- Email: [email protected]
Target grain size dependence of the morphology, crystallinity and magnetic properties of yttrium iron garnet films
Author’s Accepted Manuscript Target grain size dependence of the morphology, crystallinity and magnetic properties of yttrium iron garnet films Qianwen Guo, Hui Zheng, Liang Zheng, Peng Zheng, Qiong Wu www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)33074-8 https://doi.org/10.1016/j.ceramint.2018.10.255 CERI19967
To appear in: Ceramics International Received date: 31 August 2018 Revised date: 16 October 2018 Accepted date: 31 October 2018 Cite this article as: Qianwen Guo, Hui Zheng, Liang Zheng, Peng Zheng and Qiong Wu, Target grain size dependence of the morphology, crystallinity and magnetic properties of yttrium iron garnet films, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.255 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 galley proof before it is published in its final citable 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.
Target grain size dependence of the morphology, crystallinity and magnetic properties of yttrium iron garnet films Qianwen Guo a, Hui Zheng a*, Liang Zheng a, Peng Zheng a, Qiong Wu b a
Laboratory for Nanoelectronics and NanoDevices, School of Electronic Information, Hangzhou Dianzi
University, Hangzhou 310018, China b
*
Magnetism Key Laboratory of Zhejiang Province, China Jiliang University, Hangzhou 310018, China E-mail: [email protected]
Abstract: In this work, yttrium iron garnet films were prepared on a Ga3Gd5O12 substrate by pulsed laser deposition. The dependence of target grain size on the morphology, crystallinity, and magnetism of films was investigated. Firstly, X-ray diffraction revealed that the fine grain size of the target was beneficial for increasing the crystallinity of films. Secondly, scanning electron microscopy and roughness measurements displayed the film prepared by the small grain size of the target has better morphology and lower roughness. Lastly, magnetic hysteresis loops and ferromagnetic resonance measurements further shown that a good microstructure and fine grain size of target facilitated the growth of film with greater magnetic properties. Consequently, YIG film deposited by 1.0 m-target has magnetic properties with a high saturation magnetization of 155 emu/cm3, a 1
low saturation magnetic field of 1077 Oe, and low ferromagnetic resonance linewidth of 42 Oe.
Key words: Ferrite film, Target material, Pulsed laser deposition, Grain size, Microstructure
1.Introduction Since of the P. C. Dorsey first reported high quality of yttrium iron garnet (Y3Fe5O12, YIG) films by pulsed laser deposition (PLD) method in 1993, it has been widely studied by researchers over the past 25 years [1]. Recently, this material attracts researchers’ attention once again for its remarkable microwave properties applied in microwave devices. It owns appropriate saturation magnetization (Ms), high resistivity and low ferromagnetic resonance (FMR) linewidth, which is suitable for using in spin-wave device [2-5]. The research efforts are mainly focused on preparation and study the high quality of YIG film [6-8]. PLD has been recognized as one of the best technical choices for deposition of high-quality films [9]. In PLD, the sufficiently high energy density laser is focused on the target of the material to be deposited. A small amount of the material is evaporated or ablated in each laser pulse 2
irradiation to create a plasma plume. The ablated material flows to the substrate in a highly forward-directed plume and deposits [10]. Therefore, the target materials using in PLD should have a uniform texture to ensure a long-term stable plasma plume and be chemically suitable to make long tapes with reproducible critical properties [11, 12]. In a word, the quality of film is directly affected by the performance of the target. Several types of research have confirmed this point by experiments. J. Xu and Z. Yang have fabricated ITO (Tin-doped indium oxide) films by three kinds of targets grain size and found the small grain size contributed to improving the uniformity of electrical and optical properties [13]. M. Reza and Z. Sajuri also has prepared thin Ni film and observed that sputtering target with smaller grain size produced thin films with smaller particle size and larger coercivity value [14]. Therefore, the microstructure of the target materials, especially grain size, plays an important role in the determining of the film properties, however, related research has rarely reported. In this paper, high density of YIG targets with different grain size was self-fabricated using various methods; then the YIG films were deposited on Ga3Gd5O12 (GGG) substrate by PLD method under the different targets. The dependence of target grain size on the morphology, crystallinity and magnetic properties of YIG films will be studied in detail.
3
2. Experimental methods YIG targets with different grain sizes were prepared using a different method, as seen in Table 1. The target A with 1 m of grain size was prepared by using the sol-gel method [15, 16] for YIG powder and the two-step sintering (TSS) [17-20] for YIG ceramic. The TSS process is rapidly rising to a high temperature (T1), then immediately cooling to a lower temperature (T2) and holding it for a long time (t) [17]. The 2.3 m-target B was prepared based on the solid-state reaction method for YIG powder and the TSS method for YIG ceramic. The 5.0 m-target C and the 10.0 m-target D were synthesized based on the solid-state reaction method for YIG powder and the one-step sintering (OSS) for YIG ceramic. The details of the prepared parameters can be seen in Table 1. The condition of 1300 ℃-1225 °C-18 h means the T1=1300 °C, T2=1225 °C, and t=18 h. The condition of 1350 °C-12 h implies the temperature rises to 1350 °C in one step and keeps for 12 hours. Table 1 Preparation methods of YIG targets YIG targets
Preparation methods
Parameters
A
Sol-gel and TSS
1300 °C-1225 °C-18 h [21]
B
Solid state reaction and TSS
1350 °C-1300 °C-18 h [22]
C
Solid state reaction and OSS
1350 °C-12 h
D
Solid state reaction and OSS
1400 °C-12 h 4
YIG films were prepared on Ga3Gd5O12 (GGG) substrate by using PLD with KrF excimer laser (248 nm) [23]. Specific experimental parameters are listed in Table 2. The distance between the target and the substrate was 50 mm. It kept the laser energy at 200 mJ and frequency at 5 Hz. The substrate temperature and oxygen pressure were controlled at 750 °C and 1 Pa respectively. When the depositions were completed, the films were annealed at 750 °C for 3 h in the air with a heating rate of 2 °C/min, then cooled it down at the rate of 2 °C/min until reaching room temperature. Table 2 Deposition parameters of YIG films Parameters
Values
Substrate temperature (°C)
750
Laser energy (mJ)
200
Laser frequency (Hz)
5
Oxygen pressure (Pa)
1
The distance between target and substrate (mm)
50
Film thickness (nm)
400
The surface morphologies of YIG targets and films were measured by a scanning electron microscope (SEM, JSM-6460LV). The crystal structures of the targets materials and films were characterized by X-ray diffraction (XRD, Ultima IV) from 20 to 60 °. A vibrating film magnetometer (VSM, Lake Shore 7410) was employed to measure the magnetic hysteresis loops 5
of the films. A profile-system (Dektak XT) was applied to estimate the films thickness and surface roughness. Ferromagnetic resonance (FMR) measurements were carried out using an electron spin resonance (ESR) spectrometer at 9.78 GHz with field parallel to the film plane.
3. Results and discussions Fig. 1 shows the morphology and crystallinity of the prepared YIG targets under different conditions. Firstly, all targets have a homogeneous microstructure and high density which measured to be 99% by the Archimedes method. The grain sizes of the targets were also calculated to be 1.0, 2.3, 5.0, and 10.0 m respectively. The obtained 1.0 m-YIG target can be ascribed to the sol-gel method for nano-size of YIG powder and the two-step sintering method for sintering. As is known, the nano-size of powder has the higher activity than the micron-size powder (prepared by the solid-state method) due to the more upper superficial area. In addition, the two-step sintering method is benefit to grain-boundary diffusion while not for grain-boundary migration, which is suitable for sintering ceramics with high density and fine grain size [17]. Therefore, the target A with a fine grain of 1.0 m was obtained using the sol-gel combined the TSS method. The XRD result of the targets and the corresponding standard peaks (PDF#43-0507) were also inserted into Fig. 1, which indicated the 6
single-phase of the targets.
Fig. 1. Morphology and crystal structure of YIG targets prepared under different conditions: (a) 1.0 m; (b) 2.3 m; (c) 5.0 m; (d) 10.0 m; (e) XRD pattern
Fig. 2 displays the XRD patterns of YIG films prepared by using the above four targets. As it can be observed, except of YIG (444) peak which was near GGG substrate (444) peak, no other diffraction peak of YIG was detected. It is indicated that the growth of YIG films was highly oriented on the substrate. Also, the degree of the YIG (444) peak was decreased with the increase of target grain size, which was marked by the black dotted line. It can be concluded that the film made by 1.0 m-target has the best crystallinity. The dependence of YIG target grain size on the crystallinity of YIG films can be ascribed to the different crystallization temperature of the films deposited by using different targets. It is well known, the adsorbed particles have primarily consisted of atomic, molecular, and other low-mass 7
species during the sputtering. Usually the low-mass species, which is a small piece of an ablated particle, needs more energy to combine with the film completely. As the target grain size increases, the number and size of low-mass species increase accordingly and it causes the increase in crystallization temperature. It is known that low crystallization temperature makes it easier to grow films with good crystallinity. In this case, the film deposited by the small grain size of the target will be more uniform and present better crystallinity.
Fig. 2. XRD patterns of YIG films with a different grain size of targets. The inset shows the enlarged pattern with 2 Theta from 48 to 53o
The surface of the films prepared by different grain size of targets are displayed in Fig. 3. The film prepared by 1 m-target has a smooth surface and uniform grain without conspicuous protruding grain on the surface. 8
However, the morphology of the film is deteriorated with the increase in the grain size of the target. Some protruding grains can be seen on the film surface in Fig. 3(c), and the number and size of the protruding grains are significantly increased in Fig. 3(d). Protruding grains are formed by large size sputtered particles which depend on the target. The target in fine grain size is a benefit to reduce the size of sputtered particles and improve the crystallinity of films. The roughness of films was also measured and inserted in the Figure. The YIG film prepared by 1.0 m-target exhibits a roughness (Ra) only ～2.0 nm, while increases to ～16.1 nm by using 10.0 m-target.
Fig. 3. SEM patterns and roughness data of YIG films with a different target grain sizes (a)1.0 m; (b) 2.3 m; (c) 5.0 m; (d) 10.0 m. The inset shows the roughness profiles of the prepared films. 9
Fig. 4. Hysteresis loops for prepared YIG films with different grain size of targets. The inset shows the enlarged curves of M-H loops with H varies from -200 to 200 Oe.
The magnetic hysteresis loops of the films prepared by different targets are shown in Fig. 4. It can be seen that the magnetism of the films, especially Ms and saturation magnetic field (Hs), are closely related to target grain sizes. Besides, the distinct loops can also be observed from the inset in which the values of H varied from -200 to 200 Oe. The Hc values of these four films are less than 30 Oe, which exhibit the typical soft magnetic property. Fig. 5 shows the dependence of the Ms and Hs on target grain sizes. Firstly, the Ms of films fabricated by 1.0 m-target reaches 155 emu/cm3, which is 11% bigger than that of bulk material (139 emu/cm3) [24]. As the grain size of the target increases, the Ms value reduces. From the above XRD results, it is known that the crystallinity of the film is 10
improved with the reduction of target grain size, which would result in the advance of Ms value. Secondly, the Hs of films fabricated by 1.0 m-target is 1077 Oe, and the value rises with the increasing target grain size. According to the SEM result, the films roughness decreases and uniformity increases when target grain size reduces, which is the main reasons for this phenomenon.
Fig. 5. Dependence of the target grain sizes on the Ms and Hs of the film
Fig. 6 represents the FMR spectra of YIG films measured at 9.78 GHz with field parallel to the film plane. A strong resonance peak of the film prepared by 1m-target was detected in the applied magnetic field with a strength of 2665 Oe. There were other resonance peaks present at the edge of the highest resonance peak, which may be ascribed to the non-uniform of the film thickness. The peak-to-peak linewidth (H) of 1m-target-film 11
is 42 Oe, which is comparable with the value of 38 [email protected] m YIG film by A. Sposito [25] and 30~50 Oe @0.25 m YIG film by B.Bhoi [26]. The dependence of FMR linewidth on the target grain size is also displayed in the inset of Fig. 6, which indicates that the FMR linewidth increases with the target grain size increasing. As is well known, FMR linewidth is affected by two magnetron scattering process, which is influenced by the anisotropy of the randomly oriented crystalline grains, racks, pores, and grain size distribution [27, 28]. The film prepared by 1m-target presents the best crystallinity and microstructure which are the main reasons for its high magnetic and FMR properties.
Fig. 6. FMR pattern of YIG film with 1.0 m grain sizes. The inset shows the dependence of grain size and FMR linewidth.
4. Conclusion In this paper, the dependence of target grain sizes on the morphology, 12
crystallinity and magnetic properties of films was investigated. Measurement results show high crystallinity, excellent morphology, and great magnetism properties can be obtained using small grain size of the target. Specifically, oriented (400 nm) YIG film with high Ms of 155 emu/cm3, low Hs of 1077 Oe and low FMR linewidth of 42 Oe has been prepared by 1.0 m-target.
Acknowledgments This work is funded by National Natural Science Foundation of China (Grant No. 51702075), the Key R&D Program of Zhejiang Province of China (No. 2017C01004).
References [1] P.C. Dorsey, S.E. Bushnell, R.G. Seed, C. Vittoria, Epitaxial yttrium iron garnet films grown by pulsed laser deposition, J. Appl. Phys. 74 (1993) 1242-1246. [2] M. Collet, O. Gladii, M. Evelt, V. Bessonov, L. Soumah, P. Bortolotti, S.O. Demokritov, Y. Henry, V. Cros, M. Bailleul, Spin-wave propagation in ultra-thin YIG based waveguides, Appl. Phys. Lett. 110 (2017) 260301. [3] A. Krysztofik, H. Głowiński, P. Kuświk, S. Ziętek, L.E. Coy, J.N. Rychły, S. Jurga, T.W. Stobiecki, J. Dubowik, Characterization of spin wave propagation in (111) YIG thin films with large anisotropy, J. Phys. D-Appl. Phys. 50 (2017) 235004. [4] A.B. Ustinov, A.A. Nikitin, B.A. Kalinikos, Magnetically Tunable Microwave
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Spin-Wave Photonic Oscillator, IEEE Magn. Lett. 6 (2015) 1-4. [5] Y. Yang, Z. Yu, Q. Guo, K. Sun, R. Guo, X. Jiang, Y. Liu, H. Liu, G. Wu, Z. Lan, Thermomagnetization Characteristics and Ferromagnetic Resonance Linewidth Broadening Mechanism for Ca-Sn Co-substituted YIG Ferrites, Ceram. Int. 44 (2018) 11718-11723. [6] M. Lang, M. Montazeri, M.C. Onbasli, X. Kou, Y. Fan, P. Upadhyaya, K. Yao, F. Liu, Y. Jiang, W. Jiang, Proximity Induced High-Temperature Magnetic Order in Topological Insulator - Ferrimagnetic Insulator Heterostructure, Nano Lett. 14 (2014) 3459-3465. [7] K. Ando, S. Watanabe, S. Mooser, E. Saitoh, H. Sirringhaus, Solution-processed organic spin–charge converter, Nat. Mater. 12 (2013) 622. [8] J. Lian, Y. Chen, Z. Liu, M. Zhu, G. Wang, W. Zhang, X. Dong, Annealing effects on the microstructure and magnetic properties of Y3Fe5O12 films deposited on Si/SiO2 substrates by RF magnetron sputtering, Ceram. Int. 43 (2017) 7477-7481. [9] Z. Z, N. A, T. J, Single cuprous oxide films synthesized by radical oxidation at low temperature for PV application, Opt. Express. 21 (2013) 11448-11456. [10] Y. Umeda, F. Mitsugi, T. Ikegami, Properties of GZO Thin Film Deposited at Various Positions in the Plasma Plume in PLD Method, Can. J. Chem. 66 (2009) 651-654. [11] S.V. Shavkin, A.K. Shikov, I.A. Chernykh, V.V. Guryev, E.S. Kovalenko, E.V. Yakovenko, M.L. Zanaveskin, D.N. Rakov, A.E. Vorobieva, Diagnostics of target inhomogeneity and influence of YBCO target oxygen content on properties of HTSC
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2G samples fabricated by PLD technique, J. Phys.: Conf. Ser. 507 (2014) 022030. [12] Y. Le, K. Tanabe, Influence of target processing on properties of NdBa2Cu3O7-delta films grown by PLD: Comparison of sintered ceramic and single crystal targets, IEEE Trans. Appl. Supercond. 9 (1999) 1586-1589. [13] J. Xu, Z. Yang, X. Zhang, H. Wang, H. Xu, Grain size control in ITO targets and its effect on electrical and optical properties of deposited ITO films, J. Mater. Sci.-Mater. Electron. 25 (2014) 710-716. [14] M. Reza, Z. Sajuri, J. Yunas, J. Syarif, Effect of sputtering target's grain size on the sputtering yield, particle size and coercivity (Hc) of Ni and Ni20Al thin films, Mater. Sci. Eng. 114 (2016) 012116. [15] S.H. Vajargah, H.R.M. Hosseini, Z.A. Nemati, Preparation and characterization of yttrium iron garnet (YIG) nanocrystalline powders by auto-combustion of nitrate-citrate gel, J. Alloy. Compd. 430 (2007) 339-343. [16] L. Fernandez-Garcia, M. Suarez, J.L. Menendez, Synthesis of mono and multidomain YIG particles by chemical coprecipitation or ceramic procedure, J. Alloy. Compd. 495 (2010) 196-199. [17] I.W. Chen, X.H. Wang, Sintering dense nanocrystalline ceramics without final-stage grain growth, Nature 404 (2000) 168. [18] H.T. Kim, Y.H. Han, Sintering of nanocrystalline BaTiO3, Ceram. Int. 30 (2004) 1719-1723. [19] X.H. Wang, X.Y. Deng, H.L. Bai, H. Zhou, W.G. Qu, L.T. Li, I.W. Chen, Two‐ Step
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Sintering of Ceramics with Constant Grain‐ Size, II: BaTiO3 and Ni–Cu–Zn Ferrite, J. Am. Ceram. Soc. 89 (2010) 438-443. [20] P. Durán, F. Capel, D. Gutierrez, J. Tartaj, C. Moure, Cerium (IV) oxide synthesis and sinterable powders prepared by the polymeric organic complex solution method, J. Eur. Ceram. Soc. 22 (2002) 1711-1721. [21] R. Chen, J. Zhou, L. Zheng, H. Zheng, P. Zheng, Z. Ying, J. Deng, Two-Step Sintering Behavior of Sol–Gel Derived Dense and Submicron-Grained YIG Ceramics, J. Electron. Mater. 47 (2018) 2411-2416. [22] E. Zhou, H. Zheng, L. Zheng, P. Zheng, Z. Ying, J. Deng, J. Zhou, Synthesis of dense, fine‐ grained hexagonal barium ferrite ceramics by two‐ step sintering process, Int. J. Appl. Ceram. Technol. 15 (2018) 1023-1029. [23] R. Peña-Garcia, A. Delgado, Y. Guerra, E. Padrón-Hernández, Yig Films With Low Magnetic Damping Obtained By Solgel On Silicon (100), Mater. Lett. 161 (2015) 384-386. [24] H. Zheng, J.X. Deng, L. Zheng, H.B. Qin, J. Wu, J.J. Zhou, Influence of Pre-Sintering Temperature and Time on the Properties of YIG Target, Adv. Mater. Res. 476-478 (2012) 1150-1153. [25] A. Sposito, T. C. May-Smith, G. B. G. Stenning, P. A. J. de Groot , R. W. Eason, Opt. Mater. Express 3 (2013) 624-632. [26] B. Bhoi, B. Sahu, N. Venkataramani, R.P.R.C. Aiyar, and Shiva Prasad, IEEE Trans. Magn. PP(99) (2018) 1-5 DOI: 10.1109/TMAG.2018.2842260.
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[27] M.J. Hurben, C.E. Patton, Theory of two magnon scattering microwave relaxation and ferromagnetic resonance linewidth in magnetic thin films, J. Appl. Phys. 1998, 83(8):4344-4365. [28] A. Azevedo, A.B. Oliveira, F.M. De Aguiar, S.M. Rezende, Extrinsic contributions to spin-wave damping and renormalization in thin Ni50Fe50 Films. Phys. Rev. B 62, (2000) 5331-5333.
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PII: DOI: Reference:
S0272-8842(18)33074-8 https://doi.org/10.1016/j.ceramint.2018.10.255 CERI19967
To appear in: Ceramics International Received date: 31 August 2018 Revised date: 16 October 2018 Accepted date: 31 October 2018 Cite this article as: Qianwen Guo, Hui Zheng, Liang Zheng, Peng Zheng and Qiong Wu, Target grain size dependence of the morphology, crystallinity and magnetic properties of yttrium iron garnet films, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.255 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 galley proof before it is published in its final citable 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.
Target grain size dependence of the morphology, crystallinity and magnetic properties of yttrium iron garnet films Qianwen Guo a, Hui Zheng a*, Liang Zheng a, Peng Zheng a, Qiong Wu b a
Laboratory for Nanoelectronics and NanoDevices, School of Electronic Information, Hangzhou Dianzi
University, Hangzhou 310018, China b
*
Magnetism Key Laboratory of Zhejiang Province, China Jiliang University, Hangzhou 310018, China E-mail: [email protected]
Abstract: In this work, yttrium iron garnet films were prepared on a Ga3Gd5O12 substrate by pulsed laser deposition. The dependence of target grain size on the morphology, crystallinity, and magnetism of films was investigated. Firstly, X-ray diffraction revealed that the fine grain size of the target was beneficial for increasing the crystallinity of films. Secondly, scanning electron microscopy and roughness measurements displayed the film prepared by the small grain size of the target has better morphology and lower roughness. Lastly, magnetic hysteresis loops and ferromagnetic resonance measurements further shown that a good microstructure and fine grain size of target facilitated the growth of film with greater magnetic properties. Consequently, YIG film deposited by 1.0 m-target has magnetic properties with a high saturation magnetization of 155 emu/cm3, a 1
low saturation magnetic field of 1077 Oe, and low ferromagnetic resonance linewidth of 42 Oe.
Key words: Ferrite film, Target material, Pulsed laser deposition, Grain size, Microstructure
1.Introduction Since of the P. C. Dorsey first reported high quality of yttrium iron garnet (Y3Fe5O12, YIG) films by pulsed laser deposition (PLD) method in 1993, it has been widely studied by researchers over the past 25 years [1]. Recently, this material attracts researchers’ attention once again for its remarkable microwave properties applied in microwave devices. It owns appropriate saturation magnetization (Ms), high resistivity and low ferromagnetic resonance (FMR) linewidth, which is suitable for using in spin-wave device [2-5]. The research efforts are mainly focused on preparation and study the high quality of YIG film [6-8]. PLD has been recognized as one of the best technical choices for deposition of high-quality films [9]. In PLD, the sufficiently high energy density laser is focused on the target of the material to be deposited. A small amount of the material is evaporated or ablated in each laser pulse 2
irradiation to create a plasma plume. The ablated material flows to the substrate in a highly forward-directed plume and deposits [10]. Therefore, the target materials using in PLD should have a uniform texture to ensure a long-term stable plasma plume and be chemically suitable to make long tapes with reproducible critical properties [11, 12]. In a word, the quality of film is directly affected by the performance of the target. Several types of research have confirmed this point by experiments. J. Xu and Z. Yang have fabricated ITO (Tin-doped indium oxide) films by three kinds of targets grain size and found the small grain size contributed to improving the uniformity of electrical and optical properties [13]. M. Reza and Z. Sajuri also has prepared thin Ni film and observed that sputtering target with smaller grain size produced thin films with smaller particle size and larger coercivity value [14]. Therefore, the microstructure of the target materials, especially grain size, plays an important role in the determining of the film properties, however, related research has rarely reported. In this paper, high density of YIG targets with different grain size was self-fabricated using various methods; then the YIG films were deposited on Ga3Gd5O12 (GGG) substrate by PLD method under the different targets. The dependence of target grain size on the morphology, crystallinity and magnetic properties of YIG films will be studied in detail.
3
2. Experimental methods YIG targets with different grain sizes were prepared using a different method, as seen in Table 1. The target A with 1 m of grain size was prepared by using the sol-gel method [15, 16] for YIG powder and the two-step sintering (TSS) [17-20] for YIG ceramic. The TSS process is rapidly rising to a high temperature (T1), then immediately cooling to a lower temperature (T2) and holding it for a long time (t) [17]. The 2.3 m-target B was prepared based on the solid-state reaction method for YIG powder and the TSS method for YIG ceramic. The 5.0 m-target C and the 10.0 m-target D were synthesized based on the solid-state reaction method for YIG powder and the one-step sintering (OSS) for YIG ceramic. The details of the prepared parameters can be seen in Table 1. The condition of 1300 ℃-1225 °C-18 h means the T1=1300 °C, T2=1225 °C, and t=18 h. The condition of 1350 °C-12 h implies the temperature rises to 1350 °C in one step and keeps for 12 hours. Table 1 Preparation methods of YIG targets YIG targets
Preparation methods
Parameters
A
Sol-gel and TSS
1300 °C-1225 °C-18 h [21]
B
Solid state reaction and TSS
1350 °C-1300 °C-18 h [22]
C
Solid state reaction and OSS
1350 °C-12 h
D
Solid state reaction and OSS
1400 °C-12 h 4
YIG films were prepared on Ga3Gd5O12 (GGG) substrate by using PLD with KrF excimer laser (248 nm) [23]. Specific experimental parameters are listed in Table 2. The distance between the target and the substrate was 50 mm. It kept the laser energy at 200 mJ and frequency at 5 Hz. The substrate temperature and oxygen pressure were controlled at 750 °C and 1 Pa respectively. When the depositions were completed, the films were annealed at 750 °C for 3 h in the air with a heating rate of 2 °C/min, then cooled it down at the rate of 2 °C/min until reaching room temperature. Table 2 Deposition parameters of YIG films Parameters
Values
Substrate temperature (°C)
750
Laser energy (mJ)
200
Laser frequency (Hz)
5
Oxygen pressure (Pa)
1
The distance between target and substrate (mm)
50
Film thickness (nm)
400
The surface morphologies of YIG targets and films were measured by a scanning electron microscope (SEM, JSM-6460LV). The crystal structures of the targets materials and films were characterized by X-ray diffraction (XRD, Ultima IV) from 20 to 60 °. A vibrating film magnetometer (VSM, Lake Shore 7410) was employed to measure the magnetic hysteresis loops 5
of the films. A profile-system (Dektak XT) was applied to estimate the films thickness and surface roughness. Ferromagnetic resonance (FMR) measurements were carried out using an electron spin resonance (ESR) spectrometer at 9.78 GHz with field parallel to the film plane.
3. Results and discussions Fig. 1 shows the morphology and crystallinity of the prepared YIG targets under different conditions. Firstly, all targets have a homogeneous microstructure and high density which measured to be 99% by the Archimedes method. The grain sizes of the targets were also calculated to be 1.0, 2.3, 5.0, and 10.0 m respectively. The obtained 1.0 m-YIG target can be ascribed to the sol-gel method for nano-size of YIG powder and the two-step sintering method for sintering. As is known, the nano-size of powder has the higher activity than the micron-size powder (prepared by the solid-state method) due to the more upper superficial area. In addition, the two-step sintering method is benefit to grain-boundary diffusion while not for grain-boundary migration, which is suitable for sintering ceramics with high density and fine grain size [17]. Therefore, the target A with a fine grain of 1.0 m was obtained using the sol-gel combined the TSS method. The XRD result of the targets and the corresponding standard peaks (PDF#43-0507) were also inserted into Fig. 1, which indicated the 6
single-phase of the targets.
Fig. 1. Morphology and crystal structure of YIG targets prepared under different conditions: (a) 1.0 m; (b) 2.3 m; (c) 5.0 m; (d) 10.0 m; (e) XRD pattern
Fig. 2 displays the XRD patterns of YIG films prepared by using the above four targets. As it can be observed, except of YIG (444) peak which was near GGG substrate (444) peak, no other diffraction peak of YIG was detected. It is indicated that the growth of YIG films was highly oriented on the substrate. Also, the degree of the YIG (444) peak was decreased with the increase of target grain size, which was marked by the black dotted line. It can be concluded that the film made by 1.0 m-target has the best crystallinity. The dependence of YIG target grain size on the crystallinity of YIG films can be ascribed to the different crystallization temperature of the films deposited by using different targets. It is well known, the adsorbed particles have primarily consisted of atomic, molecular, and other low-mass 7
species during the sputtering. Usually the low-mass species, which is a small piece of an ablated particle, needs more energy to combine with the film completely. As the target grain size increases, the number and size of low-mass species increase accordingly and it causes the increase in crystallization temperature. It is known that low crystallization temperature makes it easier to grow films with good crystallinity. In this case, the film deposited by the small grain size of the target will be more uniform and present better crystallinity.
Fig. 2. XRD patterns of YIG films with a different grain size of targets. The inset shows the enlarged pattern with 2 Theta from 48 to 53o
The surface of the films prepared by different grain size of targets are displayed in Fig. 3. The film prepared by 1 m-target has a smooth surface and uniform grain without conspicuous protruding grain on the surface. 8
However, the morphology of the film is deteriorated with the increase in the grain size of the target. Some protruding grains can be seen on the film surface in Fig. 3(c), and the number and size of the protruding grains are significantly increased in Fig. 3(d). Protruding grains are formed by large size sputtered particles which depend on the target. The target in fine grain size is a benefit to reduce the size of sputtered particles and improve the crystallinity of films. The roughness of films was also measured and inserted in the Figure. The YIG film prepared by 1.0 m-target exhibits a roughness (Ra) only ～2.0 nm, while increases to ～16.1 nm by using 10.0 m-target.
Fig. 3. SEM patterns and roughness data of YIG films with a different target grain sizes (a)1.0 m; (b) 2.3 m; (c) 5.0 m; (d) 10.0 m. The inset shows the roughness profiles of the prepared films. 9
Fig. 4. Hysteresis loops for prepared YIG films with different grain size of targets. The inset shows the enlarged curves of M-H loops with H varies from -200 to 200 Oe.
The magnetic hysteresis loops of the films prepared by different targets are shown in Fig. 4. It can be seen that the magnetism of the films, especially Ms and saturation magnetic field (Hs), are closely related to target grain sizes. Besides, the distinct loops can also be observed from the inset in which the values of H varied from -200 to 200 Oe. The Hc values of these four films are less than 30 Oe, which exhibit the typical soft magnetic property. Fig. 5 shows the dependence of the Ms and Hs on target grain sizes. Firstly, the Ms of films fabricated by 1.0 m-target reaches 155 emu/cm3, which is 11% bigger than that of bulk material (139 emu/cm3) [24]. As the grain size of the target increases, the Ms value reduces. From the above XRD results, it is known that the crystallinity of the film is 10
improved with the reduction of target grain size, which would result in the advance of Ms value. Secondly, the Hs of films fabricated by 1.0 m-target is 1077 Oe, and the value rises with the increasing target grain size. According to the SEM result, the films roughness decreases and uniformity increases when target grain size reduces, which is the main reasons for this phenomenon.
Fig. 5. Dependence of the target grain sizes on the Ms and Hs of the film
Fig. 6 represents the FMR spectra of YIG films measured at 9.78 GHz with field parallel to the film plane. A strong resonance peak of the film prepared by 1m-target was detected in the applied magnetic field with a strength of 2665 Oe. There were other resonance peaks present at the edge of the highest resonance peak, which may be ascribed to the non-uniform of the film thickness. The peak-to-peak linewidth (H) of 1m-target-film 11
is 42 Oe, which is comparable with the value of 38 [email protected] m YIG film by A. Sposito [25] and 30~50 Oe @0.25 m YIG film by B.Bhoi [26]. The dependence of FMR linewidth on the target grain size is also displayed in the inset of Fig. 6, which indicates that the FMR linewidth increases with the target grain size increasing. As is well known, FMR linewidth is affected by two magnetron scattering process, which is influenced by the anisotropy of the randomly oriented crystalline grains, racks, pores, and grain size distribution [27, 28]. The film prepared by 1m-target presents the best crystallinity and microstructure which are the main reasons for its high magnetic and FMR properties.
Fig. 6. FMR pattern of YIG film with 1.0 m grain sizes. The inset shows the dependence of grain size and FMR linewidth.
4. Conclusion In this paper, the dependence of target grain sizes on the morphology, 12
crystallinity and magnetic properties of films was investigated. Measurement results show high crystallinity, excellent morphology, and great magnetism properties can be obtained using small grain size of the target. Specifically, oriented (400 nm) YIG film with high Ms of 155 emu/cm3, low Hs of 1077 Oe and low FMR linewidth of 42 Oe has been prepared by 1.0 m-target.
Acknowledgments This work is funded by National Natural Science Foundation of China (Grant No. 51702075), the Key R&D Program of Zhejiang Province of China (No. 2017C01004).
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