#!/usr/bin/env python
# coding: utf-8

# # Fixed subregions
# 
# It is sometimes necessary to "fix" specific regions of the mesh to ensure they do not change during driving. In order to do that, the first step is to create a mesh and specify subregions we want to keep fixed.
# 
# As an example, let us simulate a simple one-dimensional sample and define subregions in such a way that the first and the last discretisation cell remain fixed.

# In[1]:


import oommfc as mc
import discretisedfield as df
import micromagneticmodel as mm

region = df.Region(p1=(-30e-9, 0, 0), p2=(30e-9, 3e-9, 3e-9))
cell = (3e-9, 3e-9, 3e-9)
subregions = {
    "fixed1": df.Region(p1=(-30e-9, 0, 0), p2=(-27e-9, 3e-9, 3e-9)),
    "free": df.Region(p1=(-27e-9, 0, 0), p2=(27e-9, 3e-9, 3e-9)),
    "fixed2": df.Region(p1=(27e-9, 0, 0), p2=(30e-9, 3e-9, 3e-9)),
}
mesh = df.Mesh(region=region, cell=cell, subregions=subregions)


# In[2]:


mesh.mpl.subregions(box_aspect=(10, 3, 3), figsize=(12, 12))


# Now, let us define a system whose energy equation consists of only exchange energy:

# In[3]:


system = mm.System(name="fixed_spins")
system.energy = mm.Exchange(A=1e-12)


# We are going to initialise the magnetisation in `fixed1` region to be $(0, 0, 1)$ and in `fixed2` region $(0, 0, -1)$. In the `free` region, we are going to choose $(1, 0, 0)$ for initial magnetisation.

# In[4]:


m = {"fixed1": (0, 0, 1), "fixed2": (0, 0, -1), "free": (1, 0, 0)}
system.m = df.Field(mesh, nvdim=3, value=m, norm=1e6)


# The magnetisation is now:

# In[5]:


system.m.sel("y").mpl(
    figsize=(12, 2),
    scalar_kw={"colorbar_label": "$m_y$"},
)


# Finally, we can drive the system. In this case, we are going to choose `MinDriver`. When we call `drive` method, we have to pass `fixed_subregions` argument, which is a list of subregions names we want to keep fixed duriing relaxation.

# In[6]:


md = mc.MinDriver()
md.drive(system, fixed_subregions=["fixed1", "fixed2"])


# The relaxed magnetisation is:

# In[7]:


system.m.sel("y").mpl(
    figsize=(12, 2),
    scalar_kw={"colorbar_label": "$m_y$"},
)


# From the resulting magnetisation field, we can see that the first and the last spin have remained the same as in the initial magnetisation and a Neel domain wall has formed in between due to the exchange energy.