CoO thin films deposited onto self-assembled nanosphere arrays

Journal Pre-proofs Exchange bias in Co/CoO thin films deposited onto self-assembled nanosphere arrays A. Sharma, J. Tripathi, S. Tripathi, Yogesh Kumar, K.C. Ugochukwu, D. Kumar, M. Gupta, R.J. Chaudhary PII: DOI: Reference:

S0304-8853(19)32320-0 https://doi.org/10.1016/j.jmmm.2020.166599 MAGMA 166599

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

6 July 2019 7 January 2020 8 February 2020

Please cite this article as: A. Sharma, J. Tripathi, S. Tripathi, Y. Kumar, K.C. Ugochukwu, D. Kumar, M. Gupta, R.J. Chaudhary, Exchange bias in Co/CoO thin films deposited onto self-assembled nanosphere arrays, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/j.jmmm.2020.166599

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2020 Published by Elsevier B.V.

Exchange bias in Co/CoO thin films deposited onto self-assembled nanosphere arrays A. Sharma1, J. Tripathi2*, S. Tripathi3, Yogesh Kumar1, K. C. Ugochukwu4, D. Kumar5, M. Gupta5 and R. J. Chaudhary5 1Department 2Dept. 3Atomic

of Physics, Manipal University Jaipur, Jaipur, India. of Physics, ISLE, IPS Academy, Indore, India.

and Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India.

4Department

of Physics, School of Physical Sciences, Federal University of Technology, Owerri, Nigeria. 5UGC

DAE CSR, University campus, Khandwa Road, Indore, India.

Abstract Looking at the importance of exchange bias (EB) studies in novel magnetic systems, magnetic nanocaps consisting of antiferromagnetic (AF)/ferromagnetic (FM) layers were synthesized using nanosphere lithography, followed by ion beam sputtering. For this, CoO/Co/Si and CoO/Co/polystyrene nanosphere (PS) (800nm diameter) ultra-thin and thick films were prepared by in-situ oxidation. Room temperature magnetization measurements exhibit a clear distinction between the films on PS and those on plane base Si substrate (reference films). Low temperature field cooled measurements on CoO/Co(5nm)/PS film show a negative EB in both original (-37.4 mT) and trained hysteresis loops (-31.4 mT). However, corresponding reference films show larger EB values in comparison with those of PS substrate counterparts. Due to a higher Co layer thickness, the CoO/Co(100nm)/PS film shows a lower EB value (-18.2 mT) in comparison with its corresponding CoO/Co(5nm)/PS ultra-thin film (-37.4 mT). However, for the corresponding reference films, the ultra–thin film shows a higher EB (-68.5 mT) than the thick film (-12.6 mT), which is a generally observed behavior. The overall results are discussed in terms of the curvature induced modifications 1

in the microstructural properties, which cause drastic changes in the magnetic properties of such nanostructures. Keywords: Co/CoO; Exchange bias; Nanosphere; Magnetic; XRD; XRR; VSM

*Corresponding

author: J. Tripathi, E-mail address: [email protected]

1. Introduction Exchange bias (EB) is a well-established phenomenon in which a shift in hysteresis loop is observed when a system consisting of adjacent antiferromagnetic (AF) and ferromagnetic (FM) layers is cooled down through the Neel temperature of the AF material, in the presence of external magnetic field [12]. Since its discovery in CoO/Co system in 1956 [3], it has gained increasing importance, owing to its applications in magnetic data storage devices, sensors and spintronics [4], which is further advanced by the introduction of magnetic nanostructures, such as bilayers, multilayers, alloys and core-shell nanoparticles [5-8]. For device applications, the microstructure of these systems should be precisely controllable, as it is one of the deciding factors that governs the overall performance of the resulting device [9-16]. These microstructural properties include layer thicknesses, composition, concentration, interlayer coupling, grain size, growth /deposition parameters and layer properties [1718]. Although, a wide range of literature exists on the studies of magnetic properties of thin films containing cobalt, fabricated on plane substrates, but only a few can be found on patterned substrates [19-23]. A combination of AF CoO and FM Co on periodic modulated substrate, prepared by Choi et al. (using electrodeposition method) showed unidirectional as well as perpendicular magnetic anisotropy, leading to two asymmetries in low temperature hysteresis loops [24]. They discussed the modifications in coercivity and EB in comparison with a reference plane film and attributed the 2

anomaly to angular distribution of Co and CoO spins. Another exchange bias study on CoO/Co bilayer system was performed by Chang et al., who studied layer thickness dependent magnetic properties and found that reducing the volume of AF grains in ultrathin CoO/Co bilayer enhanced exchange bias field (HE) due to increased AF/FM pinning sites [25]. In yet another study, the influence of underlying sphere size on EB was investigated in order to understand the effect of plane film dots deposited in between the nanospheres, and the study found that the plane film dots made no or negligible contribution to the HE values, due to very small layer thickness [26]. Curved surfaces have been shown to greatly increase HE values up to 4 times the value obtainable from films of same thickness deposited on plane substrate [27]. The same study showed that HE value is enhanced in nanocaps on samples comprising of bigger nanosphere size as compared to smaller nanosphere size samples. This was ascribed to localized uncompensated AF spins in the samples. Apart from this particular CoO/Co system, several reports exist in the literature covering the magnetic properties of thin films on polystyrene nano or microspheres, including those of Co/Pt multilayers, [Pt/Co]5/IrMn multilayers and IrMn/CoFe bilayers [28-29]. However, a detailed investigation on the dependence of exchange bias on the film thickness in these films on curved surface is still lacking. These magnetic nanocaps show a large variation in film thickness across the top hemisphere of the nanospheres surface, otherwise known as nanocap. Looking at this lack of detailed literature on the relationship between exchange bias and magnetic nanocap film thickness, the present study is aimed at detailed investigation on structural properties and magnetic properties of CoO/Co films deposited on polystyrene nanospheres. The focus is on how thickness of the nanocap film affects the exchange bias in ultra-thin (5nm) and thicker (100nm) nanocap films.

3

2. Experimental To ensure the preparation of high purity films on clean substrates, Si wafers were thoroughly cleaned by standard process using ultrasonic agitation in acetone, then in methanol, followed by drying in N2 gas. To prepare curved substrates, nanosphere lithography method was utilized. Selfassembled polystyrene nanosphere of 800 nm diameter were first deposited on ultrasonically cleaned Si, which then acted as curved substrates for depositing Co(5 nm) and Co(100 nm) by ion beam sputtering technique. Before deposition of Co films, the base pressure in the ion beam sputtering deposition chamber was 2.25 x 10-7 Torr, which increased to 1. 5 x 10-5 Torr during deposition. The Argon ion beam parameters were: 1 keV (beam energy) and 40 mA (beam current). The distance between the substrate and ion gun was fixed at 12 cm, the deposition rate of Co was 2.6 nm/min. Reference films of Co(5 nm) and Co(100 nm) were also deposited directly on Si substrate under the same vacuum conditions as described above. The nanosphere lithographed samples and reference films were then subjected to in situ oxidation by introducing O2 partial pressure of 7.5 x10-4 Torr into the chamber for 50 sec., resulting in the formation of outer anti-ferromagnetic (CoO) layer adjacent to the inner ferromagnetic (Co) layer in both the reference and curved substrate films. The morphological changes between plane reference and curved films were compared using atomic force microscopy (AFM) performed on Veeco instrument INC USA (Model: multimode SPM Nanoscope IVA). X- ray diffraction and X-ray reflectivity measurements were carried out using Cu kα (0.154 nm ) radiation on Bruker D8 advance diffractometer with 40 kV, 40 mA setting. Magnetic measurements at room and low temperatures were carried out on MPMS 7T SQUID-VSM, QUANTUM DESIGN instrument. For these measurements, applied magnetic field was kept in a direction parallel to the sample surface. For low temperature exchange bias

4

measurements, a cooling field strength of 1.0 T was applied and the sample temperature was lowered to 5 K before measurements were made.

3. Results and discussion: 3.1 AFM measurements: Figure 1 shows the schematic of Co/PS films, where curvature dependent thickness variation along the nanosphere is clearly depicted. As seen, the thickness deposited is highest at the top, as this portion received the highest flux, while it decreases along the circumference of the nanosphere, practically reaching no deposition condition at the areas where adjacent nanospheres touch each other (by the sides), thus forming nanocap structures which are isolated from each other. However, on base Si substrates, there is no variation in reference film thickness and the film is uniform all over the substrate surface. The morphology of the films strongly depends on the morphology of underlying substrates, and this is clearly visible from the representative AFM micrographs shown in figure 2(a) and (b) for the PS and the plane reference films respectively. These results taken from our previous study [30] on [CoO/Co(100nm)] film deposited on plane reference Si substrate [figure 2(b)], looks smooth with many bigger grains spread uniformly over the imaged area, some tens of nanometer in diameter. The calculated root mean square (RMS) roughness of this film is small, only about 2.8 nm. On the other hand, the corresponding curved film [CoO/Co(100nm)/PS] of figure 2(a), shows a different morphology owing to uniformity and compactness. These nanospheres show hexagonal morphology and this dominates the overall morphology of the samples. The surface of the nanospheres provides a highly rough underlayer for the growth of deposited films, thereby resulting in a very high surface RMS roughness of about 70.2 nm. This roughness mainly

5

originated from the wavy nature of the nanosphere and a small contribution comes from the grains inside the film. To ascertain the average diameter of the nanosphere after the Co deposition, several AFM measurements were done on different portions of the curved film [CoO/Co(100nm)/PS] and analyzed, the estimated diameters in the two directions were compared by plotting profiles from the horizontal and vertical directions of the images. It was found that some of the diameters along the profiles were a bit larger (834.3 nm, 830 nm and 840 nm) than the designated diameter of the nanospheres (800 nm) due to the contribution of the overlaying Co film which increased the lateral dimension which is measured in AFM profile. Corresponding height scale (shown adjacent to the images) also confirms the curvature induced morphological modifications as depicted in the schematic of the thin film growth model depicted in figure 1.

3.2 XRD measurements: Before performing layer characterization, the crystalline properties were also checked using the same diffractometer settings. The X-ray diffraction measurements (see figure 3) revealed the polycrystalline nature of deposited films with the combination of CoO and Co hcp crystalline peaks. However, a Co peak with (101) orientation was observed as the strongest peak at 47.7o 2θ position and also Co (002) reflection at 44.47°, CoO (111) at 46.04° and CoO (110) at 46.33° 2θ positions were observed. All these observed peaks have been referenced with JCPDS file no: 01-071-4239, 78-0431 and 72-1474. This also indicated that no crystalline impurity phase could be detected. Magnetic properties of thin films are sensitive to small changes in structure such as introduction/removal of lattice strain, and this plays an important role in deciding the overall magnetic properties of the film [31]. Perpendicular magnetic anisotropy (PMA) in Co film has

6

been attributed to the presence of large strain in the film [32]. Spin reorientation transition (SRT) occurs from in plane to out of plane magnetization with strain relaxation [33]. In order to investigate this effect, we have calculated the strain in the Co films. This strain can be calculated from the XRD patterns. There are two factors which contribute to the total peak broadening in Xray diffraction pattern. Apart from average crystallite size effect, peak broadening occurs due to lattice strain which is always present in thin films. Strain induced broadening and particle induced broadening can be distinguished with Williamson-Hall plot [34] which was drawn for the Co films deposited on both substrates and can be seen in the inset of each respective figure. To calculate the strain in Co and CoO (101), (002), (111) and (110) diffraction peaks, the following equation was used: 𝛽𝑐𝑜𝑠 𝜃 = 2𝜀𝑠𝑖𝑛 𝜃 +

𝐾𝜆 𝐿

(1)

In equation (1), ε is the strain, 𝛽 is full width at half maximum (FWHM), the term

Κ𝜆 𝐿

represents

the grain size and K is a correction factors whose general value is taken as 1. The strain values calculated for Co peaks using the above relation are found to be -3.1 x 10-3 and -3.4 x 10-3 respectively for the 5 nm film and 100 nm respectively, for the Co films deposited on PS. However, in the case of the Co 5nm and 100 nm films deposited on Plane Si substrates, the strain values were -3.3 x 10-3 and -4.7 x 10-3 respectively. The small values of the strain, which is compressive, calculated from W. H. loops suggest that in the present case the main source of peak broadening is due to grain size, while the effect of strain is negligible.

3.3 XRR measurements: XRR measurements were performed to understand the influence of curvature on the surface and interface properties of CoO/Co/PS films. For comparison, similar measurements were also carried 7

out on plane reference films [see figure 4]. However, due to very high roughness, the fitting was not possible on curved samples and their data were used for qualitative comparison only. As seen in the figure, the reference films show the presence of oscillations beyond critical angle (known as Kiessig oscillations), which are far apart in ultra-thin film (5nm), but are closer and more in number in the thick film (100 nm). The presence of these Kiessig fringes is a clear indication of the interface quality. In all these films, there are three interfaces namely air/CoO, CoO/Co and Co/substrate interfaces. In reference films, the interfaces are sharper than those on PS due to smoother morphology of the Si substrate in comparison with the PS surface. Reflectivity data were analyzed using the Parratt 32 XRR profile fitting program [35]. The layer parameters such as thickness, surface or interface roughness and electron density were calculated from the fitted model and are given in Table 1. The densities of these films are less than their corresponding bulk Co (8.9 g/cm3) and CoO (6.44 g/cm3) films, which agrees with the fact that thin films have several defects leading to less dense samples. It is also seen that with increasing film thickness, the roughness also increases due to bigger grain growth. In the case of the films on PS substrates, the samples are very rough and hence Kiessig oscillations disappear. The decrement in overall reflection of any sample is a combined effect of layer smoothness, surface /interface roughness and defects/dislocations present in the sample as well as the electron density contrast between adjacent layers. Although both types of films were deposited on the substrates using normal flux incidence, there is a thickness variation along the circumference of the nanospheres of the PS substrate samples. Because of this, in a single lateral direction, for example; several different thicknesses are encountered and hence their combined contribution appears smeared out in the oscillations from the CoO/Co/PS samples. The X-ray wave front covers a large area on the sample, covering a large amount of such nanospheres thereby producing a mixed layer structure rather

8

than sharp individual layer structure. A very interesting feature of the XRR patterns on PS samples is the appearance of two critical angles as compared to single angle generally observed in thin films. Out of these two critical angles (marked in respective curves of figure 4), one is due to the presence of Co or CoO and the other one is due to the nanospheres (which are made of polystyrene polymer). This critical angle depends on the electron density of the material and its interaction with the incident X-rays. This complex interaction in the CoO/Co reference films is different from that in the CoO/Co nanosphere film, giving rise to two critical angles falling at two different 2θ positions. The critical angle is also different from corresponding bulk values, which can be attributed to intermixing, thickness variation, curvature as well as modification in electron density contrast in these samples [36-37].

3.4. Magnetic measurements: For room temperature hysteresis loop recording, VSM instrument was used, while for low temperature measurements, better sensitivity of SQUID-VSM instrument was utilized. To record the hysteresis loops during room temperature measurements, the magnetic field was applied in parallel direction with respect to the sample surface and the hysteresis was recorded till the complete saturation of magnetic moment was achieved. As already established by several researchers in the past [38-40], Co thin films exhibit a square shaped loop with switching action as reflected in figure 5 (magnetization loop of Co films on plane substrates). However, when curved under layer is introduced, the square shape changes to an “S” type shape (figure 5, magnetization loop of Co films on PS substrates). For the reference films, the ultrathin and thick films show similar remanence and coercivity values (2.6 mT in Co 5nm, while 2.5 mT in Co 100 nm film). Compared to these, the films on PS substrate show reduction in remanence

9

values, which are thickness dependent and decrease drastically in thicker film (Co 100nm). At this thickness, the hysteresis loop shape becomes tilted. The coercivity values show significant increment as compared to reference samples, becoming 6.5 mT and 5.4 mT for the 5 nm and 100 nm Co samples on PS substrates respectively [30]. It is well known that in continuous films, the magnetization reversal process takes place through domain wall propagation, which overcomes nucleation barriers at low coordination sites; like defects or grain boundaries [41-43]. In patterned films like in the present case of films on PS, the magnetization reversal process is not well understood yet, as further complexity is introduced by the nanostructure (thin film) formation. In such systems, each nanocaps behaves as a tiny magnet containing a domain. The switching of the individual domains results in an increment in coercivity. As the film thickness increases, due to addition of more atoms during the deposition process, the coercivity decreases. In contrast to the reference film Co 100 nm (that acts more like a bulk material), the corresponding patterned Co 100 nm on PS still exhibits different properties, which arise from the fact that the thickness of the deposited layer varies at every point along the circumference of the nanospheres. To investigate the exchange bias properties, samples were cooled down in the presence of a cooling field (1.0 T). At low temperature (5 K) the field cooled hysteresis loops are recorded and shown in figure 6. The reference film (Co, 5nm) seems to show square loop, while the nanosphere patterned (Co, 5nm) film has an “S” shaped loop. However, in decreasing branch, there is visible deviation from the “S” type shape. In the decreasing branch, it shows anomaly and a careful observation reveals the presence of two hysteresis loops, which are joint together. This is because Co deposited on nanosphere assembly also gets deposited in between the nanospheres and thus there is contribution from patterned Co as well as plane Co dots deposited on the exposed Si substrate in between the nanospheres. This effect is pronounced at low temperature on ultra-thin film.

10

Trained loops in all the patterned films show decrement in exchange bias as compared with the fist loops scan, which is ~ 84% reduction in the case of ultra-thin film on PS. On the other hand, plane films do not show any appreciable changes between the two successive hysteresis loops. The magnetic parameters calculated from these hysteresis loops are presented in table 2. The higher exchange bias in the case of the PS substrate is described using the model shown in figure 7. It schematically presents the possible distribution of magnetic spin on and inside the CoO/Co films on PS. For ultra- thin film, it may easily be assumed that along the edges, due to very small deposition, the film has oxidized completely, forming a non-magnetic CoO layer. Since it does not contribute to magnetization, it can be referred to as “magnetically dead region”. The PS patterned films are thickest at the PS apex resulting in non-uniform thickness of the Co layer on the PS after the oxidation process. Insitu oxidation of Co films always produces a self-passivating CoO layer of about 2 nm [44], hence leaving about 3 nm of Co layer underneath the CoO layer in the 5 nm Co nanocap film. This Co layer contributes to magnetization and is therefore termed magnetically active region. In higher thickness films (100 nm), both the PS structured film and the reference films showed reduction in the EB value in line with the well kown 1/𝑡𝐹𝑀 dependence of EB value in bilayer CoO/Co films [45]. The interface roughness affects the EB through the modification of AF and FM spin arrangement, there by producing uncompensated spins at the interfaces. As supported by XRR results, PS patterned films possess very rough interfaces which strengthen the interfacial spin coupling between the AF/FM layers. This introduces higher exchange bias. Studies on Co/CoO films deposited on plane substrate have revealed that even in plane films, surface roughness plays an important role as exchange bias depends on it [46]. It has been clearly observed in Co/CoO/Co trilayer that upper interfaces were more disordered than the lower interfaces, and hence exhibited more exchange bias

11

(EB) and that the presence of uncompensated AF spins strengthens the coupling thereby increasing the exchange bias [47]. These results suggest that exchange bias can be controlled in patterned magnetic nanocaps by controlling the film parameters such as film thickness, surface/interface roughness, diameter of the nanospheres and the deposition parameters. Conclusion Exchange bias studies supported by structural and morphological characterizations on CoO/Co/PS and CoO/Co/Si ultra-thin and thick films are reported. The obtained exchange bias values show drastic modifications in both ultra-thin and thick films, where the underlying substrate was changed from base plane Si to curved 800 nm diameter PS nanospheres, which provide curvature induced roughness to the over-layer. The hexagonal symmetry in the film deposited on PS as observed by AFM is in agreement with the concept of magnetic nanocaps as also reported in literature. Due to this curved behavior, nanosphere impart very high surface /interface roughness on CoO/Co/PS films. The magnetic properties are also discussed in terms of their film thickness dependency. The observed variations, mainly the exchange bias values are correlated with the curvature induced variations in structural properties.

Acknowledgment The authors A. Sharma, J. Tripathi and Yogesh Kumar are thankful to UGC-DAE-CSR, Indore (M.P.), India (Ref. No. CSR-IC/CRS-148/2015-16/05 dated 26/03/2016) for providing grant for research work. We would also like to thank Dr. V. R. Reddy, UGC-DAE CSR, Indore for XRR measurements.

12

References: 1. J. Nogués and Ivan K.Schuller, J. Magn. Magn. Mater. 192 (1999) 203 2. M.D. Baro Phys. Reports 422 (2005)65; J. I. Martín, J. Nogués, Kai Liu, J. L. Vicent and Ivan K. Schuller, J. Magn. Magn. Mater. 256 (2003) 449. 3. W.H. Meiklejohn and C.P. Bean, Phys. Rev., 102 (1956) 1413. 4. Ch. Binek and B. Doudin, J. Appl. Phys., 97 (2005) 10C514. 5. H. Saglam, J. C. Rojas-Sanchez, S. Petit, M. Hehn, W. Zhang, J. E. Pearson, S. Mangin, and A. Hoffmann, Phys. Rev. B 98 (2018) 094407. 6. H. Gao, T. Harumoto, W. Luo, R. Lan, H. Feng, Y. Du, Y. Nakamura and J. Shi, J. Magn. Magn. Mater. 473 (2019) 490. 7. B. Y. Wang, C. W. Shih, M. S. Tsai, C. J. Chen, K. Lin, J. Jhuan, L. C. W. Huang, W. C. Lin and S. C. Weng, J. Alloy and Comp. 748 (2018) 223. 8. R. Wu, S. Ding, Y. Lai, G. Tian and J. Yang, Phys. Rev. B 97 (2018) 024428. 9. J. Yu, A. D. Kent and S. S. P. Parkin, J. Appl. Phys. 87 (2000) 5049. 10. J. C. S. Kools, J. Appl. Phys. 77 (1995) 2993. 11. K. Wang, J. G. M. Sanderink, T. Bolhuis, W. G. van der Wiel and M. P. de Jong, Scientific Reports 5 (2015) 15498. 12. K. Motohashi, N. Ikeda, T. Sato, D. Shiga, H. Ono and S. Onodera, J. Mag. Mag. Mat. 320 (2008) 3004. 13. A. Manchon, J. Železný, I. M. Miron, T. Jungwirth, J. Sinova, A. Thiaville, K. Garello and P. Gambardella Rev. Mod. Phys. 91 (2019) 035004. 14. K. Shimizu, J. Tachibana, Y. Arisaka and K. Sato IEEE Transactions on Magnetics 40 (2004) 2398. 13

15. R. Q. Zhang, J. Su, J. W. Cai, G. Y. Shi, F. Li, L. Y. Liao, F. Pan and C. Song, Appl. Phys. Lett. 114, (2019) 092404. 16. S. Parkin, Xin Jiang, C. Kaiser, A. Panchula, K. Roche, M. Samant, Proceedings of the IEEE 91 (2003) 661. 17. J. V. Driel, F. R. de Boer, K.-M. H. Lenssen and R. Coehoorn, J. Appl. Phys. 88 (2000) 975. 18. D. Martien, K. Takano and A. E. Berkowitz and D. J. Smith, J. Appl. Phys. 74 (1999) 1314. 19. M. Albrecht, G. Hu, I. L. Guhr, T. C. Ulbrich, J. Boneberg, P. Leiderer and G. Schatz, Nat. Mater. 4 (2005) 203. 20. A. Sharma, J. Tripathi, K. C. Ugochukwu and S. Tripathi, Solid State Commun., 254 (2017) 1. 21. A. Sharma, S. Tripathi, K. C. Ugochukwu, J. Magn. Magn. Mater., 326 (2013) 97. 22. M. M. Soares, E. de Biasi, L. N. Coelho, M. C. dosSantos, F. S. De Menezes, M. Knobel, L. C. Sampaio and F. Garcia Phys. Rev. B 77 (2008) 224405. 23. E. Amaladass, B. Ludescher, G. Schütz, T. Tyliszczak, and T. Eimüller, Appl. Phys. Lett. 91 (2007) 172514. 24. H. I. Choi, M. Huang, X. Liu, S. Moeendarbari, J. P. Liu and Y. Hao, J. Nanosci. Nanotech, 17 (2017) 5115. 25. C. H. T. Chang, S. C. Chang, J. S. Tsay and Y. D. Yao, Appl. Surf. Sci., 405 (2017) 316. 26. Y. X. Wang, Y. J. Zhang, X. Ding, Y. Cheng and J. H. Yang, Solid State Communications, 148 (2008) 83.

14

27. Y. X. Wang, N. N. Yang, M. B. Wei, Y. J. Zhang, H.B. Liu, Y. Liu, J. H. Yang, J. Alloy and Comp., 509 (2011) 6626. 28. W. J. Li, Q. Zhang, K. Javed, H. Wei, A. Chen, Y. Zhao and X. F. Han, IEEE Magn. Lett, 6 (2015) 1. 29. S. Thomas, D. Nissen and M. Albrecht, App. Phys. Lett., 105 (2014) 022405. 30. A. Sharma, J. Tripathi, Y. Kumar, D. Kumar, M. Gupta, R. J. Chaudhary, and S. Tripathi, AIP Conference Proceedings, 2115 (2019) 030312. 31. G. Andersson and B. Hjorvarsson, Phase Transitions, 81 (2008) 679. 32. A. Walczak, T. Lusarski, A. L. Szweykowska and G. Kamieniarz, Acta Physica Polonica A 121 (2012) 653. 33. J. Miyawaki, D. Matsumura, H. Abe, T. Ohtsuki, E. Sakai, K. Amemiya and T. Ohta, Phys. Rev B 80, (2009) 020408. 34. A. Sharma, J. Tripathi, S. Tripathi, K.C. Ugochukwu, Physica B: Condensed Matter 560 (2019) 81. 35. C. Braun, PARRATT32-the reflectivity tool, HMI Berlin, 1997. 36. C. Wiemer S. Ferrari, M. Fanciulli, G. Pavia, L. Lutterotti, Thin Solid Films, 450 (2004) 134–137. 37. A. C. Ferrari, A. Libassi, B. K. Tanner, V. Stolojan, J. Yuan, L. M. Brown, S. E. Rodil, B. Kleinsorge and J. Robertson, Phys. Rev. B, 62 (2000) 1189. 38. V. Neu and S. A. Shaheen, J. Appl. Phys., 86 (1999) 7006. 39. A. Singh, V. Neu, S. Fähler, K. Nenkov, L. Schultz and B. Holzapfel, Phys. Rev B, 77 (2008)104443.

15

40. Y. L. Iunin, Y. P. Kabanov, V. I. Nikitenko, X. M. Cheng, D. Clarke, O. A. Tretiakov, O. Tchernyshyov, A. J. Shapiro, R. D. Shull and C. L. Chien, Phys. Rev. Lett., 98 (2007) 117204. 41. M. Tofizur Rahman, Nazmun N. Shams, Ding Shuo Wang, and Chih-Huang Lai, Appl. Phys. Lett., 94 (2009) 082503. 42. J. Sort, B. Dieny, M. Fraune, C. Koenig, F. Lunnebach, B. Beschoten, and G. Guntherodt, Appl. Phys. Lett., 84 (2004) 3696. 43. M Tofizur Rahman, Nazmun N Shams and Chih-Huang Lai, Nanotechnol., 19 (2008) 325302. 44. S. Brems, D. Buntinx, K.

Temst, and C. Van Haesendonck, Phy.Rev. Lett., 95

(2005)157202. 45. K. A. Seu, H. Huang, J. F. Lesoine, H. D. Showman, W. F. Egelhoff, Jr. Gan, L. Gan, and A. C. Reilly, J. Appl. Phys. 19 (2003) 6611. 46. D. Kumar, S. Singh, and A. Gupta, J. Appl. Phys., 120 (2016) 085307. 47. J. Wang, T. Sannomiya, J. Shi and Y. Nakamura, J. Appl. Phys., 111 (2012) 07D725.

16

Figure 1: A model showing different growth of the Co film on plane and polystyrene substrates.

(a)

(b)

Figure 2: 2d AFM images of Co (100 nm) films deposited on (a) PS and (b) plane Si substrate respectively.

17

 = -3.4 X 10 0.40

Cos

o

0.0018

CoO/Co(5nm)/PS

0.0024 0.0020

0.0016

Cos

-3

Co(101)

 = -3.1 X 10

50

CoO/Co(5nm)/Si (002)

 = -3.3 X 10

-3

0.0014

(101) 0.0012 0.0010

Co(002)

Sin

0.40

0.41

(110)

0.39

Sin

(002) - JCPDS file n : 01-071-4239 o (111) - JCPDS file n : 78-0431 o (110) - JCPDS file n : 72-1474 o

44

0.38

Co(002)

0.39

CoO(110)

0.38

0.40

0.41

CoO(110)

(111)

(101)

(111)

(002)

CoO(111)

0.0016

42

48

46

44

42

(110)

CoO(111)

Cos

50

48

46

0.0036

0.0012

0.41

(002) - JCPDS file n : 01-071-4239 o (111) - JCPDS file n : 78-0431 o (110) - JCPDS file n : 72-1474

44

0.0028

0.40

Sin

o

0.0032

0.39

Co(101)

Co(002)

0.38

(002) - JCPDS file n : 01-071-4239 o (111) - JCPDS file n : 78-0431 o (110) - JCPDS file n : 72-1474

42

-3

(110)

0.0006

0.41

CoO(111)

Sin

 = -4.7 X 10

0.0008

CoO(110)

0.39

(111)

0.0010

CoO(111)

0.38

0.0014 0.0012

Co(002)

0.0006

-3

Co(101)

(110)

CoO/Co(100nm)/Si

(101)

(002)

CoO(110)

Cos

(111)

0.0010 0.0008

Intensity (arb.u)

0.0016

(101)

0.0014 0.0012

0.0018

CoO/Co(100nm)/PS

(002)

Co(101)

0.0018 0.0016

(002) - JCPDS file n : 01-071-4239 o (111) - JCPDS file n : 78-0431 o (110) - JCPDS file n : 72-1474 o

46

48

50

42

44

46

48

2 (degree) Figure 3: XRD patterns of CoO/Co (5 nm and 100 nm)/films deposited on PS and CoO/Co (5 nm and 100 nm)/films deposited on Si substrates. Inset shows the W-H plot used to calculate lattice strain in each sample. Symbol ϵ represents lattice strain calculated from W-H plot.

18

50

1

Exp. Fitted

CoO/Co (100nm) / Si

CoO/Co (100nm) / PS

1 0.1

0.1

Normalized Reflectivity (a.u)

0.01 0.01

1E-3 1E-4

1E-3 1E-5 1E-4

1E-6 0.05

0.10

0.15

0.20

0.05

0.10

0.15

0.20 10

10

CoO/Co (5nm) / Si

CoO/Co (5nm) / PS

1

Exp. Fitted

1 0.1

0.1

0.01

0.01

1E-3 1E-3 1E-4 1E-4

1E-5

1E-5

1E-6

1E-6

1E-7

1E-7 0.05

0.10

0.15

0.20

0.05

0.10

0.15

1E-8 0.20

-1 qz(Ao)

Figure 4: XRR patterns of CoO/Co (5 nm and 100 nm)/films deposited on PS substrates and CoO/Co (5 nm and 100 nm)/films deposited on Si substrates.

19

1.0

CoO/Co (100nm) / PS

1.0

0.5

0.5

Normalized moment(arb.u.)

CoO/Co (100nm) / Si

0.0

0.0 1.0 0.5

-0.5

-0.5

0.0 -0.5

-1.0

-1.0

-1.0 -150 -100 -50

-50 1.0

-40

-30

-20

-10

0

10

20

0

30

50

100 150

40

50

CoO /Co (5nm) / PS

-50

1.0

0.5

0.5

0.0

0.0

-0.5

-0.5

-1.0

-1.0

-50

-40

-30

-20

-10

0

10

20

30

40

50

-40

-30

-20

-10

0

10

20

30

40

50

0

10

20

30

40

50

CoO/Co (5nm) / Si

-50

-40

-30

-20

-10

0H(mT)

Figure 5: Room temperature magnetization measurements done on CoO/ Co (5 nm and 100 nm)/ thin films deposited on PS and CoO/ Co (5 nm and 100 nm)/ thin films deposited on plane Si substrates. Inset shows the saturated hysteresis loop.

20

FC1 FC2

1.0

Normalized moment(arb.u.)

CoO/Co (100nm) / PS

CoO/Co (100nm) / Si

0.5

0.5

0.0

0.0

-0.5

-0.5

-1.0

-1.0

-600

-200

0

200

400

600

FC1 FC2

1.0

0.5

-400

FC1 FC2

1.0

-600

-400

-200

0

200

400

600

0

200

400

600

FC1 FC2

1.0

CoO/Co (5nm) / Si

CoO/Co (5nm) / PS

0.5

0.0

0.0 1.0

0.5

-0.5

-0.5 0.0

-0.5

-1.0

-1.0

-1.0 -1000 -800 -600 -400 -200

-600

-400

-200

0

0

200 400 600 800 1000

200

400

600

-600

-400

-200

0H(mT)

Figure 6: Low temperature (5 K) measurements done on CoO/ Co (5 nm and 100 nm)/ thin films deposited on PS and CoO/ Co (5 nm and 100 nm)/ thin films deposited on plane Si substrates. Inset shows the saturated hysteresis loop.

21

Active region

Inactive region

Inactive region

Active region

5 nm

100 nm

Figure 7: Different orientation of Co spins in 5nm and 100 nm thin films deposited on PS substrates.

22

Table 1: Different microstructure parameters calculated from XRR patterns of Co films deposited on Si substrate. Thickness

5nm

100nm

Thickness (d/Å ) Air CoO Co Bulk Air CoO Co Bulk

N/A 38.11 55.47 N/A N/A 44.63 966.33 N/A

Electron density (ρ/ -2) 0E+0 6.189E-5 7.254E-5 5.266E-5 0E+0 2.695E-5 6.413E-5 2.015E-5

Å

Imaginary electron density [(ρ)/ Å -2]

Roughness (σ / Å)

0E+0 2.74E-06 9.129E-06 4.588E-07 0E+0 2.74E-06 7.317E-06 4.588E-07

N/A 3.668 9.951 5 N/A 4.012 6.34 5

Table 2: Magnetization parameters calculated from hysteresis loops. Sample Name

FC1(at 5 K)

FC2 (at 5 K) Room temperature

(IBS deposited) Exchange bias (mT) CoO/Co(100nm)/PS -18.2 CoO/Co(100nm)/Si -12.6 CoO/Co(5nm)/PS -37.4 CoO/Co(5nm)/Si -68.5

Coercivity (mT) 50.4 44.6 31.8 35.7

Exchange bias (mT) -18.2 -9.4 -31.4 -68.5

Coercivity (mT) 50.4 33.2 21.8 35.4

Coercivity (mT) 5.4 2.5 6.5 2.6

A. Sharma: Conceptualization, Methodology J. Tripathi: Supervision, S. Tripathi: Writing- Original draft preparation Yogesh Kuma: Investigation K. C. Ugochukwu: Writing- Reviewing and Editing D. Kumar: Data curation, Investigation M. Gupta: Data curation, Investigation and R. J. Chaudhary: Data curation, Investigation

23

The authors A. Sharma, J. Tripathi and Yogesh Kumar are thankful to UGC-DAE-CSR, Indore (M.P.), India (Ref. No. CSR-IC/CRS-148/2015-16/05 dated 26/03/2016) for providing grant for research work.

Highlights:     

Antiferromagnetic/ferromagnetic nanocaps were synthesized by nanosphere lithography Exchange bias on CoO/Co/PS and CoO/Co/Si ultra-thin and thick films were studied These ultra-thin and thick films were prepared by in-situ oxidation Exchange bias values show drastic modifications on magnetic nanocaps Results are correlated with curvature induced variations in structural properties

24