Ultrafast Magnetization Switching in Ferro and Ferri-magnets

Ultrafast helicity-independent all-optical switching (HI-AOS) as well as ps current-pulse driven magnetization reversal of ferrimagnets (FEMs) suggest the pathway of realizing ultrafast magnetization reversal of a magnetic tunnel junction (MTJ) based memory element. However, due to low spin-polarization, FEMs aren’t an ideal choice for the free layer of a MTJ device. Now, the spin-polarization of the free layer could be enhanced by coupling a ferromagnet (FM) with the FEM, given a reliable coupling mechanism. Here, we focus on RKKY exchange coupling mediated HI-AOS of a FM, which is magnetically coupled with a FEM. A Co/Pt, coupled to a GdFeCo, had been switched within ~7 ps after ~100 fs optical excitation. However, GdFeCo is not a desirable choice for building a nano-patterned memory cell because of its unstable perpendicular magnetic anisotropy (PMA) below 1 mm, whereas CoGd or synthetic ferrimagnets seems to be a good choice. We recently demonstrated ultrafast magnetization switching in a Co/Pt layer which is magnetically coupled to a CoGd layer via a Pt spacer. CoGd reverses its magnetization after ~1.5 ps following ~100 fs optical excitation. The Co/Pt shows a unique two-step mechanism related to the RKKY coupling driven switching. It demagnetizes much faster then CoGd, then stays demagnetized for some time before actually switching at ~3.5 ps. Considering inter-sublattice, intra-sublattice and indirect RKKY exchange scatterings, we explain the magnetization reversal dynamics of different magnetic components of ferromagnetically and antiferromagnetically coupled as well as decoupled FM-FEM heterostructures with a modified microscopic three-temperature model. The calculated values of switching times agree with the experimental observations. New material systems are constantly being studied which can work as the free layer of the MTJ. Recently, HI-AOS has been demonstrated in a MTJ structure, using Co/Gd bilayer coupled with CoFeB as the free layer. Co/Tb multilayers also show HI-AOS both with ~fs and ~ps optical pulse excitation and thereby expands the material choice of the free layer of a MTJ device.