Journal of Magnetism and Magnetic Materials 123 (1993) 159-164 North-Holland On the magnetic behaviour of ACo 2 (A = Y, Lu, Zr, Sc and Hf) compounds E. Burzo a,b, E. Gratz c and V. Pop a,b a Facultatea de Fizic~, Universitatea Cluj-Napoca, 3400 Cluj-Napoca, Romania b Laboratoire de Magn#tisme, BP 166X, 38042 Grenoble-Cedex, France c Institut fiir Experimentalphysik, Technische Hochschule Wien, Wiedner Hauptstrasse, A-1040 Vienna, Austria Received 14 July 1992 The magnetic properties of AGo 2 (A = Y, Lu, Zr, Sc, Hf) compoundswere investigated in the temperature range 2-1100 K and fields up to 70 kOe. In order to avoid a possible alteration of the magnetic susceptibilities, X, caused by the presence of magnetic ordered impurities, the 2' values were obtained from magnetization isotherms, by extrapolation to H- 1_.. 0. At high temperatures, the ACo2 (A = Y, Lu, Sc and Hf) compounds show temperature dependences of magnetic susceptibili- ties of Curie-Weiss type. In ZrCo2, a more complexthermal variation of magnetic susceptibility is evidenced. At T < 15 K, the X values for YCo 2 and LuCo 2 vary accordingto the relation X = Xo(1 + aT2). The experimental data are analysed in the spin fluctuations model. 1. Introduction The AGo 2 compounds, where A = Y, Lu, Sc, Hf and Zr crystallize in a cubic C-15 type Laves phase structure. In this lattice, both A and Co atoms occupy only one type of site. Because of the high symmetry of the crystalline cell, as well as due to their interesting physical properties, these systems were extensively investigated. As a common feature, the ACo 2 compounds show an exchange enhanced paramagnetism. In the temperature dependences of the magnetic susceptibilities for YCo 2 [1-5], LuCo 2 [4,6] and ScCo 2 [4,7,8] the maxima were evidenced. The ZrCo 2 compound has been reported to be not magnetic [3,9-12]. The deviation from stoichiom- etry influences sensitively the magnetic behaviour of the above compound. The magnetic suscepti- bility seems to decrease by increasing the temper- ature up to 300 K [3,13]. By NMR studies Ya- mada and Ohmae [13] concluded that cobalt atoms are not magnetic in ZrCo2. Similar as for Correspondence to: Prof. E. Burzo, Facultatea de Fizic~t, Uni- versitatea Cluj-Napoca, 3400 Cluj-Napoca, Romania. ZrCo2, the magnetic properties of HfCo 2 are sensitive to the deviation from stoichiometry. A decrease of the magnetic susceptibility for the stoichiometric compound, when increasing tem- perature was emphasized [14]. Theoretical studies were also devoted to the analysis of the magnetic properties of these sys- tems. Yamada et al. [15,16] calculated the elec- tronic structures of d-electrons in ACo 2 com- pounds by using the self-consistent tight binding approximation. Starting from the calculated den- sity of states (DOS) and taking into account the effect of spin fluctuations the authors showed that a maximum in the temperature dependence of the magnetic susceptibility for ACo 2 (A = Y, Lu, Sc) and a modified Curie-Weiss law for A = Zr, Hf, may be explained by the shape of the DOS around the Fermi level. The different be- haviour of the ACo 2 (A = Y, Lu, Sc) and A = Zr, Hf, respectively, is mainly attributed to the differ- ence between atomic potentials of A and Co. For ACo 2 with A = Y, Lu and Sc, the atomic poten- tials are larger than for A = Zr and Hf and hy- bridization, i.e. the mixing between the d bands of Y, Lu, Sc and Co is weaker than for Zr and 0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved