Anomalous Magnetic Properties Of Quasi-one-dimensional Spin-chain Compounds Ca₃Co_2-xMn_xO₆

The magnetic properties of quasi-one-dimensional spin-chain compounds Ca₃Co2O6can be studied by adjusting the Ising spin chain characteristics, of which the simplest andmost direct way is to substitute elements of Co ions. And the research of Mn-dopedcompound is the most studied one in the alternative study of all the elements, this isbecause apart from the ferroelectric effect, Ca₃Co_2-xMn_xO₆also exhibits many abnormalmagnetic behaviors, such as―disorder-induce-order‖phenomenon, spin freezing and3:1magnetization plateau. This paper investigates systematically the magnetic properties ofCa₃Co_2-xMn_xO₆, and finds other interesting phenomena in addition to the abovephenomenon. The specific contents are listed as following:（1） Abnormal behaviors of inverse dc susceptibility （H/M） have been investigatedsystematically in quasi-one-dimensional spin-chain compounds Ca₃Co_2-xMn_xO₆（x=0.0¹.0）. Polycrystalline samples of Ca₃Co_2-xMn_xO₆（x=0.0¹.0） were prepared by thecitrate-gel method （anhydrous ethanol as the solvent）. The XRD data shows the degree ofsolid solution of the compounds Ca₃Co_2-xMn_xO₆is x=1.0, and all the samples are singlephase with a rhombohedral structure （space group R-3c）. The inverse direct currentsusceptibility measurement results showed that for the H/M curves of Ca₃Co_2-xMn_xO₆（x=0.0¹.0）, the magnitude of the deviation from the PM Curie-weiss behavior wasassociated with Mn-doping amount. As x>0.33, the pure paramagnetic （PM） Curietemperature changes its sign from positive to negative. Therefore, there is a strongcompetition between anti-ferromagnetic （AFM） and ferromagnetic （FM） interactionsaround this composition. According to the character of the H/M and M（T） curves, andwith the Griffiths singularity, H/M=（T-TrandC）1-（0λ<1）, the Griffiths-like phase andnon-Griffiths-like clusters behavior in three different regions have been discussed. Finally,it has been proposed that these Mn–doping dependent anomalies in the H/M curvesclosely correlated with complex competing magnetic interaction.（2） The magnetic relaxation of the compounds Ca₃Co_2-xMn_xO₆（x=0.0¹.0） has beenstudied by the dc magnetization and ac magnetic susceptibility measurements. Firstly, the presence of spin-glass-like freezing in the compounds has been illustrated, and themagnetic relaxation due to spin freezing is affected by the doping amount. Themagnetization in the intermediate Mn-doping relaxes more slowly than those in x=0.0andx=1.0. Thus, the spin freezing in intermediately doped compounds is closely associatedwith chemical disorder.（3） We carried out a systematic study on the magnetic anisotropy and magneticdynamic of single crystal with x⁰.33. The single crystal with a composition of x=0.38was grown by flux method. The SEM and EDS shows that the chemical composition withx=0.38and the morphology is a relatively regular hexagonal prism. The dc magnetizationand ac magnetic susceptibility measurements shows, there is a strong magnetic anisotropyin both ordered and PM states, and a meta-stable magnetization state due to spin-glass-likebelow TN=17K in the single crystal Ca₃Co_1.62Mn_0.38O₆.Comparable to those of x=0.0andx=1.0, the magnetic anisotropy of x=0.38is not much modified when the FM spin chain isdiluted. For x=0.38, the mechanism is different from the cases of x=0.0and of x=1.0, thespin-glass-like freezing is proposed to be a result of chemical disorder and competingAFM and FM interactions.