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We study the noise performance of amorphous FeCoB soft underlayers (SULs) with radial magnetic anisotropy. 200 nm thick FeCoB films are sputter deposited and optionally postannealed for 8 s at different annealing powers. The correlation of SUL read-back noise with the magnetic and structural properties is studied using spin stand testing, in-plane magneto-optical Kerr effect measurements, magnetic force microscopy, and x-ray diffraction. The effects of annealing to achieve low read-back noise are examined. It is found that as-prepared films show large dc noise associated with stripe domains due to stress-induced perpendicular anisotropy. Thermal annealing reduces the internal stress and the films become magnetically anisotropic in the radial direction. The SUL-induced dc noise drops to the electronic noise floor. dc noise is found to decrease with an increase in annealing power until the films start to crystallize.

A method for probing exchange anisotropy based on reversible susceptibility tensor is proposed. The experimental results obtained on exchange biased IrMn/FeCo multilayers are supported by the theoretical analysis of the transverse and longitudinal susceptibilities in the case of exchange-coupled systems. The vectorial mapping of the exchange bias in the studied sample is performed by detecting a composite signal containing both transverse and longitudinal susceptibilities for different orientations of the applied field. This procedure, that is demonstrated both theoretically and experimentally, allows one to obtain the critical curve of the switching fields. The critical curve constitutes the fingerprint of the switching processes from which both magnitude and direction of the directional anisotropy can be determined.