Ultrafast Subpicosecond Magnetisation Of a Two-Dimensional Ferromagnet.

Strong spin-charge interactions in several ferromagnets are expected to lead to subpicosecond (sub-ps) magnetisation of the magnetic materials through control of the carrier characteristics via electrical means, which is essential for ultrafast spin-based electronic devices. Thus far, ultrafast control of magnetisation has been realized by optically pumping a large number of carriers into the d or f orbitals of a ferromagnet; however, it is extremely challenging to be implemented by electrical gating. This work demonstrates a new method for sub-ps magnetisation manipulation, called wavefunction engineering, in which w e control only the spatial distribution (wavefunction) of s (or p) electrons and require no change in the total carrier density. Using a ferromagnetic semiconductor (FMS) (In,Fe)As quantum well (QW), w e observe instant enhancement, as fast as 600 fs, of the magnetisation upon irradiating a femtosecond (fs) laser pulse. O ur analysis shows that the instant enhancement of the magnetisation is induced when the two-dimensional (2D) electron wavefunctions (WFs) in the FMS QW are rapidly moved by a photo-Dember electric field formed by an asymmetric distribution of the photocarriers. Because this WF engineering method can be equivalently implemented by applying a gate electric field, o ur results open a new way to realise ultrafast magnetic storage and spin-based information processing in present electronic systems. This article is protected by copyright. All rights reserved.