Abstract
High-quality KFe2As2 (K122) single crystals synthesized by different techniques have been studied by magnetization and specific heat (SH) measurements. The adopted phenomenological analysis of the normal state properties shows that there are two types of samples both affected by disordered magnetic phases: (i) cluster-glass (CG) like or (ii) Griffiths phase (G) like. For (i) at low applied magnetic fields the T-dependence of the zero-field-cooled (ZFC) linear susceptibility chi(1) exhibits an anomaly with an irreversible behavior in ZFC and field-cooled (FC) data. This anomaly is related to the freezing temperature T-f of a CG. For the investigated samples the extrapolated T-f to B = 0 varies between 56 and 90 K. Below T-f we observed a magnetic hysteresis in the field dependence of the isothermal magnetization M(B). The frequency shift of the freezing temperature delta T-f = Delta T-f/T-f Delta(ln)]similar to 0.05 has an intermediate value, which provides evidence for the formation of a CG-like state in the K122 samples of type (i). The frequency dependence of their T-f follows a conventional power-law divergence of critical slowing down tau = tau(0)[T-f(n)/T-f(0) with the critical exponent zn/ approximate to 10 and a relatively long characteristic time constant tau(0) approximate to 6.9 x 10(-11) s also supporting a CG behavior. The large value of the Sommerfeld coefficient obtained from SH measurements of these samples was related to the magnetic contribution from a CG. Samples from (ii) did not show a hysteresis behavior for chi(1)(T) and M(B). Below some crossover temperature T approximate to 50-80K a power-law dependence in the chi(1)/TlG-1, with a non-universal lG was observed, suggesting a quantum G-like behavior. In this case chi(1) and M(B) can be scaled using the scaling function M-s(T, B)=B1-lambda GY(mu B=k(b)T) with the scaling moment mu similar to 3.5 mu(B). The same non-universal exponent was found also in SH measurements, where the magnetic contribution C/T proportional to T lambda G-1. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim