X-ray imaging of spin currents and magnetisation dynamics at the nanoscale

Understanding how spins move in time and space is the aim of both fundamental and applied research in modern magnetism. Over the past three decades, research in this field has led to technological advances that have had a major impact on our society, while improving the understanding of the fundamentals of spin physics. However, important questions still remain unanswered, because it is experimentally challenging to directly observe spins and their motion with a combined high spatial and temporal resolution. In this article, we present an overview of the recent advances in X-ray microscopy that allow researchers to directly watch spins move in time and space at the microscopically relevant scales. We discuss scanning X-ray transmission microscopy (STXM) at resonant soft X-ray edges, which is available at most modern synchrotron light sources. This technique measures magnetic contrast through the X-ray magnetic circular dichroism (XMCD) effect at the resonant absorption edges, while focusing the X-ray radiation at the nanometre scale, and using the intrinsic pulsed structure of synchrotron-generated X-rays to create time-resolved images of magnetism at the nanoscale. In particular, we discuss how the presence of spin currents can be detected by imaging spin accumulation, and how the magnetisation dynamics in thin ferromagnetic films can be directly imaged. We discuss how a direct look at the phenomena allows for a deeper understanding of the the physics at play, that is not accessible to other, more indirect techniques. Finally, we present an overview of the exciting opportunities that lie ahead to further understand the fundamentals of novel spin physics, opportunities offered by the appearance of diffraction limited storage rings and free electron lasers.Comment: 21 pages, 10 figure

Space-enhanced terrestrial solar power for equatorial regions

This Paper investigates the concept of solar mirrors in an Earth orbit to provide large-scale terrestrial equatorial solar farms with additional solar power during the hours of darkness. A flower constellation of mirrors is considered in highly eccentric orbits (semimajor axis=20,270.4  km) in order to increase the time of visibility over the solar farms, and through this architecture, only two mirrors are needed to provide complete night coverage over three equatorial locations. Selecting the proper value for the orbit eccentricity, solar radiation pressure and Earth’s oblateness perturbations act on the mirrors so that the apsidal motion of the orbit due to these perturbations is synchronized with the apparent motion of the sun. Therefore, it can be guaranteed that the perigee always points toward the sun and that the mirrors orbit mostly above the night side of the Earth. With respect to geostationary orbit, the family of orbits considered in this Paper allows a passive means to overcome issues related to orbital perturbations. Moreover, because of the large slant range from geostationary orbits, a larger mirror is required to deliver the same energy that could be delivered from a lower orbit with a smaller mirror. As a result, a single antiheliotropic flower constellation composed of two mirrors of 50  km2 would be able to deliver energy in the range of 4.60–5.20 GW·h per day to 1000  km3 solar farms on the equator. Finally, it is estimated that, deploying 90 of these constellations, the price of electricity could be reduced from 9.1 cents to 6 cents per kW⋅h

Multiple input control strategies for robust and adaptive climate engineering in a low order 3-box model

A low-order 3-box energy balance model for the climate system is employed with a multivariable control scheme for the evaluation of new robust and adaptive climate engineering strategies using solar radiation management. The climate engineering measures are deployed in three boxes thus representing northern, southern and central bands. It is shown that, through heat transport between the boxes, it is possible to effect a degree of latitudinal control through the reduction of insolation. The approach employed consists of a closed-loop system with an adaptive controller, where the required control intervention is estimated under the RCP4.5 radiative scenario. Through the online estimation of the controller parameters, adaptive control can overcome key issues related to uncertainties of the climate model, the external radiative forcing and the dynamics of the actuator used. In fact, the use of adaptive control offers a robust means of dealing with unforeseeable abrupt perturbations, as well as the parametrization of the model considered, to counteract the RCP4.5 scenario, while still providing bounds on stability and control performance. Moreover, applying multivariable control theory also allows the formal controllability and observability of the system to be investigated in order to identify all feasible control strategies

Generalized gradient flow structure of internal energy driven phase field systems

In this paper we introduce a general abstract formulation of a variational thermomechanical model, by means of a unified derivation via a generalization of the principle of virtual powers for all the variables of the system, including the thermal one. In particular, choosing as thermal variable the entropy of the system, and as driving functional the internal energy, we get a gradient flow structure (in a suitable abstract setting) for the whole nonlinear PDE system. We prove a global in time existence of (weak) solutions result for the Cauchy problem associated to the abstract PDE system as well as uniqueness in case of suitable smoothness assumptions on the functionals

A continuous latitudinal energy balance model to explore non-uniform climate engineering strategies

In order to investigate the effects of solar radiation management (SRM) technologies for climate engineering, an analytical model describing the main latitu8 dinal dynamics of the Earth’s climate with closed-loop control has been developed. The model is a time-dependent Energy BalanceModel (EBM) with latitudinal resolution and allows for the evaluation of non-uniform climate engineering strategies. The resulting partial differential equation is solved using a Green’s function approach. This model offers an efficient analytical approach to design strategies that counter act climate change on a latitudinal basis to overcome regional disparities in cooling. Multi-objective analyses are considered and time-dependent analytical expressions of control functions with latitudinal resolution can be obtained in several circumstances. Results broadly comparable with the literature are found, demonstrating the utility of the model in rapidly assessing new climate engineering controls laws and strategies. For example, the model is also used to quickly assess the trade-off between the number of degrees of freedom of SRM and the rms error in latitudinal temperature compensation. Moreover, using the EBM the dynamics of the ice line can be investigated and a Lyapunov stability analysis is employed to estimate the maximum reduction of solar insolation through climate engineering before the current climate falls into an ice-covered state. This provides an extreme operational boundary to future climate engineering ventures

A Dissipative Model for Hydrogen Storage: Existence and Regularity Results

We prove global existence of a solution to an initial and boundary value problem for a highly nonlinear PDE system. The problem arises from a thermomechanical dissipative model describing hydrogen storage by use of metal hydrides. In order to treat the model from an analytical point of view, we formulate it as a phase transition phenomenon thanks to the introduction of a suitable phase variable. Continuum mechanics laws lead to an evolutionary problem involving three state variables: the temperature, the phase parameter and the pressure. The problem thus consists of three coupled partial differential equations combined with initial and boundary conditions. Existence and regularity of the solutions are here investigated by means of a time discretization-a priori estimates-passage to the limit procedure joined with compactness and monotonicity arguments

Space-Enhanced Solar Power for Equatorial Regions

This paper examines the concept of solar mirrors in a Earth orbit to provide solar farms with additional solar power during the hours of darkness. The design of the orbit is key for the purposes of the mission: the mirror needs continuous access to the Sun and the solar farm simultaneously. Therefore, orbits with high-eccentricity will be considered to increase the visibility time. Also, since the most convenient locations for solar power farms are about the equator, a suitable orbit should have a low inclination. This issue can be addressed through the concept of anti-heliotropic orbits that exploits mainly solar radiation pressure perturbations to generate highly-eccentric equatorial orbits able to maintain the orientation with respect to the Sun. The considered configuration consists in two space mirrors in a flower constellation rotating with the Earth to deliver a repeat ground track