The Effect of Pd, Pt, and Co on the Superconducting and Magnetic Ordering Temperatures inRNi₂B₂C (R=Tm and Er)

We have investigated the nearly isotropic superconducting intermetallicRNi_2-xM_xB₂C system (M=Pd, Pt, and Co,x=0.05-0.2) by complementary experimental techniques. For TmNi₂B₂C,T_C=10.7 K and the antiferromagnetic (AFM) order of the Tm spins develops belowT_N=1.48 K. Pt as a dopant affectsT_Cvery little, whereas, forM=Pd,T_Cis reduced to 9.9 and 9.1 K forx=0.05 and 0.1, respectively. For TmNi₂B₂C and TmNi_1.9Pt_0.1B₂C, theH_c1values exhibit a maximum at 3 K. While the AFM transition for TmNi₂B₂C atT_Nis clearly identified by specific heat measurements, no specific heat anomaly was observed for TmNi_1.9Pd_0.1B₂C down to 0.8 K, indicating that Pd as a dopant drastically reducesT_CandT_Nof the system. For ErNi₂B₂C,T_C=10.9 K andT_N=6.3 K (T_C>T_N). In ErNi_2-xM_xB₂C, both Pt and Pd as dopants (x=0.2) reduceT_Cto ~8 K and shiftT_Nto ~11 K. ForM=Co andx=0.05T_Cis reduced to 4.8 K andT_Nincreases to 8.2 K. For higher Co concentrations (x=0.2 and 0.5), the compounds are not superconducting, andT_Nincreases to 10.9 and 14.8 K respectively. It appears that for ErNi_2-xM_xB₂C (i) forx=0.2 the sameT_N~11. K is obtained regardless ofM, and that (ii) in contrast toR=Tm, all dopants studied lead to a reduction ofT_Cand to an increase ofT_Nof the Er sublattice, thusT_C<T_N. The delicate interplay between superconductivity and AFM in this system is discussed.