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

✍ Scribed by Alexandros Lappas; Kosmas Prassides; Fredy N. Gygax; Alexander Schenck

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 393 KB
• Volume: 145
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1999.8240

✦ Synopsis

A series of oxygen-rich phases with formal stoichiometry La 2 Co x Cu 1؊x O 4؉ has been prepared. The excess of oxygen defects (0.064 40.20) that can be accommodated in the structure is higher than that found in the parent superconducting La 2 CuO 4؉ phase. The ac and dc susceptibility measurements reveal a rich magnetic phase diagram. The early members of the series (x40.25) order antiferromagnetically with localized magnetic moments per ion site of 0.5 B . The ordering temperature T N is rapidly reduced and the boundary of the paramagnetic-toantiferromagnetic (AF) phase transition is smeared out as the cobalt content increases from x ‫؍‬ 0.25 to 0.5. Further increase of the cobalt content (0.54x40.90) 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 La 2 Co 0.5 Cu 0.5 O 4.18 (T f ‫؍‬ 18 K) and La 2 Co 0.75 Cu 0.25 O 4.16 (T f ‫؍‬ 30 K) were also investigated by the muon spin relaxation ( ؉ SR) technique. When T f is approached from above, the ؉ spin dynamics show a nonexponential relaxation described by a power-law dependence of the muon spin polarization, G(t) ‫؍‬ A 0 e ؊( d t) . 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 T f to 0.5 (square root exponential) at T&1.3 T f , and 5nally 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.