1 Strong variation of s pin-orbit torques with relative spin relaxation rates in ferrimagnet s Lijun Zhu12 and Daniel C. Ralph34

2025-04-30 0 0 821.1KB 11 页 10玖币

侵权投诉

Strong variation of spin-orbit torques with relative spin relaxation rates in ferrimagnets

Lijun Zhu1,2* and Daniel C. Ralph3,4

1. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese

Academy of Sciences, Beijing 100083, China

2. College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences,

Beijing 100049, China

3. Cornell University, Ithaca, New York 14850, USA

4. Kavli Institute at Cornell, Ithaca, New York 14850, USA

*ljzhu@semi.ac.cn

Spin-orbit torques (SOTs) have been widely understood as an interfacial transfer of spin that is

independent of the bulk properties of the magnetic layer. Here, we report that SOTs acting on

ferrimagnetic FexTb1-x layers decrease and vanish upon approaching the magnetic compensation point

because the rate of spin transfer to the magnetization becomes slower than the rate of spin relaxation

into the crystal lattice due to spin-orbit scattering. These results indicate that the relative rates of

competing spin relaxation processes within magnetic layers play a critical role in determining the

strength of SOTs, which provides a unified understanding for the diverse and even seemingly puzzling

SOT phenomena in ferromagnetic and compensated systems. Our work indicates that spin-orbit

scattering within the magnet should be minimized for efficient SOT devices. We also find that the

interfacial spin-mixing conductance of interfaces of ferrimagnetic alloys (such as FexTb1-x) is as large as

that of 3d ferromagnets and insensitive to the degree of magnetic compensation.

Introduction

Efficient manipulation of magnetic materials is essential for spintronic devices. While spin-orbit torques (SOTs)1,2

are well established to be an effective tool to manipulate metallic 3d ferromagnets (FMs), whether they can

effectively control antiferromagnetically-ordered systems has remained elusive despite the recent blooming of

interest in ferrimagnets (FIMs) and antiferromagnets (AFs)3-9. Experimentally, for reasons unclear, the SOTs

exerted on nearly compensated FIMs5-7,10 are often measured to be considerably weaker than those on 3d FMs for

a given spin-current generator (by up to >20 times, see below). More strikingly, it remains under debate whether

uniform, perfectly compensated FIMs (Ms = 0 emu/cm3) can be switched at all by SOTs 11-13.

Microscopically, SOTs have been widely assumed as an interfacial transfer of spin (i.e., spin dephasing

length λdp ≈ 0 nm for transverse spin current) that is independent of the bulk properties of the magnetic layer, such

as in drift-diffusion analyses14-16. Under this assumption, spin current entering the magnet from an adjacent spin-

generating layer is absorbed fully by the magnetization via dephasing to generate SOTs, and the dampinglike SOT

efficiency per current density (𝜉𝐷𝐿

𝑗) will depend only on the spin Hall ratio (θSH) of the spin-generating layer and

the spin transparency (Tint) of the interface which determines what fraction of the spin current enters the magnet

17,18, i.e.,

𝜉𝐷𝐿

𝑗= TintθSH. (1)

This picture is a reasonable approximation for sufficiently thick metallic FMs that have a short λdp (≤1 nm) due to

strong exchange coupling 19-21 and a long spin diffusion length associated with spin relaxation due to spin-orbit

scattering22,23. However, in antiferromagnetically ordered systems λdp can be quite long, as predicted more than a

decade ago24-27, which, as we discuss below, questions the widely accepted models of “interfacial torques”, at

least, in FIMs and AFMs. So far, any roles of the bulk properties of the magnetic layer, e.g., the competing spin

relaxation rates, in the determination of 𝜉𝐷𝐿

𝑗 have been overlooked in SOT analyses.

Here, we report measurements of SOTs acting on ferrimagnetic FexTb1-x layers with strong spin-orbit coupling

(SOC)8 by tuning the FexTb1-x composition and temperature (T). We find that, in contrast to the prediction of Eq.

(1), 𝜉𝐷𝐿

𝑗varies strongly with the degree of magnetic compensation for a given Tint, due to changes in the fraction

of spin current that relaxes directly to the lattice via SOC instead of being absorbed by the magnetization to apply

SOTs. These results uncover the critical role of spin relaxation rates of the magnetic layer and provide a unified

understanding for the diverse SOT phenomena in ferromagnetic and antiferromagnetically ordered systems.

Sample details

For this work, we sputter-deposited Pt0.75Ti0.25 (5.6 nm)/FexTb1-x (8 nm) bilayers with different Fe volumetric

concentrations (x = 0.3-1). The Pt0.75Ti0.25 layer, a dirty-limit Pt alloy with strong intrinsic spin Hall effect17,

sources spin current that exerts SOT on the FIM FexTb1-x (the spin diffusion length is expected to be ≤ 8 nm at

temperatures in this study 22). Each sample was deposited by co-sputtering on an oxidized Si substrate with a 1

nm Ta seed layer, and protected by a 2 nm MgO and a 1.5 nm Ta layer that was oxidized upon exposure to

atmosphere. For electrical measurements, the samples were patterned into 5×60 µm2 Hall bars by

photolithography and ion milling with a water-cooled stage. After processing, the magnetization hysteresis of the

FexTb1-x measured from the anomalous Hall voltage (VAH) in patterned Hall bars shows fairly close coercivity

(perpendicular depinning field) and squareness as the magnetization of unpatterned regions of the films measured

by a superconducting quantum interference device (see Figs. 1(a) and 1(b), more details about the magnetization

measurements can be found in Sec. 1 in the Supplementary Materials). As shown in Figs. 1(a)-1(d), the FexTb1-x

has strong bulk perpendicular magnetic anisotropy (PMA) for 0.3 ≤ x ≤ 0.62 and well-defined in-plane magnetic

anisotropy for 0.75 ≤ x ≤ 1. All the PMA samples have large anisotropy fields (14.4-72.2 kOe, as estimated from

the fits in Fig. S3) and square hysteresis loops for both the out-of-plane magnetization and anomalous Hall voltage.

Fig. 1. (a) Magnetization vs out-of-plane field (Hz) and (b) Anomalous Hall voltage (VAH) vs Hz for Pt0.75Ti0.25 (5.6

nm)/Fe0.59Tb0.41 (8 nm), indicating strong perpendicular magnetic anisotropy and a high coercivity of ≈1 kOe. VAH

vs Hz for Pt0.75Ti0.25 (5.6 nm)/FexTb1-x (8 nm) with (c) perpendicular (x = 0.3, 0.43, and 0.61) and (d) in-plane

magnetic anisotropy (x = 0.75, 0.85, and 1).

Strong Variation of Spin-Orbit Torques with Composition and temperature

Fig. 2. (a) Ms and (b) 𝜉DL

𝑗 for Pt0.75Ti0.25/FexTb1-x with different Fe concentration (x) at 300 K. (c) Ms and (d) 𝜉DL

𝑗

for Pt0.75Ti0.25/Fe0.59Tb0.41 at different temperatures. (e) Frequency dependence of ferromagnetic resonance

linewidth (∆H) of the FeCoB layer in FeCoB (5.2 nm)/Ti (1 nm), FeCoB (5.2 nm)/Ti (1 nm)/Fe0.5Tb0.5 (8 nm),

and FeCoB (5.2 nm)/Ti (1 nm)/Fe0.61Tb0.39 (8 nm) samples. The solid lines represent linear fits, the slopes of which

yield the damping. In (a)-(e) some error bars are smaller than the data points. (f) 𝐺eff

↑↓ of the FeCoB/Ti/FexTb1-x

interfaces measured from spin pumping into the FexTb1-x. The blue circles are for the composition series (300 K)

and the red dots for the temperature series (x = 0.59). The blue dashed line represents 𝐺eff

↑↓ = 0.31×1015 Ω-1 m-2

previously reported for typical Pt/3d FM interfaces [41].

We performed harmonic Hall voltage response (HHVR) measurements37,38 by carefully separating out any

thermoelectric effects (see details in Sec. 1 in the Supplementary Materials). We calculate the SOT efficiency

using 𝜉DL

𝑗 = (2e/ℏ)Ms tFeTb HDL/jc, 18 where e is elementary charge, ℏ reduced Plank’s constant, tFeTb the FexTb1-x

thickness, Ms the saturation magnetization of the FexTb1-x (see Sec. 2 in in the Supplementary Materials), and jc

the current density in the Pt0.75Ti0.25. HDL is the current-driven damping-like SOT field. The “planar Hall correction”

32,33,38-40 is negligible for the PMA FexTb1-x samples (VPh/VAH<0.1, see Fig. S4 in in the Supplementary Materials).

In Figs. 2(a) and 2(b) we show the measured values of Ms and 𝜉DL

𝑗 at 300 K for the Pt0.75Ti0.25/FexTb1-x bilayers

with different FexTb1-x compositions (we refer to this as the composition series). Ms decreases monotonically by a

factor of 33, from 1560 emu/cm3 for x = 1 (pure Fe, 3d FM) to 47 emu/cm3 for x = 0.5 (nearly full compensation),

and then increases slowly as x further decreases. More details about the composition dependent magnetic properties

are shown in Sec. 2 in in the Supplementary Materials. As x decreases in the Fe-dominated regime (x ≥ 0.5), 𝜉DL

𝑗

decreases by a factor of 7 at 300 K, first slowly from 0.38 ± 0.02 for x =1 to 0.27 ± 0.01 for x = 0.61 and then more

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：

1Strongvariationofspin-orbittorqueswithrelativespinrelaxationratesinferrimagnetsLijunZhu1,2*andDanielC.Ralph3,41.StateKeyLaboratoryofSuperlatticesandMicrostructures,InstituteofSemiconductors,ChineseAcademyofSciences,Beijing100083,China2.CollegeofMaterialsScienceandOpto-ElectronicTechnology,Universit...

展开>> 收起<<

1 Strong variation of s pin-orbit torques with relative spin relaxation rates in ferrimagnet s Lijun Zhu12 and Daniel C. Ralph34.pdf

共11页,预览3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！

1 Strong variation of s pin-orbit torques with relative spin relaxation rates in ferrimagnet s Lijun Zhu12 and Daniel C. Ralph34

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: