Ferromagnetic ordering in CeIr2B2: Transport, magnetization, specific heat, and NMR studies
ABSTRACT We present a complete characterization of the ferromagnetic system CeIr2B2 using powder x-ray diffraction (XRD), magnetic susceptibility χ(T), isothermal magnetization M(H), specific heat C(T), electrical resistivity ρ(T,H), and thermoelectric power S(T) measurements. Furthermore, 11B 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 χ(T), C(T), and ρ(T) data confirm bulk ferromagnetic ordering with Tc=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 γ = 73(4) mJ/mol K2, which is much smaller than the previously reported value of γ = 180 mJ/mol K2 deduced from the specific heat measurement down to 2.5 K. For LaIr2B2, γ = 6(1) mJ/mol K2, which implies the density of states at the Fermi level D(EF)=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 4 f correlations in CeIr2B2 is ≈12. The Kondo temperature TK∼4 K is estimated from the jump in specific heat of CeIr2B2 at Tc. Both C(T) and ρ(T) data exhibit a gapped-magnon behavior in the magnetically ordered state with an energy gap Eg∼3.5 K. The ρ 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∼7μV/K), indicating a weak Kondo interaction. A shoulder in S(T) at about 30 K, followed by a minimum at ∼10 K, is attributed to crystal electric field effects and the onset of magnetic ordering. 11B NMR line broadening provides strong evidence of ferromagnetic correlations below 40 K.