Abstract
Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: “small” NPs with diameters in the range of 2–3 nm and “large” ones in the range of 5–8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600–650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min.
Highlights
Due to their attractive catalytic properties for oxygen reduction reactions (ORR) [1,2] as well as their high magnetocrystalline anisotropy energy density (MAE), which promises application for next-generation magnetic data storage [3,4,5], improved fabrication processes of FePt alloy films and nanoparticles (NPs) with approximately equiatomic composition are a prerequisite in pursuit of optimized functionality
While ultrathin FePt films usually exhibit large magnetic domains, the use of well-separated or at least exchangedecoupled FePt NPs potentially enables the storage of one bit per particle [4,12,13] and the critical particle size for superparamagnetism decreases to 3–4 nm assuming a bulk MAE [5,11]
In the following first part we present a high-resolution transmission electron microscopy (HRTEM) study of a few FePt NPs on a MgO(001) substrate proving the possibility of NP reorientation on a single crystalline support by post-deposition annealing
Summary
Due to their attractive catalytic properties for oxygen reduction reactions (ORR) [1,2] as well as their high magnetocrystalline anisotropy energy density (MAE), which promises application for next-generation magnetic data storage [3,4,5], improved fabrication processes of FePt alloy films and nanoparticles (NPs) with approximately equiatomic composition are a prerequisite in pursuit of optimized functionality. In the present study we investigate the possibility of a structural (re)orientation of FePt NPs and thin films on a-SiO2/ Si(001), MgO(001), and sapphire(0001) after different in situ annealing steps by HRTEM and RHEED.
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