Topologically stable helices in exchange coupled rare-earth/rare-earth multilayer with superspin-glass like ordering

Abstract

Existence of 2π-planar domain walls (DWs) are often reported for transition metal-rare-earth (TM/RE) layered systems. The magnetization process of such two-dimensional randomly anisotropical system in the form of 2π-DWs is directly correlated with topologically stable helices. Here, instead of TM/RE, we have investigated [Dy/Tb] 10 multilayers involving two different anisotropic layers of rare-earth/rare-earth (RE/RE). Using magnetization and susceptibility as function of temperature along with thermo-remanent magnetization measurements we have confirmed superspin-glass type of behavior within this RE/RE system. Additionally, an exchange bias field up to-0.88 kOe (-88 mT) was also revealed for such rare-earth based multilayers. Interestingly, using detailed analysis of the polarized neutron reflectometry profiles, we find evidences of superimposed helical magnetic configurations within both materials of Dy and Tb associated with spin-frustrated interfaces. Furthermore, magnetization fluctuations around the mean magnetization from vertically uncorrelated domains were observed with polarized off-specular neutron scattering. We believe that coexistence of helical ground states with superspin-glass-like ordering are fundamentally instrumental for topologically stability in RE/RE systems, which in principle, can be exploited in all-spin-based technology.

Non-collinear magnetic configuration mediated exchange coupling at the interface of antiferromagnet and rare-earth nanolayers

“…It was shown to be more readily observable in non-collinear in Fe/MnO than in collinear AF-coupled systems 25 . Earlier, polarized neutron reflectivity revealed direct evidence of helices in the form of planar 2 -DWs within both layers of Fe and Dy 6 , 7 and also within both materials of RE–RE 9 . Thus, the factor, in turn, can be regarded as a footprint of non-collinearity or exchange-coupled helices within a RE-based system.…”

“…The bulk values of magnetization are 2.58 /atom and around 2.3 /atom for thin film in CoFe 21 . For Dy, they are 10.6 /atom in bulk 22 and 7.1 /atom for thin film 9 .

Figure 11 Hysteresis loops, , , for S 1.

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“…The microscopic origin of the exchange bias effect in the prototypical AF–FM system is a small number of irreversible moments in AF sublattice (Mn Ir), which couples with a collinear FM (CoFe) 10 . Strong exchange bias coupling along with non-collinear spin configuration in the form of a helix has also been established in recent times for various FM–RE systems (viz., Fe-Tb, Fe-Dy) 6 , 7 or RE–RE systems (viz., Dy-Tb) 9 . Owing to non-collinear magnetic sublattice in the form of a helix within an RE, exchange bias within coupled FM–RE systems is caused by a small statistical imbalance in the number of irreversible moments in RE causing a net field at the interface that pins the FM (in FM–RE) or an RE in its FM phase (in RE–RE).…”

“…Earlier, evidence of superimposed helical magnetic configurations or 2 domain walls within both materials of FM–RE or RE–RE were found 6 – 9 . Interestingly, magnetic investigations had also revealed an exchange bias coupling with superparamagnetic (SPM) or super spin-glass (SSG) like behavior, attributed to spin-frustrated interfaces 9 . However, exchange bias coupling between the non-collinear or collinear spin texture of Mn Ir 10 – 13 and the helical spin structure of RE was rarely investigated.…”

Exchange Coupling at the Interfaces of Er–Tb and Er–CoFe Multilayers

“…Aided by the RKKY interaction, the magnetic modulations usually propagate coherently over a long range, even within the intervening nonmagnetic or magnetic layers. An exchange coupling strength of around 1.0−1.5 kOe was observed in FM−RE (Fe−Dy and Fe−Tb) or RE−RE (Dy−Tb) around the temperature range (below 150 K in Fe− Tb, 1,2 below 50 K in Fe−Dy, 3 and below 80 K in Dy−Tb 4 ), where both interfacial layers in the multilayer stack were essentially ferromagnetic. Disappearance of the exchange coupling was found around the temperature range where the REs exhibit helical phases, i.e., 80−180 K for Dy and 229−221 K for Tb.…”