High-temperature induced ferromagnetism on gamma-Fe precipitates in FeCu solid solutions

Abstract

Experimental results of magnetization and neutron diffraction in the temperature range 300-1100 K evidence an anomalous high-temperature irreversible magnetic behavior on metastable FeCu solid solutions. When the temperature is increased above 500 K, a segregation process takes place in the as-milled sample which gives rise to the appearance of Fe (bcc) and Cu (fcc) phases. Further heating shows that the magnetization at 850 K falls down due to the temperature dependence of the bcc-Fe magnetization and the onset of the alpha-gamma martensite transformation. The temperature of this martensite phase transition (1020 K) is more 100 K lower than that of pure alpha-Fe (1183 K). On cooling from high temperatures (1100 K), the magnetization does not appreciably increase its value until the temperature is lowered below 900 K, showing a broad hysteresis between the forward (warming) and the reverse (cooling) transformations. Apart of the above mentioned bcc-Fe and fcc-Cu phases, on cooling, a small amount of isolated gamma-Fe precipitates (approximate to 5%) is detected. Further heating above 600 K show a large magnetization enhancement, reaching a value 50% higher with respect to the value measured at room temperature. During cooling from 1100 K the maximum value of magnetization is not recovered. The origin of this anomalous high temperature magnetic behavior is explained on the basis of strong magnetovolume instabilities in gamma-Fe. Furthermore, the thermal expansion coefficient of the gamma-Fe precipitates (21-23x10(-6) K-1), obtained from the neutron-diffraction patterns, is in excellent agreement with that calculated theoretically (20-24x10(-6) K-1, along the studied temperature range 300-1100 K). This fact is a signature of an anti-Invar behavior in gamma-Fe precipitates that could explain this surprising magnetic response.