Ferromagnetic ordering in CeIr2B2: Transport, magnetization, specific heat, and NMR studies

We present a complete characterization of the ferromagnetic system CeIr2B2 using powder x-ray diffraction (XRD), magnetic susceptibility chi(T), isothermal magnetization M(H), specific heat C(T), electrical resistivity rho(T, H), and thermoelectric power S(T) measurements. Furthermore, B-11 NMR study was performed to probe the magnetism on a microscopic scale. Rietveld refinement of powder XRD data confirms that CeIr2B2 crystallizes in CaRh2B2-type orthorhombic structure (space group fddd). The chi(T), C(T), and rho(T) data confirm bulk ferromagnetic ordering with T-c = 5.1 K. Ce ions in CeIr2B2 are in a stable trivalent state. Our low-temperature C(T) data measured down to 0.4 K yield a Sommerfeld coefficient gamma = 73(4) mJ/mol K-2, which is much smaller than the previously reported value of gamma = 180 mJ/mol K-2 deduced from the specific heat measurement down to 2.5 K. For LaIr2B2, gamma = 6(1) mJ/mol K-2, which implies the density of states at the Fermi level D(E-F) = 2.54 states/(eV f.u.) for both spin directions. The renormalization factor for quasiparticle density of states and hence for quasiparticle mass due to 4f correlations in CeIr2B2 is approximate to 12. The Kondo temperature T-K similar to 4 K is estimated from the jump in specific heat of CeIr2B2 at T-c. Both C(T) and rho(T) data exhibit a gapped-magnon behavior in the magnetically ordered state with an energy gap E-g similar to 3.5 K. The rho data as a function of magnetic field H indicate a large negative magnetoresistance (MR) which is highest for T = 5 K. While at 5 K the negative MR keeps on increasing up to 10 T, at 2 K an upturn is observed near H = 3.5 T. On the other hand, the thermoelectric power data have small absolute values (S similar to 7 mu V/K), indicating a weak Kondo interaction. A shoulder in S(T) at about 30 K, followed by a minimum at similar to 10 K, is attributed to crystal electric field effects and the onset of magnetic ordering. B-11 NMR line broadening provides strong evidence of ferromagnetic correlations below 40 K.