Magnetothermodynamics of ferroelectric, ferroelastic, antiferromagnetic β-terbium molybdate. I. Heat capacity, entropy, magnetic moment of the electrically polarized form from 0.4 to 4.2 °K with fields to 90 kG along the c crystal axis

Abstract

The magnetic moment and heat capacity of a 1.170 cm diam spherical single crystal of electrically polarized β-Tb2(MoO4)3 have been measured at stabilized fields of 500, 1000, 1500, 2500, 5000, 10 000, 15 000, 25 000, 40 000, 65 000, and 90 000 G, along the c crystal axis, and over the range 0.4–4.2 °K. The heat capacity at zero field was also measured over the same temperature range. The temperature dependent magnetic moment reached saturation at fields of 65 and 90 kG. Its value, 43 050 G cm3/mole Tb3+, corresponds to a splitting factor of gc= 15.41. The small temperature independent magnetic susceptibility χTind = 0.0070 cm3/mole Tb3+ indicates that the upper levels of the 7F6 multiplet are well above the ground doublet. As expected, any effect of reversing the magnetic field with respect to the electrical polarization was too small to be detected. The T3 lattice heat capacity was evaluated at saturating fields of 65 and 90 kG since the electronic heat capacity vanished at these high fields owing to the high g value. This also enabled evaluation of the zero entropy of the electron–lattice systems, and the effective hyperfine field which causes nuclear spin polarization. It is known from x-ray data that Tb3+ has two equally divided environments in β-Tb2(MoO4)3. The magnetocalorimetric data indicate two largely independent electronic systems. The data have been separated into the two electronic, the nuclear, and lattice components of the heat capacity. Smoothed correlated values of the heat capacity, entropy, enthalpy, internal energy, magnetic moment and its isoerstedic temperature coefficient, differential isothermal magnetic susceptibility, and the isothermal work of magnetization have been tabulated over the range 0–90 kG and 0.5–4.2 °K. Several magnetic moment measurements were obtained over the range 0.5–1.4 °K with fields to 25 kG along the a crystal axis.