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:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Commands_all.html)

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fix precession/spin command :h3

[Syntax:]

fix ID group precession/spin style args :pre

ID, group are documented in "fix"_fix.html command :ulb,l
precession/spin = style name of this fix command :l
style = {zeeman} or {anisotropy} or {cubic} :l
  {zeeman} args = H x y z
    H = intensity of the magnetic field (in Tesla)
    x y z = vector direction of the field
  {anisotropy} args = K x y z
    K = intensity of the magnetic anisotropy (in eV)
    x y z = vector direction of the anisotropy :pre
  {cubic} args = K1 K2c n1x n1y n1x n2x n2y n2z n3x n3y n3z
    K1 and K2c = intensity of the magnetic anisotropy (in eV)
    n1x to n3z = three direction vectors of the cubic anisotropy :pre
:ule

[Examples:]

fix 1 all precession/spin zeeman 0.1 0.0 0.0 1.0
fix 1 3 precession/spin anisotropy 0.001 0.0 0.0 1.0
fix 1 iron precession/spin cubic 0.001 0.0005 1.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0
fix 1 all precession/spin zeeman 0.1 0.0 0.0 1.0 anisotropy 0.001 0.0 0.0 1.0 :pre

[Description:]

This fix applies a precession torque to each magnetic spin in the group.

Style {zeeman} is used for the simulation of the interaction
between the magnetic spins in the defined group and an external
magnetic field:

:c,image(Eqs/force_spin_zeeman.jpg)

with mu0 the vacuum permeability, muB the Bohr magneton (muB = 5.788 eV/T
in metal units).

Style {anisotropy} is used to simulate an easy axis or an easy plane
for the magnetic spins in the defined group:

:c,image(Eqs/force_spin_aniso.jpg)

with n defining the direction of the anisotropy, and K (in eV) its intensity.
If K>0, an easy axis is defined, and if K<0, an easy plane is defined.

Style {cubic} is used to simulate a cubic anisotropy, with three
possible easy axis for the magnetic spins in the defined group:

:c,image(Eqs/fix_spin_cubic.jpg)

with K1 and K2c (in eV) the intensity coefficients and
n1, n2 and n3 defining the three anisotropic directions
defined by the command (from n1x to n3z).
For n1 = (100), n2 = (010), and n3 = (001), K1 < 0 defines an
iron type anisotropy (easy axis along the (001)-type cube
edges), and K1 > 0 defines a nickel type anisotropy (easy axis
along the (111)-type cube diagonals).
K2^c > 0 also defines easy axis along the (111)-type cube
diagonals.
See chapter 2 of "(Skomski)"_#Skomski1 for more details on cubic
anisotropies.

In all cases, the choice of (x y z) only imposes the vector
directions for the forces. Only the direction of the vector is
important; it's length is ignored (the entered vectors are
normalized).

Those styles can be combined within one single command line.

:line

[Restart, fix_modify, output, run start/stop, minimize info:]

By default, the energy associated to this fix is not added to the potential
energy of the system.
The "fix_modify"_fix_modify.html {energy} option is supported by this fix
to add this magnetic potential energy to the potential energy of the system,

fix             1 all precession/spin zeeman 1.0 0.0 0.0 1.0
fix_modify      1 energy yes :pre

This fix computes a global scalar which can be accessed by various
"output commands"_Howto_output.html.

No information about this fix is written to "binary restart
files"_restart.html.

[Restrictions:]

The {precession/spin} style is part of the SPIN package.  This style
is only enabled if LAMMPS was built with this package, and if the
atom_style "spin" was declared.  See the "Build
package"_Build_package.html doc page for more info.

[Related commands:]

"atom_style spin"_atom_style.html

[Default:] none

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:link(Skomski1)
[(Skomski)] Skomski, R. (2008). Simple models of magnetism.
Oxford University Press.
