"The external magnetic field acts here as a boundary condition for the model. Let us first consider the model in (none/a/the) zero magnetic field. We see that one possible solution for the magnetization is zero. But (none/a/the) zero magnetization is not a single possible solution here: (none/a/the) non-zero magnetization is possible also."
I would make it:
Let us first consider the model in a zero magnetic field.
Here, the article simply goes with
zero magnetic applies adjectively to
But zero magnetization is not a single possible solution here: non-zero magnetization is possible also.
Here, no article is needed, as you refer to magnetization as a whole.
I do not really see how
zero makes this in any way special. If I substitute strong or weak (and without physics background, that is no problem :P ):
Let us first consider the model in a strong magnetic field.
But strong magnetism is not a single possible solution here: weak magnetism is also possible .
Zero is not used to count the magnetic field(s) here. Then the phrase would be:
Let us first consider the model in zero magnetic fields.
This means something completely different. I understand we are talking about a magnetic field with the specific property of being
zero, like it could have been