Signatures of spin-glass behaviour in PrIr₂B₂and heavy fermion behaviour in PrIr₂B₂C

The magnetic and transport properties of PrIr₂B₂ and PrIr₂B₂C have been investigated by dc and ac magnetic susceptibility, specific heat, electrical resistivity and magnetoresistance measurements. PrIr₂B₂ forms in CaRh₂B ₂-type orthorhombic crystal structure (space group Fddd). At low fields the dc magnetic susceptibility of PrIr₂B₂ exhibits a sharp anomaly near 46K which is followed by an abrupt increase below 10K with a peak at 6K, and split-up in ZFC and FC data below 46K. In contrast, the specific heat exhibits only a broad Schottky type hump near 9K which indicates that there is no long range magnetic order in this compound. The thermo-remanent magnetization is found to decay very slowly with a mean relaxation time τ =3917s. An ac magnetic susceptibility measurement also observes two sharp anomalies; the peak positions strongly depend on the frequency and shift towards high temperature with an increase in frequency, obeying the VogelFulcher law as expected for a canonical spin-glass system. The two spin-glass transitions occur at freezing temperatures T_f1=36K and T_f2=3.5K with shifts in the freezing temperatures per decade of frequency δT _f1=0.044 and δT_f2=0.09. An analysis of the frequency dependence of the transition temperature with critical slowing down, τ_max/τ₀=[(T_fT_SG)/T _SG]^z, gives ₀=10⁷s and exponent z=8, and the VogelFulcher law gives an activation energy of 84K for T_f1 and 27.5K for T_f2. While z=8 is typical for spin-glass system, the characteristic relaxation time τ₀=10⁷s is very large and comparable to that of superspin-glass systems. An addition of C in PrIr ₂B₂ leads to PrIr₂B₂C which forms in LuNi₂B₂C-type tetragonal structure (space group I4/mmm) and remains paramagnetic down to 2K. The specific heat data show a broad Schottky type anomaly, which could be fairly reproduced with CEF analysis which suggests that the ground state is a CEF-split singlet and the first excited state singlet is situated 15K above the ground state. The Sommerfeld coefficient 300mJmol¹K² of PrIr₂B₂C is very high and reflects a heavy fermion behaviour in this compound. We believe that the heavy fermion state in PrIr₂B₂C has its origin in low lying crystal field excitations as has been observed in PrRh₂B ₂C.