# Magnetic spins

The magnetic spin simulations are enabled by the SPIN package, whose
implementation is detailed in [Tranchida](Tranchida).

The model represents the simulation of atomic magnetic spins coupled to
lattice vibrations. The dynamics of those magnetic spins can be used to
simulate a broad range a phenomena related to magneto-elasticity, or or
to study the influence of defects on the magnetic properties of
materials.

The magnetic spins are interacting with each others and with the lattice
via pair interactions. Typically, the magnetic exchange interaction can
be defined using the [pair/spin/exchange](pair_spin_exchange) command.
This exchange applies a magnetic torque to a given spin, considering the
orientation of its neighboring spins and their relative distances. It
also applies a force on the atoms as a function of the spin orientations
and their associated inter-atomic distances.

The command [fix precession/spin](fix_precession_spin) allows to apply a
constant magnetic torque on all the spins in the system. This torque can
be an external magnetic field (Zeeman interaction), and an uniaxial or
cubic magnetic anisotropy.

A Langevin thermostat can be applied to those magnetic spins using [fix
langevin/spin](fix_langevin_spin). Typically, this thermostat can be
coupled to another Langevin thermostat applied to the atoms using [fix
langevin](fix_langevin) in order to simulate thermostatted spin-lattice
systems.

The magnetic damping can also be applied using [fix
langevin/spin](fix_langevin_spin). It allows to either dissipate the
thermal energy of the Langevin thermostat, or to perform a relaxation of
the magnetic configuration toward an equilibrium state.

The command [fix setforce/spin](fix_setforce) allows to set the
components of the magnetic precession vectors (while erasing and
replacing the previously computed magnetic precession vectors on the
atom). This command can be used to freeze the magnetic moment of certain
atoms in the simulation by zeroing their precession vector.

The command [fix nve/spin](fix_nve_spin) can be used to perform a
symplectic integration of the combined dynamics of spins and atomic
motions.

The minimization style [min/spin](min_spin) can be applied to the spins
to perform a minimization of the spin configuration.

All the computed magnetic properties can be output by two main commands.
The first one is [compute spin](compute_spin), that enables to evaluate
magnetic averaged quantities, such as the total magnetization of the
system along x, y, or z, the spin temperature, or the magnetic energy.
The second command is [compute property/atom](compute_property_atom). It
enables to output all the per atom magnetic quantities. Typically, the
orientation of a given magnetic spin, or the magnetic force acting on
this spin.

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::: {#Tranchida}
**(Tranchida)** Tranchida, Plimpton, Thibaudeau and Thompson, Journal of
Computational Physics, 372, 406-425, (2018).
:::