Realizing topological stability of magnetic helices in exchange-coupled multilayers for all-spin-based system

Abstract

Topologically stabilized spin configurations like helices in the form of planar domain walls (DWs) or vortex-like structures with magnetic functionalities are more often a theoretical prediction rather than experimental realization. In this paper we report on the exchange coupling and helical phase characteristics within Dy-Fe multilayers. The magnetic hysteresis loops with temperature show an exchange bias field of around 1.0 kOe at 10 K. Polarized neutron reflectivity reveal (i) ferrimagnetic alignment of the layers at low fields forming twisted magnetic helices and a more complicated but stable continuous helical arrangement at higher fields (ii) direct evidence of helices in the form of planar 2π-DWs within both layers of Fe and Dy. The helices within the Fe layers are topologically stabilized by the reasonably strong induced in-plane magnetocrystalline anisotropy of Dy and the exchange coupling at the Fe-Dy interfaces. The helices in Dy are plausibly reminiscent of the helical ordering at higher temperatures induced by the field history and interfacial strain. Stability of the helical order even at large fields have resulted in an effective modulation of the periodicity of the spin-density like waves and subsequent increase in storage energy. This opens broad perspectives for future scientific and technological applications in increasing the energy density for systems in the field of all-spin-based engineering which has the potential for energy-storing elements on nanometer length scales.

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

“…2a for [Dy10/Tb10] 10 . Apparently, there are no distinct DHLs as observed earlier for TM/RE multilayers 6,8 Fig. 2b) goes to zero also at 200 K (a similar behavior was seen for the [Dy5/ Tb5] 10 multilayer as well).…”

“…Since no additional parameters were included other than those mentioned we believe that this is the most logical consequence. Earlier, helical magnetic structures or 2π-DWs have been reported for similar systems 2,3,8 .…”

“…The range of stability of the helical state in Tb is quite restricted compared to Dy. An applied field of less than 1 kOe (100 mT) will produce a transition to the ferromagnetic state in Tb, while in Dy the reminiscent of the helical state can be found even at lower temperatures 8 . Moreover, due to basal-plane anisotropies existing both in Dy (a axis) and Tb (b axis), a combination of these two RE elements may lead to an exchange coupling as well.…”

“…The temperature ranges where Tb and Dy show AF helical magnetization, are distinctly different in bulk. For Tb, this range hovers around 220 K, whereas for Dy it ranges approximately between 180-80 K, below which both are ferromagnetic 6,8 . We found that the multilayer is magnetically very hard, with a saturation magnetization beyond 50 kOe (5000 mT).…”

“…The energy it takes to form a DW within the soft layer, which remains frozen upon cooling, results in the EB shift. Subsequently, exchange coupling and helical phase characteristics were also reported within both layers of Fe and Dy, comprising the multilayer 8 . At low fields, the TM/RE layers exhibit ferrimagnetic spin alignment, which form twisted magnetic helices in the form of planar 2π-DWs.…”