Bilayer: FM/AFM

[Vishesh Saxena]

Hello users,

I am trying to get the bilayer FM on AFM spin model. But when I get the spin model, I do not see the AFM spins. All the spins are aligned in one direction in both the layers. Why is that? I have used the values as given on the tutorials webpage for the 2 materials. And as you can see the exchange constatbt us negative for material 2.

material[1]:exchange-matrix[1]=+11.2e-21 

material[1]:exchange-matrix[2]=+11.2e-21 

material[2]:exchange-matrix[1]=+11.2e-21 

material[2]:exchange-matrix[2]=-11.2e-21

I have attached below the image I got using paraview.

Any help would be really appreciated!

Best,

Vishesh

[milton...@gmail.com]

Hi! Could you post the files, please?

[Richard Evans]

Hi Vishesh,

Yes - you need some torque to get the spins to move a bit first. So, I would run a time series with equilibration temperature at 1K - this should add a bit of thermal noise and the system will then get to its ground state.

Best,

Richard

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<bilayer_1.png>

[Vishesh Saxena]

Hi Richard,

Thanks for letting me know.

 But the thing you said about having some torques to get the AFM state, is it something specific to VAMPIRE? Because I don't know if you really need some thermal noise or some torques to get an AFM ground state no? 

Because I imagine that if I state the exchange constant to be negative, that itself should be sufficient to get an AFM ground state.

Could you maybe comment on this? 

Thanks,

Best,

Vishesh 

[Richard Evans]

Hi Vishesh,

The problem is general with the LLG equation - its like balancing a ball on top of a hill. An infinitesimal nudge (torque) will make the ball roll down the hill, but at equilibrium the system is stable. The issue is SxB == 0 if S || B, so a small torque should do the trick. You could also use the Monte Carlo solver as this should sample different spin states near the minimum, and of course due to the AFM interaction this will cause the AF state to form.

All the best,

Richard

[Vishesh Saxena]

Hey Richard,

Thanks for this explanation. It makes things clear now. 👌🏻 

Best,

Vishesh

[Vishesh Saxena]

Hello Richard,

I am back to my office and I just checked the input file to include the time series operation. But I realized that the input file already had the commands for the Monte-carlo equilibriation.

I have attached the input file below. I am not sure now what is the problem.

Best,

Vishesh

[gabo...@gmail.com]

Your input file looks okay.  Maybe Paraview is not plotting the data as expected.  Have you tried plotting the output from the bilayer tutorial [1] using povray instead?  Povray gave the attached spins1.png when it was ran as follows.

username@computername:~/Desktop/bilayer$ ls -l
total 8
-rw-rw-r-- 1 username username 1172 Oct 11 03:26 bilayer.mat
-rw-rw-r-- 1 username username 1102 Oct 11 05:36 input
username@computername:~/Desktop/bilayer$ vampire-serial
                                                _          
                                               (_)        
                    __   ____ _ _ __ ___  _ __  _ _ __ ___
                    \ \ / / _` | '_ ` _ \| '_ \| | '__/ _ \
                     \ V / (_| | | | | | | |_) | | | |  __/
                      \_/ \__,_|_| |_| |_| .__/|_|_|  \___|
                                         | |              
                                         |_|              

                      Version 6.0.0 May 15 2022 09:35:26

             Git commit: 4c9651daecc86c3e6b6df0c8055fcd9e89fd900c

  Licensed under the GNU Public License(v2). See licence file for details.

  Lead Developer: Richard F L Evans <richar...@york.ac.uk>

  Contributors: Sarah Jenkins, Andrea Meo, Andrew Naden, Matthew Ellis,
                Oscar Arbelaez, Sam Morris, Rory Pond, Weijia Fan,
                Phanwadee Chureemart, Pawel Sobieszczyk, Joe Barker,
                Thomas Ostler, Andreas Biternas, Roy W Chantrell,
                Wu Hong-Ye, Razvan Ababei, Sam Westmoreland,
                Daniel Meilak
 
                Compiled with:  GNU C++ Compiler
                Compiler Flags:

  Vampire includes a copy of the qhull library from C.B. Barber and The
  Geometry Center and may be obtained via http from www.qhull.org.

================================================================================
Tue Oct 11 05:37:09 2022
================================================================================
Initialising system variables
Creating system
Generating neighbour list..........done!
Copying system data to optimised data structures.
Using generic/normalised form of exchange interaction with 6 total interactions.
Number of atoms generated: 4176
Starting Simulation with Program Time-Series...
Simulation run time [s]: 28.5498
Monte Carlo statistics:
    Total moves: 45936000
    49.9351% Accepted
    50.0649% Rejected
Simulation ended gracefully.
username@computername:~/Desktop/bilayer$ ~/vampire/util/vdc/vdc --povray
|------------------------------------------------------------|
|              Vampire Data Converter for v5+                |
|------------------------------------------------------------|
Processing snapshot 00000000
Processing snapshot 00000001
username@computername:~/Desktop/bilayer$ povray spins
...
POV-Ray finished
username@computername:~/Desktop/bilayer$ ls -l spins1.png
-rw-rw-r-- 1 username username 234558 Oct 11 05:38 spins1.png

[1] https://vampire.york.ac.uk/tutorials/bilayer-system-generation/

Kind Regards,

Gavin

VAMPIRE user

[Vishesh Saxena]

Hey Gavin,

Yes, I used povray to run the file and it works. I can see some AFM aligned spins but I do not see them clearly. The image is blurred when I zoom in.

trying to fix that now.

Thanks for the tip!

Best,

Vishesh

[Vishesh Saxena]

I have two further questions.

1. I compared my output with the output shown in the previous email regrding the successful run of monte-carlo.

In my output the accepted and rejected % is very different. I have an accepted % of around 1 percent and the output file shared by Gavin has an acceptance of around 40%. Our inputs file are the same. 

Then what could be the issue?

2. The spins.png file generated by using povray is very pixelated. Any ideas on how to fix this? I have attached the png file below.

Best regards,

Vishesh

[gabo...@gmail.com]

Currently, I don't know the solution for the blurred image.  However, you might have a look at slide 14 in [1] and the povray documentation [2] to see if you can find something there for resolving that.

[1] https://vampire.york.ac.uk/resources/workshop2017_day4B.pdf

[2] http://www.povray.org/documentation/view/3.6.1/223/

Kind Regards,

Gavin

VAMPIRE user

[Richard Evans]

Hi all,

Yes - you need more pixels when rendering. Eg

povray -W1600 -H1200 +A0.3 spins.pov

The W and H are in pixels, and you can increases these as you need to sharpen the image (render time increases too).

All the best,

Richard 

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[Vishesh Saxena]

hey thanks! This surely increases the resolution!

Best,

Vishesh