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Multiphysics simulations of magnetic nanostructures by Matteo Franchin In recent years the research on magnetism has seen a new trend emerging, characterised by considerable effort in developing new nanostructures and nding new ways to control and manipulate their magnetisation, such as using spin polarised currents or light pulses. The field of magnetism is thus moving towards the multiphysics direction, since it is increasingly studied in conjunction with other types of physics, such as electric and spin transport, electromagnetic waves generation and absorption, heat generation and diffusion. Understanding these new phenomena is intriguing and may lead to major technological advances. Computer simulations are often invaluable to such research, since they offer a way to predict and understand the physics of magnetic nanostructures and help in the design and optimisation of new devices. For the preparation of this thesis the Nmag multiphysics micromagnetic simulation package has been further developed and improved by the author. The software has also been extended in order to model exchange spring systems. Using Nmag, we carried out micromagnetic simulations in order to characterise the magnetisation dynamics in exchange spring systems and derived analytical models to validate and gain further insight into the numerical results. We found that the average magnetisation moves in spiral trajectories near equilibrium and becomes particularly soft (low oscillation frequency and damping, high amplitude) when the applied field is close to a particular value, called the bending field. We studied spin transport in exchange spring systems and investigated new geometries and setups in order to maximise the interaction between spin polarised current and magnetisation. We found that by engineering a trilayer exchange spring system in the form of a cylindrical nanopillar, it is possible to obtain microwave emission with frequencies of 5-35 GHz for applied current densities between 0.5-2.0 x 10 (superscript 11) A/m2 and without the need for an externally applied magnetic field. We proposed a one dimensional analytical model and found a formula which relates the emission frequency to the geometrical parameters and the current density.

We review design and development decisions and their impact for the open source code Nmag from a software engineering in computational science point of view. We summarise lessons learned and recommendations for future computational science projects. Key lessons include that encapsulating the simulation functionality in a library of a general purpose language, here Python, provides great flexibility in using the software. The choice of Python for the top-level user interface was very well received by users from the science and engineering community. The from-source installation in which required external libraries and dependencies are compiled from a tarball was remarkably robust. In places, the code is a lot more ambitious than necessary, which introduces unnecessary complexity and reduces main- tainability. Tests distributed with the package are useful, although more unit tests and continuous integration would have been desirable. The detailed documentation, together with a tutorial for the usage of the system, was perceived as one of its main strengths by the community.

We present a simple and fast method to simulate spin-torque driven magnetisation dynamics in nano-pillar spin-valve structures. The approach is based on the coupling between a spin transport code based on random matrix theory and a micromagnetics finite-elements software. In this way the spatial dependence of both spin transport and magnetisation dynamics is properly taken into account. Our results are compared with experiments. The excitation of the spin-wave modes, in- cluding the threshold current for steady state magnetisation precession and the nonlinear frequency shift of the modes are reproduced correctly. The giant magneto resistance effect and the magnetisa- tion switching also agree with experiment. The similarities with recently described spin-caloritronics devices are also discussed.

Nowadays, micromagnetic simulations are a common tool for studying a wide range of different magnetic phenomena, including the ferromagnetic resonance. A technique for evaluating reliability and validity of different micromagnetic simulation tools is the simulation of proposed standard problems. We propose a new standard problem by providing a detailed specification and analysis of a sufficiently simple problem. By analyzing the magnetization dynamics in a thin permalloy square sample, triggered by a well defined excitation, we obtain the ferromagnetic resonance spectrum and identify the resonance modes via Fourier transform. Simulations are performed using both finite difference and finite element numerical methods, with \\textsf{OOMMF} and \\textsf{Nmag} simulators, respectively. We report the effects of initial conditions and simulation parameters on the character of the observed resonance modes for this standard problem. We provide detailed instructions and code to assist in using the results for evaluation of new simulator tools, and to help with numerical calculation of ferromagnetic resonance spectra and modes in general.

We study the effect of Joule heating from electric currents flowing through ferromagnetic nanowires on the temperature of the nanowires and on the temperature of the substrate on which the nanowires are grown. The spatial current density distribution, the associated heat generation, and diffusion of heat is simulated within the nanowire and the substrate. We study several different nanowire and constriction geometries as well as different substrates: (thin) silicon nitride membranes, (thick) silicon wafers, and (thick) diamond wafers. The spatially resolved increase in temperature as a function of time is computed. For effectively three-dimensional substrates (where the substrate thickness greatly exceeds the nanowire length), we identify three different regimes of heat propagation through the substrate: regime (i), where the nanowire temperature increases approximately logarithmically as a function of time. In this regime, the nanowire temperature is well-described analytically by You et al. [APL 89, 222513 (2006)]. We provide an analytical expression for the time t_c that marks the upper applicability limit of the You model. After t_c, the heat flow enters regime (ii), where the nanowire temperature stays constant while a hemispherical heat front carries the heat away from the wire and into the substrate. As the heat front reaches the boundary of the substrate, regime (iii) is entered where the nanowire and substrate temperature start to increase rapidly. For effectively two-dimensional substrates (where the nanowire length greatly exceeds the substrate thickness), there is only one regime in which the temperature increases logarithmically with time for large times, before the heat front reaches the substrate boundary. We provide an analytical expression, valid for all pulse durations, that allows one to accurately compute this temperature increase in the nanowire on thin substrates.

Using micromagnetic simulations, we have investigated spin dynamics in a nanostructure in the presence of thermal fluctuations. In particular, we have studied the effects of a uniform temperature and of a uniform thermal gradient. In both cases, the stochastic field leads to an increase of the precession angle of the magnetization, and to a mild decreas of the linewidth of the resonance peaks. Our results indicate that the Gilbert damping parameter plays the role of control parameter for the amplification of spin waves.

Recent studies have predicted extraordinary properties for transverse domain walls in cylindrical nanowires: zero depinning current, the absence of the Walker breakdown, and applications as domain wall oscillators. In order to reliably control the domain wall motion, it is important to understand how they interact with energy barriers. In this paper, we study the motion and depinning of transverse domain walls through potential barriers in ferromagnetic cylindrical nanowires. We use magnetic fields and spin-polarized currents to drive the domain walls along the wire. Using magnetic fields, we find that the minimum and the maximum fields required to push the domain wall through the barrier differ by 30 %. On the contrary, using spin-polarized currents, we find variations of a factor 130 between the minimum value of the depinning current density and the maximum value. We study the depinning current density as a function of the height of the energy barrier using numerical and analytical methods. We find that, for a barrier of 40 k_B T, a depinning current density of about 5 uA is sufficient to depin the domain wall. We reveal and explain the mechanism that leads to these unusually low depinning currents. One requirement for this new depinning mechanism is for the domain wall to be able to rotate around its own axis. With the right barrier design, the spin torque transfer term is acting exactly against the damping in the micromagnetic system, and thus the low current density is sufficient to accumulate enough energy quickly. These key insights may be crucial in furthering the development of novel memory technologies, such as the racetrack memory, that can be controlled through low current densities.

Surface plasmon resonance (SPR) is a common and useful measurement technique to perform fast and sensitive optical detection. SPR instrumentations usually comprise optical systems of mirrors and lenses which are quite expensive and impractical for point-of-care applications. In this work, we presented a novel and compact SPR device called SPECTRA, designed as a spectrophotometer add-on with a grating coupling configuration. The device is conceived as a marketable solution to perform quick SPR measurements in grating configuration without the requirement of complex instrumentation. The device can be customized either in a vertical structure to reach lower incident light angles, or in a horizontal configuration, which is suitable for SPR analysis using liquid solutions. The SPECTRA performance was evaluated through SPR measurements in typical applications. The vertical SPECTRA system was employed to detect different functionalization molecules on gold 720 nm-period grating devices. Meanwhile, the horizontal SPECTRA configuration was exploited to carry out fluid-dynamic measurements using a microfluidic cell with glycerol solutions at increasing concentrations to account for different refractive indexes. The experimental tests confirmed that the SPECTRA design is suitable for SPR measurements, demonstrating its capability to detect the presence of analytes and changes in surface properties both in static and dynamic set-ups.

PurposeWe report on the first identified cluster of the B.1.1.7 variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections in the northeast of Italy.MethodsThe cluster was recognized in January 2021 with an epidemiological started from the hospitalization of a 68-year-old man suffering from coronavirus disease 2019 (COVID-19) related pneumonia and we surprisingly found three families involved in the same cluster.ResultsWe retrospectively rebuilt the pathway of infection and performed a virological analysis.ConclusionThis allow us to make clear the very high attack rate and the great infective capacity of this B.1.1.7 variant of SARS-CoV-2.

Background We present a comparison of renal function outcomes during HTAR with the use of a new hybrid vascular graft (GHVG) or standard graft. Methods It is a multicenter, retrospective, observational study. Between January 2015 and March 2019, 36 patients were treated with HTAR. We compared HTAR performed with the use of the GHVG and with the use of standard bypass graft. Primary outcome measures were hospital mortality, acute kidney injury (AKI) at 30 days and GHVG patency. Results Mean GHVG ischemia time was significantly lower for both renal arteries (right: GHVG, 4 ± 2 vs. standard graft, 15 ± 7 min; 95% CI 2.23–6.69, P  < 0.001; left: GHVG, 3 ± 2 vs. standard graft, 13 ± 7 min; 95% CI 2.44–5.03, P  < 0.001). Hospital mortality was 17% (6/36); while mortality did not differ between the two groups, postoperative acute kidney injury rate was 30.5% (11/36 patients) and was more common in the standard graft group (7% vs. 29%; OR 3.2, P  = 0.074). Estimated primary patency was 92% ± 2 (95% CI 79.5–97%) at 36 months and was not different between the two groups (GHVG 94% ± 6 vs. standard graft 91% ± 6; log-rank χ 2  = 0.260, P  = 0.610). Conclusions In our experience of HTAR, ischemia time was significantly shorter and postoperative AKI occurrence was lower with GHVG if compared to standard graft bypass, with satisfactory midterm patency rate comparable to that of standard graft bypass.