arXiv subjects

Yukari Fujioka

Publications and source records attributed to Yukari Fujioka.

7 recordsLinked to original sources

arXiv2019

Room-temperature insulating ferromagnetic (Ni,Co)1+2xTi1-xO3 thin films

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. We show that the uniaxial $A^{1+2x}$Ti$^{4+}$$_{1-x}$O$_3$ (ATO), $A=$Ni$^{2+}$,Co$^{2+}$ and $0.6<x \leq 1$, thin films are electrically insulating ferromagnets already at room-temperature. The octahedra network of the ATO and ilmenite structures are similar 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.

cond-mat.mtrl-sci↗

arXiv2019

In Quest of a Ferromagnetic Insulator -- Structure Controlled Magnetism in Mg-Ti-O Thin Films

Ferromagnetic insulator thin films can convey information by spin waves, avoiding charge displacement and Eddy current losses. The sparsity of high-temperature insulating ferromagnetic materials hinders the development of spin wave based devices. Stoichiometric magnesium titanate, MgTiO$_3$, has an electronic-energy-band structure in which all bands are either full or empty, being a paramagnetic insulator. The MgTiO$_3$ ilmenite consists of ordered octahedra and cation network in which one third of the octahedra are vacant, one third host magnesium and one third titanium. By giving up these characteristics, a rich variety of different magnetic structures can be formed. Our experiments and electronic-energy-band-structure computations show that the magnetic and electric properties of Mg-Ti-O films can drastically be changed and controlled by Mg- and Ti-cation arrangement and abundancy in the octahedra. Insulating titanium- and semiconducting magnesium-rich films were ferromagnetic up to elevated temperatures. The presence and origin of ferromagnetic insulating phase in the films is not apparent - the expectation, based on the well-established rules set by Goodenough and Kanamori, is paramagnetic or antiferromagnetic ordering. We show that ferro- and paramagnetic phases, possessing the same stoichiometry, can be obtained by merely rearranging the cations, thus allowing defect-free interfaces in multilayer structures.

cond-mat.mtrl-sci↗

arXiv2016

Nickel-cobalt-titanate thin films - new sustainable magnetic oxides

Single phase nickel-cobalt-titanate thin films with a formula A1+2xTi1-xO3, where A is Ni2+,Co2+ and -0.25 0) or emptying filled (x<0) octahedra. When x = 1 all octahedra are filled. Two factors controlling the magnetism and crystal distortion are identified. First is a direct overlap between the adjacent cation d-orbitals resulting in a bond formation and magnetic interactions between the cations. This is most clearly revealed as a crystal distortion in the x approximately 0 compositions with approximately equal amounts of Ni and Co: the distortion of the x approximately 0 compound is a function of Ni/Co ratio. The second factor is x, which controls the cation shift towards a vacant octahedron. The displacement decreases and the symmetry increases with decreasing Ti content as was revealed by x-ray diffraction and Raman spectroscopy. When all octahedra are filled the cations prefer octahedron center positions. Also the number density of cations has increased by a factor of 50 percent when compared to the ilmenite structure. The number density ratios of Ni/Co cations between x=1 and x=0 compounds is 3. The Raman and x-ray diffraction data collected on samples with x = 1 or close to 1 are interpreted in terms of P63/mmc space group.

cond-mat.mtrl-sci↗

arXiv2015

X-ray and neutron scattering on disordered nanosize clusters: a case study of lead-zirconate-titanate solid solutions

Defects and defect models of solids are reviewed. A numerical method able to treat non-periodical solids possessing several simultaneous defect types is given for simulating scattering in nanosize clusters. The approach takes particle size, shape, and defects into account and isolates element specific signals. Examples illustrating how laboratory scale facilities can be used to extract crucial information about defects are given. As a case study a statistical approximation model for lead-zirconate-titanate (PZT) is introduced. PZT is a material possessing several defect types, including substitutional, displacement and surface defects. Spatial composition variation is taken into account by introducing a model in which the edge lengths of each cell depend on the distribution of Zr and Ti ions in the cluster. Spatially varying edge lengths and angles are referred to as microstrain. The Pb, Zr and Ti cation positions were adjusted by bond-valence sum (BVS) model to fullfil nominal valence requirement. The model is applied to compute the scattering from ellipsoid shaped PZT clusters and to simulate the structural changes as a function of average composition. Two-phase co-existence range, the so called morphotropic phase boundary (MPB) composition is given correctly. To make a comparison with commonly used x-ray and neutron diffraction data selected Bragg reflection intensities and line shapes were simulated. Examples of the effect of size and shape of the scattering clusters on diffraction patterns are given and the particle dimensions, computed through Scherrer equation, are compared with the exact cluster dimensions. Scattering from two types of 180 degree domains in spherical particles, one type assigned to Ti-rich PZT and the second to the MPB and Zr rich PZT, is computed. We show how the method can be used for modelling polarization reversal.

cond-mat.dis-nn↗

arXiv2014

Triclinic Ni0.6Co0.4TiO3 Ilmenite Oxide

Forming a solid-solution of NiTiO3 and CoTiO3, two isostructural (ilmenite) and isosymmetrical (space group R-3) compounds, result in a single-phase compound with a remarkably low crystal symmetry. By neutron and X-ray synchrotron powder diffraction techniques, the space group symmetry of the Ni0.6Co0.4TiO3 sample was found to be triclinic P-1 at room temperature, far above the magnetic transition temperature. Ni and Co ions were found to prefer positions close to the octahedron center, whereas Ti ions took off-center positions. This structural distortion is the first known case in ilmenites and opens up ways to modify functional properties of magnetic oxides. Origin of the symmetry lowering is discussed.

cond-mat.mtrl-sci↗

arXiv2011

DC and AC Magnetization Study of Complex Ilmenite Oxides (Ni1-xCox)TiO3 (0.05 < x < 0.8)

Ilmenite solid-solutions, (Ni1-xCox)TiO3 (0.05 < x < 0.8), were synthesized at ambient atmosphere through solid-state reaction and were studied by energy dispersive spectroscopy of X-rays, X-ray diffraction, and DC and AC magnetometry. Temperature dependent DC magnetic measurements revealed two transitions. The first took place at around 27 K and the second at 63 K. The low-temperature phase was antiferromagnetic. The phase observed between 27 and 63 K is characteristic to the solid-solution and is not found in either of the constituent members. A fit of the data to the Curie-Weiss law gave magnetic moment values which were significantly larger than the values based on the quenched orbital momentum assumption. Zero-field-cooled magnetization measurements with weak magnetic field revealed unexpected negative magnetization at low temperatures. Below 63 K the DC magnetization exhibited time dependent behavior. Frequency and magnetic field dependent AC magnetization is also addressed.

cond-mat.str-el↗

arXiv2008

Evidence against the polarization rotation model of piezoelectric perovskites at the morphotropic phase boundary

The origin of the very large piezoelectric response observed in the vicinity of the morphotropic phase boundary (MPB) in perovskite lead zirconate titanate and related systems has been under intensive studies. Polarization rotation ideas are frequently invoked to explain the piezoelectric properties. It was recently reported that lead titanate undergoes a phase transformation sequence $P4mm\to Pm\to Cm\to R\bar{3}c$ at 10 K as a function of hydrostatic pressure [M. Ahart et al. Nature Letters. \textbf{451}, 545 (2008)]. We demonstrate that this interpretation is not correct by (i) simulating the reported diffraction patterns, and (ii) by density-functional theory computations which show that neither the $Pm$, $Cm$ nor $Pmm2$ phase is stable in the studied pressure range, and further show that octahedral tilting is the key stabilization mechanism under high pressure. Notes on a more general ground are given to demonstrate that a continuous phase transition between rhombohedral and tetragonal phases via intermediate monoclinic phase is not possible. Thus, two-phase co-existence in the vicinity of the phase transition region is probable and has an important role for electromechanical properties.

cond-mat.mtrl-sci↗