Interlayer exchange coupling and weakened surface exchange in ultrathin epitaxial Fe(100)/Pd(100)/MgO(100) structures

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Abstract
Brillouin light scattering (BLS) and the polar magneto-optic Kerr effect (MOKE) have been used to determine the interlayer exchange coupling strength at room temperature in an epitaxial wedge sample of nominal structure Pd/Fe(20 Å)/Pd/Fe(10 Å)/MgO(100) in which the Pd interlayer thickness varied between 14 and 30 Å. The constituent Fe layers were chosen to have different thicknesses and hence possessed different perpendicular anisotropy fields. This allowed both acoustic and optical spin-wave modes to be observed in the BLS experiment and also meant that the polar MOKE experiment could in principle be used to determine both antiferromagnetic and ferromagnetic interlayer coupling strengths. In order that the magnetic parameters of the individual Fe layers be independently determined, superconducting-quantum-interference-device (SQUID) magnetometry, in-plane MOKE, polar MOKE, and BLS measurements have also been made upon reference samples of nominal structure Pd/Fe(20 Å)/Pd(150 Å)/Fe(10 Å)/MgO(100) and Pd/Fe(10 Å)/MgO(100). Comparison of BLS and polar MOKE for the reference samples revealed different values for the effective demagnetizing fields in the two experiments. This is discussed in terms of a possible weakened surface exchange interaction at the Fe/Pd interface. For the wedge structure, ferromagnetic interlayer exchange coupling was observed for the entire range of Pd thicknesses studied.