Magnetization reversal of an individual exchange biased permalloy nanotube

We investigate the magnetization reversal mechanism in an individual permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is affixed to the tip of a Si cantilever and positioned in order to optimally couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the NT's volume magnetization by dynamic cantilever magnetometry and its stray flux using the nanoSQUID. We observe a training effect and temperature dependence in the magnetic hysteresis, suggesting an exchange bias. We find a low blocking temperature $T_B = 18 \pm 2$ K, indicating the presence of a thin antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy on similar samples. Furthermore, we measure changes in the shape of the magnetic hysteresis as a function of temperature and increased training. These observations show that the presence of a thin exchange-coupled native oxide modifies the magnetization reversal process at low temperatures. Complementary information obtained via cantilever and nanoSQUID magnetometry allows us to conclude that, in the absence of exchange coupling, this reversal process is nucleated at the NT's ends and propagates along its length as predicted by theory.Comment: 8 pages, 4 figure

Emergent dynamic chirality in a thermally driven artificial spin ratchet

This is the author accepted manuscript. The final version is available from Nature Research via the DOI in this record. Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice1,2 can lead to specific collective behaviour3, including emergent magnetic monopoles4,5, charge screening6,7 and transport8,9, as well as magnonic response10-12. Here, we demonstrate a spin-ice-based activematerial in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet13,14, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells.Engineering and Physical Sciences Research Council (EPSRC)Royal Society (Government)University of St PoeltenEuropean Union Horizon 2020: Marie Sklodowska-Curie grantVienna Science and Technology FundSwiss National Science Foundatio

Emergent dynamic chirality in a thermally driven artificial spin ratchet

Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice1, 2 can lead to specific collective behaviour3, including emergent magnetic monopoles4, 5, charge screening6, 7 and transport8, 9, as well as magnonic response10, 11, 12. Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet13, 14, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells

Discrete Hall resistivity contribution from Néel skyrmions in multilayer nanodiscs

Magnetic skyrmions are knot-like quasiparticles. They are candidates for non-volatile data storage in which information is moved between fixed read and write terminals. The read-out operation of skyrmion-based spintronic devices will rely on the electrical detection of a single magnetic skyrmion within a nanostructure. Here we present Pt/Co/Ir nanodiscs that support skyrmions at room temperature. We measured the Hall resistivity and simultaneously imaged the spin texture using magnetic scanning transmission X-ray microscopy. The Hall resistivity is correlated to both the presence and size of the skyrmion. The size-dependent part matches the expected anomalous Hall signal when averaging the magnetization over the entire disc. We observed a resistivity contribution that only depends on the number and sign of skyrmion-like objects present in the disc. Each skyrmion gives rise to 22 ± 2 nΩ cm irrespective of its size. This contribution needs to be considered in all-electrical detection schemes applied to skyrmion-based devices. Not only the area of Néel skyrmions but also their number and sign contribute to their Hall resistivity

Replacing the services sector and three-sector theory: urbanization and control as economic sectors

Developed during the Second World War, ‘three-sector theory’ popularized the notion of the ‘services’ sector. It has quietly underpinned understandings of economic structure ever since. The limitations and influence of this basic breakdown have led to many critiques and extensions, but no replacements. Inspired by Henri Lefebvre’s The Urban Revolution (1968), we develop a four-sector model that replaces services with sectors focused on urbanization and control. We argue that this model is a better reflection of material economic life, and a more useful way of approaching the 21st-century economy. It also offers scholars of urbanization and regional development a creative new way of seeing urbanization

Globalisation and services-driven economic growth: An introduction

Over the past several decades the weight and the role of services in the global economy have steadily gained importance. Originally, services were primarily seen as a residual category of economic output that could not be attributed to either agriculture or manufacturing. The more or less continuous growth of the sector throughout the twentieth and early stages of the twenty-first century however, has led to a gradual reappraisal (Daniels, 2004; Elfring, 1989). While interest in the role of services in economic transformation goes back to at least halfway the previous century (see for example Clark, 1940; Kuznets, 1957; Stigler, 1956), it has intensified notably from the 1970s onwards, spurred by developments such as the rapid growth of producer services and the heightened recognition of producer services’ role in innovation and competitiveness (Hertog, 2000; OECD, 2000; Wood, 2002a). More recently still, the digitisation and advances in information and communication technologies (ICT) that in combination with services trade liberalisation have allowed for the unbundling and drastic reorganisation of services production have fuelled further debate (Baldwin, 2011; Blinder, 2006; Lambregts et al., 2015). According to the latest estimates, services account for no less than 62.5 per cent of global GDP and about 43 per cent of global employment (CIA Factbook, 2015). Moreover, a wide and rapidly growing variety of services has become tradeable in the past decades, enabling countries across the world, including in the Global South, to bank on services exports as an economic growth strategy (UNCTAD, 2004). Services now feature firmly alongside commodities and manufactured goods in global and regional trade agreements and their potential role in development has started to gain the full attention from academics as well as international development agencies such as the World Bank, UNCTAD, UNESCAP, IMF, ILO, OECD and ADB. Services, in sum, have evolved from a dependent and largely neglected economic category to the world’s largest and arguably most dynamic generator of value added and jobs (ILO, 2015), a major driving force of globalisation (Blinder, 2006, Bryson, 2007; Dossani and Kenney, 2007), and – possibly – the next engine for growth in countries across the Global South (Ghani and O’Connell, 2014; Kleibert, 2015; 2 Lambregts et al., 2015). This naturally makes the world of services an increasingly relevant field of interdisciplinary scholarly enquiry, with a number of issues vying for attention. This book concentrates on the question how the global reorganisation of services production alters the relation between and generates different sets of challenges and opportunities in the Global North and the Global South. It examines and provides empirical accounts of how changes in the geography of service delivery generate new interdependencies between services producing and services consuming regions across the globe; how services help to mitigate the impact of and contribute to recovery from economic crises in the Global North; and how the unbundling and relocation of services production fosters economic development and service sector-driven modernisation processes in new locations in the Global South. The remainder of this introductory chapter further sets the scene, identifies some of the major issues at stake, defines the book’s objective and outlines the rest of the book

Conclusions: trends and ways forward in services-driven economic growth

This book has highlighted a number of recent dynamics in the service sector. It started by arguing how services have transformed from a residual category of economic output to a dynamic sector that is the world’s largest generator of added value and jobs. Case studies from different parts of the world were subsequently presented to underline how services play a role in providing new employment opportunities, are the most significant source of inward investment for many countries, and have become increasingly tradeable over a long distance. This final chapter brings together and reflects upon the theoretical and empirical insights from the case studies. The three tenets by Ghani and O’Connell (2014) that were introduced in Chapter 1 are used to structure the discussion in the first part of this final chapter