An underlayer powder for a coating-type double-layer magnetic recording medium is a powder composed of acicular nonmagnetic iron oxide particles whose average major axis length of the particles is 20 - 200 nm, and specific surface area calculated by BET method is 30 - 100 m 2 /g, which has a phosphorus content in the particulate powder of 0.1 - 5 wt%, and soluble phosphorus compound of not greater than 100 ppm based on P, and which preferably has a powder pH of less than 8, a soluble sodium content of not greater than 100 ppm based on Na and soluble sulfate of not greater than 100 ppm based on SO 4 .
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机译：用于涂布型双层磁记录介质的底层粉末是由针状非磁性氧化铁颗粒组成的粉末，该颗粒的平均长轴长度为20-200 nm，通过BET法计算的比表面积为30-100 m 2 / g，其粉状粉末中的磷含量为0.1-5 wt％，可溶性磷化合物基于P的含量不大于100 ppm，且粉末pH优选小于8，可溶性钠含量以Na为基准，为100ppm以下，以SO 4为基准，可溶性硫酸盐为100ppm以下。
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