Nonstoichiometry Driven Ferromagnetism in Double Perovskite La₂Ni_1–xMn_1+xO₆ Insulating Thin Films

Abstract

In this work we report on the epitaxial growth of La 2 NiMnO 6 double perovskite thin films on top of (001) oriented SrTiO 3 substrates by RF magnetron sputtering. The influence of oxygen pressure (P O 2 ) and growth temperature on the microstructure, stoichiometry of the films, and magnetic and transport properties is thoroughly investigated. It is found that high oxygen pressure promotes the growth of stoichiometric films, with a Ni/Mn ratio almost equal to 1. However, these films exhibit poor ferromagnetic properties with respect to the expected optimum values corresponding to ferromagnetic ordering mediated by superexchange interaction between Mn 4+ and Ni 2+ according to the Goodenough-Kanamori rules. Most interestingly, films grown at low P O 2 exhibit Ni/Mn ratios below 1, but ferromagnetic properties close to the optimal ones. The valence balance between Ni and Mn ions in nonstoichiometric sample has been elucidated by X-ray absorption spectroscopy. The results indicate that Ni deficiency plays a crucial role in the puzzling insulating ferromagnetic behavior observed in nonstoichiometric samples.

Cluster spin-glass-like dynamics in Mn-rich La2Ni0.5Mn1.5O6 ferromagnetic insulator

“…The magnitudes of ρ 0 and T 0 were 4.83 × 10 −12 Ω m and 1.12 × 10 8 K, respectively. The T 0 value is in agreement with those observed for similar double perovskites [57,59]. The density of localized states N(E F ) at the Fermi level is related to T 0 as, T 0 = 24/[πK B N(E F )ξ 3 ], where ξ is the decay length and assumed to be nearly equal to the average distance between the neighboring Ni or Mn ions estimated from the Rietveld analysis.…”

“…The temperature dependence of resistivity ρ(T ) of La 2 Ni 0.5 Mn 1.5 O 6 in the interval of 130-325 K is plotted in figure 9(a). With decreasing temperature, it increased, indicating the insulating behavior of the material in the entire temperature range [57,58]. The various conduction models such as Efros-Shklovskii Hopping (ESH), thermal activation (TA), and variable range hopping (VRH) were attempted to describe the transport mechanism in the sample.…”

B-Site Cation Ordering in Films, Superlattices, and Layer-by-Layer-Grown Double Perovskites

“…However, the problem of oxygen deficiency and/or preparation-induced defects for PLD-grown films probably still exist in order to obtain strong magnetic properties; for example, in LCMO films [31] a post-annealing in 500 mbar of oxygen at 700 • C was shown to be a necessary condition. Several reports on the growth of LCMO [32] and LNMO [33] films by rf-sputtering under pO 2 ~0.1-0.5 mbar also pointed out the importance of post-annealing in the improvement of oxygen stoichiometry and magnetism in these films. Truong et al [25] reported the influence of growth conditions (temperature/pO 2 ) on the Ni/Mn ordering, studied by Raman spectroscopy, in the PLDgrown LNMO(111)/STO(111)films.…”

Unraveling the impact of nonmagnetic Sc substitution on the magnetic properties of La2NiMnO6 double perovskite

“…The saturation magnetization, Curie temperature, and evolution of other phases like spin glass in La 2 NiMnO 6 depend on the cation ordering and cation valences. The synthesis processes or conditions affect the cation order and valences [15,28,29]. The physical properties of La 2 NiMnO 6 can also be tuned by partial doping, such that it can be made applicable for a variety of spintronic applications.…”