Observation of 1/f magnetic fluctuations in a spin glass

We observe spontaneous magnetic fluctuations at 4,2 K in zero field for the amorphous spin glass (Al 2 O 3 ) 0,1 (MNO) 0,5 (SiO 2 ) 0,4 and for the crystalline spin glass compound CsNiFeF 6 . In both cases the noise power spectrum exhibits an 1/f dependence in the range 10 −2 Hz to 10 2 Hz. The 1/f noise amplitude is in good agreement with the measured value of the out of phase susceptibility. This observation supports the applicability of the fluctuation dissipation theorem to spin glasses and suggests the magnetic noise as a new area of investigation Observation de fluctuations spontanees de l'aimantation a 4,2 K, en champ nul, dans le verre de spin amorphe (Al 2 O 3 ) 0,1 (MnO) 0,5 (SiO 2 ) 0,4 et dans le verre de spin cristallin CsNiFeF 6 : variation en 1/F de la densite spectrale pour des frequences de 10 −2 −10 2 Hz