Competition between ferromagnetism and antiferromagnetism in FePt

FePt in the L1(0) (tetragonal) phase is of interest due to its ease of synthesis as nanoparticles and because superparamagnetism is suppressed by the large magnetoanisotropy energy (MAE) of the L1(0) phase. Here we present the results of first-principles electronic-structure calculations that reveal a competition between ferromagnetic (FM) and antiferromagnetic (AFM) ordering of the alternating Fe planes, with energy differences less than room-temperature thermal energy. We find that the FM state is stabilized relative to the AFM state as L1(0) tetragonal distortion decreases or chemical (antisite) disorder increases on Pt planes. Inherently, then, there is a competition between stabilizing the FM state and maintaining the large MAE in FePt, and optimally maximizing both is important for nanomagnetic applications.