Abstract
The magnetic state of exchanged biased $\mathrm{CoO}(20\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Co}({d}_{F})$ bilayers (${d}_{F}=5\ensuremath{-}20\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) was studied by means of polarized neutron reflectometry. By introducing a Nb(20 nm) spacer layer between the CoO/Co bilayer and the ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrate, we designed a resonator structure with significantly enhanced intensity of the spin-flip (SF) scattering at the position of the optical resonances. For the trained sample with thinnest Co layer (${d}_{F}=5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$), we detected strong SF scattering at the resonance position to the amount of 30% of the incoming intensity, pointing to a high degree of non-collinearity of the magnetization. With increasing ${d}_{F}$, the intensity of the SF scattering decreases linearly. Furthermore, an unconventional asymmetry of up-down and down-up scattering channels at the resonance positions was observed, which we ascribe to the out-of-plane magnetic stray field generated by chiral Bloch domain walls. This field leads to Zeeman splitting of the neutron energies depending on the initial neutron spin polarization. The chirality of the domain walls is assigned to the Dzyaloshinskii-Moriya interaction emerging at the CoO/Co interface. Our observations might prove useful for the design of spintronic devices based on the exchange bias effect.
Highlights
The exchange bias (EB) effect arises at the interface of antiferromagnetic (AF) and ferromagnetic (F) magnetic layers, and leads to a horizontal shift of the magnetic hysteresis loop by the exchange bias field Heb
As during the polarized neutron reflectometry (PNR) experiment a negative external field at the sample would depolarize the neutron beam, we used for PNR at effective H < 0 the procedure outlined in prior work, with cooling of the sample in negative magnetic field. (B) The noncollinear magnetic state with α = 90◦ was obtained by cooling the sample in Hmax to T = 13 K and rotating the sample by 90◦ around the z axis with H = 0, such that the magnetization of the Co layer was aligned parallel to the x axis
In this work we systematically studied the magnetization reversal of the EB bilayers CoO(20 nm)/Co(dF ) with dF = 5−20 nm by means of waveguide-enhanced neutron SF scattering
Summary
The exchange bias (EB) effect arises at the interface of antiferromagnetic (AF) and ferromagnetic (F) magnetic layers, and leads to a horizontal shift of the magnetic hysteresis loop by the exchange bias field Heb (see review [1]). The depth resolved magnetization profiles M (z) and M⊥(z) and the nuclear SLD ρ(z) are obtained by a combined fit of a model potential to the four reflectivity curves Rμν (Q). Such a fit requires knowledge of the sample structure and of the resolution properties of the neutron reflectometer. To resolve ambiguities in the reconstruction of the nuclear and magnetic profiles and to limit the number of potential models, additional data from complementary techniques, such as x-ray reflectometry and/or superconducting quantum interference device (SQUID) magnetrometry, are required. Since a waveguide requires a minimum thickness of 10 nm to form a neutron standing wave at the resonance condition, we introduced a Nb(20 nm) spacer layer in the design.
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