A new magnetic nanodot (MND) memory with FePt nanodots was proposed. The FePt nanodots dispersed in SiO2 insulating film was successfully fabricated by self-assembled nanodot deposition (SAND). The size of the FePt nanodot can be controlled by SAND with a different target area ratio of the FePt pellets area in the SiO2 target. Thermal annealing converts the magnetic properties of the FePt nanodots from antiferromagnetic into high coercivity ferromagnetic without thermal agglomeration. An L10 face-centered tetragonal (fct) FePt MND film was successfully formed which acted as a charge retention layer. Furthermore, the fundamental characteristics of the MND memory were investigated using magnetic metal oxide semiconductor (MOS) capacitor devices.

Fe 50 Pt 50 nanodots dispersed in a SiO2 film (Fe50Pt50 nanodot film) were formed by a self-assembled nanodot deposition (SAND) method in which Fe50Pt50 and SiO2 are cosputtered in a high vacuum rf magnetron sputtering equipment. Fe50Pt50 pellets are laid on a SiO2 target in a sputtering chamber to form the Fe50Pt50 nanodot film in the SAND method. The size and density of Fe50Pt50 nanodot were controlled by changing the ratio of the total area of Fe50Pt50 pellets to that of SiO2 target. The Fe50Pt50 nanodot size decreases and its density increases when the ratio decreases. As-deposited Fe50Pt50 nanodots self-assembled to a face-centered-cubic phase of single-crystal structure. The Fe50Pt50 nanodot films were annealed to evaluate the nanodot size controllability, the magnetic anisotropy, and the thermal stability. Fully ordered L10 face-centered-tetragonal Fe50Pt50 nanodots with high magnetocrystalline anisotropy (Ku≅8.7×107ergs∕cm3) were obtained by in situ annealing at 600°C for 1h in a high vacuum ambience. Furthermore, the Fe50Pt50 nanodot film with a monolayer of Fe50Pt50 nanodots was formed by annealing at 800°C due to the agglomeration of Fe50Pt50 nanodots in the SiO2 film.

The chemical states of Fe and Pt in in situ annealed L10 structured FePt nanodots formed by self-assembled nanodot deposition method have been systematically investigated by angle resolved x-ray photoelectron spectroscopy. From the Fe3p and the Pt4f core level x-ray photoelectron (XP) spectra, it is evident that both the Fe and Pt of the nanodots were oxidized in the as-grown state. After the in situ annealing under high vacuum, a peak corresponding to metallic Fe begins to appear, and subsequently the metallic peak fraction increased with the increase in the annealing temperature. In line with this, the peak fraction of the respective oxides is drastically decreased. Irrespective of the annealing temperatures, it is inferred from the intensity of the XP spectrum that the Fe atom of the FePt nanodots is highly prone to oxidation than the Pt atom. Nevertheless, the valence band spectra of the as-grown FePt nanodot film clearly depict the presence of metallic Fe–Pt alloy. We would like to explain the results of the core level and valence band XP spectra as follows: only the peripheral Fe and Pt atoms of the as-formed FePt nanodots are bonded to the oxygen of the cosputtered SiO2 matrix, whereas the metallic core of the as-formed FePt nanodots is always preserved. The very good vacuum ambient during postannealing highly promotes the dissociation of oxygen from the metal oxides via reduction reaction. This results into an increase in the fraction of metallic Fe and Pt at the periphery of FePt nanodots and the formation of high quality SiO2 matrix after annealing. Similar results were also observed for the monatomic W as well as Pt nanodots embedded in SiO2 matrix. Hence, this simple, rather effective method of in situ annealing of metal dots dispersed in an insulating matrix can be readily employed in the fabrication of high-density nanodot memory devices.