Chiral Spin Textures in Magnetic Nanostructures

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is investigated in thin film heterostructures with perpendicular magnetic anisotropy. The DMI is influenced by the atomic arrangement at the interface. Also, the DMI is insensitive to the antiferromagnetic spin order.
Ta/CoFeB/MgO heterostructures were annealed at different temperatures ranging from 210-280 °C to bring about changes in the atomic arrangement at the interfaces. DMI fields were then measured using a field-driven creep regime domain expansion technique. The DMI peaks at D = 0.057 +/- 0.003 mJ/m^2 at an annealing temperature of 230 °C. The DMI field and the anisotropy field follow a similar trend as a function of annealing temperature. These experiments allow the conclusion that the underlying mechanisms for the behaviour of the DMI and the anisotropy are related to interfacial crystal ordering and B expulsion out of the CoFeB layer and consequent accumulation at the interfaces as the annealing temperature is increased.
The DMI was also explored in systems containing an antiferromagnet (AFM) layer. Trilayers of Pt/Co/IrMn and Pt/Co/FeMn were fabricated which exhibit large perpendicular exchange bias of 40 mT and 140 mT, respectively, in the as-grown state. Different phases of the antiferromagnetic IrMn and FeMn layers were accessed by varying the thickness. The magnetic domain morphology of the ferromagnetic Co layer is influenced by the spin order of the AFM layers. These variations in domain texture are related to the ferromagnet-antiferromagnet (FM-AFM) inter-layer exchange coupling and the anisotropy energy of the AFM layer. In contrast, the DMI remains unchanged, within experimental uncertainty, for different phases of the AFM layers. The DMI was measured by Brillouin light scattering spectroscopy. From these experiments, it is inferred that the DMI is insensitive to both AFM spin order and exchange bias, in agreement with theory. The DMI is different in the two systems: D = -1.14 +/- 0.05 mJ/m^2 for Pt/Co/IrMn and D = -1.50 +/- 0.08 mJ/m^2 for Pt/Co/FeMn; this is when IrMn and FeMn layers are paramagnetic. The difference in DMI is due to different Mn concentration at the FM/AFM interface of the two systems, and also due to Fe segregation from the FeMn layer.