High-resolution imaging of remanent state and magnetization reversal of superdomain structures in high-density cobalt antidot arrays

Abstract

Remanent state and magnetization reversal processes of a series of cobalt antidot arrays with a fixed hole diameter (d ≈ 55 nm) and an array periodicity (p) ranging between 95 and 524 nm have been studied by in situ Lorentz microscopy (LM) as a function of the magnetic field. At remanence, defocused LM images showed the periodicity dependence of the magnetic states inside the lattice. A remarkable transition was observed in the type of domain structures as a function of p: for the large periodicities (p > 300 nm), conventional 90º and 180º domain were formed, whereas in small-period antidot arrays (p ≦160 nm) magnetic superdomain walls (SDWs) were nucleated to separate regions with different average magnetization direction, the so-called magnetic superdomains. In the SDW regime, a low-frequency Fourier filtering method was implemented to allow a quantitative analysis of the LM images by the transport of intensity equation method. In situ LM experiments under applied magnetic fields were performed to study the reversal magnetization process in a particular array (p = 160 nm), and clear differences were observed as a function of the magnetic field orientation. The switching process under magnetic fields parallel to the horizontal antidot rows occurs in two stages: the system first nucleates and Página 2 de 29 18/03/2018 propagates horizontal SDWs, parallel to the field. Then, at higher magnetic fields, vertical SDWs, perpendicular to the field, appear before saturation. When the magnetics field is applied at 45º with respect to the antidot rows, both horizontal and vertical SDWs are nucleated and propagated simultaneously. All the experiments were successfully correlated with micromagnetic simulations. The current study sheds new light on the magnetization reversal processes of antidot arrays and opens new possibilities of exploiting the potential of high-resolution in situ LM and new data analysis procedures to probe magnetization processes in nanomagnetism, particularly in periodic arrays of nanomagnets.

Recent advances in Lorentz microscopy

“…Of interest also are magnetic antidot arrays, in which small regions of a ferromagnetic film are removed, creating pinning sites. Rodríguez et al explored the behavior of antidot arrays milled into Co thin films with a range of periodicities [55]. For periodicities greater than 300 nm, conventional domain walls were observed running between the pinning sites, but as the periodicity was reduced, so the domain configuration changed such that each domain contained many antidots (refered to as superdomains by the authors).…”

Influence of lattice defects on the ferromagnetic resonance behaviour of 2D magnonic crystals

“…The recent advances in nanofabrication techniques have given a boost to the study of artificially patterned magnetic films for future applications in high density magnetic storage, sensor technology and magneto-logic devices. Attention has been focused on magnetic antidot arrays (magnetic thin films with periodic non-magnetic inclusions or embedded holes), whose hysteresis, anisotropy, magnetization reversal and magnetotransport behaviour can be engineered by properly varying the hole arrangement 1 2 3 4 . The introduction of local shape anisotropies makes these systems interesting also for magnetization dynamics and spin wave propagation properties, which can be exploited for the design of magnonic-crystal waveguides, spin wave emitters, nanoscale microwave filters and frequency-based magnetic nanoparticle detectors 5 6 7 8 9 .…”

Influence of Ga+ milling on the spin waves modes in a Co2MnSi Heusler magnonic crystal

“…In general, for ferromagnetic metals, it is known that Ga + irradiation may affect the magnetic properties of thin films through a reduced saturation magnetization, exchange interactions and magneto crystalline anisotropy 61,[71][72][73][74] . Many different mechanisms such as amorphization, modifications of local electronic properties due to ion implantation, chemical mixing or vacancy generation [75][76][77] can be at the origin of these magnetic variations. Recently, Ga + ion milling was assumed to be the reason for the apparition of unexpected localized spin waves modes in magnetic antidots patterned in Co/Pd multilayers 71 .…”