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We systematically investigate the influence of annealing effect on terahertz (THz) generation from CoFeB/heavy metal heterostructures driven by femtosecond laser pulses. The THz yield is achieved to increase triply in W/CoFeB through annealing effect, and doubly in Pt/CoFeB. The annealing effect originates from both the decrease of synthetic effect of THz absorption and the increase of hot electron mean free path induced by crystallization, with the latter being dominant, which is experimentally corroborated by THz transmission measurement of time-domain spectrum and four-probe resistivity t. Our observations not only deepen understand the spintronic THz radiation mechanism but also provide a novel platform for high speed spintronic opto-electronic devices.
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Keywords:
- terahertz emission /
- ferromagnetic/heavy metal heterostructure /
- annealing effect /
- super-diffusion model
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Google Scholar
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图 1 实验装置示意图以及样品结构信息 (a) 太赫兹发射实验装置: BD, 平衡探测器; WP, 沃拉斯顿棱镜;
$ \lambda /4 $ , 1/4波片; SW, 硅片; M, 铝反射镜; P1—P4, 90°离轴抛物面镜; S, 样品; (b), (c) 对n–和n+的定义: 抽运激光入射到玻璃基底定义为n–, 入射异质结薄膜定义为n+. HM, 重金属, 包括钨和铂; FM, 铁磁性金属, 即钴铁硼Fig. 1. Schematic diagram of experimental setup and sample structure information. (a) Experimental setup of the terahertz emission system. BD, balanced detector; WP, Wollaston prism; λ/4, quarter wave plate; SW, silicon wafer; M, aluminum mirror; P1–P4, 90° off-axis parabolic mirrors; S, sample; (b) and (c) definitions of n– and n+. When the laser pulses first incident onto the glass substrate, it is defined as n–, while the other case is defined as n+. HM, heavy metals including W and Pt; FM, ferromagnetic metal CoFeB.
图 2 (a), (e) 钴铁硼(2.0)/钨(2.2)和钴铁硼(2.2)/铂(4.0)双层结构示意图; (b), (c) 退火和未退火的钴铁硼(2.0)/钨(2.2)样品辐射的时域太赫兹波形, 分别对应n+和n–情况; (f), (g) 退火和未退火的铂(4.0)/钴铁硼(2.2)样品辐射的时域太赫兹波形, 分别对应n+和n–情况; (d), (h) 退火和未退火钴铁硼(2.0)/钨(2.2)、钴铁硼(2.2)/铂(4.0)样品辐射太赫兹波的频谱
Fig. 2. (a), (e) Bilayer structure diagram of CoFeB(2.0)/W(2.2) and CoFeB(2.2)/Pt(4.0); (b), (c) terahertz temporal waveforms from the CoFeB(2.0)/W(2.2) samples with and without annealing, for the cases of n+ and n–, respectively; (f), (g) terahertz temporal waveforms from the Pt(4.0)/CoFeB(2.2) samples with and without annealing, for the cases of n+ and n–, respectively; (d), (h) terahertz spectra obtained from CoFeB(2.0)/W(2.2) and CoFeB(2.2)/Pt(4.0), respectively, with and without annealing.
图 3 (a), (b) 当两个样品都经退火处理, 飞秒激光脉冲分别从正(n+)和负(n–)侧入射到钨(4.0)/钴铁硼(2.2)和铂(4.0)/钴铁硼(2.2)纳米薄膜上产生的太赫兹时域信号; (d), (e)未退火样品的结果; (c), (f)退火和未退火的钴铁硼(2.2)/钨(4.0)和钴铁硼(2.2)/铂(4.0)辐射太赫兹波的频谱
Fig. 3. (a), (b) terahertz temporal waveforms in the annealed W(4.0)/CoFeB(2.2) and Pt(4.0)/CoFeB(2.2) nanofilms from the positive (n+) and negative (n–) sides, respectively; (d), (e) results from both the samples without annealing; (c), (f) terahertz spectra radiated from annealed and unannealed CoFeB(2.2)/W(4.0) and CoFeB(2.2)/Pt(4.0), respectively.
图 4 (a), (b) 退火和未退火状态下, 钨(4.0)/钴铁硼(2.2)和铂(4.0)/钴铁硼(2.2)样品透射的太赫兹时域信号, 插图为峰值放大图; (c) 样品电阻率的退火温度依赖性, 插图为样品的磁光克尔测量结果; (d) 退火前后, 钴铁硼从非晶态结晶转为结晶态的原子结构示意图; (e)
$z = {z_{\rm{s}}}$ 处, 电子密度的模拟结果, 其中电子速度v = 0.5 nm/fsFig. 4. (a) Transmitted terahertz signals in W(4.0)/CoFeB(2.2) and (b) Pt(4.0)/CoFeB(2.2) with and without annealing. Inset: Enlarged peak values. (c) The annealing temperature dependence of the resistivity of samples. Inset: result of magneto-optic Kerr measurement of the samples. (d) Schematic diagram of the atomization of CoFeB crystallize from amorphous state to crystalline state before and after annealing. (e) The simulation results of electron density at
$z = {z_{\rm{s}}}$ . The electron velocity v = 0.5 nm/fs. -
[1] Zhang X C, Shkurinov A, Zhang Y 2017 Nat. Photonics 11 16
Google Scholar
[2] Wu X J, Ma J L, Zhang B L, Chai S S, Li Y T 2017 Opt. Express 26 7107
[3] Hafez H A, Chai X, Ibrahim A, Mondal S, Férachou D, Ropagnol X, Ozaki T 2016 J. Opt. 18 093004
Google Scholar
[4] Wu X, Quan B, Xu X, Hu F, Li W 2013 Appl. Surf. Sci. 285 853
Google Scholar
[5] Wu X, Xu X, Lu X, Wang L 2013 Appl. Surf. Sci. 279 92
Google Scholar
[6] 张顺浓, 朱伟骅, 李炬赓, 金钻明, 戴晔, 张宗芝 2018 67 197202
Google Scholar
Zhang S N, Zhu W H, Li J G, Jin Z M, Dai Y, Zhang Z Z 2018 Acta Phys. Sin. 67 197202
Google Scholar
[7] Kampfrath T, Battiato M, Maldonado P, Eilers G, Nötzold J, Mährlein S, Zbarsky V, Freimuth F, Mokrousov Y, Blügel S 2013 Nat. Nanotechnol. 8 256
Google Scholar
[8] Seifert T, Jaiswal S, Martens U, Hannegan J, Braun L, Maldonado P, Freimuth F, Kronenberg A, Henrizi J, Radu I 2016 Nat. Photonics 10 483
Google Scholar
[9] Huisman T J, Mikhaylovskiy R V, Costa J D, Freimuth F, Paz E, Ventura J, Freitas P P, Blügel S, Mokrousov Y, Rasing T 2016 Nat. Nanotechnol. 11 455
Google Scholar
[10] Wu Y, Elyasi M, Qiu X, Chen M, Liu Y, Ke L, Yang H 2017 Adv. Mater. 29 1603031
Google Scholar
[11] Sasaki Y, Suzuki K, Mizukami S 2017 Appl. Phys. Lett. 111 102401
Google Scholar
[12] Feng Z, Yu R, Zhou Y, Lu H, Tan W, Deng H, Liu Q C, Zhai Z H, Zhu L G, Cai J W, Miao B F, Ding H F 2018 Adv. Opt. Mater. 6 1800965
Google Scholar
[13] Qiu H S, Kato K, Hirota K, Sarukura N, Yoshimura M, Nakajima M 2018 Opt. Express 26 15247
Google Scholar
[14] Zhou C, Liu Y P, Wang Z, Ma S J, Jia M W, Wu R Q, Zhou L, Zhang W, Liu M K, Wu Y Z, Qi J 2018 Phys. Rev. Lett. 121 086801
Google Scholar
[15] Torosyan G, Keller S, Scheuer L, Beigang R, Papaioannou E T 2018 Sci. Rep. 8 1311
Google Scholar
[16] Zhang S, Jin Z, Zhu Z, Zhu W, Zhang Z, Ma G, Yao J 2017 J. Phys. D: Appl. Phys. 51 034001
[17] Jungfleisch M B, Zhang Q, Zhang W, Pearson J E, Schaller R D, Wen H, Hoffmann A 2018 Phys. Rev. Lett. 120 207207
Google Scholar
[18] Battiato M, Carva K, Oppeneer P 2010 Phys. Rev. Lett. 105 27203
Google Scholar
[19] Kong D Y, Wu X J, Wang B, Nie T X, Xiao M, Pandey C, Gao Y, Wen L G, Zhao W S, Ruan C J, Miao J G, Li Y T, Wang L 2019 Adv. Opt. Mater. 7 1900487
Google Scholar
[20] Chen X H, Wu X J, Shan S Y, Guo F W, Kong D Y, Wang C, Nie T X, Pandey C, Wen L G, Zhao W S, Ruan C J, Miao J G, Li Y T, Wang L 2019 Appl. Phys. Lett. 115 221104
Google Scholar
[21] Wang B, Shan S S, Wu X J, Wang C, Pandey C, Nie T X, Zhao W S, Li Y T, Miao J G, and Wang L 2019 Appl. Phys. Lett. 115 121104
Google Scholar
[22] Yuasa S, Suzuki Y, Katayama T, Ando K 2005 Appl. Phys. Lett. 87 242503
Google Scholar
[23] Wu Q, Zhang X C 1995 Appl. Phys. Lett. 67 3523
Google Scholar
[24] Sinova J, Valenzuela S O, Wunderlich J, Back C, Jungwirth T 2015 Rev. Mod. Phys. 87 1213
Google Scholar
[25] Belmeguenai M, Aitoukaci K, Zighem F, Gabor M, Petrisor J T, Mos R, Tiusan C 2018 J. Appl. Phys. 123 113905
Google Scholar
[26] Swamy G V, Pandey H, Srivastava A, Dalai M, Maurya K, Rashmi, Rakshit R 2013 AIP Advances 3 072129
Google Scholar
[27] Huang S X, Chen T Y, Chien C L 2008 Appl. Phys. Lett. 92 242509
Google Scholar
[28] Hao Q, Chen W, Xiao G 2015 Appl. Phys. Lett. 106 182403
Google Scholar
[29] Guo F W, Pandey C, Wang C, Nie T X, Wen L G, Zhao W S, Miao J G, Wang L, Wu X J 2020 OSA Continuum. 3 893
Google Scholar
[30] Bonetti S, Hoffmann M, Sher M J, Chen Z, Yang S H, Samant M, Parkin S, Dürr H 2016 Phys. Rev. Lett. 117 087205
Google Scholar
[31] Battiato M, Carva K, Oppeneer P M 2012 Phys. Rev. B 86 022404
[32] Zega T J, Hanbicki A T, Erwin S C, Zutic I U, Kioseoglou G, Li C H, Jonker B T, Stroud R M 2006 Microsc. Microanal. 12 972
Google Scholar
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