Paramagnetic neighborhood

Our conclusion is, therefore, that the Weiss constant is a measure, in these systems, of the coordination number with respect to paramagnetic ions of the same charge. In the corundum structure, thanks to the sharp change in direction shown by the plot of paramagnetic neighborhood against number of atom layers, it is possible to determine when an oxide layer is only three or four atom layers thick. 

Below 6 per cent nickel the first observation is that the Weiss constant is zero. The form of the susceptibility isotherm is thus in the case of nickel in no way related to the exchange interaction between adjacent nickel ions. This is not to say that the nickel ions are at infinite magnetic dilution. For nickel in massive nickel oxide the exchange integral, the paramagnetic neighborhood (z), and the number of unpaired electrons are smaller than they are for chromium ions in massive chromia. The quantity. A, is understandably smaller for the case of nickel, and it... 

Interpretation of these data must wait for a better understanding of the phenomena. But some preliminary statements may be made. It will be noted that the Weiss constant is not zero in the most dilute gels. This probably means that there must be a certain degree of interaction between adjacent chromium atoms, or, to put it another way, there must be a certain minimum value to the paramagnetic neighborhood, in order... 

As the gel is dehydrated the Weiss constant slowly rises. This must mean either that the chromium-chromium distance is decreasing slightly, or perhaps that the paramagnetic neighborhood is increasing. Some results on iron oxide gels to be described later indicate that the latter explanation may be more nearly correct. 

Fig. 28. Paramagnetic susceptibilities of (a) yttrium and (b) gadolinium ferrogarnets. Solid lines are theoretical curves based on molecular field model and empirically determined molecular field constants. Dashed lines are experimental curves indicating effects of short-range order in the neighborhood of the Curie temperature. (After A16onard (5).)...

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