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 p erovskit e La 2 MnCoO 6 . La 2 Mn 1−x Cu x CoO 6 (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 La 2 MnCoO 6 transformed to the rhombohedral structure. The ionic size difference between Mn 4+ (0.53 Å) and Cu 2+ (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: 3+ , where x is the doping concentration. There is also the Co 2+ + Mn 4+ = Co 3+ + Mn 3+ 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 Mn 4+ OCo 2+ ferromagnetic superexchange interaction in the ordered state of Mn 4+ and Co 2+ . 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 Cu 2+ and Mn 3+ and intermediate-spin-state Co 3+ ions.

Entanglement of cation ordering and manipulation of the magnetic properties through a temperature-controlled topotactic interface reaction in nanocomposite perovskite oxides

“…A slight decrease in T C for the 1000 °C annealed sample may be related to the anti-site disordering effect in La 2 MnCoO 6 . 42,49,86,87 The isothermal magnetization curves shown in Fig. 13(j)–(l) are in corroboration with the reported hysteresis loops for La 2 MnCoO 6 .…”

“…13(b)–(d) resembles the typical behaviour reported for La 2 MnCoO 6 . 42,49,86,87 There is a huge increase in magnetic transition temperature from 80 K for the 600 °C annealed LMO–LCO to 200 K for the 700 °C annealed sample (Fig. 13(b)).…”

“…LaMnO 3 is an antiferromagnetic insulator with T N B 140 K. 83 The rhombohedral R% 3c phase of LaCoO 3 exhibits a spin state transition over a wide temperature range. 84,85 On the other hand, the monoclinic P2 1 /n phase of La 2 MnCoO 6 with an ordered stacking of the B-site Mn 4+ and Co 2+ exhibits a ferromagnetic transition at T C = 225 K. 42 However, the orthorhombic Pbnm phase with a random distribution of B-site cations has T C B 150 K, along with the spin glass-type behavior at low temperature. 42,49,86,87 In the present investigation, we have also prepared nanocomposites with the ratio of 1 : 1 of LaMnO 3 and LaCoO 3 , i.e., LMO-LCO, and these were annealed at four different temperatures under a similar condition to that used for the LCMO-LFO nanocomposite discussed in the preceding sections.…”

“…This is because of the fact that polycrystalline LaMn 0.5 Fe 0.5 O 3 fails to exhibit long range spin ordering, unlike other LaB 0.5 B 0 0.5 O 3 compositions. [43][44][45][46] LaMn 0.5 Cr 0.5 O 3 , LaMn 0.5 Co 0.5 O 3 and LaMn 0.5 Ni 0.5 O 3 exhibit the ferromagnetic T C at around 120, 225 and 280 K, respectively, 41,42,[47][48][49][50] whereas LaCo 0.5 Fe 0.5 O 3 , LaCo 0.5 Cr 0.5 O 3 and LaFe 0.5 Ni 0.5 O 3 are antiferromagnetic with the T N 370, 295 and B100 K, respectively. [51][52][53] However, LaMn 0.5 Fe 0.5 O 3 with uniform Fe and Mn distribution was found to be of nonmagnetic behavior, where cluster-like ferromagnetic or antiferromagnetic states appear for inhomogeneous cationic distribution.…”

“…37,38 However, the partial displacement between Fe and Mn with the decrease in the number of LaMnO 3 -LaFeO 3 interfaces results in reduced magnetization, which corresponds to the antisite disordering, as observed in La 2 MnMO 6 (M = Co and Ni). [39][40][41][42] The observation of long range ferromagnetic ordering in LaMn 0.5 Fe 0.5 O 3 through the Fe 3+ (d 5 )-O-Mn 3+ (d 4 ) superexchange interaction, which is achieved by cationic ordering across the LaMnO 3 -LaFeO 3 interface in a thin film is of significant importance. This is because of the fact that polycrystalline LaMn 0.5 Fe 0.5 O 3 fails to exhibit long range spin ordering, unlike other LaB 0.5 B 0 0.5 O 3 compositions.…”