Elastic softening in HoFe₂Al₁₀ due to the quadrupole interaction under an orthorhombic crystal electric field

To investigate 4f electronic states in HoFe₂Al₁₀ under an orthorhombic crystal electric field (CEF), we measured the specific heat, magnetic susceptibility, magnetization, and elastic modulus of single-crystalline samples. We found elastic softening of the transverse elastic moduli C₅₅ and C₆₆ below 20 and 130 K, respectively. With further decreasing temperature, C₆₆ shows further elastic softening below 5 K. We observed two Schottky peaks in the specific heat at 2.2 and 20 K and small anisotropy of the magnetic susceptibility and magnetization in the paramagnetic region. By analyzing these experimental data, we obtained the CEF parameters of HoFe₂Al₁₀. From the analysis, we clarified that the softening of C₅₅ and C₆₆ originates from indirect quadrupole interactions of O_zx and O_xy, and propose that the overall CEF splitting is about 85 K. (author)