Finite-temperature magnetization of imperfect bcc Fe lattices

Defects and imperfections in ferromagnetic solids can have drastic effects on the magnetic state and performance with critical implications for materials used in high-temperature and radiation environments. In this work, Monte Carlo simulations of large-scale three-dimensional spin models were performed to compute the finite-temperature magnetization of pure and alloyed bcc Fe and examine the effects of vacancies and voids. At the atomic limit, individual Fe vacancies disrupt magnetic order leading to a pronounced decrease of the Curie temperature with increasing global porosity. For larger voids measuring several unit cells in size, a distinction is drawn between pure Fe, where defects are hollow voids, and alloyed Fe, where voids are alloy precipitates that can incorporate diffusing Fe atoms. The magnetic order in pure Fe is highly robust against defects, even up to extreme levels of global porosity (40%), whereas alloyed Fe exhibits a reduced finite-temperature magnetization even for small porosity levels (5%) and with increasing porosity the shape of the magnetization curve is drastically modified. These findings provide insight into how the magnetic state at finite temperature is affected by structural imperfections and provide a link between the magnetization and structural degradation on the nanoscale, which can in turn enable non-destructive structural testing.