Magnetic phase transitions and magnetocaloric effect in Ho2Co17: A mean-field with Magneto-Voulme coupling study

In this work, we present a theoretical study on the magnetic, magnetocaloric properties and the nature of phase transitions in Ho₂Co₁₇ compound. We have used the mean- field Bean- Rodbell model, with taking into account the magneto-volume coupling, in our calculation. The temperature and magnetic field-dependences of the magnetization, heat capacity, entropy, isothermal change in entropy ΔS_m, adiabatic temperature change ΔT_ad and the relative cooling powers are reported. An interesting feature of a first-order phase transition (FOPT), around 650 K, followed by a second order (SOPT) one at the Curie temperature is reported. We have used the direct subtraction method, of the entropies at different fields, to calculate of both ΔS_m and ΔT_ad. The maximum ΔS_m values, for a 5 T field change, are around 5.98 J/mol. K and 0.15 J/mol. K at the FOPT and the SOPT respectively. At the FOPT the adiabatic temperature change is around 8.7 K for a 5 T field change. In order to calculate the adiabatic change in temperature, the electronic and lattice contributions to the total heat capacity must be determined. These two quantities are dependent on the electronic density of states and Debye temperature respectively. For this purpose, we performed calculation on the elastic constants C_ij, elastic moduli, and the electronic density of states (DOS) using the VASP code. For example, the Voigt bulk and shear moduli, and the Debye temperature are found to be 85.518 and 55.23 GPa, and 346.80 K respectively.