Magnetic order induced by magnetic impurities in the Haldane chain compound SrNi₂V₂O₈

Understanding the topological properties of Haldane spin systems offers promising avenues for exploring exotic quantum phases. We investigate the impact of S = 5/2 Mn²⁺ impurities on the magnetic properties of SrNi₂V₂O₈, a well-known Haldane spin gapped system. Structural characterizations of SrNi_2-xMn_xV₂O₈ (x = 0.05, 0.15, 0.30 and 0.60) through Rietveld analyses of X-ray diffraction data confirm the preservation of the I4₁cd space group for all studied x, along with a preferential insertion of Mn²⁺ ions into Ni²⁺ sites. Magnetic susceptibility and magnetization data unveil a magnetic transition occurring in the temperature range of T = 93–99.3 K for x = 0.05–0.60. Dielectric data evince the absence of coupling between magnetic and ferroelectric phases. Notably, the magnetic ordering temperature varies little over a wide range of Mn concentrations, while notable anomalies are observed at x = 0.15. This peculiar behavior suggests that, across low-to-high concentrations, disorder-induced magnetic moments compete with the formation of spin singlets between the nearest Mn²⁺ impurities, which cancel out the magnetic moments.