The anomalous Hall effect of wires patterned from ðCo 90 Fe 10 =PtÞ n multilayers, with 10 n 50, is used to determine the magnetization process in a small volume of maze domains. Time-independent 1=f noise appears in samples with a quality factor Q < 1 at points on the hysteresis loop where the magnetization reverses continuously. The magnetic noise is associated with reversible excursions of segments of a domain wall $100 nm long. Barkhausen jumps are observed close to either the switching field or the saturation field where the noise power spectrum varies as 1=f 1:7 , and its magnitude decays with time. DOI: 10.1103/PhysRevLett.104.047202 PACS numbers: 75.60.Ej, 75.75.Àc, 75.50.Ww, 85.75.Nn Hysteresis is the defining property of a ferromagnet. The irreversible, nonlinear, and time-dependent response of the magnetization to an external magnetic field is traced as a time-or frequency-dependent open loop, which represents the sequence of metastable states occupied by the magnet as it adapts its domain configuration to the external field. Magnetic noise associated with the loop was first heard by Barkhausen in 1919, when he detected irreversible domain wall movement in nickel wires via the emf generated in a pickup coil by the flux jumps [1,2]. Barkhausen noise provides the basis for a method of nondestructive testing of steel parts under ac excitation [3]. It has been used to study correlations in magnetization reversal cascades by recording the jumps with a magneto-optical microscope [4].Single-domain particles of hard magnets may exhibit coherent magnetization reversal-the classical StonerWohlfarth behavior [5]-when they are about 10 nm in size, or less [6]. Uniformly magnetized films of similar thickness having spontaneous magnetization M and perpendicular anisotropy K u exhibit perpendicular magnetization when the quality factor Q ¼ 2K u = 0 M 2 is greater than 1. The hysteresis may then be a simple square loop like that of a Stoner-Wohlfarth particle, with an abrupt switch from magnetization þM to ÀM at the coercive field H c , when the applied field is along the easy axis. Perpendicular magnetization remains possible when Q < 1, but in a multidomain state, which is stabilized by the stray field created by a maze of stripe domains. The magnetization switches from a uniformly magnetized state in a positive applied field to a maze domain state, which is then progressively demagnetized by domain wall motion. The reverse stripes broaden and the normal stripes narrow before breaking up into segments and bubbles, and eventually disappear as the sample reaches saturation in the reverse direction [7,8]. Thin film multilayers of Fe or Co and Pt or Pd with perpendicular magnetization are of great interest for magnetoelectronics, magnetic recording and studies of the physics of magnetic interactions at the nanoscale level. In this Letter, we show how the magnetic noise due to fluctuations in the magnetization of a small sample volume changes as we go around the hysteresis loop. The fluctuations are measured using t...