Experimental results of magnetization and neutron diffraction in the temperature range $300–1100\mathrm{K}$ evidence an anomalous high-temperature irreversible magnetic behavior on metastable FeCu solid solutions. When the temperature is increased above $500\mathrm{K}$, 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 $850\mathrm{K}$ falls down due to the temperature dependence of the bcc-Fe magnetization and the onset of the $\alpha \text{−}\gamma$ martensite transformation. The temperature of this martensite phase transition $\left(1020\mathrm{K}\right)$ is more $100\mathrm{K}$ lower than that of pure $\alpha \text{−}\mathrm{Fe}$ $\left(1183\mathrm{K}\right)$. On cooling from high temperatures $\left(1100\mathrm{K}\right)$, the magnetization does not appreciably increase its value until the temperature is lowered below $900\mathrm{K}$, 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 $\gamma \text{−}\mathrm{Fe}$ precipitates $(\approx 5\%)$ is detected. Further heating above $600\mathrm{K}$ show a large magnetization enhancement, reaching a value 50% higher with respect to the value measured at room temperature. During cooling from $1100\mathrm{K}$ 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 $\gamma \text{−}\mathrm{Fe}$. Furthermore, the thermal expansion coefficient of the $\gamma \text{−}\mathrm{Fe}$ precipitates $(21–23\times {10}^{-6}{\mathrm{K}}^{-1})$, obtained from the neutron-diffraction patterns, is in excellent agreement with that calculated theoretically ($20–24\times {10}^{-6}{\mathrm{K}}^{-1}$, along the studied temperature range $300–1100\mathrm{K}$). This fact is a signature of an anti-Invar behavior in $\gamma \text{−}\mathrm{Fe}$ precipitates that could explain this surprising magnetic response.

• Received 26 August 2004

DOI:https://doi.org/10.1103/PhysRevB.72.014401