Oscillatory Exchange Bias in Fe/Cr Bilayers

We have measured the magnetization of Fe{(001),50 \AA}/Cr{(001),t$_{Cr}$(\AA)} bilayers grown on MgO(001) using the magneto-optical Kerr effect (MOKE). Samples were grown by UHV dc magnetron sputtering at 400 C with a wedge of Cr (t$_{Cr}$ = 0-1000 \AA) on top of a 50 \AA Fe layer. Structural characterization of the films was performed by grazing incidence X-ray diffraction (GIXRD), wide angle XRD, and atomic force microscopy (AFM). We observe small ($\sim$1 Oe) periodic oscillations ($\sim$70 K period) in the exchange bias field (H$_E$) as a function of temperature below the blocking temperature (T$_B$ $\sim$ 300K), which is attributed to variations in the wavelength, $\Lambda$, of the incommensurate spin density wave (SDW) spin structure in the antiferromagnetic Cr layer. Using scanning MOKE we have measured and compared the change in oscillation period for different t$_{Cr}$ in a single bilayer. We observe a peak in the coercive field as a function of temperature for thicker Cr layers. This effect may be related to the spin-flip transition between longitudinal and transverse spin density waves in the Cr layer. *Research supported by the NSF MRSEC program under DMR-0212032.