Accurate measurements of the susceptibility of a single crystal sphere of Dy(C2HsSO4)s9H~O have been made from I°K to 0.12°K. The temperature in these experiments was determined from the measured susceptibility of a single crystal sphere of cerium magnesium nitrate in good thermal contact with the specimen. The susceptibility showed considerable deviation from Curie's law which can be accounted for almost entirely by magnetic dipole-dipole interaction using a model of loosely coupled Ising chains. At 0.136°K there is a maximum in the susceptibility corresponding to a value of T* -~ O. 126 °. The specific heat of the isolated sphere of Dy(C~HsSOa)89H~O was measured between T* = 0.05 ° and T* ----0.7 ° using r-ray heating, electrical heating and audiofrequency relaxation heating. The results by the three methods were in excellent agreement.
Direct S-T* measurements were also made using fields of up to 10 kilogauss to extract 98% of the electronic entropy. S at the minimum T* was 1/3 R in 2. Preliminary measurements on a 2:1 ellipsoid have been made.
84. Saturation Magnetization and Molecular
Fields of the Acetylacetonates of Chromium and Iron. WARREN E. HENRY.