Studies of stress, composition, and magnetic resonance have been made on amorphous Gd‐Co thin films implanted with nitrogen ions at fluences between 1010 and 1016 ions/cm2. A new mode of magnetic resonance is produced in a small portion of the implanted region. In this portion HA−4πM decreases with fluence and ultimately achieves a negative value considerably larger in magnitude than 4πM in the film prior to the implantation. This behavior is interpreted as arising from a decrease in the Gd‐Co exchange interaction which reduces the alignment of the Gd spin system.