High-Density Ordered Arrays of CoPt₃ Nanoparticles with Individually Addressable Out-of-Plane Magnetization

The bit-patterned media (BPM) technology is a promising approach for developing high-density memory devices. Porous templates such as anodized aluminum oxide and self-assembled block copolymer films have been explored for use in BPM. In this work, in order to further increase the pore density, we used a mesoporous silica thin film (MSTF) with 8 nm sized regularly ordered pores and 4 nm thick walls as a template to grow CoPt3 nanoparticles (NPs) into two-dimensional hexagonal arrays. The use of a Au(111)/SiO2 substrate induced epitaxial growth of single-crystalline CoPt3 NPs in the face-centered cubic structure, as evidenced by high-resolution transmission electron microscopy and grazing-incidence X-ray scattering data. Direction-dependent magnetic measurements showed that the CoPt3 NPs had out-of-plane magnetic polarization. Magnetic force microscopy (MFM) data indicated that individual CoPt3 NPs could be addressed independently. First-principles electronic structure calculations indicated that the observed out-of-plane magnetic polarization of CoPt3 NPs originated from the tensile stress induced by the lattice mismatch between CoPt3 NPs and Au(111). The array of CoPt3 NPs had a remarkably high density of 5 × 1012 NPs/in2.