Formation and Superstructural Development of Epitaxially Grown FePt Nanoparticles

FORMATION AND SUPERSTRUCTURAL DEVELOPMENT OF EPITAXIALLY GROWN FePt NANOPARTICLES 

Flence, the islands are behaving like particles. The increase in coercivity for each thickness due to temperature is attributed to a higher degree of chemical ordering. 

Looking at the microstructure for samples above and below the maximum in coercivity, Fig. 21 shows that the FePt islands become interconnected above the coercivity maximum while below the maximum the islands are well separated. In the insets of Fig. 21(a) and (b) are the selected area diffraction (SAD) patterns for the samples. These indicate a single crystal FCT pattern with (001) orientation. Adjacent to the FePt diffraction spots are the (001) MgO single crystal spots indicating a slight mismatch in the lattice spacing of the two materials and a good epitaxial relationship between the two. 

The hysteresis loops of selected samples are shown in Fig. 22. For each of the thicknesses shown the loops are square with high coercivity when measured with the applied field perpendicular to the film plane. When magnetization curves are measured parallel to the plane of the film a straight line results which is characteristic of a hard axis loop. The SAD patterns in Fig. 21 showed a (001) texture which corresponds to the c axis of FePt along the direction perpendicular to the plane. Since the c axis is the easy axis of the FCT the loops in Fig. 22 confirm the perpendicular orientation seen in the electron diffraction pattern. 

Silver has been reported to reduce the transformation temperature of FePt nanoparticles made by both sputtering and chemical synthesis [15, 16]. Furthermore, Ag, MgO and FePt have very similar lattice parameters (aAg = 0.40 nm) and therefore allows for a study of the transformation temperature without destroying the perpendicular texture. Here we describe the study of FePt/Ag multilayers deposited on MgO single crystal substrates at temperatures up to 400 °C. The X-ray diffraction patterns of the (FePt 2 nm/Ag 16 nm) pseudo-multilayers deposited on MgO [001] substrates at different temperatures are shown in Fig. 23. The superlattice (001) peak is present in all samples at temperatures above 280 °C, indicating the formation of Z,lo phase. 

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