  Controlling the magnetic ground states at the nanoscale is a long-standing
basic research problem and an important issue in magnetic storage technologies.
Here, we designed a nanostructured material that exhibits very unusual
hysteresis loops due to a transition between vortex and double pole states.
Arrays of 700 nm diamond-shape nanodots consisting of Py(30 nm)/Ru(tRu)/Py(30
nm) (Py, permalloy (Ni80Fe20)) trilayers were fabricated by interference
lithography and e-beam evaporation. We show that varying the Ru interlayer
spacer thickness (tRu) governs the interaction between the Py layers. We found
this interaction mainly mediated by two mechanisms: magnetostatic interaction
that favors antiparallel (antiferromagnetic, AFM) alignment of the Py layers
and exchange interaction that oscillates between ferromagnetic (FM) and AFM
couplings. For a certain range of Ru thicknesses, FM coupling dominates and
forms magnetic vortices in the upper and lower Py layers. For Ru thicknesses at
which AFM coupling dominates, the magnetic state in remanence is a double pole
structure. Our results showed that the interlayer exchange coupling interaction
remains finite even at 4 nm Ru thickness. The magnetic states in remanence,
observed by Magnetic Force Microscopy (MFM), are in good agreement with
corresponding hysteresis loops obtained by Magneto-Optic Kerr Effect (MOKE) and
micromagnetic simulations.
