ELECTRONIC STRUCTURE AND MAGNETIC PROPERTIES OF THE Y (Fe1-xCox)2 ALLOYS

We report on a theoretical investigation of the electronic structure and magnetic properties of the pseudobinary Y (Fel-,Co,)2 Laves phase alloys. The magnetic properties are found to be well described by a Slater-Pauling curve with a maximum of the magnetisation at z = 0.25. A Fixed Spin Moment (FSM) calculation is used to describe the disappearance of the magnetic moment as a function of alloy concentration. Yttrium forms cubic Laves phase compounds with Mn, Fe, Co and Ni. Experimentally YNi2 is known to be a weak paramagnet, YCo2 an exchange enhanced paramagnet, YFez a ferromagnet and, finally, YMn2 is an antiferromagnet. Several authors [I] have predicted that YCoz should undergo a metamagnetic transition .at a magnetic field of the order of 100 T. Calculations [2] of the magnetic moment for YFe2 agree well with experiment [3J. We report here on calculations of the magnetic properties for the pseudo-binary Laves phase alloy system Y (Fel-,Cox), . This is made by simulating the electronic structure of a random alloy by that of an ordered compound with the same stoichiometry. The primitive cell of the yttrium-iron Laves phase compound contains 2 yttrium atoms and 4 iron atoms, i.e. Y2Fe4. In this cell we progressively replace iron with cobalt, i.e. we consider the ordered ternary compounds: YzFeaCo, Y2FezCo2, YzFeCo3 and, finally, Y2Co4. A systematic study of the magnetic properties of these ordered systems allows us to study the magnetism as a function of the concentration, x. The calculational techniques have been described elsewhere [4]. The calculated magnetic moments for the mentioned ordered compounds are displayed in figure 1, together with the experimental data for the pseudo-binary alloys. The calculated moment for YFe2 is 2.94 p ~ , which is in good agreement with experimental data [3]. The general trend of the measured total magnetic moment, with a maximum at approximately a = 0.30, is aIso reproduced. From table I it becomes clear that Table I. Calculated magnetic moments per atom, major i ty, minor i ty and total 3d band occupation numbers for the Y (F~~,CO, )~ allogs. P ( F ~ ) (PB) 1.68 2.09 2.01 1.91 P 0) (PB) 1.22 1.23 1.21 p ( Y ) (pB) -0.43 -0.45 -0.42 -0.38 n3d (maj.) 16.8 17.6 17.6 17.6 n3d (min.) 9.8 9.9 10.9 11.9 n3d I =-atom 6.64 6.87 7.13 7.37 7.74 Y(Fe Co ) 1-K X 2