Raman Study of the Structural Distortion in the Ni 1– x Co x TiO 3 Solid Solution

Physica Status Solidi (b)

et al. 2017

Self Cite

Single phase nickel‐cobalt‐titanate thin films with a formula A1+2xTi1−xO3, where A is Ni2+,Co2+, and −0.25 < x < 1, were grown by pulsed laser deposition on sapphire substrates. There is a large window in which Ni/Co ratio and x can be chosen independently, allowing a control of properties by the synthesis. In the prototype ilmenite and corundum structures one third of the octahedra are vacant. The reported structure is obtained by filling vacant octahedra (x > 0) or emptying filled octahedra (x < 0). Films with x = 1 have all octahedra filled. X‐ray photoelectron spectroscopy, X‐ray diffraction measurements and Raman scattering techniques were applied to address the structure as a function of x. Data collected on samples with x ≈ 1 are interpreted in terms of hexagonal P63/mmc space group. The hexagonal structure was obtained when films contained both Ni and Co: pure cobalt oxide thin films possessed the spinel structure. Two factors controlling the magnetism and crystal distortion are identified: a direct overlap between the adjacent cation d‐orbitals resulting in a bond formation and magnetic interactions, and x controlling the cation shift toward the vacant octahedron. The displacement decreases and the symmetry increases with increasing x. When x ≈ 1 the cations prefer octahedron center positions.

“…The low‐frequency band suggests that the symmetry of the film is lower than

R \bar{3}

Section: Resultssupporting

confidence: 86%

Section: Original Papersupporting

confidence: 74%

Section: Resultsmentioning

confidence: 53%

“…Neutron and synchrotron X‐ray diffraction and Raman scattering studies revealed that NCT in a bulk form possesses a lower crystal symmetry than

R \bar{3}

and also two magnetic phases absent in NiTiO 3 and CoTiO 3 . NiTiO 3 and CoTiO 3 are reported to have

R \bar{3}

symmetry.…”

Section: Why Ilmenites − Structural Aspectsmentioning

confidence: 99%

See 2 more Smart Citations

Vacancy filled nickel‐cobalt‐titanate thin films

Fujioka¹,

Frantti²,

Physica Status Solidi (b)

et al. 2017

Self Cite

“…The appearance of ferromagnetism is consistent with the idea that the anomaly is not related to a covalent bond formation. Structural distortions, present already at room temperature, are observed also in the bulk Ni 1 −x Co x TiO 3 Solid Solution . In thin films, similar distortions appear in the ilmenite‐like sample J(0.03).…”

Section: Resultsmentioning

confidence: 61%

Room‐Temperature Insulating Ferromagnetic (Ni,Co)₁₊₂_xTi₁₋_xO₃ Thin Films

Fujioka¹,

Frantti²,

et al. 2019

Annalen der Physik

Self Cite

Insulating uniaxial room-temperature ferromagnets are a prerequisite for commonplace spin wave-based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. It is shown that the uniaxial A 1 + 2x Ti 4+ 1 − x O 3 (ATO), A = Ni 2+ ,Co 2+ , and 0.6 < x ≤ 1, thin films are electrically insulating ferromagnets already at room temperature. The octahedra network of the ATO and the corundum and ilmenite structures are the same yet different octahedra-filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra can continuously be filled up to x = 1, or vacated (−0.24 < x < 0) in the ATO structure. TiO-layers, which separate the ferromagnetic (Ni,Co)O-layers and intermediate the antiferromagnetic coupling between the ferromagnetic layers in the NiTiO 3 and CoTiO 3 ilmenites, can continuously be replaced by (Ni,Co)O-layers to convert the ATO-films to ferromagnetic insulator with abundant direct cation interactions.

Simulation of the Impact of Point Defects and Edge Dislocations on X‐Ray Diffraction in Hexagonal (Ni,Co)_1+2xTi_1−xO₃ Thin Films

Frantti¹,

Fujioka²,

Physica Status Solidi (b)

et al. 2022

Self Cite

Understanding and controlling crystal defects are essential in material engineering, the classical example being semiconductor devices created by a spatially well-controlled point defect distribution. [1] 2D defects include domain wall and grain boundaries. Polarization reversal in ferroelectric materials is essentially dictated by domain walls, whose dynamics can be highly complex. [2] The meaning of domains and domain boundaries for the polarization reversal in ferroelectrics is as crucial as the role of dislocations for the plasticity of metals. [3] Edge dislocations are classified as line defects which are frequently formed in strained thin films. In case the lattice mismatch exceeds the critical value, edge dislocations are formed. After exceeding the critical thickness, the lattice parameters relax to their natural values. A spatially extended displacement field accompanies dislocation, which in diffraction measurements is often seen as an asymmetric line shape. Determination of the strain and dislocation density are key parameters in semiconductor devices based on the epitaxial thin films.Crystal defects have a decisive role for the properties of thin films, yet their nondestructive structural characterization is challenging as the volume is characteristically small and the films are grown on a substrate. X-ray diffraction (XRD) is the prevailing route for nondestructively characterizing crystals and their defects. [4][5][6][7] XRD analysis relies on results extracted from bulk samples, single crystals, and powders. XRD data collected on thin-film samples possessing several layers are commonly considered to be consisting of noninteracting layers, the result being a sum of the patterns from each layer (or phase). To estimate the phase fractions, standard profile functions (e.g., Lorentz, Pearson or pseudo-Voigt) are commonly applied to model reflection intensities. The approach suits for the polycrystalline materials lacking features resulting from long-range order (e.g., subsidiary peaks in thin films) but is an overly coarse approach for highly ordered nanoscale structures. For instance, while asymmetric line shapes can routinely be fit by the standard line profiles, it is often hard to ascertain the origin of the asymmetry.In this article, another approach is taken to model defected films. The films, with spatially varying composition and structure, are taken as a single scattering unit. As the dimensions are small, between 10 and 100 nm, the scattered X-ray intensity profile is to an excellent accuracy determined by the sample, and no preassumed profile function is applied. We first simulate diffraction patterns from the homogeneous (Ni 0.42 Co 0.58 ) 2.22 Ti 0.39 O 3 film possessing atomic-scale disorder and second from hexagonal (Ni,Co) 3 O 3 thin films under compressive biaxial strain and edge dislocations. The situation contrasts to the case where no stress relaxation by dislocation has taken place. Thus, the simulations suit for cases where the standard measurements in which d-spacing for a chosen ...