Memory Effect And Magnetic Relaxation In Ising Spin System Ca₃Co₂O₆and The Doped Compounds

The spin-chain compound Ca3Co2O6is another important system presentinggeometrical frustration. Some important aspects have been reported, for instance, strongIsing character, magnetization quantum tunneling, thermoelectric characteristic, H/Mcurves of Mn-doping compounds and so on. The most interesting issues are the PDA stateand magnetizations steps.We report memory effect and magnetic relaxation in Ising spin-chain compoundCa3Co2O6and doped compounds Ca3Co2-xTxO6(T=Mn, Fe, Al, Mg) with x=0.02and0.04,and Ca3-xAxCo2O6(A=Sr, Na) with x=0.06and0.12.Polycrystalline samples of Ca3Co2O6and doped compounds Ca3Co2-xTxO6(T=Mn, Fe,Al, Mg) with x=0.02and0.04, and Ca3-xAxCo2O6(A=Sr, Na) with x=0.06and0.12weresynthesized using citrate-gel method. The XRD data shows that all the samples are singlephase with rhombohedral structure. Recent magnetization measurements showed that timevariation behavior of magnetization is not much modified by small amount of doping inCa3Co2-xTxO6(T=Ga, Mg and Al) compared with Ca3Co2O6, implying that spin-glassbehavior is mainly associated not with chemical disorder but with spin frustration due tocompeting magnetic interactions.In this paper, we report memory effect, which further supports the spin-glass scenario,and the magnetic relaxation phenomenon. Magnetization relaxation is also studied bytime-dependent magnetization measurements. Long-time magnetic relaxation is observed.The initial stage of relaxation can be described approximately by exponential function,whereas, the later relaxation gradually follows the stretched exponential function. Themagnetic relaxation is indeed influenced by disorder induced by small amount of doping.Among the present doping elements, Fe has a biggest influence upon the relaxation. Thisis probably because Fe ion is an "AFM" ion, substitutes the high-spin Cotrigat the trigonalsite and is antiferromagnetically coupled with the other ions, so that it has a stronginfluence on the FM character of the spin chain.We plot the temperature dependence of the first-derivative ESR spectra ofCa3Co1.98Fe0.02O6and Ca3Co1.96Fe0.04O6, which exhibit strong impurity spin resonance of Fe3+. While the temperature dependence of the first-derivative ESR spectra ofCa3Co1.6Fe0.4O6exhibits strong impurity spin resonance of Co3+. What we doped differentamount of Fe, measured different formants. Fe is an isolated particle with small amount ofdoping, which have weak interactions between the free particles. Otherwise, when dopedof Fe with x=0.4, they are not isolated particles, but Fe and Co have formed the interactionbetween lattices. So the small amount of doping compounds exhibit spin resonance of Fe3+,while the ESR spectra of Ca3Co1.6Fe0.4O6exhibits strong impurity spin resonance ofCo3+.