Control led Ordering of Room -Temperature Magnetic Skyrmions in a Polar Van der Waals Magnet Peter Meisenheimer1 Hongrui Zhang1 David Raftrey23 Xiang Chen25 Ying -Ting

2025-05-06 0 0 3.74MB 26 页 10玖币

侵权投诉

Controlled Ordering of Room-Temperature Magnetic Skyrmions in a

Polar Van der Waals Magnet

Peter Meisenheimer1,*,†, Hongrui Zhang1,*,†, David Raftrey2,3,†, Xiang Chen2,5,†, Ying-Ting

Chan4, Rui Chen1, Reed Yalisove1, Mary C. Scott1,6, Jie Yao1, Weida Wu4, Peter

Fischer2,3, Robert J. Birgeneau2,5, Ramamoorthy Ramesh1,2,5

† Authors contributed equally to this work

1 Department of Materials Science and Engineering, Univ. of California, Berkeley

2 Materials Sciences Division, Lawrence Berkeley National Laboratory

3 Department of Physics, Univ. of California, Santa Cruz

4 Department of Physics Rutgers University

5 Department of Physics Univ. of California, Berkeley

6 Molecular Foundry, Lawrence Berkeley National Laboratory

* Corresponding Authors; meisep@berkeley.edu, hongruizhang@berkeley.edu

Abstract:

Control and understanding of ensembles of skyrmions is important for realization of future

technologies. In particular, the order-disorder transition associated with the 2D lattice of

magnetic skyrmions can have significant implications for transport and other dynamic

functionalities. To date, skyrmion ensembles have been primarily studied in bulk crystals,

or as isolated skyrmions in thin film devices. Here, we investigate the condensation of the

skyrmion phase at room temperature and zero field in a polar, Van der Waals magnet.

We demonstrate that we can engineer an ordered skyrmion crystal through structural

confinement on the 𝜇m scale, showing control over this order-disorder transition on scales

relevant for device applications.

1. Introduction

Skyrmions are topologically nontrivial quasiparticles made up of a collection of rotating

magnetic spins[1–3] or, more recently, electric dipoles[4,5]. In many magnetic systems, this

stems from broken inversion symmetry in the crystal, resulting in a Dzyaloshinskii-Moriya

interaction (DMI), or at interfaces, leading to the Rashba effect, that can stabilize such

topologically nontrivial spin configurations[2,6]. Due to the topological protection from weak

fluctuations and the ability to manipulate them with a magnetic field or charge current,

magnetic skyrmions are of significant interest for next generation information

technologies[7–11]. Neuromorphic computing architectures based on skyrmions have been

proposed, using the diffusion of skyrmions and the resulting magnetoresistance to carry

out probabilistic computing[12–14]. Additionally, magnetic skyrmions can be used to control

spins through the topological Hall effect, where an electron interacts with the Berry

curvature of the skyrmion to generate a parallel spin current[15–17]. These phenomena,

both skyrmion motion and the topological Hall effect, are sensitive to the ground state of

the system, wherein the arrangement of the skyrmions can have significant

consequences on the resulting properties[6,18–20]. A fundamental understanding and

pathways to control such ensembles of skyrmions is then important for realization of

advanced computational devices based on skyrmion kinetics or spin transport. In this vein,

the arrangement of magnetic skyrmions becomes exciting from perspectives of both

fundamental physics and device engineering.

Figure 1: Layered polar magnet. a The unit cell of FCGT, showing the AA’ stacking which breaks

inversion symmetry and allows for a nonzero DMI. b Hexagonally packed lattice of Néel skyrmions

which, if extended, shows the 2D solid phase. c In a real sample, these hexagonally packed domains

are broken up by topological defects. Lattice vectors are shown in pink, which are bent by the

presence of dislocations in the skyrmion lattice. The scale bar is 2 𝜇m.

Since their discovery, control of the shape[21–24] and long range ordering[25–27] of magnetic

skyrmion ensembles has been an open challenge, often explored through magnetic field,

temperature and thin film interactions. In two-dimensional (2D) systems, in the absence

of structural defects or kinetic limitations, the behavior of such 2D ensembles can be

described by the Hohenberg-Mermin-Wagner theorem, which states that true long-range,

crystalline order cannot exist due to logarithmic fluctuations of the order parameter. This

occurs because the energy associated with site displacement is small enough in 2D that

the increase in entropy associated with the formation of density fluctuations becomes

favorable[28–30]. An extension of this is the Kosterlitz–Thouless–Halperin–Nelson–Young

(KTHNY) theory, which describes the melting of 2D objects with continuous symmetry

through the unbinding of topological defects due to thermal fluctuations[31–33]. KTHNY

melting is distinct from 3D phase transitions in that it proceeds from an ordered solid to a

disordered liquid through a quasi-ordered hexatic phase that is present in 2D systems.

The solid-to-hexatic phase transition is driven by the dissociation of dislocation pairs;

when dislocations with opposite Burgers’ vectors are coupled, as in the 2D solid phase,

the topology of the lattice of skyrmions remains invariant and order is preserved. Upon

dissociation of the dislocations, long-range translational order is destroyed, but

orientational order is preserved. The hexatic-to-liquid phase transition is then driven by

thermal unbinding of these dislocations into disclinations, which destroys any remaining

orientational order[25,31–35]. This quasi-long-range degree of crystallinity in the hexatic

phase distinguishes it from the liquid phase quantitatively, as the orientational correlation

function decays algebraically instead of exponentially (Sup Figure 1).

As a 2D arrangement of quasi-particles, one could envision that the ensemble of magnetic

skyrmions can be explained within the framework of the KTHNY theory. This has been

observed in certain cases in magnetic skyrmions[25], as well as in other 2D systems such

as colloids, liquid crystals, atomic monolayers, and superconducting vortices[34–39]. The

presence of crystallographic and/or magnetic disorder, however, will interfere with the

ideal KTHNY behavior. Pinning sites that can affect the thermal motion and

(dis)association of dislocations and disclinations will prevent model KTHNY phase

transitions, and subsequently limit ordering to short-range[40,41]. In this way, phase

transitions will be suggestive of hexatic and ordered phases, but the ordered state may

not be realized on experimental timescales[27,42]. Understanding the emergence of long

range order in the FCGT skyrmion ensemble is the focus of this work.

It has been previously shown, using Lorentz transmission electron microscopy, that

Bloch-type magnetic skyrmions in bulk, single crystal Cu2OSeO3 can undergo KTHNY-

type melting with magnetic field[25]. Additionally, skyrmion ordering has been studied in

the ultrathin magnetic film Ta (5 nm)/Co20Fe60B20 (0.9 nm)/Ta (0.08 nm)/MgO (2 nm)/Ta

(5 nm), but KTHNY behavior was only alluded to and not fully realized[27]. The authors

claim that the increased size of orientational domains in the sample is indicative of a

transition to the hexatic phase, but the timescales required for the system to fully relax

kinetically are not reasonably achievable. With this as background, in this work, we study

the formation and solidification of Néel-type magnetic skyrmions in a novel layered, polar

magnetic metal, (Fe0.5Co0.5)GeTe2. We observe an exponential change in the order

parameters through the phase transition, suggesting that an ordered phase may be

achievable at room temperature, if the role of structural/chemical defects or kinetic

obstacles can be overcome. If the phase is frozen due to quenched disorder, there is

some energy associated with the pinning and we hypothesize that we can overcome it to

stabilize an ordered phase. Through spatial confinement of the device which imposes an

ordering director field, we are able to overcome this pinning energy and engineer an

ordered skyrmion crystal at room temperature, demonstrating control over this order-

disorder transition on scales relevant for device applications.

Here, (Fe0.5Co0.5)GeTe2 (FCGT) is studied as a polar magnetic metal that hosts stable

magnetic skyrmions at room temperature[43,44]. The parent material Fe5GeTe2 is a

centrosymmetric ferromagnet[45,46], but breaking of spatial inversion symmetry emerges

at 50% Co alloying, likely due to chemical ordering of the transition metal site and a switch

from ABC to AA’ stacking of the layers[44] (Figure 1a). Broken inversion symmetry gives

rise to a bulk Dzyalozhinski-Moriya interaction (DMI), allowing for stabilization of stripe

helimagnetic domains and/or a Néel skyrmion phase at room temperature[2]. Both the

stripe domains and skyrmions can be observed using magnetic force microscopy (MFM),

which probes the magnetostatic force on a scanning probe tip. Here, we use this

technique to study the quasi-static phase changes of the skyrmion ensemble as a function

of temperature and to evaluate our control over their ordering (Figure 1b,c).

2. Formation of topological defects

Figure 2: Temperature dependence of topological defects. a MFM images of the FCGT

skyrmion lattice at different temperatures. Scale bars are 2 𝜇m. b Voronoi polyhedra showing the

number of nearest neighbors for each skyrmion in the image. Green(purple) sites have more(less)

than 6 nearest neighbors. When the green and the purple sites neighbor one another, a

dislocation is formed from bound topological defects, which is indicative of the disordering in the

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

ControlledOrderingofRoom-TemperatureMagneticSkyrmionsinaPolarVanderWaalsMagnetPeterMeisenheimer1,*,†,HongruiZhang1,*,†,DavidRaftrey2,3,†,XiangChen2,5,†,Ying-TingChan4,RuiChen1,ReedYalisove1,MaryC.Scott1,6,JieYao1,WeidaWu4,PeterFischer2,3,RobertJ.Birgeneau2,5,RamamoorthyRamesh1,2,5†Authorscontributed...

展开>> 收起<<

Control led Ordering of Room -Temperature Magnetic Skyrmions in a Polar Van der Waals Magnet Peter Meisenheimer1 Hongrui Zhang1 David Raftrey23 Xiang Chen25 Ying -Ting.pdf

共26页,预览5页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

Control led Ordering of Room -Temperature Magnetic Skyrmions in a Polar Van der Waals Magnet Peter Meisenheimer1 Hongrui Zhang1 David Raftrey23 Xiang Chen25 Ying -Ting

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: