arXiv subjects

Harry M. Meyer

Publications and source records attributed to Harry M. Meyer.

3 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↗