Scholarly article on topic 'Antiferromagnetic proximity effect in epitaxial CoO/NiO/MgO(001) systems'

Antiferromagnetic proximity effect in epitaxial CoO/NiO/MgO(001) systems Academic research paper on "Nano-technology"

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Academic research paper on topic "Antiferromagnetic proximity effect in epitaxial CoO/NiO/MgO(001) systems"

SCIENTIFIC REPpRTS

OPEN Antiferromagnetic proximity effect in epitaxial CoO/NiO/MgO(001) systems

Received: 26 October 2015 Accepted: 12 February 2016 Published: 02 March 2016

Q. Li1, J. H. Liang1, Y. M. Luo1, Z. Ding1, T. Gu1, Z. Hu2, C. Y. Hua2'3, H.-J. Lin3, T. W. Pi3, S. P. Kang4, C. Won4 & Y. Z. Wu1

Magnetic proximity effect between two magnetic layers is an important focus of research for discovering new physical properties of magnetic systems. Antiferromagnets (AFMs) are fundamental systems with magnetic ordering and promising candidate materials in the emerging field of antiferromagnetic spintronics. However, the magnetic proximity effect between antiferromagnetic bilayers is rarely studied because detecting the spin orientation of AFMs is challenging. Using X-ray linear dichroism and magneto-optical Kerr effect measurements, we investigated antiferromagnetic proximity effects in epitaxial CoO/NiO/MgO(001) systems. We found the antiferromagnetic spin of the NiO underwent a spin reorientation transition from in-plane to out-of-plane with increasing NiO thickness, with the existence of vertical exchange spring spin alignment in thick NiO. More interestingly, the Neel temperature of the CoO layer was greatly enhanced by the adjacent NiO layer, with the extent of the enhancement closely dependent on the spin orientation of NiO layer. This phenomenon was attributed to different exchange coupling strengths at the AFM/AFM interface depending on the relative spin directions. Our results indicate a new route for modifying the spin configuration and ordering temperature of AFMs through the magnetic proximity effect near room temperature, which should further benefit the design of AFM spintronic devices.

Contact between materials with different magnetic orderings can modify their properties near the interface owing to exchange coupling. This phenomenon is usually called magnetic proximity effect. Investigation of the magnetic proximity effect has resulted in the discovery of many new rich physical phenomena1, such as the moments induced in heavy non-magnetic metals by proximity to ferromagnets (FMs) reported in a recent magneto-transport study2'3, and the enhancement of the Curie temperature (Tc) of diluted magnetic semiconductors due to the magnetic proximity effect of FMs1,4,5. The modification of magnetic properties influenced by magnetic proximity effect with FMs has been widely studied. For example, the Tc of FM has been found to be significantly enhanced in FM/FM bilayers6'7, which could be exploited to optimize magnetic properties in spintronic devices. Another well-known example is the manifestation of a shifted hysteresis loop (i.e., exchange bias)8,9 combined with enhanced coercivity, stronger magnetic anisotropy10,11 and spin reorientation transition12,13 for FMs in ferromagnet/antiferromagnet (AFM) bilayers owing to exchange coupling.

Antiferromagnetic materials, the other class of fundamental magnetic systems, have been widely used in advanced magnetic storage and sensor devices8. Recently, spintronic devices based on AFMs have been proposed14-16, which are predicted to realize stable high-density memory integration due to their zero moment and other nontrivial properties. However, tuning AFM magnetic properties is considered a great challenge due to its zero moment. Using exchange coupling with neighbor spins in FM/AFM bilayers, the Neel temperature (TN) of AFMs can be greatly enhanced17-20, and their spin orientation can be modulated through the formation of a lateral exchange spring spin structure14,15,21,22. Nevertheless, the tuning of the magnetic properties of AFMs without FM layer has drawn little attention. So far, there have only been few studies on the modulation of AFM magnetic properties using the AFM magnetic proximity effect in AFM/AFM bilayers. In AFM superlattices comprising two AFM materials with different TN, such as [CoO/NiO]n23-26 or [FeF2/CoF2]n27,28, the ordering temperature of the

department of Physics, State Key Laboratory of Surface Physics and Collaborative Innovation Center of Advanced Microstructures, Fudan University, Shanghai 200433, People's Republic of China. 2Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, Dresden 01187, Germany. 3National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic of China. 4Department of Physics, Kyung Hee University, Seoul 130-701, Republic of Korea. Correspondence and requests for materials should be addressed toY.Z.W. (email: wuyizheng@fudan.edu.cn)

Figure 1. Schematics of XMLD measurement and AFM spin configuration, and obtained XAS spectra.

(a) Schematic of XMLD measurement geometry. (b-d) Schematics of AFM spin configurations with arrows representing the AFM spin alignments. In (d), 0 is defined as the spin canting angle between the film surface and spin orientation and z is the depth from the CoO/NiO interface. XAS spectra with 9 = 0° and 9 = 60° or 70° of (e) Co2+ L3 edge and (f) Ni2+ L2 edge in Ni0(co0)/Mg0(001) and Co0/Ni0/Mg0(001) samples at 78 K. The unit of the thickness in (e,f) is nm. The XMLD measurements of Ni0, Co0, and Co0/Ni0 were performed on different samples.

AFM layer with lower TN can be enhanced by the other AFM layer, and the AFM order has a long propagation length of several nanometers. Zhu et al. reported that interface coupling can induce AFM spin reorientation transition in the Ni0/Co0/Mg0(001) system29. However, how the spin orientation can be influenced by exchange coupling between AFM spins remains an unsettled question. It is also unclear whether the spin orientation of one AFM layer can affect the TN of the proximate AFM layer.

In this work, in order to detect the AFM properties of top Co0 layers, we fabricated a single-crystalline Co0/Ni0 bilayer on Mg0(001) substrate. Ni0 on Mg0(001) substrate has out-of-plane AFM spin orientation owing to tensile strain30, whereas Co0 on Mg0(001) has in-plane AFM spin orientation owing to compressive strain31,32. Bulk Ni0 has a TN of ~523 K, and bulk Co0 has a lower TN of ~293 K. These distinct spin orientations and TNs enable us to study the AFM proximity effect on AFM spin orientation and TN in this system. Using the element-specific X-ray magnetic linear dichroism (XMLD) effect, we demonstrate that the AFM spin orientation of Ni0 changes from in-plane to out-of-plane with increasing Ni0 thickness, and the existence of a vertical exchange spring alignment in thick Ni0. The spin reorientation transition (SRT) in Ni0 layer dramatically lowers the TN of the adjacent Co0 layer. Further confirmed by systematical magneto-optical Kerr effect (M0KE) measurements of the Fe/Co0/Ni0/Mg0(001) system, this phenomenon of spin-orientation-dependent TN was attributed to the exchange coupling strength dependent on the spin orientation. The AFM order propagation length was explored by studying the Co0-thickness dependent TN. 0ur results provide clear evidence that the magnetic proximity effect in AFM/AFM bilayers can influence the spin direction and ordering temperature within 2-3 nm of the interface, which is crucial for designing AFM spintronic devices.

Results

Antiferromagnetic spin orientation transition in CoO/NiO bilayer. We applied XMLD measurements to determine the AFM spin orientation in a single-crystalline Co0/Ni0 bilayer on a Mg0(001) surface using the total electron yield (TEY) mode. In XMLD measurements, the obtained X-ray absorption spectrum (XAS) is strongly dependent on the relative orientation of the AFM spin axis and the linear polarization vector of the X-rays used29-36. Thus, to determine whether the AFM spin was in-plane or out-of-plane, XAS was measured as a function of the incident angle (9), defined as the angle between the propagation direction of the X-rays and the direction normal to the sample surface, as shown in Fig. 1(a). To investigate the magnetic properties of the Co0 layer, Co0 L3 edge spectra were measured with X-rays at normal incidence (9 = 0°) and grazing incidence (9 = 70° or 9 = 60°). Figure 1 shows the results of XMLD measurements performed at 78 K, which is much lower than the TN of both Ni0 and Co0 films in our samples13,29,35. As shown in Fig. 1(e), the XAS spectra observed for 2 nm Co0 proximate to 2 nm and 12 nm Ni0 were similar to that of Co0/Mg0, which exhibits in-plane spin orientation29,32. Thus, at low temperature, the Co0 AFM spin was aligned in-plane on top of both 2 nm and 12 nm

Figure 2. Change of CoO AFM order and NiO spin reorientation transition with increasing NiO thickness.

(a) CoO L3 ratio and (c) NiO L2 ratio with 0 = 0° and 6 = 70° as a function of NiO thickness at 78 K. Difference in (b) CoO L3 ratio and (d) NiO L2 ratio as a function of NiO thickness at 80 K, 300 K, and 400 K. The green line in (d) was calculated based on Monte Carlo simulation results.

NiO. Figure 1(f) shows the typical XAS measured at the Ni2+ L2 edge, in which the doublet spectra have been normalized to the intensity of the lower energy peak. The second peak of the XAS obtained from the 2 nm NiO in the CoO/NiO bilayer exhibited a clearly reversed intensity for 0 = 0° and 0 = 70° compared with those from the NiO film grown on MgO substrate, which indicates that AFM spin of 2 nm NiO was aligned out of plane on MgO substrate29,30 [Fig. 1(b)] and in plane in AFM bilayer [Fig. 1(c)]. However, the second peak in the NiO XAS spectrum of the CoO (2 nm)/NiO (12 nm) bilayer showed similar intensity at 0 = 0° and 0 = 70°. Thus, we can conclude that the AFM spins of the 12 nm NiO were in the canting state with comparable in-plane and out-of-plane components. This result is significantly different from that reported for a reversed growth order in NiO (12 nm)/ CoO bilayer on MgO(001) substrate, which showed out-of-plane AFM spin orientation of the NiO29. Due to the limited electron escape length in NiO35, the observed intensity of the xAs measurements in TEY mode mostly came from a several nanometer thickness of the NiO films. Thus, the XAS from the 12-nm NiO layer shown in Fig. 1(f) reflects the properties of the NiO near the CoO/NiO interface, whereas previous measurements on NiO/ CoO bilayers only revealed the information for the NiO away from the NiO/CoO interface29.

The XMLD effect of CoO can be quantified by the L3 ratio RL , which is defined as the ratio of the intensities of the XAS peaks at 777 eV and 779.6 eV [marked as A and B in Fig. 1(e), respectively]32. For NiO, the XMLD effect is quantitatively described by the NiO L2 ratio RL , which is defined as the ratio of the intensity of the lower energy peak divided by that of the higher energy peak30,35. Figure 2(a,c) respectively show the NiO-thickness-dependent CoO L3 ratio RLi (0) and NiO L2 ratio RL (0) measured at incidence angles of 0o and 70o at 78 K. In Fig. 2(a), the CoO Rl (0°) is always larger than rl (70°) at various NiO thicknesses. Consequently, the difference of the CoO L3 ratio ARL =RL (0°) — RL (70°) is positive for all NiO thicknesses in Fig. 2(b). This result indicates that the CoO spin aligns in plane irrespective of the NiO thickness, which may be attributed to the large anisotropy of the CoO layer37,38. The XMLD of the NiO film showed a more complicated behavior. In Fig. 2(c), RL (0°) was smaller than RL (70°) for NiO thicknesses smaller than 4 nm, after which RL (0°) began to increase while RL (70°) began to decrease, and they crossed over at dNiO~7.0 nm. Correspondingly, in Fig. 2(d), the difference of NiO L2 ratio, ARL = RL (0°) — RL (70°), changed from a negative value to a small positive value at 78 K. It has been shown that the NiO L2 ratio RL should be smaller for EIISAFM than £ ± SAPM when the NiO spin is aligned along the

Figure 3. Spin-orientation-dependent Neel temperature by XMLD measurements. Temperature dependence of (a-c) Co0 L3 ratio difference ARL and (d-f) Ni0 L2 ratio difference ARL for the systems Co0/ Mg0(001), Ni0/Mg0(001), and Co0/Ni0/Mg03(001) with layer thicknesses indicated o2n the top of each panel. Arrows indicate the AFM ordering temperature TN.

< 100> direction29,30,34. Thus, the data in Fig. 2(c,d) indicate that the Ni0 spin orientation switched from in-plane to a canting orientation at 78 K. Ni0-thickness-dependent experiments were also performed at 300 K and 400 K. As shown in Fig. 2(d), the Ni0 ARL at 300 K changed from negative to positive, indicating that the Ni0 spin switched from in-plane to out-of-plane. The Ni0 ARL at 400 K showed a rapid increase at dNi0~2.0 nm, which was related to the establishment of Ni0 AFM order29.

NiO-spin-orientation-dependent enhancement of CoO TN. The XMLD effect observed at the Co2+

L3 edge and Ni2+ L2 edge can generally be attributed to AFM ordering and the crystal-field effect33,39. While the AFM contribution decreases with increasing temperature and vanishes above TN, the contribution of the crystal field only slightly decreases at higher temperature. Thus, the TN of the AFM layer could be identified from the transition point of the Co0 ARL (T) curve and Ni0 ARL (T) curve29. Temperature-dependent XMLD measurements were performed as shown in Fig. 3. In Fig. 3(a-c), the Co2+ ARL was always positive, indicating in-plane aligned Co2+ AFM spins. The different Co0 ARL value at high temperature could be attributed to a change of the crystal field influenced by different strain in the Co0 layer. The single 2-nm Co0 layer exhibited a TN of 230 K [Fig. 3(a)]. When the Co0 layer was proximate to 2 nm Ni0, its TN was dramatically enhanced to 400 K [Fig. 3(b)], whereas the TN of the Co0 layer on top of 12 nm Ni0 was only enhanced to 300 K [Fig. 3(c)]. This result seems unexpected because the Neel temperature of 12 nm Ni0 is evidently higher than that of 2 nm Ni0 owing to the finite-size scaling13.

Using the advantages of XMLD with elementary specification, we also measured the temperature-dependent ARl of the Ni0 layer in Co0/Ni0 bilayer as shown in Fig. 3(d-f). 0ur measurements indicated that the 2 nm Ni0 had a TN of ~400 K, which coincides with the changes in the Ni0 ARL seen in Fig. 2(d). 0wing to the temperature-dependent crystal field effect31,33, the ARL of the 2 nm Ni0 still decreased above its TN. The ARL of 2 nm Ni0 in the AFM bilayer was negative at low temperature and became positive approaching 400 K, which indicates that the Ni0 spin had in-plane orientation at low temperature, and then switched to out-of-plane at high temperature [Fig. 3(e)]. In Fig. 3(f), the ARL of 12 nm Ni0 had a small positive value at low temperature, and started to increase when the temperature approached 300 K. This indicates that the Ni0 spin at the Co0/Ni0 interface changed from the canting state to an out-of-plane oriented state. Note that the SRT of the Ni0 layer occurred exactly at the TN of the proximate Co0 layer, which could be ascribed to the decreased interfacial coupling strength in the AFM bilayer related to the weakened Co0 AFM order. Based on these results, we can conclude that the Ni0 layer with in-plane aligned spin orientation largely enhanced the TN of the adjacent Co0 layer by 170 K whereas the Ni0 layer with out-of-plane spin orientation only enhanced the TN of the Co0 layer by 70 K. It should be pointed out that, as a result of thickness dependent lattice relaxation, the strain of the film grown on thick Ni0 may be different to that of the film grown on thin Ni0. However, the observed different TNs are unlikely to be attributable to the different strains of the two systems. High energy electron diffraction (RHEED) patterns of the 2- and 12-nm Ni0 films showed that the difference in lattice constant was less than 0.6%. However, as reported in ref. 31, while the difference in lattice constant between bulk Mn0 and Ag can be 8.2%, the TN of Co0 film grown on Ag(001) substrate is only 20 K higher than that grown on Mn0(001) film.

Figure 4. Spin-orientation-dependent Neel temperature by MOKE measurements. Typical hysteresis loops for the systems (a) Fe/Co0/Mg0(001) and (b,c) Fe/Co0/Ni0/Mg0(001) with indicated layer thicknesses at different temperatures. (d) Temperature-dependent Hc of Fe/Co0/Ni0/Mg0(001) with different Ni0 thickness. Inset shows an enlargement of the Hc around TN. Arrows denote the Co0 TN evaluated from the discontinuity of the slopes of the Hc(T) curves. (e) Co0 TN as a function of Ni0 thickness for the Fe/Co0/ Ni0(wedge)/Mg0(001) sample. Inset in (e) shows the structure of the sample.

In order to systematically investigate how the TN of the Co0 film changed with Ni0 thickness, a sample of Fe (2 nm)/Co0(1.5 nm)/Ni0/Mg0(001) was prepared with a wedge-shaped Ni0 layer. It is well known that below the TN, the antiferromagnetic spins in a FM/AFM bilayer can dramatically increase the coercivity (Hc) of FM layer through the exchange coupling between the FM layer and AFM layer. Thus, the TN of the Co0 layer could be determined from temperature-dependent behavior of its Hc8,20. Figure 4(a-c) show typical hysteresis loops obtained at different temperatures for Fe/Co0, Fe/Co0/Ni0 (2 nm), and Fe/Co0/Ni0 (12 nm), respectively. The samples exhibited easy hysteresis loops with negligible exchange bias field under a sweeping field H parallel to the cooling field HFC along the Co0< 110> directions11,32. The temperature-dependent Hc is plotted in Fig. 4(d), the inset of which shows the enlarged region around the TN of the Co0 layer, which is defined as the discontinuity point of the slope of the Hc(T) curve. From this plot, the TN of the 1.5-nm Co0 single layer can be judged to be about 175 K. For Co0 on top of 2 nm Ni0, the TN was enhanced to 415 K, while that of Co0 on top of 12 nm Ni0 was enhanced to 343 K.

Figure 4(e) shows the results of the systematical measurement of Co0 TN as a function of Ni0 thickness. The TN of the 1.5-nm Co0 layer first increased to ~400 K with Ni0 thickness, began to decrease for Ni0 thicker than 4 nm, and finally saturated to ~340 K for Ni0 thicker than 8 nm. The TN of Co0 decreased by ~80 K on thicker Ni0, which is consistent with the XMLD results. Recalling the fact that the SRT of Ni0 occurred at a thickness range of 4-8 nm at temperatures of 300-400 K in Fig. 2(d), the results here further confirm that the TN of the Co0

Figure 5. Study of AFM order propagation in CoO layer. (a) Schematic of AFM order in the AFM bilayer. (b) CoO L3 ratio difference ARL as a function of temperature for CoO/NiO (2 nm)/MgO(001) with a different CoO thickness. (c) Temperature dependence of Hc for Fe/CoO/NiO (2 nm)/MgO(001) with a different CoO thickness. Inset shows an enlargement of the Hc around TN. (d) TN of CoO layer as a function of CoO thickness with and without a proximate 2-nm NiO layer. Inset in (d) shows the structure of the sample.

layer was closely related to the spin orientation of the proximate NiO layer. Therefore, in Fig. 2(b), for the CoO layer on top of thick NiO film, the ARL for CoO at 300 K is similar to that observed at 400 K, indicating the paramagnetic state of CoO at 300 K. However, for CoO layers proximate to NiO layers thinner than 6 nm, we found a small difference in the CoO ARL measured at 300 K and 400 K, which indicates that the AFM order of the CoO layer on top of thin NiO film persisted above 300 K.

Propagation of CoO AFM order in CoO/NiO bilayer. We have shown that exchange coupling in the CoO/NiO bilayer enhanced the CoO TN. This enhancement of the CoO ordering temperature was expected to be at a maximum near the CoO/NiO interface owing to the interfacial nature of exchange coupling. Thus, the ordering temperature of the CoO should gradually change along the thickness direction, as indicated by the schematic in Fig. 5(a). For a sufficiently thick CoO film, the ordering temperature of the outer part of the CoO layer is expected to be close to that of bulk CoO. However, this theory is hard to prove using XMLD measurements. Figure 4(b) shows the temperature-dependent ARL of 2 nm CoO in the CoO/NiO bilayer had a clear transition which indicated a TN of ~400 K. In contrast, it only gradually decreased with temperature for 4 nm CoO, and no obvious transition could be distinguished. Thus, it was difficult to determine the ordering temperature of 4 nm CoO using XMLD measurements. As indicated in Fig. 5(a), different parts of the 4-nm CoO layer had different ordering temperature; thus, the XMLD measurements detected all the AFM information, and did not show the single phase transition.

To determine the CoO AFM order away from the AFM interface, we grew 2 nm Fe on top of CoO/NiO (2 nm)/ MgO(001) with the CoO grown into a wedge shape, and used MOKE measurements to determine the ordering temperature of the CoO spins at the Fe/CoO interface through the interfacial exchange coupling. As shown in Fig. 5(c), the temperature-dependent Hc obtained from the magnetic hysteresis loops showed a clear transition. The TN of 1.5 nm, 2 nm, and 5 nm CoO was determined to be 430 K, 380 K, and 330 K, respectively. Half of the sample was grown as Fe (2 nm)/CoO(wedge) without the NiO underlayer, as indicated by the inset in Fig. 5(d), thus both the Fe/CoO/NiO and Fe/CoO samples had identical growth conditions. Figure 5(d) shows the TN at the

Fe/Co0 interface as a function of Co0 thickness for both Fe/Co0/Ni0 and Fe/Co0 samples. The TN of Co0 in the AFM bilayer was always higher than that without the Ni0 layer. Therefore, our results demonstrate that the exchange coupling at the Co0/Ni0 interface can significantly enhance the ordering temperature of Co0. Without the Ni0 underlayer, the TN of the Co0 layer followed the finite size scaling law well, as previously reported13. However, with the Ni0 layer, the TN of the Co0 first increased with thickness and then decreased exponentially when the Co0 thickness was larger than 1.5 nm. In both Fe/Co0/Ni0 and Fe/Co0 samples, the determined TN for thick Co0 saturated at ~300 K, which is close to the bulk TN of Co0. The TN measured for the Co0/Ni0 bilayer first increased with Co0 thickness owing to the finite size effect. However, the induced AFM order of the Co0 in Co0/Ni0 bilayer should not have a higher ordering temperature than the Ni0 layer, and the AFM order of the Co0 spins in the outer layer weakens in thick Co0 film, so the measured TN decreased for Co0 thicknesses above 1.5 nm. By fitting TN(dCo0) with an exponential decay function for dCo0 > 1.5 nm, we obtained a characteristic decaying thickness of ~0.7 nm. 0ur results indicate that the propagation length of AFM order in Co0 is about 2-3 nm.

Discussion

The anisotropy of Co0 is about two orders of magnitude stronger than that of Ni038, thus the Co0 spins in the Co0/Ni0 bilayer were always in-plane, whereas the spin orientation of the Ni0 layer was influenced by the competition between interfacial exchange coupling and lattice-strain-induced out-of-plane anisotropy29. The interfacial exchange coupling should be collinear coupling, because the thin Ni0 spins were driven to align along the film plane by the in-plane Co0 spins. However, in the AFM bilayer with thick Ni0, the AFM proximity effect weakened with distance from the Co0/Ni0 interface24-26, while the strain-induced anisotropy in the Ni0 persisted above 20 nm in our experiment. Thus, for Ni0 far from the Co0/Ni0 interface, the AFM Ni0 spin was aligned out of plane, while near the interface the AFM spin was in the canting state [Fig. 1(f)]. In this case, it is natural to suppose a vertical exchange spring spin alignment existed in the thicker Ni0 layers, as shown in Fig. 1(d), which has never been reported before. Within the detection length of the XMLD measurements, the Ni0 AFM spins were found to change from in-plane to the canting orientation with increasing Ni0 thickness at low temperature.

Figure 3(e,f) show that the Ni0 spins changed from in-plane (canting state) to out-of-plane with increasing temperature; this can be understood by the weakened exchange coupling strength at the Ni0/Co0 interface at high temperature. The AFM exchange coupling strength was also related to the AFM spin orientation, as described by H = — /intS ' ■ Sj, where S' and S . represent Co0 and Ni0 spin, and /int is the interfacial exchange coupling constant. Because the Co0 spin was aligned in-plane, the AFM exchange coupling strength should have become smaller when the Ni0 spin rotated from in-plane to out-of-plane. It is therefore reasonable that a drop in Co0 TN was observed when the spin orientation of the proximate Ni0 layer changed from in-plane to out-of-plane. Monte Carlo simulation with some typical parameters qualitatively confirmed our experimental results (see Supplementary Fig. S2). The length of AFM order propagation in the Co0 layer of the Co0/Ni0 bilayer was estimated to be 2-3 nm. In terms of previous reports on Co0/Ni0 superlattices, this value is similar to that in ref. 26 and smaller than that in ref. 25, which can be attributed to different film qualities resulting from the use of different growth methods. Moreover, such AFM order propagation in the AFM bilayer was also confirmed qualitatively by Monte Carlo simulation (see Supplementary Fig. S3).

In general, the XMLD signal should reflect the AFM spin alignment, so if the spin configuration in the Ni0 film is known, the Ni0-thickness dependent XMLD signal in Fig. 2 can be quantitatively simulated. The Ni0 XMLD signal could be expressed as ARL (dN°) = JdN'° ARL (z) e~z/xdz/dN°40, where A is the electron escape depth and ARL (z) is the XMLD signal of the Ni0 layer at the position with depth z away from the Co0/Ni0 interface, and depends on the local spin canting angle at z, i.e. (z), as defined in Fig. 1(d). ARL (z)may be reasonably expressed as A cos2(^(z)) + B33. In Fig. 2(d), ARL = — 0.21 for in-plane aligned Ni0 spin, and ARl = 0.3 for out-of-plane aligned Ni0 spin at a higher temperature. Thus, we can determine B = 0.3 and A = -0.51. The dN°-dependent spin profile was calculated by Monte Carlo simulation (see Supplementary part 2), and the dN°-dependent ARL (dN°) was then calculated by choosing a suitable value for A. The green line in Fig. 2(d) shows the calculated Arl (dN°) with A = 4.0 nm, which agrees well with the experimental result observed at low temperature. 0ur calculations further verified the model with the AFM exchange spring structure shown in Fig. 1(d). Based on the XMLD transition thickness in Fig. 2(d), the length scale of the AFM exchange spring was estimated to be ~6 nm at 78 K, and became smaller at higher temperature owing to the weakening of the exchange coupling.

It should be noted that the ARL of thick Ni0 film depends on the vertical spin profile, which could be different in the systems with different Ni0 thickness. In our Monte Carlo simulations, the dN°-dependent spin profile was strongly dependent on the exchange coupling constants and AFM magnetic anisotropies, and these parameters are difficult to determine experimentally. XMLD measurements can only indicate the average AFM properties, and quantitatively determining the AFM spin profile across the film requires other techniques with layer specification. We note that the FM magnetization profile of magnetic heterostructures can be investigated by X-ray resonant magnetic reflectometry based on the X-ray magnetic circular dichroism effect41. Thus, the study of non-uniform AFM spin alignment in AFM films is likely to be possible using X-ray resonant magnetic reflec-tometry based on the XMLD effect.

In summary, we investigated the AFM proximity effect in a Co0/Ni0/Mg0(001) system on the basis of AFM spin orientation, Neel temperature, and AFM order propagation length. 0wing to AFM interfacial exchange coupling, the Ni0 AFM spin underwent SRT from in-plane to out-of-plane with increasing Ni0 thickness, with the existence of a vertical exchange spring structure in thick Ni0. The SRT had a great influence on the TN of the

CoO layer, and influenced the AFM order of the CoO within a limited depth of about 2-3 nm from the interface. The present work is expected to allow new designs for AFM-based devices that exploit the AFM proximity effect in AFM/AFM bilayers, where the AFM spin orientation and AFM ordering temperature can be manipulated without involving any FM layer.

Methods

Sample preparation. CoO/NiO/MgO(001) films were prepared by molecular beam epitaxy (MBE) in an ultra-high vacuum (UHV) chamber with a base pressure of 2 x 10-10 Torr. The single-crystal MgO(001) substrate was first annealed at 600 °C for 30 min in the UHV chamber, followed by the growth of a 10-nm MgO seed layer at 500 °C by e-beam evaporation. Subsequently, the NiO and CoO layers were prepared by reactive deposition of Ni and Co at an oxygen pressure of 1 x 10-6 Torr at room temperature (RT)11'29'32. A smooth surface was confirmed by the sharp reflection RHEED patterns. Film thickness was determined by the deposition rate (~1.0-2.0 A/min), and was measured using a calibrated quartz thickness monitor. For thickness-dependence measurements, the CoO(NiO) film was grown in a wedge shape by moving the substrate behind a knife-edge shutter. To study the exchange coupling between CoO AFM spins and Fe FM spins, Fe film was grown on the bilayers by MBE at RT. In situ RHEED patterns indicated high quality epitaxial growth of both the fcc structured CoO (NiO) films and the bcc Fe film with an epitaxial relationship of Fe[100]//CoG[110]//NiG[110]//MgG[110] (see Supplementary Fig. S1). Finally, the sample was capped with a 3-nm MgO layer as a protective layer for MOKE measurements. For the samples subjected to XMLD measurements, a Al (4-nm)/MgO (1-nm) bilayer was capped on top of the CoO layer to prevent charging effects, and a 1-nm MgO layer was used to prevent a chemical reaction between the Al and CoO.

XMLD measurements. XMLD measurements were performed at the bending-magnet Beamline 08B of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The X-ray polarization was fixed along the horizontal direction and spectra were collected in total electron yield mode. The absorption spectra were collected for both normal (9 = 0°) and grazing (9 = 60° or 70°) X-ray incidence29,31,32. The linear background has been subtracted for all of the XAS spectra shown in this paper. Samples were mounted with the slope of the NiO wedge along the vertical direction so that the NiO thickness could be easily varied by moving the sample up and down to illuminate the sample at different positions. The X-ray beam size was estimated to be 100 |im along the vertical direction and 300 |im along the horizontal direction. Accordingly, the thickness variation in the vertical direction within the X-ray beam caused by the wedge shape was less than 0.2 nm. The sample temperature was adjustable in the range of 78-460 K with a precision of less than 0.1 K.

MOKE measurements. The magnetic properties of the films were determined by MOKE measurement using a laser diode with a wavelength of 670 nm. By taking advantage of the small laser beam size (below 0.2 mm) of the MOKE apparatus, we were able to systematically perform thickness-dependent studies on the same wedge-shaped sample as that used in XMLD measurements. The sample temperature for the MOKE measurements was adjustable in the range of 80-500 K.

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Acknowledgements

This work was supported by the National Key Basic Research Program of China (Grants No. 2015CB921401 and No. 3172011CB921801), the National Science Foundation of China (Grants No. 11274074, No. 11434003, and No. 11474066), and a National Research Foundation of Korea Grant funded by the Korean Government (Grant No. 2012R1A1A2007524).

Author Contributions

W.Y.Z. designed the experiments. L.Q. and D.Z. prepared the sample. L.Q., L.J.H., L.Y.M. and D.Z. carried out the XMLD measurement helped by H.Z., H.C.Y., L.H.-J., P.T.W., L.Q. and G.T. performed the MOKE measurement. K.S.P. and W.C. made the simulation program and L.Q. did the simulation. W.Y.Z. and L.Q. analyzed the results. L.Q. and W.Y.Z. wrote the paper and all the co-authors commented on it.

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Li, Q. et al. Antiferromagnetic proximity effect in epitaxial CoO/NiO/MgO(001) systems. Sci. Rep. 6, 22355; doi: 10.1038/srep22355 (2016).

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