R&D: Resolving Anomalies in Critical Exponents of FePt Using Finite-Size Scaling in Magnetic Fields

Physical Review Applied has published an article written by J. Waters, D. Kramer, Engineering and Physical Sciences, University of Southampton, Southampton, SO17 1BJ, UK, T.J. Sluckin, School of Mathematical Sciences, University of Southampton, Southampton, SO17 1BJ, UK, and O. Hovorka, Engineering and Physical Sciences, University of Southampton, Southampton, SO17 1BJ, UK.

Results of finite-size scaling of Ising model data according to Eq. (1).
(a),(b) show the scaled curves when H=0.0005 and H=0.005, respectively.
The resulting critical parameters (c) β, (d) ν, and (e) Tbc that result
from this method are plotted for a series of external field strengths.
The error bars in (c)–(e) represent the uncertainties consistent with a 99% confidence level [11].

Abstract: “FePt is the primary material being considered for the development of information storage technologies based on heat-assisted magnetic recording (HAMR). A practical realization of HAMR requires understanding the high-temperature phase transition behavior of FePt, including critical exponents and Curie temperature distributions as the fundamental HAMR media design characteristics. The studies so far found a significant degree of variability in the values of critical exponents of FePt and remain controversial. Here, we show that at the heart of this variability is the phase transition crossover phenomenon induced by two-ion anisotropy of FePt. Through Monte Carlo simulations based on a realistic FePt effective Hamiltonian, we demonstrate that in order to identify the critical exponents accurately, it is necessary to base the analysis on field-dependent magnetization data. We have developed a two-variable finite-size scaling method that accounts for the field effect. Through the use of this method, we show unambiguously that true critical exponents of FePt are fully consistent with the three-dimensional Heisenberg universality class.“