Investigation of the Dzyaloshinskii-Moriya interaction and room temperature skyrmions in W/CoFeB/MgO thin films and microwires

Abstract

Recent studies have shown that material structures, which lack structural inversion symmetry and have high spin-orbit coupling can exhibit chiral magnetic textures and skyrmions which could be a key component for next generation storage devices. The Dzyaloshinskii-Moriya Interaction (DMI) that stabilizes skyrmions is an anti-symmetric exchange interaction favoring non-collinear orientation of neighboring spins. It has been shown that material systems with high DMI can lead to very efficient domain wall and skyrmion motion by spin-orbit torques. To engineer such devices, it is important to quantify the DMI for a given material system. Here we extract the DMI at the Heavy Metal (HM) /Ferromagnet (FM) interface using two complementary measurement schemes namely asymmetric domain wall motion and the magnetic stripe annihilation. By using the two different measurement schemes, we find for W(5 nm)/Co 20 Fe 60 B 20 (0.6 nm)/MgO(2 nm) the DMI to be 0.68 ± 0.05 mJ/m 2 and 0.73 ± 0.5 mJ/m 2 , respectively. Furthermore, we show that this DMI stabilizes skyrmions at room temperature and that there is a strong dependence of the DMI on the relative composition of the CoFeB alloy. Finally we optimize the layers and the interfaces using different growth conditions and demonstrate that a higher deposition rate leads to a more uniform film with reduced pinning and skyrmions that can be manipulated by Spin-Orbit Torques.Recent advances in thin film fabrication processes have led to the accelerated development of magnetic storage devices. This has opened exciting areas of research due to the effects occurring at the interface between a heavy metal (HM) and a ferromagnet (FM). This interface is the building block for next generation memory devices such as the Spin-Orbit Torque (SOT) MRAM 1-4 . There are a number of important phenomena associated with the interface 5 : interfacial contributions to the SOTs 6 , interfacial perpendicular anisotropy 7,8 , and interfacial Dzyaloshinskii-Moriya interaction (DMI) 9-12 . DMI is an anti-symmetric exchange interaction which favours non-collinear alignment of neighbouring spins S 1

Individual skyrmion manipulation by local magnetic field gradients

“…Multilayer M s (A/m) K eff (J/m 3 The magnitude of D s extracted for Ta/CoFeB/MgO and W/CoFeB/MgO multilayers is in line with values reported for similar systems, in experiments that quantify the DMI through current-driven or field-driven domain wall motion, as well as from measurements of equilibrium domain patterns 26,[34][35][36] . On the other hand, lower estimations of D s in these systems have been extracted through Brillouin Light Scattering (BLS) measurements 37,38 .…”

“…While MFM is well established as a tool to measure DMI and investigate nanoscale magnetic skyrmions in Co-based multilayer stacks 17,42,[44][45][46] , similar studies for CoFeB-based systems have involved using less widely available techniques, such as Scanning Transmission X-ray Microscopy (STXM) 16,18,26,27 , Magnetic Transmission soft X-ray Microscopy (MTXM) 16,19 or X-ray holography 25 . MFM imaging of soft magnetic materials like CoFeB can actually be very challenging due to image deformations caused by the probe-sample magnetic interaction.…”

Quantifying chiral exchange interaction for Néel-type skyrmions via Lorentz transmission electron microscopy

“…Magnetic multilayers are thus more technologically promising for enabling functional skyrmionic logic/memory, 13,14,24,36,37 and tremendous effort has recently been devoted towards this goal. 28,29,41,42 For example, multilayers based on Pt/Co/Ta, Pt/CoFeB/MgO, W/CoFeB/MgO, and Pt/Co/Ir trilayers have been synthesized in which electrical generation, manipulation, and detection of magnetic skyrmions were demonstrated. 28,29,[42][43][44] Here, we demonstrate the stabilization of zero-field nanoscale skyrmions in another system -Pt/Co/W multilayers.…”

“…28,29,41,42 For example, multilayers based on Pt/Co/Ta, Pt/CoFeB/MgO, W/CoFeB/MgO, and Pt/Co/Ir trilayers have been synthesized in which electrical generation, manipulation, and detection of magnetic skyrmions were demonstrated. 28,29,[42][43][44] Here, we demonstrate the stabilization of zero-field nanoscale skyrmions in another system -Pt/Co/W multilayers. Our work is motivated by the fact that Pt/Co/W trilayers are expected to be energetically advantageous for boosting the motion of magnetic skyrmions.…”

Enhanced interfacial Dzyaloshinskii—Moriya interactions in annealed Pt/Co/MgO structures

“…We quantified the strength of DMI in our samples, employing a Kerr microscope to observe asymmetric DW movement in the creep regime with an in-plane field and a perpendicular field . The dependence of DW velocities on the inplane field is found to be roughly quadratic, where the minimum occurs at a non-zero value of , which is defined as the effective DMI field [19][20][21] .…”