Magnetic anisotropy and spin-glass behavior in single crystalline U ₂PdSi₃

We present the magnetic and transport properties of single crystalline U₂PdSi₃ measured with the magnetic field (H) (or measuring current, I ) applied along two typical crystallographic directions, i.e. H ⊥ c-axis and H ∥ c-axis (or I ⊥ c-axis and I ∥ c-axis). For both directions, a spin-glass state is confirmed to form at low temperature with the same spin freezing temperature T_f (=11.5 K), initial frequency shift δT_f (=0.023) and activation energy E_a/k _B (=90.15 K) in zero dc field. Strong anisotropy in magnetic and transport behavior is found to be a significant feature of U ₂PdSi₃. The unusual ferromagnetic-like anomaly in ac susceptibility and dc magnetization curves around T_m = 71 K is observed in the case of H ∥ c-axis but not in the cases of H ⊥ c-axis. The characteristic temperature T_ir, below which evident irreversible magnetism originated from random spin freezing can be observed, shows much stronger field dependence for H ⊥ c-axis than for H ∥ c-axis. Moreover, an unusual finding is that the electrical resistivity measurements indicate the formation of magnetic Brillouin-zone boundary gaps and much larger magnetic scattering for I ⊥ c-axis, while the coherent-Kondo-effect-like behavior is obvious for I ∥ c-axis. We also emphasize that no resistivity minimum can be detected down to 2.5 K for either direction. The observed magnetic and transport behaviors are compared with those in polycrystalline U ₂PdSi₃ and other 2:1:3 intermetallic compounds.