arXiv subjects

C. L. Chien

Publications and source records attributed to C. L. Chien.

At least 19 recordsLinked to original sources

arXiv2026

Significant ordinary Nernst effect contribution to spin-orbit torque harmonic Hall measurements in metallic structures

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.

cond-mat.mes-hall↗

arXiv2024

Electrical Seebeck-Contrast Observation of Magnon Hall Effect in Topological Ferromagnet Lu$_2$V$_2$O$_7$/Heavy Metal Heterostructures

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.

cond-mat.mtrl-sci↗

arXiv2023

Vector spin Seebeck effect and spin swapping effect in antiferromagnetic insulators with non-collinear spin structure

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.

cond-mat.mtrl-sci↗

arXiv2022

Unequivocal Determination of Spin-Triplet Superconductivity Using Composite Rings

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.

cond-mat.supr-con↗

arXiv2022

Observation of Vector Spin Seebeck Effect in a Noncollinear Antiferromagnet

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.

cond-mat.mtrl-sci↗

arXiv2021

Voltage Controlled Spin-Orbit Torque Switching in W/CoFeB/MgO

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.

cond-mat.mtrl-sci↗

arXiv2021

Voltage-controlled antiferromagnetism in magnetic tunnel junctions

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.

cond-mat.mes-hall↗

arXiv2020

Fractional Little-Parks effect observed in a topological superconductor

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.

cond-mat.supr-con↗

arXiv2019

Large anomalous Nernst effect in a van der Waals ferromagnet Fe$_3$GeTe$_2$

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.

cond-mat.mtrl-sci↗

arXiv2018

Anomalous Hall Effect in Thin Film of the Weyl Antiferromagnet Mn$_3$Sn

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.

cond-mat.mtrl-sci↗

arXiv2018

Observation of Half-Quantum Flux in Unconventional Superconductor $β$-Bi$_2$Pd

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.

cond-mat.supr-con↗

arXiv2018

Triplet p-wave superconductivity with ABM state in epitaxial Bi/Ni bilayers

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.

cond-mat.supr-con↗

arXiv2018

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

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.

cond-mat.mtrl-sci↗

arXiv2018

Evidence of pure spin current

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.

cond-mat.mtrl-sci↗

arXiv2016

Electrical Detection of Spin Backflow from an Antiferromagnetic Insulator/Y3Fe5O12 Interface

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.

cond-mat.mtrl-sci↗

arXiv2016

Temperature Dependence of Angular Momentum Transport Across Interfaces

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.

cond-mat.mtrl-sci↗

arXiv2016

Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator

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.

cond-mat.mtrl-sci↗

arXiv2016

Topological Charge Analysis of Ultrafast Single Skyrmion Creation

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.

cond-mat.mes-hall↗