Spin-transfer torque and spin Hall effects combined with their reciprocal phenomena, spin pumping and inverse spin Hall effects (ISHEs), enable the reading and control of magnetic moments in spintronics. The direct observation of these effects remains elusive in antiferromagnetic-based devices. We report subterahertz spin pumping at the interface of the uniaxial insulating antiferromagnet manganese difluoride and platinum. The measured ISHE voltage arising from spin-charge conversion in the platinum layer depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized subterahertz irradiation. Our results open the door to the controlled generation of coherent, pure spin currents at terahertz frequencies.

For over ten years, arrays of interacting single-domain nanomagnets, referred to as artificial spin ices, have been engineered with the aim to study frustration in model spin systems.Here, we use Fresnel imaging to study the reversal process in 'pinwheel' artificial spin ice, a modified square ASI structure obtained by rotating each island by some angle about its midpoint. Our results demonstrate that a simple 45 • rotation changes the magnetic ordering from antiferromagnetic to ferromagnetic, creating a superferromagnet which exhibits mesoscopic domain growth mediated by domain wall nucleation and coherent domain propagation. We observe several domain-wall configurations, most of which are direct analogues to those seen in continuous ferromagnetic films. However, novel charged walls also appear due to the geometric constraints of the system. Changing the orientation of the external magnetic field allows control of the nature of the spin reversal with the emergence of either 1-D or 2-D avalanches. This unique property of pinwheel ASI could be employed to tune devices based on magnetotransport phenomena such as Hall circuits.Artificial spin ice (ASI) systems have been used not only as a route to new physical phenomena, but also to gain insight into fundamental physics. Such capabilities are only possible because these structures are able to emulate the behaviour of assemblies of the individual spins in atomic 1 arXiv:1808.10490v1 [cond-mat.dis-nn]

We report on the crossover from the thermal to athermal regime of an artificial spin ice formed from a square array of magnetic islands whose lateral size, 30 nm × 70 nm, is small enough that they are dynamic at room temperature. We used resonant magnetic soft x-ray photon correlation spectroscopy (XPCS) as a method to observe the time-time correlations of the fluctuating magnetic configurations of spin ice during cooling, which are found to slow abruptly as a freezing temperature T 0 = 178 ± 5 K is approached. This slowing is well-described by a Vogel-Fulcher-Tammann law, implying that the frozen state is glassy, with the freezing temperature being commensurate with the strength of magnetostatic interaction energies in the array. The activation temperature, T A = 40 ± 10 K, is much less than that expected from a Stoner-Wohlfarth coherent rotation model. Zerofield-cooled/field-cooled magnetometry reveals a freeing up of fluctuations of states within islands above this temperature, caused by variation in the local anisotropy axes at the oxidised edges.This Vogel-Fulcher-Tammann behavior implies that the system enters a glassy state on freezing, which is unexpected for a system with a well-defined ground state.

Artificial frustrated systems offer a playground to study the emergent properties of interacting systems. Most work to date has been on spatially periodic systems, known as artificial spin ices when the interacting elements are magnetic. Here we have studied artificial magnetic quasicrystals based on quasiperiodic Penrose tiling patterns of interacting nanomagnets. We construct a low energy configuration from a stepby-step approach that we propose as a ground state. Topologically induced emergent frustration means that this configuration cannot be constructed from vertices in their ground states. It has two parts, a quasi-one-dimensional "skeleton" that spans the entire pattern and is capable of long-range order, surrounding "flippable" clusters of macrospins that lead to macroscopic degeneracy. Magnetic force microscopy imaging of Penrose tiling arrays revealed superdomains that are larger for more strongly coupled arrays, especially after annealing the array above its blocking temperature. G eometrical frustration not only exists in crystalline materials such as the Ice I h phase of water and the rare-earth pyrochlore spin ices 1,2 , but can also be realised in a wide range of artificial systems 3-6 . Since they are built using nanotechnology, the structure of such a system, and the frustrated interactions it embodies, can be designed rather than discovered. When magnetic, such systems are known as artificial spin ices 7 .Two different analogs of the pyrochlores are the square 3,8-13 and kagomé 14-18 ice arrays, which have been widely studied. The square ice array has a long-range ordered antiferromagnetic ground state that is easily accessible through annealing 8,10,19 , although introducing a height offset between islands can restore the extensive degeneracy 13 . Meanwhile the kagomé pattern has stronger frustration and a richer phase diagram 20,21 , whose phase transitions have been probed by low energy muon spectroscopy 22 . Recent attention has focussed on thermal excitations in these systems 8,10-12,19,23-29 , as well as new lattices designed to give rise to novel phenomena 30 . These include the 'shakti' lattice, which displays topologically induced emergent frustration 31 , the 'tetris' lattice, which exhibits an emergent reduction in dimensionality 32 , and artificial charge ices suitable for data storage 33 .These arrays are spatially periodic with discrete translational symmetry. Since artificial systems may be designed arbitrarily, there is no need to always respect this constraint. Indeed, the discovery of quasicrystalline materials 34 shows that nature does not always respect it either. Quasicrystals have structures that lack translational symmetry 35 , and so possess complex forms of magnetic frustration leading to glassy behaviour [36][37][38] . Building artificial magnetic quasicrystals based on Penrose tilings brings the ability to inspect the magnetic configuration at the level of individual macrospins, allowing deeper insight into this frustration and its manifestations. Kite-and-dart tiling...