2020年度 提案書 （Web公開用）

作成者： Aleksandr Kurenkov（東北大CSIS）

研究プロジェクト名： Influence on antiferromagnet/ferromagnet
heterostructures properties on memristive switching.
概要： We propose to study spin‐orbit torque (SOT) switching in antiferromagnet/ferromagnet (AFM/FM)
heterostructures, which has non‐volatile analogue‐like properties. We have already visualized the switching in
the FM layer as well as AFM domains by means of photoemission electron microscopy (PEEM) coupled with x‐ray
magnetic circular and linear dichroism (XMCD and XMLD). The experiments were performed at the Diamond LightSource synchrotron in collaboration with Prof. Gambardella’s group of ETH. A paper with the results is being
prepared for submission. In continuation of this collaboration we plan to reveal dependence of switching on thestack structure (thicknesses of the layers) and explain it with micromagnetic simulations in Mumax3. Such an
approach will explain how and why exchange bias, anisotropy, saturation magnetization of the stack influence theswitching process and show a path towards its optimization for practical applications.

コアメンバー： Dr. Aleksandr Kurenkov (CSIS, Tohoku University), Prof. Shunsuke Fukami (RIEC,Tohoku University), Prof. Pietro Gambardella and members of his group (ETH Zürich, Switzerland), Dr.
Francesco Maccherozzi (Diamond Light Source, United Kingdom)
期待される研究成果： Results of this study will
improve fundamental understanding ofexchange bias and SOT switching and allow
tuning of memristive switching in AFM/FMdevices for specific needs. Combination ofexperimental PEEM XMCD/XMLD methods andnumerical simulations allows clarifying detailsof the system which are below time or spaceresolution of any experimental method.
Therefore we expect to obtain valuable
information about switching dynamics in thestudied material system at the nanoscale.

（図面）

Composite imageof ferromagnetic
domains in
PtMn/[Co/Ni]
bilayer, preparedby overlaying
several XMCD
images. Illustrates
switchingproperties ofdifferent FM
regions.

2020年度 報告書 （Web公開用）

作成者： Aleksandr Kurenkov（東北大CSIS）

研究プロジェクト名： Influence on antiferromagnet/ferromagnet
heterostructures properties on memristive switching
概要： This research aims to explain the unusual mechanism of spin-orbit torque (SOT)switching in antiferromagnet/ferromagnet (AFM/FM) heterostructures. Its main features
are reproducible “memristive” behavior and fine scale of domain patterns, which is not
typical for FM or heavy metal/FM stacks.
研究成果（実施状況）： Previously, we explained field-free SOT
switching in AFM/FM by exchange bias (EB). We also provedthat the EB changes locally at 200 nm or smaller scale. Here, bymeans of x-ray magnetic circular and linear dichroism, andmicromagnetic simulations in Mumax, we show that the actual
scale of EB variation is most probably considerably smaller thanFM domain size. This can be explained by the polycrystalline
structure of the AFM PtMn. Transmission electron microscopy
measurements show that the crystallites of AFM are around 15
nm in diameter and X-ray diffraction shows their disordered(111) orientation. Considering that spin structure in an AFM
depends on its crystalline structure, these results suggest
variation of Neel vector (and thus EB) at the crystalline scale i.e.
~15 nm. Combining these results in a micromagnetic simulationand comparing it to the experiment (see Figure on the right),
we conclude that polycrystalline structure of the stack is thesource of memristive behavior and that the latter can be tuned
by addressing the crystallographic properties of the devices.
主要発表論文等： [1] G. K. Krishnaswamy, A. Kurenkov et al., Phys. Rev. Appl. 14, 044036 (2020).