Nanostructured Mn—Al, Mn—Al—C permanent magnets are disclosed. The magnets have high coercivities (about 4.8 kOe and 5.2 kOe) and high magnetization values. An intennetallic composition includes a ternary transition metal modified manganese aluminum alloy Mn—Al—Fe, Mn—Al—Ni, or Mn—Al—Co having at least about 80% of a magnetic τ phase and permanent magnetic properties. The alloy may have a saturation magnetization value of at least 96 emu/g with approximately 5% ternary transition metal replacing Al. The alloy may also have a saturation magnetization value of at least 105 emu/g with 10% ternary transition metal replacing Al.

A sintered magnet body (RaT1bMcBd) coated with a powder mixture of an intermetallic compound (R1iM1j, R1xT2yM1z, R1iM1jHk), alloy (M1dM2e) or metal (M1) powder and a rare earth (R2) oxide is diffusion treated. The R2 oxide is partially reduced during the diffusion treatment, so a significant amount of R2 can be introduced near interfaces of primary phase grains within the magnet through the passages in the form of grain boundaries. The coercive force is increased while minimizing a decline of remanence.