The magnetization reorientation process (MRP) of magnetic nanostructures displays novel features thanks to confinements effects due to nanostructures reduced dimensionality. As the applied field varies, the nanostructures magnetic configuration changes via domain walls (DWs) nucleation or annihilation; if magnetic materials with negligible magnetocrystalline anisotropy are used, this process is usually ruled just by the nanostructures shape. To make the MRP more reproducible and less sensitive to shape defects and to possible high energy magnetic configurations, e.g. vortex cores, the ring geometry is preferred. In this case, the magnetization process develops through magnetic configurations that are quite independent of the specific ring geometry [1,2], the so called onion state (OS) and the vortex state (VS). When inter-ring distance is small (less than one half of ring size) the dipolar interaction influence on the MRP becomes significant. In particular, during the MRP of triangular rings the DWs move and are trapped by ring corners so that the stray magnetic field may be intense. The influence of inter-ring interactions changes due to the rings shape, relative position and magnetic configuration [2]. The fields that produce the switch between OS and VS may accordingly change and a ring magnetic configuration may get less stable or even suppressed. In this contribution, we present the results obtained on an hexagonal array (HA) of Py equilateral triangular rings. The rings thickness is 25 nm, their side is 1.8 μm and their width is 230 nm. We adopt the hexagonal pattern and a reduced corner to corner distance (50 nm) in order to maximize the effect of inter-ring magnetic interactions. The magnetic reversal process was monitored for different directions of the in-plane applied field (H) through MFM measurements and longitudinal and diffraction magneto-optical Kerr effect, LMOKE and DMOKE, respectively. LMOKE gives access to the H dependence of the in-plane magnetization components; DMOKE to the H dependence of the magnetic form factor, also related to the symmetry of the nanostructure magnetic configuration [2]. Due to the presence of strong dipolar interactions, the rings of the HA display a long range magnetic order and, for a given H, they all display the same configuration. The MRP proceeds through the development of a VS in between two specularly symmetric OS; the VS is featured by a well-defined step in the LMOKE loops and by a peak in the DMOKE loops. For H parallel to the rings side (H//), the H range where the VS is stable is narrower than that observed in the isolated rings case [2]. For H perpendicular to the rings side (H_perp), the VS disappears whereas in the isolated case the VS was clearly observed. If the angle between H and a ring side, θ, is varied from 0, i.e. H//, to 30, i.e. H_perp, the width of the VS range of stability smoothly decreases and then goes abruptly to 0 for θ ~ 30. If θ is slightly varied around 30, the VS is not observed, as well. The observed difference between the H// and H_perp geometries may be due to the interplay between shape anisotropy energy and dipolar interaction energy; in H// shape anisotropy stabilizes the VS whilst in H_perp, as this direction is a symmetry axis of the triangular ring, shape anisotropy is less effective and the dipolar term prevails, stabilizing the OS configurations and canceling the VS. Indeed, the OS is the configuration where dipolar interaction are stronger, as 2 out of the 3 ring corners display a DW. The data will be discussed together with the results of micromagnetic calculations performed with the OOMMF software. The HA was simulated using groups of N rings (N = 3, 7) having different relative positions, to access different flux-closure configurations. The results will be discussed in terms of the stability of the vortex state. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°228673 (MAGNONICS). [1] M.F. Lai et al., J. Magn. Magn. Mater., 322 (2010) 92 [2] P. Vavassori, D. Bisero et al., Phys. Rev. B 78 (2008) 174403.

Vortex state suppression in an hexagonal array of interacting Py triangular rings

SPIZZO, Federico;BISERO, Diego;VAVASSORI, Paolo;
2013

Abstract

The magnetization reorientation process (MRP) of magnetic nanostructures displays novel features thanks to confinements effects due to nanostructures reduced dimensionality. As the applied field varies, the nanostructures magnetic configuration changes via domain walls (DWs) nucleation or annihilation; if magnetic materials with negligible magnetocrystalline anisotropy are used, this process is usually ruled just by the nanostructures shape. To make the MRP more reproducible and less sensitive to shape defects and to possible high energy magnetic configurations, e.g. vortex cores, the ring geometry is preferred. In this case, the magnetization process develops through magnetic configurations that are quite independent of the specific ring geometry [1,2], the so called onion state (OS) and the vortex state (VS). When inter-ring distance is small (less than one half of ring size) the dipolar interaction influence on the MRP becomes significant. In particular, during the MRP of triangular rings the DWs move and are trapped by ring corners so that the stray magnetic field may be intense. The influence of inter-ring interactions changes due to the rings shape, relative position and magnetic configuration [2]. The fields that produce the switch between OS and VS may accordingly change and a ring magnetic configuration may get less stable or even suppressed. In this contribution, we present the results obtained on an hexagonal array (HA) of Py equilateral triangular rings. The rings thickness is 25 nm, their side is 1.8 μm and their width is 230 nm. We adopt the hexagonal pattern and a reduced corner to corner distance (50 nm) in order to maximize the effect of inter-ring magnetic interactions. The magnetic reversal process was monitored for different directions of the in-plane applied field (H) through MFM measurements and longitudinal and diffraction magneto-optical Kerr effect, LMOKE and DMOKE, respectively. LMOKE gives access to the H dependence of the in-plane magnetization components; DMOKE to the H dependence of the magnetic form factor, also related to the symmetry of the nanostructure magnetic configuration [2]. Due to the presence of strong dipolar interactions, the rings of the HA display a long range magnetic order and, for a given H, they all display the same configuration. The MRP proceeds through the development of a VS in between two specularly symmetric OS; the VS is featured by a well-defined step in the LMOKE loops and by a peak in the DMOKE loops. For H parallel to the rings side (H//), the H range where the VS is stable is narrower than that observed in the isolated rings case [2]. For H perpendicular to the rings side (H_perp), the VS disappears whereas in the isolated case the VS was clearly observed. If the angle between H and a ring side, θ, is varied from 0, i.e. H//, to 30, i.e. H_perp, the width of the VS range of stability smoothly decreases and then goes abruptly to 0 for θ ~ 30. If θ is slightly varied around 30, the VS is not observed, as well. The observed difference between the H// and H_perp geometries may be due to the interplay between shape anisotropy energy and dipolar interaction energy; in H// shape anisotropy stabilizes the VS whilst in H_perp, as this direction is a symmetry axis of the triangular ring, shape anisotropy is less effective and the dipolar term prevails, stabilizing the OS configurations and canceling the VS. Indeed, the OS is the configuration where dipolar interaction are stronger, as 2 out of the 3 ring corners display a DW. The data will be discussed together with the results of micromagnetic calculations performed with the OOMMF software. The HA was simulated using groups of N rings (N = 3, 7) having different relative positions, to access different flux-closure configurations. The results will be discussed in terms of the stability of the vortex state. This work was supported by the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement n°228673 (MAGNONICS). [1] M.F. Lai et al., J. Magn. Magn. Mater., 322 (2010) 92 [2] P. Vavassori, D. Bisero et al., Phys. Rev. B 78 (2008) 174403.
2013
Arrays of magnetic nanoparticles; Diffracted-MOKE; Magnetization reversal
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/1662278
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