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Vartika Bansal presents

 Study of magnetic heterostructures with perpendicular anisotropy by resonant magnetic reflectivity of soft X rays

VFriday, February 3rd 2023 at 14:30

Seminar room – Building A – CNRS

The defence will be in English.



Magnetic behavior in thin-film structures has attracted considerable interest for its wide ranging technological applications. As the dimension of magnetic films reduce, they are able to exhibit various magnetic properties having no equivalent in bulk material as interfacial magnetism become more important. However, probing buried magnetic interfaces in such heterostructures can be a major challenge. Perpendicular magnetic anisotropy (PMA) is one such interface induced quantity that occurs at the interface between different types of materials. This thesis focuses on determination of the out-of-plane magnetization profile through buried ultrathin ferromagnetic films with various adjacent materials by soft X ray magnetic reflectivity (SXRMR), thus providing a direct description of the magnetism at the bottom and top interfaces. This technique combines the depth-resolved information of X-ray reflectivity with the species selectivity of X-ray magnetic circular dichroism for probing in-plane (ip) and out-of-plane (oop) magnetization profile with sub nanometer resolution in thin films.

We investigated the magnetization profile in series of samples of MgO/CoFeB/(W)/Ta before and after annealing at different temperatures by measurements at Fe L3 edge. A transition in easy magnetic axis from ip to oop for both Ta and W cap layers was observed which was accompanied by changes in Fe magnetic distribution over CoFeB thickness. Ta cap layered sample showed an enhanced and more homogeneous magnetization profile after annealing at annealing at 340°C compared to from a gradient distribution before annealing.  For W cap layer, we observed a gradient profile before as well as after annealing at 400°C however accompanied by an enhancement of magnetization at both interfaces.

Another system of Pt/Co/Gd thin film Gd which has proven to be a promising stack in the context of chiral domain wall displacement studies due to the antiferromagnetic coupling at individual atomic sites of the Co/Gd interface. With SXRMR we obtained the distribution of the out-of-plane magnetization across the Co layers from measurements at the Co L3 edge as well as Gd layers from measurements at the Gd M5 edge over various temperature from 300K to 40K. For the Co layer, whose Curie temperature is higher than the ambient temperature analysis shows a homogeneous profile over temperature evolution. On the other hand, for Gd layer, whose the Curie temperature is known to be close to room temperature, we observed an inhomogeneous magnetic profile over temperature range of 43K to 300K. Although for 300K, a pure oop magnetization profile was resolved at the immediate interface with Co. At lower temperatures we observed change in oop profile and manifestation of ip component further away from immediate Co/Gd interface interface where Gd is strongly intermixed with the Al cap layer.