- Email: [email protected]
Variable substrate temperature deposition of CoFeB film on Ta for manipulating the perpendicular coercive forces
Accepted Manuscript Variable substrate temperature deposition of CoFeB film on Ta for manipulating the Perpendicular coercive forces Saravanan Lakshmanan, Subha Krishna Rao, Manivel Raja Muthuvel, Gopalakrishnan Chandrasekaran, Helen Annal Therese PII: DOI: Reference:
S0304-8853(17)30346-3 http://dx.doi.org/10.1016/j.jmmm.2017.03.069 MAGMA 62592
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
4 February 2017 28 March 2017 28 March 2017
Please cite this article as: S. Lakshmanan, S.K. Rao, M.R. Muthuvel, G. Chandrasekaran, H.A. Therese, Variable substrate temperature deposition of CoFeB film on Ta for manipulating the Perpendicular coercive forces, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.03.069
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Variable substrate temperature deposition of CoFeB film on Ta for manipulating the Perpendicular coercive forces Saravanan Lakshmanan1, Subha Krishna Rao1, Manivel Raja Muthuvel2, Gopalakrishnan Chandrasekaran1, Helen Annal Therese1* 1
Nanotechnology Research Centre, SRM University, Kattankulathur, Chennai-603203, India
2
Defence Metallurgical Research Laboratory (DMRL), Hyderabad - 500058, India
*E-mail: [email protected], [email protected] ABSTRACT Magnetization of Ta/CoFeB/Ta trilayer films with thick layer of CoFeB deposited under different substrate temperatures (Ts) via ultra-high vacuum DC sputtering technique has been measured with the applied magnetic field parallel and perpendicular to the plane of the film respectively to study the perpendicular coercive forces of the film. The samples were further analyzed for its structural, topological, morphological, electrical transport properties. The core chemical states for the elements present in the CoFeB thin film were analyzed by XPS studies. Magnetization studies reveal the existence of perpendicular coercive forces in CoFeB films deposited only at certain temperatures such as RT, 450°C, 475°C and 500°C. CoFeB film deposited at 475°C exhibited a maximum coercivity of 315 Oe and a very low saturation magnetization (Ms) of 169emu/cc in perpendicular direction. This pronounced effect in perpendicular coercive forces observed for CoFeB475 could be attributed to the effect of temperature in enhancing the crystallization of the film at the Ta/CoFeB interfaces. However at temperatures higher than 475°C the destruction of the Ta/CoFeB interface due to intermixing of Ta and CoFeB results in the disappearance of magnetic anisotropy.
Keywords: Thick CoFeB films, perpendicular coercive forces, low saturation magnetization, DC sputtering
1
I.
INTRODUCTION
Currently, there is an intense interest in realizing thin films with an easy axis of magnetization perpendicular to the plane of the film (known as PMA) due to its potential applications in electronic industries for high density magnetic recording, information storage, and magnetic sensors [1,2]. In particular, Magnetic Tunnel Junctions (MTJs) possessing PMA have received special interest for its excellent current induced magnetization switching, low power consumption, and high density spin-torque magnetic random access memory applications. Numerous PMA materials such as L10-ordered (Co,Fe/Pd,Pt) alloys [3,4], TbFeCo and GdFeCo [5], tetragonal manganese alloys [6,7], (Co,Fe,CoFe)/(Ni,Pt,Pd) multilayer structures [8-10], and ultra-thin magnetic metal/oxidized [(Pt/Co)/MOx] layer thin films [11] have been reported as suitable materials for MTJs. However, these MTJs suffer from low magneto-resistance, low spin polarization current, large Gilbert damping constant and inadequate thermal stability. Ikeda et al. reported for the first time observation of strong PMA in Ta/CoFeB/MgO multilayer system, following which several research groups have reported the presence of PMA in CoFeB thin films [12,13].
Consequently, obtaining
Perpendicular MTJs (p-MTJs) with good thermal stability and low saturation magnetization are the potential research focus for attaining the solution to one of the most existing issues of volatility faced by magnetic data storage industry. Higher the thermal stability factor (∆), higher is the non-volatility of the stored data. Literature shows that a ∆ of 40 is required to achieve good non-volatility of the stored data for a period of a decade [1,13]. The thermal stability factor of a magnetic layer is directly proportional to the energy barrier (E) that separates the in-plane and out-of-plane magnetizations. i.e. E = MsHkV/2, ……(1) where, M s is the saturation magnetization, Hk is anisotropy energy and V the volume of the free layer. Hence, higher the Hk, larger the energy barrier between the up and down magnetization orientations and higher is the thermal stability factor. This has brought an upsurge of interest, especially on the magnetic reversal and significant enhancement of TMR due to thermal effects on the anisotropy of CoFeB films. There have been several reports of observation on CoFeB with different interfacial layers such as Ta/CoFeB/MgO, MgO/CoFeB/Ta, and Mo/CoFeB etc. which shows that the PMA of the free layer is mainly influenced by the 2
annealing effects and the crystalline structures at the interfaces [13,14]. However, the PMA occurs due to the interface effect in between the magnetic and buffer layers when the CoFeB film thickness is ≤ 1.5 nm, which limits its possible milestone applications in spintronics. Further, the effect of growth temperature on the emergence of magnetic anisotropy in Ta/CoFeB/Ta system has not been reported so far. In view of the above, the present investigation was carried out to understand the changes in the magnetic anisotropy in thicker CoFeB films at higher temperatures by controlling the growth temperature. CoFeB films exhibited large perpendicular coercive forces at higher growth temperatures of 475°C, which could originate from the microstructures of the film at the interface between the magnetic CoFeB layer and the underlying Ta layer. The structural, morphological, magnetic, electrical transport properties and core electronic state analyses of the films prove them to be a competing candidate for magnetic data storage applications. II.
EXPERIMENT DETAILS
Ta (5 nm) /Co40Fe40B20 (50 nm)/ Ta (3nm) trilayer thin films were deposited on well cleaned Si (100)/SiO2 substrates using ultra high vacuum (LJUHV SP5) DC magnetron sputtering system. Ta buffer and capping layers were deposited at RT. However, the CoFeB layers were deposited on buffer Ta layer at various growth/substrate temperatures (Ts ) (Ts = RT, 350°C, 450°C, 475°C, 500°C and 550°C) under ultra-high pure argon gas atmosphere. Prior to deposition, the SiO2 substrates were cleaned ultrasonically in isopropyl alcohol (IPA) and acetone. During deposition the base pressure of the process chamber was maintained below 7.2 -8
x10 Torr. The sputter targets CoFeB alloy of 40:40:20 atomic % composition and Ta were pre-sputtered to remove the surface impurities. Subsequently, Ta and CoFeB alloy were deposited at 25 W, 100 W sputtering powers respectively at 5mT sputtering pressure under argon gas flow of 10 sccm. The substrates were rotated (15 RPM) during the deposition of the films. Here, the CoFeB thin film is sandwiched between the Ta bottom buffer layer and top capping layer. The schematic representation of Ta/CoFeB/Ta trilayer stacks on SiO2 substrate is shown in the inset of Fig.1. The structural characterization of the films was performed using Grazing Incidence X-ray diffraction [(GI-XRD), PANalytical X’pert PRO diffractometer, Cu Kα source (λ= 1.5406 Å)]. The surface morphology of all the samples were investigated using Field 3
Emission Scanning Electron Microscopy (FESEM) [FEI Quanta 200 FE] and Atomic Force Microscopy (AFM) [Agilent technologies SPM 500 Pico LE] respectively. AFM was employed to scan the trilayer (SiO2/Ta/CoFeB/Ta) stack in contact mode. The sample surface was scanned over 1x1 µm2. The magnetization studies were conducted using Vibrating Sample Magnetometer (VSM) [ADE technologies, EV9 VSM] at RT. The electrical transport properties were also investigated by the four probe Hall resistivity setup [Ecopia HMS-3000] at ambient temperature. The presence of the elements Ta, Co, Fe and B and their electronic states were analyzed using X ray Photoelectron spectroscopy (XPS) with Al-Kα radiation (hʋ = 1486.6 eV) [ESCA-14, Omicron nanotechnology, Germany]. III.
RESULTS AND DISCUSSION
Fig. 1 shows, the GI-XRD patterns of the Ta/CoFeB/Ta trilayers at different Ts. The samples deposited at different substrate temperatures such as RT, 350°C, 450°C, 475°C, 500°C and 550°C (hereafter represented as TCFBRT, TCFB350, TCFB450, TCFB475, TCFB500, and TCFB550 respectively in the following descriptions). The data shows that the samples TCFBRT, TCFB350, TCFB450, and TCFB475 are x-ray amorphous. TCFB550 shows crystalline nature and the reflection at 2θ = 44.9° is attributed to the CoFe with bcc structure along (110) direction. Nano-crystallites of CoFe embedded in amorphous CoFeB matrix was observed for films annealed above 300°C [15], and we believe that the sample TCFB550 might be similarly affected by temperature. The surface morphological analyses were carried out using FESEM and AFM and the images are displayed in Fig. 2. Significant influence of Ts on the surface morphology of various CoFeB films was observed both in the AFM topographic images 2(a-f) as well as in FESEM images 2(p-u). The surface morphology of the samples from AFM matches well with the FESEM images. Several differences in the size and morphology of the particles can be clearly seen in these images. For samples deposited at RT, the individual grains are not clearly visible. In TCFB350, the grains are slightly elongated and show an oriented growth along the plane of the film. TCFB450, TCFB475 and TCFB500 have well separated spherical shaped grains of ≈ 36 nm, ≈ 44 nm and ≈ 50 nm sizes respectively. The surface morphology of TCFB550 is different from that of the other samples in having bigger and fused particles. Smooth surface augments and the roughed surface degrades the magnetic anisotropy [16]. 4
The relation between RMS roughness (Rrms) and Ts is shown in fig.3. On an average the Rrms of the films falls between 0.7 nm to 2.25 nm. The RMS roughness (Rrms) of the films calculated from the AFM images are ≈ 0.9 nm, ≈ 1.6 nm, ≈ 3.5 nm, ≈ 2.2 nm, ≈ 1.5 nm and ≈ 4.6 nm respectively for TCFBRT to TCFB550 respectively. Magnetization measurements of the trilayer stacks conducted as a function of magnetic field (H) applied parallel (in-plane) and perpendicular (out-of-plane) to the film is shown in Fig.4. The magnetization behavior is as reported for similar CoFeB polycrystalline system with small H values sufficient for magnetic saturation (Ms) along the easy axis direction and large values of H required for Ms in the direction perpendicular to the easy axis. In Fig 4(a) and 4(e) both the in-plane and out-of-plane measurements show hysteresis prominently, while in fig. 4(d) the out-of-plane and in fig. 4(f) in-plane magnetization exhibits prominent hysteresis. The in-plane hysteresis loops have high Ms values and low coercivity while the out-of-plane loops exhibit very low Ms values and high perpendicular magnetic coercive forces. Fig.4 displays the normalized M-H loops for all the samples. The coercivity (Hc (//)) values of TCFBRT to TCFB500 range from ≈ 10 to ≈ 12 Oe, whereas, TCFB550 has shown the highest coercivity value (≈187 Oe) for in-plane measurement. The magnetization of TCFBRT, TCFB350, TCFB450, TCFB475 and TCFB500 indicate that easy axis of the film is parallel to the plane of the film. The sudden jump in the coercivity value of TCFB550 could be attributed to the changes that occur in the degree of local disorder during the crystallization of CoFe at 550°C [17]. Hysteresis loops with perpendicular coercive forces are observed only for certain CoFeB films (see the out-of-plane magnetization data in fig. 4) indicating the presence of magnetic anisotropy in these films. CoFeB deposited at Ts = RT exhibit perpendicular coercive forces, which disappeared at Ts = 350°C. Furthermore, one may see that the perpendicular coercive forces is developed monotonously at higher growth temperature (450°C) and then became stronger at 475°C and once again disappeared completely at 550°C [Fig. 4(c)-(f)]. The saturation magnetization in perpendicular direction increases for samples deposited at higher Ts when compared to the one with Ts=RT and is in the range of 169 to 209 emu/cm3. These Ms values are significantly lower than the other Cobalt based thin films such as Pt/Co (900 emu/cc), Co/Ni (660 emu/cc) multilayers, TbFeCo and CoFeB films, which exhibit PMA [18-21]. On contrary to the in-plane coercivity values, the out-of5
plane coercivity values are higher and are in the range of 68 Oe to 315 Oe. The most striking feature is the sudden increase in the out-of plane coercivity value of TCFB475 to 315 Oe. Such an abrupt increase in Hc at higher temperatures can be attributed to the enhancement in the degree of crystallization of CoFeB [22-23]. However, the migration of boron atoms as well as the degree of crystallization of CoFeB influences the magnetic moment of these films. The smaller size of boron atoms compared with Co, Fe and Ta atoms enables a faster and easier mobility of Boron (B) leading to variation in magnetic moment during thermal treatment [22, 24]. Observation of perpendicular magnetic anisotropy (PMA) for the as-deposited film has been already reported for Ta/CoFeB/MgO system with CoFeB thickness of 1.2 nm [25]. Similarly in our study the magnetic anisotropy observed for TCFBRT sample could probably originate from the partial crystallization of CoFeB at the top and bottom interfaces thus resulting in observed mixed anisotropy. At Ts = 350°C, the perturbation in crystallization due to mild thermal heating of the substrate during deposition results in the disappearance of perpendicular coercivity in TCFB350. This is further supported by the AFM and FESEM images of TCFB350 with an oriented particle growth. The emergence of large perpendicular coercive forces at 475°C could be attributed to further enhanced crystallization of CoFeB magnetic layer at Ta/CoFeB interfaces. This observation is very much in line with the studies reported [25- 27] where an increase in perpendicular coercivity was observed when the annealing temperature was higher than 350°C due to crystallization of CoFeB layer. However, at elevated temperatures the intermixing of Ta and CoFeB at their interfaces and the formation of CoFe nano-crystals could explain the disappearance of perpendicular coercivity. The absence of any XRD peaks revealing the crystallized layers can be explained by their very low thickness, which makes them non-detectable by conventional XRD instruments. However, in-plane anisotropy prevails in all cases, as the high thickness of the CoFeB film implies prevalence of shape magnetic anisotropy. The formation of CoFe at 550°C is further confirmed by GI-XRD, where the peak at 2θ = 44.9° corresponding to (110) bcc structured CoFe. For film with a bcc structured CoFe, a very high tensile strain of - 2.44x10-4 has been observed due to Schottky defects generated in the lattice, presumably by boron leaving from the CoFeB into the interface [28] which could further explain the disappearance of hysteresis loop in TCFB550. 6
The electrical transport properties were studied via four probe Hall resistivity measurement system using the van der pauw method. All trilayer films were subjected to the resistivity measurement at 0.5 T of applied magnetic field. The resistivity versus Ts of CoFeB film is plotted in Fig. 5. From the plot the following inferences can be made; firstly, that all the resistivity values of the films are larger than 100 µΩ-cm (except the resistivity of TCFB550 film), indicating that the major phase of CoFeB film is amorphous as reported by Jen et al., [29] which is again confirmed by the X-Ray diffraction studies. Secondly, a) the resistivity of the films decreased rapidly from 222 µΩ - cm to 143 µΩ - cm with the increase in Ts up to 475°C, which may be due to surface oxidation / roughness effect. b) The resistivity value increased suddenly to 206 µΩ - cm 500°C, which corresponds to the anomalous Hall resistivity due to the presence of spin-orbit interaction [30- 32]. This anomalous Hall resistivity of Ta and CoFeB layer strongly depends on the magnetization orientation in the plane perpendicular to the current direction. The observed conductivity of samples with increase in Ts reflects that the preferred orientation of the distribution of particles such that the easy axis of the majority of them aligns parallel to the plane of the film. Similar spin-orbit interaction behaviour has been reported in the Heavy metal (HM)/Ferromagnet (FM) systems such as Ta/CoFeB and W/CoFeB films [33-36]. Nevertheless, TCFB550 shows an abrupt drop in resistivity (95 µΩ - cm) owing to the formation of CoFe nano-crystallites in CoFeB layer and the intermixing of relieved boron atoms with Ta layer at Ta/CoFeB/Ta interface, which is supported by XPS studies.
XPS studies were conducted for the samples TCFB500 and TCFB550 as the crystalline nature and the magnetic property of these two samples differ. The core-level spectra of the trilayer stacks (Fig. 6) show the presence of Fe, Co, B and Ta in both of them. The difference spectra of both Co and Fe exhibit 2 P3/2 emission peaks at 778.2 eV and 706.7 eV binding energy values respectively indicating that the Fe and Co content in the CoFeB layer are rather stable against variation in the Ts. For B 1s two components could be resolved – the first one, at ~188 eV for B in the CoFeB layer and the second one, at ~192 eV associated with boron in its oxidized form (BOx). Due to the low intensity values, the chemical shift in B 1s could be challenging to detect. Similarly, core-level spectra of Tantalum display two peaks ~ at 26.7 eV and 28.5 eV for Ta 4f7/2 and Ta 4f5/2 respectively 7
(Fig. 4 (e)) for TCFB500. These binding energy values match very well with the values reported for Ta2O5. The presence of a much broader peak and a chemical shift of 0.6 eV towards lower binding energies observed for TCFB550 indicate the existence of Tantalum in a slightly lower oxidation state (Ta2O5-x) [37]. Further, the reduction of Ta is observed at 550°C owing to the possible intermixing of Ta with boron and the formation of Ta-O. However, the presence of a peak corresponding to BOx in both samples indicates that the partial crystallization of CoFe is initiated at 500°C itself. Hence, the presence of CoFe nanocrystallites embedded in CoFeB matrix further explains the increase in Ms values and decrease in Hc and Rrms values for TCFB500 compared to TCFB475. IV.
SUMMARY AND CONCLUSION
In summary, we have observed perpendicular coercive forces with very low saturation magnetization values in Ta/CoFeB/Ta stacks with 50 nm thick CoFeB films deposited at Ts such as RT, 450°C, 475°C and 500°C. The large perpendicular coercive forces exhibited by TCFB 475 could be attributed to the enhancement in crystallization of CoFeB at Ta/CoFeB interfaces at 475°C. The intermixing of Ta and CoFeB at their interfaces and the appearance of a Schottky defects generated due to boron leaving from the CoFeB into the Ta interface of the film could account the disappearance of perpendicular coercivity in TCFB550 film. Large anomalous Hall resistivity (206 µΩ - cm) is observed for TCFB500 film owing to the phenomenon of spin-orbit interaction. The presence of B in the form of CoFeB and BOx observed by XPS for samples prepared at high Ts, confirms that the crystallization of CoFe is initiated at 500°C itself. Hence, the Ta/CoFeB(50 nm)/Ta system exhibiting high perpendicular coercive forces at higher growth temperatures could be a highly promising candidate for spintronic devices with low power consumption. Moreover, the amorphous nature of the films could obstruct the magnetization reversal by significantly reducing the pinning site density, thus can enhance the spin switching speed. ACKNOWLEDGEMENTS The authors would like to thank Dr. K. Ramamoorthy for his support in four probe Hall resistivity measurement at the school of physics, Bharathidasan University, Tamilnadu, India.
8
REFERENCES [1] M. Wang, Y. Zhang, X. Zhao and W. Zhao, Micromachines. 6,1023 (2015). [2] A.T. Hindmarch, A.W. Rushforth, R.P. Campion, C.H. Marrows and B.L. Gallagher, Phys. Rev. B 83, 212404 (2011). [3] M. Yoshikawa, E. Kitagawa, T. Nagase, T. Daibou, M. Nagamine, K. Nishiyama, T. Kishi and H. Yoda, IEEE Trans. Magn. 44, 2573 (2008). [4] G. Kim, Y. Sakuraba, M. Oogane, Y. Ando and T. Miyazaki, Appl. Phys. Lett. 92, 172502 (2008). [5] H. Ohmori, T. Hatori and S. Nakagawa, J. Appl. Phys. 103, 07A911 (2008). [6] T. Kubota, Q.L. Ma, S. Mizukami, X.M. Zhang, H. Naganuma, M. Oogane, Y. Ando and T. Miyazaki, J. Phys. D: Appl. Phys. 46, 155001 (2013). [7] Q.L. Ma, T. Kubota, S. Mizukami, X.M. Zhang, H. Naganuma, M. Oogane, Y. Ando and T. Miyazaki, Appl. Phys. Lett. 101, 032402 (2012). [8] J.H. Park, C. Park, T. Jeong, M.T. Moneck, N.T. Nufer and J.G. Zhu, J. Appl. Phys. 103, 07A917 (2008). [9] Z.R. Tadisina, A. Natarajarathinam, B.D. Clark, A.L. Highsmith, T. Mewes, S. Gupta, E. Chen and S. Wang, J. Appl. Phys. 107, 09C703 (2010). [10] L. You, R.C. Sousa, S. Bandiera, B. Rodmacq and B. Dieny, Appl. Phys. Lett. 100, 172411 (2012). [11] A. Manchon, C. Ducruet, L. Lombard, S. Auffret, B. Rodmacq, B. Dieny, S. Pizzini, J. Vogel, V. Uhlíř, M. Hochstrasser, and G. Panaccione, J. Appl. Phys. 104, 043914 (2008). [12] Y. Liu, H. Liang and C. Jiangwei, AIP Advances 6, 045008 (2016). [13] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H.D. Gan, M. Endo, S. kanai, J. Hayakawa, F. Matsukura, H. Ohno, Nature Mater. 9, 721 (2010). [14] C.W. Cheng, W. Feng, G. Chern, C.M. Lee, T.H. Wu, J. Appl. Phys. 110, 033916 (2011) ; T. Liu, Y. Zhang, J.W. Cai, H.Y. Pan, Sci. Rep. 4, 5895 (2014). [15] J. Sinha, M. Gruber, M. Kodzuka T. Ohkubo, S. Mitani, K. Hono and M. Hayashi, J. Appl. Phys. 117, 043913 (2015). [16] Y. Fu-Te, L. Yi-Hung, J.K. Mei, H. Jen-Hwa and P.C. Kuo, J. Appl. Phys. 111, 07C111 (2012). 9
[17] K. Lalminthang, H. Fulara, M. Raju, S. Chaudhary, J. Magn. Magn. M ater.324, 3118 (2012). [18] P. J. Metaxas, J. P. Jamet, A. Mougin, M. Cormier, J. Ferré, V. Baltz, B. Rodmacq, B. Dieny, and R. L. Stamps, Phys. Rev. Lett. 99, 217208 (2007). [19] R. Sbiaa, H. Meng, and S. N. Piramanayagam, Phys. Stat. Sol. (RRL) 5, 413 (2011). [20] D-T. Ngo, K. Ikeda, and H. Awano, Appl. Phys. Express. 4, 093002 (2011). [21] M. Yamanouchi, A. Jander, P. Dhagat, S. Ikeda, F. Matsukura, and H. Ohno, IEEE Magn. Lett. 2, 3000304 (2011). [22] Y.-H. Wang, W.-C. Chen, S.-Y. Yang, K.-H. Shen, C. Park, M.-J. Kao, and M.- J. Tsai J.Appl.Phys. 99, 08M307 (2006). [23] M. Yamanouchi, R. Koizumi, S. Ikeda, H. Sato, K. Mizunuma, K. Miura, H. D. Gan, F. Matsukura, and H. Ohno J.Appl.Phys. 109, 07C712 (2011). [24] F. Bensmina, P. Humbert, A. Dinia, D. Muller, V. S. Speriosu, and B. A. Gurney, J. Magn. Magn. Mater., 338, 198 (1999). [25] A. Kaidatzis, C. Bran, V. Psycharis, M. Vazquez, J. M. Garcia-Martin and D. Niarchos Appl. Phys. Lett., 106, 262401, (2015). [26] Y.-H. Wang, W.-C. Chen, S.-Y. Yang and K.-H. Shen, J. Appl. Phys. 99, 08M307 (2006). [27] W.X. Wang, Y.Yang, H.Naganuma, Y.Ando, R.C. Yu, and X.F.Han, Appl. Phys. Lett. 99, 012502 (2011). [28] G. Venkat Swamy, H. Pandey, A.K. Srivastava, M.K. Dalai, K.K. Maurya, Rashmi and R.K. Rakshit, AIP Advances 3, 072129 (2013). [29] S. U. Jen, Y. D. Yao, Y. T. Chen, J. M. Wu, C. C. Lee, T. L. Tsai and Y. C. Chang, J.Appl.Phys. 99, 053701 (2006). [30] G. X. Miao and G. Xiao, Appl. Phys. Lett. 85, 73 (2004). [31] C. L. Canedy, G. Q. Gong, J. Q. Wang and G. Xiao J. Appl. Phys. 79, 6126 (1996). [32] G. Su, Y. Li, D. Hou, X. Jin, H. Liu, and S. Wang, Phys. Rev. B 90, 214410 (2014). [33] J. Kim, P. Sheng, S. Takahashi, S. Mitani and M. Hayashi, Phys. Rev. Lett. 116, 097201 (2016). [34] L. Liu, C.F Pai,Y. Li, H.W. Tseng, D.C. Ralph, and R.A Buhrman et al., Science. 336, 555 (2012). 10
[35] S. Cho, S.H.C. Baek, K.D. Lee, Y. Jo and B.G. Park, Sci. Rep. 5, 14668 (2015). [36] T. Zhu Chin. Phys. B 23, 047504 (2014). [37] N. Miyakawa, D. C. Worledge, and K. Kita, IEEE Magn. Lett. 4, 1000104 (2013).
11
FIGURE CAPTIONS
Fig. 1. GI-XRD patterns of SiO2/Ta (5 nm)/Co40Fe40B20(50 nm)/Ta (3 nm) Ta/CoFeB/Ta films deposited at different Ts (Schematic representation of Ta/CoFeB/Ta trilayer structure is shown in the inset).
Fig. 2. AFM and FESEM images of Ta/CoFeB/Ta trilayers at different Ts of ; RT (a)&(p) 350°C (b)&(q), 450°C (c)&(r), 475°C (d)&(s), 500°C (e)&(t) and 550°C (f)&(u).
Fig. 3. Rrms as a function of Ts is plotted from AFM analysis.
Fig. 4. Representative M- H loops of the Ta/CoFeB/Ta stacks deposited at a Ts of RT (a), 350°C (b), 450°C (c), 475°C (d), 500°C (e), and at 550°C (f).
Fig. 5. (a) Plot of Hall Resistivity Vs Ts. Fig. 6. XPS spectra in the regions of Co (a), Fe (b), B (c) and Ta (d) core levels for the Ta/CoFeB/Ta films deposited at Ts = 500°C and 550°C.
12
Highlights
Ta/CoFeB(50nm)/Ta thin films were deposited at various substrate temperatures (Ts). CoFeB films deposited at Ts such as RT, 450°C, 475°C and 500°C exhibited perpendicular coercivity. CoFeB deposited at 475°C displayed a higher coercivity of 315 Oe and a low saturation magnetization (Ms) of 169 emu/cc. The presence of higher perpendicular coercive forces in CoFeB films originate from the enhanced crystallization of the film at the Ta/CoFeB interface.
13
S0304-8853(17)30346-3 http://dx.doi.org/10.1016/j.jmmm.2017.03.069 MAGMA 62592
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
4 February 2017 28 March 2017 28 March 2017
Please cite this article as: S. Lakshmanan, S.K. Rao, M.R. Muthuvel, G. Chandrasekaran, H.A. Therese, Variable substrate temperature deposition of CoFeB film on Ta for manipulating the Perpendicular coercive forces, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.03.069
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
Variable substrate temperature deposition of CoFeB film on Ta for manipulating the Perpendicular coercive forces Saravanan Lakshmanan1, Subha Krishna Rao1, Manivel Raja Muthuvel2, Gopalakrishnan Chandrasekaran1, Helen Annal Therese1* 1
Nanotechnology Research Centre, SRM University, Kattankulathur, Chennai-603203, India
2
Defence Metallurgical Research Laboratory (DMRL), Hyderabad - 500058, India
*E-mail: [email protected], [email protected] ABSTRACT Magnetization of Ta/CoFeB/Ta trilayer films with thick layer of CoFeB deposited under different substrate temperatures (Ts) via ultra-high vacuum DC sputtering technique has been measured with the applied magnetic field parallel and perpendicular to the plane of the film respectively to study the perpendicular coercive forces of the film. The samples were further analyzed for its structural, topological, morphological, electrical transport properties. The core chemical states for the elements present in the CoFeB thin film were analyzed by XPS studies. Magnetization studies reveal the existence of perpendicular coercive forces in CoFeB films deposited only at certain temperatures such as RT, 450°C, 475°C and 500°C. CoFeB film deposited at 475°C exhibited a maximum coercivity of 315 Oe and a very low saturation magnetization (Ms) of 169emu/cc in perpendicular direction. This pronounced effect in perpendicular coercive forces observed for CoFeB475 could be attributed to the effect of temperature in enhancing the crystallization of the film at the Ta/CoFeB interfaces. However at temperatures higher than 475°C the destruction of the Ta/CoFeB interface due to intermixing of Ta and CoFeB results in the disappearance of magnetic anisotropy.
Keywords: Thick CoFeB films, perpendicular coercive forces, low saturation magnetization, DC sputtering
1
I.
INTRODUCTION
Currently, there is an intense interest in realizing thin films with an easy axis of magnetization perpendicular to the plane of the film (known as PMA) due to its potential applications in electronic industries for high density magnetic recording, information storage, and magnetic sensors [1,2]. In particular, Magnetic Tunnel Junctions (MTJs) possessing PMA have received special interest for its excellent current induced magnetization switching, low power consumption, and high density spin-torque magnetic random access memory applications. Numerous PMA materials such as L10-ordered (Co,Fe/Pd,Pt) alloys [3,4], TbFeCo and GdFeCo [5], tetragonal manganese alloys [6,7], (Co,Fe,CoFe)/(Ni,Pt,Pd) multilayer structures [8-10], and ultra-thin magnetic metal/oxidized [(Pt/Co)/MOx] layer thin films [11] have been reported as suitable materials for MTJs. However, these MTJs suffer from low magneto-resistance, low spin polarization current, large Gilbert damping constant and inadequate thermal stability. Ikeda et al. reported for the first time observation of strong PMA in Ta/CoFeB/MgO multilayer system, following which several research groups have reported the presence of PMA in CoFeB thin films [12,13].
Consequently, obtaining
Perpendicular MTJs (p-MTJs) with good thermal stability and low saturation magnetization are the potential research focus for attaining the solution to one of the most existing issues of volatility faced by magnetic data storage industry. Higher the thermal stability factor (∆), higher is the non-volatility of the stored data. Literature shows that a ∆ of 40 is required to achieve good non-volatility of the stored data for a period of a decade [1,13]. The thermal stability factor of a magnetic layer is directly proportional to the energy barrier (E) that separates the in-plane and out-of-plane magnetizations. i.e. E = MsHkV/2, ……(1) where, M s is the saturation magnetization, Hk is anisotropy energy and V the volume of the free layer. Hence, higher the Hk, larger the energy barrier between the up and down magnetization orientations and higher is the thermal stability factor. This has brought an upsurge of interest, especially on the magnetic reversal and significant enhancement of TMR due to thermal effects on the anisotropy of CoFeB films. There have been several reports of observation on CoFeB with different interfacial layers such as Ta/CoFeB/MgO, MgO/CoFeB/Ta, and Mo/CoFeB etc. which shows that the PMA of the free layer is mainly influenced by the 2
annealing effects and the crystalline structures at the interfaces [13,14]. However, the PMA occurs due to the interface effect in between the magnetic and buffer layers when the CoFeB film thickness is ≤ 1.5 nm, which limits its possible milestone applications in spintronics. Further, the effect of growth temperature on the emergence of magnetic anisotropy in Ta/CoFeB/Ta system has not been reported so far. In view of the above, the present investigation was carried out to understand the changes in the magnetic anisotropy in thicker CoFeB films at higher temperatures by controlling the growth temperature. CoFeB films exhibited large perpendicular coercive forces at higher growth temperatures of 475°C, which could originate from the microstructures of the film at the interface between the magnetic CoFeB layer and the underlying Ta layer. The structural, morphological, magnetic, electrical transport properties and core electronic state analyses of the films prove them to be a competing candidate for magnetic data storage applications. II.
EXPERIMENT DETAILS
Ta (5 nm) /Co40Fe40B20 (50 nm)/ Ta (3nm) trilayer thin films were deposited on well cleaned Si (100)/SiO2 substrates using ultra high vacuum (LJUHV SP5) DC magnetron sputtering system. Ta buffer and capping layers were deposited at RT. However, the CoFeB layers were deposited on buffer Ta layer at various growth/substrate temperatures (Ts ) (Ts = RT, 350°C, 450°C, 475°C, 500°C and 550°C) under ultra-high pure argon gas atmosphere. Prior to deposition, the SiO2 substrates were cleaned ultrasonically in isopropyl alcohol (IPA) and acetone. During deposition the base pressure of the process chamber was maintained below 7.2 -8
x10 Torr. The sputter targets CoFeB alloy of 40:40:20 atomic % composition and Ta were pre-sputtered to remove the surface impurities. Subsequently, Ta and CoFeB alloy were deposited at 25 W, 100 W sputtering powers respectively at 5mT sputtering pressure under argon gas flow of 10 sccm. The substrates were rotated (15 RPM) during the deposition of the films. Here, the CoFeB thin film is sandwiched between the Ta bottom buffer layer and top capping layer. The schematic representation of Ta/CoFeB/Ta trilayer stacks on SiO2 substrate is shown in the inset of Fig.1. The structural characterization of the films was performed using Grazing Incidence X-ray diffraction [(GI-XRD), PANalytical X’pert PRO diffractometer, Cu Kα source (λ= 1.5406 Å)]. The surface morphology of all the samples were investigated using Field 3
Emission Scanning Electron Microscopy (FESEM) [FEI Quanta 200 FE] and Atomic Force Microscopy (AFM) [Agilent technologies SPM 500 Pico LE] respectively. AFM was employed to scan the trilayer (SiO2/Ta/CoFeB/Ta) stack in contact mode. The sample surface was scanned over 1x1 µm2. The magnetization studies were conducted using Vibrating Sample Magnetometer (VSM) [ADE technologies, EV9 VSM] at RT. The electrical transport properties were also investigated by the four probe Hall resistivity setup [Ecopia HMS-3000] at ambient temperature. The presence of the elements Ta, Co, Fe and B and their electronic states were analyzed using X ray Photoelectron spectroscopy (XPS) with Al-Kα radiation (hʋ = 1486.6 eV) [ESCA-14, Omicron nanotechnology, Germany]. III.
RESULTS AND DISCUSSION
Fig. 1 shows, the GI-XRD patterns of the Ta/CoFeB/Ta trilayers at different Ts. The samples deposited at different substrate temperatures such as RT, 350°C, 450°C, 475°C, 500°C and 550°C (hereafter represented as TCFBRT, TCFB350, TCFB450, TCFB475, TCFB500, and TCFB550 respectively in the following descriptions). The data shows that the samples TCFBRT, TCFB350, TCFB450, and TCFB475 are x-ray amorphous. TCFB550 shows crystalline nature and the reflection at 2θ = 44.9° is attributed to the CoFe with bcc structure along (110) direction. Nano-crystallites of CoFe embedded in amorphous CoFeB matrix was observed for films annealed above 300°C [15], and we believe that the sample TCFB550 might be similarly affected by temperature. The surface morphological analyses were carried out using FESEM and AFM and the images are displayed in Fig. 2. Significant influence of Ts on the surface morphology of various CoFeB films was observed both in the AFM topographic images 2(a-f) as well as in FESEM images 2(p-u). The surface morphology of the samples from AFM matches well with the FESEM images. Several differences in the size and morphology of the particles can be clearly seen in these images. For samples deposited at RT, the individual grains are not clearly visible. In TCFB350, the grains are slightly elongated and show an oriented growth along the plane of the film. TCFB450, TCFB475 and TCFB500 have well separated spherical shaped grains of ≈ 36 nm, ≈ 44 nm and ≈ 50 nm sizes respectively. The surface morphology of TCFB550 is different from that of the other samples in having bigger and fused particles. Smooth surface augments and the roughed surface degrades the magnetic anisotropy [16]. 4
The relation between RMS roughness (Rrms) and Ts is shown in fig.3. On an average the Rrms of the films falls between 0.7 nm to 2.25 nm. The RMS roughness (Rrms) of the films calculated from the AFM images are ≈ 0.9 nm, ≈ 1.6 nm, ≈ 3.5 nm, ≈ 2.2 nm, ≈ 1.5 nm and ≈ 4.6 nm respectively for TCFBRT to TCFB550 respectively. Magnetization measurements of the trilayer stacks conducted as a function of magnetic field (H) applied parallel (in-plane) and perpendicular (out-of-plane) to the film is shown in Fig.4. The magnetization behavior is as reported for similar CoFeB polycrystalline system with small H values sufficient for magnetic saturation (Ms) along the easy axis direction and large values of H required for Ms in the direction perpendicular to the easy axis. In Fig 4(a) and 4(e) both the in-plane and out-of-plane measurements show hysteresis prominently, while in fig. 4(d) the out-of-plane and in fig. 4(f) in-plane magnetization exhibits prominent hysteresis. The in-plane hysteresis loops have high Ms values and low coercivity while the out-of-plane loops exhibit very low Ms values and high perpendicular magnetic coercive forces. Fig.4 displays the normalized M-H loops for all the samples. The coercivity (Hc (//)) values of TCFBRT to TCFB500 range from ≈ 10 to ≈ 12 Oe, whereas, TCFB550 has shown the highest coercivity value (≈187 Oe) for in-plane measurement. The magnetization of TCFBRT, TCFB350, TCFB450, TCFB475 and TCFB500 indicate that easy axis of the film is parallel to the plane of the film. The sudden jump in the coercivity value of TCFB550 could be attributed to the changes that occur in the degree of local disorder during the crystallization of CoFe at 550°C [17]. Hysteresis loops with perpendicular coercive forces are observed only for certain CoFeB films (see the out-of-plane magnetization data in fig. 4) indicating the presence of magnetic anisotropy in these films. CoFeB deposited at Ts = RT exhibit perpendicular coercive forces, which disappeared at Ts = 350°C. Furthermore, one may see that the perpendicular coercive forces is developed monotonously at higher growth temperature (450°C) and then became stronger at 475°C and once again disappeared completely at 550°C [Fig. 4(c)-(f)]. The saturation magnetization in perpendicular direction increases for samples deposited at higher Ts when compared to the one with Ts=RT and is in the range of 169 to 209 emu/cm3. These Ms values are significantly lower than the other Cobalt based thin films such as Pt/Co (900 emu/cc), Co/Ni (660 emu/cc) multilayers, TbFeCo and CoFeB films, which exhibit PMA [18-21]. On contrary to the in-plane coercivity values, the out-of5
plane coercivity values are higher and are in the range of 68 Oe to 315 Oe. The most striking feature is the sudden increase in the out-of plane coercivity value of TCFB475 to 315 Oe. Such an abrupt increase in Hc at higher temperatures can be attributed to the enhancement in the degree of crystallization of CoFeB [22-23]. However, the migration of boron atoms as well as the degree of crystallization of CoFeB influences the magnetic moment of these films. The smaller size of boron atoms compared with Co, Fe and Ta atoms enables a faster and easier mobility of Boron (B) leading to variation in magnetic moment during thermal treatment [22, 24]. Observation of perpendicular magnetic anisotropy (PMA) for the as-deposited film has been already reported for Ta/CoFeB/MgO system with CoFeB thickness of 1.2 nm [25]. Similarly in our study the magnetic anisotropy observed for TCFBRT sample could probably originate from the partial crystallization of CoFeB at the top and bottom interfaces thus resulting in observed mixed anisotropy. At Ts = 350°C, the perturbation in crystallization due to mild thermal heating of the substrate during deposition results in the disappearance of perpendicular coercivity in TCFB350. This is further supported by the AFM and FESEM images of TCFB350 with an oriented particle growth. The emergence of large perpendicular coercive forces at 475°C could be attributed to further enhanced crystallization of CoFeB magnetic layer at Ta/CoFeB interfaces. This observation is very much in line with the studies reported [25- 27] where an increase in perpendicular coercivity was observed when the annealing temperature was higher than 350°C due to crystallization of CoFeB layer. However, at elevated temperatures the intermixing of Ta and CoFeB at their interfaces and the formation of CoFe nano-crystals could explain the disappearance of perpendicular coercivity. The absence of any XRD peaks revealing the crystallized layers can be explained by their very low thickness, which makes them non-detectable by conventional XRD instruments. However, in-plane anisotropy prevails in all cases, as the high thickness of the CoFeB film implies prevalence of shape magnetic anisotropy. The formation of CoFe at 550°C is further confirmed by GI-XRD, where the peak at 2θ = 44.9° corresponding to (110) bcc structured CoFe. For film with a bcc structured CoFe, a very high tensile strain of - 2.44x10-4 has been observed due to Schottky defects generated in the lattice, presumably by boron leaving from the CoFeB into the interface [28] which could further explain the disappearance of hysteresis loop in TCFB550. 6
The electrical transport properties were studied via four probe Hall resistivity measurement system using the van der pauw method. All trilayer films were subjected to the resistivity measurement at 0.5 T of applied magnetic field. The resistivity versus Ts of CoFeB film is plotted in Fig. 5. From the plot the following inferences can be made; firstly, that all the resistivity values of the films are larger than 100 µΩ-cm (except the resistivity of TCFB550 film), indicating that the major phase of CoFeB film is amorphous as reported by Jen et al., [29] which is again confirmed by the X-Ray diffraction studies. Secondly, a) the resistivity of the films decreased rapidly from 222 µΩ - cm to 143 µΩ - cm with the increase in Ts up to 475°C, which may be due to surface oxidation / roughness effect. b) The resistivity value increased suddenly to 206 µΩ - cm 500°C, which corresponds to the anomalous Hall resistivity due to the presence of spin-orbit interaction [30- 32]. This anomalous Hall resistivity of Ta and CoFeB layer strongly depends on the magnetization orientation in the plane perpendicular to the current direction. The observed conductivity of samples with increase in Ts reflects that the preferred orientation of the distribution of particles such that the easy axis of the majority of them aligns parallel to the plane of the film. Similar spin-orbit interaction behaviour has been reported in the Heavy metal (HM)/Ferromagnet (FM) systems such as Ta/CoFeB and W/CoFeB films [33-36]. Nevertheless, TCFB550 shows an abrupt drop in resistivity (95 µΩ - cm) owing to the formation of CoFe nano-crystallites in CoFeB layer and the intermixing of relieved boron atoms with Ta layer at Ta/CoFeB/Ta interface, which is supported by XPS studies.
XPS studies were conducted for the samples TCFB500 and TCFB550 as the crystalline nature and the magnetic property of these two samples differ. The core-level spectra of the trilayer stacks (Fig. 6) show the presence of Fe, Co, B and Ta in both of them. The difference spectra of both Co and Fe exhibit 2 P3/2 emission peaks at 778.2 eV and 706.7 eV binding energy values respectively indicating that the Fe and Co content in the CoFeB layer are rather stable against variation in the Ts. For B 1s two components could be resolved – the first one, at ~188 eV for B in the CoFeB layer and the second one, at ~192 eV associated with boron in its oxidized form (BOx). Due to the low intensity values, the chemical shift in B 1s could be challenging to detect. Similarly, core-level spectra of Tantalum display two peaks ~ at 26.7 eV and 28.5 eV for Ta 4f7/2 and Ta 4f5/2 respectively 7
(Fig. 4 (e)) for TCFB500. These binding energy values match very well with the values reported for Ta2O5. The presence of a much broader peak and a chemical shift of 0.6 eV towards lower binding energies observed for TCFB550 indicate the existence of Tantalum in a slightly lower oxidation state (Ta2O5-x) [37]. Further, the reduction of Ta is observed at 550°C owing to the possible intermixing of Ta with boron and the formation of Ta-O. However, the presence of a peak corresponding to BOx in both samples indicates that the partial crystallization of CoFe is initiated at 500°C itself. Hence, the presence of CoFe nanocrystallites embedded in CoFeB matrix further explains the increase in Ms values and decrease in Hc and Rrms values for TCFB500 compared to TCFB475. IV.
SUMMARY AND CONCLUSION
In summary, we have observed perpendicular coercive forces with very low saturation magnetization values in Ta/CoFeB/Ta stacks with 50 nm thick CoFeB films deposited at Ts such as RT, 450°C, 475°C and 500°C. The large perpendicular coercive forces exhibited by TCFB 475 could be attributed to the enhancement in crystallization of CoFeB at Ta/CoFeB interfaces at 475°C. The intermixing of Ta and CoFeB at their interfaces and the appearance of a Schottky defects generated due to boron leaving from the CoFeB into the Ta interface of the film could account the disappearance of perpendicular coercivity in TCFB550 film. Large anomalous Hall resistivity (206 µΩ - cm) is observed for TCFB500 film owing to the phenomenon of spin-orbit interaction. The presence of B in the form of CoFeB and BOx observed by XPS for samples prepared at high Ts, confirms that the crystallization of CoFe is initiated at 500°C itself. Hence, the Ta/CoFeB(50 nm)/Ta system exhibiting high perpendicular coercive forces at higher growth temperatures could be a highly promising candidate for spintronic devices with low power consumption. Moreover, the amorphous nature of the films could obstruct the magnetization reversal by significantly reducing the pinning site density, thus can enhance the spin switching speed. ACKNOWLEDGEMENTS The authors would like to thank Dr. K. Ramamoorthy for his support in four probe Hall resistivity measurement at the school of physics, Bharathidasan University, Tamilnadu, India.
8
REFERENCES [1] M. Wang, Y. Zhang, X. Zhao and W. Zhao, Micromachines. 6,1023 (2015). [2] A.T. Hindmarch, A.W. Rushforth, R.P. Campion, C.H. Marrows and B.L. Gallagher, Phys. Rev. B 83, 212404 (2011). [3] M. Yoshikawa, E. Kitagawa, T. Nagase, T. Daibou, M. Nagamine, K. Nishiyama, T. Kishi and H. Yoda, IEEE Trans. Magn. 44, 2573 (2008). [4] G. Kim, Y. Sakuraba, M. Oogane, Y. Ando and T. Miyazaki, Appl. Phys. Lett. 92, 172502 (2008). [5] H. Ohmori, T. Hatori and S. Nakagawa, J. Appl. Phys. 103, 07A911 (2008). [6] T. Kubota, Q.L. Ma, S. Mizukami, X.M. Zhang, H. Naganuma, M. Oogane, Y. Ando and T. Miyazaki, J. Phys. D: Appl. Phys. 46, 155001 (2013). [7] Q.L. Ma, T. Kubota, S. Mizukami, X.M. Zhang, H. Naganuma, M. Oogane, Y. Ando and T. Miyazaki, Appl. Phys. Lett. 101, 032402 (2012). [8] J.H. Park, C. Park, T. Jeong, M.T. Moneck, N.T. Nufer and J.G. Zhu, J. Appl. Phys. 103, 07A917 (2008). [9] Z.R. Tadisina, A. Natarajarathinam, B.D. Clark, A.L. Highsmith, T. Mewes, S. Gupta, E. Chen and S. Wang, J. Appl. Phys. 107, 09C703 (2010). [10] L. You, R.C. Sousa, S. Bandiera, B. Rodmacq and B. Dieny, Appl. Phys. Lett. 100, 172411 (2012). [11] A. Manchon, C. Ducruet, L. Lombard, S. Auffret, B. Rodmacq, B. Dieny, S. Pizzini, J. Vogel, V. Uhlíř, M. Hochstrasser, and G. Panaccione, J. Appl. Phys. 104, 043914 (2008). [12] Y. Liu, H. Liang and C. Jiangwei, AIP Advances 6, 045008 (2016). [13] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H.D. Gan, M. Endo, S. kanai, J. Hayakawa, F. Matsukura, H. Ohno, Nature Mater. 9, 721 (2010). [14] C.W. Cheng, W. Feng, G. Chern, C.M. Lee, T.H. Wu, J. Appl. Phys. 110, 033916 (2011) ; T. Liu, Y. Zhang, J.W. Cai, H.Y. Pan, Sci. Rep. 4, 5895 (2014). [15] J. Sinha, M. Gruber, M. Kodzuka T. Ohkubo, S. Mitani, K. Hono and M. Hayashi, J. Appl. Phys. 117, 043913 (2015). [16] Y. Fu-Te, L. Yi-Hung, J.K. Mei, H. Jen-Hwa and P.C. Kuo, J. Appl. Phys. 111, 07C111 (2012). 9
[17] K. Lalminthang, H. Fulara, M. Raju, S. Chaudhary, J. Magn. Magn. M ater.324, 3118 (2012). [18] P. J. Metaxas, J. P. Jamet, A. Mougin, M. Cormier, J. Ferré, V. Baltz, B. Rodmacq, B. Dieny, and R. L. Stamps, Phys. Rev. Lett. 99, 217208 (2007). [19] R. Sbiaa, H. Meng, and S. N. Piramanayagam, Phys. Stat. Sol. (RRL) 5, 413 (2011). [20] D-T. Ngo, K. Ikeda, and H. Awano, Appl. Phys. Express. 4, 093002 (2011). [21] M. Yamanouchi, A. Jander, P. Dhagat, S. Ikeda, F. Matsukura, and H. Ohno, IEEE Magn. Lett. 2, 3000304 (2011). [22] Y.-H. Wang, W.-C. Chen, S.-Y. Yang, K.-H. Shen, C. Park, M.-J. Kao, and M.- J. Tsai J.Appl.Phys. 99, 08M307 (2006). [23] M. Yamanouchi, R. Koizumi, S. Ikeda, H. Sato, K. Mizunuma, K. Miura, H. D. Gan, F. Matsukura, and H. Ohno J.Appl.Phys. 109, 07C712 (2011). [24] F. Bensmina, P. Humbert, A. Dinia, D. Muller, V. S. Speriosu, and B. A. Gurney, J. Magn. Magn. Mater., 338, 198 (1999). [25] A. Kaidatzis, C. Bran, V. Psycharis, M. Vazquez, J. M. Garcia-Martin and D. Niarchos Appl. Phys. Lett., 106, 262401, (2015). [26] Y.-H. Wang, W.-C. Chen, S.-Y. Yang and K.-H. Shen, J. Appl. Phys. 99, 08M307 (2006). [27] W.X. Wang, Y.Yang, H.Naganuma, Y.Ando, R.C. Yu, and X.F.Han, Appl. Phys. Lett. 99, 012502 (2011). [28] G. Venkat Swamy, H. Pandey, A.K. Srivastava, M.K. Dalai, K.K. Maurya, Rashmi and R.K. Rakshit, AIP Advances 3, 072129 (2013). [29] S. U. Jen, Y. D. Yao, Y. T. Chen, J. M. Wu, C. C. Lee, T. L. Tsai and Y. C. Chang, J.Appl.Phys. 99, 053701 (2006). [30] G. X. Miao and G. Xiao, Appl. Phys. Lett. 85, 73 (2004). [31] C. L. Canedy, G. Q. Gong, J. Q. Wang and G. Xiao J. Appl. Phys. 79, 6126 (1996). [32] G. Su, Y. Li, D. Hou, X. Jin, H. Liu, and S. Wang, Phys. Rev. B 90, 214410 (2014). [33] J. Kim, P. Sheng, S. Takahashi, S. Mitani and M. Hayashi, Phys. Rev. Lett. 116, 097201 (2016). [34] L. Liu, C.F Pai,Y. Li, H.W. Tseng, D.C. Ralph, and R.A Buhrman et al., Science. 336, 555 (2012). 10
[35] S. Cho, S.H.C. Baek, K.D. Lee, Y. Jo and B.G. Park, Sci. Rep. 5, 14668 (2015). [36] T. Zhu Chin. Phys. B 23, 047504 (2014). [37] N. Miyakawa, D. C. Worledge, and K. Kita, IEEE Magn. Lett. 4, 1000104 (2013).
11
FIGURE CAPTIONS
Fig. 1. GI-XRD patterns of SiO2/Ta (5 nm)/Co40Fe40B20(50 nm)/Ta (3 nm) Ta/CoFeB/Ta films deposited at different Ts (Schematic representation of Ta/CoFeB/Ta trilayer structure is shown in the inset).
Fig. 2. AFM and FESEM images of Ta/CoFeB/Ta trilayers at different Ts of ; RT (a)&(p) 350°C (b)&(q), 450°C (c)&(r), 475°C (d)&(s), 500°C (e)&(t) and 550°C (f)&(u).
Fig. 3. Rrms as a function of Ts is plotted from AFM analysis.
Fig. 4. Representative M- H loops of the Ta/CoFeB/Ta stacks deposited at a Ts of RT (a), 350°C (b), 450°C (c), 475°C (d), 500°C (e), and at 550°C (f).
Fig. 5. (a) Plot of Hall Resistivity Vs Ts. Fig. 6. XPS spectra in the regions of Co (a), Fe (b), B (c) and Ta (d) core levels for the Ta/CoFeB/Ta films deposited at Ts = 500°C and 550°C.
12
Highlights
Ta/CoFeB(50nm)/Ta thin films were deposited at various substrate temperatures (Ts). CoFeB films deposited at Ts such as RT, 450°C, 475°C and 500°C exhibited perpendicular coercivity. CoFeB deposited at 475°C displayed a higher coercivity of 315 Oe and a low saturation magnetization (Ms) of 169 emu/cc. The presence of higher perpendicular coercive forces in CoFeB films originate from the enhanced crystallization of the film at the Ta/CoFeB interface.
13