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Significant ordinary Nernst effect contribution to spin-orbit torque harmonic Hall measurements in metallic structures
Authors:
Igor Lyalin,
C. C. Chiang,
C. L. Chien
Abstract:
The harmonic Hall measurements are commonly used to quantify the spin-orbit torque in ferromagnet/normal metal bilayers. The contribution from ordinary Nernst effect to the second harmonic voltages is usually assumed to be negligible and omitted in the data analysis. We show that in Cr/Co bilayers the ordinary Nernst effect cannot be neglected and can lead to largely exaggerated values of spin-orb…
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The harmonic Hall measurements are commonly used to quantify the spin-orbit torque in ferromagnet/normal metal bilayers. The contribution from ordinary Nernst effect to the second harmonic voltages is usually assumed to be negligible and omitted in the data analysis. We show that in Cr/Co bilayers the ordinary Nernst effect cannot be neglected and can lead to largely exaggerated values of spin-orbit torque efficiency. Conducting additional experiments, we estimate the Nernst coefficient of Cr and find it comparable to the values reported for other metals. Consequently, the ordinary Nernst effect should be carefully considered when employing the harmonic Hall technique to quantify spin-orbit torque in metals, while the results of some previous works might need to be re-examined.
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Submitted 30 July, 2026;
originally announced July 2026.
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Vector spin Seebeck effect and spin swapping effect in antiferromagnetic insulators with non-collinear spin structure
Authors:
Jinsong Xu,
Weiwei Lin,
Jiaming He,
J. -S. Zhou,
Danru Qu,
Ssu-Yen Huang,
C. L. Chien
Abstract:
Antiferromagnets (AFs) are prospective for next-generation high-density and high-speed spintronic applications due to their negligible stray field and ultrafast spin dynamics, notwithstanding the challenges in detecting and manipulating AF order with no magnetization (M = 0). Among the AFs, non-collinear AFs are of particular interest because of their unique properties arising from the non-colline…
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Antiferromagnets (AFs) are prospective for next-generation high-density and high-speed spintronic applications due to their negligible stray field and ultrafast spin dynamics, notwithstanding the challenges in detecting and manipulating AF order with no magnetization (M = 0). Among the AFs, non-collinear AFs are of particular interest because of their unique properties arising from the non-collinear spin structure and the small magnetization M. In this work, we describe the recently observed vector spin Seebeck effect in non-collinear LuFeO$_3$, where the magneto-thermovoltage under an in-plane temperature gradient, not previously observed, is consistent with the predicted spin swapping effect. Our results shed light on the importance of the non-collinear spin structure in the emerging spin phenomena in non-collinear AFs and offer a new class of materials for AF spintronics and spin caloritronics.
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Submitted 6 September, 2023;
originally announced September 2023.
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Unequivocal Determination of Spin-Triplet Superconductivity Using Composite Rings
Authors:
Xiaoying Xu,
Yufan Li,
C. L. Chien
Abstract:
Phase-sensitive measurements on a composite ring made of a superconductor of interest connected by a known singlet $s$-wave superconductor can unambiguously determine its pairing symmetry. In composite rings with epitaxial $β-$Bi$_2$Pd and $s$-wave Nb, we have observed half-integer quantum flux when Nb is connected to the opposite crystalline ends of $β-$Bi$_2$Pd and integer-quantum flux when Nb i…
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Phase-sensitive measurements on a composite ring made of a superconductor of interest connected by a known singlet $s$-wave superconductor can unambiguously determine its pairing symmetry. In composite rings with epitaxial $β-$Bi$_2$Pd and $s$-wave Nb, we have observed half-integer quantum flux when Nb is connected to the opposite crystalline ends of $β-$Bi$_2$Pd and integer-quantum flux when Nb is connected to the same crystalline ends of $β-$Bi$_2$Pd. These findings provide unequivocal evidence of odd-parity pairing state of the triplet superconductor $β-$Bi$_2$Pd.
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Submitted 17 October, 2022;
originally announced October 2022.
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Electrical Seebeck-Contrast Observation of Magnon Hall Effect in Topological Ferromagnet Lu$_2$V$_2$O$_7$/Heavy Metal Heterostructures
Authors:
Jinsong Xu,
Jiaming He,
J. -S. Zhou,
Danru Qu,
Ssu-Yen Huang,
C. L. Chien
Abstract:
The observation of the magnon Hall effect (MHE) has relied solely on the challenging measurement of the thermal Hall conductivity. Here, we report a highly sensitive electrical Seebeck-contrast method for the observation of MHE in Lu$_2$V$_2$O$_7$/heavy metal heterostructures, that is highly desirable for the exploration of new MHE materials and their applications. Using measuring wires with very…
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The observation of the magnon Hall effect (MHE) has relied solely on the challenging measurement of the thermal Hall conductivity. Here, we report a highly sensitive electrical Seebeck-contrast method for the observation of MHE in Lu$_2$V$_2$O$_7$/heavy metal heterostructures, that is highly desirable for the exploration of new MHE materials and their applications. Using measuring wires with very different Seebeck coefficients, we established a general method that can separate contributions (e.g., MHE) that generates a lateral temperature drop, from those [e.g., anomalous Nernst effect (ANE) and spin Seebeck effect (SSE)] that generate a lateral electric field. We show that a suitable heavy metal overlayer can eliminate the inherent ANE and SSE signals from the semiconducting Lu$_2$V$_2$O$_7$. The MHE in Lu$_2$V$_2$O$_7$ is quasi-isotropic among crystals with different orientations. In addition to the previously reported transverse MHE under an in-plane temperature gradient, we have uncovered longitudinal MHE under an out-of-plane temperature gradient.
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Submitted 4 January, 2024; v1 submitted 12 October, 2022;
originally announced October 2022.
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Observation of Vector Spin Seebeck Effect in a Noncollinear Antiferromagnet
Authors:
Jinsong Xu,
Jiaming He,
J. -S. Zhou,
Danru Qu,
Ssu-Yen Huang,
C. L. Chien
Abstract:
Spintronic phenomena to date have been established in magnets with collinear moments, where the spin injection through the spin Seebeck effect (SSE) is always along the out-of-plane direction. Here, we report the observation of a vector SSE in a noncollinear antiferromagnet (AF) LuFeO$_3$, where temperature gradient along the out-of-plane and also the in-plane directions can both inject a pure spi…
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Spintronic phenomena to date have been established in magnets with collinear moments, where the spin injection through the spin Seebeck effect (SSE) is always along the out-of-plane direction. Here, we report the observation of a vector SSE in a noncollinear antiferromagnet (AF) LuFeO$_3$, where temperature gradient along the out-of-plane and also the in-plane directions can both inject a pure spin current and generate a voltage in the heavy metal via the inverse spin Hall effect (ISHE). We show that the thermovoltages are due to the magnetization from canted spins in LuFeO$_3$. Furthermore, in contrast to the challenges of generating, manipulating and detecting spin current in collinear AFs, the vector SSE in LuFeO$_3$ is readily viable in zero magnetic field and can be controlled by a small magnetic field of about 150 Oe at room temperature. The noncollinear AFs expand new realms for exploring spin phenomena and provide a new route to low-field antiferromagnetic spin caloritronics and magnonics.
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Submitted 12 October, 2022;
originally announced October 2022.
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Voltage Controlled Spin-Orbit Torque Switching in W/CoFeB/MgO
Authors:
Jinsong Xu,
C. L. Chien
Abstract:
Voltage control of magnetism and spintronics have been highly desirable, but rarely realized. In this work, we show voltage-controlled spin-orbit torque (SOT) switching in W/CoFeB/MgO films with perpendicular magnetic anisotropy (PMA) with voltage administered through SrTiO3 with a high dielectric constant. We show that a DC voltage can significantly lower PMA by 45%, reduce switching current by 2…
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Voltage control of magnetism and spintronics have been highly desirable, but rarely realized. In this work, we show voltage-controlled spin-orbit torque (SOT) switching in W/CoFeB/MgO films with perpendicular magnetic anisotropy (PMA) with voltage administered through SrTiO3 with a high dielectric constant. We show that a DC voltage can significantly lower PMA by 45%, reduce switching current by 23%, and increase the damping-like torque as revealed by the first and second-harmonic measurements. These are characteristics that are prerequisites for voltage-controlled and voltage-select SOT switching spintronic devices.
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Submitted 28 January, 2021;
originally announced January 2021.
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Voltage-controlled antiferromagnetism in magnetic tunnel junctions
Authors:
Meng Xu,
Mingen Li,
Pravin Khanal,
Ali Habiboglu,
Blake Insana,
Yuzan Xiong,
Thomas Peterson,
Jason C. Myers,
Deborah Ortega,
Hongwei Qu,
C. L. Chien,
Wei Zhang,
Jian-Ping Wang,
W. G. Wang
Abstract:
We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)Ox formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly-magnetized CoFeB allows sensitive detection of the exchange b…
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We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)Ox formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly-magnetized CoFeB allows sensitive detection of the exchange bias. It is found that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system.
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Submitted 26 January, 2021;
originally announced January 2021.
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Fractional Little-Parks effect observed in a topological superconductor
Authors:
Yufan Li,
Xiaoying Xu,
Shu-Ping Lee,
C. L. Chien
Abstract:
In superconductors, the condensation of Cooper pairs gives rise to fluxoid quantization in discrete units of $Φ_0 = hc / 2e$. The denominator of $2e$ is the signature of electron pairing, which is evidenced by a number of macroscopic quantum phenomena, such as the Little-Parks effect and the Josephson effect, where the critical temperature or the critical current oscillates in the period of $Φ_0$.…
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In superconductors, the condensation of Cooper pairs gives rise to fluxoid quantization in discrete units of $Φ_0 = hc / 2e$. The denominator of $2e$ is the signature of electron pairing, which is evidenced by a number of macroscopic quantum phenomena, such as the Little-Parks effect and the Josephson effect, where the critical temperature or the critical current oscillates in the period of $Φ_0$. Here we report the observation of fractional Little-Parks effect in mesoscopic rings of epitaxial $β$-Bi$_2$Pd, a topological superconductor. Besides $Φ_0$, novel Little-Parks oscillation periodicities of $2Φ_0$, $3Φ_0$ and $4Φ_0$ are also observed, implying quasiparticles with effective charges being a fraction of a Cooper pair. We show that the fractional Little-Parks effect may be closely related to the fractional Josephson effect, which is a key signature of chiral Majorana edge states.
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Submitted 1 March, 2020;
originally announced March 2020.
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Large anomalous Nernst effect in a van der Waals ferromagnet Fe$_3$GeTe$_2$
Authors:
Jinsong Xu,
W. Adam Phelan,
C. L. Chien
Abstract:
Anomalous Nernst effect, a result of charge current driven by temperature gradient, provides a probe of the topological nature of materials due to its sensitivity to the Berry curvature near the Fermi level. Fe3GeTe2, one important member of the recently discovered two-dimensional van der Waals magnetic materials, offers a unique platform for anomalous Nernst effect because of its metallic and top…
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Anomalous Nernst effect, a result of charge current driven by temperature gradient, provides a probe of the topological nature of materials due to its sensitivity to the Berry curvature near the Fermi level. Fe3GeTe2, one important member of the recently discovered two-dimensional van der Waals magnetic materials, offers a unique platform for anomalous Nernst effect because of its metallic and topological nature. Here, we report the observation of large anomalous Nernst effect in Fe3GeTe2. The anomalous Hall angle and anomalous Nernst angle are about 0.07 and 0.09 respectively, far larger than those in common ferromagnets. By utilizing the Mott relation, these large angles indicate a large Berry curvature near the Fermi level, consistent with the recent proposal for Fe3GeTe2 as a topological nodal line semimetal candidate. Our work provides evidence of Fe3GeTe2 as a topological ferromagnet, and demonstrates the feasibility of using two-dimensional magnetic materials and their band topology for spin caloritronics applications.
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Submitted 29 October, 2019;
originally announced October 2019.
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Anomalous Hall Effect in Thin Film of the Weyl Antiferromagnet Mn$_3$Sn
Authors:
Tomoya Higo,
Danru Qu,
Yufan Li,
C. L. Chien,
Yoshichika Otani,
Satoru Nakatsuji
Abstract:
The Weyl antiferromagnet Mn$_3$Sn has recently attracted significant attention as it exhibits various useful functions such as large anomalous Hall effect that are normally absent in antiferromagnets. Here we report the thin film fabrication of the single phase of Mn$_3$Sn and the observation of the large anomalous Hall effect at room temperature despite its vanishingly small magnetization. Our wo…
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The Weyl antiferromagnet Mn$_3$Sn has recently attracted significant attention as it exhibits various useful functions such as large anomalous Hall effect that are normally absent in antiferromagnets. Here we report the thin film fabrication of the single phase of Mn$_3$Sn and the observation of the large anomalous Hall effect at room temperature despite its vanishingly small magnetization. Our work on the high-quality thin film growth of the Weyl antiferromagnet paves the path for developing the antiferromagnetic spintronics.
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Submitted 27 October, 2018;
originally announced October 2018.
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Observation of Half-Quantum Flux in Unconventional Superconductor $β$-Bi$_2$Pd
Authors:
Yufan Li,
Xiaoying Xu,
C. L. Chien
Abstract:
We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting $β$-Bi$_2$Pd thin films. The half-quantum fluxoid manifests itself as a $π$ phase shift in the quantum oscillation of the critical temperature. This result verifies unconventional superconductivity of $β$-Bi$_2$Pd, in accord with the expectation of a topological superconductor. We also discu…
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We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting $β$-Bi$_2$Pd thin films. The half-quantum fluxoid manifests itself as a $π$ phase shift in the quantum oscillation of the critical temperature. This result verifies unconventional superconductivity of $β$-Bi$_2$Pd, in accord with the expectation of a topological superconductor. We also discuss the strong indication that $β$-Bi$_2$Pd is a spin-triplet superconductor.
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Submitted 26 October, 2018;
originally announced October 2018.
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Triplet p-wave superconductivity with ABM state in epitaxial Bi/Ni bilayers
Authors:
G. J. Zhao,
X. X. Gong,
J. C. He,
J. A. Gifford,
H. X. Zhou,
Y. Chen,
X. F. Jin,
C. L. Chien,
T. Y. Chen
Abstract:
We report observation of spin triplet superconductivity in epitaxial Bi/Ni bilayers with TC up to 4 K and 2Delta/kBTC = 12. Andreev reflection spectroscopy (ARS) with ballistic injection of unpolarized and spin-polarized electrons conclusively reveals spin triplet p-wave superconductivity. The gap structure measured by ARS in multiple crystal directions shows the ABM (Anderson-Brinkman-Morel) stat…
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We report observation of spin triplet superconductivity in epitaxial Bi/Ni bilayers with TC up to 4 K and 2Delta/kBTC = 12. Andreev reflection spectroscopy (ARS) with ballistic injection of unpolarized and spin-polarized electrons conclusively reveals spin triplet p-wave superconductivity. The gap structure measured by ARS in multiple crystal directions shows the ABM (Anderson-Brinkman-Morel) state, the same as that in superfluid 3He.
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Submitted 24 October, 2018;
originally announced October 2018.
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Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal
Authors:
Tomoya Higo,
Huiyuan Man,
Daniel B. Gopman,
Liang Wu,
Takashi Koretsune,
Olaf M. J. van 't Erve,
Yury P. Kabanov,
Dylan Rees,
Yufan Li,
Michi-To Suzuki,
Shreyas Patankar,
Muhammad Ikhlas,
C. L. Chien,
Ryotaro Arita,
Robert D. Shull,
Joseph Orenstein,
Satoru Nakatsuji
Abstract:
When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties as well as for various applications including magneto-optical record…
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When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties as well as for various applications including magneto-optical recording. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of $M \sim$0.002 $μ_{\rm B}$/Mn, the non-collinear AF metal Mn$_3$Sn exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Neel state may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.
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Submitted 17 May, 2018;
originally announced May 2018.
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Evidence of pure spin current
Authors:
Weiwei Lin,
C. L. Chien
Abstract:
Evidences of pure spin current are indistinguishable from those of many parasitic effects. Proper choices of materials and methods are essential for exploring pure spin current phenomena and devices.
Evidences of pure spin current are indistinguishable from those of many parasitic effects. Proper choices of materials and methods are essential for exploring pure spin current phenomena and devices.
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Submitted 28 March, 2018;
originally announced April 2018.
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Electrical Detection of Spin Backflow from an Antiferromagnetic Insulator/Y3Fe5O12 Interface
Authors:
Weiwei Lin,
C. L. Chien
Abstract:
Spin Hall magnetoresistance (SMR) has been observed in Pt/NiO/Y3Fe5O12 (YIG) heterostructures with characteristics very different from those in Pt/YIG. We show that the SMR in Pt/NiO/YIG strongly correlates with spin conductance, both sharing very strong temperature dependence due to antiferromagnetic magnons and spin fluctuation. This phenomenon indicates that spin current generated by spin Hall…
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Spin Hall magnetoresistance (SMR) has been observed in Pt/NiO/Y3Fe5O12 (YIG) heterostructures with characteristics very different from those in Pt/YIG. We show that the SMR in Pt/NiO/YIG strongly correlates with spin conductance, both sharing very strong temperature dependence due to antiferromagnetic magnons and spin fluctuation. This phenomenon indicates that spin current generated by spin Hall effect in the Pt transmits through the insulating NiO and is reflected from the NiO/YIG interface. Inverted SMR has been observed below a temperature which increases with the NiO thickness, suggesting spin-flip reflection from the antiferromagnetic NiO exchange coupled with the YIG.
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Submitted 25 November, 2016;
originally announced November 2016.
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Temperature Dependence of Angular Momentum Transport Across Interfaces
Authors:
Kai Chen,
Weiwei Lin,
C. L. Chien,
Shufeng Zhang
Abstract:
Angular momentum transport in magnetic multilayered structures plays a central role in spintronic physics and devices. The angular momentum currents or spin currents are carried by either quasi-particles such as electrons and magnons, or by macroscopic order parameters such as local magnetization of ferromagnets. Based on the generic interface exchange interaction, we develop a microscopic theory…
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Angular momentum transport in magnetic multilayered structures plays a central role in spintronic physics and devices. The angular momentum currents or spin currents are carried by either quasi-particles such as electrons and magnons, or by macroscopic order parameters such as local magnetization of ferromagnets. Based on the generic interface exchange interaction, we develop a microscopic theory that describes interfacial spin conductance for various interfaces among non-magnetic metals, ferromagnetic and antiferromagnetic insulators. Spin conductance and its temperature dependence are obtained for different spin batteries including spin pumping, temperature gradient and spin Hall effect. As an application of our theory, we calculate the spin current in a trilayer made of a ferromagnetic insulator, an antiferromagnetic insulator and a non-magnetic heavy metal. The calculated results on the temperature dependence of spin conductance quantitatively agree with the existing experiments.
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Submitted 8 April, 2016; v1 submitted 6 April, 2016;
originally announced April 2016.
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Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator
Authors:
Weiwei Lin,
Kai Chen,
Shufeng Zhang,
C. L. Chien
Abstract:
We report large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet(AF)/yttrium iron garnet(YIG), where a thin AF insulating layer of NiO or CoO can enhance spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the Néel temperature of the thin AF layer, has been found to scale linearly with t…
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We report large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet(AF)/yttrium iron garnet(YIG), where a thin AF insulating layer of NiO or CoO can enhance spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the Néel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM/YIG interface for NM = 3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results.
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Submitted 4 April, 2016; v1 submitted 2 March, 2016;
originally announced March 2016.
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Topological Charge Analysis of Ultrafast Single Skyrmion Creation
Authors:
Gen Yin,
Yufan Li,
Lingyao Kong,
Roger K. Lake,
C. L. Chien,
Jiadong Zang
Abstract:
Magnetic skyrmions are topologically non-trivial spin textures of potential interest for future information storage applications, and for such purposes, the control and understanding of single skyrmion creation is required. A scheme is analyzed to create single Néel-type and Bloch-type skyrmions in helimagnetic thin films utilizing the dynamical excitations induced by the Oersted field and the spi…
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Magnetic skyrmions are topologically non-trivial spin textures of potential interest for future information storage applications, and for such purposes, the control and understanding of single skyrmion creation is required. A scheme is analyzed to create single Néel-type and Bloch-type skyrmions in helimagnetic thin films utilizing the dynamical excitations induced by the Oersted field and the spin transfer torque given by a vertically injected spin-polarized current. A topological charge analysis using a lattice version of the topological charge provides insight into the locally triggered transition from a trivial to a non-trivial topological spin texture of the Néel or Bloch type skyrmion. The topological protection of the magnetic skyrmion is determined by the symmetric Heisenberg exchange energy. The critical switching current density is $\sim10^{7}\thinspace\textrm{A/cm}^{2}$, which decreases with the easy-plane type uniaxial anisotropy and thermal fluctuations. In-plane spin polarization of the injected current performs better than out-of-plane polarization, and it provides ultrafast switching times (within 100 ps) and reliable switching outcomes.
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Submitted 7 March, 2016; v1 submitted 28 November, 2014;
originally announced November 2014.
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Universal Ratio of Intrinsic Resistivities of Spin Helix in B20 (Fe-Co)Si Magnets
Authors:
S. X. Huang,
Jian Kang,
Fei Chen,
Jiadong Zang,
G. J. Shu,
F. C. Chou,
S. V. Grigoriev,
V. A. Dyadkin,
C. L. Chien
Abstract:
The B20 magnets with the Dzyaloshinskii-Moriya (D-M) interaction exhibit spin helix and Skyrmion spin textures unattainable in traditional Heisenberg ferromagnets. We have determined the intrinsic resistivity of the spin helix, which is a macroscopic Bloch domain wall, in B20 (Fe-Co)Si magnets. We found a universal resistance ratio of gamma = 1.35 with current parallel and perpendicular to the hel…
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The B20 magnets with the Dzyaloshinskii-Moriya (D-M) interaction exhibit spin helix and Skyrmion spin textures unattainable in traditional Heisenberg ferromagnets. We have determined the intrinsic resistivity of the spin helix, which is a macroscopic Bloch domain wall, in B20 (Fe-Co)Si magnets. We found a universal resistance ratio of gamma = 1.35 with current parallel and perpendicular to the helix, independent of composition and temperature. This gamma value is much smaller than 3, the well-known minimum value for domain wall resistivity in traditional ferromagnets, due to the significant spin-orbit coupling in the B20 magnets.
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Submitted 27 September, 2014;
originally announced September 2014.
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Magnetoresistance due to Broken C4 Symmetry in Cubic B20 Chiral Magnets
Authors:
S. X. Huang,
Fei Chen,
Jian Kang,
Jiadong Zang,
G. J. Shu,
F. C. Chou,
C. L. Chien
Abstract:
The B20 chiral magnets with broken inversion symmetry and C4 rotation symmetry have attracted much attention. The broken inversion symmetry leads to the Dzyaloshinskii-Moriya that gives rise to the helical and Skyrmion states. We report the unusual magnetoresistance (MR) of B20 chiral magnet Fe0.85Co0.15Si that directly reveals the broken C4 rotation symmetry. We present a microscopic theory, a mi…
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The B20 chiral magnets with broken inversion symmetry and C4 rotation symmetry have attracted much attention. The broken inversion symmetry leads to the Dzyaloshinskii-Moriya that gives rise to the helical and Skyrmion states. We report the unusual magnetoresistance (MR) of B20 chiral magnet Fe0.85Co0.15Si that directly reveals the broken C4 rotation symmetry. We present a microscopic theory, a minimal theory with two spin-orbit terms, that satisfies all the symmetry requirements and accounts for the transport experiments.
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Submitted 27 September, 2014;
originally announced September 2014.
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Electrical Detection of Direct and Alternating Spin Current Injected from a Ferromagnetic Insulator into a Ferromagnetic Metal
Authors:
P. Hyde,
Lihui Bai,
D. M. J. Kumar,
B. W. Southern,
S. Y. Huang,
B. F. Miao,
C. L. Chien,
C. -M. Hu
Abstract:
We report room temperature electrical detection of spin injection from a ferromagnetic insulator (YIG) into a ferromagnetic metal (Permalloy, Py). Non-equilibrium spins with both static and precessional spin polarizations are dynamically generated by the ferromagnetic resonance of YIG magnetization, and electrically detected by Py as dc and ac spin currents, respectively. The dc spin current is el…
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We report room temperature electrical detection of spin injection from a ferromagnetic insulator (YIG) into a ferromagnetic metal (Permalloy, Py). Non-equilibrium spins with both static and precessional spin polarizations are dynamically generated by the ferromagnetic resonance of YIG magnetization, and electrically detected by Py as dc and ac spin currents, respectively. The dc spin current is electrically detected via the inverse spin Hall effect of Py, while the ac spin current is converted to a dc voltage via the spin rectification effect of Py which is resonantly enhanced by dynamic exchange interaction between the ac spin current and the Py magnetization. Our results reveal a new path for developing insulator spintronics, which is distinct from the prevalent but controversial approach of using Pt as the spin current detector.
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Submitted 17 October, 2013;
originally announced October 2013.
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Reply to the Comment on "Unified Formulism of Andreev Reflection at a Ferromagnetic/Superconductor Interface" by Eschrig et al
Authors:
T. Y. Chen,
C. L. Chien
Abstract:
Reply to the Comment on "Unified Formulism of Andreev Reflection at a Ferromagnetic/Superconductor Interface" by Eschrig et al
Reply to the Comment on "Unified Formulism of Andreev Reflection at a Ferromagnetic/Superconductor Interface" by Eschrig et al
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Submitted 8 February, 2013;
originally announced February 2013.
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Intrinsic Spin Seebeck Effect in Au/YIG
Authors:
D. Qu,
S. Y. Huang,
Jun Hu,
Ruqian Wu,
C. L. Chien
Abstract:
The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin-polarized density-functional calculations…
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The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin-polarized density-functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG.
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Submitted 25 January, 2013;
originally announced January 2013.
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Pressure effects on strained FeSe0.5Te0.5 thin films
Authors:
M. Gooch,
B. Lorenz,
S. X. Huang,
C. L. Chien,
C. W. Chu
Abstract:
The pressure effect on the resistivity and superconducting Tc of prestrained thin films of the iron chalcogenide superconductor FeSe0.5Te0.5 is studied. Films with different anion heights above the Fe layer showing different values of ambient pressure Tc's are compressed up to a pressure of 1.7 GPa. All films exhibit a significant increase of Tc with pressure. The results cannot solely be explaine…
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The pressure effect on the resistivity and superconducting Tc of prestrained thin films of the iron chalcogenide superconductor FeSe0.5Te0.5 is studied. Films with different anion heights above the Fe layer showing different values of ambient pressure Tc's are compressed up to a pressure of 1.7 GPa. All films exhibit a significant increase of Tc with pressure. The results cannot solely be explained by a pressure-induced decrease of the anion height but other parameters have to be considered to explain the data for all films.
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Submitted 22 August, 2011;
originally announced August 2011.
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Enhancement of vortex pinning in superconductor/ferromagnet bilayers via angled demagnetization
Authors:
Marta Z. Cieplak,
L. Y. Zhu,
Z. Adamus,
M. Konczykowski,
C. L. Chien
Abstract:
We use local and global magnetometry measurements to study the influence of magnetic domain width w on the domain-induced vortex pinning in superconducting/ferromagnetic bilayers, built of a Nb film and a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy, with an insulating layer to eliminate proximity effect. The quasi-periodic domain patterns with different and systematically…
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We use local and global magnetometry measurements to study the influence of magnetic domain width w on the domain-induced vortex pinning in superconducting/ferromagnetic bilayers, built of a Nb film and a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy, with an insulating layer to eliminate proximity effect. The quasi-periodic domain patterns with different and systematically adjustable width w, as acquired by a special demagnetization procedure, exert tunable vortex pinning on a superconducting layer. The largest enhancement of vortex pinning, by a factor of more than 10, occurs when w ~ 310 nm is close to the magnetic penetration depth.
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Submitted 19 July, 2011;
originally announced July 2011.
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Tunable phase diagram and vortex pinning in a superconductor-ferromagnet bilayer
Authors:
L. Y. Zhu,
Marta Z. Cieplak,
C. L. Chien
Abstract:
We have observed the evolution of phase diagram and vortex pinning using a single ferromagnet/superconductor bilayer of [Co/Pt]8/Nb through a special demagnetization procedure. It induces a continuous and reversible change of the domain width with equal positive/negative domains enabling the observation of the predicted tunable phase diagram. The tunable domain pattern also systematically affects…
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We have observed the evolution of phase diagram and vortex pinning using a single ferromagnet/superconductor bilayer of [Co/Pt]8/Nb through a special demagnetization procedure. It induces a continuous and reversible change of the domain width with equal positive/negative domains enabling the observation of the predicted tunable phase diagram. The tunable domain pattern also systematically affects vortex pinning. We have determined the dependence of the activation energy of vortex pinning on domain width, temperature, and magnetic field.
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Submitted 13 July, 2010;
originally announced July 2010.
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Control of tetrahedral coordination and superconductivity in FeSe0.5Te0.5 thin films
Authors:
S. X. Huang,
C. L. Chien,
V. Thampy,
C. Broholm
Abstract:
We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the theo…
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We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the theoretical proposal that (pi,pi) spin fluctuations promote superconductivity in Fe superconductors.
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Submitted 12 February, 2010;
originally announced February 2010.
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Determination of Superconducting Gap of SmFeAsFxO1-x Superconductors by Andreev Reflection Spectroscopy
Authors:
T. Y. Chen,
S. X. Huang,
Z. Tesanovic,
R. H. Liu,
X. H. Chen,
C. L. Chien
Abstract:
The superconducting gap in FeAs-based superconductor SmFeAs(O1-xFx) (x = 0.15 and 0.30) and the temperature dependence of the sample with x = 0.15 have been measured by Andreev reflection spectroscopy. The intrinsic superconducting gap is independent of contacts while many other "gap-like" features vary appreciably for different contacts. The determined gap value of 2D = 13.34 +/-0.47 meV for Sm…
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The superconducting gap in FeAs-based superconductor SmFeAs(O1-xFx) (x = 0.15 and 0.30) and the temperature dependence of the sample with x = 0.15 have been measured by Andreev reflection spectroscopy. The intrinsic superconducting gap is independent of contacts while many other "gap-like" features vary appreciably for different contacts. The determined gap value of 2D = 13.34 +/-0.47 meV for SmFeAs(O0.85F0.15) gives 2D/kBTC = 3.68, close to the BCS prediction of 3.53. The superconducting gap decreases with temperature and vanishes at TC, in a manner similar to the BCS behavior but dramatically different from that of the nodal pseudogap behavior in cuprate superconductors.
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Submitted 23 February, 2009;
originally announced February 2009.
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The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
Authors:
T. Y. Chen,
Z. Tesanovic,
R. H. Liu,
X. H. Chen,
C. L. Chien
Abstract:
Since the discovery of superconductivity in the cuprates two decades ago, it has been firmly established that the CuO_2 plane is consequential for high T_C superconductivity and a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO_(1-x)F_x has recently been discovered but with the conspicuous lacking of the CuO_2 planes, thus raising th…
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Since the discovery of superconductivity in the cuprates two decades ago, it has been firmly established that the CuO_2 plane is consequential for high T_C superconductivity and a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO_(1-x)F_x has recently been discovered but with the conspicuous lacking of the CuO_2 planes, thus raising the tantalizing questions of the different pairing mechanisms in these oxypnictide superconductors. Intimately related to pairing in a superconductor are the superconducting gap, its value, structure, and temperature dependence. Here we report the observation of a single gap in the superconductor SmFeAsO_0.85F_0.15 with T_C = 42 K as measured by Andreev spectroscopy. The gap value of 2Delta = 13.34+/-0.3 meV gives 2Delta/k_BT_C = 3.68, close to the BCS prediction of 3.53. The gap decreases with temperature and vanishes at T_C in a manner consistent with the Bardeen-Cooper-Schrieffer (BCS) prediction but dramatically different from that of the pseudogap behavior in the cuprate superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, t-J model, and the like, originally designed for cuprates.
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Submitted 29 May, 2008;
originally announced May 2008.
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Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings
Authors:
F. Q. Zhu,
G. W. Chern,
O. Tchernyshyov,
X. C. Zhu,
J. G. Zhu,
C. L. Chien
Abstract:
Magnetization reversals through the formation of vortex state and the rotation of onion state are two processes with comparable probabilities for symmetric magnetic nanorings with radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in…
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Magnetization reversals through the formation of vortex state and the rotation of onion state are two processes with comparable probabilities for symmetric magnetic nanorings with radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and can not be altered. In this work, we report the magnetic behavior of a novel type of nanorings -- asymmetric nanorings. By tuning the asymmetry we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain wall energy on the local cross section area of nanoring for which we have provided theoretical calculations.
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Submitted 9 August, 2005;
originally announced August 2005.
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Controllable High-Speed Rotation of Nanowires
Authors:
D. L. Fan,
F. Q. Zhu,
R. C. Cammarata,
C. L. Chien
Abstract:
We report a versatile method for executing controllable high-speed rotation of nanowires by AC voltages applied to multiple electrodes. The rotation of the nanowires can be instantly switched on or off with precisely controlled rotation speed (to at least 1800 rpm), definite chirality, and total angle of rotation. We have determined the torque due to the fluidic drag force on nanowire of differe…
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We report a versatile method for executing controllable high-speed rotation of nanowires by AC voltages applied to multiple electrodes. The rotation of the nanowires can be instantly switched on or off with precisely controlled rotation speed (to at least 1800 rpm), definite chirality, and total angle of rotation. We have determined the torque due to the fluidic drag force on nanowire of different lengths. We also demonstrate a micromotor using a rotating nanowires driving a dust particle into circular motion. This method has been used to rotate magnetic and nonmagnetic nanowires as well as carbon nanotubes.
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Submitted 7 March, 2005;
originally announced March 2005.
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Antisymmetric magnetoresistance in magnetic multilayers with perpendicular anisotropy
Authors:
X. M. Cheng,
S. Urazhdin,
O. Tchernyshyov,
C. L. Chien,
V. I. Nikitenko,
A. J. Shapiro,
R. D. Shull
Abstract:
While magnetoresistance (MR) has generally been found to be symmetric in applied field in non-magnetic or magnetic metals, we have observed antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and transport measurements show that the antisymmetric MR is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only i…
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While magnetoresistance (MR) has generally been found to be symmetric in applied field in non-magnetic or magnetic metals, we have observed antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and transport measurements show that the antisymmetric MR is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only in materials with perpendicular magnetic anisotropy. As a result, the extraordinary Hall effect (EHE) gives rise to circulating currents in the vicinity of the domain walls that contributes to the MR. The antisymmetric MR and EHE have been quantitatively accounted for by a theoretical model.
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Submitted 9 December, 2004;
originally announced December 2004.
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Effects of Antiferromagnetic Spin Rotation on Anisotropy of Ferromagnetic/Antiferromagnetic Bilayers
Authors:
S. Urazhdin,
C. L. Chien
Abstract:
In epitaxial (111) oriented Ni$_{80}$Fe$_{20}$/Fe$_{50}$Mn$_{50}$ bilayers, we separate two distinct behaviors: unidirectional anisotropy (exchange bias) in thick Fe$_{50}$Mn$_{50}$, and enhanced coercivity in thin Fe$_{50}$Mn$_{50}$. By measuring the magnetization response to a rotating magnetic field, we quantitatively determine the relevant anisotropies, and demonstrate that the enhanced coer…
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In epitaxial (111) oriented Ni$_{80}$Fe$_{20}$/Fe$_{50}$Mn$_{50}$ bilayers, we separate two distinct behaviors: unidirectional anisotropy (exchange bias) in thick Fe$_{50}$Mn$_{50}$, and enhanced coercivity in thin Fe$_{50}$Mn$_{50}$. By measuring the magnetization response to a rotating magnetic field, we quantitatively determine the relevant anisotropies, and demonstrate that the enhanced coercivity is related to the rotatable magnetic anisotropy of Fe$_{50}$Mn$_{50}$. We also demonstrate the consequences of the anisotropy changes with temperature.
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Submitted 4 October, 2004;
originally announced October 2004.
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Room temperature domain wall pinning in bent ferromagnetic nanowires
Authors:
D. M. Silevitch,
M. Tanase,
C. L. Chien,
D. H. Reich
Abstract:
Mechanically bent nickel nanowires show clear features in their room temperature magnetoresistance when a domain wall is pinned at the location of the bend. By varying the direction of an applied magnetic field, the wire can be prepared either in a single-domain state or a two-domain state. The presence or absence of the domain wall acts to shift the switching fields of the nanowire. In addition…
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Mechanically bent nickel nanowires show clear features in their room temperature magnetoresistance when a domain wall is pinned at the location of the bend. By varying the direction of an applied magnetic field, the wire can be prepared either in a single-domain state or a two-domain state. The presence or absence of the domain wall acts to shift the switching fields of the nanowire. In addition, a comparison of the magnetoresistance of the nanowire with and without a domain wall shows a shift in the resistance correlated with the presence of a wall. The resistance is decreased by 20-30 milli-Ohms when a wall is present, compared to an overall resistance of 40-60 Ohms. A model of the magnetization was developed that allowed calculation of the magnetostatic energy of the nanowires, giving an estimate for the nucleation energy of a domain wall.
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Submitted 27 August, 2003;
originally announced August 2003.
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Current-induced spin-wave excitations in a single ferromagnetic layer
Authors:
Y. Ji,
C. L. Chien
Abstract:
A new current induced spin-torque transfer effect has been observed in a single ferromagnetic layer without resorting to multilayers. At a specific current density of one polarity injected from a point contact, abrupt resistance changes due to current-induced spin wave excitations have been observed. The critical current at the onset of spin-wave excitations depends linearly on the external fiel…
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A new current induced spin-torque transfer effect has been observed in a single ferromagnetic layer without resorting to multilayers. At a specific current density of one polarity injected from a point contact, abrupt resistance changes due to current-induced spin wave excitations have been observed. The critical current at the onset of spin-wave excitations depends linearly on the external field applied perpendicular to the layer. The observed effect is due to current-driven heterogeneity in an otherwise uniform ferromagnetic layer.
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Submitted 4 October, 2002;
originally announced October 2002.