Achiral tilted domain walls in perpendicularly magnetized nanowires

Abstract

Perpendicularly magnetized nanowires exhibit distinct domain wall types depending on the geometry. Wide wires contain Bloch walls, narrow wires Néel walls. Here, the transition region is investigated by direct imaging of the wall structure using high-resolution spin-polarized scanning electron microscopy. An achiral intermediate wall type is discovered that is unpredicted by established theoretical models. With the help of micromagnetic simulations, the formation of this novel wall type is explained.In recent years, domain walls in perpendicularly magnetized materials have been intensely investigated because they are narrower than in in-plane systems and therefore, when used to store a data bit, promise higher storage density. In perpendicularly magnetized systems, domain walls are of Bloch type, i.e., the magnetization rotates within the wall plane. In in-plane magnetized systems, in contrast, diverse wall types exist. In bulk, again Bloch walls prevail, whereas in thin films, the energetically favored wall type is a Néel wall, i.e., the magnetization rotates perpendicularly to the plane of the wall. In between, a finite film-thickness range exists in which domain walls are neither of Bloch nor of Néel type. They are characterized by more complex arrangements of spins, such as zigzag patterns [1], cross-ties [2] or continuous asymmetric deformations [3]. The Bloch wall is the energetically preferred state in perpendicularly magnetized films irrespective of film thickness. Néel walls can be made the ground state by changing the geometry to wires [4,5] or adding constrictions [6], by applying magnetic fields [7,8], or by introducing Dzyaloshinskii-Moriya exchange interaction (DMI) [9].The Bloch-Néel wall transition in perpendicularly magnetized nanowires, as a function of the wire width, was indirectly observed by measuring a change in the anisotropic magnetoresistance (AMR) [10]. Most recently, it was studied analytically [5]. A direct observation of this transition in real space is missing. Both Bloch and Néel walls were observed in thin films by spinpolarized scanning tunneling microscopy [11], in multilayers by spin-polarized low-energy electron microscopy [12], and in nanowires by optically monitoring the Zeeman shift of the electron spin in a nitrogen-vacancy defect in diamond [13].In this work, we investigate domain walls at the BlochNéel wall transition in flat nanowires (or "nanostrips") with perpendicular magnetic anisotropy as a function of the nanowire width. We image the wall structure in real space using high-resolution spin-polarized scanning electron microscopy (spin-SEM), which is capable of determining the specimen's magnetization by measuring the

Switching between Magnetic Bloch and Néel Domain Walls with Anisotropy Modulations

“…4(a), the transition between Bloch and Néel DWs is not as sharp as in the thin film case. This has already been observed as a function of w [47]. As shown in Fig.…”

Measuring the Dzyaloshinskii-Moriya interaction of the epitaxial Co/Ir(111) interface

“…It is a surface sensitive technique that measures two components of the magnetization simultaneously [43][44][45] with a depth of information of about 0.5 nm [42,46] Recently, also the nanosecond magnetization dynamics became accessible [47][48][49]. The SEMPA system used here is sensitive to the in-plane spin polarization and allows for vectorial imaging of the in-plane magnetization orientation with 4° precision for each pixel of an image [50], which is prerequisite for the investigation of the angle of DW magnetization [17,38,51].…”