Structural and Magnetic Investigations on FePt Nanoparticles

Magnetic nanoparticles and thin films exhibiting a large magnetic anisotropy energy density (MAE) have attracted enormous attention over the last years due to their potential use in magnetic data storage or sensing applications. FePt alloy nanoparticles and films have a huge MAE in the chemically ordered L10 phase. This ordered phase is typically obtained by annealing the as-prepared chemically disordered fcc FePt nanoparticles at temperatures above 600°C. Besides standard colloidal approaches [1], such fcc FePt nanoparticles can be fabricated by a micellar technique resulting in regular arrays on various substrates [2,3]. This approach offers the advantage that the interparticle distance can be tuned between 20-100 nm. Due to the larger separation particle agglomeration can be avoided during the annealing induced transformation into the L10 phase. Moreover, the particles are magnetostatically decoupled. We investigated the formation of the L10 phase for 2-10 nm FePt particles as a function of the annealing temperature and time on Si/SiOx substrates. Hysteresis loops at T = 15K reveal strongly enhanced coercive field after an annealing step at 700°C for 30 min compared to the as-prepared particles. The effective MAE is found to be 1-2×10 J/m using simple Stoner-Wohlfarth modeling that is a factor of 3-5 smaller than the FePt bulk MAE. In order to correlate the magnetic properties to the structure of the particles we performed transmission electron microscopy (TEM) investigations of FePt particles on both, Si/SiOx and MgO(001) substrates. Basically, the lowered MAE could originate from defective crystal structures, imperfect ordering of Feand Pt-planes (chemical ordering parameter), and/or inhomogeneous segregation of Fe/Pt within the particles. The applied TEM methods included conventional TEM (bright-field, dark-field, electron diffraction), spherical aberration corrected high resolution imaging and scanning TEM (Z-contrast). It was found that most particles contain defects such as stacking faults and multiple twins (Figure 1). Calculated TEM images based on various models of FePt nanoparticles (Figure 2) were generated and compared to experimental images especially to analyze and to determine the chemical ordering parameter of the L10 phase, crystal structure and defects.