Magnetic and transport properties of Yb₅T₄Ge ₁₀ (T=Co, Rh, Ir)

The magnetic and transport properties of Yb₅T₄Ge ₁₀ and Lu₅T₄Ge₁₀ (T=Co, Rh, Ir) have been investigated. All Yb compounds are metals with the paramagnetic state above 1.2 K. Some magnetic transition is commonly expected below 1.2 K from the result of the specific heat. Single crystal of Yb₅Rh ₄Ge₁₀ has been grown by the Bridgman method. Magnetic susceptibility of Yb₅Rh₄Ge₁₀ shows a strong anisotropy with the easy-plane type within the c-plane. At 2 K, magnetization of Yb₅Rh₄Ge₁₀ for H∥a gradually increases as increasing the magnetic field and saturates to a constant value of 4.5μ_B/f.u. in H=50kOe in contrast to magnetization for H∥c, which keeps increasing to 0.9μ_B/f.u. in H=50kOe. The magnetic part of the specific heat for Yb₅Rh₄Ge₁₀ shows the Schottky anomaly centered at about 60 K due to the CEF effect, which acts on the eight-hold multiplet of Yb³⁺ ion. Furthermore, a broad peak appears at low temperatures by the application of the magnetic field. Height of this peak is very low as compared to the Schottky peak expected for the two-levels split from the ground state doublet for five Yb³⁺ ions in formula unit. The magnetic entropy gain in the magnetic field for Yb₅Rh ₄Ge₁₀ at 300 K does not attain to 5R ln8, therefore, there is a possibility that the mixed valence state consisted of three or four Yb³⁺ ions among five Yb ions in formula unit is realized in all temperature range. A similar situation is found in Yb₅Co ₄Ge₁₀ and Yb₅Ir₄Ge₁₀.