Suppression of the spin waves nonreciprocity due to interfacial Dzyaloshinskii Moriya interaction by lateral confinement in magnetic nanostructures

Despite the huge recent interest towards chiral magnetism related to the interfacial Dzyaloshinskii Moriya interaction (iDMI) in layered systems, there is a lack of experimental data on the effect of iDMI on the spin waves eigenmodes of laterally confined nanostructures. Here we exploit Brillouin Light Scattering (BLS) to analyze the spin wave eigenmodes of non-interacting circular and elliptical dots, as well as of long stripes, patterned starting from a Pt(3.4 nm)/CoFeB(0.8 nm) bilayer, with lateral dimensions ranging from 100 nm to 400 nm. Our experimental results, corroborated by micromagnetic simulations based on the GPU-accelerated MuMax3 software package, provide evidence for a strong suppression of the frequency asymmetry between counter-propagating spin waves (corresponding to either Stokes or anti-Stokes peaks in BLS spectra), when the lateral confinement is reduced from 400 nm to 100 nm, i.e. when it becomes lower than the light wavelength. Such an evolution reflects the modification of the spin wave character from propagating to stationary and indicates that the BLS based method of quantifying the i-DMI strength from the frequency difference of counter propagating spin waves is not applicable in the case of magnetic elements with lateral dimension below about 400 nm.Comment: Accepted for pubblication by: Physical Review

Disorder-independent control of magnetic monopole defect population in artificial spin-ice honeycombs

Breakdown of the ice rule in artificial spin-ice nanostructures results in magnetic monopole defects with zero magnetic moment. Such defects exist during the magnetic switching process in some nanostructures and yet are absent in other apparently similar arrays having the same geometry and made from the same material components. One explanation proposed for this discrepancy is that it is due to the variation of disorder across samples, with monopole defect formation occuring only in highly disordered samples. Although disorder can indeed play a role in the determination of monopole density, in this paper we show, by experiment and simulation, that in samples of similar, low disorder, the factor controlling the nature of magnetic switching is whether the domain walls are in the transverse wall regime or in the vortex wall regime. This work illustrates that monopole formation can be controlled by intrinsic micro-magnetic behaviour as well as by extrinsic quenched disorder

Self-Similar Nested Flux Closure Structures in a Tetragonal Ferroelectric

In specific solid-state materials, under the right conditions, collections of magnetic dipoles are known to spontaneously form into a variety of rather complex geometrical patterns, exemplified by vortex and skyrmion structures. While theoretically, similar patterns should be expected to form from electrical dipoles, they have not been clearly observed to date: the need for continued experimental exploration is therefore clear. In this Letter we report the discovery of a rather complex domain arrangement that has spontaneously formed along the edges of a thin single crystal ferroelectric sheet, due to surface-related depolarizing fields. Polarization patterns are such that nanoscale "flux-closure" loops are nested within a larger mesoscale flux closure object Despite the orders of magnitude differences in size, the geometric forms of the dual-scale flux closure entities are rather similar.</p