Thermodynamic properties of ferroics described by the transverse Ising model and their applications for CoNb2O6

The temperature and transverse field dependence of the entropy and the specific heat of ferroics in the quantum paramagnetic (QPa) state is investigated using the transverse Ising model (TIM) with different spin values within mean field and Gaussian spin fluctuation approximations. A maximum peak of the temperature dependence of the specific heat curves is enhanced in the QPa state due to spin fluctuations. This peak shifts to higher temperature region and its magnitude reduces with increasing transverse field. In addition, the temperature corresponding to this maximum depends linearly on the deviation of the transverse field from its critical value. The obtained specific heat qualitatively agrees with the experimental observation for the quasi-one-dimensional (1D) Ising ferromagnet CoNb$_2$O$_6$ in the QPa phase. It is also shown that the spin-1/2 three dimensional (3D) TIM clearly describes the specific heat of CoNb$_2$O$_6$ in the QPa states near the critical temperature. However, the spin-3/2 3D-TIM is more adequate than the spin-1/2 3D-TIM for describing the thermal behavior of CoNb$_2$O$_6$ in the QPa states at high fields and at elevated temperatures.