Fe, Co, and Ni ultrafine particles (UFP) were prepared by a gas evaporation method in an inert atmosphere. The different preparing conditions of the Fe, Co, and Ni UFP included inert gas pressures and kinds were investigated by means of x-ray diffraction, transmission electron microscopy (TEM), and magnetic measurements with low or high temperatures. The experimental results show that coercivities Hc of the Fe, Co, and Ni UFP, depending on particle sizes, obey a single-domain theory. The maximum Hc was 450, 1000, and 1500 Oe, respectively, for Ni (310 Å), Fe (210 Å), and Co (200 Å) at room temperature. The coercivities came up to high in the low temperatures. The saturation magnetizations Ms of the Fe, Co, and Ni UFP decreased with small particle sizes. A face-centered structure of Co UFP (200–300 Å) was confirmed by x-ray diffraction. Some Fe UFP in the shape of balls (200–400 Å) observed in the TEM may be an amorphous structure.
Fe100−xCrx ultrafine particles (UFP) with x=5–55 and a size in the range of 100–500 Å have been prepared by the gas evaporation method using Ar gas pressures P=2, 5, and 8 Torr. Transmission electron microscopy with energy dispersive x-ray analysis showed that the composition and structure of the UFPs was the same as in the original bulk alloy. The coercivity Hc at room temperature for the Fe95Cr5, Fe90Cr10, and Fe80Cr20 UFPs reached values up to 1250, 780, and 625 Oe, respectively. The temperature coefficients of these UFPs α(Hc) = −0.09%/°C were much smaller than those of Fe in the temperature range of 20–450 °C when x≤20. A strong antiferromagnetic coupling between Fe and Cr appeared in the small particles. Both the coercivity and saturation magnetization were found to decrease with Cr addition. For samples with x=50%–60%, Hc = 200–300 Oe, and Ms = 30–60 emu/g.
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