AbstractIntegrating magnetic functionalities with silicon holds the promise of developing, in the most dominant semiconductor, a paradigm-shift information technology based on the manipulation and control of electron spin and charge. Here, we demonstrate an ion implantation approach enabling the synthesis of a ferromagnetic layer within a defect free Si environment by exploiting an additional implant of hydrogen in a region deep below the metal implanted layer. Upon post-implantation annealing, nanocavities created within the H-implanted region act as trapping sites for gettering the implanted metal species, resulting in the formation of metal nanoparticles in a Si region of excellent crystal quality. This is exemplified by the synthesis of magnetic nickel nanoparticles in Si implanted with H+ (range: ~850 nm; dose: 1.5 x 10 16 cm -2) and Ni+ (range: ~60 nm; dose: 2 10 15 cm -2). Following annealing, the H implanted regions populated with Ni nanoparticles of size (~10–25 nm) and density (~10 11/cm 2) typical of those achievable via conventional thin film deposition and growth techniques. In particular, a maximum amount of gettered Ni atoms occurs after annealing at 900 degrees C, yielding strong ferromagnetism persisting even at room temperature, as well as fully recovered crystalline Si environments adjacent to these Ni nanoparticles. Furthermore, Ni nanoparticles capsulated within a high-quality crystalline Si layer exhibit a very high magnetic switching energy barrier of ~0.86 eV, an increase by about one order of magnitude as compared to their counterparts on a Si surface or in a highly defective Si environment.