Characterizing Temporal Heterogeneity by Quantifying Nanoscale Fluctuations in Amorphous Fe-Ge Magnetic Films

摘要

Equilibrium phase transitions are influenced by fluctuations and often discussedwithin the framework of the Gibbs free energy, wherein the exchangeof energy between system and thermal bath is stationary and all regionsof the sample exhibit the same phase. Presence of spatial heterogeneity inthe magnetic structures such as pinning centers, domain walls, topologicaldefects, etc. may cause temporal heterogeneity that modifies the nature ofthe magnetic phase transition. This study reports that interplay of nanoscalethermodynamics with spatio-temporal heterogeneity gives rise to complexphase transition pathways in amorphous Fe_xGe_(1-x) thin films with temperatureand Fe-concentration (x). Coherent resonant soft X-ray scattering experimentsthat have simultaneous spatial, temporal, and spectral sensitivity show thatthe origin of helical to paramagnetic phase transition in amorphous Fe-Gethin films lies in the appearance of enhanced-fluctuation spots deep insidethe ordered state. The fluctuations are heterogeneous, starting over a smallfraction of the domains that increases and becomes isotropic over the entirefilm as the temperature increases or the Fe-concentration decreases. Thefluctuating-fraction, when normalized to magnetization for different Fe-concentrations,follows a single power law behavior, suggesting that the natureof the transition can be described in terms of the underlying spatio-temporalfluctuations.