Deterministic Switching of Perpendicular Magnetic Anisotropy by

Apr 10, 2017 - Daniel B. GopmanPeijie ChenJune W. LauAndres C. ChavezGregory P. CarmanPeter FinkelMargo StaruchRobert D. Shull. ACS Applied ...
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Deterministic Switching of Perpendicular Magnetic Anisotropy by Voltage Control of Spin Reorientation Transition in (Co/Pt)3/ Pb(Mg1/3Nb2/3)O3−PbTiO3 Multiferroic Heterostructures Bin Peng,† Ziyao Zhou,† Tianxiang Nan,‡ Guohua Dong,† Mengmeng Feng,† Qu Yang,† Xinjun Wang,‡ Shishun Zhao,† Dan Xian,§ Zhuang-De Jiang,§ Wei Ren,†,§ Zuo-Guang Ye,⊥,† Nian X. Sun,‡ and Ming Liu*,†,§ †

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China ‡ Department of Electrical and Computer Engineering, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115, United States § Collaborative Innovation Center of High-End Manufacturing Equipment, Xi’an Jiaotong University, Xi’an 710049, China ⊥ Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada S Supporting Information *

ABSTRACT: One of the central challenges in realizing multiferroics-based magnetoelectric memories is to switch perpendicular magnetic anisotropy (PMA) with a control voltage. In this study, we demonstrate electrical flipping of magnetization between the out-of-plane and the in-plane directions in (Co/ Pt)3/(011) Pb(Mg1/3Nb2/3)O3−PbTiO3 multiferroic heterostructures through a voltage-controllable spin reorientation transition (SRT). The SRT onset temperature can be dramatically suppressed at least 200 K by applying an electric field, accompanied by a giant electric-field-induced effective magnetic anisotropy field (ΔHeff) up to 1100 Oe at 100 K. In comparison with conventional strain-mediated magnetoelastic coupling that provides a ΔHeff of only 110 Oe, that enormous effective field is mainly related to the interface effect of electric field modification of spin−orbit coupling from Co/Pt interfacial hybridization via strain. Moreover, electric field control of SRT is also achieved at room temperature, resulting in a ΔHeff of nearly 550 Oe. In addition, ferroelastically nonvolatile switching of PMA has been demonstrated in this system. E-field control of PMA and SRT in multiferroic heterostructures not only provides a platform to study strain effect and interfacial effect on magnetic anisotropy of the ultrathin ferromagnetic films but also enables the realization of power efficient PMA magnetoelectric and spintronic devices. KEYWORDS: perpendicular magnetic anisotropy, spin reorientation transition, multiferroic heterostructure, magnetoelectric coupling, ferromagnetic resonance, spin−orbit coupling

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multiferroic heterostructures are attractive for their strong magnetoelectric (ME) coupling effect at room temperature and feasibility for various ferromagnetic materials.8−21 In the past decade, E-field control of magnetic anisotropy and E-field

erpendicular magnetic anisotropy (PMA) is of great technological importance for enhancing thermal stability and achieving ultrahigh density of spintronic and logic devices.1−7 The operating electrical current is power-dissipating and results in overheating that becomes one major concern for further miniaturization of spintronic devices.6 The electric field (E-field), instead of the current, control of magnetism in multiferroics provides the desired solution.8 Among many studied multiferroic materials, ferromagnetic/ferroelectric © 2017 American Chemical Society

Received: March 4, 2017 Accepted: April 10, 2017 Published: April 10, 2017 4337

DOI: 10.1021/acsnano.7b01547 ACS Nano 2017, 11, 4337−4345

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Figure 1. Microstructure and magnetic anisotropy of as-deposited (Co/Pt)3/(011) PMN−PT multiferroic heterostructures. (a) Crosssectional HAADF-STEM image of a t = 0.8 nm sample and EDX line profiles of Co, Pt, and Ta elements across the films. (b) In-plane and outof-plane magnetic hysteresis loops for a t = 0.9 nm multiferroic heterostructure with PMA. (c) Dynamic magnetization reversal of a t = 0.9 nm multiferroic heterostructure observed by magneto-optic Kerr effect. (d) Angular-dependent ferromagnetic resonance fields for as-deposited multiferroic heterostructures with different Co thickness.

manipulate spin interactions effectively and results in greater than 1 order of magnitude stronger ME coupling strength than that caused by strain effect.34 For Co/Pt multilayers, the spin− orbit coupling between Co and Pt dominates PMA, especially from the interfacial Co 3d−Pt 5d hybridization localized at the Co/Pt interface,35 which is also sensitive to lattice strain and introduces a strong interface effect.36 Therefore, E-field modification of spin−orbit coupling via strain could be very effective to modulate PMA. However, this effect has not been experimentally revealed. In this study, we observed a dramatic E-field control of SRT and PMA in (Co/Pt)3/(011) Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN−PT) multiferroic heterostructures. The SRT onset temperature was suppressed at least 200 K by applying an E = 12 kV cm−1, corresponding to a coupling coefficient of 16.7 K cm kV−1 and a huge ΔHeff of about 1100 Oe. Completely reversible magnetization rotations between in-plane and out-ofplane directions were also achieved at room temperature. Compared to the control sample with thicker Co in which the strain effect only contributes about 110 Oe, E-field modification of spin−orbit coupling enables giant SRT and PMA changes, resulting in a nearly 1000 Oe effective magnetic anisotropy field.

switching of magnetization were widely studied in multiferroic heterostructures, and strain-mediated magnetoelastic coupling effect (strain effect) is the primary driving force for which Efield-induced strain in ferroelectric layers transfers to ferromagnetic layers, thus changing the magnetoelastic anisotropy.10−20 Although E-field switching of in-plane magnetization has been demonstrated17,21 and giant E-field-induced effective magnetic field has been reported,10 E-field switching of PMA is still challenging despite some local observations.22,23 Thickness-driven spin reorientation transition (SRT) in ultrathin ferromagnetic films is a typical method to achieve PMA as well as perpendicular magnetization switching at room temperature.24−26 Tuned by strain effect, E-field control of SRT was theoretically predicted for many ferromagnetic materials,27,28 and it was experimentally demonstrated in a Ni/Cu/ BaTiO3 multiferroic heterostructure.29 Nevertheless, it is hardly achieved in the mostly studied PMA systems (e.g., Co/X (X = Pt, Pd and Ni) multilayers5,7 and CoFeB)2 because, for these systems, strain effect introduces a relatively small ME coupling strength due to their small magnetostriction unless extremely large strain could be applied. For example, theoretical calculations have revealed that a critical strain of nearly 1% is required for E-field switching of PMA in Co films,6 which cannot be achieved by any existing ferroelectric/piezoelectric materials. Although, for Co ultrathin film, combining the strain effect with the charge effect30−33 will increase the ME coupling strength, strain effect still plays a dominant role near the critical thickness of SRT.28 The charge effect is only active for the ferromagnetic/ferroelectric interface and is ineffective for multilayer PMA systems that are widely used in spin valves and magnetic tunnel junctions. Other than strain effect and charge effect, it was reported recently that lattice strain could

RESULTS AND DISCUSSION Microstructure and Magnetic Anisotropy of AsDeposited Multiferroic Heterostructures. Ta(5 nm)/Pt(1 nm)/[Co(t)/Pt(1 nm)]3/Ta(3 nm) multilayers with different Co thickness were deposited onto (011) PMN-PT piezoelectric substrates by magnetron sputtering. Figure 1a shows typical cross-sectional observations for a t = 0.8 nm multiferroic heterostructure by the atomic resolution high-angle annular 4338

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Figure 2. E-field control of SRT in t = 1.1 nm (Co/Pt)3/(011) PMN−PT multiferroic heterostructures. Typical temperature and angulardependent FMR fields measured at (a,c) E = 0 kV cm−1 and (b,d) E = 12 kV cm−1. The inset shows angular-dependent FMR measurement setup in which x-, y-, and z-axes are parallel to [100], [01−1] and [011] directions of (011) PMN−PT substrates, respectively. Magnetic anisotropy was fully characterized by angular-dependent FMR within the x−z plane and y−z plane.

evidenced by a four-fold symmetric Hr−θH curve for t = 1.1 nm samples. E-Field Control of Spin Reorientation Transition. Efield control of SRT was first examined in t = 1.1 nm (Co/Pt)3/ PMN−PT multiferroic heterostructures. Magnetic anisotropy was fully characterized by angular-dependent FMR under E = 0 and 12 kV cm−1, as summarized in Figure 2. The inset shows FMR measurement configuration in which x-, y-, and z-axes are parallel to [100], [01−1], and [011] directions of (011) PMN− PT, respectively; θ and φ is the polar and the azimuth angle of magnetization M, and magnetic field H is rotated in the x−z or y−z plane. Once the E-field is applied to PMN−PT and then returns zero, the magnetization state of multiferroic heterostructures differs from that of as-deposited, establishing a stable effective coupling between ferroelectric and ferromagnetic domains.29 At room temperature and E = 0 kV cm−1, Co/Pt multilayers favored an in-plane magnetic easy axis along the [100] direction as evidenced by a smaller Hr,x = 2760 Oe compared to Hr,y = 3100 Oe and Hr,z = 3820 Oe. Obvious temperature-driven SRT could be observed during cooling with the magnetic easy axis rotated gradually from in-plane to out-ofplane, with significant FMR fields shift: Hr,x and Hr,y, respectively, increased about 890 and 250 Oe whereas Hr,z decreased 760 Oe. E-field-controllable SRT has been observed after applying E = 12 kV cm−1 to PMN−PT. At room temperature, the in-plane magnetic easy axis was stabilized along the [100] direction with Hr,x decreasing 58 Oe, Hr,y increasing 20 Oe, and Hr,z increasing 75 Oe, which was an expected tendency because of the compressive strain along the [100] direction and the tensile strain along the [01−1] direction combined with a negative magnetostriction of Co.38 With temperature decreasing from 300 to 100 K, Hr,x decreased less than 50 Oe, Hr,z decreased

dark-field scanning transmission electron microscopy (HAADF-STEM). The layered structure of Ta/Pt/Co/Pt/ Co/Pt/Co/Pt/Ta from top to bottom can be clearly identified. Energy-dispersive X-ray (EDX) line profiles were taken to confirm relative elemental distribution among the individual layers. The compositional distribution of Pt, Co, and Ta elements across the layers is also plotted in Figure 1a, confirming that the multilayer structure remains chemically sharpened. Both microstructure and element distribution observations verify a clear interface between Co and Pt in our multiferroic heterostructures, which is crucial for exhibiting PMA. The magnetization state could be precisely controlled in those multiferroic heterostructures. Figure 1b shows in-plane and out-of-plane magnetic hysteresis loops for a t = 0.9 nm (Co/Pt)3/PMN−PT multiferroic heterostructure, whereas others with different Co thickness are shown in Figure S1 (see Supporting Information). PMA multiferroic heterostructures were obtained at room temperature with t < 1.1 nm. Figure 1c shows a dynamic magnetization reversal of this t = 0.9 nm multiferroic heterostructure observed by magneto-optic Kerr effect microscopy, confirming uniform domains in this PMA multiferroic heterostructure. The magnetic anisotropy of as-deposited multiferroic heterostructures were examined by angular-dependent ferromagnetic resonance (FMR) measurement, which serves as a quantitative magnetism determination method and offers spatial magnetic anisotropic information with good precision ( 1300 Oe (see section 4 of Supporting Information). In thinner samples with t = 1.1 nm (as shown in Figure 3), the PMA is strong at low temperature, which indicates a large orbital moment and an effective interfacial hybridization. The applied E-field on (011) PMN−PT favors the in-plane magnetization, thus significantly reducing the PMA. This could be explained by the lattice-strain-induced suppression of the interfacial hybridization, which is evidenced by the large change of the magnetic anisotropy field and SRT onset temperature. Considering that ME coupling in the control sample was due to pure strain effect, lattice strain modification of interfacial hybridization-induced interface effect can be separated from the total ΔHeff during E-field control of SRT, as shown in Figure 3f. Because the contribution of conventional pure strain effect is only 116 Oe as estimated from thicker Co sample as shown in Figure S3 (see Supporting Information), the contribution of E-field modification of spin−orbit coupling

direction was about 95 Oe at room temperature, and it just increased slightly to 116 Oe at 100 K. Lastly, charge effect30,32,33,41,42 can be excluded in this study because of a 3 nm Ta buffer layer and a 1 nm Pt layer separated the bottom cobalt layer from PMN−PT substrate, preventing interfacial charge accumulation at the Co surface. It has been demonstrated that Co 3d−Pt 5d interfacial hybridization could induce a strong perpendicular orbital moment which increases rapidly with the decrease of Co thickness and enhances the PMA.35 This kind of spin−orbit coupling is sensitive to lattice strain as the change of interatomic distance could directly modify interfacial hybridization as well as corresponding PMA, revealed by a firstprinciple calculation.36 For (011) PMN−PT, the large in-plane compressive strain will induce out-of-plane tensile strain and enlarge the interplanar distance between Co and Pt, and this will weaken the Co 3d−Pt 5d interfacial hybridization as well as PMA. On the other hand, the sensitivity of such an interface effect to strain will be weak if multiferroic heterostructures have an in-plane magnetic easy axis when Co/Pt interface magnetic anisotropy or contribution of spin−orbit coupling itself cannot overcome shape anisotropy. Therefore, it is likely that for this work the E-field could modify spin−orbit coupling at the Co− Pt interface via strain, and this effect becomes noticeable when 4341

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Figure 5. Ferroelastic domain switching-induced nonvolatile magnetization switching for t = 1.1 nm (Co/Pt)3/(011) PMN−PT multiferroic heterostructures at room temperature. (a) E-field-dependent in-plane and out-of-plane FMR fields (Hr,x and Hr,z). (b) Out-of-plane FMR field (Hr,z) shift during repeated E-field impulse switching of +12 and −2 kV/cm. Schematics of domain structures of PMN−PT after (c) positively poled and (d) ferroelastic domain switching and their corresponding reciprocal space mapping about (022) reflections of (011) PMN−PT.

field was applied, Hr,x decreased and Hr,y and Hr,z increased as expected, making the [100] direction a magnetic easy axis at E = 16.7 kV cm−1. The voltage-induced ΔHeff values were −548, 200, and 286 Oe along the x-, y-, and z-axes, respectively. The much larger FMR field shift, compared with the strain effect in the control sample (see Figure S3 of Supporting Information), may also come from lattice strain modification of spin−orbit coupling. This magnetic easy axis rotation was further confirmed by E-field-dependent magnetic hysteresis loops shown in Figure 4e,f. In the beginning, magnetic anisotropy was much easier in the out-of-plane [011] direction than in the in-plane [100] direction, and it was opposite after application of the E-field. These results proved a voltage-induced deterministic magnetization switching between out-of-plane and in-plane directions at room temperature. Nonvolatile Switch of Perpendicular Magnetic Anisotropy. Nonvolatile switching of PMA will further reduce power consumption in ME devices, where manipulation of magnetism by voltage impulse, instead of voltage bias, is superior. Here, nonvolatile magnetization switching was successfully achieved in t = 1.1 nm (Co/Pt)3/PMN−PT heterostructures by ferroelastic domain switching of PMN−PT.11 Figure 5a shows both in-plane Hr,x and out-of-plane Hr,z under bipolar and unipolar E-fields, and they changed slightly and linearly with E-field after PMN−PT was fully poled. However, by reversing the E-field close to its coercive field around ±2 kV

can be deduced to be about 1000 Oe, which is nearly 10 times that of the conventional strain effect. In this study, (011) PMN−PT was deliberately selected because of the in-plane biaxial strain with opposite sign. Applying positive E-field on (011) PMN−PT will generate compressive strain/stress along the [100] direction and tensile strain/stress along the [01−1] direction.10,11,38 As Co has a negative magnetostriction, both [100] compressive strain/stress and [01−1] tensile strain/stress contribute to E-field-induced effective magnetic field along the [100] direction and stabilize the [100] magnetic easy axis. Hence, during E-field control of SRT, the magnetic easy axis will rotate in the x−z plane (see section 2 of Supporting Information). E-Field Switching of Perpendicular Magnetization at Room Temperature. We further demonstrated magnetization switching at room temperature by E-field control of PMA and SRT for a t = 1.05 nm (Co/Pt)3/PMN−PT multiferroic heterostructure. Figure 4a,b shows the angular dependence of FMR fields in x−z and y−z planes, respectively. By carefully controlling the thickness of the Co layer, a moderate PMA was obtained at room temperature with Hr,z 460 Oe smaller than that of Hr,x at E = 0 kV cm−1. Applying E = 16.7 kV cm−1, the magnetic easy axis rotated from the out-of-plane [011] direction to the in-plane [100] direction. Figure 4c,d shows E-field dependence of in-plane and out-of-plane FMR fields and E-field-induced effective magnetic field, respectively. When E4342

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ACS Nano cm−1, an obvious FMR field shift was observed with Hr,x increased about 300 Oe and Hr,z decreased about 260 Oe. Similar behavior could be observed by applying unipolar E-field between +12 and −2 kV cm−1, where removing the E-field from +12 kV cm−1 to zero field set a remnant state “A” and removing E-field from −2 kV cm−1 to zero field set another remnant state “B”. Those two distinct remnant states were stable during repeated E-field impulse switching of +12 and −2 kV cm−1, as shown in Figure 5b. Additionally, those two distinct remnant states correspond to different magnetization states, and nonvolatile E-field-driven SRT could be achieved as revealed in Figure S4a (see Supporting Information) with a four-fold symmetric magnetic anisotropy pattern. High-resolution X-ray diffraction measurements were used to determine the polarization switching pathway and lattice strain during ferroelastic domain switching of PMN−PT. In rhombohedral PMN−PT single crystals, polarization can align along eight body diagonal directions of the pseudocubic unit cell with four structural domains (r1, r2, r3, r4),11,43 as shown in Figure 5c,d. According to the lattice parameter and rhombohedral distortion, structural domains r1/r2 (polarization points to the out-of-plane direction) and r3/r4 (polarization stays in the plane) can be distinguished by the spot distribution about the (022) and (222) reflections, based on the difference in d-spacing of these distorted domains.11,43 Figure 5c,d also shows the E-field dependence of the reciprocal space mapping in the vicinity of the (022) reflections of the rhombohedral PMN−PT, and Figure S4b,c (see Supporting Information) shows the (222) reflections, corresponding to the remnant states “A” and “B”, respectively. In a positively poled state, r1+/r2+ domains are favored, corresponding to a lower Q022 value. Applying an impulse of −2 kV cm−1 induces non180° (71 and 109°) ferroelastic domain switching, with polarizations rotating from out-of-plane to in-plane and results in an increasing fraction of r3/r4 domains at the expense of r1/ r2 domains. Compared to r1/r2 domains, r3/r4 domain structures have larger d-spacing, inducing expansion along inplane directions. Therefore, it was confirmed that non-180° ferroelastic domain switching creates two stable and distinct remnant states, which is responsible for the remnant states “A” and “B”. Additionally, the linear piezostrain-effect-induced ΔHeff was more than 5 times smaller than that induced by ferroelastic domain switching-induced strain. Considering the fact that the linear piezostrain is usually 2−3 times smaller than ferroelastic domain switching-induced strain in (011) PMN−PT,44 that observation also verifies the important role of lattice-strainsensitive spin−orbit coupling to PMA as discussed above, highlighting the possibility of E-field control of interface anisotropy in Co/Pt multilayers as well as many other PMA magnetic multilayers.

modulation of spin−orbit coupling at the Co/Pt interfaces. Nonvolatile switching of PMA at room temperature was also demonstrated by utilizing ferroelastic domain switching in PMN−PT. E-field control of SRT in multiferroic heterostructures provides a platform to study the interface effect on magnetic anisotropy of the ultrathin ferromagnetic films, and Efield switching of perpendicular magnetization will enable the realization of power-efficient PMA magnetoelectric and spintronic devices.

METHODS Sample Fabrication. Ta(5 nm)/Pt(1 nm)/[Co(t)/Pt(1 nm)]3/ Ta(3 nm) multilayers with different Co thickness were deposited onto (011) PMN−PT piezoelectric substrates by magnetron sputtering with a base pressure of