Crystal and thermodynamic properties of Tb2Ni2X, (X = Al, Ga)

Abstract

The crystal and thermodynamic properties of ${\text{Tb}}_2{\text{Ni}}_2\text{X}$ (X = Al, Ga) are reported through measurements of X-ray diffraction (XRD), magnetic susceptibility, $\chi \left(T\right)$, magnetization, $M\left({\mu }_{0}H\right)$ and heat capacity, $C_p\left(T\right)$. XRD pattern analysis confirms the orthorhombic ${\text{W}}_2{\text{CoB}}_2$-type structure with the space group of $Immm$. $\chi \left(T\right)$ at high temperature for both compounds follows the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent Tb ion. The low-temperature $\chi \left(C_p\right)$ data indicate that both compounds order antiferromagnetically at $T_N$ = 41 K (40.4 K) and 41.5 K (41.4 K) for Al and Ga compounds, respectively. $C_p\left(T\right)$ data of the nonmagnetic counterparts ${\text{Y}}_2{\text{Ni}}_2\text{X}$ (X = Al, Ga) are well described by the Debye model giving a Debye temperature, ${\theta }_D$ = 236.9(4) K and 225.3(2) K for Al and Ga compounds, respectively. The low-temperature part of the $4f$-magnetic contribution to the total heat capacity, $C_{4f}\left(T\right)$ can be described by the antiferromagnetic spin-wave dispersion, giving an energy gap ${\Delta }_{sw}$ = 47(3) K and 26(2) K for Al and Ga compounds, respectively. The $4f$ – magnetic entropy $S_{4f}\left(T\right)$ for both compounds reaches the value of $2Rln\left(2\right)$ close to their respective $T_N$ values.