1. A rare earth SmCo₅ permanent magnet comprised of binder-free consolidated and sintered powder particles and having a magnet composition and sintered microstructure characterized by a heterogeneous multi-modal grain structure that is present throughout the sintered microstructure without added grain boundary modifier composition different from the magnet composition wherein the multi-modal grain structure of the sintered microstructure includes at least two distinct grain regions having the same composition as the magnet composition but having at least one most coarse grain region having a plurality of relatively most coarse grains aggregated together with an average most coarse grain size of about 10 to about 30 microns and at least one fine grain region having a plurality of relatively fine grains aggregated together with an average fine grain size that is about 25% to about 50% of the average coarse grain size wherein the at least one fine grain region is present as a strengthening microstructural feature in the sintered microstructure with a periphery thereof forming a fine grain-to-coarse grain boundary with the at least one most coarse grain region and is present in controlled proportion of the sintered microstructure to increase a mechanical property comprising at least one of fracture toughness and mechanical strength of said magnet having the heterogeneous multi-modal grain structure as compared to a sintered magnet having said magnet composition but a single modal grain structure with a single average grain size of about 40 microns, without altering the magnet composition, wherein the SmCo₅ permanent magnet has a flexural strength of 182 to 214 MPa at 20° C.