We set up the theory of ultrafast spin dynamics in the ferromagnetic Ni and antiferromagnetic NiO. Investigating the speed limits of spin dynamics is of a very high importance for future computer memory designs, such as Magnetic Random Access Memories. We find magnetic dephasing in Ni and dephasing-rephasing processes in NiO on the femtosecond timescales. Thus, NiO presents a very interesting feature of fast spin dynamics accompanied by a long lasting (till nanoseconds) coherence, which may find applications e.g. in quantum computing. These effects can be probed e.g. by nonlinear magneto-optical methods such as Second Harmonic Generation (SHG). Additionally, we demonstrate a laser-driven remagnetization in Ni, which is the first step towards spin control.