyen Minh Hong, Phan Ngoc Minh and Nguyen Phu Thuy Cryogenic Laboratory, Faculty of Physics, University of Hanoi, 90 Nguyen Trai, Hanoi, Vietnam (Received 13 November 1991 by P.Burlet) Magnetic phase transitions in Yl.xGdxCO4 B with x ranging from 0 to 1 have been determined in the temperature range of 77 to 600K. The compensation temperature is found in the compounds with x ~ 0.6,while in the rest compounds a spin reorientation from the c-axis towards the basal plane upon decreasing temperature is observed. On the basis of the observed Curie and compensation temperatures the intersublattice exchange interaction coefficient has been derived and the contribution of the anisotropic exchange interaction to the anisotropy energy of the Gd containing compounds has been estimated. 1.INTRODUCTION The RCo4B compounds crystallize in the CeCo4B type of structure which can be derived from the CaCu 5 structure by an ordered substitution of two Co atoms in every second layer containing R-Co by two B atoms [1]. Although the RCos and RCo4B compounds are so closely related in structure, the Co magnetism in these two types of compound differs largely [2,3,4]. The striking feature of the Co sublattice in YCo4B is the existence of the transition of the magnetization direction from the c-axis to the basal plane as lowering temperature through about 150 K [2,4,8].In the previous study we have reported on the large influence of the lattice deformation on the Co anisotropy in Yl.xLaxCo4B in which accompanying with an increase in the a parameter (a total increase 0f6.9% in volume or decrease of 3.5% in the c/a ratio) an axial anisotropy in the whole temperature range is manifested in LaCo4B [5]. It is interesting that the axial anisotropy in the whole temperature range is also found in GdCo4B in which Gd ions are of S-state and do not contribute to the resultant anisotropy [2,10].The origin of the magnetocrystaUine anisotropy in the latter compound is still an opened question. The Yl.xGdxCo4B have been studied by Burzo et al. [6] who reported the Curie temperature, saturation and paramagnetic moments. In this contribution the magnetic phase transitions in Yl.xGdxCO4 B will be .presented. The observed Curie and compensation temperatures are analyzed in the frame of a molecular field approach and the role of the exchange anisotropy on the concentration variation of the anisotropy energy is estimated. 211 2. EXPERIMENTAL The Yl.xGdxCO4B compounds with x ffi 0, 0.2, 0.4,0.6,0.8and 1.0have been prepare, byare-melting the constituents of nominal composition. The purity of the starting materials is 99.9% for Y and Gd, 99.99% for Co and 99% for B. In order to compensate the lost during the melting an access amount of lwt% was added for the rare earths. The obtained ingots were then annealed in purified argon atmosphere for 50 hours at 900°C. All the samples are of single phase of the CeCo4B type of structure as revealed by the X-ray diffractogram and thermo-magnetic analysis. Values for the lattice parameters are listed in Table 1. We note that upon increasing Gd concentration the c value is nearly unchanged, while the a o n e slightly increases giving rise to an total increase of 1.8% in the volume or a decrease of 0.8% in the c/a ratio. Magnetic measurement has been carried out on the bulk samples in a low magnetic feld (0.1 '1") in the temperature range from 77 to 550 K and the results are shown in Fig. 1( a and b). The Curie temperature is determined by extrapolating from the B/a(T) curve [7]. We note that in the measured range of temperature an anomaly appears in the compounds with x -- 0, 0.2 and 0.4 (fig.la) while in the remaining compounds a compensation temperature is observed. As confirmed by further measurements on the aligned powder samples this anomaly reflects the spin re, orientation process in the corresponding compound. The results of these measurements are shown in fig.2 in which we present the dependence of the magnetization of the aligned powder sample on the angle between the alignment and the applied field directions at 77 K and 300 K. The interchange of the minima and maxima of