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Title: Interface morphology and Dzyaloshinskii-Moriya interaction in Pt/Co thin films
Author: Wells, Adam William John
ISNI:       0000 0004 6497 3020
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2017
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The interfacial morphology and the Dzyaloshinskii-Moriya interaction (DMI) have been investigated in thin films of perpendicularly-magnetised epitaxial Pt(30 A)/Co(7 A)/Pt(10 A) and Pt(30 A)/Co(7, 9, 11 A)/Ir20Mn80(20, 50 A). In Pt/Co/Pt, altering the substrate temperature during deposition of the top Co/Pt bilayer in the range of 100 C to 300 C formed a peak in total interfacial disorder with deposition temperature of 3.3 A at around 250 C. The coercivity increased from 230 +/- 60 Oe to 400 +/- 60 Oe whilst the anisotropy and saturation magnetisation stayed constant at 15.0 +/- 0.3 kOe and 90 +/- 10 microemu, respectively, when the substrate temperature at deposition was reduced in value. In contrast, the anisotropy increased with deposition temperature if either the substrate temperature was increased, or the deposition temperature of only the top Pt layer was varied, though not both simultaneously. The bubble domain expansion technique was used to measure domain wall velocity with applied magnetic fields in various in-plane (IP) and out-of-plane (OOP) configurations. Of the three prominent creep models used, it was found that the most basic model fitted best. A moderate linear correlation was seen between DMI and the difference in disorder between the top and bottom Co interfaces, indicating that as the disorder in one interface increases, its contribution to DMI decreases, allowing the relative interface disorders to dictate the strength, and even sign, of the net DMI. The net DMI increased with increasing deposition temperature, also, showing that such a growth parameter can be used to fine-tune the DMI. Using Pt/Co/Ir20Mn80 samples, the growth temperature and working pressure were optimised for DMI measurements. Samples with 50 A of IrMn, with exchange bias strengths up to 1000 Oe, showed a high density of small, rough domains forming during magnetisation reversal. Samples with 20 A of IrMn, so without exchange bias, produced well-formed bubble domains for Co thicknesses below 9 A, with a Voronoi-like network seen in an 11 A sample. Bubble domain expansion showed severely skewed velocity against IP field graphs, with behaviour at higher fields not predicted by current models, giving DMI constant values between -0.6 and -1.0 mJ/m^2.
Supervisor: Moore, Thomas ; Marrows, Christopher Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available