FM: ω(k) = 4J·sin²(ka/2) | AFM: ω(k) = 2J·|sin(ka)|
Spin chain — arrows show Sₓ, S_y, Sz components (color = z-component)
Dispersion relation ω(k)
Group velocity v_g = dω/dk and phase velocity v_φ = ω/k
ω = —
vₚ = —
v_g = —