Physical and magnetic properties of frustrated triangular-lattice antiferromagnets R₃Cu (R = Ce, Pr)

We report the synthesis, crystal structure, and thermodynamic properties of the two binary laves phases Ce₃Cu and Pr₃Cu, which crystallize in the tetragonal I4/mmm (space group: 139). We have investigated the compounds using dc magnetic susceptibility χ_dc(T), ac magnetic susceptibility χ_ac(T), isothermal magnetization M(B) and specific heat C_p(T) measurements. Both compounds show features of strong geometrical frustration, which is driven by the arrangement of the rare-earth ions on triangular lattice sites. Ce₃Cu shows a phase transition at T_N = 5 K and its M(B) at 2 K reveals the presence of a metamagnetic transition near 2 T. Pr₃Cu, on the other hand, shows a spin-glass behaviour with a freezing temperature of T_f1 ≃ 12 K and with an irreversible behavior in zero-field-cooled (ZFC) and field-cooled (FC) χ_dc(T) below about 13 K. The real part of χ_ac(T) reveals a frequency dependence of T_f. The result, however shows the presence of another frequency-dependent phase below T_f2 ≃ 9.35 K, a feature which signals a re-entrant spin-glass behaviour in the compound. Furthermore, the frequency dependence of T_f1 follows the Vogel-Fulcher law, with an activation energy E_a∕k_B = 5.3 K. This observation along with the estimated relative shift in T_f1 per decade in frequency δT_f = 0.0105, irreversible behaviour in ZFC-FC χ_dc(T) and a slow decay in thermo-remnant magnetization provide supports for the formation of a cluster spin-glass in Pr₃Cu. In addition, we observe the enhancement of the Sommerfeld coefficient, γ in both compounds with values of 0.1892(5) J/(mol_Ce K²) and 0.5078(1) J/(mol_Pr K²). The estimation of the Sommerfeld-Wilson ratio W_R yield values of 0.576 and 1.17 for Ce₃Cu and Pr₃Cu, respectively.