Journal of Science: Advanced Materials and Devices (2016) 57e60 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original article Enhanced perpendicular coercivity of ultrathin perpendicularly magnetized TbeFeeCo films on silicon substrates using a thin Pt underlayer Bang Do a, b, *, Hiroyuki Awano a a b Toyota Technological Institute, Nagoya 468-8511, Japan Institute of Materials Science, VAST, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received 14 March 2016 Accepted 23 March 2016 Available online 11 April 2016 We have studied the effect of an additional thin Pt underlayer on the magnetic properties of ultrathin perpendicularly magnetized TbFeCo films grown on silicon substrates We have demonstrated that the presence of a thin Pt underlayer can enhance the perpendicular coercivity of ultrathin TbFeCo films with thickness down to nm while preserving their low saturation magnetization This can be attributed to the hybridization of Pt and Co orbitals, which induces the strong interfacial perpendicular anisotropy of Co/Pt and/or CoFe/Pt These characteristics are potentially useful to reduce the critical current density induced magnetization switching and enhance the thermal stability of a magnetic tunnel junction and the current-induced domain wall motion in a nanowire made out of the ultrathin TbFeCo magnetic layer © 2016 Vietnam National University, Hanoi Publishing services by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Perpendicular coercivity TbeFeeCo Perpendicular magnetic anisotropy Introduction Perpendicular magnetic anisotropy materials have been studied for spintronics applications due to their excellent thermal stability and low critical current density of spin-torque-induced magnetization switching [1e5] An effective way to reduce the critical current density is reducing thickness of the magnetic layer while maintaining its desirable perpendicular anisotropy for thermal stability [6] For practical applications, further reduction to the critical current density must be reached, representing an important but difficult task Amorphous TbFeCo alloys, which have low saturation magnetization and large perpendicular anisotropy [7,8], are expected to fulfill the aforementioned requirements [9e11] It has been reported that the perpendicular anisotropy can still be realized in TbFeCo films with thickness of just a few nanometers [10,11] In order to reduce film thickness while preserving a large perpendicular coercivity of the TbFeCo film, underlayers, such as Ti, SiN [12], Al [12,13], and Ag [14] have been added onto Si/SiO2 substrates In particular, Lee et al [12] have shown that the presence of Al and Ti underlayers can enhance the perpendicular coercivity of ultrathin TbFeCo films with thickness down to nm * Corresponding author Toyota Technological Institute, Nagoya 468-8511, Japan E-mail address: dobang@toyota-ti.ac.jp (B Do) Peer review under responsibility of Vietnam National University, Hanoi This enhancement of the perpendicular coercivity is attributed to the formation of a Ti, Al island-like surface which effectively prevents oxidation of Tb atoms It has been otherwise shown that the amount of Tb is reduced in TbFeCo alloy, leading to the formation of in-plane anisotropy of the FeCo phase while reducing the perpendicular anisotropy of the ultrathin TbFeCo film [15] According to the recent studies, the hybridization of Pt and Co orbitals can induce the interfacial perpendicular anisotropy of Co/Pt [16] and CoFe/Pt [17] multilayers, where the thickness of the magnetic layers is less than nm The enhanced interfacial anisotropy is attributed to the strong spineorbit coupling and the least reactive metal of Pt These studies have motivated us to perform a systematic investigation into the effect of an additional Pt underlayer on the magnetic properties of TbFeCo films grown on silicon substrates In this study, we demonstrate the enhancement of perpendicular coercivity in Pt/TbFeCo bilayer films with thickness down to nm while preserving their low saturation magnetization Experimental The Tb26Fe66.8Co7.2 films were directly grown on naturally oxidized Si(100) substrates without and with Pt underlayers by using RF/DC magnetron sputtering During the deposition, highly purified Ar gas was kept at a fixed pressure of mTorr The layer http://dx.doi.org/10.1016/j.jsamd.2016.03.001 2468-2179/© 2016 Vietnam National University, Hanoi Publishing services by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons org/licenses/by/4.0/) 58 B Do, H Awano / Journal of Science: Advanced Materials and Devices (2016) 57e60 thicknesses of the TbFeCo and Pt underlayer were varied in the ranges of tTbFeCo ¼ 0.5e27 nm and tPt ¼ 0.5e2 nm, respectively A nm-thick Pt film was subsequently capped on the TbFeCo layer to prevent oxidization The magnetic properties of the films were investigated using a magneto-optical Kerr effect (MOKE) and an alternating gradient force magnetometer (AGFM) in out-of-plane fields of up to ±15 kOe The surface topologies of the oxidized Si substrate and the nm-thick underlayer were observed by atomic force microscopy (AFM) Results and discussion Fig 1(a,b) shows the MOKE hysteresis loops of the TbFeCo films with different thicknesses without and with the nm-thick Pt underlayer It can be seen in Fig 1(a) that while the TbFeCo films with thickness down to nm, grown directly on the oxidized Si substrates possess well-defined square hysteresis loops, the thinner films show a dominant in-plane loop of the CoFe phase, which has been suggested to be formed due to reduction of Tb content in the alloy [11] To prevent oxidization of Tb, a 2-nm-thick Pt underlayer was first grown on the oxidized Si substrate Fig 1(b) shows the MOKE hysteresis loops of TbFeCo films grown on the Pt underlayers As one can see clearly in this figure, the presence of the Pt underlayer enables the well-defined square loops in the TbFeCo films with thickness reduced to 1.5 nm In this case, the Pt underlayer is much more efficient in preventing the oxidation of Tb than the Al and Ti underlayers as reported in Ref [12] It has been reported [12,13] that the island-like surfaces caused by the Al or Ti underlayer can increase number of pinning sites that restrict motion of domain walls and hence increase the perpendicular coercivity The same mechanism is expected to be responsible for the case of our Pt/TbFeCo films Fig shows the AFM images of the naturally oxidized Si substrate, without and with the 2-nm-thick Pt underlayer The mean surface roughnesses are determined to be 0.156 and 0.301 nm for the oxidized Si and Pt, respectively Similar to the Al and Ti underlayers [12,13], the large and high island-like structure of the Pt surface is expected to reduce contact areas between the TbFeCo layer and the oxidized Si substrate, resulting in the observed magnetic properties of the TbFeCo film We also grown 2-nm-thick TbFeCo layers on much thinner Pt underlayers and measured their MOKE hysteresis loops (data not shown) and found that the Pt underlayer of 0.5 nm thickness can also improve the perpendicular coercivity of the 2-nm-thick TbFeCo layer In Fig 3, one can see that the perpendicular coercivities of the TbFeCo films grown on the Pt underlayers are larger than those grown directly on the oxidized Si substrate The enhancement of perpendicular coercivity was kept for all the film thicknesses of Fig Magneto-optical Kerr effect hysteresis loops for TbFeCo films grown on the oxidized Si(100) substrate (a) without and (b) with the 2-nm-thick Pt underlayer Fig AFM images of the oxidized Si substrate (a) without and (b) with the 2-nm-thick Pt layer B Do, H Awano / Journal of Science: Advanced Materials and Devices (2016) 57e60 Fig The thickness dependence of perpendicular coercivity for TbFeCo films grown on the oxidized Si(100) substrate without (circles) and with (squares) the 2-nm-thick Pt underlayer The lines are shown as a guide to the eyes 2e27 nm, except for the 12-nm-thick film This enhancement can be attributed to the hybridization of Pt and Co orbitals, which induces the strong interfacial perpendicular anisotropy of Co/Pt [16] and CoFe/Pt [17] multilayers, where the thickness of the magnetic layers is less than nm In particular, Fang et al [17] suggested that the nanoscaled interface structure of a metal layer and an TbFeCo layer will decrease the exchange interaction between the Tb atom and the Fe/Co atom locally since the non-magnetic metal is inserted The magnetic pinning sites will be introduced at these weak exchange interaction locations, giving rise to the enhanced perpendicular coercivity Gadetsky et al [18] reported that the TbFeCo film exhibited a good perpendicular anisotropy when the Tb content varied between 15 and 38 at.% The authors also showed that with increasing Tb content, the coercivity first increased, reached a maximum at 23 at.% Tb e which is a compensation composition, and then decreased for higher Tb contents For our samples, we used an alloy target of 26 at.% Tb We observed a remarkable change in the polarity of Kerr rotation angle for both the TbFeCo films grown on the oxidized Si substrate without and with the Pt underlayer, when the thickness of the film was increased from to nm This switched polarity is attributed to the change of film magnetization configuration from the CoeFe transitional-metal (TM)-rich to Tb-rare-earth (RE)-rich composition It can be seen that the enhancement of corecivity is larger for the TbFeCo films with thicknesses less than nm This provides solid evidence for the usefulness of the Pt underlayer in preventing oxidization of Tb from residual oxygen on the bare oxidized Si substrate It is worth mentioning here that the increase of perpendicular anisotropy can be an effective way to increase the thermal stability and reduce the intrinsic critical current density for magnetization switching with decrease in the saturation magnetization of the magnetic layer in a magnetic tunnel junction Fig shows the thickness dependence of saturation magnetization (Ms) for both TbFeCo films grown without and with the 2nm-thick Pt underlayer It can be seen that Ms strongly depends on thickness for the films grown without the 2-nm-thick Pt underlayer The Ms was largest for the thinnest film, which had the smallest Tb content due to oxidization as noted above As compared to the case of the film grown without the Pt underlayer, Ms of the TbFeCo film grown with the Pt underlayer (~95 emu/c.c.) was found to be almost unchanged with respect to variation in film thickness, because the presence of the Pt underlayer prevents oxidization of Tb These results are in good agreement with those reported in Refs 59 Fig The thickness dependence of perpendicular saturation magnetization for TbFeCo films grown on the oxidized Si(100) substrate without (circles) and with (squares) the 2-nm-thick Pt underlayer [15] and [18] In those studies, the Ms was reported to decrease with the increase of Tb content for the TM-rich composition An important consequence that emerges from our study is that we demonstrate the addition of a thin Pt underlayer can effectively enhance the perpendicular coercivity of ultrathin TbFeCo films while preserving their low saturation magnetization These characteristics are desirable for reducing the critical current density induced switching magnetization and domain wall motion, as well as for enhancing the thermal stability of the TbFeCo magnetic layer Conclusions In summary, we studied the effect of adding a Pt underlayer on the magnetic properties of ultrathin TbFeCo films The Pt underlayer was formed as an island-like structure, which created magnetic pinning sites The presence of the Pt underlayer effectively enhanced the perpendicular coercivity of ultrathin TbFeCo films with thicknesses down to nm while preserving their low saturation magnetization The improved characteristics have potential to reduce the critical current density induced switching magnetization and enhance the thermal stability of the TbFeCo magnetic layer in a magnetic tunnel junction and the current-induced domain wall motion in a nanowire made out of the TbFeCo thin film Acknowledgments This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan e KAKENHI No 26630137 (2014-2016) References [1] S Ikeda, K Miura, H Yamamoto, K Mizunuma, H.D Gan, M Endo, S Kanai, J Hayakawa, F Matsukuna, H Ohno, A perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction, Nat Mater (2010) 721e724 [2] P Weinberger, Ultrafast switching of a spin valve with perpendicular anisotropy, Phys Rev B 81 (2010) 104417 [3] S Emori, U Bauer, S.-M Ahn, 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H Awano / Journal of Science: Advanced Materials and Devices (2016) 57e60 [6] M Nakayama, T Kai, N Shinomura, M Amano, E Kitagawa, T Nagase, M Yoshikawa, T Kishi, S Ikegawa, H Yoda, Spin transfer... composition An important consequence that emerges from our study is that we demonstrate the addition of a thin Pt underlayer can effectively enhance the perpendicular coercivity of ultrathin TbFeCo... thermal stability of the TbFeCo magnetic layer Conclusions In summary, we studied the effect of adding a Pt underlayer on the magnetic properties of ultrathin TbFeCo films The Pt underlayer was