Magnetodynamical response of large-area close-packed arrays of circular dots fabricated by nanosphere lithography

We report a combined experimental and theoretical study of the quasistatic hysteresis and dynamic excitations in large-area arrays of NiFe nanodisks forming a hexagonal lattice with the lattice constant of 390 nm. Arrays were fabricated by patterning a 20-nm-thick NiFe film using the etched nanosphere lithography. We have studied a close-packed (edge-to-edge separation between disks dcp = 65 nm) and an ultraclosed packed (ducp = 20 nm) array. Hysteresis loops for both arrays were qualitatively similar and nearly isotropic, i.e., independent on the in-plane external field orientation. The shape of these loops revealed that magnetization reversal is governed by the formation and expulsion of vortices inside the nanodisks. When we assumed that the nanodisks’ magnetization significantly decreases near their edges, micromagnetic simulations with material parameters deter-mined independently from continuous film measu-rements could satisfactorily reproduce the hysteresis. Despite the isotropic hysteresis, significant in-plane anisotropy of the dynamic response of the ultraclose-packed array was found experimentally by the all-electrical spin-wave spectroscopy and Brillouin light scattering. Dynamical simulations could successfully reproduce the difference between excitation spectra for fields directed along the two main symmetry axes of the hexagonal lattice. Simulations revealed that this difference is caused by the magnetodipolar interaction between nanodisks, which leads to a strong variation of the spatial distribution of the oscillation power both for bulk and edge modes as a function of the bias field orientation. Comparison of simulated and measured frequencies enabled the unambiguous identification of experimentally observed modes. Results of this systematic research are relevant both for fundamental studies of spin-wave modes in patterned magnetic structures and for the design of magnonic crystals for potential applications as, e.g., spin-wave guides and filters.

Atomic layer deposition of Ni and Ni80Fe20 for tubular spin-wave nanocavities

Maria Carmen Giordano

Magnetic thin films and magnetic nanostructures have become essential components of modern technological applications. Modern branches of magnetisms focus on spin-charge coupling (spintronics) and the collective excitation of spin waves in magnetically ordered materials (magnonics). In particular, spin waves represent a promising charge-free medium to encode and transmit information in the GHz frequency regime, relevant for microwaves technologies. The ever-increasing demand for compact and portable electronic and telecommunication devices leads to the need for further miniaturization and high integration density of their functional components. These aspects have prompted nanomagnetism to venture the third dimension. This development is also motivated by novel physical phenomena and functionalities predicted to arise from three-dimensional (3D) complex magnetic configurations. In order to advance the research in 3D spintronics and 3D magnonics, it will be essential to have control over the fabrication of 3D nanoarchitectures and to access the new properties of individual nanostructures emerging from new complex 3D spin-textures. These two challenges are addressed in this thesis. Ferromagnetic nanotubes (NTs) represent the ideal 3D system for the study of shape dependent static and dynamic magnetic properties. Their magnetic configurations, as well as the spin wave confinement and propagation inside them, can be engineered by changing their geometrical parameters. First, in order to fabricate 3D magnetic device architectures, we explored atomic layer deposition (ALD). The thickness and quality of the thin films deposited with ALD are irrespective of the geometry of the surface on which they are deposited. This characteristic makes it ideal to fabricate 3D nanomagnets by magnetically coating 3D nanotemplates, e.g. nanowires (NWs) in the case of NTs. In this thesis we propose ALD processes for the conformal coating by means of two ferromagnetic materials conventionally used in planar spintronics and magnonics systems: Ni and permalloy (Py, Ni80Fe20), the second material expected to have a lower spin wave damping. By identifying correlations between ALD process parameters and thin film properties we optimized the materials in terms of conformality, electrical resistivity, anisotropic magnetoresistance (AMR) effect, spin wave damping and, in the case of permalloy, also stoichiometry. The optimized materials were then transferred onto GaAs nanowires templates, obtaining ferromagnetic NTs with hexagonal cross sections. We achieved Ni NTs with the lowest resistivity ever reported in similar ALD-grown Ni systems and a large AMR effect. We observed the lowest spin waves damping for permalloy NTs, exhibithing a value comparable with permalloy thin films achieved with conventional planar deposition techniques. Second, we investigated magnetic states and spin waves confinement in nanotubes experimentally and via micromagnetic simulations. We combined magnetoresistance measurements with Brillouin light scattering spectroscopy (BLS) and micromagnetic simulations to study Ni and Py NTs. BLS measurements, performed while irradiating NTs with microwaves, show that these nanotubes form spin-wave nanocavities which impose discrete wave vectors and confine GHz microwave signals on the nanoscale. Our experimental results show that the confinement can be engineered by changing the NT's geometrical parameters and magnetic states.

EPFL2021

Magnetic states and spin-wave modes in single ferromagnetic nanotubes

Daniel Rüffer

In this thesis the electrical properties, magnetic states and spin wave resonances of individual magnetically hollow ferromagnetic nanotubes have been studied. They were prepared from the different materials Nickel (Ni), Permalloy (Py) and Cobalt-Iron-Boron (CoFeB), deposited as shells onto non-magnetic Gallium-Arsenide (GaAs) semiconductor nanowires via Atomic Layer Deposition (ALD), thermal evaporation and magnetron sputtering, respectively. The resulting nanotubes had lengths between 10 to 20 μm, diameters of 150 to 400 nm and tube walls (shells) which were 20 to 40nm thick. Structural analysis of the tubes by Transmission Electron Microscopy revealed a poly(nano)crystalline (Ni, Py) and amorphous (CoFeB) structure. Electrical transport experiments as a function of temperature revealed different transport mechanisms for each of the materials. Electron-phonon scattering dominated the temperature dependence of the resistivity in Ni, while a clear evidence for electron magnon scattering was observed in Py. Electron-electron interaction in granular and amorphous media was identified as the major contribution to the temperature dependence in CoFeB. The Anisotropic Magnetoresistance (AMR) ratios have been determined for all tubes and different temperatures. Ni nanotubes exhibited a large relative AMR effect of 1.4% at room temperature. The AMR measurements provided information about the magnetic configurations as well as the magnetization reversal mechanism. Indications for the formation of vortex segments in Ni tubes were found for the magnetization reversal when the magnetic field was perpendicular to the nanotube axis. In cooperation with the Poggio group in Basel, cantilever magnetometry has been used for the further characterization of the nanotube magnetization. The magnetization curves were compared to the AMR measurements and finite element method (FEM) micromagnetic simulations. The comparison between the experimental results and the simulations suggested that the roughness of Ni tubes gave rise to segmented magnetic switching. An almost perfect axial alignment of the remanent magnetization has been observed in Py and CoFeB nanotubes. The influence of the inhomogeneous internal field in transverse magnetic fields was investigated by simulation. The segment-wise alignment of spins with the field direction is argued to provoke characteristic kinks in the hysteresis curve and measured AMR effect. Magnetothermal spatial mapping experiments using the anomalous Nernst effect (ANE) complemented the magnetotransport experiments in cooperation with the group of Prof. Grundler in Munich. Here, first evidence of end-vortices entering the nanotube before reversal could be found. Electrically detected spin wave resonance experiments have been performed in cooperation with the group of Prof. Grundler on individual nanotubes. The detected voltage, generated by the spin rectification effect, revealed multiple resonances in the GHz frequency. The experimentally observed resonances were compared to calculated ones extracted from dynamic simulations. With this comparison, the signatures could be attributed to azimuthally confined spin-wave modes. The deduced dispersion relation suggested the quantization of exchange dominated spin waves in that resonance frequencies follow roughly a quadratic dependence on the wave vector.

EPFL2014

Propagation of Spin Waves Excited in a Permalloy Film by a Finite-Ground Coplanar Waveguide: A Combined Phase-Sensitive Micro-Focused Brillouin Light Scattering and Micromagnetic Study

Dirk Grundler

The excitation of spin waves by a microwave current injected into a coplanar waveguide with finite-width ground lines on a continuous Permalloy film is investigated both experimentally and numerically. Phase sensitive micro-focused Brillouin light scattering has been employed to reveal the spatial profile of the propagating spin waves in the magnetostatic surface wave geometry. The experimental results have been satisfactorily reproduced by means of micromagnetic simulations. The exciting microwave field used in this simulation has the spatial profile defined by the coplanar waveguide and user-defined periodic boundary conditions were employed in order to simulate the extended system. The resulting space and time dependent evolution of the magnetization has been analyzed by means of one and two dimensional fast Fourier transform algorithm in order to obtain the spatial profile and the frequency spectrum of the excited spin waves as well as their dispersion relations. Evidence is given to asymmetric emission from the two sides of the coplanar waveguide due to the symmetry breaking related to the sense of precession of the dynamical magnetization, as well as to the near-field effects of the extended spin wave emitter.