Evidence of antiferromagnetic interactions in iron-implanted kapton

Iron ions were implanted into a polymer, Kapton, at 300 keV to a dose of 10¹⁷ atoms cm^-2 in order to modify magnetic properties. Magnetization curves were measured by a superconducting quantum interference device magnetometer at 4.2, 77 and 300 K in fields up to 40 kG. The as-implanted sample exhibits paramagnetic behaviour at all temperatures. Annealing in vacuum caused ferromagnetic behaviour at 4.2 K. Unexpectantly, it has been found that the magnetic moments of all samples are abnormally small at low temperatures. The magnetization curves can be explained by taking into account the existence of magnetic clusters and free iron ions with antiferromagnetic interactions.