Dipolar Interactions at Ferromagnetic Critical Points

The critical behavior for T>Tc and $H=0$ of ferromagnets with both isotropic exchange coupling and dipolar interactions is studied by exact renormalization-group techniques in d=4-ε dimensions (ε>0) with ($n=d$)-component spins. A crossover from short-range isotropic (Heisenberg) to characteristic dipolar behavior occurs. The asymptotic spin-spin correlation function develops a factor δ_αβ-q^αq^βq² which suppresses longitudinal fluctuations. Experiments on EuO and EuS are considered.