Unlocking Bloch-type chirality in ultrathin magnets through uniaxial strain

Abstract

Chiral magnetic domain walls are of great interest because lifting the energetic degeneracy of left-and right-handed spin textures in magnetic domain walls enables fast current-driven domain wall propagation. Although two types of magnetic domain walls are known to exist in magnetic thin films, Bloch-and Néel-walls, up to now the stabilization of homochirality was restricted to Néel-type domain walls. Since the driving mechanism of thin-film magnetic chirality, the interfacial Dzyaloshinskii-Moriya interaction, is thought to vanish in Bloch-type walls, homochiral Bloch walls have remained elusive. Here we use real-space imaging of the spin texture in iron/nickel bilayers on tungsten to show that chiral domain walls of mixed Bloch-type and Néel-type can indeed be stabilized by adding uniaxial strain in the presence of interfacial Dzyaloshinskii-Moriya interaction. Our findings introduce Bloch-type chirality as a new spin texture, which may open up new opportunities to design spin-orbitronics devices.

Out-of-plane chiral domain wall spin-structures in ultrathin in-plane magnets

“…2a), with domain walls exhibiting Néel wall texture, reorients to an out-of-plane easy axis (shaded blue in Fig. 2a), also with Néel type domain walls17222324. This is consistent with experimental observations of domain wall textures near spin reorientation transitions in Fe/Ni bilayers on Cu(100) crystals where K u ≈0 (ref.…”

“…Propagating along the W[001] direction from domains magnetized along [001] (cyan in the figures) to domains magnetized along [00–1] (red in the figures), the domain wall magnetization always points down (white in the figures); and propagating from domains magnetized along [00–1] to domains magnetized along [001] the magnetization always points up (black in the figures). This pattern in the handedness of the spin textures of these domain walls suggests that the interfacial DMI at the Ni/W interface22 plays an important role in these structures.…”

“…In the Fe/Ni/W(110) system, additional contributions to the magnetic free energy might play a role to stabilize the domain wall structure. These include perpendicular magnetic anisotropy of the Fe layer2122, uniaxial in-plane anisotropy due to epitaxial strain at the Ni(111)/W(110) interface2122 and the interfacial DMI at the Ni/W interface22. Looking at Fig.…”

“…However, in most thin film systems the energy penalty associated with rotating spins into a hard axis is smaller than the dipolar energy penalty associated with rotating spins into an out-of-plane alignment. One contribution that can counter this dipolar energy penalty is perpendicular magnetic anisotropy, which is indeed present in Fe layers grown in this orientation2122.…”

Spin–Orbit Effects in CoFeB/MgO Heterostructures with Heavy Metal Underlayers

“…The Dzyaloshinskii-Moriya interaction, 36,37 an anti-symmetric exchange interaction which favors orthogonal alignment of neighboring magnetic moments, develops if the interface contains materials with strong spin orbit coupling. 38 In addition to the broken structural inversion symmetry of the system, strong interfacial DMI are responsible for the emergence of chiral magnetic structures, such as chiral domain walls, [27][28][29][39][40][41][42][43][44][45][46] spin spirals 47 and skyrmions. [48][49][50][51][52] Here we review and summarize recent experimental results on the spin-orbit e®ects in heavy metal (HM)/CoFeB/MgO heterostructures.…”