Magnetic properties of the thulium layered compound Tm2Al11B6: An AlB2-type analogue

(Tm2AlB6)-B-11 with the Y2ReB6-type crystal structure was synthesized, and the magnetic properties, specific heat, and magnetic entropy were investigated. Recent developments in rare earth aluminoboride compounds with two-dimensional boron layers have attracted interest due to the heavy fermion behavior superconductivity discovered in YbAlB4 and the multiple anomalies manifesting below the Neel temperatures in TmAlB4 attributed to be due to intrinsic building defects. (Tm2AlB6)-B-11 has a strong structural analogy to the AlB2-type structure similar to the REAlB4 compounds. Multiple antiferromagnetic transitions were found in (Tm2AlB6)-B-11 at T-N1 = 3.5 K and T-N2 = 2.8 K at zero field. Interestingly, the magnetic field dependence of the two transitions differed sharply. A low field of 4 kOe suppresses the low temperature transition, whereas T-N1 initially reduces with applied field but then exhibits a surprising robustness (i.e., steep phase boundary) versus the magnetic field above 20 kOe with the peak temperature hardly shifting up to 70 kOe. This indicates that a high field phase exists with extreme stability. The complex behavior is indicated to be influenced by different factors compared to related systems TmB4 and TmAlB4, e. g., crystal field effects or surprisingly strong magnetic interactions above T-N1 indicated by the specific heat behavior. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3073664]