Enhancement of spin orbit torques in a tb co alloy magnetic wire by controlling its tb composition

Enhancement of spin orbit torques in a Tb Co alloy magnetic wire by controlling its Tb composition Enhancement of spin orbit torques in a Tb Co alloy magnetic wire by controlling its Tb composition Yu[.]

Enhancement of spin orbit torques in a Tb-Co alloy magnetic wire by controlling its

Tb composition

Yuichiro Kurokawa, Akihiro Shibata, and Hiroyuki Awano

Citation: AIP Advances 7, 055917 (2017); doi: 10.1063/1.4974280

View online: http://dx.doi.org/10.1063/1.4974280

View Table of Contents: http://aip.scitation.org/toc/adv/7/5

Published by the American Institute of Physics

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AIP Advances 7, 055916055916 (2017); 10.1063/1.4974067

Enhancement of spin orbit torques in a Tb-Co alloy

magnetic wire by controlling its Tb composition

Yuichiro Kurokawa,aAkihiro Shibata, and Hiroyuki Awano

Information Storage Materials Laboratory, Toyota Technological Institute, 2-12-1 Hisakata,

Tenpaku-ku, Nagoya 468-8511, Japan

(Presented 3 November 2016; received 22 September 2016; accepted 27 October 2016;

published online 11 January 2017)

We investigated the current-induced domain wall motion (CIDWM) in Pt(3 nm)/

TbxCo1-x (6 nm) alloy wires with various Tb composition (x) We found that the thresh-old current density (Jth) for the CIDWM in the TbxCo1-xalloy wires decreases with

increasing x In particular, the Jth with x = 0.37 is almost 3 times smaller than that with x = 0.23 We estimated Dzyaloshinskii-Moriya interaction (DMI) effective field (HDMI) by measuring CIDWM in a longitudinal magnetic field We found that DMI

constant (D) estimated by the HDMIalso strongly depends on x The size of the DMI

may be modified by changing electronegativity or local atomic arrangement in Tb-Co

alloy These results suggest that Tb can induce strong HDMI and effectively affect CIDWM in TbxCo1-x alloy wires © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4974280]

I INTRODUCTION

Current induced domain wall motion (CIDWM) is a subject of great interests because of its potential applications such as new magnetic memories and logic.1 3 A number of experimental investigations for the CIDWM have been carried out using magnetic wires.4 14Recently, the CIDWM

in perpendicular magnetized wires with heavy metal layer has been reported.15 – 22 In the case of magnetic wires without heavy metal layers, domain wall (DW) driving forces can be explained by spin transfer torques (STTs),5on the other hand, that in perpendicular magnetized wires with heavy metal layers can be explained by spin orbit torques (SOTs).20It is well known that the SOTs is more effective for CIDWM than the STTs For example, the DW velocity in perpendicular magnetized magnetic wire with heavy metal layers is faster than the one in magnetic wire without heavy metal layers.21The driving force of CIDWM by the SOTs originates from the spin hall effect (SHE) and Dzyaloshinskii-Moriya interaction (DMI) from ferromagnetic/heavy metal interface.20

In previous study, we investigated the CIDWM by the SOTs in rare-earth transition-metal mag-netic wires.23–26 Recently, we reported that Tb/Co multilayered wires have low threshold current

density (Jth) for the CIDWM and high DW velocity.27We suggested that this result is attributed to large SOTs generated by extrinsic SHE from the Tb-Co inner interfaces.27 On the other hand, in contrast to the Tb/Co multilayered wires, the CIDWM in Tb-Co alloy has not been clarified

There-fore, in this study, we investigated the CIDWM in amorphous Tb-Co alloy wires with various Tb

compositions

II EXPERIMENTAL DETAILS

6 nm-thick TbxCo1-xalloy wires were deposited on thermal oxide Si substrates using DC mag-netron sputtering with an Ar pressure of 2 mTorr Before depositing Tb-Co alloy wires, the substrates were etched using sputtering to remove the residual contamination on their surfaces It is expected that the oxidations of metals in the present Tb-Co alloy wires would be suppressed compared with

a Author to whom correspondence should be addressed Electronic mail: ykurokawa@toyota-ti.ac.jp

2158-3226/2017/7(5)/055917/5 7, 055917-1 © Author(s) 2017

055917-2 Kurokawa, Shibata, and Awano AIP Advances 7, 055917 (2017)

FIG 1 Normalized Hall resistance (RH ) of TbxCo1-x wires with x = 0.23 and 0.30, as a function of out-of-plane magnetic field (H).

TABLE I A comparison of the coercivity (Hc) and the DMI effective field (HDMI ) in TbxCo1-xwires.

the one in our previous study.27The magnetic wires were capped by a 3 nm-thick Pt layer using RF magnetron sputtering First, 3 µm-wide patterned wires were fabricated using electron beam lithog-raphy for a lift-off process The magnetic properties of the wires were measured by a Hall effect measurement and an alternating gradient field magnetometer (AGFM) The CIDWM in the Pt/Tb-Co wire was observed by a Kerr microscope The compositions of the Tb-Co alloy wires were estimated

by an electron probe micro analyzer (EPMA)

FIG 2 Kerr images of the TbxCo1-x wire with x = 0.30 (a) before applying pulse current, and (b) after applying pulse current

for 100 ns pulse current of 2.7 × 10 11 A/m 2(two pulses) The arrow indicates the current direction (c) Velocity (v) of domain

wall in Tb Co wires as a function of current density (J).

III RESULTS

Figure 1 shows the normalized anomalous Hall resistance (RH) of the TbxCo1-x wires with

x = 0.23 and 0.30, as a function of the out-of-plane magnetic field (H).

According to Fig.1, the Tb-Co wires clearly have perpendicular magnetic anisotropy Its

coer-civitis (Hc) are shown in TableI The Tb-Co alloy is ferrimagnet whose Tb magnetization direction is opposite that of Co Because of this arrangement, its net magnetization can be switched by changing

the alloy composition The sign of RHis opposite with x = 0.23 and x = 0.30 because the magnetization

of wires was changed from the transition-metal-dominant to the rare-earth-metal-dominant side

The DW velocity (v) in the Tb-Co wires was measured as follows First, we applied an

out-of-plane magnetic field that is much higher than the coercivity to saturate the wire magnetization Then,

a DW was induced by applying an oppositely weak out-of-plane magnetic field that is lower than the coercivity but higher than the DW propagation field The DW was driven in the wire by a 100

ns pulse current Then, the DW displacement was observed using a Kerr microscope Finally, v was

calculated from the DW displacement and pulse width

Figures2(a)and2(b)show the Kerr images of the TbxCo1-x wire with x = 0.30 The DW was

driven in the wire by two pulses of 100 ns duration current of 2.7 × 1011A/m2 Figs.2(a)and2(b) clearly indicate that the DW was driven by the current The directions of CIDWM in the Tb-Co wires were along the current direction This result indicates that the DW driving force in Tb-Co wire can

be attributed to the SHE and DMI which originates from ferromagnetic/heavy metal interfaces.19 , 20

FIG 3 Velocity (v) of domain wall in Tb xCo1-x wires with (a) x = 0.23, (b) x = 0.30 and (c) x = 0.37, as a function of longitudinal in-plane magnetic field (Hx) under J = 3.2 × 1011 A/m 2 , 2.6 × 10 11 A/m 2 and 1.5 × 10 11 A/m 2 , respectively Squares and diamonds represent up-down and down-up DWs, respectively.

055917-4 Kurokawa, Shibata, and Awano AIP Advances 7, 055917 (2017)

Figure2(c)shows v in the Tb xCo1-x wires with x = 0.23, 0.25, 0.30, 0.32 and 0.37, as a function of current density (J) As shown in Fig.2(c), the threshold current density (Jth) values of the TbxCo1-x

wire are much lower than those of other magnetic wires, for example, FeNi (6.7 × 1011A/m2).12

In the case of the CIDWM induced by the SHE and DMI, the DWs should be changed to chiral

Neel walls under the DMI effective field (HDMI).20In this study, to estimate the HDMI, we

investi-gated the longitudinal in-plane field (Hx) dependence of DW velocity v in the Tb xCo1-xwires with

x = 0.23, 0.30 and 0.37 are shown in Figs.3(a)–3(c), respectively, as a function of the Hx As shown

in Figs.3(a)–3(c), the v value strongly depends on Hx Moreover, the Hxdependences of the v values

were reversed when DW polarity was switched This indicates that the SHE-induced

perpendicu-lar effective field becomes strong/weak because the Neel wall is enhanced/canceled by the Hx We

defined the Hx corresponding to v = 0 as the HDMI The HDMIestimated by the Hx dependence of

DW velocity are shown in TableI As shown in TableI, the HDMIincreases as x increases.

IV DISCUSSION

It is well known that DMI constant (D) is expressed by D ∼ µ0MsHDMI∆, where Msis saturation magnetization, µ0is permeability of vacuum and ∆ is domain wall width.20In this study, we estimated

D of the Tb xCo1-xwires by using ∆= 5 nm.28Figures4(a)and4(b)show the DW propagation field

(Hp), Ms, Jth and D of the Tb xCo1-x wires as a function of x, respectively As shown in Fig.4(b),

the D values of the Tb xCo1-x wires increase as x increases Especially, the D at x = 0.37 is almost 15 times higher than that at x = 0.23 Moreover, the Jthat x = 0.37 is almost 3 times smaller than that at

x = 0.23 Kim et al have reported that the Jthis proportional to the Hp.16In the region of x > 0.30, the x dependences of Hp and Jth are similar On the other hand, in the region of 0.23 < x < 0.30, the x dependences of Hpand Jth are different However, the Jth also depends on the HDMIand the

Jth decreases as the HDMIincreases.18 As shown in TableI, the HDMI increases as the x increases Therefore, we expected that the x dependence of Jth is probably caused by the x dependence of Hp

and HDMI

According to Fig.4(b), the D is strongly depends on x It probably means that Tb atoms affect interfacial DMI between the Pt and Tb-Co layer The enhancement of the D value can be associated

with several possible mechanisms One possibility is that the charge localization of the interface atoms may be changed Torrejon et al have reported that the electronegativity may play an important

FIG 4 (a) Saturation magnetization (Ms), domain wall propagation field (Hp), (b) threshold current density (Jth ) and DMI

constant (D) of Tb xCo1-x wires as a function of x The black broken lines indicate compensation composition (xcomp ) of

Tb Co alloy.

role for DMI.18In our case, the change of the electronegativity in Tb-Co layer may affect the size

of the DMI because the electronegativity of Tb (= 1.1) is much different from that of Co (= 1.7).29 Another possibility is that atomic configuration may be changed because the crystal structure of atoms in interface also affects DMI.30It suggests that Tb atoms change the local atomic arrangement

of the Tb-Co layer and enhance the D value.

V CONCLUSIONS

We investigated the current-induced domain wall motion in Pt/TbxCo1-xalloy wires with various

Tb compositions We found that the threshold current density in the TbxCo1-xalloy wires decreases

with increasing x In particular, the Jthwith x = 0.37 is almost 3 times smaller than that with x = 0.23 Moreover, we also found that DMI constant strongly depends on x These results suggest that Tb can

induce strong DMI and effectively affect CIDWM in TbxCo1-xalloy wires

ACKNOWLEDGMENTS

We thank D Bang and S Sumi for useful discussions and their technical help This work was financially supported by the MEXT-Supported Program for Strategic Research Foundation at Private University (2014-2020) and MEXT KAKENHI Grant Number 26630137 (2014-2016)

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