Nanostructured Materials Lab > Research

Research

Recent Research Areas and Representative Publications (since 1992)

I. Giant Magnetoresistance in Granular Systems:

After the discovery of giant magnetoresistance (GMR) by Fert and Grünberg (2007 Nobel Prize in Physics) in 1988, we have discovered GMR in granular systems in 1992 demonstrating that GMR is a general phenomena in magnetic nanostructures with a non-aligned spin structure for mediating spin-dependent scattering.

John Q. Xiao, J. Samuel Jiang, and C. L. Chien, “Giant Magnetoresistance in Non-Multilayer Magnetic Systems,” Phys. Rev. Lett. 68, 3749 (1992).
John Q. Xiao, J. Samuel Jiang, and C. L. Chien, “Giant Magnetoresistance in Granular Co-Ag System,” Phys. Rev. B 46, 9266 (1992).
P. Xiong, G. Xiao, J. Q. Wang, J. Q. Xiao, J. S. Jiang, and C. L. Chien, “Extraordinary Hall Effect and Giant Magnetoresistance in Granular Co-Ag System,” Phys. Rev. Lett. 69, 3220 (1992).
C. L. Chien. John Q. Xiao and J. Samuel Jiang, “Giant Negative Magneto-resistance in Granular Magnetic Solids,” J. Appl. Phys. 73, 5309 (1993).
C. L. Chien, “Magnetism and Giant Magneto-Transport Properties in Granular Solids,” Annual Review of Materials Science, 25, 129 (1995).

II. Arrays of Magnetic Nanowires:

We have pioneered arrays of magnetic nanowires, which may be single-material or multi-segmented, suitable for a wide variety of magnetic, chemical, biomedical, and MEMS applications, and indeed the focused research area of more than ten research centers worldwide.

T. M. Whitney, J. S. Jiang, P. C. Searson, and C. L. Chien, “Fabrication and Magnetic Properties of Arrays of Metallic Nanowires,” Science, 261, 1316 (1993).
K. Liu, K. Nagodawithana, P. C. Searson, and C. L. Chien, “Perpendicular Giant Magnetoresistance of Multilayered Co/Cu Nanowires,” Phys. Rev. (Rapid Commun.) B 51, 7381 (1995).
Kai Liu, C. L. Chien, and P. C. Searson, “Finite-Size Effects in Bismuth Nanowires,” Phys. Rev. B 58 (Rapid Communications), 14681 (1998).
L. Sun, P. C. Searson and C. L. Chien, “Finite-Size Effects in Nickel Nanowire Arrays,” Phys. Rev. B 61 (Rapid Commun). R6463 (2000).
L. Sun, C. L. Chien, and P. C. Searson, “Magnetic Anisotropy in Prismatic Ni Nanowires,” Appl. Phys. Lett. 79 , 4429 (2001).
J. Mallet, T. Eagleton, K. Yu-Zhang, C. L. Chien, and P. C. Searson, “Fabrication and Magnetic Properties of fcc CoXPt1-X Nanowires,” Appl. Phys. Lett.,84, 3900 (2004).
L. Sun, Y. Hao, C. L. Chien. And P. C. Searson, “Tuning the properties of magnetic nanowires,” IBM J. Res. and Develp., 49, 79 (2005).
L. Sun, P. C. Searson, and C. L. Chien, “Asymmetry of magnetic hysteresis in exchange-biased multilayers with out-of-plane applied field,” Phys. Rev. B (Rap. Comm.) 71, 012417 (2005).

III. Proximity Effects in Superconductor/Ferromagnet Multilayers:

We have revealed the intriguing interactions occurring in the proximity of a superconductor and a ferromagnet, including Josephson coupling, &pi-phase coupling, and interlayer coupling across a superconducting layer.

J. S. Jiang, D. Davidovic, D. H. Reich and C. L. Chien, “Oscillatory Superconducting Transition Temperature in Nb/Gd Multilayers,” Phys. Rev. Lett. 74, 314 (1995).
J. Q. Xiao and C. L. Chien, “Proximity Effects of Superconductor/Magnetic Semiconductor NbN/GdN Multilayers,” Phys. Rev. Lett. 76, 1727 (1996).
C. L. Chien and D. H. Reich, “Proximity Effects in Superconducting/Magnetic Multilayers,” J. Mag. Mag. Mat. 200, 83-94 (1999).
Marta Z. Cieplak, X. M. Cheng, C. L. Chien, and H. Sang, “Origin of Pinning enhancement in ferromagnet-superocnductor bilayer,” J. Appl. Phys. 97, 026105 (2005).

IV. Physics of Exchange Bias:

Exchange bias occurring across the interface between a ferromagnet and an antiferromagnet is an intriguing phenomenon of scientific and technological importance. We have uncovered some of the rich physics of exchange bias, including the memory effect, exchange bias in the paramagnetic state, spiraling spin structure, and oscillatory exchange bias.

T. Ambrose and C. L. Chien, “Finite-Size Effects and Uncompensated Magnetization in Thin Antiferromagnetic CoO Layers,” Phys. Rev. Lett. 76, 1743 (1996).
N. J. Gokemeijer, T. Ambrose, and C. L. Chien, “Long-Range Exchange Bias Across a Spacer Layer,” Phys. Rev. Lett. 79, 4270 (1997).
X. W. Wu and C. L. Chien, “Exchange Coupling in Ferromagnet/Antiferromagnet Bilayers with Comparable TC and TN,” Phys. Rev. Lett. 81, 2795 (1998).
V. I. Nikitenko, V. S. Gornakov, A. J. Shapiro, R. D. Shull, Kai Liu, S. M. Zhou, and C. L. Chien, “Asymmetry in the Elementary Events of Magnetization Reversal in Ferromagnetic/Antiferromagnetic Bilayers,” Phys. Rev. Lett. 84, 765 (2000).
F. Y. Yang and C. L. Chien, “Spiraling Spin Structure in an Exchange-Coupled Antiferromagnetic Layer,” Phys. Rev. Lett., 85, 2597 (2000).
F. Y. Yang and C. L. Chien, “Oscillatory Exchange Bias due to an Antiferromagnet with Incommensurate Spin Density wave,” Phys. Rev. Lett. 90, 147201 (2003).
V. S. Gornakov, Yu. P. Kabanov, O. A. Tikhomirov, V. I. Nikitenko, S. V. Urazhdin, F. Y. Yang, C. L. Chien, A. J. Shapiro, and R. D. Shull, “Experimental study of the microscopic mechanisms of magnetization reversal in FeNi/FeMn exchange-biased ferromagnet/antiferromagnet polycrystalline bilayers using the magneto-optical indicator film technique,” Phys. Rev. B 73, 184428 (2006).

V. Andreev Reflection Spectroscopy:

Andreev reflection spectroscopy (ARS) utilizes the conversion of a supercurrent into a normal current. We have developed ARS into a quantitative technique for measuring the spin polarization of a metal as well as the superconducting gap of a superconductor.

G. J. Strijkers, Y. Ji, F. Y. Yang, C. L. Chien, and J. M. Byers, “Andreev Reflections at Metal/Superconductor Point-Contacts: Measurement and Analysis,” Phys. Rev. B 63, 104510 (2001).
Y. Ji, G. J. Strijkers, F. Y. Yang, C. L. Chien, J. M. Byers, A. Anguelouch, G. Xiao, and A. Gupta, “Determination of the Spin Polarization of Half-Metallic CrO2 by point Contact Andreev Reflection,” Phys. Rev. Lett. 86, 5585 (2001).
S. X. Huang, T. Y. Chen, and C. L. Chien, “Spin polarization of amorphous CoFeB determined by point-contact Andreev reflection,” Appl. Phys. Lett. 92, 242509 (2008).
T. Y. Chen, S. X. Huang, and C. L. Chien, “Pronounced effects of additional resistance in Andreev reflection spectroscopy,” Phys. Rev. B 81, 214444 (2010).

VI. Magneto-Transport Properties of Single-Crystal Bi Thin Films:

Bi is a semimetal with unusual Fermi surfaces and electrons and holes of low effective mass and carrier density. We have accomplished in high quality Bi thin films enormous carrier mean path necessary for capturing extremely large magnetoresistance (400,000%), Shubnikov-de Haas oscillations, spin Hall effect, and quantum transport.

F. Y. Yang, Kai Liu, C. L. Chien, and P. C. Searson, “Large Magnetoresistance and Finite-Size Effects in Electrodeposited Single-Crystal Bi Thin Films,” Phys. Rev. Lett. 82, 3328 (1999).
F. Y. Yang, Kai Liu, Kimin Hong, D. H. Reich, P. C. Searson, and C. L. Chien, “Large Magnetoresistance of Electrodeposited Single-Crystal Bismuth Thin Films,” Science 284, 1335 (1999).
F. Y. Yang, Kai Liu, Kimin Hong, D. H. Reich, P. C. Searson, C. L. Chien, Y. Leprince-Wang, Kui Yu-Zhang, and Ke Han, “Shubnikov-de Haas Oscillations in Electrodeposited Single-Crystal Bismuth Films,” Phys. Rev. B 61, 6631 (2000).

VII. CrO2 and Other Half-Metals:

In a half-metal with only one spin band at the Fermi energy, the electrons are fully polarized and thus the ultimate material for spintronics. We have measured and identified CrO2 as a true half-metal as well as several others with exceptionally high spin polarization.

Y. Ji, G. J. Strijkers, F. Y. Yang, C. L. Chien, J. M. Byers, A. Anguelouch, G. Xiao, and A. Gupta, “Determination of the Spin Polarization of Half-Metallic CrO2 by point Contact Andreev Reflection,” Phys. Rev. Lett.86 , 5585 (2001).
F. Y. Yang, C. L. Chien, X. W. Li, A. Gupta, and G. Xiao, “Critical Behavior of Epitaxial Half-Metallic Ferromagnetic CrO2 Films,” Phys. Rev. B 63, 92403 (2001).
Y. Ji, C. L. Chien, Y. Tomioka and Y. Tokura, “Measurement of Spin Polarization of Single Crystals of La0.7Sr0.3MnO3 and La0.6Sr0.4MnO3” Phys. Rev. B 66, 12410 (2002).
J. M. D. Coey and C. L. Chien, “Half-Metallic Ferromagnetic Oxides,” in Spin-Polarized Materials for Spintronics in MRS Bulletin 28 (no.10), 720 (October 2003).
L. Wang, K. Umemoto, R. M. Wentzcovitch, T. Y. Chen, C. L. Chien, J. G. Checkelsky, J. C. Eckert, E. D. Dahlberg, and C. Leighton, “Co1-xFexS2: a tunable source of highly spin polarized electrons,” Phys. Rev. Lett., 94, 056602 (2005).

VIII. Spin-Transfer Torque Effects:

In the spin-transfer torque (STT) effect the spin angular momentum carried by a spin-polarized current can exert a torque to switch nanomagnet, induce spin precession and generate microwave radiation, all accomplished without using an external magnetic field. We have made the first observation of STT effect in a single ferromagnetic layer and in granular solid.

Y. Ji, C. L. Chien, and M. D. Stiles, “Current Induced Spin Wave Excitations in a Single Ferromagnetic Layer,” Phys. Rev. Lett., 90, 106601(2003).
T. Y. Chen, Y. Ji, and C. L. Chien, “Reversible Switching in Continuous Films by Point Contact Spin Injection,” Appl. Phys. Lett. 84, 380 (2004).
T. Y. Chen, Y. Ji, C. L. Chien, and M. D. Stiles, “Current-driven switching in a single exchange-biased ferromagnetic layer,” Phys. Rev. Lett.,93, 026601 (2004).
S. Urazhdin, C. L. Chien, K. Y. Guslienko, and L. Novozhilova, “Effects of current on the magnetic states of permalloy nanodiscs,” Phys., Rev. B 73, 054416 (2006).
T. Y. Chen, S. X. Huang, C. L. Chien and M. D. Stiles, “Enhanced magnetoresistance induced by spin transfer torque in granular films with a magnetic field,” Phys. Rev. Lett.96, 207203 (2006).

IX. Patterned Nanomagnets:

Patterned nanomagnets with sizes in the range of 100 nm to a few &mum acquire unique magnetic configuration and switching characteristics as a result of the delicate balance of exchange energy and magnetostatic energy.

F. Q. Zhu, Z. Shang, D. Monet, and C. L. Chien, “Large enhancement of coercivity of magnetic Co/Pt nanodots with perpendicular anisotropy,” J. Appl. Phys. 101, 09J101 (2007).
C. L. Chien, F. Q. Zhu, and J. G. Zhu, “Patterned Nanomagnets,” Physics Today 60, 40 (2007); Japanese translation in Parity 23 (no.2) 10 (2008).

X. Magnetic Nanorings and Tunnel Junctions:

We have pioneered a new method for fabricating nanorings with the largest number (1010), the smallest ring (100 nm) and narrowest ring width (20 nm). We also made the first demonstration of nanoring tunnel junctions, exploiting the new memory states and switching characteristics unattainable in disk tunnel junctions.

F. Q. Zhu, D. L. Fan, X. C. Zhu, J. G. Zhu, R. C. Cammarata, C. L. Chien, “Ultrahigh density arrays of ferromagnetic nanorings on a macroscopic area,” Adv. Mater. 16, 2155 (2004).
F. Q. Zhu, G. W. Chern, O. Tchernyshyov, X. C. Zhu, J. G. Zhu, and C. L. Chien, “Magnetic Bistability and Controllable Reversal of Asymmetric Ferromagnetic Nanorings,” Phys. Rev. Lett., 96, 027203 (2006).
H. X. Wei, F. Q. Zhu, X. F. Han, Z. C. Wen, and C. L. Chien, “Current-induced multiple spin structures in 100 nm ring magnetic tunnel junctions,” Phys. Rev. B. 77, 224432 (2008).

XI. Manipulation of Nanowires in Suspension by Electric Tweezers:

Manipulation of nanoentities in suspension is in the realm of extremely low Reynolds numbers (10-5 ) where viscous force overwhelms. We have developed the technique of electric tweezers using electrical voltages to manipulate nanowires with precision (better than 150 nm). Electric tweezers is a new technique for a wide range of biomedical, MEMS, and fluid mechanics applications.

D. L. Fan, F. Q. Zhu. R. C. Cammarata, and C. L. Chien, “Manipulation of Nanowires in Suspension by AC Electric Fields,” Appl. Phys. Lett. 85, 4175 (2004).
D. L. Fan, F. Q. Zhu, R. C. Cammarata, and C. L. Chien, “Controllable High-Speed Rotation of Nanowires,” Phys. Rev. Lett., 94, 247208 (2005).
D. L. Fan, F. Q. Zhu, R. C. Cammarata, and C. L. Chien, “Efficiency of assembling of nanowires in suspension by AC electric fields,” Appl. Phys. Lett. 89, 223115 (2006).
D. L. Fan, R. C. Cammarata, and C. L. Chien, “Precision transport and assembling of nanowires in suspension by electric field,” Appl. Phys. Lett. 92, 093115 (2008).
D. L. Fan, R. C. Cammarata, and C. L. Chien, “Controlled manipulation of nanoentities in suspension,” in Biomagnetism and Magnetic Biosystems Based on Molecular Recognition Processes, p. 44-51, eds J. A. C. Bland and A. Ionescu, AIP Conf. Proc. 1025 (2008).
D. L. Fan, Z. Z. Yin, R. Cheong, F. Q. Zhu, R. C. Cammarata, C. L. Chien, and A. Levchenko, “Sub-cellular resolution delivery of a cytokine via precisely manipulated nanowires,” Nature Nanotechnology 5, 545 (2010).
Featured story, “Nanowires have cells in their sights,” Nature Nanotechnology 5, 481 (2010).

XII. Materials with Perpendicular Magnetic Anisotropy:

While most ferromagnetic materials have in-plane anisotropy, a few materials, among them Co/Pt multilayers, exhibit perpendicular magnetic anisotropy, which is suitable for the studies of Bloch domain walls, the interplay in ferromagnet-superconductor hybrids, and with relevance to perpendicular magnetic recording.

X. M. Cheng, S. Urazhdin, O. Tchernyshyov, C.L. Chien, V.I. Nikitenko, A.J. Shapiro and R.D. Shull, “Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy,” Phys. Rev. Lett., 94, 017203 (2005).
Y. L. Iunin, Y. P. Kabanov, V. I. Nikitenko, X. M. Cheng, D. Clarke, O. A. Tretiakov, O. Tchernyshyov, A. J. Shapiro, R. D. Shull, and C. L. Chien, “Asymmetric domain nucleation and unusual magnetization reversal in ultrathin Co films with perpendicular anisotropy,” Phys. Rev. Lett., 98, 117204 (2007).
L. Y. Zhu, T. Y. Chen, and C. L. Chien, “Altering the superconducting transition temperature by domain-wall arrangement in hybrid ferromagnet-superconductor structures,” Phys. Rev. Lett. 101, 017004 (2008).
L. Y. Zhu, M. Z. Cieplak, and C. L. Chien, “Tunable phase diagram and vortex pinning in ferromagnet-superconductor bilayer,” Phys. Rev. B (Rapid Commun.) 82, 060503 (2010).

XIII. New Fe Superconductors:

In 2008, a new family of Fe superconductors have been discovered, with characteristics very different from those the conventional (s-wave) and the cuprate (d-wave) superconductors. We are among the first group that have demonstrated that the gap of the new Fe superconductors have the s-wave symmetry.

T. Y. Chen, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien, “A BCS-like gap in the superconducting SmFeAsO0.85F0.15” Nature, 453, 1224 (2008).
T. Y. Chen, S. X. Huang, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien, “Determination of Superconducting Gap of SmFeAsFxO1-x Superconductors by Andreev Reflection Spectroscopy,” Physica C 469, 521 (2009).
S. X. Huang, C. L. Chien, V. Thampy, and C. Broholm, “Control of tetrahedral coordination and superconductivity in FeSe0.5Te0.5 thin films,” Phys. Rev. Lett., 104, 217002, (2010).

XIV. Spin Caloritronics:

Following spintronics, spin caloritronics exploits interactions among charge, spin and heat.  The transverse spin Seebeck effect (SSE) was first reported in 2008 in magnetic thin films on substrates.  We showed that the observed signal was dominated by the anomalous Nernst effect with no conclusive signal from SSE [Phys. Rev. Lett., 107, 216604 (2011)].  We subsequently established the first observation of a longitudinal SSE in Au/YIG [Phys. Rev. Lett., 110, 067206 (2013)].  We also discovered an intriguing magnetic proximity effect [Phys. Rev. Lett., 109, 107204 (2012); Phys. Rev. Lett., 110, 147207 (2013)] in Pt/YIG with magnetoresistance characteristics unlike those of other magnetoresistance phenomena.

394. S. Y. Huang, W. G. Wang, S. F. Lee, J. R. Kwo, and C. L. Chien, “Intrinsic spin-dependent thermal transport,” Phys. Rev. Lett. 107, 216604 (2011).

399. S. Y. Huang, X. Fan, D. Qu, Y. P. Chen, W. G. Wang, J. Wu, T. Y. Chen, J. Q. Xiao, and C. L. Chien, “Transport Magnetic Proximity Effects in Platinum,” Phys. Rev. Lett. 109, 107204 (2012).

402. D. Qu, S. Y. Huang, J. Hu, R. Wu, and C. L. Chien, “Intrinsic spin Seebeck effect in Au/YIG.” Phys. Rev. Lett. 110, 067206 (2013).

406. Y. M. Lu, Y. Choi, C. M. Ortega, X. M. Cheng, J. W. Cai, S. Y. Huang, L. Sun, and C. L. Chien, “Pt Magnetic Polarization on Y3Fe5O12 and Magnetotransport Characteristics,” Phys. Rev. Lett. 110, 147207 (2013).

408. S. Y. Huang, D. Qu, and C. L. Chien, “Charge, Spin, and Heat Transport in the Proximity of Metal/Ferromagnet Interface,” Solid State Physics, 64, 53 (2013)

412. B. F. Miao, S. Y. Huang, D. Qu, and C. L. Chien, “Physical Origins of the New Magnetoresistance in Pt/YIG,” Phys. Rev. Lett. 112, 236601 (2014).

XV. Voltage-Controlled Spintronic Devices:

Spintronic devices have evolved from field switching to current switching by the spin transfer torque (STT) effect. But the critical switching current density is too high at 106 – 107 A/cm2 to be useful. We have achieved voltage-controlled spintronic devices where low voltages (less than 1.5 V) can alter the magnetic properties and thereby controlling spintronic properties with current density in the range of 104 A/cm2, two orders of magnitude lower [Nature Mater. 11, 64 (2012)].

395. W. G. Wang, M. Li, S. Hageman, and C. L. Chien, “Electric field assisted switching in magnetic tunnel junctions,” Nature Mater. 11, 64 (2012).

401. W. G. Wang and C. L. Chien, “Voltage induced switching in magnetic tunnel junctions with perpendicular magnetic anisotropy,” J. Phys. D: Appl. Phys. 46, 074004 (2013).

XVI. Skyrmion Thin Flims:

Exotic magnetic skyrmions with a new type of topological spin texture have been realized in cubic B20 magnets (e.g., MnSi and FeGe). These materials lack a center of inversion symmetry and with the presence of the Dzyaloshinskii-Moriya (DM) interaction, which leads to a helical ground state. Skyrmions, with a double-twist spin texture carry a topological charge and a Berry phase in real space. These magnetic skyrmions not only provide a novel route to study the topological nature of magnetic defects but also exhibit unusual static and dynamic properties. However, the skyrmion phase in bulk crystals exists only in a very small region of a few K and a narrow magnetic field range in the phase space. We have achieved epitaxial B20 FeGe thin films with greatly expanded skyrmion phase. We show that the skyrmion states, as revealed by the topological Hall effect and the small angle neutron scattering are stabilized in a dramatically larger region in phase space in FeGe thin films, including the entire temperature range up to TC, and in a large field range that includes zero magnetic field.

398. S. X. Huang and C. L. Chien, “Extended Skyrmion phase in epitaxial FeGe(111) thin films,” Phys. Rev. Lett. 108, 267201 (2012).

423. Gen Yin, Yufan Li, Lingyao Kong, Roger K. Lake, C. L. Chien, and Jiadong Zang, “Topological charge analysis of ultrafast single skyrmion creation,” Phys. Rev. B 93, 174403 (2016).

425. S. X. Huang, Fei Chen, Jian Kang, Jiadong Zang, G. J. Shu, F. C. Chou, and C. L. Chien, “Unusual magnetoresistance in cubic B20 Fe0.85Co0.15Si chiral magnets,” New J. Phys. 18, 065010 (2016).

427. S. X. Huang and C. L. Chien, “Chapter 6. Epitaxial thin films of the cubic B20 chiral magnets,” invited chapter in a book, “Skyrmions: Topological Structures, Properties, and Applications” by CRC Press | Taylor & Francis Group LLC (2016).

XVII. Spin Hall effect and Inverse Spin Hall Effect:

Inverse spin Hall effect (ISHE) has been observed in non-magnetic metals, such as Pt and W. We reported the first ISHE in a ferromagnetic metal [Phys. Rev. Lett. 111, 066602, (2013)] and demonstrated the utility for spin current detection.  We have reported a new and self-consistent method for measuring spin Hall angles [Phys. Rev. B 89, 140407(R) (2014)].  We have also showed that antiferromagnetic metal of Cr also exhibit very large spin Hall effect, independent of antiferromagnetic ordering [Phys. Rev. B 92, 020418(R) (2015)].

409. B. F. Miao, S. Y. Huang, D. Qu, and C. L. Chien, “Inverse spin Hall effect in a ferromagnetic metal,” Phys. Rev. Lett. 111, 066602, (2013).

410. D. Qu, S. Y. Huang, B. F. Miao, S. X. Huang, and C. L. Chien, “Self-consistent determination of spin Hall angles in selected 5d metals by thermal spin injection,” Phys. Rev. B 89, 140407(R) (2014).

417. D. Qu, S. Y. Huang, and C. L. Chien, “Inverse spin Hall effect in Cr: independence of antiferromagnetic ordering,” Phys. Rev. B 92, 020418(R) (2015).

XVIII. Transmission and Amplification of Pure Spin Current in Antiferromagnetic Insulators:

Pure spin current delivers angular momentum with the fewest number of charge carriers in metals, but decays within the spin diffusion length of the order of nm in many metals. We demonstrated amplification of pure spin current up to a factor of 10 by a thin layer of AF insulator such as NiO and CoO, between the metal layer and YIG, from which the spin current is injected [Phys. Rev. Lett. 116, 186601 (2016)] and detected spin backflow from the AF/YIG interface [Phys. Rev. Lett. 118, 067202 (2017)].

424. Weiwei Lin, Kai Chen, Shufeng Zhang, and C. L. Chien, “Enhancement of thermally injected spin current through an antiferromagnetic insulator,” Phys. Rev. Lett. 116, 186601 (2016).

429. Kai Chen, Weiwei Lin, C. L. Chien, and Shufeng Zhang, “Temperature dependence of angular momentum transport across interfaces,” Phys. Rev. B 94, 054413 (2016).

432. Weiwei Lin and C. L. Chien, “Electrical Detection of Spin Backflow from an Antiferromagnetic Insulator/Y3Fe5O12 Interface,” Phys. Rev. Lett. 118, 067202 (2017).

XIX. Spin Orbit Torque Switching of Ferromagnetic Layers:

Switching of ferromagnetic (FM) layers can be accomplished by a magnetic field, and by a current via the spin transfer torque effect.  The newest advent is switching by the spin orbit torque (SOT) via a pure spin current.  SOT switching of FM layers with in-plane magnetization can be readily accomplished.  SOT switching of FM layers with PMA requires an external field, which is technologically unacceptable.  Recently, we show [Phys. Rev. Lett. 120, 117703 (2018)] competing spin current from two heavy metals with opposite spin Hall angle, instead of cancelling the spin currents, can actually accomplish zero-field current switching.

  1.  Qinli Ma, Yufan Li, D. B. Gopman, Yu. P. Kabanov, R. D. Shull, and C. L. Chien, “Switching a perpendicular ferromagnetic layer by competing spin currents,” Phys. Rev. Lett. 120, 117703 (2018).

XX. Strong Spin Orbit Torque from Topological Kondo Insulator SmB6 Thin Films:

Bulk SmB6 crystals exhibit an uncanny feature of a resistance plateau at low temperatures, suggestive of possibly a true topological insulator with insulating interior and conducting surface.  Our SmB6 thin films exhibit resistance plateau, but not surface only conduction.  However, SmB6 thin films exhibit strong (even stronger than those of some heavy metals) spin orbit torque that can switch ferromagnetic layers [Sci. Adv. 4, eaap8294 (2018)] and exhibit non-trivial topological surface state [Phys. Rev. Lett. 120, 207206 (2018)].

436. Yufan Li, Qinli Ma, S. X. Huang, C. L. Chien, “Thin films of topological Kondo insulator candidate SmB6: strong spin-orbit torque without exclusive surface conduction,” Sci. Adv. 4, eaap8294 (2018).

439. T. Liu, Yufan Li, L. Gu, J. J. Ding, H. C. Chang, P. A. P. Janantha, B. Kalinikos, V. Novosad, A. Hoffmann, R. Q. Wu, C. L. Chien, and M. Z. Wu, “Nontrivial Nature and Penetration Depth of Topological Surface States in SmB6 Thin Films,” Phys. Rev. Lett. 120, 207206 (2018).

XXI. Weyl Antiferromagnets with Time Reversal-Odd Topological Responses:

Mn3Sn is among the Weyl magnets, where the Weyl fermions near the Fermi energy give rise to time reversal-odd topological responses, such as large anomalous Hall effect, anomalous Nernst effect, remnant magnetization, and large MOKE optical response.

437. T. Higo, H. Y. Man, D. B. Gopman, L. Wu, T. Koretsune, O. M. J. van’t Erve, Y. P. Kabanov, D. Rees, Yufan Li, M. Suzuki, S. Patankar, M. Ikhlas, C. L. Chien, R. Arita, R. D. Shull, J. Orenstein. and S. Nakatsuji, “Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal.” Nature Photonics 12, 73 (2018).

442. Tomoya Higo, Danru Qu, Yufan Li, C. L. Chien, Yoshichika Otani, and Satoru Nakatsuji, “Anomalous Hall effect in thin films of the Weyl antiferromagnet Mn3Sn,” Appl. Phys. Lett. 113, 202402 (2018).

XXII. Spin Triplet p-wave Superconductors:

The Cooper pairs in a suoperconductor (SC) can be either spin singlet with even parity or spin triplet with odd parity.  The known SCs are overwhelmingly singlet, mostly s-wave (e.g., Nb) and some d-wave (e.g., cuprates).  Triplet p-wave SCs, important in Majorana fermions and quantum computing, are very rare.  We have realized triplet SCs in b-Bi2Pd with the observation of half quantum flux in the Little-Parks experiments.

445. Yufan Li, Xiaoying Xu, M. H. Lee, M. W. Chu, and C. L. Chien, “Observation of Half-Quantum Flux in Unconventional Superconductor b-Bi2Pd,” Science 366, 238 (2019).

448. Xiaoying Xu, Yufan Li, and C. L. Chien, “Spin-Triplet Pairing State Evidenced by Half-Quantum Flux in a Noncentrosymmetric Superconductor,” Phys. Rev. Lett. 124, 167001 (2020).

XXIII. Absence of Evidence of Electric Switching of AF Néel Vector:

Recently, there has been numerous reports of electrical switching of antiferromagnetic (AF) Néel vector via spin-orbit torque (SOT) attracting worldwide attention.  By applying a write current in the AFM layer or the normal metal (NM)/AF bilayer, the measured resistance in a patterned multiterminal structure shows recurring signals due to supposedly electrical switching of the AF Néel vector.  We demonstrate that similar signals can be observed in such structures, with and without the AF layer. This widely held switching signal are not conclusive evidence of SOT AF switching but the thermal artifacts of patterned metal structure on substrate. We show that under a large writing current density beyond the Ohmic regime, the multiterminal devices can generate unintended anisotropic thermal gradients and voltages. As a consequence, the strength of the signal is greatly affected by the thermal conductivity of the substrates. Our results seriously question the validity, and indeed the prospect, of SOT switching of AF Néel vector.

447. C. C. Chiang, S. Y. Huang, D. Qu, P. H. Wu, and C. L. Chien, “Absence of evidence of electrical switching of antiferromagnetic Néel vector,” Phys. Rev. Lett. 123, 227203 (2019) [Editor’s Suggestion, Featured in Physics].