Specific Heat of Antiferromagnetic-like TbB41Si1.2, a B12 Icosahedral Boron-Rich Compound

A detailed measurement of the specific heat of the B12 icosahedral compound TbB41Si1.2 was made. TbB41Si1.2 crystals were grown by the floating zone method. Magnetic susceptibility measurements indicated that an antiferromagnetic-like transition occurs at 18 K in these crystals. TbB50 and isostructural TbB41Si1.2 are the first higher borides in which a magnetic transition was observed to occur. A hump in the specific heat is observed below 20 K, supporting the conclusion that a magnetic transition occurs in this system. An interesting magnetic field dependence was observed. Near the metamagnetic critical field of 31 kG, determined from magnetization measurements, the hump shifts to lower temperatures approaching 0 K. At 40 kG no hump is apparent. However, on further increasing the magnetic field to 55 and 70 kG, a new hump in the specific heat appears, indicating the possibility of the existence of an additional transition at high fields. Magnetization measurements of REB50 (RE=Tb, Dy, Ho, Er) up to 55 kG showed that the compounds saturated universally at around only half the saturation magnetization value of free rare-earth ions, which also indicated the likelihood of a further transition. The structure of the REB50type compounds has a one-dimensional character with the rare-earth atoms forming an alternating bond length chain along the c-axis. These results are quite interesting in the context with the recent high activity in Haldane gap related compounds in which fundamental quantum phenomena are observed.