Present status of the experimental aspect ofRFe₂O₄study

Abstract

We give a brief review of the experimental research on a triangular mixed valence iron oxide RFe2O4 (R = Y, Dy, Ho, Er, Tm, Yb, Lu, Sc or In). Interest in this material has been increasing every year because of the fascinating but complicated interaction between spin, charge and the orbital state of iron ions in frustrated geometry. Reports collected in this review cover experimental research on crystallography, chemical analysis, bulk and thin film preparation, magnetic, dielectric, diffraction with neutrons, x-ray and electron, optical and x-ray absorption, Mössbauer spectroscopy and other methods that incorporate the use of modern scientific technology and knowledge. The report mainly focuses on experimental facts since 1990 on which an early review by Siratori has been published (Kimizuka et al 1990 Handbook on the Physics and Chemistry of Rare Earths vol 13, ed K A Gschneidner Jr and L Eyring (Amsterdam: North-Holland/Elsevier) pp 283-384).

Structural, magnetic and optical properties of Y bFe2O4 films deposited by spin coating

“…19 The energy features below ∼1.5 eV are assigned to Fe 2+ → Fe 3+ charge transfer excitations, while the diffuse bumps at energies slightly above ∼1.5 eV are assigned to Fe 2+ on-site excitations with crystal field splitting. 6 Strong ligand (O 2p) to metal (Fe 3d) charge transfer transitions occur above 3.4 eV, as determined by the direct bandgap plot of (αhν) 2 transfer has also been identified for the LuFe 2 O 4 bulk crystal. 19 A plot of (αhν) 1/2 against hν for the obtained film gave an indirect bandgap of ∼0.4 eV, which is in good agreement with that of the bulk crystal.…”

“…Epitaxial YbFe 2 O 4 films were prepared on α-Al 2 O 3 (001) substrates by the spin-coating method using an aqueous-based YbFe 2 …”

“…1,2 The crystal structure of RFe 2 O 4 consists of alternating stacking of a double layer (W-layer) of FeO 5 bipyramids and a hexagonal layer of RO 6 octahedra. Equal amounts of Fe 2+ and Fe 3+ ions are distributed over the W-layer and form a triangular lattice.…”

Iron vacancy effect on the magnetization of YbFe₂O₄

“…As the charge ordering in the triangular lattice possesses spontaneous polarization, a new type of ferroelectricity, so-called electronic ferroelectricity, is expected. As the electronic ferroelectricity is vastly different from conventional displaced type ferroelectrics, such as BaTiO 3 , RFe 2 O 4 has attracted the attention of researchers [3]. Since the electronic ferroelectricity had firstly reported [4], there has been much discussion about the charge-ordering model.…”

Terahertz Spectra Revealing the Collective Excitation Mode in Charge‐Density‐Wave Single Crystal LuFe 2 O 4

“…The multiferroics are widely used in modern electronics, such as memory elements, filtering devices, and high‐performance insulators . Mixed valence material LuFe 2 O 4 is one of the novel multiferroics, which originates from peculiar polarization mechanism compared with the typical ferroelectrics . Raman, infrared (IR), and neutron diffraction spectroscopy investigations have been carried out on LuFe 2 O 4 , illustrating its structural, charge ordering, and ferrimagnetic order properties.…”