Field from Magnetization (3D):
computefield
Inspired by the 2D-solver based
mag2hfield, the
computefield application uses the
3D Oxs solver engine to compute demagnetization and other fields from a
magnetization snapshot. Various options allow fields to be computed
outside the original simulation volume (say the stray field above a thin
film sample) or to isolate the field arising from one portion of the
sample (for example from one layer in a multilayer structure).
Launching
The computefield launch command is:
tclsh oommf.tcl computefield [standard options] \
[-cwd directory] [-meshOvf meshovffile] [-Msfile Msinputfile] \
[-Msmask maskexpr] [-Msspec spec] [-outdemagenergy demagenergyfile] \
[-outM magfile] [-outmif miffile] [-pbc axes] [-runboxsi <0|1>] \
[-runopts boxsiopts] [-simulationbox xmin ymin zmin xmax ymax zmax] \
[-useroutputs "name1 file1 ..."] \
[-userscriptfile energyscriptfile] <m0_infile> [outdemagfile]
where
- -cwd directory
- Make directory the active working directory.
- -meshOvf meshovffile
- Normally the mesh cell sizes and simulation dimensions are inferred
from the m0_infile, but the -meshOvf option overrides the
default with values from meshovffile. This is useful
primarily in settings where the initial magnetization is prescribed
generically using a coarser mesh than the one intended for the
simulation—for example, the left half of the simulation is to have
magnetization pointing up, the right half pointing down, defined via
a mesh with only two (large) cells. In this case a separate
OVF file may be used as an argument to both -Msfile and
-meshOvf to define the part geometry and meshing.
- -Msfile Msinputfile
- Set pointwise saturation magnetization values from
Msinputfile, overriding
values from m0_infile. Any
points outside the span of Msinputfile have set to 0.
- -Msmask maskexpr
- At each point in the simulation volume, the Tcl expr-expression
maskexpr is evaluated to rescale (multiply) the otherwise set
saturation magnetization . The expr-expression may include the
special variables $x, $y, $z, $rx, $ry,
$rz, $Ms, $mx, $my, and $mz. The first
three are the raw pointwise location in meters, the second three are
the relative location (each in the range [0,1]), $Ms is the
saturation magnetization at the point, and ($mx,$my,$mz) is
the reduced (unit normalized) magnetization. Analogous to the
-wgtfunc option of avf2odt, maskexpr is typically a 0/1 valued
function used to mask off a portion of the simulation part. See the
first example below.
- -Msspec spec
- Override the values from m0_infile with the (quoted)
spec string in the form of an inline
Specify block. Note: If spec is a
single numeric value representing a uniform saturation magnetization
(in A/m), then that value is applied but restricted to the
simulation volume described by m0_infile or meshovffile,
with set to 0 A/m in any extended volume requested by the
-simulationbox option.
At most one of -Msfile and -Msspec may be used.
- -outdemagenergy demagenergyfile
- Write the demagnetization energy density scalar field (.oef) to
demagenergyfile.
- -outM magfile
- Saves a copy of the magnetization as used in the computation,
incorporating m0_infile, meshovffile, and
Ms/file/spec/mask options. This can be helpful for checking
that input parameters, in particular -Msmask, are behaving as
expected.
- -outmif miffile
- The computefield application creates a temporary MIF used as
input for running boxsi. This MIF file is deleted after
boxsi exits, but the -outmif option will save a copy in
miffile.
- -pbc axes
- Creates a periodic mesh for the simulation. The axes
should be a string of one or more of the letters `x',
`y', and `z', denoting the periodic direction(s), as
used by the periodic option to the
Oxs_PeriodicRectangularMesh
Specify block.
- -runboxsi <0|1>
- By default, computefield runs
boxsi on its
constructed MIF file to compute the requested output fields. This
behavior is short circuited with -runboxsi 0, which can be used in
conjunction with -outmif to allow additional user edits to the
MIF file preceding a manual boxsi run.
- -runopts boxsiopts
- Command line options passed to the automatic boxsi run. The
boxsiopts string must be quoted as a single argument to
computefield.
- -simulationbox xmin ymin zmin xmax ymax zmax
- Use the specified values for the simulation volume, overriding the
extents in the input m0_infile or meshovffile files. The
x/y/z/min/max values can be listed as either six individual values
or quoted as a single element to computefield. Any entry
specified as a single hyphen (`-') will inherit the corresponding
value from m0_infile/meshovffile. (The -info option
to the command line utility avf2ovf can be used to view OVF file extents.)
The simulationbox option is typically used to compute stray
fields outside the original simulation volume. If the original volume
extends outside the requested volume, then magnetization outside the
requested volume is ignored and a warning is printed.
- -useroutputs
"name1 file1 ..."
- A list, quoted as a single element on the command line, of alternating
outputs and filenames, referring to Oxs_Energy objects specified
in the separate -userscriptfile. This command line option
augments and amends any user_outputs setting in the script file.
- -userscriptfile energyscriptfile
- The only outputs computefield directly supports on the command
line are demagnetization field, demagnetization energy density, and
magnetization. However, the user can create an
energyscriptfile, which is a separate Tcl script with
additional Oxs_Energy objects to be included in the run. (See
second example below.) The name of each desired output and the
associated filename should be appended to the “user_outputs”
Tcl list, either directly inside the script file, or via the
-useroutputs command line option.
- m0infile
- Input magnetization (.omf) file. Required. This file sets the
magnetization direction at each point, i.e., the MIF m0 value
in the Oxs_TimeDriver and
Oxs_MinDriver Specify blocks.
The pointwise saturation magnetization (MIF Ms option) and
simulation volume extents (MIF atlas object) are also derived from
this file unless overridden by other options.
- outdemagfile
- Demagnetization output file name. Optional. If not specified then the
demagnetization field is not output.
Examples
Consider a multilayer thin film having total thickness 30 nm, with the
top layer 10 nm thick. A simulation is run and the magnetization at some
interesting point is saved to the file foo.omf. For efficiency, the
simulation volume was set to contain only the film itself, but for
analysis we want to know the stray field out to 150 nm above the top of
the film. Moreover, we would like to know the portion of the stray field
coming from just the top 10 nm layer. The command
tclsh oommf.tcl computefield -simulationbox - - - - - 180e-9 \
-Msmask '20e-9<$z && $z<30e-9 ? 1 : 0' foo.omf foo-toplayer.ohf
will compute this and save the results in foo-toplayer.ohf. (On
the Windows command line, remove or replace the backslash line
continuation character with a caret, ^, and replace the
single quotes with double quotes.)
As another example, suppose bar.omf holds Oxs_MinDriver::Spin
normalized magnetization output for a sample with uniform saturation
magnetization of 1400 kA/m. If we want to know the magnetocrystalline
anisotropy field at that magnetization state, we can construct a
separate file, say barK.tcl, that contains the anisotropy block from
the original simulation:
Specify Oxs_UniaxialAnisotropy:K {
K1 520e3
axis {0 0 1}
}
lappend user_outputs Oxs_UniaxialAnisotropy:K:Field barK.ohf
The last line requests the field output be written to the file
barK.ohf. We will feed this file to computefield using the
-userscriptfile option. After closing barK.tcl we decide we
would also like to have the anisotropy energy density, so we'll use the
-useroutput option to request that, and for good measure request
the demagnetization energy density with -outdemagenergy as
well. The accompanying computefield command is
tclsh oommf.tcl computefield -Msspec 1.4e6 -userscriptfile barK.tcl \
-useroutputs "Oxs_UniaxialAnisotropy:K:Energy\ density barK.oef" \
-outdemagenergy bar-demag.oef bar.omf
Note that the demagnetization field is not requested and so won't be
saved. (If that field is wanted, just add the filename after
bar.omf.)
OOMMF Documentation Team
September 27, 2024
