Measurements of noise in magnetic thin lms have variously reported 1=f noise, white
noise, and random telegraph noise. The 1=f noise experiments claim magnetic noise by
relating the magnetic noise to the susceptibility with use of the
theorem. However, neither the linearity of the susceptibility necessary for application
uctuation-dissipation nor the frequency dependence of susceptibility was explored.
To investigate more fully the frequency dependence and linearity of the magnetic susceptibility
of a magnetic lm, we measured the AC susceptibility of a 100 nm thick
lm of permalloy as a function of the magnitude HAC and frequency f of an applied
AC magnetic eld over the full hysteresis loop of the lms, i.e., the measurements were
performed while slowly varying an applied DC magnetic eld, HDC. The AC frequency
range was from 20 Hz to 5 kHz while the AC eld range was between 0.2 G and 1 G.
At HDC = 0 the response of the system was measured as a function of HAC at 200
Hz. It was found to be nonlinear but became reasonably close to linear for HAC < 1G
- the coercive eld was approximately 15 G. The in-phase and out-of-phase components
approximately follow power laws with frequency, with exponents 0.69 and -0.2.
The behavior of the out-of-phase component is roughly consistent with previously measured
white magnetic noise and a simple harmonic oscillator model, but the in-phase
component does not follow the prediction of this model.