preparation of l11 copt mgo l11 copt tri layer film on ru 0001 underlayer

The lower and the upper CoPt layers involve metastable L11structure, whereas the crystal structure of MgO layer is B1.. The present study shows that an epitaxial L11-CoPt/MgO/L11-CoPt tr

Preparation of L11-CoPt/MgO/L11-CoPt tri-layer film on Ru(0001) underlayer

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Citation: AIP Advances 6, 056103 (2016); doi: 10.1063/1.4943060

View online: http://dx.doi.org/10.1063/1.4943060

View Table of Contents: http://aip.scitation.org/toc/adv/6/5

Published by the American Institute of Physics

Preparation of L11-CoPt/MgO/L11-CoPt tri-layer film

on Ru(0001) underlayer

Mitsuru Ohtake (大竹充 ),1,2, aDaisuke Suzuki (鈴木大輔 ),2

Masaaki Futamoto (二本正昭 ),2Fumiyoshi Kirino (桐野文良 ),3

and Nobuyuki Inaba (稲葉信幸 )4

1Faculty of Engineering, Kogakuin University, Hachioji, Tokyo 192-0015, Japan

2Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112-8551, Japan

3Graduate School of Fine Arts, Tokyo University of the Arts, Taito-ku, Tokyo 110-8714, Japan

4Faculty of Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan

(Presented 15 January 2016; received 6 November 2015; accepted 4 December 2015;

published online 25 February 2016)

A CoPt/MgO/CoPt tri-layer film is prepared on an Ru(0001) single-crystal underlayer

at 300 ◦C by ultra-high vacuum magnetron sputtering The growth behavior and the crystallographic properties are investigated by reflection high-energy electron

diffraction, x-ray diffraction, and cross-sectional transmission electron microscopy

A fully epitaxial CoPt/MgO/CoPt film is formed on the Ru underlayer The lower CoPt, the MgO, and the upper CoPt layers consist of two (111) variants whose atomic stacking sequences of close-packed plane along the perpendicular direction are ABCABC and ACBACB The lower and the upper CoPt layers involve metastable L11structure, whereas the crystal structure of MgO layer is B1 Flat and atomically sharp interfaces are formed between the layers The tri-layer film shows a strong perpendicular magnetic anisotropy reflecting the magnetocrystalline anisotropy of L11 crystal The present study shows that an epitaxial L11-CoPt/MgO/L11-CoPt tri-layer film with perpendicular magnetic anisotropy can be formed by using a low substrate temperature of 300◦C.C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4943060]

I INTRODUCTION

Magnetic tunnel junctions consist of tunnel barrier and ferromagnetic electrode layers and have been studied for applications like tunnel magnetoresistance (TMR) sensors and magnetoresistive random access memory (MRAM) devices In order to achieve high TMR ratios, highly-oriented polycrystalline1or epitaxial single-crystal2MgO layer is effective as the barrier layer For MRAM device applications, magnetic materials with high uniaxial magnetocrystalline anisotropy (Ku) ener-gies are required as the electrode material L10 ordered FePt3and CoPt4 alloys show Ku greater than 107erg/cm3along [001] However to promote L10ordering, it is necessary to process these materials at an elevated temperature around 600◦C,5 , 6which enhances the layer interface roughness and the atomic diffusion On the contrary, CoPt crystal with metastable L11phase shows Kugreater than 107 erg/cm3along [111]7,8and the (111)-oriented films have been prepared by employing a lower temperature around 300◦C.7 14CoPt alloy with L11phase is thus one of the candidates for electrode material In the present study, a CoPt/MgO/CoPt tri-layer film is prepared on an Ru(0001) single-crystal underlayer The growth behavior, the crystal structure, the microstructure, and the magnetic properties are investigated

a E-mail: mitsuru@cc.kogakuin.ac.jp / ohtake@futamoto.elect.chuo-u.ac.jp

2158-3226/2016/6(5)/056103/5 6, 056103-1 © Author(s) 2016

056103-2 Ohtake et al. AIP Advances 6, 056103 (2016)

II EXPERIMENTAL PROCEDURE

A radio frequency (RF) magnetron sputtering system equipped with a reflection high-energy electron diffraction (RHEED) facility was used for film preparation The base pressure was lower than 4 × 10−7Pa Co50Pt50(at %) and MgO targets of 3 inch diameter were employed The distance between target and substrate was set at 150 mm The RF powers for Co50Pt50and MgO targets were respectively fixed at 45 and 200 W The Ar gas pressure was kept constant at 0.67 Pa Under the conditions, the deposition rates were 0.020 and 0.015 nm/s for the CoPt and the MgO materials, respectively

SrTiO3(111) single-crystal substrates were used Before film formation, substrates were heated

at 600◦C for 1 hour to obtain clean surfaces Ru underlayers of 10 nm thickness were formed on the substrates CoPt single-layer, MgO/CoPt bi-layer, and CoPt/MgO/CoPt tri-layer films were prepared

on the underlayers The deposition temperature was 300◦C The thicknesses of CoPt and MgO layers were fixed at 40 and 2 nm, respectively The CoPt layer compositions were confirmed by energy dispersive x-ray spectroscopy and the errors were less than 4 at % from the target composition The crystallographic orientation relationship was studied by RHEED The crystal structure was investigated by 2θ/ω-scan out-of-plane x-ray diffraction (XRD) with Cu-Kα radiation (λ

= 0.15418 nm) The surface morphology and the cross-sectional microstructure were observed by atomic force microscopy (AFM) and transmission electron microscopy (TEM), respectively The TEM sample was first thinned mechanically and then ion-milled to be transparent for the elect-ron beam accelerated at 300 kV The magnetization curves were measured by vibrating sample magnetometry

In the present paper, fcc-based notations of plane and direction are applied to the L11and the L10structures for simple comparison with the disordered A1 structure, though the accurate struc-tures are rhombohedral and tetragonal, respectively Furthermore, the unit cell shown in Fig.1(a)is used for the crystal structures

III RESULTS AND DISCUSSION

Figure1(b)shows the RHEED pattern of an Ru underlayer deposited on SrTiO3(111) substrate observed by making the incident electron beam parallel to SrTiO3[1¯10] A diffraction pattern with spots and streaks is recognized Figure 1(f) shows the schematic diagram of diffraction pattern simulated for a (0001) single-crystal with hcp-based disordered A3 structure The experimental datum is in agreement with the simulation result An Ru(0001) single-crystal underlayer is formed

on the substrate

Figure 1(c) shows the RHEED pattern observed for a CoPt single-layer film deposited on Ru(0001) underlayer A diffraction pattern composed of streaks is recognized An epitaxial CoPt layer is formed on the Ru underlayer Figures1(g)–1(i)show the schematic diagrams of diffraction patterns calculated for (111) crystals with A1, L11, and L10 structures, respectively The atomic

FIG 1 (a) Unit cell of fcc-based structure used in the present study (b)–(e) RHEED patterns observed for (b) an Ru under-layer deposited on SrTiO 3 (111) substrate at 300 ◦ C and (c) CoPt single-layer, (d) MgO/CoPt bi-layer, and (e) CoPt/MgO/CoPt tri-layer films deposited on Ru underlayers at 300◦C The incident electron beam is parallel to SrTiO 3 [1¯10] (f)–(j) Schematic diagrams of RHEED patterns simulated for (f) A3 (0001), (g) A1(111), (h) L1 1 (111), (i) L1 0 (111), and (j) B1(111) surfaces The filled and the open circles correspond to fundamental and superlattice reflections, respectively The incident electron beam is parallel to (f) [11¯20], [(g), (h), (j)] [1¯10] + [¯110], or (i) [1¯10] + [¯110] + [¯101] + [10¯1] + [01¯1] + [1¯11].

arrangement of A3(0001) surface shows a six-fold symmetry with respect to the perpendicular direction, whereas those of A1(111), L11(111), and L10(111) surfaces, respectively, show three-, three-, and one-fold symmetries Therefore, the diffraction patterns simulated by making the inci-dent electron beam parallel to[1¯10] and [¯110] are overlapped in Figs.1(g)and1(h), whereas those simulated by making the incident beam parallel to[1¯10], [¯110], [¯101], [10¯1], [01¯1], and [1¯11] are overlapped in Fig 1(i) The details of the simulations are reported in our previous paper.13 The observed pattern of Fig.1(c)does not agree with the calculated pattern of Fig.1(i), because streaks shown by the arrows in Fig 1(i)are absent The experimental datum of Fig 1(c) corresponds

to the simulation results of Figs 1(g)and/or1(h) The crystallographic orientation relationship is thus determined as CoPt(111)[1¯10], [¯110]∥Ru(0001)[11¯20] The CoPt layer consists of two (111) variants whose atomic stacking sequences of close-packed plane along the perpendicular direction are ABCABC and ACBACB However, it is not easy to assign the crystal structure from the RHEED datum, since the diffraction patterns from A1(111) and L11(111) surfaces are very similar when streaks appear In order to characterize the crystal structure, XRD measurement was carried out The reflection intensity is proportional to the structure factor (F) and the complex conjugate (F∗) The F values of A1-CoPt(111), A1-CoPt(222), L11-CoPt(111), and L11-CoPt(222) are respec-tively calculated to be 0, 16( fPt+ fCo), 16S( fPt− fCo), 16( fPt+ fCo),14 where S is the long-range order degree and f is the atomic scattering factor of Co or Pt Therefore, the XRD reflections from CoPt(111) and CoPt(222) are respectively superlattice and fundamental Figure2(a)shows the XRD pattern measured for the CoPt/Ru/SrTiO3(111) specimen CoPt(111) superlattice reflection

is clearly observed in addition to CoPt(222) + Ru(0002) and SrTiO3(111) fundamental reflections The result shows that the CoPt layer involves L11structure

Figure1(d)shows the RHEED pattern observed for an MgO/CoPt bi-layer film A diffraction pattern including spots is recognized Figure1(j)shows the schematic diagram of diffraction pattern simulated for a B1 crystal with two (111) variants The observed pattern agrees with the simulated pattern Therefore, the MgO layer grows epitaxially in the crystallographic orientation relationship

of MgO(111)[1¯10], [¯110]∥CoPt(111)[1¯10], [¯110]

Figure1(e)shows the RHEED pattern observed for a CoPt/MgO/CoPt tri-layer film A diffrac-tion pattern similar to the case of CoPt single-layer [Fig 1(c)] is recognized A fully epitaxial CoPt/MgO/CoPt tri-layer is formed Figure2(b)shows the XRD pattern measured for the tri-layer film CoPt(111) superlattice reflection from the upper CoPt layer is considered to be overlapped with that from the lower CoPt layer In order to confirm L11ordering in the upper CoPt layer, lower

FIG 2 [(a), (b)] XRD patterns of (a) CoPt single-layer and (b) CoPt/MgO/CoPt tri-layer films deposited on Ru underlayers

at 300 ◦ C (c) XRD pattern of a CoPt single-layer film prepared by deposition at 300 ◦ C followed by annealing at 600 ◦ C (d) XRD pattern of a CoPt /MgO/CoPt tri-layer film, where the lower CoPt layer was annealed at 600 ◦ C before the deposition

of MgO and upper CoPt layers at 300 ◦ C The intensity is shown in logarithmic scale.

056103-4 Ohtake et al. AIP Advances 6, 056103 (2016)

FIG 3 (a) AFM and (b) cross-sectional TEM images of a CoPt/MgO/CoPt tri-layer film deposited on Ru underlayer at

300◦C (c) High-resolution TEM image around the MgO layer observed along Ru[11¯20] (d) Magnetization curves measured for the tri-layer film.

CoPt layer was annealed at 600◦C to proceed a reverse transformation from L11 to A1 phase.12

Then, MgO and upper CoPt layers were sequentially deposited on the disordered CoPt layer at

300 ◦C Figure2(c) shows the XRD pattern measured for a CoPt single-layer film prepared by deposition at 300◦C followed by annealing at 600◦C CoPt(111) superlattice reflection is absent, suggesting that the CoPt layer consists of A1 crystal Figure 2(d) shows the XRD pattern of a CoPt/Ru/CoPt tri-layer film, where the lower CoPt layer was annealed at 600◦C before the deposi-tion of MgO and upper CoPt layers at 300◦C CoPt(111) superlattice reflection is recognized The results indicate that the upper CoPt layer involves L11structure

Figure 3(a) shows the AFM image of the CoPt/MgO/CoPt tri-layer film without annealing The arithmetical mean surface roughness value is 0.2 nm A flat surface is realized Figure 3(b) shows the cross-sectional TEM image observed for the CoPt/MgO/CoPt film Flat layer inter-faces are recognized Figure 3(c) shows the high-resolution TEM image around the MgO layer observed along Ru[11¯20] Atomically sharp boundaries are recognized at the CoPt/MgO interfaces Figure 3(d) shows the magnetization curves measured by applying the magnetic field along the perpendicular or the in-plane direction A strong perpendicular magnetic anisotropy is recognized, which is considered to be reflecting the magnetocrystalline anisotropy of L11crystal

IV CONCLUSION

A CoPt/MgO/CoPt tri-layer film is prepared on an Ru(0001) underlayer at 300◦C The struc-ture and the magnetic properties are investigated A fully epitaxial CoPt/MgO/CoPt film is formed The CoPt and the MgO layers consist of two (111) variants whose atomic stacking sequences of close-packed plane along the perpendicular direction are ABCABC and ACBACB The crystal structure of lower and upper CoPt layer is L11, whereas that of MgO layer is B1 Flat and atomically sharp interfaces are formed between the layers The tri-layer film shows a strong perpendicular magnetic anisotropy reflecting the magnetocrystalline anisotropy of L11crystal The present study shows that a CoPt/MgO/CoPt tri-layer film with perpendicular magnetic anisotropy can be formed

by using a low substrate temperature of 300◦C

ACKNOWLEDGMENTS

A part of this work was supported by JSPS KAKENHI Grant Numbers 25420294 and 26820117 and Chuo University Grant for Special Research

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