Magnetothermodynamics of α-NiSO4·6H2O. V. Proton Spin Polarization Rate and Activation Enthalpy as a Function of Temperature and Field to 90 kG along the a Axis

Abstract

The rate of protonpolarization in α‐NiSO4·6H2O has been measured as a function of magnetic field and temperature by means of the spontaneous heat evolution and change of temperature during polarization. Fields of 90, 65, 40, 25, 15, and 11 kG were directed along the a crystal axis of the 4‐cm‐diam spherical single crystal at temperatures over the range 1.5–0.4°K. The rate observations have been represented by equations of the form, k A = Z exp (− ΔH / RT) . The activation enthalpies ΔH were found to correspond to the electron terms, W 1 –W 0 , previously derived from the authors' magnetothermodynamic measurements on this crystal. The proportionality factors, Z , in the above equation were field dependent with a minimum near 50 kG. The procedure involved cooling the high‐temperature proton distribution in zero field with subsequent application of the field at temperatures sufficiently low so that the rate of polarization was very small.