  Zero-field-cooled (ZFC) and field-cooled (FC) magnetization along with
ac-magnetization vs. temperature and m-h loop measurements are reported for two
series of ion-beam sputtered Fe-Cr GMR multilayers where the interface
roughness is different. The exchange coupling between the Fe layers varies from
ferromagnetic (FC) to antiferromagnetic (AF) depending upon the Cr layer
thickness. The ZFC and FC magnetization data follow different curves below an
irreversible temperature ($T_{irr}$). The FC data shows a $T^{3/2}$ thermal
demagnetization behavior at lower temperatures with very small spin-wave
stiffness constant (as compared with that of bulk Fe obtained from Bloch's
$T^{3/2}$ law) but it goes as 1/T at higher temperatures (above ($T_{irr}$)).
This behavior is interpreted in terms of the coexistence of spin-glass
(SG)/superparamagnetic, FM and AF phases. ac-magnetization vs. temperature
shows a peak at $T_g$. This peak shifts towards higher temperatures and its
amplitude decreases with increasing frequency of the driving ac field.
