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 Articles you may be interested in Electric-current-induced dynamics of bubble domains in a ferrimagnetic Tb/Co multilayer wire below and above the magnetic compensation point AIP Advances 7, 055916055916 (2017); 10.1063/1.4974067 AIP ADVANCES 7, 055917 (2017) Enhancement of spin orbit torques in a Tb-Co alloy magnetic wire by controlling its Tb composition Yuichiro Kurokawa,a Akihiro Shibata, and Hiroyuki Awano Information Storage Materials Laboratory, Toyota Technological Institute, 2-12-1 Hisakata, Tenpaku-ku, Nagoya 468-8511, Japan (Presented 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)/ Tbx Co1-x (6 nm) alloy wires with various Tb composition (x) We found that the threshold current density (J th ) for the CIDWM in the Tbx Co1-x alloy wires decreases with increasing x In particular, the J th with x = 0.37 is almost times smaller than that with x = 0.23 We estimated Dzyaloshinskii-Moriya interaction (DMI) effective field (H DMI ) by measuring CIDWM in a longitudinal magnetic field We found that DMI constant (D) estimated by the H DMI also 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 H DMI and effectively affect CIDWM in Tbx Co1-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.4–14 Recently, 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),5 on the other hand, that in perpendicular magnetized wires with heavy metal layers can be explained by spin orbit torques (SOTs).20 It 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.21 The 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 magnetic wires.23–26 Recently, we reported that Tb/Co multilayered wires have low threshold current density (J th ) for the CIDWM and high DW velocity.27 We 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 Therefore, in this study, we investigated the CIDWM in amorphous Tb-Co alloy wires with various Tb compositions II EXPERIMENTAL DETAILS nm-thick Tbx Co1-x alloy wires were deposited on thermal oxide Si substrates using DC magnetron sputtering with an Ar pressure of 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 Normalized Hall resistance (RH ) of Tbx Co1-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 (H c ) and the DMI effective field (H DMI ) in Tbx Co1-x wires x (at.%) 0.23 0.25 0.30 0.32 0.37 H c (kOe) H DMI (kOe) 2.0 5.5 2.5 1.4 0.36 0.03 0.05 0.25 0.23 0.94 the one in our previous study.27 The magnetic wires were capped by a nm-thick Pt layer using RF magnetron sputtering First, µm-wide patterned wires were fabricated using electron beam lithography 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 Kerr images of the Tbx Co1-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 × 1011 A/m2 (two pulses) The arrow indicates the current direction (c) Velocity (v) of domain wall in Tbx Co1-x wires as a function of current density (J) 055917-3 Kurokawa, Shibata, and Awano AIP Advances 7, 055917 (2017) III RESULTS Figure shows the normalized anomalous Hall resistance (RH ) of the Tbx Co1-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 coercivitis (H c ) are shown in Table I 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 RH is 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-ofplane 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 Figures 2(a) and 2(b) show the Kerr images of the Tbx Co1-x wire with x = 0.30 The DW was driven in the wire by two pulses of 100 ns duration current of 2.7 × 1011 A/m2 Figs 2(a) and 2(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 Velocity (v) of domain wall in Tbx Co1-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 (H x ) under J = 3.2 × 1011 A/m2 , 2.6 × 1011 A/m2 and 1.5 × 1011 A/m2 , respectively Squares and diamonds represent up-down and down-up DWs, respectively 055917-4 Kurokawa, Shibata, and Awano AIP Advances 7, 055917 (2017) Figure 2(c) shows v in the Tbx Co1-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 (J th ) values of the Tbx Co1-x wire are much lower than those of other magnetic wires, for example, FeNi (6.7 × 1011 A/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 (H DMI ).20 In this study, to estimate the H DMI , we investigated the longitudinal in-plane field (H x ) dependence of DW velocity v in the Tbx Co1-x wires with x = 0.23, 0.30 and 0.37 are shown in Figs 3(a)–3(c), respectively, as a function of the H x As shown in Figs 3(a)–3(c), the v value strongly depends on H x Moreover, the H x dependences of the v values were reversed when DW polarity was switched This indicates that the SHE-induced perpendicular effective field becomes strong/weak because the Neel wall is enhanced/canceled by the H x We defined the H x corresponding to v = as the H DMI The H DMI estimated by the H x dependence of DW velocity are shown in Table I As shown in Table I, the H DMI increases as x increases IV DISCUSSION It is well known that DMI constant (D) is expressed by D ∼ µ0 Ms HDMI ∆, where M s is saturation magnetization, µ0 is permeability of vacuum and ∆ is domain wall width.20 In this study, we estimated D of the Tbx Co1-x wires by using ∆ = nm.28 Figures 4(a) and 4(b) show the DW propagation field (H p ), M s , J th and D of the Tbx Co1-x wires as a function of x, respectively As shown in Fig 4(b), the D values of the Tbx Co1-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 J th at x = 0.37 is almost times smaller than that at x = 0.23 Kim et al have reported that the J th is proportional to the H p 16 In the region of x > 0.30, the x dependences of H p and J th are similar On the other hand, in the region of 0.23 < x < 0.30, the x dependences of H p and J th are different However, the J th also depends on the H DMI and the J th decreases as the H DMI increases.18 As shown in Table I, the H DMI increases as the x increases Therefore, we expected that the x dependence of J th is probably caused by the x dependence of H p and H DMI 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 (a) Saturation magnetization (M s ), domain wall propagation field (H p ), (b) threshold current density (J th ) and DMI constant (D) of Tbx Co1-x wires as a function of x The black broken lines indicate compensation composition (x comp ) of Tbx Co1-x alloy 055917-5 Kurokawa, Shibata, and Awano AIP Advances 7, 055917 (2017) role for DMI.18 In 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.30 It 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/Tbx Co1-x alloy wires with various Tb compositions We found that the threshold current density in the Tbx Co1-x alloy wires decreases with increasing x In particular, the J th with x = 0.37 is almost 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 Tbx Co1-x alloy 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) S S P Parkin, M Hayashi, and L Thomas, Science 320, 190 (2008) A Allwood, G Xiong, C C Faulkner, D Atkinson, D Petit, and R P Cowburn, Science 309, 1688 (2005) J Jaworowicz, 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torques in a Tb- Co alloy magnetic wire by controlling its Tb composition Yuichiro Kurokawa ,a Akihiro Shibata, and Hiroyuki Awano Information... constant (D) of Tbx Co1 -x wires as a function of x The black broken lines indicate compensation composition (x comp ) of Tbx Co1 -x alloy 055917-5 Kurokawa, Shibata, and Awano AIP Advances 7, 055917... 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