Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting multilayer T. J. Hicken1 2 3M. N. Wilson1 4Z. Salman3S. L. Zhang5S. J. R. Holt6

2025-04-26 0 0 1.01MB 9 页 10玖币

侵权投诉

Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting

multilayer

T. J. Hicken,1, 2, 3 M. N. Wilson,1, 4 Z. Salman,3S. L. Zhang,5S. J. R. Holt,6

T. Prokscha,3A. Suter,3F. L. Pratt,7G. van der Laan,8T. Hesjedal,9and T. Lancaster1

1Department of Physics, Centre for Materials Physics,

Durham University, Durham, DH1 3LE, United Kingdom

2Department of Physics, Royal Holloway, University of London, Egham, TW20 0EX, United Kingdom

3Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen PSI, Switzerland

4Department of Physics and Physical Oceanography,

Memorial University of Newfoundland, St. John’s, NL A1B 3X7, Canada

5School of Physical Science and Technology, ShanghaiTech University,

Shanghai, 201210, China; ShanghaiTech Laboratory for Topological Physics,

ShanghaiTech University, Shanghai 200031, China

6Max Planck Institute for the Structure and Dynamics of Matter,

Luruper Chaussee 149, 22761 Hamburg, Germany

7ISIS Pulsed Neutron and Muon Facility, STFC Rutherford Appleton Laboratory,

Harwell Oxford, Didcot, OX11 OQX, United Kingdom

8Diamond Light Source, Harwell Science and Innovation Campus,

Didcot, Oxfordshire, OX11 0DE, United Kingdom

9Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, OX1 3PU, United Kingdom

(Dated: March 28, 2024)

Skyrmion-hosting multilayer stacks are promising avenues for applications, although little is known

about the depth dependence of the magnetism. We address this by reporting the results of circular

dichroic resonant elastic x-ray scattering (CD-REXS), micromagnetic simulations, and low-energy

muon-spin rotation (LE-µ+SR) measurements on a stack comprising [Ta/CoFeB/MgO]16/Ta on a

Si substrate. Energy-dependent CD-REXS shows a continuous, monotonic evolution of the domain-

wall helicity angle with incident energy, consistent with a three-dimensional hybrid domain-wall-like

structure that changes from N´eel-like near the surface to Bloch-like deeper within the sample. LE-

µ+SR reveals that the magnetic ﬁeld distribution in the trilayers near the surface of the stack is

distinct from that in trilayers deeper within the sample. Our micromagnetic simulations support

a quantitative analysis of the µ+SR results. By increasing the applied magnetic ﬁeld, we ﬁnd a

reduction in the volume occupied by domain walls at all depths, consistent with a crossover into a

region dominated by skyrmions above approximately 180 mT.

I. INTRODUCTION

Several material classes have emerged that host mag-

netic skyrmion excitations [1, 2], but for applications it

is necessary to stabilise skyrmions at room temperature.

For this purpose, multilayer systems show promise [3].

These comprise a repeated stack of a magnetic layer

and one or more heavy elements, e.g. Ta/CoFeB/MgO.

Here, a Dzyaloshinskii-Moriya interaction (DMI) arises

from inversion symmetry-breaking at interfaces between

the layers, leading to skyrmions that can exist over a

wide range of temperature, extending up to 500 K [4].

While skyrmions are often assumed to be a quasi-two-

dimensional texture that identically repeats in the third

dimension, recent work on Ta/CoFeB/MgO multilayers

suggests a variation of the spin texture across multiple

repeats of the multilayer [5], with micromagnetic simula-

tions predicting that the helicity changes from N´eel-type

at the surface, to Bloch-type deeper in the stack.

There are many diﬀerent techniques with which one

can characterise three-dimensional spin textures such as

skyrmions, which all have diﬀerent strengths and weak-

nesses [6–16]. In the case of Ta/CoFeB/MgO multilayer

stacks, circular dichroic resonant elastic x-ray scattering

(CD-REXS) measurements [17–19] have previous been

employed to study the system [5, 20–22]. In this system,

alternating non-magnetic spacers establish a pronounced

dipole-dipole interaction among the CoFeB layers, lead-

ing to a unique hybrid three-dimensional (3D) domain

wall conﬁguration with a Bloch layer that is located in

the middle of the stack [5, 15]. The helicity angle χ,

which describes the character of domain walls as well

as skyrmions, continuously rotates between the two ex-

tremes throughout the stack, with a characteristic pro-

ﬁle χ(z), where zis the depth into the sample. In this

case, the top CoFeB layer favors a N´eel-type domain wall

with a helicity angle of χ= 180◦or 0◦, while the middle

trilayer that hosts the Bloch point exhibits a Bloch-type

domain wall with χ=±90◦. The bottom trilayer is again

of N´eel-type, but with χchanged by 180◦from the sur-

face [5, 23]. Introducing interfacial DMI moves the Bloch

conﬁguration in z[5, 15]. In nonzero external magnetic

ﬁelds, a similar 3D magnetization characteristic is also

expected for skyrmions.

Despite the predictions from micromagnetic sim-

ulations of Ta/CoFeB/MgO multilayer stacks being

consistent with the CD-REXS measurements, a di-

rect study of the depth-resolved magnetic environ-

arXiv:2210.06070v2 [cond-mat.str-el] 27 Mar 2024

ment in the stack has hitherto been lacking. Here

we provide these measurements through an investi-

gation of depth-dependent magnetism in the multi-

layer [Ta(2)/CoFeB(1.5)/MgO(2)]16/Ta(5) (thicknesses

in nm) using low-energy muon-spin rotation (LE-

µ+SR) [24–26]. Previous work [27–30] has shown that

bulk µ+SR [24, 31] is a useful tool in the study of

skyrmion systems and their dynamics, while LE-µ+SR

has probed depth-dependent magnetism at interfaces [32]

and the behavior of superconducting [33] and mag-

netic [34] superlattices. However, these studies do not

address the evolution of the properties with depth within

a repeating structure. Combining our measurements on

[Ta(2)/CoFeB(1.5)/MgO(2)]16 with energy-resolved CD-

REXS, and interpreting them with support from micro-

magnetic simulations, we show here how the domain-wall

angle and magnetic structure varies with depth, and re-

veal a crossover in magnetic properties as a function of

depth.

II. EXPERIMENTAL AND COMPUTATIONAL

DETAILS

The [Ta(2)/CoFeB(1.5)/MgO(2)]16/Ta(5) (thicknesses

in nm) multilayered thin ﬁlms were grown by magnetron

sputtering onto Si wafers [5, 35]. A Ta layer was always

chosen as the ﬁnal layer to prevent oxidation. The base

pressure of the deposition system was <3×10−6Pa. The

sputter gas, Ar, was used at a pressure of 0.3 Pa. The

deposition rates for Ta, CoFeB, and MgO were 0.48, 0.30,

and 0.05 ˚

A/s respectively. A sputtering power of 100 W

was used.

The CD-REXS experiments were carried out in the

RASOR diﬀractometer on beamline I10 at the Diamond

Light Source (UK). The CD-REXS patterns shown are

obtained from the diﬀerence between the scattering in-

tensities for left- and right-circularly polarized x-rays for

varying photon energies around the Fe L3absorption

edge. The dichroism extinction condition directly reveals

the twisting angle of the magnetic structure averaged

over the probed depth.

Micromagnetic simulations were performed using the

Ubermag package [36], with oommf [37] as the compu-

tational backend. The same material parameters were

used as in Ref. [5], with the layer thicknesses changed to

be appropriate for our sample. The parameters included

are exchange A= 10 pJm−1, interfacial (Cnv ) DMI

D= 0.4 mJm−2, and out-of-plane uniaxial anisotropy

Ku= 0.9 MJm−3, and the calculations also included de-

magnetisation in the Hamiltonian. In magnetic layers,

the magnetic saturation was Ms= 1.18 MAm−1. Open

boundary conditions were employed on samples of lateral

size 300 ×300 nm, and the system used a discretization

of 4 ×4×0.5 nm. The complete code used can be found

in the Supplemental Material [17].

Low-energy (LE)-µ+SR measurements of were carried

out at the Swiss Muon Source, Paul Scherrer Institut,

Switzerland, using the LEM instrument, which allows the

implantation depth of the muons to be altered by chang-

ing the energy of the incoming muons [25, 26]. Mea-

surements were performed at 290 K, either in the ZF or

TF geometry. Data analysis was carried out using both

the WiMDA program [38], making use of the MINUIT

algorithm [39] via the iminuit [40] Python interface for

global reﬁnement of parameters, and musrﬁt [41]. Stop-

ping proﬁles of the muons in the multilayer stack were

modeled using the TRIM.SP software [42]. A model of

the multilayer stack was made in the software by insert-

ing layers matching the density of each of the materials at

the correct thickness to match the experimental system.

TRIM.SP then calculates histograms of the stopping pro-

ﬁles for a particular incident muon energy; we varied this

parameter to investigate the changes in stopping proﬁles.

For these calculations, at all incident energies, the energy

variance was set to 0.45 keV, and muons were incident

perpendicular to the surface with a variance of 15◦.

III. RESULTS & DISCUSSION

A. Circular dichroic resonant elastic x-ray

scattering

We studied the depth-dependent character of the

magnetic domain walls in the multilayer system

[Ta(2)/CoFeB(1.5)/MgO(2)]16/Ta(5) on a Si substrate

using CD-REXS [20–22, 43], which is sensitive to the

twisting angle of spin spirals and skyrmions, and was

previously applied to retrieve χof a magnetic twisted

domain state [21]. Figure 1(a) shows energy-dependent

dichroic scattering patterns, obtained on the multilayer

sample at room temperature and in zero applied mag-

netic ﬁeld. The ring-like magnetic scattering patterns

were obtained by integrating several azimuthal angles

using the ϕ-axis of the diﬀractometer. The dichroic pat-

terns feature a dividing vector that separates the blue

and red regions of negative and positive dichroic con-

trast, governed by the dichroism extinction rule [20], see

Fig. S2 in Ref. [17]. The azimuthal direction of the di-

viding vector uniquely reveals the value of χ[21] (Fig. S2

in Ref. [17]). For multilayer systems with a non-uniform

χ(z) proﬁle, the measured χmis the average χfrom all

trilayers with diﬀerent weightings. Experimentally, by

varying the photon energy across the Fe L3edge, or the

x-rays incidence angle θ, the weight changes due to the

varied sampling depth of the soft x-rays, leading to a dif-

ferent measured average χm[22, 23, 44]. For the investi-

gated [Ta(2)/CoFeB(1.5)/MgO(2)]16/Ta(5) heterostruc-

ture, the attenuation length of the x-rays, Λ, is 79 nm

at the Fe L3edge (707.8 eV) at normal incidence, and

152 nm oﬀ-edge (700 eV). The sampling depth varies with

the angle between the x-ray beam and surface normal, α,

as Λ cos(α)/2 [45]. A systematic variation of these pa-

rameters therefore provides a strategy for investigating

the depth-dependent structural property of a 3D hybrid

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：

Depth-dependentmagneticcrossoverinaroom-temperatureskyrmion-hostingmultilayerT.J.Hicken,1,2,3M.N.Wilson,1,4Z.Salman,3S.L.Zhang,5S.J.R.Holt,6T.Prokscha,3A.Suter,3F.L.Pratt,7G.vanderLaan,8T.Hesjedal,9andT.Lancaster11DepartmentofPhysics,CentreforMaterialsPhysics,DurhamUniversity,Durham,DH13LE,UnitedKin...

展开>> 收起<<

Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting multilayer T. J. Hicken1 2 3M. N. Wilson1 4Z. Salman3S. L. Zhang5S. J. R. Holt6.pdf

共9页,预览2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！

Depth-dependent magnetic crossover in a room-temperature skyrmion-hosting multilayer T. J. Hicken1 2 3M. N. Wilson1 4Z. Salman3S. L. Zhang5S. J. R. Holt6

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: