Antiferromagnetic skyrmions in Mumax3

[Sanaa Sharma]

Greetings!

I am trying to replicate the results from the paper attached below, wherein they've tried to nucleate antiferromagnetic skyrmions using in-plane spin-polarized current. The code I came up with is as follows:-

N := 64

setgridsize(N,N,2)

c := 1e-9

setcellsize(c,c,2e-9)

setpbc(0,0,2)

DefRegion(1, Layer(0)) //AFM1 layer

DefRegion(2, Layer(1)) //AFM2 layer

defregion(3, circle(0.5*N*c))

save(regions)

Msat.SetRegion(1, 580e3)

Msat.SetRegion(2, 580e3)

Aex.SetRegion(1, -1e-11)

Aex.SetRegion(2, -1e-11)

RKKY := 3.75e-3

scale := (RKKY*1e-9)/ (2* Aex.Average())

ext_scaleExchange(1,2,scale)

Ku1.setRegion(1, 0.8e6)

Ku1.setRegion(2, 0.8e6)

AnisU.setregion(1,vector(0,0,1))

AnisU.setregion(2,vector(0,0,1))

alpha.setregion(1,0.56)

alpha.setregion(2,0.56)

m.setregion(1, uniform(0,0,1))

m.setregion(2, uniform(0,0,1)) 

saveas(m, "m_initial")

minimize()

j.setregion(1,vector(1.3e13,0,0))

j.setregion(2,vector(1.3e13,0,0))

Pol.setRegion(1, 0.56)

Pol.setRegion(2, 0.56)

run(5.5e-12)

autosave(m, 2e-10)

tableAutosave(1e-10)

tableAdd(ext_topologicalcharge)

run(2e-11)

However, I am not able to generate files for subsequent timestamps after the injection of current. It'll be really kind of you to help me with this.

Thanks in advance!

Sanaa

[Nasir Mehmood]

Hello!

"t" in "autosave(m, t)" should be smaller or equal to "run(t)".

try it, hope it'll fix the issue

regards

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Dr. Nasir Mehmood
Post Doc Researcher

Lanzhou University, GS, PRC

[Sanaa Sharma]

Thanks Nasir! I am getting the subsequent files now but still can not come up with a skyrmion on the antiferromagnetic layer. Can you please help identify what's wrong?

Thanks a ton!

Sanaa

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[Nasir Mehmood]

Hello!

In order to reproduce results of a paper, one should have to exactly follow the method used there. However, there are several points in the paper which are seemingly not present in your code.

1. The authors considered a DMI constant (3.5mJ/m2)

2. utilized CPP (current perpendicular to plane) geometry

3. square film dimensions used are 512 nm x 512 nm

4. Did finite temperature simulation (considered some temperature)

See "Methods" section of the paper for details.

Also, I'm not clear about region3 (circle) in your code. See the example of RKKY on mumax examples page. If a material parameter is same in all of the layers then it does not need to set it region-wise.

Hope these points will help you to sort out the issue.

Regards

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Dr. Nasir Mehmood
Post Doc Researcher

Lanzhou University, GS, PRC

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[Nasir Mehmood]

Moreover, in my opinion one does not need to use layered structure in this case, as Aex is already negative. You may try it without using layers and RKKY.

[nico...@gmail.com]

I have a question, does it make physical sense to place a negative Aex in mumax3 to simulate an antiferromagnetic?.  I have understand that to define the Aex in mumax3 we use the continuum approximation that is not totally valid for AFM.

Hello!

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Dr. Nasir Mehmood
Post Doc Researcher

Lanzhou University, GS, PRC

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Dr. Nasir Mehmood
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Lanzhou University, GS, PRC

[Sanaa Sharma]

Thanks a ton! This was really helpful.

Regards 

Sanaa

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Dr. Nasir Mehmood
Post Doc Researcher

Lanzhou University, GS, PRC

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