Impurity induced antiferromagnetic order in Haldane gap compound SrNi2-xMgxV2O8

The effect of nonmagnetic Mg2+ doping in SrNi2V2O8, a Haldane gap system with a disordered ground state, was investigated using DC magnetic susceptibility and heat capacity measurements in polycrystalline samples of SrNi2-xMgxV2O8 with x=0.0,0.0,0.0,0.1 and 0.14. The results clearly reveal that the substitution of Ni2+ (S=1) ion by Mg2+ (S=0) ion induces a magnetic phase transition with the ordering temperatures lying in the range 3.4-4.3K, for the samples with lowest and highest value of x. The intrachain exchange constant (J/kB) and the Haldane gap (Delta) for all the compounds were estimated to be 98+/-2 and 25K, respectively, which are close to that of the undoped compound. The magnetization data further suggest that the compounds exhibit metamagnetic behavior below TN, supporting a picture of antiferromagnet with significant magnetic anisotropy and competing intrachain and interchain interactions.