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在铁磁/超导异质结中, 铁磁体的交换场通过近邻效应将导致超导体准粒子态密度的塞曼劈裂. 基于该效应, 在外磁场不强的情况下, 通过外加磁场可以有效地调节铁磁/超导界面处的交换作用, 从而实现超导体在正常态和超导态之间转换, 产生极大磁电阻. 本文利用脉冲激光沉积方法制备了EuS/Ta异质结并研究了其电磁特性. Ta在3.6 K以下为超导态, EuS在20 K以下为铁磁态. 在2 K时, EuS/Ta异质结中可观测蝴蝶型磁滞回线, 证明在低磁场下(< ±0.18 T)异质结中EuS铁磁态和Ta超导态共存. 磁输运测试表明, 通过施加外磁场可以有效调节EuS的交换场, 随着交换场的增大, 同时也加强了界面处的交换作用, 从而抑制Ta的超导态, 实现了Ta在超导态和正常态之间的转变, 在EuS/Ta异质结中观测到了高达144000%的磁电阻. 本文制备的EuS/Ta异质结具有极大磁电阻效应, 在自旋电子学器件中有潜在的应用前景.
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关键词:
- 铁磁绝缘体/超导异质结 /
- 交换场 /
- 磁电阻效应
Based on the proximity effect, the exchange interaction at the interface between a ferromagnetic insulator (FI) and a superconductor (S) could enhance the Zeeman splitting of the superconducting quasiparticle density of states. The superconducting electrons feel the exchange field on the surface of the S layer. Therefore, tuning the internal exchange field at the FI/S interface could switch the superconductor from a superconducting state to a normal state, leading to an infinite magnetoresistance in FI/S heterostructure. Here in this work, we fabricate the EuS/Ta heterojunction by the pulsed laser deposition, and perform the magnetotransport measurements. In the EuS/Ta heterojunction, Ta film as a typical BSC supercenter exhibits the superconducting transition under 3.6 K, and the EuS film is ferromagnetic under 20 K. The magnetization of EuS is suppressed by superconductivity of Ta at 0.01 T below 3 K. In addition, the butterfly-type hysteresis loop is observed at 2 K. And the decrease of the saturation magnetization of EuS/Ta heterostructure is observed by comparing with the EuS single layer. It is caused by a reconstruction of homogeneous ferromagnetic order in the EuS ferromagnetic layer due to the proximity effect with the Ta superconducting layer. The above measurement results show that the competition between the ferromagnetism of EuS film and superconductivity of Ta film below Tc of Ta film. If the exchange field of the FI is sufficiently strong, it tries to align the spins of the electrons of a Cooper pair in S layer parallel to each other, thus destroying the superconductivity. Meanwhile, the superconductivity in S layer will be recovered when the exchange field of the FI is weak. The resistance at a specific value of the magnetic field (1 T) steeply drops to zero, and clear hysteresis behavior is observed in EuS/Ta heterostructure, resulting in an infinite magnetoresistance up to 144000%, by tuning the internal exchange field at EuS/Ta interface. Meanwhile, the anomalous Hall effect with hysteresis behavior is observed at 2 K, indicating that the electron in Ta film is spin polarized due to the magnetic proximity effect near the EuS/Ta interface. Our results show that the EuS/Ta heterostructure with infinite magnetoresistance could be a good candidate for spintronic devices.-
Keywords:
- ferromagnetic insulator/superconductor heterostructure /
- exchange field /
- magnetoresistance
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图 1 EuS/Ta异质结结构以及输运测试示意图
Fig. 1. Schematic diagram of the EuS/Ta heterostructure geometry.
图 2 EuS/Ta异质结的电阻随温度的变化曲线; 插图为超导转变温度放大图
Fig. 2. Resistance as a function of temperature for EuS/Ta heterostructure.Inset: zoom of R-T curve at low temperature range.
图 3 (a) EuS/Ta异质结的热磁曲线, 插图是低温区域的放大, 黑色箭头指向热磁曲线的拐点; (b) 2 K时EuS/Ta异质结和EuS单层膜的磁滞回线
Fig. 3. (a) Magnetization of temperature dependence of the EuS/Ta heterostructure. Inset: zoom of M-T curve at low temperature, the black arrow marks the kick point of M-T curve with H = 0.01 T; (b) magnetic hysteresis loop of the EuS/Ta heterostructure and EuS single layer at 2 K, respectively.
图 4 EuS/Ta异质结的在2 K时的磁电阻和霍尔效应
Fig. 4. MR and Hall effect as a function of field of EuS/Ta heterostructure at 2 K.
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[1] Deutscher G, de Gennes P G 1969 Superconductivity (Vols. 1 and 2) (New York: Marcel Dekker, Inc.) pp1005−1034
[2] Dotsch H, Bahlmann N, Zhuromskyy O, Hammer M, Wilkens L, Gerhardt R, Hertel P, Popkov A F 2005 J. Opt. Soc. Am. B:Opt. Phys. 22 240
Google Scholar
[3] de Gennes P G 1964 Rev. Mod. Phys. 36 225
Google Scholar
[4] de Gennes P G 1966 Phys. Lett. 23 10
Google Scholar
[5] Buzdin A I 2005 Rev. Mod. Phys. 77 935
Google Scholar
[6] Bergeret F S, Volkov A F, Efetov K B 2005 Rev. Mod. Phys. 77 1321
Google Scholar
[7] 李晓薇 2005 54 2313
Google Scholar
Li X W 2005 Acta Phys. Sin. 54 2313
Google Scholar
[8] 金霞, 董正超, 梁志鹏, 仲崇贵 2013 62 047401
Google Scholar
Jin X, Dong Z C, Liang Z P, Zhong C G 2013 Acta Phys. Sin. 62 047401
Google Scholar
[9] Gu J Y, You C Y, Jiang J S, Pearson J, Bazaliy Y B, Bader S D 2002 Phys. Rev. Lett. 89 267001
Google Scholar
[10] Rusanov A Y, Habraken S, Aarts J 2006 Phys. Rev. B 73 060505
Google Scholar
[11] Hao X, Moodera J S, Meservey R 1990 Phys. Rev. B 42 8235
Google Scholar
[12] Moodera J S, Hao X, Gibson G A, Meservey R 1988 Phys. Rev. Lett. 61 637
Google Scholar
[13] Tedrow P M, Tkaczyk J E, Kumar A 1986 Phys. Rev. Lett. 56 1746
Google Scholar
[14] Hauser J J 1969 Phys. Rev. Lett. 23 374
Google Scholar
[15] Li B, Roschewsky N, Assaf B A, Eich M, Epstein-Martin M, Heiman D, Munzenberg M, Moodera J S 2013 Phys. Rev. Lett. 110 097001
Google Scholar
[16] Yang Q I, Zhao J, Zhang L, Dolev M, Fried A D, Marshall A F, Risbud S H, Kapitulnik A 2014 Appl. Phys. Lett. 104 082402
Google Scholar
[17] Zhang X, Shi X 2020 J. Supercond. Novel. Magn. 33 217
Google Scholar
[18] 张裕恒 2009 超导物理 (中国科学技术大学出版社) 第7页
Zhang Y H 2009 Superconducting Physics (University of Science and Technology of China Press) p7 (in Chinese)
[19] Stachow-Wójcik A, Story T, Dobrowolski W, Arciszewska M, Gałązka R R, Kreijveld M W, Swüste C H W, Swagten H J M, de Jonge W J M, Twardowski A, Sipatov A Y 1999 Phys. Rev. B 60 15220
Google Scholar
[20] Smits C J P, Filip A T, Kohlhepp J T, Swagten H J M, Koopmans B, and de Jonge W J M 2004 J. Appl. Phys. 95 7405
Google Scholar
[21] Miao G X, Moodera J S 2009 Appl. Phys. Lett. 94 182504
Google Scholar
[22] Di Bernardo A, Diesch S, Gu Y, Linder J, Divitini G, Ducati C, Scheer E, Blamire M G, Robinson J W A 2015 Nat. Commun. 6 9053
Google Scholar
[23] Di Bernardo A, Salman Z, Wang X L, Amado M, Egilmez M, Flokstra M G, Suter A, Lee S L, Zhao J H, Prokscha T, Morenzoni E, Blamire M G, Linder J, Robinson J W A 2015 Phys. Rev. X 5 041021
Google Scholar
[24] Diesch S, Machon P, Wolz M, Surgers C, Beckmann D, Belzig W, Scheer E 2018 Nat. Commun. 9 5248
Google Scholar
[25] Strambini E, Golovach V N, De Simoni G, Moodera J S, Bergeret F S, Giazotto F 2017 Phys. Rev. Mater. 1 054402
Google Scholar
[26] 郭子政, 胡旭波 2013 62 057501
Google Scholar
Guo Z Z, Hu X B 2013 Acta Phys. Sin. 62 057501
Google Scholar
[27] Garifullin I A, Tikhonov D A, Garif’yanov N N, Fattakhov M Z, Theis-Bröhl K, Westerholt K, Zabel H 2002 Appl. Magn. Reson. 22 439
Google Scholar
[28] Mühge T, Garif'yanov N N, Goryunov Y V, Theis-Bröhl K, Westerholt K, Garifullin I A, Zabel H 1998 Physica C 296 325
Google Scholar
[29] Bergeret F S, Volkov A F, Efetov K B 2001 Phys. Rev. B 64 134506
Google Scholar
[30] Lu Y M, Choi Y, Ortega C M, Cheng X M, Cai J W, Huang S Y, Sun L, Chien C L 2013 Phy. Rev. Lett. 110 147207
Google Scholar
[31] Huang S Y, Fan X, Qu D, Chen Y P, Wang W G, Wu J, Chen T Y, Xiao J Q, Chien C L 2012 Phys. Rev. Lett. 109 107204
Google Scholar
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