We have used ferromagnet/antiferromagnet/ferromagnet trilayers and ferromagnet/antiferromagnet multilayers to probe the grain size dependence of exchange bias in polycrystalline Co/Fe50Mn50. X-ray diffraction and transmission electron microscopy show that the Fe50Mn50 (FeMn) grain size increases with increasing FeMn thickness in the Co (30 Å)/FeMn system. Hence, in Co(30 Å)/FeMn(tAF Å)/Co(30 Å) trilayers the two Co layers sample different FeMn grain sizes at the two antiferromagnet/ferromagnet interfaces. For FeMn thicknesses above 100 Å, where simple bilayers have a thickness-independent exchange bias, we are therefore able to deduce the influence of FeMn grain size on the exchange bias and coercivity (and their temperature dependence) simply by measuring trilayer and multilayer samples with varying FeMn thicknesses. This can be done while maintaining the (1 1 1) orientation, and with little variation in interface roughness. Increasing the average grain size from 90 to 135 Å results in a fourfold decrease in exchange bias, following an inverse grain size dependence. We interpret the results as being due to a decrease in uncompensated spin density with increasing antiferromagnet grain size, further evidence for the importance of defect-generated uncompensated spins.
Bibliographical noteFunding Information:
The work was supported primarily by the MRSEC program of the National Science Foundation under award number DMR-0212302. We would like to acknowledge additional support from Fuji Electric Advanced Technology through membership in the University of Minnesota Center for Micromagnetics and Information Technologies. MAH is grateful for additional financial support from 3 M. AAF thanks the University of Minnesota UROP. Helpful conversations with R. Victora, J. Saha, P. Crowell and J. Parker are gratefully acknowledged.
- Exchange bias
- Grain size
- Uncompensated spins