Spin glass magnetism in the oxygen-rich La2CoxCu1-xO4+delta layered oxides: Magnetic susceptibility and muon-spin-relaxation studies

A series of oxygen-rich phases with formal stoichiometry La2CoxCu1-xO4+delt a has been prepared. The excess of oxygen defects (0.06 less than or equal to delta less than or equal to 0.20) that can be accommodated in the struct ure is higher than that found in the parent superconducting La2CuO4+delta p hase. The ac and de susceptibility measurements reveal a rich magnetic phas e diagram. The early members of the series(x less than or equal to 0.25) or der antiferromagnetically with localized magnetic moments per ion site of 0 .5 mu(B). The ordering temperature T-N is rapidly reduced and the boundary of the paramagnetic-to-antiferromagnetic (AF) phase transition is smeared o ut as the cobalt content increases from x = 0.25 to 0.5. Further increase o f the cobalt content (0.5 Ix 10.90) leads to suppression of the AF state an d the appearance of a spin glass at very low temperatures. This is attribut ed 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 (T-f = 18 K) and L2Co0.75 Cu0.25O4.16 (T-f = 30 K) were also investigated by the muon spin relaxation (mu(+)SR) technique. When T-f is approached from above, the mu(+) spin dyn amics show a nonexponential relaxation described by a power-law dependence of the muon spin polarization, G(t) = A(0)e(-(lambda dt)beta). The observed rapid growth of the correlation times tau(c) is reminiscent of the spin fr eezing process in Ising spin glasses. A continuous drop in the value of the exponent beta is also encountered, changing from 1.0 (simple exponential) at T similar to 3.3 T-f to 0.5 (square root exponential) at T similar to 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. (C) 1 999 Academic Press.