Remnant states and magnetic coupling in Co/Cu multilayered nanowires observed by electron holography

Abstract

Magnetic nanowires (NWs) are of great interest due to their potential applications in technological devices and fundamental analysis in the spintronic field[1]–[3]. Among the wide diversity of NWs, the multilayered ones appear as good candidates for studying the spin torque phenomenon, and increasing the radio frequency (RF) output power by designing nano‐oscilators connected in series[4], [5]. For this propose the fundamental study of the magnetic states and coupling in cylindrical multilayered NWs is required. The resulting remanent states in these one‐dimensional multilayered systems will be the result of the competition between the shape anisotropy, crystal anisotropy, exchange interactions and the dipolar coupling that occurs due to the interlayer character. In this work we have used electron holography (EH) technique to reveal the remnant states of Co/Cu multilayered NWs after applying a saturation magnetic field perpendicular (PP) and parallel (PL) to the wire axis. Cylindrical Co/Cu multilayered NWs were prepared by electrodeposition technique using the single bath method, and polycarbonate membrane to allow the wires growth. Local chemical analysis has been performed by energy‐filtered TEM (EFTEM) to distinguish and measure the thickness of the Co(Cu) layers founding 42 nm (46 nm) as its average value. Figure 1 c shows a EFTEM map with the positions of cobalt and copper layers for a wire with 80nm of diameter. For elucidating the different local remnant states along the wire, magnetic phase shift images retrieved by EH experiments were compared with those calculated from micromagnetic simulations (See Figs 1 a and b). We found that the resulting remnant states present a strong dependence with the local morphology of the layers (Co and Cu thicknesses, NW diameter, Co/Cu interface tilting) and the magnetic coupling between consecutives Co layers. Surprisingly only a weak effect due to the applied magnetic field direction was observed. For the studied nanowires, the ferromagnetic layers can present either a monodomain state perpendicularly to the NW axis, or a vortex state where the core orientation is determined by the layer morphology and the magnetic interaction with neighbors Co layers. This latter state is the most observed magnetic state. Thus, the remnant magnetic state of the multilayered NW is formed by a mixture of monodomain and vortex states. Figure 1 d shows the 3D representation of the simulation, in which there are two vortices with different chirality and polarization, the cores of the vortices are tilted respect to the Z axis.