10.5.15. Magnetic spins

The magnetic spin simulations are enabled by the SPIN package, whose implementation is detailed in 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 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 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. Typically, this thermostat can be coupled to another Langevin thermostat applied to the atoms using fix langevin in order to simulate thermostatted spin-lattice systems.

The magnetic damping can also be applied using 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 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 can be used to perform a symplectic integration of the combined dynamics of spins and atomic motions.

The minimization style min/spin can be applied to the spins to perform a minimization of the spin configuration.

Inertial spin dynamics (TSPIN)

Added in version 13Nov2026.

TSPIN provides auxiliary inertial dynamics for equilibrium sampling of spin-lattice systems with variable spin magnitudes. Both the direction and magnitude evolve under a Newtonian equation of motion. The spin velocities and masses are auxiliary sampling variables; their trajectories do not represent physical Landau-Lifshitz precession or relaxation times.

TSPIN uses the existing atom_style spin and requires magnetic interactions that provide the full Cartesian spin force, including its longitudinal component. See fix nve/tspin for the force convention and compatibility requirements.

The following commands support TSPIN simulations:

The following fixed-lattice example assumes that the simulation box, atoms, masses, and initial spins have been defined. It demonstrates commands using a harmonic modulus potential without spin-spin interactions.

pair_style zero 4.0
pair_coeff * *
fix 1 all spring/tspin 1.0 2.2
fix_modify 1 energy yes
fix 2 all nve/tspin lattice frozen
velocity/tspin all create 300.0 12345 spinmass 0.0075
fix 3 all langevin/tspin 300.0 300.0 0.05 48279
compute 1 all temp/tspin
thermo_style custom step pe c_1 c_1[1]

Magnetic output

compute spin reports averages of unit spin directions, selected magnetic energies, and a torque-based temperature. For variable-magnitude spins, these directional averages are not modulus-weighted magnetic moments, and this temperature is not the TSPIN kinetic temperature. Use compute property/atom to access spin magnitudes and directions.

The usual ke, etotal, and econserve exclude auxiliary spin kinetic energy. See compute temp/tspin for energy conservation checks and the kinetic spin temperature.

Do not combine inertial and fixed-modulus spin integrators on the same atoms. With SPIN pair styles, this restriction also applies to disjoint groups. Choose spin masses and timesteps for stable integration and converged equilibrium averages.

Note

Do not unfix the automatically created TSPIN_STATE fix while TSPIN styles are in use.

(Tranchida) Tranchida, Plimpton, Thibaudeau and Thompson, Journal of Computational Physics, 372, 406-425, (2018).