We report the structure and physical properties of two quasi-two-dimensional triangular antiferromagnetic materials, Co0.66Al2Se3.53 and Ni0.61Al2Se3.55, which show highly magnetically frustrated characters. Powder X-ray diffractions demonstrate that Co0.66Al2Se3.53 and Ni0.61Al2Se3.55 possess identical space group of P-3m1 with lattice parameters a = 3.8089(1) Å, c = 12.676(1) Å and a = 3.7880(1) Å, c = 12.650(1) Å, respectively. Analyzing the susceptibility data of Co0.66Al2Se3.53 reveal a Curie Weiss temperature of −216 K, and a spin-freezing transition temperature of 4.5 K, giving a frustration index f = −θcw/Tf ≈ 48. Ni0.61Al2Se3.55 possesses an effective moment of 2.38 µB, a Curie–Weiss temperature of −62 K with no sign of spin-freezing transition down to 2 K. The AC susceptibility data of Co0.66Al2Se3.53 suggest a spin glass-like transition, but no intersite mixing between Co2+ and Al3+ was observed from the X-ray photoelectron spectroscopy measurements.
High temperature spin-freezing transition in pyrochlore Eu2Ti2O7: A new observation from ac-susceptibility
