In-situ mapping of Bloch and Neel domain walls in magnetic thin films

Abstract text (incl. figure legends and references)

Magnetic domain walls (DWs) in ferromagnetic thin films exhibit a rich variety of configurations and corresponding dynamic properties depending on parameters like film thickness, defect density, magnetocrystalline anisotropy, exchange stiffness, and Dzyaloshinskii–Moriya interaction (DMI). These may be tuned by corresponding synthesis strategies such as multilayer growth, strain engineering, etc., which holds great potential for next-generation magnetic memory or logical devices utilizing moving DWs.

Here, we study epitaxial ferromagnetic multilayer devices of La_0.7Sr_0.3Mn_1-xRu_xO₃, consisting 8 nm thick manganite layers with varying Ru/Mn content, in order to engineer symmetric and antisymmetric exchange interaction and magnetic anisotropy across the multilayer stack. We particularly map the DW states as a function of temperature and external out-of-plane magnetic fields employing high-resolution magnetic imaging in the Transmission Electron Microscopy (TEM), in order to reveal the dominating magnetic interaction mechanisms and the impact of inversion symmetry breaking at the interfaces. Moreover, we develop and apply a high-frequency in-situ electric biasing holder facilitating the manipulation of magnetic textures and show first results on in-situ domain wall motion in soft magnetic thin films.

Lorentz TEM and transport of intensity phase reconstruction is used to characterize the magnetic domains and DWs formed as a function of temperature and perpendicular magnetic field strength. Mapping of magnetization dynamics at the nanometer scale, including DWs, is performed by application of very short current pulses in a specially designed high-frequency electric biasing holder and parallel imaging via Lorentz microscopy.

Fig. 1 shows a characteristic pattern of stripe domains. Varying the external out-of-plane magnetic field from 0 mT to -350 mT (and back) leads to hysteretic growth of one domain type with respect to the other allowing to identify the domains as out-of-plane type stabilized by magnetocrystalline anisotropy. Careful analysis of the DW contrast assisted by micromagnetic simulations furthermore allows to identify the DWs as chiral Néel type. This observation indicates the crucial impact of interfacial DMI in their formation, which is corroborated by micromagnetic simulations. We finally observe a split of the domains into bubbles of diameter smaller than 100 nms with magnetic texture characteristic of Néel skyrmions at certain field strengths.

To demonstrate the possibility of DW manipulation by (ultrashort) electric pulses and the induced spin-torque effect, we prepared thin Ni and Permalloy (Py) films on a Si₃N₄ membrane and observed the current induced DW variations by Lorentz TEM. Here, the DW shifts are quantitatively analyzed with respect to the current density and also with heat energy. We demonstrate that we can achieve current densities in the range of 10¹¹-10¹³ A/m² as required for spin torque DW motion in Py.

High-resolution magnetic field mapping of La_0.7Sr_0.3Mn_1-xRu_xO₃ multilayer system demonstrates the possibility to engineer chiral Neel domain walls and skyrmions. First steps toward spin-torque driven in-situ manipulation of DWs thin film is demonstrated using a dedicated biasing holder and sample design.