Direct Observation of Unconventional Topological Spin Structure in Coupled Magnetic Discs

Confined magnetic thin films are known to exhibit a variety of fascinating topological spin states such as Skyrmions, vortices, and antivortices. Such topological excitations are fundamentally important to our understanding of quantum critical phenomenon and related phase transitions. Here we report on the direct observation of an unconventional topological spin state and its behavior in antiferromagnetically coupled NiFe discs at room temperature. The observed spin structure is similar to the theoretically predicted merons which have not yet been observed directly. We have used in situ Lorentz microscopy magnetizing experiments combined with micromagnetic simulations to follow the stability and the behavior of the meron state. The work presented in this paper will open new opportunities for direct experimental investigation of various topological states that can provide insights into the fundamental physics of their interactions.

Figure 1

(a) Schematic showing the spin structure of a meronlike state with opposite chirality; (b) schematic showing the trilayer discs and direction of the electron beam for imaging, and (c) LTEM under-focus image, and (d) reconstructed magnetic induction color map.

Figure 2

Micromagnetic simulation results showing the magnetization in top and bottom disc along with simulated LTEM underfocus image and magnetic induction map for various interlayer coupling strengths (a) $+15.6\mathrm{Oe}$, (b) and $-15.6\mathrm{Oe}$.

Figure 3

Micromagnetic simulations showing the evolution of the meronlike state from a vortex state in the bottom disc for $H_{\mathrm{IEC}}=-15.6\mathrm{Oe}$. The numbers in the top right corner show the simulation time steps in nanoseconds.

Figure 4

(a) LTEM underfocus images recorded as a function of applied in-plane magnetic field, and (b) micromagnetic simulations for a disc in meronlike state with interlayer coupling strength of $-15.6\mathrm{Oe}$.