Magnetic properties of some rare earth-iron-cobalt and rare earth-cobalt-nickel compounds ;

Abstract

The design and construction of a high field pulse magnetometer are described. This instrument was used for investigating the magnetic properties of the various pseudobinary compounds in the alloy systems Y(_x)(Fe,Co)(_I-X) and Y(_x)(Co,Ni), as well as the systems Gd(_3)(Fe,Co) and Gd(_3)(Co,Ni), over the entire ranges of solid solubility. The variation of spontaneous moment in the yttrium compounds as a function of 3d electron concentration shows a continuous change in form from the YB(_2) compounds to the transition metal rich Y(_2)B(_17) compounds, Where B denotes(Fe,Co) or (Co,Ni). This change indicates a gradual progression to the well-known (Fe, Co) and (Co,Ni) moment variation of the Slater-Pauling curve. In the Y(_2)Co,Ni)(_7) and Y(_2)(Co,Ni)(_7) compounds the spontaneous moment decreases with increasing 3d electron concentration, becoming zero for about 80% Ni, and then reappears with a small value for the compounds YNi(_3)and Y(_2)Ni(_7). This phenomenon is very similar to that found in the compounds Y(_x) Ni(_1-x) as x is increased. The results are interpreted within the rigid band model and it is suggested that the disappearance of ordering in the Y(Co,Ni)(_3) and Y(_2)(Co,Ni)(_7) systems is due to a deep minimum in the density of states curve. The 3d moment in the Gd(_x)B(_1-x) and Y(_x)B(_1-x) compounds is deduced and an attempt is made, for B = Co or Ni, to isolate it from the moment contribution due to the conduction electron polarization. A unified picture is presented in which the 3d band is gradually populated by some of the valence electrons from the tripositive Gd and Y ions, as x is increased. The compounds for which the 3d band is filled are indicated, not by the absence of any spontaneous moment in the yttrium compounds, but rather, by the inability of gadolinium to induce a 3d moment when Gd is substituted for Y. The magnetic results of the Gd(_3)(Fe, Co) and Gd(_3)(Co,Ni) systems indicate that these compounds are antiferromagnetic and the results are interpreted quantitatively in terms of the Neel two-sublattice theory of antiferromagnetism. Values are deduced for the anisotropy arid the inter- and intra-sublattice exchange coefficients. There is a close correlation between structural stability and the existence of antiferromagnetism. When the latter disappears (at 10% Fe) the structure becomes unstable for higher concentrations of iron. From this correlation, a rule is deduced for predicting when a structure might become unstable. This rule is found to be applicable also to the other stoichiometries of the gadolinium-rich compounds.