Permanent magnet for accelerating corpuscular beam

A permanent magnet having superior resistance to radioactive deterioration of magnetic properties when subjected to a corpuscular beam having a wave length of not more than about 1.ANG., said magnet having a composition consisting essentially of R.sub.a Fe.sub.bal. Co.sub.b B.sub.c Ga.sub.d M.sub.e where R is at least one element selected from the group consisting of Nd, Pr, Dy, Tb, Ho, and Ce, and M is at least one element selected from the group consisting of Al, Si, Nb, Ta, Ti, Zr, Hf, and W, with the proviso that 12.ltoreq.a.ltoreq.18, 0.ltoreq.b.ltoreq.30, 4.ltoreq.c.ltoreq.10, 0.01.ltoreq.d.ltoreq.3, and 0.ltoreq.e.ltoreq.2 in terms of atomic percent, said magnet having a microstructure comprised of fine crystal grains having an average grain size of about 0.01 .mu.m to about 0.5 .mu.m and being magnetically anisotropic, said crystal grains being rendered magnetically anisotropic by plastically deforming said magnet at a temperature in the range from about 600.degree. C. to about 800.degree. C. at a strain rate in the range from about 1.times.10.sup.-4 sec.sup.-1 to about 1.times.10.sup.-1 sec.sup.-1, the plastic working ratio h.sub.o /h, where h.sub.o is the height of said magnet before plastic deformation and h is the height of said magnet after plastic deformation, being about 2 or more.