Deriving a dynamics term#

All dynamics terms in micromagneticmodel are in micromagneticmodel/dynamics directory. They are all derived from micromagneticmodel.dynamics.DynamicsTerm base class.

For instance, let us say we want to implement a dynamics term with following specifications:

property	value
name	`SpecialDynamics`
expression	$\xi\mathbf{m}\times\mathbf{v}$
parameters	$\xi$, $\mathbf{v}$
parameter properties	$\xi \ge 0$, can be spatially varying
	$\mathbf{v}$ is three-dimensional, can be spatially varying

The dynamics term class would be:

[1]:

import micromagneticmodel as mm


class SpecialDynamics(mm.DynamicsTerm):
    pass

Now, we can try to instantiate it

[2]:

try:
    sd = SpecialDynamics(xi=0.1, v=(0, 0, 1))
except TypeError:
    print("Exception raised.")

Exception raised.

An exception was raised because _reprlatex and _allowed_attributes properties must be implemented. Therefore, an extended implementation of the class is:

[3]:

class SpecialDynamics(mm.DynamicsTerm):
    _reprlatex = r"$\xi\mathbf{m}\times\mathbf{v}$"
    _allowed_attributes = ["xi", "v"]

    def dmdt(self, m):
        raise NotImplementedError

We can try to instantiate the class again:

[4]:

sd = SpecialDynamics(xi=0.1, v=(0, 0, 1))
sd

[4]:

$$\xi\mathbf{m}\times\mathbf{v}$$

The dynamics object is created. The last thing we have to impose on the energy class is the typesystem. More precisely, we have to make sure no negative $\xi$ values are allowed, $\mathbf{v}$ is three-dimensional and that name attribute accepts only valid Python variable names. This is done by using ubermagutil. More details can be found here.

[5]:

import discretisedfield as df
import ubermagutil.typesystem as ts


@ts.typesystem(
    xi=ts.Parameter(descriptor=ts.Scalar(unsigned=True), otherwise=df.Field),
    v=ts.Parameter(descriptor=ts.Vector(size=3), otherwise=df.Field),
)
class SpecialDynamics(mm.DynamicsTerm):
    _reprlatex = r"$\xi\mathbf{m}\times\mathbf{v}$"
    _allowed_attributes = ["xi", "v"]

    def dmdt(self, m):
        raise NotImplementedError

If we now attempt to pass invalid input arguments, exceptions are raised.

[6]:

try:
    sd = SpecialDynamics(xi=-0.1, v=(0, 0, 1))  # negative xi
except ValueError:
    print("Exception raised.")

Exception raised.

[7]:

try:
    sd = SpecialDynamics(xi=0.1, v=(0, 1))  # 2D v
except ValueError:
    print("Exception raised.")

Exception raised.

Some of the properties and methods of the implemented energy term are:

[8]:

sd = SpecialDynamics(xi=0.1, v=(0, 1, 1))

[9]:

sd.xi

[9]:

0.1

[10]:

sd.v

[10]:

(0, 1, 1)

[11]:

sd.name

[11]:

'specialdynamics'

[12]:

sd

[12]:

$$\xi\mathbf{m}\times\mathbf{v}$$

[13]:

repr(sd)

[13]:

'SpecialDynamics(xi=0.1, v=(0, 1, 1))'

Deriving a dynamics term#

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