  Two-dimensional magnetic skyrmions are particle-like magnetic domains in
magnetic thin films. The kinetic property of the magnetic skyrmions at finite
temperature is well described by the Thiele equation, including a stochastic
field and a finite mass. In this paper, the validity of the constant-mass
approximation is examined by comparing the Fourier spectrum of Brownian motions
described by the Thiele equation and the Landau-Lifshitz- Gilbert equation.
Then, the 4-dimensional Fokker-Planck equation of the skyrmion is derived from
the Thiele equation with a mass-term. Finally, an expression of the diffusion
flow and diffusion constant in a tensor form are derived under a local
thermalization approximation. The result is also applicable for the chiral
diffusions, such as a diffusion of the classical electron gas in a magnetic
field.
