Spin injection is one of the key phenomena exploiting the electron spin degree of freedom in future electronic devices.¹ A critical parameter that determines the efficiency of spin-injection is the degree of spin-polarization carried by the current. Significant interest has been addressed to the spin injection into semiconductors,² and recent developments in the field have demonstrated the possibility of efficient spin-injection and spin-detection in various electronic systems.^3,4 All the above results rely however on a 'passive' spin-injection where the degree of transport spin polarization is determined by the spin-polarization of the injector and the detector, and the electronic properties of the interface. Adjustable spin-injection with a controllable degree of spin-polarization would be appealing from the scientific point of view and useful for applications.