Spin-freezing below the ferromagnetic transition determined by the imaginary component of ac magnetic susceptibility

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Abstract
The temperature dependences of the real and imaginary components of ac magnetic susceptibility have been measured for a number of amorphous Fe-Mn and Fe-Ni alloys. The alloys have paramagnetic, ferromagnetic, and spin-glass intervals as a function of decreasing temperature. The spin-freezing temperatures Tfg are identified by examining the imaginary susceptiblility χ″. With decreasing temperature, χ″, representative of losses, begins to increase at the Curie temperature TC, and reaches a peak at a temperature identified as Tfg. No peak is seen at TC. The real susceptibility χ′ increases rapidly at TC but decreases before reaching Tfg. For frequencies of 10 and 20 Hz, the peak magnitude of χ″ is about half that of χ′. The results suggest that Tfg may be unambiguously defined by a peak in χ″ in the case of spin glasses with intervening ferromagnetic states.