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 occuring 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).

Spin Caloritronics:

In spintronics, both elect charge and spin are manipulated.  On the heal of spintronics, we now have spin caloritronics where one exploits the interaction between heat transport and the charge or spin degree of freedom.  Spin Seebeck effect and spin-dependent spin transport are two very recent examples.

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).
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)

XIV. Voltage-Controlled Spintronic Devices:

Spintronic devices have evolved from first-generation (1G) field devices, driven by magnetic field to second-generation (2G) current devices driven by the spin transfer torque (STT) effect.  Unfortunately, the critical switching current density in the 2G current devices is too high at 10 6 – 10 7 A/cm2 to be useful.  Very recently, voltage-controlled spintronic devices have been achieved 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 to three orders of magnitude lower.

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