mumax3

[Wanlu Yu]

Dear All,

I hope this email finds you well.
I am a beginner in using MuMax3 for micromagnetic simulations, and I have encountered a question regarding skyrmion initialization. I am trying to place a Néel-type skyrmion at the center of my structure and stabilize it. However, I am unsure about the initial radius of the skyrmion as defined in MuMax3.
In my current code, I use the following command to create a skyrmion:
text
m = NeelSkyrmion(1, -1).scale(0.2, 0.2, 1)

But I am not clear what the default radius is, or how to set it explicitly to a specific value, for example, 17.5 nm.
Could you please advise me on how to modify the code to initialize a Néel skyrmion with a radius of 17.5 nm? Any guidance or example would be greatly appreciated.

Thank you very much for your time and support.

[Josh Lauzier]

Hi,

Unfortunately there isn't a way to explicitly set a value in nm. You need to use scale as you are. The default for the radius (defining 'radius' to be mz=0) is 6.6 cellsizes. So you will need to scale it appropriately based on your cellsize. The function that mumax uses internally is given by:

mz := 2 * pol * e^(-r2/w2) - 0.5

where r2 is the radial distance squared (x^2+y^2), and w2 is w squared, where w is defined as 8*cellsize  (it uses cellsize in the x-direction for the value of "cellsize") . mz will be 0 when r=w * (ln(2))^(0.5) . So if your cellsize is 5nm, it would be something like 

m=NeelSkyrmion(1, -1).scale(0.63, 0.63, 1) 

because 8*5* (ln(2))^(0.5) =33.3, and then 17.5/33.3=0.63. But if your cellsize is something like 7nm, something like

m=NeelSkyrmion(1, -1).scale(0.375, 0.375, 1) 

Best,
Josh L.