Alexander Kozhanov

Transport in ferromagnetic GdTiO3 / SrTiO3 heterostructures

Epitaxial GdTiO3/SrTiO3 structures with different SrTiO3 layer thicknesses are grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate surfaces by hybrid molecular beam epitaxy. It is shown that the formation of the pyrochlore (Gd2Ti2O7) phase can be avoided if GdTiO3 is grown by shuttered growth, supplying alternating monolayer doses of Gd and of the metalorganic precursor that supplies both Ti and O. Phase-pure GdTiO3 films grown by this approach exhibit magnetic ordering with a Curie temperature of 30 K. The electrical transport characteristics can be understood as being dominated by a conductive interface layer within the SrTiO3.

Dispersion in magnetostatic CoTaZr spin waveguides

Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion and up to three different modes are identified. Attenuation lengths of the order of 3 μm are several orders of magnitude shorter than that predicted from eddy currents in these thin wires.

Dispersion and spin wave “tunneling” in nanostructured magnetostatic spin waveguides

Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion. The measured attenuation lengths, of the order of 3 μm, are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires. Spin waves effectively “tunnel” through air gaps, produced by focused ion beam etching, as large as 1.5 μm.

Gaps and pseudogaps in perovskite rare earth nickelates

We report on tunneling measurements that reveal the evolution of the quasiparticle state density in two rare earth perovskite nickelates, NdNiO3 and LaNiO3, that are close to a bandwidth controlled metal to insulator transition. We measure the opening of a sharp gap of ∼30 meV in NdNiO3 in its insulating ground state. LaNiO3, which remains a correlated metal at all practical temperatures, exhibits a pseudogap of the same order. The results point to both types of gaps arising from a common origin, namely, a quantum critical point associated with the T = 0 K metal-insulator transition. The results support theoretical models of the quantum phase transition in terms of spin and charge instabilities of an itinerant Fermi surface.

Cross Junction Spin Wave Logic Architecture

Backward volume magnetostatic spin wave interference in ferromagnetic cross is explored. Spin waves are excited at the two input arms of the cross, interference signal amplitude, and phase are detected in the other two, output arms of the cross. Spin wave scattering defines the out of phase interference in the output arms of the cross: the constructive (destructive) interference is observed at different input wave phase shifts in different output arms of the cross. Numeric simulations are in a good agreement with experiment. Based on the observed interference, we propose the cross junction spin wave logic device that operates as an OR or NOR logic gates. Spin wave phase represents the logic information. Destructive and constructive interference in the cross center determines the phase of the device output.

Spin wave modes in ferromagnetic tubes

Resonances are observed in the transmission between two coplanar waveguides coupled by ferromagnetic Co90Ta5Zr5 tubes that wrap around their shorted ends. The resonances are assigned to the magnetostatic surface waves that counter propagate along the tube perimeter. We use a model based on an infinite ferromagnetic tube, with elliptical cross section of roughly the same dimensions as the studied structure. Additional theoretical analysis of the fundamental precession mode observed in experiment is carried out. Periodic boundary conditions dictated by the tube perimeter and applied to magnetostatic surface waves quantitatively account for the experimentally observed bandwidth of excited modes, despite the contorted tubular shape. The tubular topology appears to be more important than the shape details.

Micro-structured ferromagnetic tubes for spin wave excitation

Micron scale ferromagnetic tubes placed on the ends of ferromagnetic CoTaZr spin waveguides are explored in order to enhance the excitation of backward volume magnetostatic spin waves. The tubes produce a closed magnetic circuit about the signal line of the coplanar waveguide and are, at the same time, magnetically contiguous with the spin waveguide. This results in a ten-fold increase in spin wave amplitude. However, the tube geometry distorts the magnetic field near the spin waveguide, and relatively high biasing magnetic fields are required to establish well-defined spin waves. Only the lowest (uniform) spin wave mode is excited.

Spin wave scattering and interference in ferromagnetic cross

Magnetostatic spin wave scattering and interference across a CoTaZr ferromagnetic spin wave waveguide cross junction were investigated experimentally and by micromagnetic simulations. It is observed that the phase of the scattered waves is dependent on the wavelength, geometry of the junction, and scattering direction. It is found that destructive and constructive interference of the spin waves generates switching characteristics modulated by the input phase of the spin waves. Micromagnetic simulations are used to analyze experimental data and simulate the spin wave scattering and interference.

Martensite transformation of epitaxial Ni-Ti films

The structure and phase transformations of thin Ni–Ti shape memory alloy films grown by molecular beam epitaxy are investigated for compositions from 43 to 56 at. % Ti. Despite the substrate constraint, temperature dependent x-ray diffraction and resistivity measurements reveal reversible, martensitic phase transformations. The results suggest that these occur by an in-plane shear which does not disturb the lattice coherence at interfaces.

Magnetostatic Spin-Wave Modes in Ferromagnetic Tube

Magnetostatic spin-wave modes were excited and detected in micron size ferromagnetic Co90Ta5 Zr5 rectangular tubes that wrapped around the shorted ends of coplanar waveguides. The observed modes can be assigned to surface spin waves of an infinite ferromagnetic film but with periodic boundary conditions and quantized wave vector imposed by the finite circumference of the tube.