Emerging Chirality in Artificial Spin Ice

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 50 kelvin. 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

The non-random walk of chiral magnetic charge carriers in artificial spin ice

The flow of magnetic charge carriers (dubbed magnetic monopoles) through frustrated spin ice lattices, governed simply by Coulombic forces, represents a new direction in electromagnetism. Artificial spin ice nanoarrays realise this effect at room temperature, where the magnetic charge is carried by domain walls. Control of domain wall path is one important element of utilizing this new medium. By imaging the transit of domain walls across different connected 2D honeycomb structures we contribute an important aspect which will enable that control to be realized. Although apparently equivalent paths are presented to a domain wall as it approaches a Y-shaped vertex from a bar parallel to the field, we observe a stark non-random path distribution, which we attribute to the chirality of the magnetic charges. These observations are supported by detailed statistical modelling and micromagnetic simulations. The identification of chiral control to magnetic charge path selectivity invites analogy with spintronics

Limitations in artificial spin ice path selectivity: the challenges beyond topological control

Magnetic charge is carried through nanowire networks by domain walls, and the micromagnetic structure of a domain wall provides an opportunity to manipulate its movement. We have shown previously that magnetic monopole defects exist in artificial spin ice (ASI) and result from two bar switching at a vertex. To create and manipulate monopole defects and indeed magnetic charge in general, path selectivity of the domain wall at a vertex is required. We have recently shown that in connected ASI structures, transverse wall chirality (or topology) determines wall path direction, but a mechanism known as Walker breakdown, where a wall mutates into a wall of opposite chirality partially destroys selectivity. Recently it has been claimed that in isolated Y-shaped junctions that support vortex walls, selectivity is entirely determined by chirality (or topology), the suggestion being that vortex wall chirality is robust in the Walker breakdown process. Here we demonstrate that in Y-shaped junctions, magnetic switching in the important topologically protected regime exists only for a narrow window of field and bar geometry, and that it will be challenging to access this regime in field-driven ASI. This work has implications for the wider field of magnetic charge manipulation for high density memory storage

Study of the first paramagnetic to ferromagnetic transition in as prepared samples of Mn-Fe-P-Si magnetocaloric compounds prepared by different synthesis routes

International audienceMagnetocaloric materials with composition of Mn 1.3 Fe 0.65 P 0.5 Si 0.5 have been prepared by ball milling and solid-state reaction methods and consolidated using powder annealing, and conventional and spark plasma sintering. Magnetic and calorimetric measurements show remarkable differences upon first cooling, and slight differences on second and further coolings between the samples prepared by different synthesis routes. Further measurements using Hall probe imaging in high magnetic field have been also carried out. As-prepared samples have been cooled down just above the critical temperature, and the first phase transition has been induced by application of a magnetic field. Bulk samples show staircase isothermal magnetization curves whereas powders show smoother transition curves

Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra

As charge carriers traverse a single superconductor ferromagnet interface, they experience an additional spin-dependent phase angle that results in spin mixing and the formation of a bound state called the Andreev bound state. Here we explore whether point contact Andreev reflection can be used to detect the Andreev bound state and, within the limits of our experiment, we extract the resulting spin mixing angle. By examining spectra taken from L a 1.15 S r 1.85 M n 2 O 7 − Pb junctions, together with a compilation of literature data on highly spin polarized systems, we suggest that the existence of the Andreev bound state would resolve a number of long standing controversies in the literature of Andreev reflection, as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly, we find that for high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have chosen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances

Spin-Pumping-Induced Inverse Spin Hall Effect in Nb/Ni80Fe20 Bilayers and its Strong Decay Across the Superconducting Transition Temperature

We quantify the spin Hall angle θSH and spin-diffusion length lsd of Nb from inverse spin Hall effect (ISHE) measurements in Nb/Ni80Fe20 bilayers under ferromagnetic resonance. By varying the Nb thickness tNb and comparing to a Ni80Fe20/Pt reference sample, room temperature values of θSH and lsd for Nb are estimated to be approximately -0.001 and 30 nm, respectively. We also investigate the ISHE as a function of temperature T for different tNb. Above the superconducting transition temperature Tc of Nb, a clear tNb-dependent T evolution of the ISHE is observed whereas below Tc, the ISHE voltage drops rapidly and is below the sensitivity of our measurement setup at a lower T. This suggests the strong decay of the quasiparticle (QP) charge-imbalance relaxation length across Tc, as supported by an additional investigation of the ISHE in a different sample geometry along with model calculation. Our finding suggests careful consideration should be made when developing superconductor spin Hall devices that intend to utilize QP-mediated spin-to-charge interconversion.This work is supported by EPSRC Programme Grant EP/N017242/1

Exchange-field enhancement of superconducting spin pumping

A recent ferromagnetic resonance study [Jeon et al., Nat. Mater. 17, 499 (2018)] has reported that spin pumping into a singlet superconductor (Nb) can be greatly enhanced over the normal state when the Nb is coupled to a large spin-orbit-coupling (SOC) spin sink such as Pt. This behavior has been explained in terms of the generation of spin-polarized triplet supercurrents via SOC at the Nb/Pt interface, acting in conjunction with a nonlocally induced magnetic exchange field. Here we report the effect of adding a ferromagnet (Fe) to act as an internal source of an additional exchange field to the adjacent Pt spin sink. This dramatically enhances the spin pumping efficiency in the superconducting state compared with either Pt and Fe separately, demonstrating the critical role of the exchange field in generating superconducting spin currents in the Nb

Signatures of filamentary superconductivity in antiferromagnetic BaFe2As2 single crystals

In this paper, we present ac susceptibility and magnetotransport measurements on aged single crystals of the ferropnictide parent compound, BaFe2As2 with a paramagnetic-to-antiferromagnetic transition temperature of 134 K. The ac susceptibility shows the clear onset of a partial diamagnetic response with an onset temperature, commensurate with a subtle downturn in resistivity at approximately 20 K. Below 20 K the magnetotransport shows in-plane anisotropy, magnetic-field history dependence and a hysteretic signature. Above 20 K the crystals show the widely reported high-field linear magnetoresistance. An enhanced noise signature in ac susceptibility is observed above 20 K, which varies in character with amplitude and frequency of the ac signal. The hysteresis in magnetoresistance and the observed sensitivity of the superconducting phase to the amplitude of the ac signal are indicative characteristics of granular or weakly linked filamentary superconductivity. These features taken together with the observed noise signature above $T_{\mathrm{c}}$ suggests a link between the formation of the superconducting filamentary phase and the freezing of antiphase domain walls, known to exist in these materials