Dispersed iron oxide magnetic powder slurry and method for producing same

A dispersed iron oxide magnetic powder slurry, which is a dispersed iron oxide powder slurry, in which particles of ε-iron oxide or ε-iron oxide resulting from partial substitution of Fe sites by another metal element are dispersed in an aqueous solution, wherein the average secondary particle diameter of the particles of ε-iron oxide or ε-iron oxide resulting from partial substitution of Fe sites by another metal element measured with a dynamic light scattering particle size distribution analyzer (hereinafter referred to as a DLS average particle diameter) is 65 nm or less, and the pH at 25° C. is 4 or higher and lower than 11, wherein the dispersed iron oxide powder slurry contains a quaternary ammonium salt and a polycarboxylic acid, which is an organic acid having two or more carboxy groups in the molecule, and has one type or two types of a hydroxy group and an amino group other than the carboxy groups.