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.
Highlights
The magnetism of double perovskites (DP) is interesting due to its complex nature
The X-ray diffraction (XRD) and the XRR patterns of the LNMO films with different annealing times are shown in supplementary material Fig. S1 (a) and (b), respectively
High-quality thin films of LNMO were fabricated by pulsed laser deposition (PLD) at 900 ○C and pO2 of 500 mTorr
Summary
The magnetism of double perovskites (DP) is interesting due to its complex nature. The DP La2BMnO6 with B=Cr, Fe, Co, Ni, and Cu are reported to exhibit complex magnetic properties due to competing magnetic interactions, which are affected by structural and magnetic inhomogeneities. A near room-temperature magnetic transition (TC) with complex intrinsic and extrinsic magnetic behavior makes the DP La2NiMnO6 (LNMO) interesting. Bulk LNMO has a high-temperature magnetic transition (TC1) around 250-270 K and a low-temperature transition (TC2) around 100-150 K.3,6,7 Previous reports suggest that the TC1 is originating from the orthorhombic LNMO phase containing Ni2+ and Mn4+ cations, and the TC2 is originating from the rhombohedral LNMO phase with Ni3+ and Mn3+ cations. Recently, this material has attracted even more attention due to its oxygen-reduction and oxygen-evolution reactions caused by its favorable electronic structure. LNMO plays a role in different functional properties like magnetic, multiferroic, optical, and electrochemical activities, as summarized in book chapter. The monoclinic high-TC phase of bulk LNMO was reported to be stabilized by creating La-vacancies and accommodating extra oxygen in the lattice.. Previous reports suggest that the TC1 is originating from the orthorhombic LNMO phase containing Ni2+ and Mn4+ cations, and the TC2 is originating from the rhombohedral LNMO phase with Ni3+ and Mn3+ cations.. Previous reports suggest that the TC1 is originating from the orthorhombic LNMO phase containing Ni2+ and Mn4+ cations, and the TC2 is originating from the rhombohedral LNMO phase with Ni3+ and Mn3+ cations.6 This material has attracted even more attention due to its oxygen-reduction and oxygen-evolution reactions caused by its favorable electronic structure.. It was found that the LNMO film prefers a monoclinic structure under tensile and a rhombohedral phase under compressive strain.. The multiphase nature, off-stoichiometry, oxygen-deficiency, epitaxial strain, etc., are the controlling factors of magnetism in LNMO. The present work deals with a careful study of oxygen defect engineering in LNMO thin films and control of their magnetic behavior
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