Spin glass magnetism in the oxygen-rich La₂Co_xCu_1-xO_4+δ layered oxides: Magnetic susceptibility and muon-spin-relaxation studies

A series of oxygen-rich phases with formal stoichiometry La₂Co_xCU_1-xO_4+δ has been prepared. The excess of oxygen defects (0.06 ≤ δ ≤ 0.20) that can be accommodated in the structure is higher than that found in the parent superconducting La₂CuO_4+δ phase. The ac and dc susceptibility measurements reveal a rich magnetic phase diagram. The early members of the series (x ≤ 0.25) order antiferromagnetically with localized magnetic moments per ion site of 0.5 μB. The ordering temperature T_N is rapidly reduced and the boundary of the paramagnetic-to-antiferromagnetic (AF) phase transition is smeared out as the cobalt content increases from x = 0.25 to 0.5. Further increase of the cobalt content (0.5 ≤ x ≤ 0.90) leads to suppression of the AF state and the appearance of a spin glass at very low temperatures. This is attributed to the increased degree of structural and electronic disorder among (Co/Cu) sites, which leads to frustration of the nearest-neighbor (nn) AF bonds. The spin glass phases of the La₂Co_0.5Cu_0.5O_4.18 (T_f = 18 K) and La₂Co_0.75Cu_0.25O_4.16 (T_f = 30 K) were also investigated by the muon spin relaxation (μ⁺SR) technique. When T_f is approached from above, the μ⁺ spin dynamics show a nonexponential relaxation described by a power-law dependence of the muon spin polarization, G(t) = A₀e^-(λdt)β. The observed rapid growth of the correlation times τc is reminiscent of the spin freezing process in Ising spin glasses. A continuous drop in the value of the exponent β is also encountered, changing from 1.0 (simple exponential) at T ∼ 3.3 T_f to 0.5 (square root exponential) at T ∼ 1.3 T_f, and finally approaching 1/3 very close to T_f. A variety of chemical systems that undergo a spin glass transition are governed by spin dynamics that follow a universal picture similar to the one encountered here.