Magnetic and thermal properties of TmV₂Al₂₀ single crystals

The magnetization and specific heat of TmV₂Al₂₀ single crystals were measured in the temperature range from 0.5 to 300K in external magnetic fields up to 7 T. TmV₂Al₂₀ was found to be paramagnetic above 0.5 K. Clear magnetic anisotropy was observed along the three principal crystallographic axes in the field above 1 T at 0.5 K. The magnetically easy axis is along the [100] direction, and the hard axis is along the [111] direction. On cooling below 2K in zero external field, the magnetic part of specific heat divided by temperature, C_mag=T, increases up to 6 J=molK2 near 0.6 K. The magnetic entropy in zero field reaches R ln 5 near 10 K, suggesting that the ground state of Tm³⁺ ions is a nonmagnetic doublet state with the first excited state of a magnetic triplet state nearby (a pseudo-fivefold degenerate state). The experimental results were reproduced by the crystalline electric field calculations, and an energy level scheme was proposed. The enhanced value of C_mag=T in the lowest temperature region in zero field was explained by assuming an energy splitting of the doublet ground state.