Spin Glass Magnetism in the Oxygen-Rich La2CoxCu1−xO4+δ Layered Oxides: Magnetic Susceptibility and Muon-Spin-Relaxation Studies

Abstract

A series of oxygen-rich phases with formal stoichiometry La2CoxCu1−xO4+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 La2CuO4+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 TN 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 La2Co0.5Cu0.5O4.18 (Tf=18 K) and La2Co0.75Cu0.25O4.16 (Tf=30 K) were also investigated by the muon spin relaxation (μ+SR) technique. When Tf 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 Tf to 0.5 (square root exponential) at T∼1.3 Tf, and finally approaching 1/3 very close to Tf. 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.