spins in AFM compound are not polarized above Neel temperature even in 100 T magnetic field

[Sophie Weber]

Hello,

I am struggling to properly implement a magnetic field for a system I am working on. Basically, I want to simulate an experiment where an antiferromagnet is cooled down starting from above its Neel temperature (which I've confirmed running the simulations without an applied field is around 180 Kelvin) to below its Neel temperature, always in the presense of a magnetic field applied parallel to the easy axis. Physically, what should happen is that all sublattices are polarized parallel to the direction of the B field when they are above the Neel temperature, in the paramagnetic phase, and as we cool across the transition temperature, the systems should began to order antiferromagnetically as the exchange outcompetes the applied field (the B field should be well below the spin-flop value where the AFM order parameter would align perpendicularly).

However, I am extremely confused by the results I'm getting when trying to determine roughly the value of magnetic field that is appropriate to use. If I run a monte carlo time-series at 300 K, testing for B field from 0 to 100 Tesla in steps of 10 T, consistently for every calculation, all materials (which are the sublattices) are parallel to the B field, except for two, which are antiparallel to the field. Moreover, the sublattices which are antiparallel are always the same (2 and 8), no matter what the field value.

I can't believe that I need a field beyond 100 Tesla to fully polarize the spins, and the fact that the same two sublattices are always flipping makes me think that I am probably doing something wrong in the simulation setup, or I am thinking incorrectly about the physcs. Cooling this system down slowly in the absence of the magnetic field gives exactly what I expect, so I think it's really the magnetic field implementation.

This dropbox link goes to the input and output of the simulation I tried to do. If you see anything that is wrong with the simulation, or you catch some incorrect physics assumption I'm making, I'd be very greatful. Please met me know if you want me to plot anything to make explanations easier.

https://www.dropbox.com/scl/fo/2kj0yugcs5xykkyej93xf/APh3zN1vBDoYF7l5WlBJmTw?rlkey=qvl17e7uolijot96852udy7sn&dl=0

Thanks very much,

Sophie