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

Abstract

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 χdc(T), ac magnetic susceptibility χac(T), isothermal magnetization M(B) and specific heat Cp(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 TN = 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 Tf1 ≃ 12 K and with an irreversible behavior in zero-field-cooled (ZFC) and field-cooled (FC) χdc(T) below about 13 K. The real part of χac(T) reveals a frequency dependence of Tf. The result, however shows the presence of another frequency-dependent phase below Tf2 ≃ 9.35 K, a feature which signals a re-entrant spin-glass behaviour in the compound. Furthermore, the frequency dependence of Tf1 follows the Vogel-Fulcher law, with an activation energy Ea∕kB = 5.3 K. This observation along with the estimated relative shift in Tf1 per decade in frequency δTf = 0.0105, irreversible behaviour in ZFC-FC χdc(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/(molCe K2) and 0.5078(1) J/(molPr K2). The estimation of the Sommerfeld-Wilson ratio WR yield values of 0.576 and 1.17 for Ce3Cu and Pr3Cu, respectively.