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Tunable Ferromagnetic Transition Temperature and Vertical Hysteretic Shift in SrRuO3 Films Integrated on Si(001) Ming Zheng*,† and Wei Wang*,‡ †
Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, Singapore 117574 Laboratory of Eco-Photoelectric Technology, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China
‡
S Supporting Information *
ABSTRACT: SrRuO3 thin films have been epitaxially integrated on complementary metal oxide semiconductor (CMOS) compatible Si(001) substrates via pulsed laser deposition using a unique buffer layer (SrTiO3/TiN) approach. When the oxygen pressure during deposition was controlled, a dramatic suppression in the ferromagnetic transition temperature (TC) of up to 53 K was observed, caused by the growth-induced ruthenium vacancies rather than the oxygen vacancies. The ruthenium vacancies can also effectively tune the vertical magnetization shift (Mshift) in hysteresis loops, and thus we achieved a giant Mshift of 240%. Transport and magnetic measurements reveal that these appreciable physical phenomena are closely related to the ruthenium defect-induced local disorder and complex effects due to the strongly hybridized p-d orbitals as well as the induced lattice distortion. These observations indicate the importance of ruthenium defects in controlling the vertical magnetization shift and ferromagnetic transition temperature in this transitional metal oxide. KEYWORDS: epitaxial films, ferromagnetic transition temperature, vertical magnetization shift, lattice distortion, spintronic device sensors, etc.14−16 However, the vertical magnetization shift, which refers to an offset of the hysteresis loop along the magnetization axis, is too small in conventional FM/AFM film heterostructures and cannot be easily probed by the isothermal magnetization measurements. Fortunately, recent observations of large Mshift values, e.g., 35% and 100% for the La0.7Sr0.3FeO3/ SRO and La0.7Sr0.3MnO3/SRO systems,8,17 respectively, have triggered renewed research efforts. The conjunction of the Mshift with the EB effect can also offer an additional degree of freedom in future spintronic device applications. Notably, due to the lower cost, larger area, and volume production, Si single crystals are the most desirable platforms for the integration of multifunctional oxide-based nanoelectronic devices, which are also quite compatible with current complementary metal oxide semiconductor (CMOS) technology. Despite the large amount of effort that has been dedicated to the realization and manipulation of lattice-coupled physical parameters of SRO films by substrate-induced strain, film thickness, and oxygen vacancies, the effects of the ruthenium vacancies on the surface morphology, crystal structure, electronic transport, and magnetic properties of the SRO films integrated on CMOS compatible Si(001) substrates are still unknown and remain unaddressed. There is no doubt that an in-depth understanding of certain important issues regarding the SRO/Si hetero-
1. INTRODUCTION Among the popularly known ruthenium oxides, SrRuO3 (SRO) is currently being extensively investigated because of its striking electrical and magnetic properties. As it is the only case of a ferromagnetic metal in the 4d oxides, SRO exhibits itinerant ferromagnetism below ∼160 K, implying potential applications in magnetic tunnel junctions and magnetic random accessory devices.1−3 The growing sophistication of high-quality thin-film fabrication technologies with atomically smooth surfaces makes it possible for the SRO film to serve as an epitaxial electrode in microelectronic devices4−6 or as a component of oxide-based superlattices and heterostructures.7,8 Strong spin−orbit interactions and the electron correlation effect in this system are the most cited mechanisms underlying the sensitive dependence of magnetic and transport properties on substrate-induced strain, stoichiometry, film thickness, etc. Therefore, elucidating the mutual relationships among these factors and the film’s physical properties is crucial to harnessing macroscopic properties such as the anomalous Hall effect,9 magnetocrystalline anisotropy (MCA),10 exchange bias (EB),11 and vertical magnetization shift (Mshift)12 in novel spintronic devices. Of these intriguing properties, the exchange bias effect, which is defined as an offset of the hysteresis loop along the magnetic field axis in a system with an interface between ferromagnetic (FM) and antiferromagnetic (AFM) materials,13 has sparked a surge of research activities due to its usage in spin valves, magnetic recording media, domain stabilizers in recording heads based on anisotropic magnetoresistance, giant magnetoresistive © XXXX American Chemical Society
Received: March 1, 2016 Accepted: May 20, 2016
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DOI: 10.1021/acsami.6b02623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a−f and i) RHEED patterns of SRO films on STO/TiN bilayer buffered Si(001) deposited at various oxygen pressures with the incidence azimuth of the electron beam along ⟨010⟩, ⟨110⟩, and ⟨120⟩ of the Si(001) substrate. (g and h) Surface morphologies of the SRO films deposited at PO2 = 1 and 15 Pa, respectively. spectrometer (EDS). The crystal structure was characterized using a high-resolution Bruker D8 Discover (Cu Kα1 radiation, λ = 1.5406 Å) X-ray diffractometer (XRD). The film resistivity was measured by the standard four probe method on the physical property measurement system (PPMS-9, Quantum Design). Magnetic data were recorded using a SQUID magnetometer (MPMS XL-5, Quantum Design) with the magnetic field applied parallel to the film plane.
structures, such as the lattice distortion induced by ruthenium vacancies, the evolution of the ferromagnetic transition temperature (TC) against ruthenium vacancies, the origin of Mshift, and the effects of the ruthenium vacancies on Mshift, would help shed light on the underlying essential physics of SRO films and the design of Si-integrated multifunctional electric devices based on complex oxides. In this work, we used (001)-oriented Si single-crystal substrates to grow SRO epitaxial films and strongly manipulated the TC and Mshift of the films by exploiting the growth-induced ruthenium vacancies. The ruthenium vacancies favor the expansion of unit cell volume which, in turn, robustly modulates the resistivity and magnetization of the SRO films. Such exotic physical behaviors can be interpreted in terms of the ruthenium defect-induced strong orbital hybridization, large local disorder, and structural distortion.
3. RESULTS AND DISCUSSION Figures S1a−c of the Supporting Information show the in situ RHEED patterns of the Si, TiN, and STO layers, respectively. After deposition of the TiN and STO layers, the symmetric sharp and streaky pattern still appears, indicating epitaxial growth of the STO layer on the TiN-buffered Si(001). As depicted in Figure S1d, the STO film also has a smooth surface with a root-mean-square roughness (Rq) of ∼0.54 nm. Figures 1a−f show the in situ RHEED patterns of the SRO films on STO/TiN bilayer buffered Si(001) deposited at various oxygen pressures with the incidence azimuth of the electron beam along ⟨010⟩ and ⟨110⟩ of the Si(001) substrate. For PO2 = 0.01 Pa, some vague diffraction rings were observed (Figures 1a and d), suggesting random in-plane crystallographic orientation and poor crystallinity. As PO2 increased to 0.1 Pa, several elongated spots distributed at the diffraction rings appear in the pattern, and the pattern also shows a streaky trend, which is a clear sign of the emergence of some small crystalline islands. As PO2 was further increased to 1 Pa, a layer-by-layer growth mode was achieved, as manifested by the presence of a sharp and streaky pattern as well as the absence of diffraction rings and spots. This type of growth mode would lead to an extremely flat surface on the as-grown films, as reflected by the small Rq of ∼1.1 nm (0.98 nm) for the SRO films deposited at PO2 = 1 Pa
2. EXPERIMENTAL SECTION SRO films were fabricated on HF-cleaned Si(001) substrates by pulsed laser deposition using a unique buffer layer (SrTiO3/TiN) approach that utilizes domain matching epitaxy (DME).18 First, the TiN (10 nm) and SrTiO3 (STO) (80 nm) buffer layers were sequentially deposited on the Si substrates under optimized conditions by laser molecular beam epitaxy. SRO films were then grown on the buffered Si(001) substrates at 690 °C under oxygen pressures (PO2) ranging from 0.01 to 15 Pa followed by in situ annealing of the films in 1 atm O2 for 30 min to reduce oxygen deficiencies. The growth process for all films was in situ monitored by reflection high-energy electron diffraction (RHEED) at an electron acceleration voltage of 20 kV. The thickness and surface morphology of the films were determined using a scanning electron microscope (SEM; FEI Magellan 400) and an atomic force microscope (AFM; Nanocute SII, Seiko), respectively. The film compositions were measured by an energy-dispersive X-ray B
DOI: 10.1021/acsami.6b02623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. (a) XRD θ−2θ scans for the SRO films grown on the STO/TiN bilayer buffered Si substrates at various oxygen pressures. (b) XRD ϕscans taken on the SRO(101) and Si(202) diffraction peaks.
of the pseudocubic SRO films are summarized in Table 1 of the Supporting Information. One can see that when PO2 is increased from 1 to 15 Pa, both of the lattices along the a- and c-axes are compressed, thus giving rise to an effective unit cell contraction. It has been found that the SRO films deposited at low oxygen pressures may produce ruthenium and/or oxygen deficiencies, both of which could enhance the lattice distortion.24−27 In our case, all of the SRO films were annealed in situ in 1 atm O2 for 30 min to rule out the influence of oxygen deficiencies on lattice distortion. Accordingly, the unit cell expansion of our SRO films deposited at low oxygen pressures should stem from the growth-induced ruthenium deficiencies as previously reported for the SRO bulks and films.26,27 EDS measurements indeed show that the Ru/Sr atomic ratios for the SRO films deposited at PO2 = 1, 5, 10, and 15 Pa are estimated to be ∼0.89, 0.93, 0.95, and 0.97, respectively. The expansion of the unit cell volume induced by the ruthenium deficiencies for the SRO films deposited at low oxygen pressures is due to the charge screening effect and/or the relaxation of Sr atoms toward vacant near-neighbor Ru sites.3,26 The formation of ruthenium deficiencies is probably related to the collision of target particles under the oxygen atmosphere. Particles ejected from the SRO targets likely undergo an even higher-order laser interaction under high oxygen pressures due to a decrease in the mean free path,25 which leads to the plume shape change from the forward direction at PO2 = 1 Pa to the isotropy (spherical) at PO2 = 15 Pa, as shown in Figure S3. A Monte Carlo simulation28 indicates that the part of the particles reaching the substrate is thermalized due to these collisions of target particles with the background gas. A low ambient gas pressure of about 20 mTorr is required for thermalization of most of the light species, while a somewhat higher pressure above 100 mTorr is necessary to thermalize most of the heavy particles. Therefore, in high oxygen pressures, the deposition rate of heavy elements (e.g., Ru) decreases more quickly than that of light elements (e.g., Sr). On the contrary, in low oxygen pressures, the deposition rate of light elements decreases more rapidly, thus producing the ruthenium deficiencies in the SRO films.28 The presence of ruthenium deficiencies can also be verified by the strong dependence of the electric and magnetic properties of the SRO films on the oxygen pressure.
(15 Pa). Furthermore, the ratio of the diffraction fringe spacing to the incidence azimuth along ⟨010⟩, ⟨110⟩, and ⟨120⟩ is 1:√2:√5 (see Figures 1c, f, and i), respectively, establishing the perovskite structure of the SRO film. Although SRO is orthorhombic (a = 5.5670 Å, b = 5.5304 Å, and c = 7.8446 Å), for the sake of simplicity, we consider it as a pseudocubic structure (a ∼ b ∼ c ∼ 3.923 Å).3 Figure 2a shows the XRD θ−2θ scans for the SRO films grown on the STO/TiN bilayer buffered Si substrates under various oxygen pressures. Strong (00l) (l = 1, 2) diffraction peaks can be seen from all of the SRO films, signaling that the SRO films are of a single phase and highly (001)-oriented (pseudocubic notation). Note that a weak TiO2 diffraction peak was found at 2θ = 38°, probably due to minor oxidation of the TiN layer during the growth of the STO layer on the TiN layer in the oxygen atmosphere.19 The in-plane epitaxial relationship of the film with respect to that of the underlying substrate was further accessed by examining the XRD ϕ-scans which were taken on the SRO(101) and Si(202) diffraction peaks. As presented in Figure 2b, 4-fold symmetrical diffraction peaks recurring every 90° were observed at the same azimuthal ϕangle, implying a “cube-on-cube” epitaxial nature of all of the SRO films on the Si substrates. On the basis of the results of the XRD and RHEED measurements, the epitaxial orientations of the SRO films on Si deposited at PO2 = 1−15 Pa were obtained as follows: SRO⟨010⟩//Si⟨010⟩ (in-plane) and SRO(001)//Si(001) (out-of-plane). Such successful epitaxial growth of the SRO film on the buffered Si substrate could be attributed to the good domain match between TiN and Si and STO and TiN.20 It is worth noting that as PO2 increased from 1 to 15 Pa, the SRO(00l) (l = 1, 2) diffraction peaks shifted to large 2θ angles and eventually overlapped with the STO(00l) (l = 1, 2) diffraction peaks, which confirms the structural evolution of the SRO films. We can calculate the out-of-plane lattice parameter (c) from the out-of-plane θ−2θ scan data. The in-plane lattice parameter (a) can be deduced from the off-axis θ−2θ scan data (see Figure S2) using the equation a = 2 (d101)2 − (d002)2 ,21−23 where d101 and d002 are the lattice spacings of the (101) and (002) planes, respectively. The lattice parameters and the corresponding unit cell volume (V) C
DOI: 10.1021/acsami.6b02623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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defect scattering dominates the transport properties, thus leading to a higher resistivity. On the other hand, at room temperature, the resistivity also increases with the ruthenium defect-induced unit cell volume. This suggests that more than just scattering contributes to the resistivity at room temperature. As shown in Table 1 of the Supporting Information with the lattice parameters (a ∼ b ∼ c ∼ 3.923 Å) of the SRO bulk, the in-plane strains of the SRO films for PO2 = 15 and 1 Pa were calculated to be −0.382% (compressive strain) and 0.561% (tensile strain), respectively. Hence, the ruthenium vacancyinduced increase in resistivity at high temperatures should originate from the reduced mobility for the tensile-strain case.33 Apart from electronic transport, the magnetic properties of the SRO films are also strongly dependent on the growthinduced ruthenium vacancies. Figure 4a illustrates the zero-
Figure 3a shows the temperature dependence of the resistivity of the SRO films deposited under various oxygen
Figure 3. (a) Temperature dependence of the resistivity of the SRO films deposited under various oxygen pressures. The red solid line is the fitted result at 10 K ≤ T ≤ 50 K using ρ = 1/(G0 + aT1/2) + bT2. (b) Corresponding dρ/dT versus T curves. Inset shows TC versus PO2.
pressures. The kink in the resistivity curve corresponds to the paramagnetic to ferromagnetic phase transition near the Curie temperature (TC), representing a coupled magnetic and electrical behavior. To clearly observe this phase transition, we plotted the dρ/dT versus T curves in Figure 3b. It can be seen that when PO2 is decreased from 15 to 1 Pa, TC is progressively suppressed from 141 to 90 K, a reduction of 51 K (see inset of Figure 3b), which indicates the subsidence in ferromagnetic stability. When the EDS measurements are combined, such rapid suppression of TC is ascribed to the induced ruthenium vacancies rather than the oxygen vacancies.3,26,27,29,30 Particularly, upon closer inspection, the resistivity curve for PO2 = 1 Pa shows a low-temperature upturn at T = 26 K, probably due to the disorder-induced localization effect. To further explore the low-temperature transport mechanism, we fitted the resistivity of the SRO film in the 10 K ≤ T ≤ 50 K temperature range using the equation ρ = 1/(G0 + aT1/2) + bT2 based on the weak localization effect (WLE),31 where G0 is zero-temperature conductance and a and b are constants. The red solid line in Figure 3a is the fitted result, which is in full agreement with the experimental data, conspicuously proving the emergence of the WLE in the ruthenium-poor SRO film. The WLE should originate from the ruthenium vacancy-induced rupture of the conduction path within the RuO6 octahedra.32 Similar to that observed in ruthenium-poor SRO films,27 the low oxygen pressure grown films show a higher resistivity. At low temperatures, ruthenium
Figure 4. (a) Temperature dependence of ZFC and FC magnetization of the SRO films deposited under various oxygen pressures. The red solid line is the fitted result near TC using the scaling law M ∝ (TC − T)β. (b) Corresponding dM/dT versus T curves. Inset shows TC versus PO2 curves.
field-cooled (ZFC) and field-cooled (FC) magnetization as a function of temperature for the SRO films. Similar to that found for SRO bulk and thin films,34,35 a pronounced magnetic irreversibility was observed between the ZFC and FC curves at H = 100 Oe for all of the SRO films. The large splitting between the ZFC and FC magnetization below TC suggests the existence of a huge MCA in these systems.33,35 The FC magnetization data near TC are fitted with the scaling law M ∝ (TC − T)β (see the red line in Figure 4a),36 where M is spontaneous magnetization and β is the critical exponent. The β value of the SRO film for PO2 = 15 Pa is ∼0.6, close to that of the SRO single crystal (∼0.5),37 and thus can be interpreted based on the mean field model. When PO2 is decreased to 1 Pa, D
DOI: 10.1021/acsami.6b02623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces the β value becomes ∼0.4. Such disparity in β is presumably due to the differences in lattice strain, defects, and domain structure.32 Note that the ZFC curve for PO2 = 1 Pa exhibits a prominent peak value at T = 73 K, which is also corroborated by the presence of the peak value near T = 73 K (indicated by red arrow) in the dM/dT versus T curve (see Figure 4b). This anomaly in both the ZFC and FC curves at a low temperature hints at the existence of a hidden magnetic ordering at low temperatures.32 Again, as deduced from the dM/dT versus T curves, we found that TC is reduced considerably from 145 to 92 K when PO2 is decreased from 15 to 1 Pa (see inset of Figure 4b), which is commensurate with the resistivity data. The sharp suppression of TC can be attributed to the enhanced local disorder induced by ruthenium defects or the correlation effects due to the strongly hybridized p and d orbitals.3,26 It has been demonstrated that not only ruthenium defects but also ruthenium defect-induced lattice distortion can have a strong influence on TC due to the strong sensitivity of magnetic coupling to the interatomic distance.29,30 As mentioned above, ruthenium vacancies can generate the expansion of unit cell volume for the SRO films. This inverse proportion of TC to the unit cell volume implies that the ruthenium defect-induced lattice distortion indeed plays an important role in controlling the magnetic couplings. Such an appreciable ruthenium vacancy-induced evolution (∼53 K) of TC in SRO films integrated on Si is unprecedented and substantiates the effectiveness of ruthenium vacancy manipulation of lattice distortion. The growth-induced ruthenium vacancies not only reduce TC but also suppress magnetization of the SRO films (see Figure 4a), which is further confirmed by the magnetic hysteresis loops measured at T = 10 K shown in Figures 5a and b. This magnetization evolution is also strongly associated with the ruthenium vacancy-induced structural distortion.25,29,38 When PO2 is decreased from 15 to 1 Pa, the film structure changes from the pseudocubic (c/a ∼1.002) to the tetragonal (c/a ∼1.012) phase. The tetragonal unit cell may disfavor the crossover of the fourth spin of the half d filled Ru4+ ions from the t2g to eg energy levels due to the narrowing energy band ↑ gaps, and thus the high spin configuration of the Ru4+ (t↑↑↑ 2g eg ) ions can be suppressed by the structural distortion. 29 Additionally, the increased ruthenium vacancies may produce a frustrated spin state in this magnetic system, which also lowers the magnetic coupling. Figures 5a and b present the magnetization−magnetic field (M−H) hysteresis loops for the SRO films deposited at PO2 = 15 and 1 Pa and measured at T = 10 K after being cooled from T = 300 K with and without the application of an H = 0.5 or −0.5 T in-plane magnetic field. Remarkably, both of the ZFC M−H curves show symmetric hysteresis loops centered at the origin. However, for the FC M−H curves, we observed a vertical magnetization shift (i.e., along the magnetization axis) toward the same sign of the cooling field. This vertical hysteretic shift can be calculated using Mshift = [(M+sat + M−sat)/ 2]/[(M+sat − M−sat)/2],39 where M+sat and M−sat are the positive and negative saturation values of the hysteresis loop, respectively. It is surprising that as PO2 is decreased from 15 to 1 Pa, Mshift is enhanced markedly from 25% to 240%, an increase of 8.6×, which is much larger than that (100%) of the La0.7Sr0.3MnO3/ SRO system.17 This rare and intriguing vertical shift present in the La0.7Sr0.3MnO3/SRO system was found to vary with the thickness of the SRO layer, which could be explained by the
Figure 5. Magnetization−magnetic field (M−H) hysteresis loops for the SRO films deposited at PO2 = 15 Pa (a) and 1 Pa (b) and measured at T = 10 K after being cooled from T = 300 K with and without the application of an H = 0.5 or −0.5 T in-plane magnetic field.
strong interplay between the uniaxial magnetocrystalline anisotropy and microscopic interface domain structure.17 Further, in antiferromagnetic/ferromagnetic heterostructures such as SrMnO3/SRO superlattices7 and La0.7Sr0.3FeO3/SRO bilayers,8 the magnetization shift in the hysteresis loop was also observed, which is due to the pinned moments and uncompensated spins at the interface. Similar to that in an earlier report on polycrystalline SRO bulk,12,40 we believe this vertical shift in our SRO films is consistent with the normal behavior of ferromagnetic materials. After PO2 is decreased from 15 to 1 Pa, the induced ruthenium defects may act as pinning centers for the Ru4+ moments during the field cooling and thus enhance the Mshift. Moreover, local disorder could be introduced by the ruthenium defects, which also contributes to the large Mshift. In addition, lattice distortion is also a key factor in manipulating the magnetic coupling. It has been reported that the pseudocubic SRO films have in-plane uniaxial magnetic anisotropy, whereas the tetragonal films exhibit perpendicular uniaxial magnetic anisotropy.41,42 The distinct difference in the magnetic anisotropy of these two phases also results in the large evolution of Mshift. The ruthenium deficiencies could also change the orientation of the RuO6 octahedra and hence modify the spin−orbit coupling. Therefore, it is assumed that increased ruthenium defects, local disorder, spin−orbit coupling, and the change in magnetic anisotropy are responsible for the observed giant vertical shift (Mshift ∼240%). All of these data demonstrate that the unique vertical hysteretic shift behavior is a microscopic consequence of the ruthenium deficiency-induced lattice distortion in the SRO films. E
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(6) Eom, C. B.; Vandover, R. B.; Phillips, J. M.; Werder, D. J.; Marshall, J. H.; Chen, C. H.; Cava, R. J.; Fleming, R. M.; Fork, D. K. Fabrication and Properties of Epitaxial Ferroelectric Heterostructures with (SrRuO3) Isotropic Metallic Oxide Electrodes. Appl. Phys. Lett. 1993, 63, 2570. (7) Padhan, P.; Prellier, W. Direct Observation of Pinned/Biased Moments in Magnetic Superlattices. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 104416. (8) Rana, R.; Pandey, P.; Singh, R. P.; Rana, D. S. Positive ExchangeBias and Giant Vertical Hysteretic Shift in La0.3Sr0.7FeO3/SrRuO3 Bilayers. Sci. Rep. 2014, 4, 4138. (9) Fang, Z.; Nagaosa, N.; Takahashi, K. S.; Asamitsu, A.; Mathieu, R.; Ogasawara, T.; Yamada, H.; Kawasaki, M.; Tokura, Y.; Terakura, K. The Anomalous Hall Effect and Magnetic Monopoles in Momentum Space. Science 2003, 302, 92−95. (10) Wang, X. W.; Zhang, Y. Q.; Meng, H.; Wang, Z. J.; Li, D.; Zhang, Z. D. Magnetic Anisotropy and Transport Properties of 70 nm SrRuO3 Films Grown on Different Substrates. J. Appl. Phys. 2011, 109, 07D707. (11) Sow, C.; Pramanik, A. K.; Anil Kumar, P. S. Exchange Bias in Strained SrRuO3 Thin Films. J. Appl. Phys. 2014, 116, 194310. (12) Pi, L.; Zhang, S. X.; Tan, S.; Zhang, Y. H. Exchange Bias-Like Phenomenon in SrRuO3. Appl. Phys. Lett. 2006, 88, 102502. (13) Nogues, J.; Sort, J.; Langlais, V.; Skumryev, V.; Surinach, S.; Munoz, J. S.; Baro, M. D. Exchange Bias in Nanostructures. Phys. Rep. 2005, 422, 65. (14) Bibes, M.; Villegas, J. E.; Barthelemy, A. Ultrathin Oxide Films and Interfaces for Electronics and Spintronics. Adv. Phys. 2011, 60, 5. (15) Dieny, B.; Speriosu, V. S.; Parkin, S. S. P.; Gurney, B. A.; Wilhoit, D. R.; Mauri, D. Giant Magnetoresistive in Soft Ferromagnetic Multilayers. Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 1297. (16) Nogues, J.; Schuller, I. K. Exchange Bias. J. Magn. Magn. Mater. 1999, 192, 203. (17) Singamaneni, S. R.; Fan, W.; Prater, J. T.; Narayan, J. Complete Vertical M-H Loop Shift in La0.7Sr0.3MnO3/SrRuO3 Thin Film Heterostructures. J. Appl. Phys. 2015, 117, 17B711. (18) Narayan, J.; Tiwari, P.; Chen, X.; Singh, J.; Chowdhury, R.; Zheleva, T. Epitaxial Growth of TiN Films on (100) Silicon Substrates by Laser Physical Vapor Deposition. Appl. Phys. Lett. 1992, 61, 1290. (19) Zhang, W.; Chen, A.; Khatkhatay, F.; Tsai, C.-F.; Su, Q.; Jiao, L.; Zhang, X.; Wang, H. Integration of Self-Assembled Vertically Aligned Nanocomposite (La0.7Sr0.3MnO3)1−x:(ZnO)x Thin Films on Silicon Substrates. ACS Appl. Mater. Interfaces 2013, 5, 3995−3999. (20) Mal, S.; Yang, T.-H.; Gupta, P.; Prater, J. T.; Narayan, J. Thin Film Epitaxy and Magnetic Properties of STO/TiN Buffered ZnO on Si(001) Substrates. Acta Mater. 2011, 59, 2526−2534. (21) Specht, E. D.; Christen, H.-M.; Norton, D. P.; Boatner, L. A. XRay Diffraction Measurement of the Effect of Layer Thickness on the Ferroelectric Transition in Epitaxial KTaO3/KNbO3 Multilayers. Phys. Rev. Lett. 1998, 80, 4317. (22) Du, C. H.; Adur, R.; Wang, H. L.; Hauser, A. J.; Yang, F. Y.; Hammel, P. C. Control of Magnetocrystalline Anisotropy by Epitaxial Strain in Double Perovskite Sr2FeMoO6 Films. Phys. Rev. Lett. 2013, 110, 147204. (23) Liu, Z. Q.; Li, L.; Gai, Z.; Clarkson, J. D.; Hsu, S. L.; Wong, A. T.; Fan, L. S.; Lin, M.-W.; Rouleau, C. M.; Ward, T. Z.; Lee, H. N.; Sefat, A. S.; Christen, H. M.; Ramesh, R. Full Electroresistance Modulation in a Mixed-Phase Metallic Alloy. Phys. Rev. Lett. 2016, 116, 097203. (24) Lu, W. L.; He, K. H.; Song, W. D.; Sun, C. J.; Chow, G. M.; Chen, J. S. Effect of Oxygen Vacancies on the Electronic Structure and Transport Properties of SrRuO3 Thin Films. J. Appl. Phys. 2013, 113, 17E125. (25) Yoo, Y. Z.; Chmaissem, O.; Kolesnik, S.; Dabrowski, B.; Maxwell, M.; Kimball, C. W.; McAnelly, L.; Haji-Sheikh, M.; Genis, A. P. Contribution of Oxygen Partial Pressures Investigated Over a Wide Range to SrRuO3 Thin-Film Properties in Laser Deposition Processing. J. Appl. Phys. 2005, 97, 103525.
4. CONCLUSIONS In summary, we reported the successful epitaxial integration of SRO films on Si(001) substrates using domain matching epitaxy and realized unprecedented ruthenium deficiencyinduced evolution of the ferromagnetic transition temperature (ΔTC ∼53 K) and vertical hysteretic shift (Mshift ∼240%). When PO2 is decreased from 15 to 1 Pa, the induced ruthenium deficiencies generate the expansion of unit cell volume accompanied by the enhancement of resistivity and the reduction of magnetization. We attribute these emergent physical phenomena to ruthenium defect-induced enhanced local disorder and complex effects related to the strongly hybridized p and d orbitals as well as the induced structural distortion. Our findings provide a framework for manipulating the vertical hysteretic shift and ferromagnetic transition temperature of the SRO films by the ruthenium defects and improving the performance of related electronic devices based on complex oxides using strain engineering.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02623. RHEED patterns during the deposition of STO and TiN buffer layers on Si(001), surface morphology of the STO film on TiN-buffered Si(001), off-axis θ−2θ scan data obtained by tilting the film plane at an angle of 45° for SRO/Si heterostructures deposited under different oxygen pressures, plume shape for the SRO films deposited at 1 and 15 Pa, and lattice parameters and unit cell volumes calculated from SRO(002) and SRO(101) diffraction peaks (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (Grant 2009CB623304) and the NSFC (Grant 11090332).
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REFERENCES
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DOI: 10.1021/acsami.6b02623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.6b02623 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
