Powder for underlayer of coating-type magnetic recording medium

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A powder for an underlayer of a coating-type magnetic recording medium, which powder comprises flat-acicular iron oxide particles having an average major axis length of 20–200 nm, a short axis cross-section taken perpendicularly to the long axis that has a long width and a short width, and a short axis cross-section ratio defined as the ratio of the long width to the short width that is greater than 1.3 and substantially uniform in the long axis direction, the powder having a specific surface area measured by the BET method of 30–100 m2/g. The underlayer powder preferably contains 0.1–5.0 wt % of P and, optionally, an amount of R(R representing one or more rare earth elements, defined as including Y) such that R/Fe expressed in atomic percentage (at. %) is 0.1–10 at. %.