Iron clustering and the magnetic properties in α-Al2O3 crystals after iron implantation to high doses

Magnetization and magnetoresistance were studied in iron-ion-implanted aluminum oxide crystals. The implantation was done up to a total dose of 3.5 x 10(17) ions/cm(2) into Al2O3 substrates with projectile energy of 100 keV. Using conversion electron Mossbauer spectroscopy, it is shown that most of the implanted ions precipitate as alpha-Fe clusters and that the nano-size clusters exhibit a transition from superparamagnetic to ferromagnetic state at the dose range of (1.5-2.0) x 10(17) ions/cm(2). A maximum magnetoresistance ratio of 7.5% was observed around a dose of 1.5 x 10(17) ions/cm(2) at a field of H = 1.2 T.