AC MAGNETIC RESPONSE OF AN NI81MN19 ALLOY

Low field magnetic measurements of a re-entrant system Ni81Mn19 as a f unction of temperature are shown to exhibit two successive transitions . The ac susceptibility shows strong frequency dependence due to the f rustrated spins. The freezing temperature, Ti, shifts to higher temper atures with increasing frequency and decreasing ac field amplitude. It s dependence on frequency is associated with the viscuous relaxation o f spins during an ac cycle of the measuring field, which can be attrib uted to the magnetic relaxation of spin clusters. The results support the theory that re-entrant spin glass state is coexistent with the fer romagnetic domains and that no real phase transition to a spin glass s tate seems to occur. The interplay of magnetic domain formation and mo vement with the so-called frustrated spins appears to be responsible f or the observed dependences.