Calorimetric Determination of Isothermal Entropy Changes in High Magnetic Fields at Low Temperatures. CoSO4·7H2O

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Abstract
An apparatus has been described for the detailed calorimetric measurement of the entropy increase accompanying isothermal demagnetization from high magnetic fields. Integrated automatic heat input controlled by a helium gas thermometer, in thermal contact with the sample, divided by the selected temperature, gives a detailed description of the entropy increase. Investigation of a powdered sample of CoSO4·7H2O has shown that application of fields of the order of 100 kOe near 1°K extracts an entropy of R ln2 as an upper limit from the electronic system of this substance. No irreversible effects were found. The entropy was found to be a function of Hi/T, over the range 1.236 to 4.211°K and from 0 to 94 500 Oe. Hi refers to the magnetic field inside the sample. The resistance of a Pt(91–92%)—W(9–8%) alloy wire used for the noninductively wound calorimetric heater was investigated. The magnetoresistance was positive, very small, and linear with field at all fields. (1/R) (dR/dT) = —5×10—5 deg—1 and (1/R) (dR/dH) = +2.5×10—9 Oe—1, range 1.2 to 4.2°K and from 0 to 95 kOe. Equations for the effect of a magnetic field on the helium gas thermometer are given.