Fourier analysis of exchange biased Ni80Fe20/Fe50Mn50/Ni80Fe20 trilayers
Hysteresis loops of polycrystalline and single-crystal exchange biased Ni80Fe20 (Permalloy)/ Fe50Mn50/Ni80Fe20 (Permalloy) trilayers were measured as a function of Fe50Mn50 layer thickness with the longitudinal Kerr effect. The variation of the macroscopic pinning field H-p and the coercivity H-c was observed over a full 360 degrees in plane rotation and Fourier analyzed. The magnetization behavior of both Permalloy layers of the polycrystalline samples was analyzed, and it was found that the pinning field of the bottom layer is always greater than for the top layer, while the situation is reversed for the coercivity due to defects incorporated in the antiferromagnetic layer. The single-crystal samples were prepared on epitaxially grown Cu(111) on Si(110), and the magnetization behavior of the top Permalloy layer was studied. In contrast to the polycrystalline samples, the coercivities peak very sharply at the easy axis, which manifests itself in large higher-order Fourier coefficients. Coercivities and loop shifts show a strong linear dependence on the antiferromagnetic layer thickness. The relation of the biasing direction to the crystal axes had no influence on H-c acid H-p. This behavior is attributed to complete strain relief through the buffer layer and better crystalline growth of the trilayer as compared to the polycrystalline samples. Examination of the results with a Ginzburg-Landau energy functional verified that the Fourier coefficients obey necessary conditions to achieve energetic stability together with spontaneous magnetization. The energy functional was used to model the angular dependence of the loop shift.