Jaccarino-Walker model

A convolution of the spectrum of pure SmS with the probabilities p(m) under the assumption of additive effects (i.e. a shift m AE for m Gd nearest neighbors) does not produce any discernible structure if AE is chosen so as to make the width of the main peak near EF comparable to the experimental data, see curve (c) for x = 15.5%. A model which has some similarity with the Jaccarino-Walker model for magnetic alloys (67), in which it is assumed that only two kinds ofSm ions are present in the alloy, gives more satisfactory results, see curves (b). These curves were generated under the assumption that for m> 1 the Sm 4/states have their energy shifted relative to those with m = 0 by about 0.5 eV. Clearly such a model is an oversimplification, in part because one would expect Coulombic effects due to Gd3+ ions to be in some way additive, unless the screening length due to the extra Gd 5 <7 electron is smaller than the nn distance a/ /2 = 4.22 A for SmS. 

Cannon et al. (1975) used the Jaccarino-Walker model for the transition-metal moment in some cubic Laves phase compounds. Specifically, they showed that in the Gd(COj Ni )2 system the Co moment appears to be criticaUy dependent on the number and type of its nearest neighbours. Ichinose (1987) measured the NMR of " Al, Mn, " Co and Gd in Gd(X, Coj2 for X = Al, Mn, Fe and Ni. He found from the analysis of the NMR spectra that the concentration dependence of the Co hyperfine field is proportional to the sum of the conduction electron polarization arising from nearest-neighbour transition atoms and that the Co atoms carry a magnetic moment induced by the neighbouring X atoms. 

It is established that the rigid band model is insufficient in these systems. The disappearance of a Co moment must be connected with the crystalline state since amorphous YC02 shows a moment of 1.6ju.b. On the other hand GdCo2 shows in the amorphous state 1.4ju.b/Co. In addition grinding can cause a rise in the susceptibility of YC02 by a factor of 3. This is due to the introduction of lattice imperfections (Steiner and Ortbauer, 1974). The Jaccarino-Walker model which takes into account the various surroundings of a Co atom in the region of... 

Fig. 19. XPS spectra of pure SmS and Gd substituted SmS. Curves (a) Experiment, curves (b) prediction of a model similar to that of Jaccarino and Walker curve (c) prediction of a random model assuming additive effects.

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