The investigation of the magnetization reversal process in magnetic nanostructures represents an interesting topic. Indeed, thanks to they reduced dimensionality, those structures display new physical phenomena promoted by confinements effects. In addition, both size and physical properties favour their use for the production of novel devices, such as MRAMs [1]. Generally, the reversal process takes place through domain-wall (DW) movement, and if the nanostructure is made of a material with negligible magnetocrystalline anisotropy, its shape plays a great role on DWs movement. In addition, in patterned media also magnetic interparticle interactions may concur to affect DWs features. The study of DW is useful for spintronic applications and may also feature the possibility of nanoparticles manipulation [2], so it is of interest to analyze how DWs develop in a specific nanostructure geometry. 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 longitudinal and diffraction magneto-optical Kerr effect, LMOKE and DMOKE, respectively. The former gives access to the H dependence of the in-plane magnetization components; the latter, to the H dependence of the magnetic form factor, also related to the symmetry of the nanostructure magnetic configuration [3]. The HA magnetization reversal proceeds through the development of an intermediate stable vortex configuration; its appearance corresponds to a well-defined step in the LMOKE loops and to a peak in the DMOKE loops. For H parallel to the rings side, both the stability of the vortex configuration and the rings magnetic configurations are very close to those observed in the isolated rings case [3]. Differently, for H perpendicular to the rings side, the intermediate vortex configuration is no more stable and is not observed. The instability is confirmed by the fact that when H is slightly tilted with respect to that direction, the presence of the vortex state is alternatively observed in just one of the branches of the loops. The difference between the two configurations is possibly related to the interplay between a specific H direction and the corresponding flux-closure structures, favoured by the reduced ring inter-distance, that can develop within the HA. Those data will be discussed together with the results of micromagnetic calculations performed with the OOMMF software and magnetic force microscopy measurements. 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 corresponding stability of the vortex state, as well. [1] J.I. Martín et al, J. Magn. Magn. Mater. 256 (2003) 449. [2] P. Vavassori et al., J. Appl. Phys. 107 (2010) 09B301 [3] P. Vavassori et al., Phys. Rev. B 78 (2008) 174403.
Stability of the vortex state in an hexagonal array of Py triangular rings
SPIZZO, Federico;PATRIGNANI, Luca;BISERO, Diego;VAVASSORI, Paolo;RONCONI, Franco
2011
Abstract
The investigation of the magnetization reversal process in magnetic nanostructures represents an interesting topic. Indeed, thanks to they reduced dimensionality, those structures display new physical phenomena promoted by confinements effects. In addition, both size and physical properties favour their use for the production of novel devices, such as MRAMs [1]. Generally, the reversal process takes place through domain-wall (DW) movement, and if the nanostructure is made of a material with negligible magnetocrystalline anisotropy, its shape plays a great role on DWs movement. In addition, in patterned media also magnetic interparticle interactions may concur to affect DWs features. The study of DW is useful for spintronic applications and may also feature the possibility of nanoparticles manipulation [2], so it is of interest to analyze how DWs develop in a specific nanostructure geometry. 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 longitudinal and diffraction magneto-optical Kerr effect, LMOKE and DMOKE, respectively. The former gives access to the H dependence of the in-plane magnetization components; the latter, to the H dependence of the magnetic form factor, also related to the symmetry of the nanostructure magnetic configuration [3]. The HA magnetization reversal proceeds through the development of an intermediate stable vortex configuration; its appearance corresponds to a well-defined step in the LMOKE loops and to a peak in the DMOKE loops. For H parallel to the rings side, both the stability of the vortex configuration and the rings magnetic configurations are very close to those observed in the isolated rings case [3]. Differently, for H perpendicular to the rings side, the intermediate vortex configuration is no more stable and is not observed. The instability is confirmed by the fact that when H is slightly tilted with respect to that direction, the presence of the vortex state is alternatively observed in just one of the branches of the loops. The difference between the two configurations is possibly related to the interplay between a specific H direction and the corresponding flux-closure structures, favoured by the reduced ring inter-distance, that can develop within the HA. Those data will be discussed together with the results of micromagnetic calculations performed with the OOMMF software and magnetic force microscopy measurements. 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 corresponding stability of the vortex state, as well. [1] J.I. Martín et al, J. Magn. Magn. Mater. 256 (2003) 449. [2] P. Vavassori et al., J. Appl. Phys. 107 (2010) 09B301 [3] P. Vavassori et al., Phys. Rev. B 78 (2008) 174403.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.