Ni spin-valves

Materials Letters 240 (2019) 124–127

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Positive and negative magnetoresistances in Co/Cu/Ni spin-valves Wenbo Zhao a, Chuanchuan Liu a, Weichuan Huang a, Chuangming Hou a, Zhiwei Chen a, Zhen Luo a, Yuewei Yin a,⇑, Xiaoguang Li a,b,⇑ a Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, and CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei 230026, China b Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

a r t i c l e

i n f o

Article history: Received 17 November 2018 Received in revised form 10 December 2018 Accepted 26 December 2018 Available online 4 January 2019 Keywords: Multilayer structure Inverse magnetoresistance effect Magnetic materials Magnetization reversal

a b s t r a c t We studied the magnetoresistance effects in Co/Cu/Ni spin-valves with magnetic fields along different in-plane directions. When the magnetic easy axis directions of Co and Ni layers are parallel, only typical positive magnetoresistance effects were discovered. Interestingly, a magnetoresistance sign inverse was discovered in the spin-valve with different magnetic easy axis directions between Co and Ni layers. This anomalous behavior may be related to the multi-step magnetization reversal processes in these materials. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction In spintronic devices, such as spin-valves and magnetic tunnel junctions (MTJs), the relationship between magnetic and transport properties is important not only for fundamental investigations, but also for commercial applications [1,2]. The key functional feature of a spin-valve (or MTJ) is the resistance variation when the angle between the magnetizations of the two ferromagnetic electrodes changes, the so-called giant magnetoresistance (GMR) effect [3] (or tunneling magnetoresistance effect [4]). Generally, by sweeping magnetic field, the resistance of a spin-valve changes between anti-parallel (RAP) and parallel (RP) magnetization configurations, and the maximum GMR is given by [5]:

GMR ¼

RAP  RP 2p1 p2 ed=ks ¼ RAP 1 þ p1 p2 ed=ks

ð1Þ

d and ks denote the thickness and spin diffusion length of the nonmagnetic middle layer, respectively. p1 and p2 denote the spin polarizations of the two ferromagnetic layers (1  p1, p2  1). In principle, both positive and negative GMR could be obtained when the signs of p1 and p2 are the same or opposite. However, both positive and negative magnetoresistance (MR) effects have been ⇑ Corresponding authors at: Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, and CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei 230026, China (X. Li and Y. Yin). E-mail addresses: [email protected] (Y. Yin), [email protected] (X. Li). https://doi.org/10.1016/j.matlet.2018.12.134 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

observed in some spin-valves or MTJs even with the same structure, which may be due to different mechanisms such as orbital hybridization, band structure shifting, or pinholes, etc. [6–8]. In fact, the magnetization of the ferromagnetic layer is not always anti-parallel or parallel to the external field direction. In principle, when the MEA and magnetic field direction are not parallel, the magnetization may stay in the magnetic field direction or different MEA directions, depending on the magnitude and direction of the magnetic field [9]. Thus, the magnetizations of the electrodes in a spin-valve will experience complex reversal procedures in the sweeping magnetic field. Accordingly, the angle a between them may either increase or decrease with the sweeping magnetic field. In spin-valves, the a-dependent current-perpendicular-toplane (CPP) resistance Ra is [3]:

Ra ¼ 1  b  cos2 ða=2Þ RAP

ð2Þ

where b is a constant. Therefore, the sign of MR may be affected in this situation. However, no related result has been reported. Here, we studied the MR in Co/Cu/Ni spin-valves with collinear and non-collinear MEA directions between Co and Ni. The spin polarizations of Co and Ni are negative [10]. According to Eq.1, the MR is expected to be positive in spin-valves with collinear MEA directions, which was observed in our experiment. While in spin-valves with non-collinear MEA, both positive and negative MR effects were obtained with magnetic fields along different in-plane directions. This may be related to the multi-step magnetization reversal procedures in Co and Ni layers.

W. Zhao et al. / Materials Letters 240 (2019) 124–127

2. Material and methods The Co(15 nm)/Cu(7 nm)/Ni(30 nm) multilayer films were deposited on (0 0 1)-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 substrates using ion-beam sputtering at room temperature (RT), and a 200 Oe magnetic field was applied during growth of Co and Ni to set their MEA. The spin-valve with a size of top Co electrode of 10  10 mm2 was patterned into a cross-strip geometry to investigate the CPP resistive properties by UV photolithography and Ar ion milling [3], as shown in Fig. 1a. The high resolution transmission electron microscopy (HRTEM) image and the selected area electron diffraction (SAED) were measured with a JEOL JEM-ARM200F microscope. Rotating sample magneto-optic Kerr effect (rot-MOKE) measurements were performed for the Co and Ni in the spin-valve at RT with a NanoMOKE II system. The magnetic field dependent resistances were examined at 80 K in a physical property measurement system (PPMS). 3. Results and discussion Fig. 1b shows the cross-sectional HRTEM image and SAED pattern of an as-grown heterostructure. The interfaces are smooth, and the bright ring in the SAED pattern corresponds to the reflections from the nanocrystalline metal films [11]. By adjusting the magnetic field direction applied during the multilayer growth, the as-grown MEA of Co and Ni can be either collinear (Sample1) or non-collinear (Sample-2). Define h as the angle between the directions of sweeping magnetic field and [1 0 0]S direction of the substrate, as shown in Fig. 1a. The MEA directions of Co and Ni were confirmed by the h dependent remnant magnetization ratio (MR/MS), as shown in Fig. 1c and d, investigated by rot-MOKE technique [3]. For Sample-1, the MEA directions of Co and Ni are parallel to [1 0 0]S, as shown in Fig. 1c. While for Sample-2, as shown in Fig. 1d, a 20° difference between the MEA directions of Co and Ni (shown by the black and red arrows respectively) was obtained. Notably, the h dependent MR/MS of the Ni in Sample-2 shows small

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peaks near h = 90° and h = 275°, indicating the existence of a biaxial magnetic anisotropy. For Sample-1, the normalized magnetic hysteresis (MH) loops for Co and Ni at RT and magnetic field dependent MR (MR ¼ ðRðHÞ  Rð0ÞÞ=Rð0Þ) at 80 K were shown in Fig. 2. When h = 0°, with the magnetic field direction parallel to the MEA of Co and Ni, their magnetizations experience a ‘‘1-jump” (180°-reversal) between the MEA directions in the sweeping magnetic fields. Thus, the MH loops for Co and Ni show sharp switchings at their coercive fields (Fig. 2a). Correspondingly, the MR curve shows typical spin-valve like behavior (Fig. 2b). The sharp resistive switching fields refer to the coercive fields of Co and Ni at 80 K, which are much larger than the coercive fields at RT [11]. When h = 90°, with the magnetic field direction parallel to the magnetic hard axes (MHA) of Co and Ni, their magnetizations will gradually rotate from the magnetic field direction to the MEA directions with the magnetic field reducing to zero, and then to the reverse magnetic field direction. This leads to smooth MH loops (Fig. 2c). Correspondingly, the MR curve is also smooth, and a small positive MR is obtained (Fig. 2d). For Sample-2, when h = 0°, the magnetic field direction and the MEA directions of Co and Ni are close with a small difference (±10°). The magnetizations of Co and Ni will first experience gradual rotation from the magnetic field direction to the nearest MEA direction with reducing magnetic field to zero. Then, with increasing magnetic field in opposite direction, the magnetizations will experience combined sharp reversal to the other MEA directions with smooth rotation to the reversing magnetic field direction. Because the angle between the magnetic field direction and MEA is small, the jump between MEA directions may dominate the magnetization reversal process. The MH loop for Co (Ni) layer shows one sharp switching, at which the magnetic field is defined as HS1(Co) (HS1(Ni)), as shown in Fig. 3a. Accordingly, the MR curve also shows spin-valve like behavior (Fig. 3b). However, the resistive switching is not as sharp as in Fig. 2b, due to the gradual magnetization rotation process.

Fig. 1. (a) Schematic of a Co/Cu/Ni spin-valve. (b) Cross-sectional HRTEM image of the Co/Cu/Ni/PMN-PT heterostructure. The inset shows the SAED pattern. Angular dependent MR/MS at RT for Co and Ni in (c) Sample-1 and (d) Sample-2.

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W. Zhao et al. / Materials Letters 240 (2019) 124–127

Fig. 2. MH loops for Co and Ni at RT and MR curves at 80 K for Sample-1 with (a, b) h = 0° and (c, d) h = 90°.

Fig. 3. MH loops for Co and Ni at RT and MR curves at 80 K for Sample-2 with (a, b) h = 0° and (c, d) h = 90°.

W. Zhao et al. / Materials Letters 240 (2019) 124–127

When h = 90°, the angles between the magnetic field direction and MEA directions of Co and Ni are relatively large. Thus, the contribution of magnetization rotation from the magnetic field direction to MEA directions becomes significant. Accordingly, the MH loops are smooth in general. In addition, two kinks can be observed for the magnetization reversals, which refer to the magnetization switching in a ‘‘2-jump” reversal procedure or a multi-domain switching scenario [12,13]. As shown in Fig. 3c, when sweeping magnetic fields from 100 Oe to 100 Oe, the magnetic fields of the first and second kinks are defined as HS1(Co) and HS2(Co) (HS1(Ni) and HS2(Ni)) for Co (Ni), respectively. Interestingly, the MR curve shows roughly negative MR with multiple resistive switching fields (Fig. 3d). According to Eq. (2), the complex resistive switching may be related to the magnetization switching processes in the sweeping magnetic field. These results demonstrate that the coexistence of positive and negative MR can be obtained in Sample-2. To further demonstrate the relation between magnetization reversal and resistive switching in Sample-2, we analyzed the h dependent switching fields extracted from both MH loops at RT and MR curves at 80 K (Fig. S1a and b in the Supplementary materials), respectively. The switching magnetic fields of resistance and magnetization show similar curvature, which suggests that the multiple resistive switching and the corresponding sign reversal of MR effects are related to the complex magnetization reversal procedures of Co and Ni. 4. Conclusions We have studied the magnetic field direction dependent MR effects in Co/Cu/Ni spin-valves. When the MEA of Co and Ni are parallel to each other, only positive MR effect was obtained. On the other hand, when the MEA of Co and Ni are not the same, both positive and negative MR effects were observed with magnetic fields along different in-plane directions, which may be related to the change of the angle between the magnetizations of Co and Ni during the complex magnetization reversal procedures.

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Conflict of interest None. Acknowledgements This work was supported by the National Natural Science Foundation of China, and National Key Research and Development Program of China (2016YFA0300103 and 2015CB921201), and was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2018.12.134. References [1] S. Bhatti, R. Sbiaa, A. Hirohata, H. Ohno, S. Fukami, S.N. Piramanayagam, Mater. Today 20 (2017) 530–548. [2] D. Ielmini, H.P. Wong, Nat. Electron. 1 (6) (2018) 333. [3] W.B. Zhao, W.C. Huang, C.C. Liu, C.M. Hou, Z.W. Chen, Y.W. Yin, X.G. Li, ACS Appl. Mater. Interfaces 10 (25) (2018) 21390–21397. [4] K. Masuda, Y. Miura, Phys. Rev. B 96 (5) (2017) 054428. [5] R.G. Geng, A. Roy, W.B. Zhao, R.C. Subedi, X.G. Li, J. Locklin, T.D. Nguyen, Adv. Funct. Mater. 26 (22) (2016) 3999–4006. [6] H. Vinzelberg, J. Schumann, D. Elefant, R.B. Gangineni, J. Thomas, B. Büchner, J. Appl. Phys. 103 (9) (2008) 093720. [7] D. Pantel, S. Goetze, D. Hesse, M. Alexe, Nat. Mater. 11 (4) (2012) 289–293. [8] S.H. Liang, H.X. Yang, H.W. Yang, B.S. Tao, A. Djeffal, M. Chshiev, W.C. Huang, X. G. Li, A. Ferri, R. Desfeux, Adv. Mater. 28 (2016) 10204–10210. [9] C. Gould, C. Rüster, T. Jungwirth, E. Girgis, G.M. Schott, R. Giraud, K. Brunner, G. Schmidt, L.W. Molenkamp, Phys. Rev. Lett. 93 (11) (2004) 117203. [10] J.M. De Teresa, A. Barthélémy, A. Fert, J.P. Contour, R. Lyonnet, F. Montaigne, P. Seneor, A. Vaures, Phys. Rev. Lett. 82 (21) (1999) 4288. [11] A.V. Sergueeva, C. Song, R.Z. Valiev, A.K. Mukherjee, Mater. Sci. Eng. A 339 (1– 2) (2003) 159–165. [12] R.P. Cowburn, S.J. Gray, J. Ferré, J.A.C. Bland, J. Miltat, J. Appl. Phys. 78 (12) (1995) 7210–7219. [13] A. Hubert, R. Schäfer, Magnetic Domains: the Analysis of Magnetic Microstructures, Springer Science & Business Media, 2008.