Abstract
The magnetic moment of a 3.930 cm diam spherical single crystal of Nd(C2H5SO4)3⋅9H2O (NES) has been measured below 0.5 °K after cooling by demagnetization from fields of 25, 15, 10, and 5 kG along the a crystal axis. Small amounts of irreversibility, believed to be due to the different rates of cooling of the 79.5% spin-free Nd3+ isotopes and the 20.5% Nd3+ isotopes with nuclear spin, was measured by ’’entropy gain,’’ enabling correction to true magnetic moments on isentropes. Equilibrium between the lattice and electron system was lost at fields of about 40 kG and temperatures below 0.5 °K. Entropy gain values were recorded for 0, 1000, 2500, and 5000 G on isoerstedics. An ’’Adiabatic Demagnetization-Sample Isolation-Calorimeter’’ was used to measure enthalpy as a function of entropy by means of electrical heating after demagnetization, thus enabling the derivation of thermodynamic temperature scales at zero and constant fields of 1, 2.5, and 5 kG as a function of entropy. The temperature-dependent component of the initial susceptibility, (χa,0−χa,T-ind), obtained from the magnetic moment data leads to the expression (χa,0−0.00576) = (0.3899/T)[1−0.0028/T −2.56×10−4/T2+5.06×10−6/T3−3.15×10−8/T 4] cm3/mole NES, which is valid down to 0.014 °K. The ’’high temperature’’ limiting magnetic component of the heat capacity was found to be CH=0(mag) =2.29×10−3/T2 −9.0×10−3/T3 gibbs/mole Nd3+. After correcting for the nuclear hyperfine structure term, 1.87×10−3 gibbs °K2/mole Nd3+, the dipole–dipole interaction term ADD=0.42×10−3 gibbs °K2/mole Nd3+. This, and the similar ADD=0.40×10−3 gibbs °K2/mole Nd3+ found from c-axis data, may be compared to the theoretical value, 0.396×10−3 gibbs °K2/mole Nd3+. The agreement shows that, aside from the hyperfine nuclear effect, the magnetic interactions are essentially ideal dipole–dipole. A heat capacity maximum occurred at 0.014 °K and zero field. The thermodynamic temperature, relative enthalpy, relative internal energy, heat capacity, and magnetic moment were tabulated as a function of entropy over the range 0.01–0.5 °K for zero field. Tabulations are also included for 1000, 2500, and 5000 G. Comments on the magnetic ground state of NES are given. Evidence for ’’frozen-in’’ magnetic structure at zero field and low temperature is presented.
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