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非磁/铁磁异质结构中存在很多有趣的演生现象, 特别是, 铂/铁磁异质结构中的反常霍尔效应是一个研究热点. 采用脉冲激光沉积技术和射频磁控溅射技术制备出具有原子级接触界面的铂/锰酸锶镧异质结, 并对异质结的电输运性能进行了系统的研究. 实验发现, 铂/锰酸锶镧异质结中存在由铂贡献的反常霍尔效应, 这是由磁近邻效应诱导铂表现出铁磁性造成的. 反常霍尔电阻随着温度的降低而急剧增加, 并且在低于 40 K时改变符号. 反常霍尔电阻随铂厚度的增加而急剧降低, 证实了铂的铁磁性起源于异质结界面. 此外, 异质结在低外加磁场下可能产生了拓扑霍尔效应, 这是由异质结界面处的Dzyaloshinskii-Moriya相互作用诱导产生手性磁畴壁结构引起的. 上述研究结果为进一步理解非磁/铁磁异质结构中的电子自旋和电荷输运之间的相互作用提供了实验基础.Many emergent and novel phenomena occur in nonmagnetic/ferromagnet heterostructures. In particular, Pt/ferromagnet heterostructures where the Pt has strong spin-orbit coupling and thus can convert spin current into charge current, has attracted a great attention recently. The anomalous Hall effect (AHE) has been found in many Pt/ferromagnet heterostructures. However, the underlying physics remains elusive, so it is necessary to find more heterostructures in order to provide more experimental data. In this work, we investigate anomalous Hall resistances (AHRs) in Pt thin films sputtered on epitaxial La0.67Sr0.33MnO3 (LSMO) ferromagnetic films. High-quality Pt/LSMO heterojunctions are fabricated by pulsed laser deposition and RF-magnetron sputtering. The physical properties of LSMO films are characterized by the measurements of magnetic and transport properties. The AHR mainly contributed by Pt in the Pt/LSMO heterojunction increases sharply with temperature decreasing and changes its sign below 40 K. Furthermore, the AHR decreases sharply with the increase of Pt thickness. Those facts suggest that the ferromagnetism of Pt originates from interface due to magnetic proximity effect. Interestingly, this heterojunction can exhibit possible signal of topological Hall effect under low applied magnetic field. The above results provide an experimental basis for further understanding the interactions between electron spin and charge transport in nonmagnetic/ferromagnetic heterostructures.
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Keywords:
- anomalous Hall effect /
- magnetic proximity effect /
- topological Hall effect /
- epitaxial growth
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图 1 LSMO薄膜生长 (a) RHEED周期振荡; (b) 薄膜生长前的RHEED衍射图; (c) 薄膜生长后的RHEED衍射图
Fig. 1. The LSMO thin film growth: (a) RHEED oscillation; (b) RHEED pattern before film growth; (c) RHEED pattern after film growth.
图 2 形貌表征 (a) STO (001)衬底AFM图; (b) LSMO (40 u.c.)薄膜AFM图; (c) Pt(2 nm)/LSMO(40 u.c.)薄膜AFM图; (d)和(e)分别为(b)和(c)中薄膜表面线扫描图
Fig. 2. Morphology characterization: (a) AFM image of STO (001) substrate; (b) AFM image of LSMO (40 u.c.) film; (c) AFM image of Pt(2 nm)/LSMO(40 u.c.) film; (d) line-scan of the LSMO film in (b); (e) line-scan of the Pt/LSMO film in (c).
图 3 结构表征 (a) Pt(6 nm)/LSMO(40 u.c.)薄膜的2θ-ω扫描; (b)为(a)中(002)衍射峰的放大图, 插图为LSMO薄膜(002)衍射峰的摇摆曲线; (c) Pt/LSMO薄膜在(103)衍射峰附近的倒易空间图; (d) Pt/LSMO薄膜的XRR谱, 拟合的红线与实验数据相符
Fig. 3. Structure characterization: (a) 2θ-ω scan of Pt(6 nm)/LSMO(40 u.c.) thin films; (b) enlarged view of the (002) diffraction peak in panel (a), and the inset is a rocking curve of LSMO film around (002) diffraction peak; (c) reciprocal space map of Pt/ LSMO film around (103) diffraction peak; (d) XRR spectrum of Pt/LSMO film, and the red line is a fit to the experimental data
图 4 (a) LSMO (40 u.c.)薄膜的磁化强度的温度依赖性, 插图为磁化强度对温度的一阶微分; (b) LSMO (40 u.c.)薄膜不同温度下的磁化强度的场依赖性, 插图为3 K时曲线的中心放大图
Fig. 4. (a) Temperature dependence of magnetization of LSMO (40 u.c.) films, the inset is the first derivative of magnetization versus temperature; (b) field dependence of the magnetization of LSMO (40 u.c.) films at different temperatures, and the inset is an enlarged view of the curve at 3 K.
图 5 LSMO(40 u.c.)薄膜与Pt(2 nm)/LSMO(40 u.c.)薄膜电阻的温度依赖性
Fig. 5. Temperature dependence of resistance of LSMO (40 u.c.) film and Pt(2 nm)/LSMO(40 u.c.) film.
图 6 (a) LSMO(40 u.c.)薄膜和(b) Pt(2 nm)/LSMO(40 u.c.)薄膜在不同温度下的RAHR, 图(a)插图为2 K时测量的LSMO薄膜的Rxy
Fig. 6. RAHR of (a) LSMO (40 u.c.) film and (b) Pt(2 nm)/LSMO(40 u.c.) film at different temperatures. The inset in panel (a) is Rxy of the LSMO film measured at 2 K.
图 7 (a) 在2 K时不同Pt厚度的Pt/LSMO(40 u.c.)薄膜的RAHR, 其中4和6 nm曲线的RAHR分别扩大了4倍和5倍; (b) Pt(6 nm)/LSMO(40 u.c.)薄膜在不同温度下的RAHR
Fig. 7. (a) RAHR of Pt/LSMO(40 u.c.) film with different Pt thickness, which were measured at 2 K. RAHR of the 4 and 6 nm curves are enlarged by a factor of four and five, respectively; (b) RAHR of Pt(6 nm)/LSMO(40 u.c.) films at different temperatures.
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[1] Ohno Y, Young D K, Beschoten B, Matsukura F, Ohno H, Awschalom D D 1999 Nature 402 790
Google Scholar
[2] Jedema F J, Filip A T, Wees B J V 2001 Nature 410 345
Google Scholar
[3] Heinrich B, Tserkovnyak Y, Woltersdorf G, Brataas A, Urban R, Bauer G E W 2003 Phys. Rev. Lett. 90 187601
Google Scholar
[4] Kajiwara Y, Harii K, Takahashi S, Ohe J, Uchida K, Mizuguchi M, Umezawa H, Kawai H, Ando K, Takanashi K, Maekawa S, Saitoh E 2010 Nature 464 262
Google Scholar
[5] Heinrich B, Burrowes C, Montoya E, Kardasz B, Girt E, Song Y Y, Sun Y Y, Wu M Z 2011 Phys. Rev. Lett. 107 066604
Google Scholar
[6] Rezende S M, Rodriguez S R L, Soares M M, Vilela L L H, Ley D D, Azevedo A 2013 Appl. Phys. Lett. 102 012402
Google Scholar
[7] Uchida K, Takahashi S, Harii K, Ieda J, Koshibae W, Ando K, Maekawa S, Saitoh E 2008 Nature 455 778
Google Scholar
[8] Uchida K, Xiao J, Adachi H, Ohe J, Takahashi S, Ieda J, Ota T, Kajiwara Y, Umezawa H, Kawai H, Bauer G E W, Maekawa S, Saitoh E 2010 Nat. Mater. 9 894
Google Scholar
[9] Weng H M, Yu R, Hu X, Dai X, Fang Z 2015 Adv. Phys. 64 03227
Google Scholar
[10] Takahashi S, Maekawa S 2008 Sci. Technol. Adv. Mater. 9 014105
Google Scholar
[11] Miao B F, Huang S Y, Qu D, Chien C L 2014 Phys. Rev. Lett. 112 236601
Google Scholar
[12] Althammer M, Meyer S, Nakayama H, Schreier M, Altmannshofer S, Weiler M, Huebl H, Geprags S, Opel M, Gross R, Meier D, Klewe C, Kuschel T, Schmalhorst J M, Reiss G, Shen L M, Gupta A, Chen Y T, Bauer G E W, Saitoh E, Goennenwein S T B 2013 Phys. Rev. B 87 224401
Google Scholar
[13] Lu Y M, Choi Y, Ortega C M, Cheng X M, Cai J W, Huang S Y, Sun L, Chien C L 2013 Phys. Rev. Lett. 110 147207
Google Scholar
[14] Isasa M, Pinto A B, Velez S, Golmar F, Sanchez F, Hueso L E, Fontcuberta J, Casanova F 2014 Appl. Phys. Lett. 105 142402
Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
[18] Putter S, Geprags S, Schlitz R, Althammer M, Erb A, Gross R, Goennenwein S T B 2017 Appl. Phys. Lett. 110 012403
Google Scholar
[19] Biswas A, Yang C H, Ramesh R, Jeong M H 2017 Prog. Surf. Sci. 92 02117
Google Scholar
[20] Peng R, Xu H C, Xia M, Zhao J F, Xie X, Xu D F, Xie B P, Feng D L 2014 Appl. Phys. Lett. 104 081606
Google Scholar
[21] Snyder G J, Hiskes R, DiCarolis S, Beasley M R, Geballe T H 1996 Phys. Rev. B 53 14434
Google Scholar
[22] 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
[23] Soumyanarayanan A, Raju M, Oyarce A L G, Tan A K C, Im M Y, Petrovi A P, Ho P, Khoo K H, Tran M, Gan C K, Ernult F, Panagopoulos C 2017 Nat. Mater. 16 898
Google Scholar
[24] Zhang S, Zhang S S L 2009 Phys. Rev. Lett. 102 086601
Google Scholar
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Google Scholar
[26] Belabbes A, Bihlmayer G, Bechstedt F, Blügel S, Manchon A 2016 Phys. Rev. Lett. 117 247202
Google Scholar
[27] Meng K K, Zhao X P, Liu P F, Liu Q, Wu Y, Li Z P, Chen J K, Miao J, Xu X G, Zhao J H, Jiang Y 2018 Phys. Rev. B 97 060407
Google Scholar
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