Fine platelet particles of barium ferrite wereWe have studied the Ba-ferrite particles, which prepared by the hydrothermal precipitation technique. The were prepared by the hydrothermal precipitation magnetic properties such as magnetization, coercivity and method [9,lO]. There are some advantages in the temperature variation of the coercivity can be controlled by hydrothermal preparation : fine particles with narrow size the selection of the substitution elements. Non-stoichiometric distribution can be easily obtained. In the present paper, we Co2+-Ti4+ substitution yields the formation of the fine will report the preparation and the magnetic properties of particles, and further the replacement of Co2+ by Ni2' the Ba-ferrite particles, which are synthesized by the reduces the temperature variation degree of the coercivity. hydrothermal procedure.These fine particles are thought to be good candidates for highIn addition, fine acicular particles of metal a-Fe density recording. Furthermore, we briefly discussed on the have been considered as the other candidate for high density preparation procedure of fine a-Fe acicular particles.recording materials. We have studied the preparation of metal iron fine particles as well as Ba-ferrite particles. On some of the problems : narrowing particle size distribution, I INTRODUCTION increase in coercivity and resistance to environmental corrosion, we'll briefly present our experimental results. Magnetic iron oxide particles have been widely used in many applications such as pigments of paint, they have mechanical hardness and chemical stability. In magnetic recording media, it is well known that acicular iron oxide particles are coated on the substrate with longitudinal orientation. In t h~s configuration, the FeC136%0 and BaC1, were dissolved in distilled demagnetizing field causes instability of the remanent water, and the molar ratio of Fe3+/Ba2+ was adjusted to magnetization, as the recording density increases [l]. about 9. Some of the Fe3+ were replaced by other cations Therefore, there exists a limitation in the high density such as Co2+, Ni2+ and Ti4+ in order to inocllfy the recording of the longitudnal recording system. magnetic properties. The cationic solutions were mixedIn 1977, S. Iwasaki and Y. Nakamura reported with NaOH aqueous solution, and the alkaline suspensions the perpendicular magnetic recording system based on thin containing various amount of excess NaOH ( 3 to 9M ) were film media, in which the remanent magnetization cannot be prepared. Then the alkaline suspensions were inserted into reduced by the demagnetizing field[2]. Since then, many a stainless steel autoclave, and subjected to hydrothermal researchers have extensively studied and developed the treatment at temperature from 150 "C to 300 "C for four perpendicular recording system using thin film media of hours, followed by slow cooling to room temperature. The Co-Cr or others for high density recording [3,4]. In 1982, precipitates were separated with filtration, washed with Toshiba corporation...

High-altitude platform stations (HAPSs) are expected to provide ultrawide-coverage areas and disaster-resilient networks from the stratosphere at around 20 km by installing wireless equipment on HAPS. Because their altitude is much lower than that of communications satellites, HAPSs can provide mobile communications services directly to smartphones, which are commonly used in terrestrial networks, such as fourth generation Long Term Evolution. Considering the widespread nature of mobile broadband communications and the importance as a backup line in case of disaster, HAPSs are expected to provide a large capacity in the future. A cellular system with single-cell frequency reuse using multiple cells similar to terrestrial mobile communications should be introduced to achieve such a capacity. The number of cells that a HAPS can accommodate ranges from 1 to more than 100, depending on unmanned aerial vehicle (UAV) ability. By contrast, the optimal cell configuration, which depends on the number of available cells, has not been clarified in previous research. In this paper, we propose an optimization method for the cell configuration for HAPS mobile communications using a genetic algorithm, which can be generally applied regardless of the number of cells and can clarify the optimal cell configuration. Although many cells are required to achieve gigabit-class HAPS mobile communications, the heightened power consumption due to the large number of cells is a critical problem for UAVs. Thus, we also investigate the reduction of the total transmission power and demonstrate the feasibility of energy-efficient gigabit HAPS mobile communications with wide coverage.

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