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Spin arrangement at interfaces in layered magnetic materials is of vital importance to the emerging field of spintronics. Knowledge of how and why the interfacial spins behave in a certain way will aid in the development of future magnetic-based memories.
Much exploration has taken place in the interlayer exchange coupling (IEC) of ferromagnetic heterostructures with in-plane anisotropy. Only recently has it become apparent that to achieve the goals of increased areal density in magnetic memory a push for exploring magnetic materials with perpendicular magnetic anisotropy (PMA) must occur. An interesting and promising candidate for such a magnetic system is [Co/Pt]/NiO/[Co/Pt], where two [Co/Pt] multilayers with PMA are separated by a thin, insulating, antiferromagnetic NiO layer and display oscillatory coupling with NiO thickness. This magnetic heterostructure displays an entirely new IEC where the Ni spins within the NiO layer cant in concert with the adjacent [Co/Pt] layers, causing the periodicity of the oscillatory coupling to coincide with the NiO antiferromagnetic ordering parameter. The strength and sign of this coupling, either positive (favoring parallel alignment) or negative (favoring anti-parallel alignment), can be tuned with slight changes in the NiO layer thickness. The origin of the oscillatory IEC was investigated using advanced microscopy and spectroscopy techniques.
For antiferromagnetically coupled [Co/Pt] layers, the competition between magnetostatic coupling and IEC gives rise to a region of overlapping domains (resulting in a ferromagnetically coupled stripe). Discovered with high resolution magnetic force microscopy and quantitatively modeled with micromagnetic simulation, the width of this overlap region scales inversely with the IEC.
Heterostructures of Co/NiO/[Co/Pt], where the Co ([Co/Pt]) has in-plane (out-of-plane) anisotropy, allow for isothermal tuning of the hysteresis loop shift along the applied field axis at room temperature, as well as display a greatly enhanced blocking temperature (increase of more than 175K). The presence of the [Co/Pt] multilayer with PMA is responsible for the enhancement. In addition, these structures display temperature dependent exchange bias training effects, which have been successfully modeled using a phenomenological thermodynamic approach.