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铁磁/非磁异质结构中的超快自旋流-电荷流转换实现相干太赫兹辐射得到了广泛研究. 热自旋电子学结合了热输运与磁输运, 可以有效地产生和探测自旋的非平衡输运. 本文利用飞秒激光脉冲激发铁磁绝缘体钇铁石榴石(Y3Fe5O12, YIG)/Pt异质结构, 通过超快自旋塞贝克效应(SSE)产生太赫兹(THz)相干辐射. 实验中, THz脉冲的相位随外加磁场和激光入射样品顺序的反转而反转, 表明THz辐射与界面温度梯度的方向密切相关. 为了考察界面对THz辐射性能的影响, 系统地研究了YIG/Pt异质结构不同退火处理后的THz辐射情况. 实验发现, 生长在Gd3Ga5O12 (GGG)衬底上的YIG/Pt经退火处理后再原生一层Pt膜, 其THz辐射强度提高了一个数量级. 归因于退火后增强了YIG/Pt界面的自旋混合电导率. 此外, 还研究了生长在高阻Si衬底上退火后优化结构的能量密度与THz辐射强度的关系, 拟合得到饱和能量密度约为1.4 mJ/cm2. 实验结果表明, YIG/Pt异质结构的界面调控能够优化THz辐射特性, 为基于超快SSE自旋电子学太赫兹发射器开辟了新的途径.Recently, ferromagnetic/non-magnetic heterostructures have been widely studied for the generation of terahertz (THz) emitter based on spin-to-charge conversion. Actually, thermal spintronics effectively combines thermal transport with magnetism for creating and detecting non-equilibrium spin transport. A spin current or voltage can be induced by a temperature bias applied to a ferromagnetic material, which is called spin Seebeck effect (SSE). In this paper, we present a SSE based THz emission by using the heterostructures made of insulating ferrimagnet yttrium iron garnet (Y3Fe5O12, YIG) and platinum (Pt) with large spin orbit coupling. Upon exciting the Pt layer with a femtosecond laser pulse, a spin Seebeck current arises, applying a temperature gradient to the interface. Based on the inverse spin Hall effect, the spin Seebeck current is converted into a transient charge current and then yields the THz transients, which are detected by electrooptic sampling through using a ZnTe crystal at room temperature. The polarity of the THz pulses is flipped by 180° when the direction of the external magnetic field is reversed. By changing the direction of the pump beam excitation geometry to vary the sign of the temperature gradient at the YIG/Pt interface, the polarity of the THz signal is reversed. Fast Fourier transformation of the THz signals yields the amplitude spectra centered near 0.6 THz with a bandwidth in a range of 0.1–2.5 THz. We systematically investigate the influence of annealing effect on the THz emission from different YIG/Pt heterostructures. It can be found that the THz radiation is achieved to increase ten times in the YIG/Pt grown on a Gd3Ga5O12 (GGG) substrate through high-temperature annealing. The mechanism of annealing effect can be the increase of the spin mixing conductance of the interface between YIG and Pt. Finally, we investigate the pump fluence dependent THz peak-to-peak values for the annealed YIG/Pt grown on the Si substrate. Due to the spin accumulation effect at the interface of the YIG/Pt heterostructure, the THz radiation intensity gradually becomes saturated with the increase of pump fluence. Our results conclude that annealing optimization is of importance for increasing the THz amplitude, and open a new avenue to the future applications of spintronic THz emitters based on ultrafast SSE.
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图 1 (a) THz发射光谱实验装置图; (b) 在YIG/Pt双层膜结构中, 沿z轴方向外加面内磁场H = ± 200 mT, 飞秒激光诱导铁磁绝缘体和非磁性金属界面产生瞬态温度梯度
$ \nabla T $ (沿着–y轴; 红色表示高温, 蓝色表示低温), 超快SSE产生一个从YIG进入Pt层的自旋流(沿着–y轴), 基于ISHE, 在–x轴方向上产生瞬态电荷流; (c) 样品GGG//Pt(10)和GGG//YIG(60)/Pt(10)双层膜的THz辐射信号, +N和–N分别表示激光脉冲从Pt膜一侧和GGG衬底一侧辐照样品; (d), (e), (f) 分别表示(c)中GGG//Pt(10)和GGG//YIG(60)/Pt(10)的3种激发构置下的THz辐射原理图Fig. 1. (a) Schematic of experimental setup for THz generation; (b) schematic of the YIG/Pt bilayer sample placed in the static in-plane magnetic field of ± 200 mT. A femtosecond laser pulse excites the YIG/Pt bilayer, a temperature gradient
$ \nabla T $ is created at the interface of ferromagnetic insulator YIG and nonmagnetic metal Pt, launching a spin current (along the –y direction; the red part means the high temperature side and the blue part describes the low temperature side) from YIG layer into the Pt layer based on the SSE. Within the Pt layer, the spin current is converted into a charge current (along the –x direction) via ISHE; (c) measured electrooptic signal of THz emission from GGG//Pt(10) and GGG//YIG(60)/Pt(10) bilayer. THz emission signals are radiated with front (+N, red) and back (–N, blue) pumps; (d), (e), (f) the THz emission schematics of the three sample cases in (c).图 2 (a) GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3), GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3), GGG//YIG(40)/Pt1st(3)/Pt2nd(3)和Si//YIG(40)/Pt1st(3)/Pt2nd(3)不同结构样品所产生的THz辐射脉冲; (b) 飞秒激光脉冲激发YIG/Pt1st/Pt2nd结构辐射THz信号示意图; (c) 将图 (a) 中GGG//YIG(40)/Pt1st(3)/Pt2nd(3)和Si//YIG(40)/Pt1st(3)/Pt2nd(3)的时域谱线进行傅里叶变换后的归一化频谱图, 插图为THz发射光谱的半高全宽(ΔF)和中心频率(fc)
Fig. 2. (a) THz emitted EOS waveforms of GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3), GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3), GGG//YIG(40)/Pt1st(3)/Pt2nd(3) and Si//YIG(40)/Pt1st(3)/Pt2nd(3) heterostructures (layer thickness in nm); (b) schematic view of THz generation in YIG(40)/Pt1st(3)/Pt2nd(3) heterostructures on GGG and Si substrates via SSE; (c) normalized frequency-domain THz signals of GGG//YIG(40)/Pt1st(3)/Pt2nd(3) and Si//YIG(40)/Pt1st(3)/Pt2nd(3) heterostructures. Inset: the full width at half maximum (ΔF) and center frequency (fc) for the normalized THz amplitude spectrum.
图 3 (a) 外加磁场+H (蓝线)和–H (红线)时, GGG//YIG(40)/Pt1st(3)/Pt2nd(3)结构辐射的THz脉冲; (b) GGG//YIG(40)/Pt1st(3)/Pt2nd(3)结构在不同激光激发构置下产生的THz脉冲, 此时外加磁场固定为+H, 插图为飞秒脉冲激发样品的方向
Fig. 3. (a) THz signals emitted from the GGG//YIG(40)/Pt1st(3)/Pt2nd(3) bilayers applied with +H (blue line) and –H (red line); (b) THz emission signals with front- (blue line) and back- (orange line) pumps with +H. Insets: Schematic view of the laser pulse exciting the sample from the different sides.
表 1 5种不同结构样品的制备过程及其归一化THz振幅对比
Table 1. Preparation processes of five different sample structures and their normalized THz amplitudes.
样品
序号样品结构(厚度/nm) 生长步骤 归一化THz振幅
(强度/arb. units )① GGG//Pt(10) 沉积Pt膜 0 ② GGG//YIG(60)/Pt(10) 沉积YIG膜, 沉积Pt膜 0.076 ③ GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3) 沉积YIG膜, YIG膜退火, 沉积Pt膜 0.075, 0.045 ④ GGG//YIG(40)/Pt(3), Si//YIG(40)/Pt(3) 沉积YIG膜, 沉积Pt膜, YIG/Pt双层膜退火 0, 0 ⑤ GGG//YIG(40)/Pt1st(3)/Pt2nd(3),
Si//YIG(40)/Pt1st(3)/Pt2nd(3)沉积YIG膜, 沉积Pt膜 (1st), YIG/Pt双层膜
退火, 沉积Pt膜 (2nd)1.000, 0.121 -
[1] Saitoh E, Ueda M, Miyajima H, Tatara G 2006 Appl. Phys. Lett. 88 182509
Google Scholar
[2] Mosendz O, Pearson J E, Fradin F Y, Bauer G E W, Bader S D, Hoffmann. A 2010 Phys. Rev. Lett. 104 046601
Google Scholar
[3] Demidov V E, Urazhdin S, Ulrichs H, et al. 2012 Nat. Mater. 11 1028
Google Scholar
[4] Hirsh J E 1999 Phys. Rev. Lett. 83 1834
Google Scholar
[5] Sinova J, Valenzuela S O, Wunderlich J, Back C H, Jungwirth T 2015 Rev. Mod. Phys. 87 1213
Google Scholar
[6] Maekawa S, Adachi H. Uchida A, Ieda K, Saitoh J E 2013 J. Phys. Soc. Jpn. 82 102002
Google Scholar
[7] 韩方彬, 张文旭, 彭斌, 张万里 2015 物理学报 24 247202
Google Scholar
Han F B, Zhang W X, Peng B, Zhang W L 2015 Acta Phys. Sin. 24 247202
Google Scholar
[8] Kampfrath T, Battiato M, Maldonado P, et al. 2013 Nat. Nanotechnol. 8 256
Google Scholar
[9] Seifert T, Jaiswal S, Martens U, et al. 2016 Nat. Photon. 10 483
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[10] Battiato M, Carva K, Oppeneer P M 2010 Phys. Rev. Lett. 105 027203
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[12] Melnikov A, Razdolski I, Wehling T O, et al. 2011 Phys. Rev. Lett. 107 076601
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[19] Kikkawa T, Uchida K, Shiomi Y, et al. 2013 Phys. Rev. Lett. 110 067207
Google Scholar
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Google Scholar
[23] Uchida K, Nonaka T, Ota T, Nakayama H, Saitoh E 2010 Appl. Phys. Lett. 97 262504
Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
[34] Wu S M, Pearson J E, Bhattacharya A 2015 Phys. Rev. Let. 114 186602
Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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