Field-Free Deterministic Magnetization Switching with Ultralow

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Surfaces, Interfaces, and Applications

Field-Free Deterministic Magnetization Switching with UltraLow Current Density in Epitaxial Au/Fe4N Bilayer Films Hongwei Li, Gaili Wang, Dan Li, Ping Hu, Wenqi Zhou, Shuai Dang, Xingyuan Ma, Tian Dai, Songdan Kang, Fengmei Yu, Xiang Zhou, Shuxiang Wu, and Shuwei Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00129 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Materials & Interfaces

Field-Free Deterministic Magnetization Switching with Ultra-Low Current Density in Epitaxial Au/Fe4N Bilayer Films Hongwei Li1, Gaili Wang1, Dan Li1, Ping Hu1, Wenqi Zhou1, Shuai Dang1, Xingyuan Ma1, Tian Dai1, Songdan Kang1, Fengmei Yu2, Xiang Zhou3, Shuxiang Wu*, 1), Shuwei Li*, 1) 1State

Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, School of Materials

Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China 2Automation

College, Zhongkai University of Agriculture and Engineering, Guangzhou 510225, People’s

Republic of China 3School

of Physics and Astronomy, Sun Yat-sen University Zhuhai campus, Zhuhai 519082, People’s Republic of

China

ABSTRACT: Current-induced magnetization switching was investigated in Au/Fe4N bilayer films grown by plasma-assisted molecular beam epitaxy (PA-MBE) system. Depending on lattice distortion and interfacial coupling induced by substrates, the Fe4N layer could be divided into two sublayers having different magnetic anisotropies. The bottom sublayer shows perpendicular magnetic anisotropy (PMA), while the top one has in-plane magnetic anisotropy (IMA). Coupling between the two sublayers provides an extra in-plane effective field and enables a field-free magnetization switching in the bilayer films. By summarizing a series of Hall measurements, switching phase diagram was obtained. Temperature-dependent switching behaviors demonstrate that threshold current density for the field-free magnetization switching, which is much smaller than that of pervious reports, increases with decreasing temperature and shows similar temperature dependences to that of coercivity. 1

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KEYWORDS: Anomalous Hall effect, Spin-orbit torque, Magnetization switching, Heavy metal/ Ferromagnet bilayer films, Spintronics INTRODUCTION Manipulating and detecting magnetization of a ferromagnet (FM) merely through charge current are of great importance for applications in next-generation spintronic devices1-3. Spin transfer torque (STT), first theoretically studied by Sloncwezski4 and Berger5 in 1996, provides a promising avenue for the pure-current-control of magnetization. However, for the STT-induced magnetization switching, a spin polarizer in spin valve structure is required to produce a spin polarized charge current6 and its torque efficiency is limited by the polarization of that current7. Recent studies show that STT in combination with spin-orbit coupling (SOC), namely spin-orbit torque (SOT), allows controlling magnetization without those limitations7-8. When a FM is attached to a non-magnetic heavy metal (HM) and a charge current is applied, spin accumulations will take place at the interface between HM and FM via Rashba effect and spin Hall effect (SHE) of the HM layer due to its strong SOC. The spin accumulations at the interface give rise to a field-like torque 𝑻𝐹𝐿 = 𝛾𝐵𝐹𝒎 × (𝒛 × 𝒋) and a damping-like torque

𝑻𝐷𝐿 = 𝛾𝐵𝐷𝒎 × [(𝒛 × 𝒋) × 𝐦]

exerting on the

magnetization of the FM9-10, and thus switching it, where 𝛾 is the gyromagnetic ratio, 𝒎, 𝒛 and 𝒋 are unit vectors along the magnetization, thickness direction and current direction, respectively, 𝐵𝐹 and 𝐵𝐷 are the effective field-like and damping-like fields, respectively. SOT-induced magnetization switching, as a promising technology for the future spintronics, has been extensively studied 2


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theoretically11-13 and experimentally14-19 as low write current8, high scalability and energy efficiency2, 20-21 are allowed in SOT-based magnetic random access memory (SOT-MRAM). Although SOT makes it possible to switch the magnetization without spin injection from the spin polarizer, an external in-plane magnetic field along current direction is still required to break the switching symmetry and realize deterministic magnetization switching. However, it is undesirable for practical applications because the use of the in-plane field influences device scaling and integration22. Much effort has been expended to achieve field-free deterministic magnetization switching. When a lateral structural asymmetry is introduced in wedge-shaped HM/FM structures23-24, magnetization switching without external fields could be obtained. In addition, by inserting an antiferromagnetic (AFM) layer into the HM/FM stacks25-26, exchange bias between the FM and AFM layers could serve as an effective field; thus, zero-field switching is expected. It is also reported that field-free switching is possible if an exchange coupling between a FM layer with PMA and another FM layer with IMA is introduced27-28. The basis of these methods mentioned above is to induce an out-of-plane tilt in the magnetization through the generation of an in-plane effective field by which the up and down switching symmetry of magnetization could be destroyed22. However, all those works were performed either in relatively complex multilayer structures or simple HM/FM structures with a lateral inhomogeneity which might be inappropriate for mass production10. Here, we report field-free deterministic magnetization switching in epitaxial Au/Fe4N bilayer films grown by PA-MBE on 3



MgO substrates. Fe4N has a high Curie temperature, high saturation magnetization, high spin polarization and large negative spin polarization of the electrical conductivity, which are all of technological importance and fundamental interest29-34. However, Fe4N as a switchable ferromagnetic layer in SOT-induced magnetization switching has rarely been reported so far. In this work, owing to the special magnetic anisotropies of Fe4N, field-free switching with ultra-low threshold current density was obtained in the simple bilayer films. Temperature-dependent switching behaviors were also studied. EXPERIMENTAL METHOD The Au(𝑡𝐴𝑢)/Fe4N(3) bilayer films (numbers inside the parenthesis are layer thickness in nanometers) were grown on MgO (100) substrates by an Omicron customized multiprobe PA-MBE system. The MgO substrates were degreased and then annealed in ultrahigh vacuum chamber at 823K for 20 minutes. During the growth of Fe4N films, the Fe source was heated to 1473 K by electron beams and the substrate temperature was kept at 723 K; nitrogen partial pressure in the chamber was set at 1.2×10-5 mbar with the RF power of 300 W. After the growth of Fe4N films, Au films were grown on Fe4N films in ultrahigh vacuum chamber, during which the temperature of Au effusion cell was 1423 K and the substrate temperature was 523 K. All these conditions were maintained invariable during the whole growth procedure which was in-situ monitored by reflection high energy electron diffraction (RHEED). Chemical composition and crystal structure of the samples were ex-situ investigated by x-ray photoelectron spectroscopy (XPS) and x-ray diffraction (XRD) with Cu-Kα 4


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radiation, respectively. Magnetic properties of the samples were measured by a Superconducting Quantum Interference Device (SQUID). Hall bars with length of 3 mm and width of 0.2 mm were patterned on the samples and then Hall measurements were conducted by Physical Property Measurement System (PPMS) and Keithley 4200-SCS. Hall resistivity ρxy was calculated from the expression of ρxy = tVxy I, where t, I and Vxy are the film thickness, channel current and transverse voltage on the Hall bars, respectively. To eliminate systematic errors and magnetoresistance, the Vxy was obtained by 𝑉𝑥𝑦 = [𝑉𝑥𝑦( +𝐻) ― 𝑉𝑥𝑦( ― 𝐻)]/2, where 𝑉𝑥𝑦( ± 𝐻) represent the transverse voltage measured under opposite applied field ± 𝐻.

RESULTS AND DISCUSSION The crystal texture of the fabricated Fe4N films was characterized by RHEED and XRD and the results have been reported in our previous work35. RHEED and XRD patterns reveal the monocrystalline and high quality surface of the Fe4N films. To further confirm the chemical compositions of the Fe4N films, XPS spectra of Fe 2p and N 1s were investigated as shown in Fig. 1 (a) and (b), respectively. Fe 2p3/2 and Fe 2p1/2 peaks situate at about 706.7 eV and 720.0 eV, respectively, corresponding to unoxidized Fe in iron nitrides36. The binding energy of the N 1s is about 398.1 eV which is in agreement with previous data and indicates weak covalent Fe-N bonding in Fe4N30. In addition, the Fe/N atom ratio determined from XPS is very close to 4:1 and no other impurities were detected. Therefore, the prepared films could be determined to be single crystal Fe4N. 5



FIG. 1. XPS spectra of (a) Fe 2p and (b) N 1s measured in Au(3)/Fe4N(3) bilayer films after Ar+ ions etching.

Fe4N is a typical anti-perovskite compound in which a nitrogen atom occupies the body center and two inequivalent Fe-sites with magnetic moments about 3.0 𝜇𝐵 6


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/𝑓.𝑢. and 2.0 𝜇𝐵/𝑓.𝑢. located at the corner and face-center positions, respectively. Fe4N films usually shows IMA as indicated by literatures30, 37, but owing to crystal structure distortion and interfacial effect, PMA has also been reported38-40. Out-of-plane M-𝐻𝑧 hysteresis loops of Au(3)/Fe4N(3) bilayer films at different temperatures are shown in Fig. 2(a). Square loops indicate that PMA of Fe4N films was obtained in this work. It is noteworthy that the obtained magnetization at 5 K is larger than its theoretical value. In previous report41, an unusual increase of magnetization was also observed below 60 K, which could be attributed to the substrate impurities30. In addition to the unusual large magnetization at low temperature, another interesting feature of the magnetic hysteresis loops of Au(3)/Fe4N(3) bilayer films is shown in Fig. 2(b). In-plane and out-of-plane hysteresis loops show similar behaviors, this seems to indicate that both x-axis and z-axis are the easy axes of the Fe4N films. When the magnetization was measured in a wide field range, it shows a two-step saturation behavior as shown in Fig. 2(c). These unusual magnetic behaviors could be explained well if a sublayer structure of Fe4N films is considered. In the growth process, the Fe4N layer could be divided into two sublayers depending on lattice strain and interfacial coupling induced by the substrates. The bottom sublayer displays PMA due to the crystal structure distortion and interfacial coupling, while the top one shows IMA due to the recuperation of the crystal structure and reduction of the interfacial coupling. So, when magnetized the samples with relatively small z-axis (x-axis) fields, the bottom (top) sublayer saturated rapidly (Fig. 2(b)). However, when the applied z-axis (x-axis) field 7



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continually increased, magnetic moments of top (bottom) sublayer were also forced to align along the field, displaying the second saturation step as shown in Fig. 2(c), where arrows in orange boxes represent magnetic states of Fe4N under different external magnetic fields when the sample was magnetized along z-axis. If we assume that each sublayer has the same magnetization per unit volume, according to the two-step saturation behavior of the M-H curves. The thickness of bottom and top sublayer was estimated to be about 1.7 nm and 1.3 nm, respectively. The magnetization values shown in Fig. 2(a) and (b) were calculated by using the estimated thickness of each sublayer. Anisotropic magnetoresistance (AMR), which is defined as AMR =

ρ(θ) ― 𝜌 ⊥ 𝜌⊥

× 100%, was obtained from single Fe4N films at different temperatures, where ρ(θ) and 𝜌 ⊥ represent resistivity when the angle between applied field 𝐻𝑥𝑦 (rotating in xy plane) and current I (flowing along x-axis) is θ and 90°, respectively. The schematic measurement configuration for the AMR is shown in the inset of Fig. 2(b). In contrast to normal FMs, AMR of Fe4N films is negative as shown in Fig. 2(d). The magnitude of AMR increases with decreasing temperature and the AMR curves at all temperatures studied show two-fold symmetry. AMR could be expressed as AMR ∝ ― 𝑃𝐷𝑃𝜎, where 𝑃𝐷 and 𝑃𝜎 are the spin polarization of density of states at Fermi level and spin polarization of electrical conductivity, respectively42. 𝑃𝐷 of Fe4N has been determined to be negative43-44. Therefore, the negative AMR is closely related to the negative 𝑃𝜎 which means that minority spins of 3d electrons dominate the conduction of Fe4N44. 8


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FIG. 2. (a) M-Hz loops of samples measured along z-axis (out-of-plane) at different temperatures. (b) Room temperature magnetic hysteresis loops measured along x-axis (in-plane) and z-axis. (c) M-Hx and M-Hz loops measured in wide field range at 5 K, arrows in orange boxes represent magnetic states of Fe4N when the sample was magnetized along z-axis. All the magnetic measurements in (a)-(c) were performed on Au(3)/Fe4N(3) bilayer films. (d) AMR curves obtained at different temperatures in single Fe4N(3) films, the schematic measurement configuration for the AMR is shown in the inset of (b).

Anomalous Hall resistivity

which is regarded as a detector of

𝜌𝐴𝐻

magnetization states was measured under perpendicular applied field 𝐻𝑧 and the results are shown in Fig. 3. Note that the ordinary contributions to the Hall resistivity arising from Lorentz force have been extracted. The obtained 𝜌𝐴𝐻-𝐻𝑧 curves are opposite to previous report31, which might be caused by different measurement circuits. However, in identical measurement configurations and circuits, the sign of anomalous Hall coefficient 𝑅𝐴𝐻 (which is defined as 𝜌𝐴𝐻 = 𝑅𝐴𝐻𝑀𝑧, where 𝑀𝑧 is 9



normal component of magnetization) is opposite to that of Mn4N-based antiperovskite compounds45-46, i.e., positive (negative) 𝑀𝑧 generates negative (positive) 𝜌𝐴𝐻 in Fe4N films while positive (negative) 𝑀𝑧 generates positive (negative) 𝜌𝐴𝐻 in Mn4N-based antiperovskite compounds. This sign change of 𝑅𝐴𝐻 is attributed to the negative 𝑃𝜎 of Fe4N. It is interesting that there is a difference between M-𝐻𝑧 and 𝜌𝐴𝐻-𝐻𝑧 curves at low temperature as shown in Fig. 2 and 3. Similar results have been observed previously47, which might be caused by multiple factors that play an important role anomalous Hall effect (AHE) but less related to the magnetization, such as extrinsic scatterings. In addition, saturation values of 𝜌𝐴𝐻 as well as coercivity 𝐻𝑐 (defined as 𝐻𝑐 =

|𝐻𝑐1| + |𝐻𝑐2| 2

, where 𝐻𝑐1 and 𝐻𝑐2 are the switching

fields that switch the magnetization from up to down and down to up, respectively.) for both Fe4N and Au/Fe4N monotonously increase with decreasing temperature in the whole temperature range of 5 K to 300 K. Previous researches47-48 found similar behaviors of AHE in CoFeB thin films in which the increase of saturation 𝜌𝐴𝐻 were attributed to the increase of magnetization of CoFeB with decreasing temperature. But for 𝐻𝑐, it is nearly independent of temperature in the M-𝐻𝑧 loops while strongly dependent on it in the 𝜌𝐴𝐻-𝐻𝑧 curves just as in our work. The temperature dependence of 𝐻𝑐 obtained from Hall measurements satisfy the equation of 𝐻𝑐(𝑇) = 𝐻0(1 ― 𝐴𝑇

12

) , where A is a constant and 𝐻0 is the coercivity at 0 K, which is

consistent with a semiclassical model of thermally activated domain nucleation and domain wall motion in magnetization switching49. In this work, the temperature dependence of 𝐻𝑐 in low temperature range (5-150 K) is in well agreement with that 10


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semiclassical model while an obvious deviation was detected in relatively higher temperature range (inset of Fig. 5(b)). However, since it has been reported that the interfacial interaction at the HM/FM interface could greatly modify the 𝐻𝑐 of the FM layer50-51, the variation of

𝐻𝑐

in this work may also derive from the

temperature-dependent Au/Fe4N interface anisotropy energy. It is noteworthy that the saturation 𝜌𝐴𝐻 of Au/Fe4N bilayer films is considerably larger than that of Fe4N single films. Two mechanisms might be related to the enlargement of 𝜌𝐴𝐻: the first one is a possible magnetic proximity effect (MPE), through the MPE, the Au layer adjacent to the Au/Fe4N interface acquires magnetic features which could generate AHE just as normal FM conductors do; the other is a modification of AHE form SHE of Au since AHE and SHE share the same origins, it has been reported that in many HM/FM bilayer systems, the SHE generated in HM layer is sufficient to tune the AHE of FM layer52-54. However, since no evident differences of 𝑀𝑧 at zero field in single Fe4N films and Au/Fe4N bilayer films were observed, it might indicate that the SHE dominate the variation of 𝜌𝐴𝐻 in Au/Fe4N bilayers but the MPE is still hard to excluded completely. In addition, the anomalous Hall loops of both Fe4N and Au/Fe4N samples drift towards negative field direction, showing a detectable asymmetry in the coercivity. The drift 𝐻𝑒 (defined as 𝐻𝑒 =

𝐻𝑐1 + 𝐻𝑐2 2

) is about -40 Oe

for both Fe4N and Au/Fe4N at 300 K while increase to about -300 Oe for Au/Fe4N and -1600 Oe for Fe4N at 5 K. The asymmetry of AHE loops in Fe4N single films might be related to the exchange bias between Fe4N and naturally oxidized AFM Fe-O phases. But in Au/Fe4N bilayer films, it could be attributed to exchange coupling 11



between the two Fe4N sublayers having different anisotropies. Furthermore, in Au/Fe4N bilayer films, it is the preferred switching direction that breaks the switching symmetry and enables the field-free magnetization switching27-28.

FIG. 3. Anomalous Hall loops of (a) Fe4N(3) single films and (b) Au(3)/Fe4N(3) bilayer films at different temperatures. The inset of (a) shows the schematic measurement circuit for the AHE.

12


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Current-induced magnetization switching is shown in Fig. 4 in which 𝜌𝐴𝐻 was measured in the presence of in-plane magnetic fields 𝐻𝑥 (current direction) rather than perpendicular ones. In all Hall measurements, the magnetic field sweeping sequence is +5 T → 0 T→ -5 T → 0 T→ +5 T. When 𝐻𝑥 decreases gradually from +5 T, the magnetic moments in the bottom sublayer, which are initially in the x direction due to the large 𝐻𝑥, tend to tilt out of the film plane due to the PMA. In Fe4N single films, the magnetic moments do not favor tilting up or tilting down due to the absence of SOT, leading to zero 𝑀𝑧 in the film and, thus, zero 𝜌𝐴𝐻. The 𝜌𝐴𝐻 in Fe4N single films were measured but no evident 𝜌𝐴𝐻-𝐻𝑥 loops were detected. However, in Au/Fe4N bilayer film, SOT breaks the tilting symmetry of the magnetic moments of the bottom sublayer, resulting in non-zero 𝑀𝑧 in the film. Fig. 4(a) shows 𝜌𝐴𝐻 of Au/Fe4N bilayer films as a function of 𝐻𝑥 for two charge currents having opposite polarities at 300 K and 100 K. Considering that the direction of both filed-like and damping-like torques is dependent on the direction of input current9-10, two currents with opposite polarities will generate opposite spin torques. Therefore, 𝜌𝐴𝐻-𝐻𝑥 loops for opposite applied currents of 5 mA (solid symbols) and -5 mA (open symbols) show completely opposite evolving manners in Au/Fe4N bilayer films. The remanent 𝜌𝐴𝐻 at 300 K (red and pink symbols) is close to zero, but it shows non-zero values at 100 K (dark and light olive symbols), implying that spin states of the samples could be effectively controlled by charge current at a proper temperature. In addition, the 𝜌𝐴𝐻 decreases rapidly with the increase of the in-plane field 𝐻𝑥 in high field range, which is caused by the tilt of magnetization towards x direction in large 13



field. 𝜌𝐴𝐻 were also obtained as a function of driving current under a series of fixed 𝐻𝑥, which unambiguously confirmed the SOT-induced field-free magnetization switching. The 𝜌𝐴𝐻-𝐻𝑥 and 𝜌𝐴𝐻-I curves were measured several times, all the obtained curves almost overlapped with one another, showing a reproducible nature of the SOT-induced magnetization switching. As shown in Fig. 4(b), positive (negative) 𝐻𝑥 induce clockwise (anticlockwise) 𝜌𝐴𝐻-I loops. No deterministic switching was observed when 𝐻𝑥 = 50 Oe. From the magnetic characterizations shown in Fig. 2, the origin of the field-free switching is ascribed to the exchange coupling between the two Fe4N sublayers. The coupling provides an extra in-plane effective field which gives rise to the field-free deterministic magnetization switching. However, according to S. Fukami’s research55, when the magnetization of top sublayer with IMA is switched, its magnetization is still lies in the film plane. More specifically, the magnetization in top sublayer could just be switched from x to –x or vice versa. Therefore, during the switching process, the top sublayer with IMA has no contribution to 𝜌𝐴𝐻 which originated mainly from the bottom sublayer with PMA. It is worth to notice that the values of 𝜌𝐴𝐻 shown in Fig. 4 are much smaller than that shown in Fig. 3(b), indicating incomplete switchings. That could be attributed to three reasons: (I) 𝜌𝐴𝐻 stems from bottom sublayer in Fig. 4 while both top and bottom sublayers contribute to it in Fig. 3(b) as discussed above; (Ⅱ) a multi-domain state and small current by which the magnetizations in different magnetic domains cannot be switched completely26; (III) the charge current cannot switch the whole device 14


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area, leading to smaller 𝜌𝐴𝐻 compared to that switched by magnetic fields56. For the magnetization switching in this work, if we ignore the effect of interface scattering on the current distribution, the current density was estimated by using a (𝑡𝐴𝑢 + 𝑡𝐹𝑒𝑁)𝜌𝐹𝑒𝑁

(𝑡𝐴𝑢 + 𝑡𝐹𝑒𝑁)𝜌𝐴𝑢

simple parallel circuit model: 𝐽𝐴𝑢 = 𝐽0𝑡𝐴𝑢𝜌𝐹𝑒𝑁 + 𝑡𝐹𝑒𝑁𝜌𝐴𝑢, 𝐽𝐹𝑒𝑁 = 𝐽0𝑡𝐴𝑢𝜌𝐹𝑒𝑁 + 𝑡𝐹𝑒𝑁𝜌𝐴𝑢, Where 𝐽𝐴𝑢, 𝐽𝐹𝑒𝑁, 𝐽0, 𝑡𝐴𝑢, 𝑡𝐹𝑒𝑁, 𝜌𝐴𝑢 and 𝜌𝐹𝑒𝑁 represent current density flowing in Au, current density flowing in Fe4N, average current density, thickness of Au layer, thickness of Fe4N layer, resistivity of Au and resistivity of Fe4N, respectively. Using this model, the current density for the field-free switching in Au(3)/Fe4N(3) bilayer films is estimated to be about 6.8 × 105A/cm2, which is considerably smaller than that of pervious reports16, 24, 27, implying that this system might benefit the future low power consumption spintronics. Besides, these results indicate that although the SOC of Au is weaker than that of Pt and Ta, it is still possible to produce adequate spin torques for the SOT-induced field-free magnetization switching. In fact, conventional SOT-induced magnetization switching and field-free switching through structural engineering were realized in structures based on Mo whose SOC is even weaker than that of Au56.

15



FIG. 4. (a) ρAH as a function of in-plane fields Hx for two charge currents with opposite polarities at 300 K and 100 K. (b) Room temperature ρAH as a function of current under different fixed Hx. All the data were obtained from Au(3)/Fe4N(3) bilayer films.

Switching phase diagram of the Au(3)/Fe4N(3) bilayer films was depicted by summarizing the 𝜌𝐴𝐻-I responses under a series of fixed 𝐻𝑥. As shown in Fig. 5(a), when the applied in-plane field and current (𝐻𝑥, I) located at the purple area, the spin torques generated by current are not sufficient to deterministically switch the 16


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magnetization, therefore, both 𝑀𝑧 > 0 and 𝑀𝑧 < 0 are possible. When the (𝐻𝑥, I) located at the red (blue) areas, spin torques are sufficient to determinately switch the magnetization, resulting in 𝑀𝑧 > 0 (𝑀𝑧 < 0). Red and blue lines are boundaries between different areas which represent different magnetic states. Across these lines, the SOT-induced magnetization switching becomes deterministic. In a word, the switching phase diagram tells the current and in-plane magnetic field required to switch the magnetization of Fe4N in Au(3)/Fe4N(3) bilayer films. Fig. 5(b) shows temperature dependences of threshold current density 𝑗𝑡ℎ (left axis) for the field-free switching in Au(𝑡𝐴𝑢)/Fe4N(3) bilayer films and the coercivity 𝐻𝑐 (right axis) of Au(3)/Fe4N(3) bilayer films determined from Hall measurements. It is clear that 𝑗𝑡ℎ and 𝐻𝑐 show similar temperature dependences. Similar temperature-dependent behaviors of 𝑗𝑡ℎ and anisotropy field has been reported previously18, implying a close 1

relationship between 𝑗𝑡ℎ and anisotropy field. Inset of Fig. 5(b) shows the 𝑇2 dependence of 𝐻𝑐, linear fitting curve in the low temperature range indicates that the temperature dependence of 𝐻𝑐 in this bilayer films is in well agreement with the Haney’s semiclassical model of thermally activated domain nucleation and domain wall motion in magnetization switching49. Possibly owing to the nucleation of magnetic domains and/or enhanced Au/Fe4N interface anisotropy energy, decreasing temperature gives rise to the increase of 𝐻𝑐 at which the magnetization is switched, and then the increase of 𝐻𝑐 might cause the increase of spin torques required to switch the magnetization. Therefore, 𝑗𝑡ℎ and 𝐻𝑐 show similar temperature dependences. But the quantitative relationship between them still needs further and 17



deeper researches. In addition, 𝑗𝑡ℎ in samples with larger 𝑡𝐴𝑢 is slightly smaller than that with smaller 𝑡𝐴𝑢, but the total amount of current is considerably larger due to larger cross section area. Larger current could generate more joule heating, leading to higher temperature in the samples. According to its temperature dependence as shown in Fig. 5(b), the threshold current density in thicker Au samples decreases with increasing Au thickness.

18


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FIG. 5. (a) Switching phase diagram of the Au(3)/Fe4N(3) bilayer films. (b) Threshold current density (left axis) for the field-free switching as a function of temperature in Au(tAu)/Fe4N(3), different solid symbols represent samples with different tAu. Olive open stars (right axis) represent the temperature dependence of Hc in Au(3)/Fe4N(3) bilayer films determined from Hall 1

measurements, inset shows the T2 dependence of Hc.

19



CONCLUSIONS In summary, we have investigated the SOT-induced magnetization switching in Au/Fe4N bilayer films grown by PA-MBE system. Magnetic characterizations suggest that the Fe4N layer could be divided into two sublayers. The bottom sublayer shows PMA due to lattice distortion and interfacial coupling, while the top one has IMA due to recuperation of the lattice structure and reduction of the interfacial coupling. Exchange coupling between the two sublayers generates an extra in-plane effective field that breaks the switching symmetry and results in the field-free magnetization switching in the bilayer films. The threshold current density 𝑗𝑡ℎ for the field-free magnetization switching increases with decreasing temperature and shows similar temperature dependences to that of coercivity. As the 𝑗𝑡ℎ is much smaller than that of previous reports and no external magnetic field is needed to switch the magnetization in this bilayer system, it might shed light on a new perspective for the future low power consumption and field-free spintronic devices.

AUTHOR INFORMATION Corresponding authors *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant 20


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No. 61273310, 11304399 and 11274402), and Natural Science Foundation of Guangdong

Province

(Grant No.

2015A030313121,

2016A030310234,

and S2012020011003). The Fundamental Research Funds for Central Universities (Grant No. 17lgpy02)

REFERENCES: (1) Brataas, A.; Kent, A. D.; Ohno, H. Current-Induced Torques in Magnetic Materials. Nat. Mater. 2012, 11, 372-381 (2) Wang, K. L.; Alzate, J. G.; Khalili Amiri, P. Low-Power Non-Volatile Spintronic Memory: STT-RAM and Beyond. J. Phys. D: Appl. Phys. 2013, 46, 074003 (3) Zutic, I.; Fabian, J.; Das Sarma, S. Spintronics: Fundamentals and Applications. Rev. Mod. Phys. 2004, 76, 323-410 (4) Slonczewski, J. C. Current-Driven Excitation of Magnetic Multilayers. J. Magn. Magn. Mater. 1996, 159, L1-L7 (5) Berger, L. Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current. Phys. Rev. B 1996, 54, 9353-9358 (6) Ralph, D. C.; Stiles, M. D. Spin Transfer Torques. J. Magn. Magn. Mater. 2008, 320, 1190-1216 (7) Brataas, A.; Hals, K. M. D. Spin–Orbit Torques in Action. Nat. Nanotechnol. 2014, 9, 86-88 (8) Liu, L.; Pai, C. F.; Li, Y.; Tseng, H. W.; Ralph, D. C.; Buhrman, R. A. Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum. Science 2012, 21



Page 22 of 29

336, 555-558 (9) Garello, K.; Miron, I. M.; Avci, C. O.; Freimuth, F.; Mokrousov, Y.; Blügel, S.; Auffret, S.; Boulle, O.; Gaudin, G.; Gambardella, P. Symmetry and Magnitude of Spin–Orbit Torques in Ferromagnetic Heterostructures. Nat. Nanotechnol. 2013, 8, 587-593 (10) Oh, Y.-W.; Chris Baek, S.-h.; Kim, Y. M.; Lee, H. Y.; Lee, K.-D.; Yang, C.-G.; Park, E.-S.; Lee, K.-S.; Kim, K.-W.; Go, G.; Jeong, J.-R.; Min, B.-C.; Lee, H.-W.; Lee, K.-J.; Park, B.-G. Field-Free Switching of Perpendicular Magnetization Through Spin–Orbit

Torque

in

Antiferromagnet/Ferromagnet/Oxide

Structures.

Nat.

Nanotechnol. 2016, 11, 878-884 (11) Lee, K.-S.; Lee, S.-W.; Min, B.-C.; Lee, K.-J. Threshold Current for Switching of a Perpendicular Magnetic Layer Induced by Spin Hall Effect. Appl. Phys. Lett. 2013, 102, 112410 (12) Freimuth, F.; Blügel, S.; Mokrousov, Y. Spin-Orbit Torques and Tunable Dzyaloshinskii-Moriya Interaction in Co/Cu/Co Trilayers. Phys. Rev. B 2018, 98, 024419 (13) Ado, I. A.; Tretiakov, O. A.; Titov, M. Microscopic Theory of Spin-Orbit Torques in Two Dimensions. Phys. Rev. B 2017, 95, 094401 (14) Chen, W.; Qian, L.; Xiao, G. Deterministic Current Induced Magnetic Switching Without External Field using Giant Spin Hall Effect of β-W. Sci. Rep. 2018, 8, 8144 (15) Sheng, Y.; Li, Y. C.; Ma, X. Q.; Wang, K. Y. Current-Induced Four-State Magnetization Switching by Spin-Orbit Torques in Perpendicular Ferromagnetic 22


Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Trilayers. Appl. Phys. Lett. 2018, 113, 112406 (16) Khang, N. H. D.; Ueda, Y.; Hai, P. N. A Conductive Topological Insulator with Large Spin Hall Effect for Ultralow Power Spin–Orbit Torque Switching. Nat. Mater. 2018, 17, 808-813 (17) Li, P.; Liu, T.; Chang, H.; Kalitsov, A.; Zhang, W.; Csaba, G.; Li, W.; Richardson, D.; DeMann, A.; Rimal, G.; Dey, H.; Jiang, J. S.; Porod, W.; Field, S. B.; Tang, J.; Marconi, M. C.; Hoffmann, A.; Mryasov, O.; Wu, M. Spin–Orbit Torque-Assisted Switching in Magnetic Insulator Thin Films with Perpendicular Magnetic Anisotropy. Nat. Commun. 2016, 7, 12688 (18) Fan, Y.; Upadhyaya, P.; Kou, X.; Lang, M.; Takei, S.; Wang, Z.; Tang, J.; He, L.; Chang, L.-T.; Montazeri, M.; Yu, G.; Jiang, W.; Nie, T.; Schwartz, R. N.; Tserkovnyak, Y.; Wang, K. L. Magnetization Switching Through Giant Spin–Orbit Torque in a Magnetically Doped Topological Insulator Heterostructure. Nat. Mater. 2014, 13, 699-704 (19) Lv, W.; Jia, Z.; Wang, B.; Lu, Y.; Luo, X.; Zhang, B.; Zeng, Z.; Liu, Z. Electric-Field Control of Spin–Orbit Torques in WS2/Permalloy Bilayers. ACS Appl. Mater. Inter 2018, 10, 2843-2849 (20) Kim, J.; Sinha, J.; Hayashi, M.; Yamanouchi, M.; Fukami, S.; Suzuki, T.; Mitani, S.; Ohno, H. Layer Thickness Dependence of the Current-Induced Effective Field Vector in Ta|CoFeB|MgO. Nat. Mater. 2012, 12, 240-245 (21) Liu, L.; Lee, O. J.; Gudmundsen, T. J.; Ralph, D. C.; Buhrman, R. A. Current-Induced Switching of Perpendicularly Magnetized Magnetic Layers Using 23



Spin Torque from the Spin Hall Effect. Phys. Rev. Lett. 2012, 109, 096602 (22) Li, S.; Goolaup, S.; Kwon, J.; Luo, F.; Gan, W.; Lew, W. S. Deterministic Spin-Orbit Torque Induced Magnetization Reversal In Pt/[Co/Ni]n/Co/Ta Multilayer Hall Bars. Sci. Rep. 2017, 7, 972 (23) Akyol, M.; Yu, G.; Alzate, J. G.; Upadhyaya, P.; Li, X.; Wong, K. L.; Ekicibil, A.; Khalili Amiri, P.; Wang, K. L. Current-Induced Spin-Orbit Torque Switching of Perpendicularly Magnetized Hf|CoFeB|MgO and Hf|CoFeB|TaOx Structures. Appl. Phys. Lett. 2015, 106, 162409 (24) Yu, G.; Upadhyaya, P.; Fan, Y.; Alzate, J. G.; Jiang, W.; Wong, K. L.; Takei, S.; Bender, S. A.; Chang, L.-T.; Jiang, Y.; Lang, M.; Tang, J.; Wang, Y.; Tserkovnyak, Y.; Amiri, P. K.; Wang, K. L. Switching of Perpendicular Magnetization by Spin– Orbit Torques in the Absence of External Magnetic Fields. Nat. Nanotechnol. 2014, 9, 548-554 (25) van den Brink, A.; Vermijs, G.; Solignac, A.; Koo, J.; Kohlhepp, J. T.; Swagten, H. J. M.; Koopmans, B. Field-Free Magnetization Reversal by Spin-Hall Effect and Exchange Bias. Nat. Commun. 2016, 7, 10854 (26) Fukami, S.; Zhang, C.; DuttaGupta, S.; Kurenkov, A.; Ohno, H. Magnetization Switching by Spin–Orbit Torque in an Antiferromagnet–Ferromagnet Bilayer System. Nat. Mater. 2016, 15, 535-541 (27) Lau, Y.-C.; Betto, D.; Rode, K.; Coey, J. M. D.; Stamenov, P. Spin–Orbit Torque Switching Without an External Field Using Interlayer Exchange Coupling. Nat. Nanotechnol. 2016, 11, 758-762 24


Page 24 of 29

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Kwak, W. Y.; Kwon, J. H.; Grünberg, P.; Han, S. H.; Cho, B. K. Current-Induced

Magnetic

Switching

With

Spin-Orbit

Torque

in

an

Interlayer-Coupled Junction with a Ta Spacer Layer. Sci. Rep. 2018, 8, 3826 (29) Li, Z. R.; Feng, X. P.; Wang, X. C.; Mi, W. B. Anisotropic Magnetoresistance in Facing-Target Reactively Sputtered Epitaxial γ′-Fe4N Films. Mater. Res. Bull. 2015, 65, 175-182 (30) Dirba, I.; Yazdi, M. B.; Radetinac, A.; Komissinskiy, P.; Flege, S.; Gutfleisch, O.; Alff, L. Growth, Structure, and Magnetic Properties of γ′-Fe4N Thin Films. J. Magn. Magn. Mater. 2015, 379, 151-155 (31) Zhang, Y.; Mi, W. B.; Wang, X. C.; Zhang, X. X. Scaling of Anomalous Hall Effects in Facing-Target Reactively Sputtered Fe4N Films. PCCP 2015, 17, 15435-15441 (32) Mi, W. B.; Guo, Z. B.; Feng, X. P.; Bai, H. L. Reactively Sputtered Epitaxial γ′-Fe4N Films: Surface Morphology, Microstructure, Magnetic and Electrical Transport Properties. Acta Mater. 2013, 61, 6387-6395 (33) Takata, F.; Kabara, K.; Ito, K.; Tsunoda, M.; Suemasu, T. Negative Anisotropic Magnetoresistance Resulting from Minority Spin Transport in NixFe4−xN (x = 1 and 3) Epitaxial Films. J. Appl. Phys. 2017, 121, 023903 (34) Lai, Z.; Li, Z.; Liu, X.; Bai, L.; Tian, Y.; Mi, W. Ferromagnetic Resonance of Facing-Target Sputtered Epitaxial γ′-Fe4N Films: the Influence of Thickness and Substrates. J. Phys. D: Appl. Phys. 2018, 51, 245001 (35) Li, H.; Wang, G.; Li, D.; Hu, P.; Zhou, W.; Ma, X.; Dang, S.; Kang, S.; Dai, T.; 25



Yu, F.; Zhou, X.; Wu, S.; Li, S. Spin-Orbit Torque-Induced Magnetization Switching in Epitaxial Au/Fe4N Bilayer Films. Appl. Phys. Lett. 2019, 114, 092402 (36) Cheng, Y. H.; Zheng, R. K.; Liu, H.; Tian, Y.; Li, Z. Q. Large Extraordinary Hall Effect and Anomalous Scaling Relations Between the Hall and Longitudinal Conductivities in ε-Fe3N Nanocrystalline Films. Phys. Rev. B 2009, 80, 174412 (37) Gallego, J. M.; Grachev, S. Y.; Borsa, D. M.; Boerma, D. O.; Écija, D.; Miranda, R. Mechanisms of Epitaxial Growth and Magnetic Properties of γ′−Fe4N(100) Films on Cu(100). Phys. Rev. B 2004, 70, 115417 (38) Yin, L.; Mi, W.; Wang, X. Perpendicular Magnetic Anisotropy and High Spin Polarization in Tetragonal Fe4N/BiFeO3 Heterostructures. Phys. Rev. Appl. 2016, 6, 064022 (39) Yin, L.; Wang, X.; Mi, W. Electric-Field Tunable Perpendicular Magnetic Anisotropy in Tetragonal Fe4N/BiFeO3 Heterostructures. Appl. Phys. Lett. 2017, 111, 032404 (40) Li, Z. R.; Mi, W. B.; Bai, H. L. The Contribution of Distinct Response Characteristics of Fe Atoms to Switching of Magnetic Anisotropy in Fe4N/MgO Heterostructures. Appl. Phys. Lett. 2018, 113, 132401 (41) Costa-Krämer, J. L.; Borsa, D. M.; García-Martín, J. M.; Martín-González, M. S.; Boerma, D. O.; Briones, F. Structure and Magnetism of Single-Phase Epitaxial γ′−Fe4N. Phys. Rev. B 2004, 69, 144402 (42) Ito, K.; Kabara, K.; Sanai, T.; Toko, K.; Imai, Y.; Tsunoda, M.; Suemasu, T. Sign of the Spin-Polarization in Cobalt-Iron Nitride Films Determined by the 26


Page 26 of 29

Page 27 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Anisotropic Magnetoresistance Effect. J. Appl. Phys. 2014, 116, 053912 (43) Ito, K.; Okamoto, K.; Harada, K.; Sanai, T.; Toko, K.; Ueda, S.; Imai, Y.; Okuda, T.; Miyamoto, K.; Kimura, A.; Suemasu, T. Negative Spin Polarization at the Fermi Level in Fe4N Epitaxial Films by Spin-Resolved Photoelectron Spectroscopy. J. Appl. Phys. 2012, 112, 013911 (44) Kokado, S.; Fujima, N.; Harigaya, K.; Shimizu, H.; Sakuma, A. Theoretical Analysis of Highly Spin-Polarized Transport in the Iron Nitride Fe4N. Phys. Rev. B 2006, 73, 172410 (45) Meng, M.; Wu, S. X.; Zhou, W. Q.; Li, S. W. Scaling of the Anomalous Hall Effect in Epitaxial Antiperovskite Mn3.5Dy0.5N Involving Multiple Competing Scattering Mechanisms. Appl. Phys. Lett. 2016, 109, 082405 (46) Li, H.; Wang, G.; Hu, P.; Li, D.; Dang, S.; Ma, X.; Dai, T.; Kang, S.; Yu, F.; Zhou, X.; Wu, S.; Li, S. Suppression of Anomalous Hall Effect by Heavy-Fermion in Epitaxial Antiperovskite Mn4-xGdxN Films. J. Appl. Phys. 2018, 124, 093903 (47) Chen, W.; Xiao, G.; Zhang, Q.; Zhang, X. Temperature Study of the Giant Spin Hall Effect in the Bulk Limit of β−W. Phys. Rev. B 2018, 98, 134411 (48) Hao, Q.; Xiao, G. Giant Spin Hall Effect and Magnetotransport in a Ta/CoFeB/MgO Layered Structure: A Temperature Dependence Study. Phys. Rev. B 2015, 91, 224413 (49) Haney, P. M.; Lee, H.-W.; Lee, K.-J.; Manchon, A.; Stiles, M. D. Current Induced Torques and Interfacial Spin-Orbit Coupling: Semiclassical Modeling. Phys. Rev. B 2013, 87, 174411 27



(50) Lin, D. C.; Song, C.; Cui, B.; Wang, Y. Y.; Wang, G. Y.; Pan, F. Giant Coercivity in Perpendicularly Magnetized Cobalt Monolayer. Appl. Phys. Lett. 2012, 101, 112405 (51) Kisielewski, M.; Maziewski, A.; Kurant, Z.; Tekielak, M.; Wawro, A.; Baczewski, L. T. Magnetic Ordering in Ultrathin Cobalt Film Covered by an Overlayer of Noble Metals. J. Appl. Phys. 2003, 93, 7628-7630 (52) Meng, K. K.; Miao, J.; Xu, X. G.; Wu, Y.; Zhao, X. P.; Zhao, J. H.; Jiang, Y. Anomalous Hall Effect and Spin-Orbit Torques in MnGa/IrMn Films: Modification From Strong Spin Hall Effect of the Antiferromagnet. Phys. Rev. B 2016, 94, 214413 (53) Meng, K. K.; Miao, J.; Xu, X. G.; Xiao, J. X.; Zhao, J. H.; Jiang, Y. Anomalous Hall Effect in Mn1.5Ga/Ta and Mn1.5Ga/Pt Bilayers: Modification from Spin-Orbit Coupling of Heavy Metals. Phys. Rev. B 2016, 93, 060406(R) (54) Wang, G. L.; Wu, S. X.; Meng, M.; Li, H. W.; Li, D.; Hu, P.; Li, S. W. Tuning Anomalous Hall Effect in Bilayers Films by the Interfacial Spin-Orbital Coupling. J. Appl. Phys. 2018, 123, 113906 (55) Fukami, S.; Anekawa, T.; Zhang, C.; Ohno, H. A Spin–Orbit Torque Switching Scheme with Collinear Magnetic Easy Axis and Current Configuration. Nat. Nanotechnol. 2016, 11, 621-625 (56) Chen, T.-Y.; Chan, H.-I.; Liao, W.-B.; Pai, C.-F. Current-Induced Spin-Orbit Torque and Field-Free Switching in Mo-Based Magnetic Heterostructures. Phys. Rev. Appl. 2018, 10, 044038

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Field-Free Deterministic Magnetization Switching with Ultralow

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