Electrical detection of magnetic skyrmions by tunnelling non-collinear magnetoresistance

Abstract

Magnetic skyrmions are localized non-collinear spin textures with a high potential for future spintronic applications. Skyrmion phases have been discovered in a number of materials and a focus of current research is to prepare, detect and manipulate individual skyrmions for implementation in devices. The local experimental characterization of skyrmions has been performed by, for example, Lorentz microscopy or atomic-scale tunnel magnetoresistance measurements using spin-polarized scanning tunnelling microscopy. Here we report a drastic change of the differential tunnel conductance for magnetic skyrmions that arises from their non-collinearity: mixing between the spin channels locally alters the electronic structure, which makes a skyrmion electronically distinct from its ferromagnetic environment. We propose this tunnelling non-collinear magnetoresistance as a reliable all-electrical detection scheme for skyrmions with an easy implementation into device architectures.

Impact of the skyrmion spin texture on magnetoresistance

“…All spectra are featureless at negative sample bias (not shown) but show characteristic peaks in the unoccupied states: the ferromagnetic states have a single peak at around +0.7 V, which shifts towards higher energy for the spin spiral states and the skyrmion centers; at the same time the intensity of this peak decreases. As demonstrated previously 9 a two peak structure is found for the skyrmion center of the hcp PdFe, and this change of the electronic structure has been modeled by tight-binding based on density function theory (DFT) calculations for the ferromagnetic state which neglect SOC 9 and has been shown to originate from NCMR. In addition, the vacuum LDOS, which correlates with the experimentally measured dI/dU , has been obtained from these DFT calculations for the ferromagnetic state and a spin spiral with a period of 5.14 nm, very similar to the experimentally observed one.…”

“…Recent experimental studies have shown that discrete jumps in magnetic field dependent MR can be linked to the appearance and disappearance of individual skyrmion tubes in a wire 13 . In this work we extend the investigation of the previously studied system of PdFe on Ir(111) and present a more detailed analysis, in particular of the central finding of the response of a susceptible peak in the differential conductance and its relation to the non-collinearity 9 . First, we present data which demonstrates that the NCMR effect occurs also for the zero magnetic field spin spiral ground state and in both stackings of the Pd on Fe.…”

“…Due to its spatial resolution STM has been a key method to unravel the changes of the vacuum LDOS of a domain wall: the difference between the center of the wall (in-plane magnetization) and a domain (out-of-plane) was detected with a non-magnetic tip and it was pinned down by accompanying density functional theory (DFT) calculations of the electronic band structure to a mixing of states due to spin-orbit coupling (SOC), resulting in tunnel anisotropic magnetoresistance (TAMR) 7 . Recently, STM measurements have identified an MR effect, which originates from the non-collinearity of a spin texture 9 . The observed non-collinear magnetoresistance (NCMR), see sketch in inset to Fig.…”

Interplay between size and stability of magnetic skyrmions

“…For nanoscale skyrmions, thermal stability becomes an issue as thermal fluctuations can spontaneously destroy skyrmion states and, therefore, corrupt the stored data. Available experimental data on magnetic skyrmions in various materials and material combinations demonstrates inverse relationship between the skyrmion size and skyrmion stability: small skyrmions tend to be less stable compared to large ones [8,[11][12][13][14][15][16][17][18][19][20][21]. For example, roomtemperature skyrmions in a Pt/CoFeB/MgO heterostructure are roughly 100 nm in diameter [18], which is more than an order of magnitude larger than skyrmions observed in a Pd/Fe/Ir(111) system only at low temperatures by means of spin-polarized scanning tunneling microscopy [19,20].…”

Insights into the orbital magnetism of noncollinear magnetic systems

“…13 It also generates a topological contribution to the Hall effect, a transport signature of a skyrmion-hosting sample, 14,15 and was shown to enable the electrical detection of an isolated skyrmion. 16,17 The link between the magnetic structure and orbital electronic properties was explored for other kinds of magnetic systems before, [18][19][20][21][22][23] with renewed interest since the experimental discovery of skyrmions. [24][25][26][27][28][29][30] Inspired by the investigations of nanosized skyrmions in the PdFe/Ir(111) system, 31,32 we uncovered another manifestation of their topological nature: a new kind of orbital magnetism.…”