E-iron oxide powder, composition including the same, magnetic recording medium, and magnetic recording and reproducing device

An ε-iron oxide powder having an average particle size in a range of 5.0 to 16.0 nm, and

• an uneven distribution of an M atom in a surface layer portion,
• wherein the M atom is one or more kinds of atoms selected from the group consisting of an aluminum atom and an yttrium atom,
• a content of the M atom with respect to 100 atom % of iron atoms is in a range of 4.0 to 9.5 atom %, and
• the uneven distribution of an M atom in a surface layer portion indicates that a ratio of surface layer portion content (with respect to bulk iron atom)/bulk content is equal to or greater than 0.90,
• wherein the surface layer portion content (with respect to bulk iron atom) is a content of the M atom in a solution obtained by partially dissolving the ε-iron oxide powder with acid with respect to 100 atom % of iron atom in a solution obtained by totally dissolving the ε-iron oxide powder with acid, and
• the bulk content is a content of the M atom in a solution obtained by totally dissolving the ε-iron oxide powder with acid with respect to 100 atom % of iron atom in a solution obtained by totally dissolving the ε-iron oxide powder with acid.