Global Helium ion irradiation can tune the magnetic properties of thin films, notably their magneto-crystalline anisotropy. Helium ion irradiation through nanofabricated masks can been used to produce sub-micron planar magnetic nanostructures of various types. Among these, perpendicularly magnetized dots in a matrix of weaker magnetic anisotropy are of special interest because their quasi-static magnetization reversal is nucleation-free and proceeds by a very specific domain wall injection from the magnetically “soft” matrix, which acts as a domain wall reservoir for the “hard” dot. This guarantees a remarkably weak coercivity dispersion. This new type of irradiation-fabricated magnetic device can also be designed to achieve high magnetic switching speeds, typically below 100 ps at a moderate applied field cost. The speed is obtained through the use of a very high effective magnetic field, and high resulting precession frequencies. During magnetization reversal, the effective field incorporates a significant exchange field, storing energy in the form of a domain wall surrounding a high magnetic anisotropy nanostructure's region of interest. The exchange field accelerates the reversal and lowers the cost in reversal field. Promising applications to magnetic storage are anticipated.