  We study the background (equilibrium), linear and nonlinear spin currents in
2D Rashba spin-orbit coupled systems with Zeeman splitting and in 3D
noncentrosymmetric metals using modified spin current operator by inclusion of
the anomalous velocity. The linear spin Hall current arises due to the
anomalous velocity of charge carriers induced by the Berry curvature. The
nonlinear spin current occurs due to the band velocity and/or the anomalous
velocity. For 2D Rashba systems, the background spin current saturates at high
Fermi energy (independent of the Zeeman coupling), linear spin current exhibits
a plateau at the Zeeman gap and nonlinear spin currents are peaked at the gap
edges. The magnitude of the nonlinear spin current peaks enhances with the
strength of Zeeman interaction. The linear spin current is polarized out of
plane, while the nonlinear ones are polarized in-plane. We witness pure
anomalous nonlinear spin current with spin polarization along the direction of
propagation. In 3D noncentrosymmetric metals, background and linear spin
currents are monotonically increasing functions of Fermi energy, while
nonlinear spin currents vary non-monotonically as a function of Fermi energy
and are independent of the Berry curvature. These findings may provide useful
information to manipulate spin currents in Rashba spin-orbit coupled systems.
