When magnet is displaced by a very small angle θ, then restoring couple acting on the magnet is τ=−MBHsin‌θ Negative sign shows the restoring nature of torque. Now since τ=Iα and sin‌θ≈θ for small angular displacement ‌∴‌‌Iα=MBHθ ‌‌ or ‌α=‌ angular acceleration ‌ ‌=‌