Domain-wall-assisted giant magnetoimpedance of thin-wall ferromagnetic nanotubes

Abstract

We study the efficiency of the magnetoimpedance (MI) of thin-walled circumferentially-ordered nanotubes in sub-GHz and GHz frequency regimes, using micromagnetic simulations. We consider empty ferromagnetic tubes as well as tubes filled with non-magnetic conductors of circular cross-section (nanowire coverings), focusing on the low-field regime of MI (below a characteristic field of the low-frequency ferromagnetic resonance). In this field area, the efficient mechanism of MI is related to oscillations of the positions of (perpendicular to the tube axis) domain walls (DWs). Two mechanisms of driving the DW motion with the AC current are taken into account; the driving via the Oersted field and via the spin-transfer torque. The simulations are performed for Co nanotubes of the diameter of 300nm. Achievable low-field MI exceeds 100%, while the field region of a high sensitivity of that DW-based giant MI is of the width of tens of kA/m. The later is widely adjustable with changing the density of the driving AC current, its frequency, and the nanotube length. Of particular interest is the resonant motion of DW due to the interaction with the nanotube ends, the conditions of whom are discussed.

Domain-wall-assisted asymmetric magnetoimpedance in ferromagnetic nanostripes

“…Such a stabilization is important since the low-field MI in the nanostripes is based on the oscillatory motion of the ferromagnetic DWs, unlike GMI in micrometer-sized systems (whose low-field mechanism is dominated by the skin effect). The above statement relates to systems of a certain regime of thickness of the magnetic layer smaller than the skin depth and it is supported by our recent study of GMI in nanotubes [7]. It has confirmed the DW-based magnetic response to the alternating Oersted field to be largely sensitive to the constant axial field applied, however, its stability has been found limited by processes of the DW-pair annihilation.…”

Magnetic Nanowires and Nanotubes

“…This regime is also related to the giant magneto-impedance effect, covered in the next section (sec.3.5.1.). This was further refined in Janutka and Brzuszek (2018): starting from a simple Néel wall between two domains with azimuthal magnetization, the transition to the Walker regime involves azimuthal instabilities of the Néel wall, ending in the nucleation of vortex-antivortex pairs end the transformation into cross-tie walls. These are shown to be intrinsically unstable under OErsted fields through the azimuthal motion of vortices and antivortices, so that this highlights the transition towards the Walker regime.…”

“…The usual physical effect is interaction of the resulting OErsted field with magnetization. However, the emergence of the spin-transfer torque phenomenon requires that direct effect of the current is considered (Janutka and Brzuszek 2018). It may play a key role in thin-walled nanotubes of small diameter, in which the OErsted field is weak for a given current density in the material.…”

“…GMI Giant magneto-impedance is an effect specific to high-frequency excitation of microwires, with a complex physics related to skin depth, magnetocrystalline anisotropy and precessional dynamics (sec.3.5.1.). It has been mentioned theoretically to be applicable to tubes with orthoradial magnetization (Janutka and Brzuszek 2018), however there exists no experimental realization yet.…”

“…Magnetic microwires can be also used as wire-less stress sensors (Marín 2017). Recently, Janutka and Brzuszek (2018) calculated that the GMI can be exploited also in magnetic (nano)tubes with azimuthal magnetization, displaying multiple domain walls. Suitable samples with long length have been already fabricated by Staňo et al (2017b).…”