THERMAL-EQUILIBRIUM NOISE WITH 1 F SPECTRUM IN A FERROMAGNETIC ALLOY - ANOMALOUS TEMPERATURE-DEPENDENCE/

We report on the magnetization noise in a soft ferromagnetic alloy for frequencies f<10 Hz, temperatures T<4.2 K, and excitation field ampli tudes <150 mu A/m. The spectra agree, without any adjustable parameter , with the fluctuation dissipation formula and the measured complex su sceptibility of the material. The spectra show a lif shape coming from a frequency-independent imaginary susceptibility chi(0)''-chi(0)'' is found to be proportional to T above approximate to 2.5 K but tends to be independent of T below this value. After annealing the sample to r elieve the internal stresses, chi(0)'' is found nearly temperature ind ependent above approximate to 2.5 K and to sharply increase by decreas ing the temperature below that value. We discuss these findings in ter ms of the hopping of the magnetization by activation or tunneling.