fix spring/tspin command

Syntax

fix ID group-ID spring/tspin K S0

  • ID, group-ID are documented in fix command

  • spring/tspin = style name of this fix command

  • K = spring constant (energy per squared Bohr magneton, \(\mathrm{eV}/\mu_B^2\) in metal units)

  • S0 = equilibrium spin modulus (Bohr magnetons)

Examples

fix 1 all spring/tspin 1.0 2.2
fix_modify 1 energy yes

Description

Added in version TBD.

Apply a harmonic restoring potential to the magnitude of each magnetic spin in the group,

\[U_{L,i} = \frac{1}{2}K(|\vec{S}_i|-S_0)^2,\]

which produces the radial magnetic force

\[\vec{F}^{m}_{i,\mathrm{spring}} = -K(|\vec{S}_i|-S_0)\hat{s}_i.\]

The spring force is purely radial and produces no direct spin torque. This fix is useful with TSPIN integrators when the magnetic potential does not provide a longitudinal restoring force.

Restart, fix_modify, output, run start/stop, minimize info

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

Enable fix_modify energy yes to include the spring energy in the global potential energy pe; it is excluded by default. This fix also computes a global scalar containing the total spring energy, accessible as f_ID. This scalar is “extensive” and has energy units.

The spring force is applied during energy minimization.

Restrictions

This fix requires atom_style spin and the SPIN package; see Build package.

Default

none