pair_style spin/exchange command

Syntax

pair_style spin/exchange cutoff

• cutoff = global cutoff pair (distance in metal units)

Examples

pair_style spin/exchange 4.0
pair_coeff * * exchange 4.0 0.0446928 0.003496 1.4885
pair_coeff 1 2 exchange 6.0 -0.01575 0.0 1.965


Description

Style spin/exchange computes the exchange interaction between pairs of magnetic spins:

where si and sj are two neighboring magnetic spins of two particles, rij = ri - rj is the inter-atomic distance between the two particles, and J(rij) is a function defining the intensity and the sign of the exchange interaction for different neighboring shells. This function is defined as:

where a, b and d are the three constant coefficients defined in the associated “pair_coeff” command (see below for more explanations).

The coefficients a, b, and d need to be fitted so that the function above matches with the value of the exchange interaction for the N neighbor shells taken into account. Examples and more explanations about this function and its parameterization are reported in (Tranchida).

From this exchange interaction, each spin i will be submitted to a magnetic torque omega, and its associated atom can be submitted to a force F for spin-lattice calculations (see fix_nve_spin), such as:

with h the Planck constant (in metal units), and eij = (ri - rj)/|ri-rj| the unit vector between sites i and j.

More details about the derivation of these torques/forces are reported in (Tranchida).

For the spin/exchange pair style, the following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands, and set in the following order:

• rc (distance units)
• a (energy units)