Enhancing the blocking temperature of perpendicular exchange biased Cr2O3 thin films using buffer layers Naoki Shimomura, Satya Prakash Pati, Tomohiro Nozaki, Tatsuo Shibata, and Masashi Sahashi Citat[.]
Enhancing the blocking temperature of perpendicular-exchange biased Cr2O3 thin films using buffer layers Naoki Shimomura, Satya Prakash Pati, Tomohiro Nozaki, Tatsuo Shibata, and Masashi Sahashi Citation: AIP Advances 7, 025212 (2017); doi: 10.1063/1.4977714 View online: http://dx.doi.org/10.1063/1.4977714 View Table of Contents: http://aip.scitation.org/toc/adv/7/2 Published by the American Institute of Physics AIP ADVANCES 7, 025212 (2017) Enhancing the blocking temperature of perpendicular-exchange biased Cr2 O3 thin films using buffer layers Naoki Shimomura,1 Satya Prakash Pati,1 Tomohiro Nozaki,1,a Tatsuo Shibata,2 and Masashi Sahashi1,3 Department of Electronic Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan Advanced Technology Development Center, TDK Corporation, Chiba 272-0026, Japan ImPACT Program, Japan Science and Technology Agency, Tokyo 102-0076, Japan (Received 27 December 2016; accepted 16 February 2017; published online 24 February 2017) In this study, we investigated the effect of buffer layers on the blocking temperature (T B ) of perpendicular exchange bias of thin Cr2 O3 /Co exchange coupled films with a Ru spacer and revealed a high T B of 260 K for 20-nmthick Cr2 O3 thin films By comparing the T B values of the 20-nm-thick Cr2 O3 films on Pt and α-Fe2 O3 buffers, we investigated the lattice strain effect on the T B We show that higher T B values can be obtained using an α-Fe2 O3 buffer, which is likely because of the lattice strain-induced increase in Cr2 O3 magnetocrystalline anisotropy © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4977714] I INTRODUCTION Electric control of magnetization using the magnetoelectric (ME) effect has received considerable attention as a promising candidate for next-generation low-energy consumption magnetic recording devices.1 The linear ME effect was theoretically predicted for Cr2 O3 in 19602 and experimentally confirmed in 1961.3 Few techniques at that time were utilized to deal with antiferromagnets, so practical applications were not immediately realized Since 2005, the ME effect in Cr2 O3 has captured renewed attention because electrical switching of perpendicular exchange bias has been demonstrated in bulk Cr2 O3 single crystal/ferromagnet exchange coupled systems.4,5 Here, antiferromagnetic domain reversal by applying both magnetic and electric fields6 has been used to switch the perpendicular exchange bias of a Cr2 O3 /ferromagnet This perpendicular exchange bias switching has recently been demonstrated in several-hundred-nanometer-thick Cr2 O3 films deposited by the sputtering method, which yields more realistic device applications.7–12 One major issue for device application is simultaneous realization of both a further reduction of the Cr2 O3 film thickness and an increase in the operating temperature Further reduction of the Cr2 O3 film thickness is necessary to decrease the applied voltage and the aspect ratio of the recording bit However, for thin Cr2 O3 films, there is a rapid reduction in the blocking temperature T B of the perpendicular exchange bias with decreasing Cr2 O3 thickness.13 Here, T B represents the temperature at which the perpendicular exchange bias disappears and can be considered the upper limit of the operating temperature Thus, in addition to the enhancement of the N´eel temperature T N , for which several attempts have been reported recently,14–17 achieving high T B is also an important issue Thus far, perpendicular exchange bias has been reported for relatively thick Cr2 O3 films, with thicknesses ≥ 30 nm.10,12,13,18,19 The T B decreases with decreasing Cr2 O3 thickness;12,19 the T B a Author to whom correspondence should be addressed Electronic mail: nozaki@ecei.tohoku.ac.jp 2158-3226/2017/7(2)/025212/9 7, 025212-1 © Author(s) 2017 025212-2 Shimomura et al AIP Advances 7, 025212 (2017) for 30-nm-thick Cr2 O3 films is as low as 80 K when the unidirectional magnetic anisotropy energy J K (= H ex M s t FM ) is 0.26 erg/cm2 13 Here, H ex , M s , and t FM represent the exchange bias field, saturation magnetization, and thickness of the ferromagnetic layer, respectively If the Cr2 O3 thickness is further decreased, the perpendicular exchange bias is not observed, even at very low temperatures For Cr2 O3 , the fact that T B is much lower than T N (∼307 K) can be qualitatively understood using the Meiklejohn–Bean free-energy model (MB model).19,20 In the MB model, the T B is explained by the competition between the interface exchange coupling energy J ex and the product of the magnetic anisotropy energy K AF and thickness t AF of the antiferromagnet (K AF t AF ) When K AF t AF > J ex , the exchange bias is observed (unidirectional anisotropy) When K AF t AF < J ex , the exchange bias disappears and only the enhancement of the coercivity H c is observed (uniaxial anisotropy) Such appearance/disappearance of the exchange bias has also been observed for an antiferromagnetic Mn–Ir alloy.21 For the Mn–Ir alloy, the exchange bias at room temperature disappears and the H c increases when the Mn–Ir thickness decreases below ∼5 nm.21 That is, for a 5-nm-thick Mn–Ir film, the T B is approximately equal to the room temperature For Cr2 O3 , because of its small K AF (∼2.0 × 105 erg/cc at low temperature22 ) and large J ex , the T B becomes much lower than the T N even for a 250-nm-thick Cr2 O3 film.10,18 According to the MB model, to realize a high T B in thin Cr2 O3 layers, either the J ex needs to be decreased or the K AF needs to be increased Decreasing the J ex can be achieved by inserting a thin metallic spacer between the Cr2 O3 film and ferromagnet Increasing T B by inserting a Pt spacer layer has been reported, where the reduction in the J ex was confirmed as a reduction in the H ex 18,19,23 The H ex magnitude control has also been achieved using Cr 10 or Ru24 spacers A higher K AF can be achieved by doping and inducing lattice strain The K AF consists of the magnetic dipole anisotropy K MD and magnetocrystalline anisotropy (the fine structure anisotropy) K FS Increasing the K AF by Al doping has been reported for a Cr2 O3 bulk system, which is primarily caused by increasing the K FS 25 Artman et al calculated the change in the K MD induced by lattice parameter variation and found that the K MD increases with increasing c, decreasing a, or increasing ionic position w.22 Recently, Kota et al calculated the effect of lattice parameter variation on single-ion magnetic anisotropy (K FS ) and reported that the K FS increases with decreasing c or increasing a, which is the opposite of the trend for K MD 26 Their calculation results predict an enhancement of K FS of more than one order of magnitude by inducing a 1% change in the lattice parameter a That is, drastic improvement of the K AF of Cr2 O3 because of K FS enhancement is expected by increasing the lattice parameter a However, the lattice strain effect on the K AF of Cr2 O3 has not been reported experimentally Because the lattice strain can be easily induced for thin films by adjusting the lattice parameter of the buffer layer, a thin Cr2 O3 /Co system is favorable for investigating the effect of the lattice strain on the K AF In this study, we attempted to achieve T B ≈ T N for a 20-nm-thick Cr2 O3 thin film/Co exchange coupling system with a Ru spacer layer through enhancing the K AF by inducing lattice strain II EXPERIMENTAL PROCEDURES All of the samples were fabricated by RF-DC magnetron sputtering with a base pressure below × 10 Pa The sample structures were c-Al2 O3 substrate/Pt (25 nm) or α-Fe2 O3 (20 nm)/Cr2 O3 (20 nm)/Ru (t Ru nm)/Co (1 nm)/Pt (5 nm) In this study, using a Ru spacer layer, we controlled the J ex (J K ) H ex was observed for samples with Ru thicknesses > 1.00 nm, and the T B value and H ex magnitude increased and decreased, respectively, with increasing Ru thickness, as shown in Fig We also varied the buffer layer material (Pt or α-Fe2 O3 ) We chose the buffer layer thickness so that the lattice strain of the buffer layer induced by the Al2 O3 substrate was mostly relaxed The oxygen reactive sputtering technique was used for deposition of Cr2 O3 (α-Fe2 O3 ) by sputtering a Cr (Fe) metal target in an Ar/O2 mixed gas atmosphere The substrate temperature during deposition was 873 K for the Pt buffer layer, 773 K for the Cr2 O3 and α-Fe2 O3 oxide layers, and 406 K for the Ru spacer, Co ferromagnetic, and Pt capping layers The magnetic properties were measured by superconducting quantum interference device (SQUID) magnetometry after cooling the sample in the presence of an applied magnetic field (+10 kOe) from 340 K, which is sufficiently greater than the T N of Cr2 O3 During the measurements, the magnetic field was applied normal to the film 025212-3 Shimomura et al AIP Advances 7, 025212 (2017) FIG Temperature dependence of |H ex | (left panel) and H c (right panel) for c-Al2 O3 substrate/Pt (25 nm)/Cr2 O3 (20 nm)/Ru (t Ru nm)/Co (1 nm)/Pt (5 nm) surface Structural characterization was performed by X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM) measurements Nanobeam electron diffraction with a beam size of approximately 20 nm27 was used for refining the lattice parameters of the thin Cr2 O3 layer (20 nm thick) III RESULTS & DISCUSSION We investigated the effect of lattice strain induced by changing the buffer layer material on the K AF of Cr2 O3 For a buffer layer of Cr2 O3 , good lattice matching and oxidative resistance at 773 K in an O2 atmosphere (deposition conditions of Cr2 O3 ) are required In this study, we chose α-Fe2 O3 and Pt for the buffer layer materials Both Cr2 O3 and α-Fe2 O3 possess a corundum structure, so epitaxial growth of α-Fe2 O3 /Cr2 O3 is expected Pt is a face-centered cubic structure, but it preferentially orients along the [111] direction Thus, the lattice matching is relatively good In addition, both α-Fe2 O3 and Pt exhibit no degradation at 773 K in an O2 atmosphere Figure shows schematics of the (0001) planes of Cr2 O3 and α-Fe2 O3 , as well as the (111) plane of Pt Table I summarizes the lattice mismatch between bulk Cr2 O3 22 and the buffer layers For the lattice mismatch calculations, experimental lattice spacing values for 20-nm α-Fe2 O3 and 25-nm Pt on an Al2 O3 substrate were used Because α-Fe2 O3 (Pt) has larger (smaller) lattice parameters than Cr2 O3 , we expected to obtain an expanded (compressed) a-axis for Cr2 O3 using an α-Fe2 O3 (Pt) buffer We investigated the change in the K AF of these samples by measuring the T B and compared the results with the theoretical predictions The lattice parameters and morphology can be affected by changing the buffer layer Thus, we first characterized the structural properties Figure shows the XRD patterns for 2θ/ω (out of FIG Schematic diagrams of the (0001) plane of Cr2 O3 , (0001) plane of α-Fe2 O3 , and (111) plane of Pt The black solid line and blue broken line in the Pt (111) plane indicate a triangular lattice along the [110] and [112] directions, respectively 025212-4 Shimomura et al AIP Advances 7, 025212 (2017) TABLE I Experimental lattice spacing values of the buffer layer and lattice mismatch between bulk Cr2 O3 22 and the buffer layer Buffer (Cr2 O3 ) Pt α-Fe2 O3 Lattice spacing of buffer layer (difference from a of bulk Cr2 O3 22 ) 0.4961 nm22 0.4792(4) nm 0.5034(4) nm (-) ( 3.41%) (+1.47%) Orientation Pt [110]//Cr2 O3 [1120] α-Fe2 O3 [1010]//Cr2 O3 [1010] plane, Figure (a)) and 2θ χ /φ (in plane, Figure (b)) scans of α-Fe2 O3 -buffered and Pt-buffered samples The 2θ χ /φ scan was carried out for the Al2 O3 (1010) plane For the α-Fe2 O3 -buffered sample, good epitaxial growth was observed The epitaxial relations are Al2 O3 (0001) [1010]/αFe2 O3 (0001) [1010]/Cr2 O3 (0001) [1010] For the Pt-buffered sample, although good (111)-oriented Pt and (0001)-oriented Cr2 O3 were obtained, two types of domains appeared in the in-plane orientations The epitaxial relations are Al2 O3 (0001) [1010]/Pt (111) [110] or [112]/Cr2 O3 (0001) [1010] or [1120] These two types of domains are also reported in Ref 28 Figure shows crosssectional TEM images of the α-Fe2 O3 -buffered (Figure (a)) and Pt-buffered (Figure (b)) samples The TEM image confirmed the epitaxial growth of the α-Fe2 O3 -buffered sample and existence of the two types of domains in the Pt-buffered sample (domain sizes of several tens of nanometers) The different domain sizes can also affect the T B However, based on the MB model, the domain sizes perpendicular to the film surface are more important than the domain sizes in the film plane because we discuss the relationship between J ex and K AF t AF In this paper, we neglect the grain size effect, and additional investigations are underway We then checked the lattice parameter a of Cr2 O3 on α-Fe2 O3 and Pt We have previously reported the a value of α-Fe2 O3 - and Pt-buffered 20-nm-thick Cr2 O3 films from the XRD patterns.29 However, the a value of the α-Fe2 O3 -buffered sample could not be accurately determined because of the overlap between the Cr2 O3 (3030) and α-Fe2 O3 (3030) FIG XRD patterns of (a) 2θ/ω (out of plane) and (b) 2θ χ /φ (in-plane) scans of α-Fe2 O3 - (red line) and Pt-buffered (black line) samples The 2θ χ /φ scan was carried out for the Al2 O3 (1010) plane 025212-5 Shimomura et al AIP Advances 7, 025212 (2017) FIG Cross-sectional TEM images of (a) α-Fe2 O3 - and (b) Pt-buffered samples (c) Nano beam diffraction patterns of 20-nm-Cr2 O3 film of α-Fe2 O3 buffered sample The incident beam direction is [1120] Bragg peaks In the present study, we performed nanobeam electron diffraction measurements27 of the α-Fe2 O3 -buffered sample and more precisely determined the a value of the Cr2 O3 film The nanobeam electron diffraction patterns of a 20-nm Cr2 O3 film on α-Fe2 O3 buffer layer are shown in Fig (c) The a value of Cr2 O3 estimated from the nanobeam electron diffraction was the same as that of the α-Fe2 O3 buffer estimated from in-plane XRD Thus, we confirmed that the a value of Cr2 O3 is locked by the α-Fe2 O3 buffer layer From this result, we can conclude that both the α-Fe2 O3 (3030) and Cr2 O3 (3030) peaks are at 64.02◦ in Figure (b) The refined lattice parameter values are listed in Table II Both the present data and the data in Ref 29 show that the a values for α-Fe2 O3 -buffered Cr2 O3 films are larger than that of the Pt-buffered Cr2 O3 film, and both the samples have larger a values than bulk Cr2 O3 (a = 0.4961 nm22 ) The unexpected a value of the Pt-buffered Cr2 O3 could have resulted from lattice relaxation caused by misfit dislocations or dislocations at grain boundaries at the Pt/Cr2 O3 interface because of a relatively bad lattice matching ( 3.41 %), while it is difficult to identify the dislocations from TEM images (Fig (b)) Next, using samples with a 1.25-nm-thick Ru spacer, for which finite H ex and T B were obtained, we clarified the buffer layer effect on the T B using Pt- and α-Fe2 O3 -buffered samples with 20-nmthick Cr2 O3 Figure shows the temperature dependence of the J K for Pt- and α-Fe2 O3 -buffered 20-nm-thick Cr2 O3 (c-Al2 O3 /Pt (25 nm) or α-Fe2 O3 (20 nm)/Cr2 O3 (20 nm)/Ru (1.25 nm)/Co (1 nm)/Pt (5 nm)) For the J K calculation, we used the experimental value of M s t FM ∼ 1.4 × 10-4 emu/cm2 As shown in Figure 1, T B ≈ 150 K for the Pt-buffered sample A much higher T B ≈ 260 K was obtained for the α-Fe2 O3 -buffered samples Because the T N of the α-Fe2 O3 -buffered 20-nm-thick Cr2 O3 film decreased to 269 K because of the lattice strain,29 we managed to obtain T B ≈ T N for a 20-nm-thick Cr2 O3 film Because the J K values of these samples are almost equal, or slightly larger for the α-Fe2 O3 -buffered samples, based on the MB model, the K AF t AF of the αFe2 O3 -buffered samples must be much higher than that of the Pt-buffered samples In other words, the K AF of the Cr2 O3 films increased using the α-Fe2 O3 buffer layer If we assume Mauri’s domain wall model,30 higher K AF links to higher J K (H ex ) The slightly larger J K of α-Fe2 O3 -buffered samples may come from the higher K AF , although more work is required for confirmation Based on Mauri’s TABLE II Experimental values of the lattice parameter a of the Cr2 O3 layer and T B (this work) and T N 29 of Pt- and α-Fe2 O3 -buffered 20-nm-thick Cr2 O3 samples (Al2 O3 /Pt (25 nm) or α-Fe2 O3 (20 nm)/Cr2 O3 (20 nm)/Ru (1.25 nm)/Co (1 nm)/Pt (5 nm)) Buffer Pt α-Fe2 O3 a of Cr2 O3 from XRD (difference from a of bulk Cr2 O3 22 ) 0.4979(4) nm 0.5034(4) nm (+0.36 %) (+1.47 %) TB T N 29 150 K 260 K 294 K 269 K 025212-6 Shimomura et al AIP Advances 7, 025212 (2017) FIG Temperature dependence of J K for Pt- (blue solid circles) and α-Fe2 O3 -buffered (red open squares) 20-nm-thick Cr2 O3 samples (c-Al2 O3 substrate/Pt (25 nm) or α-Fe2 O3 (20 nm)/Cr2 O3 (20 nm)/Ru (1.25 nm)/Co (1 nm)/Pt (5 nm)) model, the T N change also affects the magnitude of the J K The lower T N of the α-Fe2 O3 -buffered sample (T N ≈ 269 K) compared with the Pt-buffered sample (T N ≈ 294 K) can lower the J K of the α-Fe2 O3 -buffered sample slightly In addition, we investigated the relations between J K and T B for the Pt- and α-Fe2 O3 -buffered samples with different Ru spacer thicknesses Figure shows the T B values of the Pt- and α-Fe2 O3 -buffered samples plotted against J K at 50 K Irrespective of the Ru spacer thickness, higher T B values were obtained for the α-Fe2 O3 -buffered samples, while the T B decreased with increasing J K for both samples These results clearly demonstrate that the K AF of the Cr2 O3 layer is higher when using an α-Fe2 O3 buffer layer than a Pt buffer layer Moreover, we investigated the Cr2 O3 layer thickness dependence of the J K and T B for the α-Fe2 O3 -buffered sample Figure shows the temperature dependence of the J K for Al2 O3 /α-Fe2 O3 (20 nm)/Cr2 O3 (t Cr2O3 nm)/Ru (1.25 nm)/Co (1 nm)/Pt (5 nm) with various Cr2 O3 thicknesses (3 ≤ t Cr2O3 ≤ 20) We observed an exchange bias for a 5-nm-thick Cr2 O3 sample (T B ≈ 10 K), although thinner Cr2 O3 samples exhibited no apparent exchange bias As expected from the MB model, the T B decreases with decreasing Cr2 O3 thickness, while the magnitude of the J K is almost unchanged in the ≤ t Cr2O3 ≤ 20 range The same trends were also reported for without spacer samples.12,19 These results indicate that the MB model is qualitatively applicable for this system, even for thin Cr2 O3 regions (t Cr2O3 ≤ 20) Because α-Fe2 O3 is an antiferromagnet, in addition to the lattice strain effect, an enhancement of K AF because of the interlayer interaction between the antiferromagnetic α-Fe2 O3 and Cr2 O3 can be considered, as experimentally demonstrated in a NiO/CoO system.31 However, the observed dependence of the T B on the Cr2 O3 thickness does not support this assumption If there is a large contribution of α-Fe2 O3 magnetic anisotropy through the interlayer interaction, the T B will not decrease with decreasing Cr2 O3 thickness because the α-Fe2 O3 magnetic anisotropy contribution becomes dominant with decreasing Cr2 O3 thickness However, our data suggest that the T B decreases FIG Relationship between T B and J K for Pt- and α-Fe2 O3 -buffered 20-nm-thick Cr2 O3 samples (c-Al2 O3 substrate/Pt (25 nm) or α-Fe2 O3 (20 nm)/Cr2 O3 (20 nm)/Ru (t Ru nm)/Co (1 nm)/Pt (5 nm)) at 50 K 025212-7 Shimomura et al AIP Advances 7, 025212 (2017) FIG Temperature dependence of J K for Al2 O3 /α-Fe2 O3 (20 nm)/Cr2 O3 (t Cr2O3 nm)/Ru (1.25 nm)/Co (1 nm)/Pt (5 nm) with various Cr2 O3 thicknesses (3 ≤ t Cr2O3 ≤ 20) with decreasing Cr2 O3 thickness Thus, the effect of the interlayer interaction is negligibly small in this case, probably because of the small K AF of α-Fe2 O3 (∼2 × 105 erg/cc at low temperature22 ) Using the MB model, we estimated the change in the K AF for 20-nm-thick Cr2 O3 films caused by the strain induced by the buffer layer According to the MB model, at the critical point where the exchange bias abruptly disappears, the relationship K AF = J ex /t AF holds true If we assume that the J ex is almost the same as the J K , we can estimate the K AF as K AF = J K /t AF In fact, the K AF values of FeMn/NiFe32 and IrMn/NiFe21 at room temperature have been estimated from K AF = J K /t AF cr by determining the critical antiferromagnet thickness t AF cr In this study, we estimated K AF = J K cr /t AF We determined the critical unidirectional magnetic anisotropy energy J K cr at 100 K by changing the Ru spacer layer thickness for Al2 O3 /Pt (25 nm) or α-Fe2 O3 (20 nm)/Cr2 O3 (20 nm)/Ru (t Ru nm)/Co (1 nm)/Pt (5 nm) structure samples The critical Ru spacer layer thickness was 0.75 nm (1.25 nm) for the α-Fe2 O3 - (Pt-) buffered sample, and J K cr = 0.37 erg/cm2 (0.09 erg/cm2 ) was obtained, as shown in Fig The estimated K AF values at 100 K were 1.9 × 105 erg/cc for the α-Fe2 O3 -buffered sample and 4.5 × 104 erg/cc for the Pt-buffered sample Note that the T B of these samples are not exactly 100 K, but between 100 K and 150 K Because of the rough determination of T B , the J K cr and estimated K AF values are slightly underestimated Because the calculation of the K AF using the MB model includes many assumptions, we could not obtain an exact absolute value of the K AF from these calculations However, these values can be used to compare two samples with similar structures In this study, only the buffer layers were different The other film properties, such as the Cr2 O3 layer thickness, spacer layer, and Co layer thickness, were maintained Because the precise characterization of the K AF of antiferromagnetic thin films is considerably difficult, we believe that FIG Temperature dependence of H ex and H c for Al2 O3 /α-Fe2 O3 (20 nm)/Cr2 O3 (20 nm)/Ru (0.75 nm)/Co (1 nm)/Pt (5 nm) (Fe2 O3 buffer) and Al2 O3 /Pt (25 nm)/Cr2 O3 (20 nm)/Ru (1.25 nm)/Co (1 nm)/Pt (5 nm) (Pt buffer) 025212-8 Shimomura et al AIP Advances 7, 025212 (2017) this is a good method to estimate the K AF Based on these concepts, it was found that the K AF of the α-Fe2 O3 -buffered sample is nearly four times that of the Pt-buffered sample Here, we compare the experimental results and theoretical predictions, assuming that the variation of the K AF primarily originates from the lattice strain The experimental a values of Cr2 O3 and T B values are summarized in Table II In our results, the T B increases with increasing a, indicating increasing K AF for Cr2 O3 This is the same trend as the K FS change with lattice strain.26 In addition, the estimated K AF value, nearly four times higher than the K AF value for α-Fe2 O3 buffered sample, is in acceptable agreement with the theoretical prediction.26 From these agreements, we believe the lattice strain effect on the K FS is the dominant contributor to the change in the K AF of Cr2 O3 Finally, we discuss the lattice strain effect on the T B and T N The T N data for the samples reported in Ref 29 are included in Table II The T N of an α-Fe2 O3 -buffered sample is approximately 25 K less than that of a Pt-buffered sample, and the T N decreases with increasing a There is a trade-off between the lattice strain effects on the T B and T N : the T B increases with increasing a, while the T N decreases This dependence appears to be unfavorable However, the lattice strain effect is stronger for the T B than the T N Compared with the Pt-buffered sample, a 100 K higher T B was obtained for the α-Fe2 O3 -buffered sample, while the T N reduction was approximately 25 K This is because, as discussed in Ref 26, the K FS of bulk Cr2 O3 in the equilibrium state is extremely small and very sensitive to changes in the lattice parameters Thus, it is possible to increase the T B with only a small reduction in the T N IV CONCLUSIONS We discovered a high T B ≈ 260 K for a 20-nm-thick Cr2 O3 thin film/Co exchange coupling system with a Ru spacer layer and an α-Fe2 O3 buffer layer By changing the buffer layer material from Pt to α-Fe2 O3 , a higher T B was attained The enhancement of the T B may be due to the lattice strain-induced K AF change, which we estimate was four times higher for α-Fe2 O3 -buffered Cr2 O3 film than for Pt-buffered Cr2 O3 film We also clarified the trade-off between the T N and T B with respect to the lattice strain of Cr2 O3 and demonstrated that the T B is more sensitive to lattice strain than the T N Such control of the T B is the first step toward utilizing the ME effect in Cr2 O3 thin films Combined with further improvement of material properties, these techniques for controlling the T B open up doors for device applications ACKNOWLEDGMENTS This research was partly supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST), the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Japan Government), the Murata Science foundation, and a Grant-in-Aid from the Japan Society for the Promotion of Science (JSPS) Fellows JSPS KAKENHI Grant Number 16H05975 X Chen, A Hochstrat, P Borisov, and W Kleemann, Appl Phys Lett 89, 202508 (2006) E Dzyaloshinskii, Sov Phys JETP 10, 628 (1960) D N Astrov, Sov Phys JETP 13, 729 (1961) P Borisov, A Hochstrat, X Chen, W Kleemann, and C Binek, Phys Rev Lett 94, 117203 (2005) X He, Y Wang, N Wu, A N Caruso, E Vescovo, K D Belashchenko, P A Dowben, and C Binek, Nat Mater 9, 579 (2010) T Martin and J C Anderson, IEEE Trans Magn 2, 446 (1966) T Ashida, M Oida, N Shimomura, T Nozaki, T Shibata, and M Sahashi, Appl Phys Lett 104, 152409 (2014) K Toyoki, Y Shiratsuchi, T Nakamura, C Mitsumata, S Harimoto, Y Takechi, Y Nishimura, H Nomura, and R Nakatani, Appl Phys Express 7, 114201 (2014) K Toyoki, Y Shiratsuchi, A Kobane, S Harimoto, S Onoue, H Nomura, and R Nakatani, J Appl Phys 117, 17D902 (2015) 10 T Ashida, M Oida, N Shimomura, T Nozaki, T Shibata, and M Sahashi, Appl Phys Lett 106, 132407 (2015) 11 K Toyoki, Y Shiratsuchi, A Kobane, C Mitsumata, Y Kotani, T Nakamura, and R Nakatani, Appl Phys Lett 106, 162404 (2015) 12 Y Shiratsuchi and R Nakatani, Mater Trans 57, 781 (2016) 13 Y Shiratsuchi, H Noutomi, H Oikawa, T Fujita, and R Nakatani, J Appl Phys 109, 07C101 (2011) 14 S Mu, A L Wysocki, and K D Belashchenko, Phys Rev B 87, 054435 (2013) 15 M Street, W Echtenkamp, T Komesu, S Cao, P A Dowben, and C Binek, Appl Phys Lett 104, 222402 (2014) I 025212-9 16 Y Shimomura et al AIP Advances 7, 025212 (2017) Kota, H Imamura, and M Sasaki, Appl Phys Express 6, 113007 (2013) Kota, H Imamura, and M Sasaki, IEEE Trans Magn 50, 2505404 (2014) 18 T Nozaki, M Oida, T Ashida, N Shimomura, T Shibata, and M Sahashi, Appl Phys Lett 105, 212406 (2014) 19 Y Shiratsuchi, T Fujita, H Noutomi, H Oikawa, and R Nakatani, IEEE Trans Magn 47, 3909 (2011) 20 W H Meiklejohn and C P Bean, Phys Rev 102, 1413 (1956) 21 M Tsunoda, Y Tsuchiya, T Hashimoto, and M Takahashi, J Appl Phys 87, 4375 (2000) 22 J O Artman, J C Murphy, and S Foner, Phys Rev 138, A912 (1965) 23 Y Shiratsuchi, Y Takechi, K Toyoki, Y Nakano, S Onoue, C Mitsumara, and R Nakatani, Appl Phys Express 6, 123004 (2013) 24 S P Pati, M Al-Mahdawi, S Ye, Y Shiokawa, T Nozaki, and M Sahashi, Phys Rev B 94, 224417 (2016) 25 J O Artman, J C Murphy, and S Foner, J Appl Phys 36, 986 (1965) 26 Y Kota and H Imamura, Appl Phys Express 10, 013002 (2017) 27 K Saitoh, H Nakahara, and N Tanaka, Microscopy 62, 533 (2013) 28 T Kosub, M Kopte, R Hă uhne, P Appel, B Shields, P Maletinsky, R Hăubner, M O Liedke, J Fassbender, O G Schmidt, and D Makarov, Nat Commun 8, 13985 (2017) 29 S P Pati, N Shimomura, T Nozaki, T Shibata, and M Sahashi, J Appl Phys 117, 17D137 (2015) 30 D Mauri, H C Siegmann, P S Bagus, and E Kay, J Appl Phys 62, 3047 (1987) 31 M J Carey and A E Berkowitz, J Appl Phys 15, 6892 (1993) 32 D Mauri, E Kay, D Scholl, and J K Howard, J Appl Phys 62, 2929 (1987) 17 Y ... investigated the effect of buffer layers on the blocking temperature (T B ) of perpendicular exchange bias of thin Cr2 O3 /Co exchange coupled films with a Ru spacer and revealed a high T B of 260...AIP ADVANCES 7, 025212 (2017) Enhancing the blocking temperature of perpendicular- exchange biased Cr2 O3 thin films using buffer layers Naoki Shimomura,1 Satya Prakash Pati,1... model, the T N change also affects the magnitude of the J K The lower T N of the α-Fe2 O3 -buffered sample (T N ≈ 269 K) compared with the Pt-buffered sample (T N ≈ 294 K) can lower the J K of the