Domain Wall Dynamics
Domain walls are naturally occurring magnetic defects that separate uniformly magnetized regions called domains in magnetic materials. They represent finite regions over which the magnetization rotates from one domain orientation to another. High speed, controlled motion of domain walls could enable non-volatile, low power, and fast magnetic memory using domain orientation (up/down or left/right) to encode bits (‘1’ or ‘0’). In a heavy metal-ferromagnet stack, the spin Hall effect in the heavy metal layer induces a torque on the ferromagnet, resulting in electric current-induced domain wall motion. The maximum domain wall velocity is limited by its ability to maintain a particular configuration known as a Néel wall. Recent work by the group has demonstrated that this speed limit vanishes in ferrimagnetic materials near angular momentum compensation.
The Néel domain wall configuration is controlled by an interaction known as the Dzyaloshinskii-Moriya Interaction (DMI) that occurs at heavy metal/ferromagnet interfaces. The DMI occurs at interfaces between ferro/ferrimagnetic atoms and heavy metal atoms exhibiting high spin-orbit coupling. In contrast to the Heisenberg interaction that yields ferromagnetic or antiferromagnetic order, the DMI favors perpendicular orientations of adjacent spins. As a result, the DMI not only stabilizes favorable domain wall configurations but also gives rise to twisting topological spin textures known as skyrmions. Our group investigates domain wall dynamics and the origins of DMI in metallic and insulating ferro/ferrimagnet on heavy metal heterostructures through domain wall time-of-flight measurements using the magneto-optical Kerr effect.
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