Signatures of spin-glass behaviour in PrIr2B2 and 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 CaRh2B2-type orthorhombic crystal structure (space group Fddd). At low fields the dc magnetic susceptibility of PrIr2B2 exhibits a sharp anomaly near 46 K which is followed by an abrupt increase below 10 K with a peak at 6 K, and split-up in ZFC and FC data below 46 K. In contrast, the specific heat exhibits only a broad Schottky type hump near 9 K 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 tau = 3917 s. 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 Vogel-Fulcher law as expected for a canonical spin-glass system. The two spin-glass transitions occur at freezing temperatures T-f1 = 36 K and T-f2 = 3.5 K with shifts in the freezing temperatures per decade of frequency delta T-f1 = 0.044 and delta T-f2 = 0.09. An analysis of the frequency dependence of the transition temperature with critical slowing down, tau(max)/tau(0) = [(T-f - T-SG)/T-SG](-z nu), gives tau(0) = 10(-7) s and exponent z nu = 8, and the Vogel-Fulcher law gives an activation energy of 84 K for T-f1 and 27.5 K for T-f2. While z nu = 8 is typical for spin-glass system, the characteristic relaxation time tau(0) = 10(-7) s is very large and comparable to that of superspin-glass systems. An addition of C in PrIr2B2 leads to PrIr2B2C which forms in LuNi2B2C-type tetragonal structure (space group I4/mmm) and remains paramagnetic down to 2 K. 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 15 K above the ground state. The Sommerfeld coefficient gamma similar to 300 mJ mol(-1) K-2 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 PrRh2B2C.