Each electron carries one negative elementary charge, whose collective motion generates electric currents that drive the operation of lights, transistors and all kinds of electronic devices. However, as being an elementary particle, electron also possesses an intrinsic angular momentum, i.e. spin of 1/2. It has been a tempting goal to manipulate electron spins for developing faster and more energy-efficient electronic devices since Datta and Das proposed the idea of spin field-effect transistor in the 1990s.