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Sputtering preparation and magnetic properties of amorphous TbFeCo films
Journal of Non-Crystalline Solids 450 (2016) 82–86
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Sputtering preparation and magnetic properties of amorphous TbFeCo films Ke Wang ⁎, Ruofei Chen, Ya Huang College of Information Science and Engineering, Huaqiao University, Xiamen City 361021, China
a r t i c l e
i n f o
Article history: Received 9 June 2016 Received in revised form 21 July 2016 Accepted 28 July 2016 Available online xxxx Keywords: Rare earth-transition metals (RE-TM) alloy films Amorphous films Magnetic films Magnetic properties'
Amorphous TbFeCo films are fabricated by DC sputtering using a composite target. The magnetic properties of the films are found to be strongly related to sputtering gas pressure and power. It shows that perpendicular films with good squareness can be fabricated at a low sputtering pressure in a wide power range. A series of perpendicular TbFeCo films with thickness ranging from 75 to 450 nm is fabricated by low pressure sputtering. The film exhibits a high perpendicular anisotropy of 106 erg/cm3 in magnitude and a moderate coercivity of 1– 4 kOe. The magnetic compensation point is observed at 105 °C and the Curie temperature is measured to be 147 °C. From the thickness dependent measurements the effective saturation magnetization and the saturation extraordinary Hall resistivity are determined to be 142.5 emu/ cm3 and 4.33 μΩ cm, respectively. Additionally, a large polar Kerr rotation angle of ~0.2° at the wavelength of 633 nm is demonstrated. 1. Introduction Ferromagnetic TbFeCo alloys have been widely studied as the media for magneto-optical recording and heat assisted magnetic recording (HAMR) due to their high perpendicular magnetic anisotropy (PMA) [1,2]. Recently, it is demonstrated to combine with CoFeB to form perpendicular magnetic tunnel junctions (p-MTJ) for the application in perpendicular magnetic random access memory (p-MRAM) [3,4]. Moreover, the large PMA in TbFeCo can generate nano-sized width of domain wall, which is desirable for high spin-torque efficient with low intrinsic current density [5]. Current-driven magnetic domain wall ⁎ Corresponding author. E-mail address: [email protected] (K. Wang).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.07.041 0022-3093/© 2016 Elsevier B.V. All rights reserved.
motions along TbFeCo wires is reported with a low critical current density down to 3 × 106 A/cm2, showing the alloys are promising for magnetic wire memory [6]. Very recently, the spin Hall effect induced switching of magnetization is also demonstrated in Ta/TbFeCo structure with strong intrinsic PMA [7]. It has been known that the PMA is associated with the microstructures of the magnetic films [8]. The PMA in crystalline ferromagnetic films can be attributed to magnetocrystalline anisotropy. For the amorphous TbFeCo the origin of the PMA has not been fully understood yet due to the uniformity of the structure [9,10]. Several physical mechanisms such as single-ion anisotropy model, strain and columnar microstructures were proposed to explain the PMA in amorphous rare earthtransition metal (RE-TM) films [2,11,12]. From the application point of view, perpendicular TbFeCo film with moderate coercivity is more desirable for the devices such as P-MTJs or wire memory [4,13]. Hence, it is necessary to further study and control magnetic properties of the amorphous TbFeCo films for the potential device applications. In this work, we investigate sputtering prepared amorphous TbFeCo films and find the magnetic properties of TbFeCo film are strongly related to the deposition conditions. The films with good perpendicular properties can be fabricated at a low gas pressure in a wide power range. The amorphous films fabricated at low pressure possess moderate coercivity and a high PMA and as well as a large Kerr rotation angle. The effective saturation magnetization and saturation extraordinary Hall resistivity are further derived from the fitting of the thickness dependent measurements. 2. Experimental details TbFeCo films were DC sputtered on silicon (100) substrates at room temperature by a magnetron sputtering system with a base pressure of 6 × 10− 6 Pa. A composite target consisting of 4 pieces of Tb chips (99.95% purity) and a Fe90Co10 alloy plate (99.9% purity) with 2 in. in diameter was used for the deposition of TbFeCo films. The shape of all the Tb chips is isosceles triangle with the vertex angle of ~28° and the leg of 0.8 in. The chips are 0.1 in. in thickness and regularly arranged on the Fe90Co10 alloy plate. The TbFeCo films with good perpendicular properties were achieved by adjusting the areas and positions of the Tb chips on the CoFe alloy disk [14]. The target-substrate distance is set to be 60 mm. The sputtering power changes from 45 W to 76 W and the working pressure of Ar gas varies from 0.2 Pa to 0.6 Pa. No protective capping layer is used after deposition of TbFeCo. The structure of the alloy film was characterized by a X-ray diffractometer (XRD, Rigaku Smartlab) with Cu Kα radiation (λ = 0.1541 nm). Magnetic and
K. Wang et al. / Journal of Non-Crystalline Solids 450 (2016) 82–86
thermomagnetic properties were measured by using vibrating sample magnetometer (VSM, Microsense EZ9). Magneto-optical Kerr effect (MOKE) loops were measured in polar geometry using a He\\Ne laser (λ = 633 nm). The sheet resistance and extraordinary Hall effect (EHE) measurements were performed based on four-point probe method. 3. Results and discussion Fig. 1a and b show magnetic and EHE loops for the TbFeCo films fabricated at different sputtering parameters. Two Ar gas pressures of 0.2 Pa and 0.6 Pa were chosen for the comparison. The thicknesses of the sputtered films are in the range between 150 nm and 180 nm. Rectangular loops are measured from the films with the field applied perpendicular to the sample plane. The polarization of the EHE loops is opposite to the corresponding VSM loops. It suggests all the sputtered films are Tb-dominant [15], which will be confirmed by the
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thermomagnetic measurement later. The films sputtered at low pressure possess better remanent ratio than those fabricated at high pressure. Fig. 1c shows variation of remanence of the loops in the films sputtered at two working pressures. For each data point the error estimates are made from several repeated measurements on the same sample. The PMA obtained in film sputtered at 0.2 Pa has a large remanence over 0.9 in a wide power range between 45.5 W and 74 W. In contrast, for the films sputtered at the high Ar pressure of 0.6 Pa the remanence is relatively low. It increases with the sputtering power reaching a peak value of 0.90 at 69 W and then falls again. The considerable difference in the magnetic properties of ferrimagnetic TbFeCo can be mainly ascribed to the change in composition caused during the deposition [8,15–19]. The variation of the Tb-toFeCo ratio in the alloy films can be caused by the re-sputtering of the growing film arising from Ar bombardment. The re-sputtering yield of Tb from the film is higher than that of FeCo [16], which consequently can cause the variation in Tb concentration of the film sputtered at
Fig. 1. Magnetic and EHE loops for the TbFeCo films sputtered at the Ar gas pressure of a) 0.2 Pa and b) 0.6 Pa. c) Comparison of the remanence of the loops measured from the films sputtered at two pressures.
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different gas pressures. On the other hand, film composition may become Tb-rich at the high powers as the sputtering yield of Tb atoms ejected from the target increases faster than that of FeCo atoms with increasing the power [16]. Additionally, at the low sputtering pressure the sputtered atoms ejected from the target have a long mean free path and are less scattered by the Ar gas. The energy flux of sputtered Tb and FeCo atoms is relatively high when they reach substrates. Hence, the difference in microstructure of the film could be produced by the sputtered atoms with different bombarding energy, which may also cause the difference in the magnetic properties. Fig. 2a presents in-plane and out-of-plane magnetic hysteresis loops of a 150-nm-thick TbFeCo film sputtered at the Ar pressure of 0.2 Pa and the power of 68 W. The magnetic loops show the magnetic easy axis is perpendicular to the film plane. The saturation magnetization Ms is measured to be 110 emu/cm3. This small saturation moment is due to the antiferromagnetic coupled RE and TM atoms in RE-TM alloys and the net moment is the difference of magnetic moment between Tb and Co/Fe atoms [20]. The out-of-plane coercive Hc of the sample is 2.2 kOe and magnetic anisotropy field HK is greater than 1.5 T. From the loops, the perpendicular magnetic anisotropy Kμ is estimated to be the order of 106 erg/cm3 in magnitude. The XRD pattern of the as-deposited film is shown in Fig. 2b. Except from the Si (400) peak from the substrate no peaks from the film can be detected, confirming the sputtered TbFeCo film is amorphous. Thermomagnetic measurements
have been widely used in the amorphous magnetic films, where the temperature for magnetic or structural phase transition can be detected [21,22]. Fig. 2c shows the thermomagnetic curve of the TbFeCo film. It shows the sputtered alloy film is Tb-dominant, in good agreement with the polarization of the EHE measurements in Fig.1. The Curie temperature TC is determined to be 147 °C. Interestingly, before the temperature reaches TC, magnetic compensation occurs at 105 °C. This magnetic compensation comes from the competition between antiferromagnetic coupling of RE with TM ions and ferromagnetic interactions between TM ions [23]. A series of films with variable thicknesses is sputtered at the low pressure of 0.2 Pa and the power of 68 W. Magnetic loops in Fig. 3 show all the films possess good PMA. Fig. 3b descripts the variation of Hc and saturation magnetic moment per unit area against film thickness. The error estimates for each data point are made from several repeated measurements on the same film. A decrease in coercivity with thickness is demonstrated for the sputtered films. The thickness dependence of coercivity was previously reported in amorphous perpendicular TbDyFeCo films with similar thickness [24]. This might be related to the thickness dependent change in microstructure and/or stress in the amorphous RE-TM films [16,25]. The saturation magnetic moment per unit area linearly increases with the thickness of the films, as expected. The effective saturation magnetization Ms is determined to be 142.5 ± 4.64 emu/cm3 from the slope of the linear fitting of thickness dependent
Fig. 2. a) In-plane and out-of-plane magnetic loops for a 150 nm TbFeCo film sputtered at the Ar pressure of 0.2 Pa and the power of 68 W. b) XRD pattern and c) thermomagnetic curve of the film.
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Fig. 3. a) Magnetic loops measured from the films with different thickness sputtered at the low pressure of 0.2 Pa and the power of 68 W. b) Variation of Hc and saturation magnetic moment per area against film thickness.
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magnetic moment per unit area. In addition, the intercept of the linear fitting with the horizontal axis is 25.7 ± 9.5 nm. This value may be attributed to magnetic dead layer, which arises from the oxidation and the intermixing of the alloy film at the surface or interfaces [26,27]. Fig. 4a and b shows the MOKE and EHE loops of the sputtered films with the thickness ranging from 75 to 450 nm. All the MOKE and EHE measurements confirm the magnetization of films is perpendicular to the sample plane. The MO Kerr rotation angles of the films are quite consistent and measured to be in the range of 0.18–0.22°. In contrast, the amplitude of the Hall resistance is measured to be in the range of 0.087–0.57 Ω and decreases with film thickness. A small slope in the MOKE loops is noticeable, which was often found in the unprotected TbFeCo film [28,29]. This comes from in-plane magnetization component due to the preferential oxidation of Tb element at the sample surface, which accordingly produces a thin FeCo-rich magnetically soft layer on top of the perpendicular TbFeCo films [29]. Fig. 4c and d plot the variation of sheet resistance and Hall resistance against film thickness, respectively. The error estimates for each data point are made from several repeated measurements on the same film. The sheet resistance decreases with the increasing thickness of the film, as we expected, for the reduced defects in the thick films. The sheet resistance of the 75-nm-thick TbFeCo film is measured to be 28.31 ± 1.02 μΩ/square, which corresponds to an electrical resistivity of 212.33 ± 7.65 μΩ cm. For the 450-nm-thick film the sheet resistance is measured to be 4.17 ± 0.15 μΩ/square, corresponding to an electrical resistivity of 187.65 ± 6.75 μΩ cm. These values agree well with the reported values of several hundred μΩ cm in amorphous RE-TM alloy films such as TbFe, SmFe and TbFeCo materials [7,30]. The high resistivity is consistent with the amorphous structure of the films, as confirmed in the XRD measurement. Additionally, the nanoscale chemical inhomogeneity, revealed in amorphous TbFeCo films, can enhance the resistivity [31,32]. The extraordinary Hall resistance can be expressed by RH = (RoH + RsMs)/d [33], where the first term represents the ordinary Hall effect, related to the Lorentz force acting on moving charge carriers with Ro being the ordinary Hall coefficient. This effect is linear in outof-plane field H and is small enough to be ignored in our experiments.
Fig. 4. a) MOKE and b) EHE loops obtained for the sputtered films in the thickness range between 75 and 450 nm. Variation of c) sheet resistance and d) Hall resistance with film thickness.
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The second term arising from the extraordinary Hall effect, which is proportional to the out-of-plane magnetization Ms with the EHE coefficient Rs. From the fitting the thickness dependent hall resistance the saturation Hall resistivity Rs Ms is derived to be 4.33 ± 0.08 μΩ cm. This value agrees well with that reported in amorphous alloy films such as TbFe and SmFe [30]. 4. Conclusions In short, amorphous TbFeCo films with good perpendicular properties are sputtered using a composite target at low Ar pressure. The films possess a high perpendicular anisotropy of ~106 erg/cm3 and an effective saturation magnetization of 142.5 emu/cm3. A large polar Kerr rotation angle of ~ 0.2° at the wavelength of 633 nm is obtained. The saturation extraordinary Hall resistivity of 4.33 μΩ cm is derived from the fitting of the thickness dependent Hall resistance. Acknowledgments This work was supported by Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (No. ZQN-YX107) and Research Start-up Funding for Highlevel Talents Project Sponsored by Huaqiao University (No. 13BS401). References [1] T. Hatori, M. Okuda, S. Nakagawa, Influence of spin polarization enhancement layer to rare earth–transition metal thin films for perpendicular tunneling magnetoresistance evaluated by ferromagnetic Hall effect, J. Appl. Phys. 99 (2006) (08C513–1–3). [2] N. Anuniwat, M. Ding, S.J. Poon, S.A. Wolf, J.W. Lu, Strain-induced enhancement of coercivity in amorphous TbFeCo films, J. Appl. Phys. 113 (2013) (043905–1–5). [3] M. Nakayama, T. Kai, N. Shimomura, M. Amano, E. Kitagawa, T. Nagase, M. Yoshikawa, T. Kishi, S. Ikegawa, H. Yoda, Spin transfer switching in TbCoFe ∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy, J. Appl. Phys. 103 (2008) (07 A710–1–3). [4] C.M. Lee, L.X. Ye, T.H. Hsieh, C.Y. Huang, T.H. Wu, Magnetic properties of TbFeCobased perpendicular magnetic tunnel junctions, J. Appl. Phys. 107 (2010) (09C712–1–3). [5] M.T. Rahman, X. Liu, M. Matsumoto, A. Morisako, Magnetoresistance in amorphous TbFeCo films with perpendicular magnetic anisotropy, J. Appl. Phys. 97 (2005) (10C515–1–3). [6] H. Awano, Investigation of domain wall motion in RE-TM magnetic wire towards a current driven memory and logic, J. Magn. Magn. Mater. 381 (2015) 50–55. [7] Z.Y. Zhao, M. Jamali, A.K. Smith, J.P. Wang, Spin Hall switching of the magnetization in Ta/TbFeCo structures with bulk perpendicular anisotropy, Appl. Phys. Lett. 106 (2015) (132404–1–4). [8] M. Murakami, M. Birukawa, Sputtering gases and pressure effects on the microstructure, magnetic properties and recording performance of TbFeCo films, J. Magn. Magn. Mater. 320 (2008) 608–611. [9] S.Q. Yin, X.Q. Li, X.G. Xu, J. Miao, Y. Jiang, Effect of Ta underlayer on perpendicular anisotropy of TbFeCo films, IEEE Trans. Magn. 47 (2011) 3129–3131. [10] L.N. Tong, T.T. Li, X.M. He, P. Deng, The effects of the growth pressure and annealing conditions on perpendicular magnetic anisotropy of sputtered NdFeCo films on Si(111), J. Alloys Compd. 605 (2014) 149–156. [11] Y. Suzuki, S. Takayama, F. Kirino, N. Ohta, Single ion model for perpendicular magnetic anisotropy in RE-TM amorphous films, IEEE Trans. Magn. 23 (1987) 2275–2277.
[12] L. Xi, J.M. Lu, J.J. Zhou, Q.J. Sun, D.S. Xue, F.S. Li, Thickness dependence of magnetic anisotropic properties of FeCoNd films, J. Magn. Magn. Mater. 322 (2010) 2272–2275. [13] K. Umadevi, S. Bysakh, J. Arout Chelvane, S.V. Kamat, V. Jayalakshmi, Tailoring magnetic anisotropy in Tb\ \Fe\ \Co thin films by rapid thermal annealing, J. Alloys Compd. 663 (2016) 430–435. [14] K. Wang, Z.Y. Li, R. Xiong, X.F. Yang, Z.Q. Hu, Optimization of the writing and erasing characteristics of MO tri-layered films using the non-magnetic intermediate layer, Mater. Sci. Eng. B 76 (2000) 53–55. [15] S. Yumoto, K. Toki, O. Okada, H. Gokan, Anomalous magnetoresistance effect in sputtered TbFeCo relating to dispersed magnetic moment, IEEE Trans. Magn. 24 (6) (1988) 2793–2795. [16] T.K. Hatwar, A.C. Palumbo, D.G. Stinson, Effect of argon sputtering pressure on the magnetic properties and morphology of TbFeCo films, IEEE Trans. Magn. 24 (1988) 2775–2777. [17] W.Y. Zhang, H. Shima, F. Takano, M. Takenaka, M. Yamazaki, K. Masuda, H. Akinaga, T. Nagahama, S. Nimori, Origin of perpendicular magnetic anisotropy and evolution of magnetic domain structure of amorphous Pr–TM–B (TM = Fe, Co) films, J. Magn. Magn. Mater. 322 (2010) 900–908. [18] H. Basumatary, J.A. Chelvane, D.V.S. Rao, S.V. Kamat, R. Ranjan, Effect of sputtering parameters on the structure, microstructure and magnetic properties of Tb\ \Fe films, Thin Solid Films 583 (2015) 1–6. [19] L.T. Tu, N.H. Duc, J.R. Jeong, C.G. Kim, Influence of working pressure on the magnetic properties of Tb(Fe0.55Co0.45)1.5 thin films, J. Magn. 13 (2008) 160–162. [20] J.M.D. Coey, J. Chapert, J.R. Rebouillat, T.S. Wang, Magnetic structure of an amorphous rare-earth transition-metal alloy, Phys. Rev. Lett. 36 (1976) 1061–1064. [21] K. Wang, S. Dong, Z. Xu, Y. Huang, Crystallization and magnetic properties in amorphous Co2MnSi alloy films, Mater. Lett. 180 (2016) 140–143. [22] K. Wang, Y. Huang, Z. Xu, Y.Y. Hu, S. Dong, Crystallization and high temperature functional properties in CoFeB films, Solid State Commun. 234-235 (2016) 31–34. [23] M.L. Ding, S.J. Poon, Tunable perpendicular magnetic anisotropy in GdFeCo amorphous films, J. Magn. Magn. Mater. 339 (2013) 51–55. [24] H. Basumatary, A.C. Jeyaramane, S.V. Kamat, R. Ranjan, Structural and magnetic properties of amorphous Tb\ \Dy\ \Fe\ \Co thin films, J. Supercond. Nov. Magn. 29 (3) (2016) 863–867. [25] Z. Liu, S.M. Zhou, X.B. Jiao, Correlation between perpendicular magnetic anisotropy and microstructure in TbFeCo and TbFeCo–SiO2 films, J. Phys. D: Appl. Phys. 42 (2009) (015008-1-5). [26] P.G. Gowtham, G.M. Stiethl, D.C. Ralph, R.A. Buhrman, Thickness-dependent magnetoelasticity and its effects on perpendicular magnetic anisotropy in Ta/ CoFeB/MgO thin films, Phys. Rev. B 93 (2015) (024404–1–5). [27] K. Wang, Z. Xu, S. Dong, Y.Z. Qiu, Y. Huang, Magnetic, thermal, electrical properties and crystallization kinetics of Co60Fe20B20 alloy films, Sci. China Mater. (2016), http://dx.doi.org/10.1007/s40843-016-5052-1. [28] C.D. Xu, Z.F. Chen, X. Yang, S.F. Li, D.X. Chen, S.M. Zhou, T.S. Lai, Structure and property changes and femtosecond laser-induced magnetization dynamics of two-yearold high coercive TbFeCo films, J. Magn. Magn. Mater. 324 (2012) 2034–2038. [29] E.E. Marinero, D.C. Miller, A.E. Bell, A. Gupta, R.N. Payne, H. Notarys, Oxidation induced magnetic and structural changes in magneto-optical alloys, IEEE Trans. Magn. 23 (1987) 2629–2631. [30] T.W. Kim, S.H. Lim, R.J. Gambino, Spontaneous Hall effect in amorphous Tb–Fe and Sm–Fe thin films, J. Appl. Phys. 89 (2001) 7212–7214. [31] X. Li, C.T. Ma, J. Lu, A. Devaraj, S.R. Spurgeon, R.B. Comes, S.J. Poon, Exchange bias and bistable magneto-resistance states in amorphous TbFeCo thin films, Appl. Phys. Lett. 108 (2016) (012401–1–5). [32] T.M. Liu, T. Wang, A.H. Reid, M. Savoini, X. Wu, B. Koene, P. Granitzka, C. Graves, D. Higley, Z. Chen, G. Razinskas, M. Hantschmann, A. Scherz, J. Stohr, A. Tsukamoto, B. Hecht, A.V. Kimel, A. Kirilyuk, T. Rasing, H.A. Durr, Nanoscale confinement of all-optical magnetic switching in TbFeCo-competition with nanoscale heterogeneity, Nano Lett. 15 (2015) 6862–6868. [33] P. Hansen, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Elsevier, Amsterdam 1991, p. 6 (Chap. 4).
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Sputtering preparation and magnetic properties of amorphous TbFeCo films Ke Wang ⁎, Ruofei Chen, Ya Huang College of Information Science and Engineering, Huaqiao University, Xiamen City 361021, China
a r t i c l e
i n f o
Article history: Received 9 June 2016 Received in revised form 21 July 2016 Accepted 28 July 2016 Available online xxxx Keywords: Rare earth-transition metals (RE-TM) alloy films Amorphous films Magnetic films Magnetic properties'
Amorphous TbFeCo films are fabricated by DC sputtering using a composite target. The magnetic properties of the films are found to be strongly related to sputtering gas pressure and power. It shows that perpendicular films with good squareness can be fabricated at a low sputtering pressure in a wide power range. A series of perpendicular TbFeCo films with thickness ranging from 75 to 450 nm is fabricated by low pressure sputtering. The film exhibits a high perpendicular anisotropy of 106 erg/cm3 in magnitude and a moderate coercivity of 1– 4 kOe. The magnetic compensation point is observed at 105 °C and the Curie temperature is measured to be 147 °C. From the thickness dependent measurements the effective saturation magnetization and the saturation extraordinary Hall resistivity are determined to be 142.5 emu/ cm3 and 4.33 μΩ cm, respectively. Additionally, a large polar Kerr rotation angle of ~0.2° at the wavelength of 633 nm is demonstrated. 1. Introduction Ferromagnetic TbFeCo alloys have been widely studied as the media for magneto-optical recording and heat assisted magnetic recording (HAMR) due to their high perpendicular magnetic anisotropy (PMA) [1,2]. Recently, it is demonstrated to combine with CoFeB to form perpendicular magnetic tunnel junctions (p-MTJ) for the application in perpendicular magnetic random access memory (p-MRAM) [3,4]. Moreover, the large PMA in TbFeCo can generate nano-sized width of domain wall, which is desirable for high spin-torque efficient with low intrinsic current density [5]. Current-driven magnetic domain wall ⁎ Corresponding author. E-mail address: [email protected] (K. Wang).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.07.041 0022-3093/© 2016 Elsevier B.V. All rights reserved.
motions along TbFeCo wires is reported with a low critical current density down to 3 × 106 A/cm2, showing the alloys are promising for magnetic wire memory [6]. Very recently, the spin Hall effect induced switching of magnetization is also demonstrated in Ta/TbFeCo structure with strong intrinsic PMA [7]. It has been known that the PMA is associated with the microstructures of the magnetic films [8]. The PMA in crystalline ferromagnetic films can be attributed to magnetocrystalline anisotropy. For the amorphous TbFeCo the origin of the PMA has not been fully understood yet due to the uniformity of the structure [9,10]. Several physical mechanisms such as single-ion anisotropy model, strain and columnar microstructures were proposed to explain the PMA in amorphous rare earthtransition metal (RE-TM) films [2,11,12]. From the application point of view, perpendicular TbFeCo film with moderate coercivity is more desirable for the devices such as P-MTJs or wire memory [4,13]. Hence, it is necessary to further study and control magnetic properties of the amorphous TbFeCo films for the potential device applications. In this work, we investigate sputtering prepared amorphous TbFeCo films and find the magnetic properties of TbFeCo film are strongly related to the deposition conditions. The films with good perpendicular properties can be fabricated at a low gas pressure in a wide power range. The amorphous films fabricated at low pressure possess moderate coercivity and a high PMA and as well as a large Kerr rotation angle. The effective saturation magnetization and saturation extraordinary Hall resistivity are further derived from the fitting of the thickness dependent measurements. 2. Experimental details TbFeCo films were DC sputtered on silicon (100) substrates at room temperature by a magnetron sputtering system with a base pressure of 6 × 10− 6 Pa. A composite target consisting of 4 pieces of Tb chips (99.95% purity) and a Fe90Co10 alloy plate (99.9% purity) with 2 in. in diameter was used for the deposition of TbFeCo films. The shape of all the Tb chips is isosceles triangle with the vertex angle of ~28° and the leg of 0.8 in. The chips are 0.1 in. in thickness and regularly arranged on the Fe90Co10 alloy plate. The TbFeCo films with good perpendicular properties were achieved by adjusting the areas and positions of the Tb chips on the CoFe alloy disk [14]. The target-substrate distance is set to be 60 mm. The sputtering power changes from 45 W to 76 W and the working pressure of Ar gas varies from 0.2 Pa to 0.6 Pa. No protective capping layer is used after deposition of TbFeCo. The structure of the alloy film was characterized by a X-ray diffractometer (XRD, Rigaku Smartlab) with Cu Kα radiation (λ = 0.1541 nm). Magnetic and
K. Wang et al. / Journal of Non-Crystalline Solids 450 (2016) 82–86
thermomagnetic properties were measured by using vibrating sample magnetometer (VSM, Microsense EZ9). Magneto-optical Kerr effect (MOKE) loops were measured in polar geometry using a He\\Ne laser (λ = 633 nm). The sheet resistance and extraordinary Hall effect (EHE) measurements were performed based on four-point probe method. 3. Results and discussion Fig. 1a and b show magnetic and EHE loops for the TbFeCo films fabricated at different sputtering parameters. Two Ar gas pressures of 0.2 Pa and 0.6 Pa were chosen for the comparison. The thicknesses of the sputtered films are in the range between 150 nm and 180 nm. Rectangular loops are measured from the films with the field applied perpendicular to the sample plane. The polarization of the EHE loops is opposite to the corresponding VSM loops. It suggests all the sputtered films are Tb-dominant [15], which will be confirmed by the
83
thermomagnetic measurement later. The films sputtered at low pressure possess better remanent ratio than those fabricated at high pressure. Fig. 1c shows variation of remanence of the loops in the films sputtered at two working pressures. For each data point the error estimates are made from several repeated measurements on the same sample. The PMA obtained in film sputtered at 0.2 Pa has a large remanence over 0.9 in a wide power range between 45.5 W and 74 W. In contrast, for the films sputtered at the high Ar pressure of 0.6 Pa the remanence is relatively low. It increases with the sputtering power reaching a peak value of 0.90 at 69 W and then falls again. The considerable difference in the magnetic properties of ferrimagnetic TbFeCo can be mainly ascribed to the change in composition caused during the deposition [8,15–19]. The variation of the Tb-toFeCo ratio in the alloy films can be caused by the re-sputtering of the growing film arising from Ar bombardment. The re-sputtering yield of Tb from the film is higher than that of FeCo [16], which consequently can cause the variation in Tb concentration of the film sputtered at
Fig. 1. Magnetic and EHE loops for the TbFeCo films sputtered at the Ar gas pressure of a) 0.2 Pa and b) 0.6 Pa. c) Comparison of the remanence of the loops measured from the films sputtered at two pressures.
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different gas pressures. On the other hand, film composition may become Tb-rich at the high powers as the sputtering yield of Tb atoms ejected from the target increases faster than that of FeCo atoms with increasing the power [16]. Additionally, at the low sputtering pressure the sputtered atoms ejected from the target have a long mean free path and are less scattered by the Ar gas. The energy flux of sputtered Tb and FeCo atoms is relatively high when they reach substrates. Hence, the difference in microstructure of the film could be produced by the sputtered atoms with different bombarding energy, which may also cause the difference in the magnetic properties. Fig. 2a presents in-plane and out-of-plane magnetic hysteresis loops of a 150-nm-thick TbFeCo film sputtered at the Ar pressure of 0.2 Pa and the power of 68 W. The magnetic loops show the magnetic easy axis is perpendicular to the film plane. The saturation magnetization Ms is measured to be 110 emu/cm3. This small saturation moment is due to the antiferromagnetic coupled RE and TM atoms in RE-TM alloys and the net moment is the difference of magnetic moment between Tb and Co/Fe atoms [20]. The out-of-plane coercive Hc of the sample is 2.2 kOe and magnetic anisotropy field HK is greater than 1.5 T. From the loops, the perpendicular magnetic anisotropy Kμ is estimated to be the order of 106 erg/cm3 in magnitude. The XRD pattern of the as-deposited film is shown in Fig. 2b. Except from the Si (400) peak from the substrate no peaks from the film can be detected, confirming the sputtered TbFeCo film is amorphous. Thermomagnetic measurements
have been widely used in the amorphous magnetic films, where the temperature for magnetic or structural phase transition can be detected [21,22]. Fig. 2c shows the thermomagnetic curve of the TbFeCo film. It shows the sputtered alloy film is Tb-dominant, in good agreement with the polarization of the EHE measurements in Fig.1. The Curie temperature TC is determined to be 147 °C. Interestingly, before the temperature reaches TC, magnetic compensation occurs at 105 °C. This magnetic compensation comes from the competition between antiferromagnetic coupling of RE with TM ions and ferromagnetic interactions between TM ions [23]. A series of films with variable thicknesses is sputtered at the low pressure of 0.2 Pa and the power of 68 W. Magnetic loops in Fig. 3 show all the films possess good PMA. Fig. 3b descripts the variation of Hc and saturation magnetic moment per unit area against film thickness. The error estimates for each data point are made from several repeated measurements on the same film. A decrease in coercivity with thickness is demonstrated for the sputtered films. The thickness dependence of coercivity was previously reported in amorphous perpendicular TbDyFeCo films with similar thickness [24]. This might be related to the thickness dependent change in microstructure and/or stress in the amorphous RE-TM films [16,25]. The saturation magnetic moment per unit area linearly increases with the thickness of the films, as expected. The effective saturation magnetization Ms is determined to be 142.5 ± 4.64 emu/cm3 from the slope of the linear fitting of thickness dependent
Fig. 2. a) In-plane and out-of-plane magnetic loops for a 150 nm TbFeCo film sputtered at the Ar pressure of 0.2 Pa and the power of 68 W. b) XRD pattern and c) thermomagnetic curve of the film.
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Fig. 3. a) Magnetic loops measured from the films with different thickness sputtered at the low pressure of 0.2 Pa and the power of 68 W. b) Variation of Hc and saturation magnetic moment per area against film thickness.
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magnetic moment per unit area. In addition, the intercept of the linear fitting with the horizontal axis is 25.7 ± 9.5 nm. This value may be attributed to magnetic dead layer, which arises from the oxidation and the intermixing of the alloy film at the surface or interfaces [26,27]. Fig. 4a and b shows the MOKE and EHE loops of the sputtered films with the thickness ranging from 75 to 450 nm. All the MOKE and EHE measurements confirm the magnetization of films is perpendicular to the sample plane. The MO Kerr rotation angles of the films are quite consistent and measured to be in the range of 0.18–0.22°. In contrast, the amplitude of the Hall resistance is measured to be in the range of 0.087–0.57 Ω and decreases with film thickness. A small slope in the MOKE loops is noticeable, which was often found in the unprotected TbFeCo film [28,29]. This comes from in-plane magnetization component due to the preferential oxidation of Tb element at the sample surface, which accordingly produces a thin FeCo-rich magnetically soft layer on top of the perpendicular TbFeCo films [29]. Fig. 4c and d plot the variation of sheet resistance and Hall resistance against film thickness, respectively. The error estimates for each data point are made from several repeated measurements on the same film. The sheet resistance decreases with the increasing thickness of the film, as we expected, for the reduced defects in the thick films. The sheet resistance of the 75-nm-thick TbFeCo film is measured to be 28.31 ± 1.02 μΩ/square, which corresponds to an electrical resistivity of 212.33 ± 7.65 μΩ cm. For the 450-nm-thick film the sheet resistance is measured to be 4.17 ± 0.15 μΩ/square, corresponding to an electrical resistivity of 187.65 ± 6.75 μΩ cm. These values agree well with the reported values of several hundred μΩ cm in amorphous RE-TM alloy films such as TbFe, SmFe and TbFeCo materials [7,30]. The high resistivity is consistent with the amorphous structure of the films, as confirmed in the XRD measurement. Additionally, the nanoscale chemical inhomogeneity, revealed in amorphous TbFeCo films, can enhance the resistivity [31,32]. The extraordinary Hall resistance can be expressed by RH = (RoH + RsMs)/d [33], where the first term represents the ordinary Hall effect, related to the Lorentz force acting on moving charge carriers with Ro being the ordinary Hall coefficient. This effect is linear in outof-plane field H and is small enough to be ignored in our experiments.
Fig. 4. a) MOKE and b) EHE loops obtained for the sputtered films in the thickness range between 75 and 450 nm. Variation of c) sheet resistance and d) Hall resistance with film thickness.
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The second term arising from the extraordinary Hall effect, which is proportional to the out-of-plane magnetization Ms with the EHE coefficient Rs. From the fitting the thickness dependent hall resistance the saturation Hall resistivity Rs Ms is derived to be 4.33 ± 0.08 μΩ cm. This value agrees well with that reported in amorphous alloy films such as TbFe and SmFe [30]. 4. Conclusions In short, amorphous TbFeCo films with good perpendicular properties are sputtered using a composite target at low Ar pressure. The films possess a high perpendicular anisotropy of ~106 erg/cm3 and an effective saturation magnetization of 142.5 emu/cm3. A large polar Kerr rotation angle of ~ 0.2° at the wavelength of 633 nm is obtained. The saturation extraordinary Hall resistivity of 4.33 μΩ cm is derived from the fitting of the thickness dependent Hall resistance. Acknowledgments This work was supported by Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (No. ZQN-YX107) and Research Start-up Funding for Highlevel Talents Project Sponsored by Huaqiao University (No. 13BS401). References [1] T. Hatori, M. Okuda, S. Nakagawa, Influence of spin polarization enhancement layer to rare earth–transition metal thin films for perpendicular tunneling magnetoresistance evaluated by ferromagnetic Hall effect, J. Appl. Phys. 99 (2006) (08C513–1–3). [2] N. Anuniwat, M. Ding, S.J. Poon, S.A. Wolf, J.W. Lu, Strain-induced enhancement of coercivity in amorphous TbFeCo films, J. Appl. Phys. 113 (2013) (043905–1–5). [3] M. Nakayama, T. Kai, N. Shimomura, M. Amano, E. Kitagawa, T. Nagase, M. Yoshikawa, T. Kishi, S. Ikegawa, H. Yoda, Spin transfer switching in TbCoFe ∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy, J. Appl. Phys. 103 (2008) (07 A710–1–3). [4] C.M. Lee, L.X. Ye, T.H. Hsieh, C.Y. Huang, T.H. Wu, Magnetic properties of TbFeCobased perpendicular magnetic tunnel junctions, J. Appl. Phys. 107 (2010) (09C712–1–3). [5] M.T. Rahman, X. Liu, M. Matsumoto, A. Morisako, Magnetoresistance in amorphous TbFeCo films with perpendicular magnetic anisotropy, J. Appl. Phys. 97 (2005) (10C515–1–3). [6] H. Awano, Investigation of domain wall motion in RE-TM magnetic wire towards a current driven memory and logic, J. Magn. Magn. Mater. 381 (2015) 50–55. [7] Z.Y. Zhao, M. Jamali, A.K. Smith, J.P. Wang, Spin Hall switching of the magnetization in Ta/TbFeCo structures with bulk perpendicular anisotropy, Appl. Phys. Lett. 106 (2015) (132404–1–4). [8] M. Murakami, M. Birukawa, Sputtering gases and pressure effects on the microstructure, magnetic properties and recording performance of TbFeCo films, J. Magn. Magn. Mater. 320 (2008) 608–611. [9] S.Q. Yin, X.Q. Li, X.G. Xu, J. Miao, Y. Jiang, Effect of Ta underlayer on perpendicular anisotropy of TbFeCo films, IEEE Trans. Magn. 47 (2011) 3129–3131. [10] L.N. Tong, T.T. Li, X.M. He, P. Deng, The effects of the growth pressure and annealing conditions on perpendicular magnetic anisotropy of sputtered NdFeCo films on Si(111), J. Alloys Compd. 605 (2014) 149–156. [11] Y. Suzuki, S. Takayama, F. Kirino, N. Ohta, Single ion model for perpendicular magnetic anisotropy in RE-TM amorphous films, IEEE Trans. Magn. 23 (1987) 2275–2277.
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