Magnetic powder includes: at least one epsilon-phase iron oxide-based compound selected from the group consisting of ε-Fe2O3 and a compound represented by Formula (1); and a surface treatment layer including a silane compound on at least a part of a surface. The magnetic powder has an average particle diameter of 8 nm to 20 nm. The content ratio of carbon atoms of the silane compound included in the surface treatment layer to iron atoms of the at least one epsilon-phase iron oxide-based compound selected from the group consisting of ε-Fe2O3 and the compound represented by Formula (1) is 0.05% to 0.5% in terms of the number of atoms. A manufacturing method thereof and applications thereof are also provided. In Formula (1), A represents at least one metal element other than Fe and a represents a number that satisfies a relationship of 0