Heat Capacity, Entropy, and Magnetic Moment of Single-Crystal CuSO4·5 H2O with Fields to 90 kG ‖ to the α Magnetic Axis

Abstract

The heat capacity and magnetic moment of CuSO₄·5 H₂O have been measured over the range 0.4°—4 2°K in stabilized magnetic fields of 0, 5, 10, 20, 30, 50, 70, and 90 kG. The field was directed along the α magnetic axis of the 3.70‐cm‐diam, spherical, single crystal. A sensitive carbon thermometer was used to measure the variation of temperature with magnetic field in a calorimetric apparatus at nearly constant entropy. Very small corrections for the entropy change of the empty calorimeter and for entropy leakage enabled accurate tabulation of entropy changes in CuSO₄·5 H₂O ove the experimental range. Magnetic saturation was used as a zero reference for the electronic entropy and the total electronic entropy above zero was found to be R ln2 when the ``tail'' above 4°K was considered. The data add further support to the suggestion of Geballe and Giauque that the two types of Cu⁺⁺ per unit cell act as essentially independent magnetic systems. Magnetic‐moment values during representative adiabatic demagnetizations to zero field are given. Values for several low‐entropy adiabatics fall on a straight line between 4 and 13 kG. An extrapolation of this line intersects the M axis at a point appreciably above one‐half the total saturation value. This fact can be explained if the g value of the ferromagnetic system is 2.101 and that of the antiferromagnetic system is 2.051 at low temperatures. In any case, the over‐all g value is 2.076. At the higher fields and lower temperatures it was necessary to consider a small amount of entropy removed by nuclear polarization. All of the observations apply to the spherical sample uncorrected for the demagnetizing effect.