Nanometer-Sized Domain Structures in LuFeMO₄(M= Cu, Co) Revealed by Energy-Filtered Transmission Electron Microscopy

Abstract

Diluted effect on the charge ordered (CO) structure in dielectric materials LuFeMO 4 (M = Cu, Co) were investigated by an energy-filtered transmission electron microscopy (TEM), in combination with a conventional TEM. It is revealed that partial substitution of Cu 2 + and Co 2 + for Fe 2 + in LuFe 2 O 4 destabilized the CO structure. In LuFeCuO 4 , there exist characteristic zigzag-shaped diffuse streaks and diffuse spots at the h/3-δ h/3-δ 0-type positions (h: integer, δ = 0.06) in reciprocal space, which originate from static cationic ordering of Fe 3 + and Cu 2 + ions in the triangular lattice. Moreover, real-space images by the energy-filtered TEM revealed that nanometer-sized domain structures with the 5∼10 nm size are formed. In contrast, honeycomb-shaped diffuse streaks were found in LuFeCoO 4 , in which no domain structures are observed in real-space images.

Dielectric, Ferroelectric, and Magnetic Characteristics of LuFeCuO4 Ceramics

“…It is known that Fe ions in LuFe 2 O 4 can be replaced by some ions to form LuFe M O 4 ( M = Mg, Mn, Cu, Zn) . In these systems, the formal valence of Fe ion is 3+, whereas that of M ions is 2+.…”

“…A previous article also reported that the dielectric constants become smaller in the order of R Fe 2 O 4 , R FeCoO 4 , R FeCuO 4 , and R GaCuO 4 ( R = Yb and Lu), which is ascribed to the suppression of charge transfer in the substituted systems. The Cu‐substituted system, LuFeCuO 4 , was reported to have a short‐ranged ordered structure in the triangular lattice . A recent work found that the number of oxygen vacancies plays an important role in the dielectric properties of LuFeCuO 4 , and the leakage current density decreases with oxygen annealing .…”

Present status of the experimental aspect ofRFe2O4study

“…Such ordering was concluded from the zigzag diffuse pattern found in the TEM diffraction observation. This result indicates that the divalent and trivalent ions in the double triangular lattice form an ordering even in the crystal growth process [131][132][133][134]. As for LuCuGaO 4 , the neutron diffraction and the muon-spin relaxation experiment were performed [135].…”