Long-range behavior of exchange bias in CoFe/FeMn-based multilayers

CoFe/FeMn-interface-based multilayers were grown in magnetic field and at room temperature. The exchange bias field $H_{\mathrm{EB}}$ depends strongly on the order of depositions and is much higher for CoFe/FeMn than FeMn/CoFe bilayers. By combining the two bilayer structures into symmetric CoFe/FeMn ($t_{\mathrm{FeMn}}$)/CoFe trilayers, $H_{\mathrm{EB}}$ is enhanced for both the top and bottom CoFe layers. Enhancements of exchange bias are also observed by reducing the FeMn thickness $t_{\mathrm{FeMn}}$ of the trilayers. These results evidence the propagation of exchange bias between the two CoFe/FeMn and FeMn/CoFe interfaces mediated by the FeMn antiferromagnetic order. Furthermore, the exchange bias is even considerably increased when a thin Al or Mg layer is inserted into the CoFe/FeMn interface (i.e., CoFe/Al/FeMn or CoFe/Mg/FeMn) and persists for the insertion layer thicknesses of up to about 1.5 nm. These results strongly indicate that exchange bias is not a pure interfacial phenomenon, but mainly governed by possible long-range couplings such as dipole-dipole and RKKY between the antiferromagnetic uncompensated spins and the ferromagnetic layer.