Chirality of Bloch domain walls in exchange biased CoO/Co bilayer seen by waveguide-enhanced neutron spin-flip scattering

TL;DR

This study used waveguide-enhanced polarized neutron reflectometry to investigate the chirality of Bloch domain walls in exchange-biased CoO/Co bilayers, revealing non-collinear magnetic states and the influence of chiral domain walls on neutron scattering asymmetries.

Contribution

It demonstrates the detection of Bloch domain wall chirality and stray fields in exchange-biased bilayers using waveguide-enhanced neutron scattering, a novel application of this technique.

Findings

Strong spin-flip scattering in thin Co layers indicating high non-collinearity.

Linear decrease of SF scattering intensity with increasing Co thickness.

Evidence of stray fields attributed to chiral Bloch domain walls.

Abstract

Magnetic state of exchanged biased CoO(20nm)/Co($d_F$) bilayer ($d_F$=5-20nm) was studied by means of polarized neutron reflectometry. By spacing of CoO/Co bilayer and Al$_2$O$_3$ substrate with Nb(20nm) layer we created waveguide structure which allowed us to significantly enhance intensity of spin-flip (SF) scattering in the position of optical resonances. For the trained sample with thinnest Co(5nm) we detected strong SF scattering at the resonance position (up to 30\% of incoming intensity) speaking about high non-collinearity of the system. As $d_F$ increases, the intensity of SF scattering linearly decreases. At the same time we observed asymmetry of up-down and down-up scattering channels at the resonance positions. We attribute this asymmetry to the Zeeman splitting of neutrons energies with different initial polarization taking place in high external field. Analysis, however, shows that the applied in the PNR experiment external field is not enough to quantitatively explain the observed asymmetry for the samples with $d_F > $ 5nm and we have to postulate presence of additional magnetic field produced by sample. We attribute this additional field to the stray field produced by chiral Bloch domain walls. The chirality of the domain walls can be explained by Dzyaloshinskii-Moriya interaction arising at the CoO/Co interface. Our results can be useful for designing of spintronic devices using exchange bias effect.