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

A series of oxygen-rich phases with formal stoichiometry La2CoxCu1-xO4+ has been prepared. The excess of oxygen defects (0.060.20) that can be accommodated in the structure is higher than that found in the parent superconducting La2CuO4+ phase. The ac and dc susceptibility measurements reveal a rich magnetic phase diagram. The early members of the series (x0.25) order antiferromagnetically with localized magnetic moments per ion site of 0.5 B. 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 (0.5x0.90) 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 (+SR) technique. When Tf is approached from above, the + spin dynamics show a nonexponential relaxation described by a power-law dependence of the muon spin polarization, G(t)=A0e-(dt). The observed rapid growth of the correlation times c is reminiscent of the spin freezing process in Ising spin glasses. A continuous drop in the value of the exponent 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.