Interface-driven magnetisation tuning in ultra-thin heavy metal/ferromagnet heterostructures

This dissertation reports a detailed study of heavy-metal/ferromagnet multilayers particularly, Pt/Co/Pt. Modification of the Co 3d orbital at the Pt/Co interface is well-known to induce perpendicular magnetic anisotropy which drives the magnetic moment out-of-plane instead of the in-plane orientation expected in thin-film magnetic systems due to dipole interaction. In the first part of this thesis, a systematic variation of the Co thickness spanning the range of this transition has been carried out. X-ray Magnetic Circular Dichroism measurements as a function of Co layer thickness and temperature showed a clear dependence of the spin and orbital moments which can be explained by a cluster-type growth of the Co layer for sub-nm films with a variable Curie temperature of the clusters. In parallel, our measurements on Pt/Co/Ta multilayers widely used in spin-orbit torque studies shows a similar behaviour albeit with a reduced orbital and spin moments arising from the quenching of the magnetism at the Co/Ta interface.

The interfacial origin of the perpendicular magnetic anisotropy in these multilayers makes it strongly sensitive to the structural quality of the interface and can be tuned by the addition of sub-nm dusting elements. In the second part, we have carried out an exhaustive study by dusting with various elements and our results show that the modification of the anisotropy depends on the specific dusting element - Au dusted samples showing a marked enhancement of the perpendicular magnetic anisotropy arising from an increased effective interfacial orbital moment revealed by X-ray Magnetic Circular Dichroism measurements.

Furthermore, temperature-dependent magnetic measurements of pristine and dusted Pt/Co/Pt structures demonstrated that the low-temperature magnetic reversal in these systems is dominated by strong domain-wall pinning at structural defects. These defects decrease with increasing Co thickness and can be tuned by addition of interfacial dusted layers. This is particularly relevant for use of Pt/Co/Pt multilayers for superconducting spintronics. The last part of this thesis includes development of Pulsed Laser Deposition growth of Pt/Co/Pt multilayers in presence of Ar ballast gas which opens the possibility to integrate these structures with oxide materials like superconductors with more complex order parameter symmetry.