Magnetic behavior of oxidized iron thin films prepared by sputtering at very low temperatures

Iron thin films have been grown on Si(1 0 0) wafers by DC magnetron sputtering at well-controlled substrate temperatures. Film thickness is less than 100 nm and preparation temperature ranges from 100 to 300 K. The magnetic properties of these films show two interesting behaviors. By decreasing the preparation substrate temperature, the magnetic coercivity presents a clear enhancement, but below 200 K, coercivity goes to very low values. Additionally, the magnetic hysteresis loops show an exchange bias, which is attributed to the film oxidation, that also depends on the preparation temperature. Exchange bias also increases when the preparation temperature goes from 300 to 200 K and suddenly decrease when the preparation is carried out at temperatures lower than 200 K. Magnetic and structural data can be explained within a qualitative model that takes into account the iron nanometric grain size forming the thin films.