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4026 - Magnetic-field imaging using novel VortexCore MFM tip

7 months 2 weeks ago - 7 months 2 weeks ago #75 by jserra
Oral Presentation
Monday, December 7
Speaker: Jan Soltys
Full Title: Magnetic-field imaging using novel VortexCore MFM tip
Last edit: 7 months 2 weeks ago by SofiaAbrunhosa.

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7 months 2 weeks ago #151 by SofiaAbrunhosa
Session questions:
  1. Is deposition of magnetic material on the side walls of the cone an issue?
  2. A vortex core typically moves within the disc in external fields, does this affect the measurement?
  3. Interesting is the difference of type of contrast between the LM probe and the vortex probe on the vortex states in squares. It is not simply a decrease of magnitude of the contrast. Any comment? Do you understand it?

We kindly ask the speakers to reply to any questions left unanswered during the talk.

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7 months 2 weeks ago - 7 months 2 weeks ago #378 by Soltys
Thank you Sofia for your questions.
1. As the cone side walls are very steep and direction of evaporated material is parallel to the tip axis there there is only very thin layer of permalloy on cone side walls and it should not have significant inluence. However, I must admit that the shape of the disk created on the tip apex is not entirely ideal regarding its flatness. We discused this fact in our paper in APL 116, 242406 (2020). I enclose here a picture from this paper. The boundary between upper Py layer and Si tip is clearly visible. The shape of the evaporated Py layer (red dashed line) is far from the ideal flat disk (green dot-dashed line). So, to understand better the vortex stability in such 3D magnetic element, we calculated magnetic ground state in such curved geometry. The results obtained have been compared with the stability of the vortex core in the ideal flat disk. The magnetization loop shows that the vortex structure is formed spontaneously also in ur non ideal curved disk, but the values of the vortex nucleation and annihilation fields differ from those for the flat disk model. An important fact is that for a rather wide interval of external fields, the vortex core state still represents the ground state of the system. For fields from interval -+ 20mT, the magnetic moment is constant; for larger external fields, the moment increases rapidly as compared to the VC moment of the ideal disk. Based on the simulations, we can conclude that the fabricated tip has a stable value of the magnetic moment also in weak external magnetic fields. Further simulations were performed for a wide range of diameters and thicknesses of such curve-shaped structures. The results show the stability of the vortex state in alltested structures.

2. Above I explained that value of mag. moment of disk is stable in certain range of external field. However, as you rightly pointed out, the same does not apply to the position of the vortex. Vortex core position can be deflected from its central position by in-plane stray field produced, for example, by samples having big domains. The core displacement results in deterioration of the MFM image, because vortex core is in other position as it is supposed to be. Therefore we plan to investigate the possibility to enhance the vortex core rigidity (position stability) with bilayer system - combination of ferromagnetic (FM) and antiferromagnetic (AF) layer. The coupling of FM to AFM enhance the stability.

3. There could be more explanation of different type of contrast between the LM probe and the VC probe. You can even get a different images when scanning the same object with probes from different manufacturers. Here is example of square taken with 3 different low moment commercial probes. You can see that the domain walls can be imaged very sharply, or conversely, very blurred (like fan-like structure). It depends on many factor and it is difficult to explain briefly.
Last edit: 7 months 2 weeks ago by Soltys.

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