8.5.9. 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), or an uniaxial 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 Gilbert 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.
All the computed magnetic properties can be output by two main commands. The first one is 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. 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.
(Tranchida) Tranchida, Plimpton, Thibaudeau and Thompson, Journal of Computational Physics, 372, 406-425, (2018).