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Coupling effects in magnetic patterned nanostructures

Final Report Summary - COEF-MAGNANO (Coupling effects in magnetic patterned nanostructures)

Magnetic properties of materials are modified when deposited on a substrate with a thin film thickness of few tens of nanometers. Moreover, if dissimilar materials are alternatively grown creating heterostructures of bilayers and multilayers, novel magnetic properties emerge due to interfacial and proximity effects just at the interface between both materials. This is the case of antiferromagnetic/ ferromagnetic (AFM/FM) bilayers, superconducting/ ferromagnetic (SC/FM) bilayers, and ferromagnetic1/ ferromagnetic2 (FM1/FM2) multilayers. In addition to this constriction in the layer thickness, it is possible to create lateral constrictions by lithographic patterning of the heterostructures. The result is the appearance of unusual spin configurations in the nanostructures due to dimensionality effects. In short, the fabrication of high quality heterostructures and their patterning into nanostructures open new possibilities to explore exclusive magnetic properties and unique spin configurations.

This project investigates magnetic coupling effects at the AFM/FM and SC/FM interface, and FM1/FM2 multilayers for both continuous and nanostructured heterostructures. Two AFM/FM systems were selected: FeF2/Py (NiFe) and IrMn/CoFe. Proximity effects were investigated in a high temperature superconductor and Co/Pt multilayers: YBaCuO/[Co/Pt]xn. For FM1/FM2 systems, two distinct FM materials were chosen, Co/Ni, in order to fabricate multilayers with perpendicular magnetic anisotropy. Main findings for each of these systems are summarized below.

FeF2/Py bilayers were deposited by electron beam evaporation. FeF2 becomes antiferromagnetic below 78 K. Then, the exchange coupling at the FeF2/Py interface induces a spring-like domain wall throughout the Py layer. It was proved, for the first time, that this kind of spin structure in the FM material breaks the inverse proportionality law of the exchange bias field with the FM thickness, even for thicknesses smaller than the domain wall width of the FM (Fig. 1). This is an important finding because the inverse proportionality law was assumed for more than fifty years. Moreover, as explained above, patterning into nanostructures might lead to unique spin configurations. This is the case for Py dots with diameters in the submicrometer scale. Py dots exhibit vortex spin configuration, i.e. spins orientate in circular paths generating a vortex singularity just in the center of the dot. The competition between spring-line domains walls and the vortex state was investigated in FeF2/Py dots at low temperature. It was found that, depending on the temperature, the spin configuration of the dot is dominated by a vortex structure or the in-depth magnetic profile, creating non-trivial helical arrangements (Fig. 2).

The crystallinity of IrMn/CoFe bilayers is an important parameter that affects the exchange coupling at the AFM/FM interface. Such effect was analyzed comparing epitaxial and polycrystalline IrMn/CoFe thin films. Polycrystalline samples were grown by sputtering. However, epitaxial films required molecular beam epitaxy techniques. IrMn is antiferromagnetic at room temperature, so it requires an annealing process to induce an exchange bias direction. It was observed that polycrystalline samples yield higher values of exchange bias than epitaxial bilayers. Patterned samples of lines were fabricated with linewidths of 700 nm. No significant variations were observed for these 1D nanostructures respect to the continuous counterparts, indicating that the important magnetic length scale in this system is smaller than the linewidth of the nanostructure.

It was predicted that Meissner currents of a superconductor should interact via magnetostatic coupling with the ferromagnetic domains in SC/FM bilayers. This effect was searched in high temperature superconductor YBaCuO and Co/Pt multilayers as ferromagnetic material. Magnetic domains were recorded at several temperatures by magneto-optical Kerr effect microscopy. It was observed that the lower the number of multilayers the larger the FM domain size. However, no signature of magnetostatic coupling was found crossing the SC critical temperature.

Co/Ni multilayers with perpendicular magnetic anisotropy (PMA) have attracted much attention because of their potential in spintronic applications. The microscopic origin of the PMA comes from the Co/Ni interface. Thus, structural and growth conditions that affect the quality of the interface have a direct impact on the PMA strength. The effect of Co and Ni thicknesses, Co/Ni thickness ratio, substrate temperature, and buffer layers was investigated. The interface roughness was studied by atomic force microscopy, and demonstrated that multilayers subjected to high temperature increase their roughness and lose PMA (Fig. 3). The magnetization reversal mechanism was analyzed in multilayers with PMA by magneto-optical Kerr effect microscopy. It was revealed that opposite domains nucleate but they do not propagate. Hence, the full reversal is achieved by massive nucleation of tiny domains. This mechanism was also observed in patterned lines and dots of Co/Ni multilayers.

It must be noticed that although this project addresses fundamental research, some results can prove validation of novel concepts. For instance, FeF2/FM dots and antidots demonstrated a way to read and write multiple digits in a single cell for magnetic storage media. Moreover, other current results of magnetically coupled nanostructures might have application in magnetic sensors and electronic compasses.