Observation of predominant long-range ordering by overcoming the magnetic frustration in Cu-doped La2MnCoO6

Abstract

We have investigated the effect of copper substitution on the structure, cation valence, and magnetic properties of the double perovskite La2MnCoO6. La2Mn1–xCuxCoO6 (0 ≤ x ≤ 0.4) were synthesized by the sol–gel techniques and characterized by PXRD, FE-SEM, EDX, XPS, and both the dc- and ac-magnetization. There is a structural transition for copper substitution, where the monoclinic structure of the parent La2MnCoO6 transformed to the rhombohedral structure. The ionic size difference between Mn4+ (0.53 Å) and Cu2+ (0.73 Å) may be ascribed to be the driving force behind the structural transition with an increase in Cu doping. EDX analysis confirms the nominal cationic compositions and chemical homogeneity of the samples. The copper substitution has a large impact on the valency of both the Mn and Co cations by following the charge neutrality condition: Mn4+ + Co2+ → (1 – x)Mn4+ + xCu2+ + (1 – 2x)Co2+ + 2xCo3+, where x is the doping concentration. There is also the Co2+ + Mn4+ = Co3+ + Mn3+ interaction. The presence of multivalent cations is evidenced from the XPS analysis. This largely influences the cationic ordering as well as the magnetic interactions. The monoclinic structure of the x = 0 and 0.1 samples show long-range ferromagnetic ordering. This is associated with the Mn4+—O—Co2+ ferromagnetic superexchange interaction in the ordered state of Mn4+ and Co2+. For small Cu doping (x = 0.1), this interaction is marginally disturbed. However, in the rhombohedral phase for x ≥ 0.2, the ferromagnetic interaction is largely suppressed with the evolution of competing antiferromagnetic interactions between the multivalent cations. Most importantly, in the panorama of multivalent mixed cations with randomness and competing exchange interactions, which are the fundamental ingredients of magnetic frustration, the antisite-disordered induced magnetic frustration for x = 0 is practically suppressed for x ≥ 0.2. The antiferromagnetic ordering over magnetic frustration can be attributed to the local lattice distortion induced by Jahn–Teller-active Cu2+ and Mn3+ and intermediate-spin-state Co3+ ions.