Oxygen defect engineered magnetism of La2NiMnO6 thin films

Abstract

The double perovskite La2NiMnO6 (LNMO) exhibits complex magnetism due to the competition of magnetic interactions that are strongly affected by structural and magnetic inhomogeneities. In this work, we study the effect of oxygen annealing on the structure and magnetism of epitaxial thin films grown by pulsed laser deposition. The key observations are that a longer annealing time leads to a reduction of saturation magnetization and an enhancement in the ferromagnetic transition temperature. We explain these results based upon epitaxial strain and oxygen defect engineering. The oxygen enrichment by annealing caused a decrease in the volume of the perovskite lattice. This increased the epitaxial strain of the films that are in-plane locked to the SrTiO3 substrate. The enhanced strain caused a reduction in the saturation magnetization due to randomly distributed anti-site defects. The reduced oxygen defects concentration in the films due to the annealing in oxygen improved the ferromagnetic long-range interaction and caused an increase in the magnetic transition temperature.

Mechanisms for point defect-induced functionality in complex perovskite oxides

“…In La 2 NiMnO 6 , the presence of excess electrons, for example, due to oxygen vacancy formation or a polar catastrophe mechanism, can alter the checkerboard to a columnar order, introducing antiferromagnetic Mn-Mn and Ni-Ni superexchange in addition to the ferromagnetic Mn-Ni interactions [64]. In agreement with this result, extended oxygen anneal resulted in a reduced oxygen vacancy concentration and lead to stronger ferromagnetism in this material [65].…”

Substitutional effect of Mg2+ on structural and magnetic properties of La2MgxNi1‐xMnO6 double‐perovskite thin films

“…As illustrated in Figure 4(f), the valence state of Mn ions can be effectively regulated in a wide range by doping Mg 2+ at B‐sites, and the fraction of Mn 4+ in the LM x NMO thin films is increased as compared with LNMO films with the maximum value of 77.4% being obtained in LM 0.3 NMO thin film. It is well known that oxygen vacancies usually exist in LNMO double perovskite films when they are ablated at a low oxygen pressure (∼100 mTorr) during PLD process 43,44 . In our PLD experiment, the oxygen pressure was fixed at 15 Pa (∼110 mTorr), a typical low oxygen pressure, which will cause moderate amount of oxygen vacancies and the subsequent lowered Mn 4+ /Mn 3+ ratio in the films according to the charge neutrality principle.…”

“…It is well known that oxygen vacancies usually exist in LNMO double perovskite films when they are ablated at a low oxygen pressure (∼100 mTorr) during PLD process. 43,44 In our PLD experiment, the oxygen pressure was fixed at 15 Pa (∼110 mTorr), a typical low oxygen pressure, which will cause moderate amount of oxygen vacancies and the subsequent lowered Mn 4+ /Mn 3+ ratio in the films according to the charge neutrality principle. Hence, when the Mg 2+ doping level is small from x = 0 to x = 0.3, it will preferably replace Ni 3+ ions in order to compensate the electron charge caused by the oxygen vacancies, which will induce the neighboring Mn 3+ to become Mn 4+ so as to maintain the charge balance.…”