Quick clarification on magnetization output

[Sophie Weber]

I had a quick clarification on understanding the output; when outputting material-magnetization, is my understanding correct that the magnetization length is the magnitude of the vector sum of all moments in the simulation supercell that belong to that material, and then mx, my, mz are the cartesian components of the unit vector parallel to that vector sum? I was quite surprised that in one simulation, the length of some of the magnetization vectors are only about .33, even though the temperature is extremely low. So I wanted to make sure I am understanding correctly. Relatedly, in the manual it says that the mean-material-magnetization will give you the time-averaged, rather than instantaneous, value. Does that time average last from the start of the simulation (excluding equilibration steps) to the last time step that was outputted, i.e. is it a running time-average (in which case presumably one can just take the values at the last time step, or is each interval between printed time steps averaged separately?

Thank you,

Sophie

[Sophie Weber]

Hello,

I wrote this about a month ago, and never got a response to it. Apologies to bother again (I'm coming back to work on this after some other projects), but could you please clarify how the magnetization length and material_magnetization and mean_material_magnetization are computed? This would helpful in debugging (do you agree that by default the magnetization length ought to be one, or close to one, at very low temperature below the transition?

Thanks very much,

Sophie

[gabo...@gmail.com]

I guess either nobody has an answer (for example, could be caused by not having enough time to look into it due to other responsibilities), there is not enough information, or the question(s) weren't understood.

The calculations I have done are the simple ones from the workshop, where magnetization happens to start with a value of 1. Since I've no experience with supercell, it is hard to say why it would be 0.33 for your Curie temperature calculation.  If it were a hysteresis loop calculation, I would say it could be due to the sim:maximum-applied-field-strength not being high enough (greater than Hc).

It it were me doing that calculation, I would likely try to visually plot the spins for perhaps seeing what is happening with them.  However, I'm still not so good at doing that with VAMPIRE.  I think povray [1] might have been what was being used for doing that.

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

Kind Regards,

Gavin

VAMPIRE user

[Sophie Weber]

Thanks this is helpful, I appreciate it. Related to your point on magnetic fields, I'm not doing a hysteresis loop calculation, I am just doing a time-series where I copy checkpoints for each temperature step as I'm cooling down, but in the presence of an applied field of 10!T along the 0 0 1 axis. However, I see that the z component of material_magnetization above the Neel temperature (where it should be paramagnetic) is not close to 1 for every sublattice i.e., the moments seem to not respond to the applied field. But if I print the applied field energy, (which I assume in the same units Joules as everything else?) it's order 10^-18, whereas my exchanges as uniaxial anisotropy is order 10^-22, so it is definitely a large field in relation to the internal magnetic parameters; so I do not understand wny the z component of the moments, and also the magnitude, is not basically one for these high temperatures.  Do you have an idea what might be going on? I can provide example input and outputs if necessary.

Thanks very much

Sophie

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