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MAGNETIC CONTRIBUTION TO THERMAL EXPANSION 5135 quantItIes. The anomalous behavior of (oa/a)eq [or g( T) ] at T M appears to be more pronounced for B= 1 than for the other two values of B.In the strong correlation limit U in Eq. (1) is replaced by an effective interaction U e ff.t2 This U_1f depends on the details of the energy band and is shown to depend on the bandwidth. 12 Thus U_1f would become dependent upon the lattice spacing. Therefore, for materials that come into the strong correlation regime the results given above would be modified to some extent; but it is anticip:lted that the above theory contains the essential features of a more elaborate treatment.

The initial pressure derivative of the Curie temperature of ZrZn 2 has been measured under hydrostatic conditions, yielding a value of -(1.95 ±0.1) °K kbar -1 for a sample with a Curie temperature of (21.5 db0.5)°K. This is in good agreement with the values of -1.8 and -2.4°K kbar -1 predicted by Wohlfarth using the model of a very weak itinerant ferromagnet and two different methods of calculation. The present measurement allows an estimate of the effective degeneracy temperature for ZrZno of 300°K and an estimate for the initial differential susceptibility of 6.2X10""" 6 emu g -1 Oe _1 .