Free monopoles have fascinated and eluded researchers since their prediction by Dirac 1 in 1931. In spin ice, the bulk frustrated magnet, local ordering principles known as ice rules-two-in/two-out for four spins arranged in a tetrahedron-minimize magnetic charge. Remarkably, recent work 2-5 shows that mobile excitations, termed 'monopole defects', emerge when the ice rules break down 2 . Using a cobalt honeycomb nanostructure we study the two-dimensional planar analogue called kagome or artificial spin ice. Here we show direct images of kagome monopole defects and the flow of magnetic charge using magnetic force microscopy. We find the local magnetic charge distribution at each vertex of the honeycomb pins the magnetic charge carriers, and opposite charges hop in opposite directions in an applied field. The parameters that enter the problem of creating and imaging monopole defects can be mapped onto a simple model that requires only the ice-rule violation energy and distribution of switching fields of the individual bars of a cobalt honeycomb lattice. As we demonstrate, it is the exquisite interplay between these energy scales in the cobalt nanostructure that leads to our experimental observations.The dipolar interactions of a given spin with all of its nearest neighbours cannot be satisfied on a triangular lattice, resulting in a frustrated magnetic state with strong correlations and a local ordering principle, but no long-range order. Owing to its equivalence to the electrical charge distribution in water ice 6 , the materials are known as 'spin ices' and the local ordering scheme as the 'ice rules'. Spin-ice materials such as Dy 2 Ti 2 O 7 have been subject to an intense research effort 7,8 and frustrated magnetism has evolved into a deeply interdisciplinary field, providing model systems for complex biological problems and a mathematical basis for the neural network algorithm from the Sherrington-Kirkpatrick model 9 .A powerful way to understand spin ice is to consider the magnetic dipole as a positive and negative magnetic charge (±q) separated by one lattice spacing. The ice rule can then be described as the local minimization at each lattice site i of the total magnetic charge (Q i, = q i ). Predictions suggest that the magnetic properties can be fractionalized, with mobile excitations carrying magnetic charge, rather than spin, and their interactions being described by a magnetic Coulomb's law 2 (equation (1))where V 0 is the self-energy and r ij is the separation. Although these topological excitations are confined to the dipole lattice, and they are compatible with Maxwell's equations 10 , their free magnetic charge character has led to the nomenclature magnetic Blackett Laboratory, Imperial College, Prince Consort Road, London SW7 2AZ, UK. *e-mail: W.Branford@imperial.ac.uk.monopole defects. Recent studies 3-5,10 in rare-earth pyrochlores strongly suggest that monopole defects exist in bulk spin ice 10 .Creating an odd number of intersecting dipoles, as in 'kagome spin ice' 11 , is interesting becaus...

Artificial spin ice, made up of planar nanostructured arrays of simple ferromagnetic bars, is a playground for rich physics associated with the spin alignment of the bars and spin texture associated with the magnetic frustration at the bar vertices. The phase diagram is exotic, showing magnetic monopole-like defects and liquid and solid phases of spins arranged in loop states with predicted chiral order. We show that magnetotransport measurements in connected honeycomb structures yield the onset of an anomalous Hall signal at 50K. The temperature scale can be attributed to the long-range dipolar ice phase.The topological Hall signal arises because chiral loops form at the sample edges, indicating a generic route to exotic states via nanoarray edge structure.

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Arrays of non-interacting nanomagnets are widespread in data storage and processing. As current technologies approach fundamental limits on size and thermal stability, enhancing functionality through embracing the strong interactions present at high array densities becomes attractive. In this respect, artificial spin ices are geometrically frustrated magnetic metamaterials that offer vast untapped potential due to their unique microstate landscapes, with intriguing prospects in applications from reconfigurable logic to magnonic devices or hardware neural networks. However, progress in such systems is impeded by the inability to access more than a fraction of the total microstate space. Here, we demonstrate that topological defect-driven magnetic writing-a scanning probe technique-provides access to all of the possible microstates in artificial spin ices and related arrays of nanomagnets. We create previously elusive configurations such as the spin-crystal ground state of artificial kagome dipolar spin ices and high-energy, low-entropy 'monopole-chain' states that exhibit negative effective temperatures.

We describe the controlled synthesis of dextran-coated superparamagnetic iron oxide nanoparticles (SPIONs) using a stable passively-driven capillary-based droplet reactor. High quality highly crystalline particles were obtained with a narrow size distribution of mean diameter 3.6 nm and standard deviation 0.8 nm. The particles were evaluated for use in MRI, and found to exhibit a large saturation magnetisation of 58 emu/g and a high T 2 relaxivity of 66 mM À1 s À1 at 4.7 T, signifying good MRI contrast enhancement properties.