Yi-Ying Chin - Academia.edu (original) (raw)

Papers by Yi-Ying Chin

Physical Review Applied

In conventional spintronic devices, ferromagnetic materials are used, which have a magnetization ... more In conventional spintronic devices, ferromagnetic materials are used, which have a magnetization dynamics timescale of around nanoseconds, setting a limit for the switching speed. Increasing the magnetization switching speed has been one of the major challenges for spintronic research. In this work we take advantage of the ultrafast magnetic dynamics in ferrimagnetic materials instead of ferromagnets, and we use femtosecond laser pulses and a plasmonic photoconductive switch to create THz electrical pulses for ferrimagnetic switching by spin-orbit torque. By anomalous Hall and magneto-optic Kerr effect (MOKE) measurement, we demonstrate the robust THz-electrical-pulse-driven magnetization switching of ferrimagnetic Gd-Fe-Co. The time-resolved MOKE shows more than 50-GHz magnetic resonance frequency of Gd-Fe-Co, indicating faster than 20-ps magnetic dynamics. X-ray magnetic circular dichroism demonstrates the antiferromagnetically coupled Fe and Gd sublattices. Our work provides a promising route to realize ultrafast operation speed for nonvolatile magnetic memory and logic applications.

Applied Physics Letters, 2021

An A-site ordered but B-site disordered quadruple perovskite oxide PbMn3Cr3MnO12 was synthesized ... more An A-site ordered but B-site disordered quadruple perovskite oxide PbMn3Cr3MnO12 was synthesized by high-pressure and high-temperature methods. The compound crystallizes in space group Im-3 with the charge distribution of Pb2+Mn3+3Cr3+3Mn4+O12. Three antiferromagnetic phase transitions are found to occur at TN1 ≈ 155, TN2 ≈ 81, and TN3 ≈ 74 K, respectively, due to the complex B-site and A′-B intersite spin interactions. Compared with the isostructural LaMn3Cr4O12 with negligible A′-B intersite spin coupling, the substitution of Mn4+, which has identical electronic configuration to that of Cr3+ (t2g3), into the B site can introduce A′-B intersite spin interactions. As a result, the A′-site spin-related ordering temperature increases significantly, while the B-site one remains little changed in the current PbMn3Cr3MnO12. This work opens up a way to enhance the A′-site spin ordering temperature in quadruple perovskite oxides.

Transition metal oxides are a fascinating class of materials showing a wealth of fascinating prop... more Transition metal oxides are a fascinating class of materials showing a wealth of fascinating properties. These include metal-insulator transition in the early transition metal oxides, the colossal magnetoresistance in the manganates, the occurrence of stripe phases in the nickelates, and the high-T c superconductivity in the cuprates. The cobaltates form a special subclass in the sense that this single subclass covers a very wide range of different properties. For example, one can find not only giant magnetoresistance in RENi 0.3 Co 0.7 O 3 , and superconductivity in Na x CoO 2 •yH 2 O, but also exceptionally high thermopower in NaCo 2 O 4 , and even unique phenomena like the spin-blockade behavior in HoBaCo 2 O 5.5 and La 1.5 Sr 0.5 CoO 4. Much of these spectacular phenomena have their origin in the interplay among the charge, orbital, and spin degrees of freedom of the constituent transition metal ions. Special to the cobaltates with respect to the other subclasses like the cuprates, nickelates and manganates, is that the Co ions possess not only the"spin-up" and "spin-down" degrees of freedom, but also the possibility for a spin-state change: Co 3+ and Co 4+ ions can be tuned to have a high spin (HS) (S=2; S=5/2), a low spin (LS) (S=0; S=1/2), and an intermediate spin (IS) (S=1; S=3/2) state. These different spin states have also quite different orbital occupation. This enormous flexibility of the Co ions to have quite different local electronic states forms a challenge for experimentalists and theorists to explain and predict the properties of cobaltates. In this thesis we investigate the electronic structure of selected cobaltates using X-ray absorption and photoemission spectroscopy combined with configuration-interaction (CI) cluster calculations to further understand their electronic and magnetic properties. This thesis starts with the study on the spin state of the Co 3+ ions in Sr 0.7 Y 0.3 CoO 2.63 , which is claimed to have orbital ordering from the octahedral IS Co 3+ ions. The Co-L 2,3 and O-K X-ray absorption spectroscopy (XAS) measurements reveal that Co 3+ ions at octahedral, pyramidal, and tetrahedral sites are all in the HS state. Then we investigate valence and spin states of Co in SrCo 0.5 Ru 0.5 O 3 to understand its magnetic behavior. Ru-L 2,3 XAS for SrCo 0.5 Ru 0.5 O 3 clearly reveals a Co 3+ /Ru 5+ state, and not a Co 4+ /Ru 4+ state. The octahedral Co 3+ ions are in the unusual HS state, which is consistent with the large CoO distance obtained from Co-K EXAFS data. Under high pressure the Co Kβ X-ray emission spectra show a collapse of spin moment, reflecting a HS to a LS transition for Co 3+ ions.

Physical Review B, 2019

We investigate and compare the electronic structure of a bulk single crystal of Y 3 Fe 5 O 12 gar... more We investigate and compare the electronic structure of a bulk single crystal of Y 3 Fe 5 O 12 garnet [YIG, a high-T C (= 560 K) ferrimagnet] with that of an epitaxial ultrathin (3.3 nm) film of YIG with a reduced ferrimagnetic temperature T C = 380 K, using bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The Fe 2p HAXPES spectrum of the bulk single crystal exhibits a purely trivalent Fe 3+ state for octahedral and tetrahedral sites. The Fe 3s spectrum shows a clear splitting which allows us to estimate the on-site Fe 3s-3d exchange interaction energy. The valence band HAXPES spectrum shows Fe 3d, O 2p, and Fe 4s derived features and a band gap of ∼2.3 eV in the occupied density of states, consistent with the known optical band gap of ∼2.7 eV. Fe Ledge XAS identifies the octahedral Fe 3+ and tetrahedral Fe 3+ site features. XMCD spectra at the Fe L 2,3 edges show that bulk single-crystal YIG exhibits antiferromagnetic coupling between the octahedral-and tetrahedral-site spins. The calculated Fe 2p HAXPES, Fe Ledge XAS, and XMCD spectra using full multiplet cluster calculations match well with the experimental results and confirm the full local spin moments. In contrast, HAXPES, XAS, and XMCD of the Pt/YIG (3.3 nm) ultrathin epitaxial film grown by a pulsed laser deposition method show a finite Fe 2+ contribution and a reduced Fe 3+ local spin moment. The Fe 2+ state is attributed to a combination of oxygen deficiency and charge transfer effects from the Pt capping layer to the ultrathin film. However, the conserved XMCD spectral shape for the ultrathin film indicates that the 3.3-nm epitaxial film is genuinely ferrimagnetic, in contrast to recent studies on films grown by radio-frequency magnetron sputtering which have shown a magnetic dead layer of ∼6 nm. The presence of Fe 2+ and the reduced local spin moment in the epitaxial ultrathin film lead to a reduced Curie temperature, quantitatively consistent with well-known mean-field theory. The results establish a coupling of the local Fe spin moments, valency, and long-range magnetic ordering temperature in bulk single crystal and epitaxial ultrathin-film YIG.

Applied Physics Letters, 2017

The unconventional doping in YBa2Cu3O7x/La0.7Ca0.3MnO3 heterostructures by termination control.

Physical Review B, 2013

Here we present linear-polarization-dependent soft x-ray absorption spectroscopy data at the Ce M... more Here we present linear-polarization-dependent soft x-ray absorption spectroscopy data at the Ce M 4,5 edges of CeOs 2 Al 10 and CeFe 2 Al 10. Despite the strong hybridization between 4f and conduction electrons and the existence of spin gaps as seen in inelastic neutron scattering, we were able to determine the crystal field ground state wave functions by combining our spectroscopy data with magnetization measurements. The results quantitatively explain the small ordered moment along c and the measured magnetic moment along the easy a axis in CeOs 2 Al 10 .

Physical Review B, 2013

The physical properties of Sr 14−x Ca x Cu 24 O 41 are determined by the hole distribution betwee... more The physical properties of Sr 14−x Ca x Cu 24 O 41 are determined by the hole distribution between the edge-shared CuO 2 chain and the corner-shared Cu 2 O 3 two-leg ladder, but inconsistent results on the hole distribution were obtained in various experimental works in recent decades. In this work we reinvestigate the hole distribution by soft x-ray absorption spectroscopy at the Cu L 3 edge. By comparing with the pure ladder and the pure chain systems, we can unambiguously distinguish between the hole distributions in the chain and the ladder. We have found that there are 5.3 holes in the edge-shared chain and 0.7 holes in the corner-shared ladder on average for Sr 14 Cu 24 O 41. Upon Ca substitution, the holes gradually transfer from the edge-shared chain to the corner-shared ladder, and there are up to 1.21 holes in the corner-shared ladder for Sr 1.8 Ca 12.2 Cu 24 O 41 .

Chinese Journal of Physics, 2021

Bulletin of the American Physical Society, 2018

The family of perovskite related Sr and Co containing compounds has received considerable attenti... more The family of perovskite related Sr and Co containing compounds has received considerable attention as oxygen-permeating membranes, sensors, or cathodes for Solid oxide fuel cell (SOFC) because of their high oxygen permeability values [1]. SrCoO3-δ phases with 3C-like crystal structures are presented as mixed conductors with very high oxygen permeability [2]. On the other hand, the SrCoO3-δ samples with 2H-like hexagonal structures at room temperature (RT) have shown to be almost non-oxygen permeable [3].

The synthesis of transition metal oxides is typically time- and energy-consuming. Recently, fast ... more The synthesis of transition metal oxides is typically time- and energy-consuming. Recently, fast sintering methods have demonstrated great potential to reduce ceramic sintering time and energy use, improving the commercial prospects of these materials. In this article, a quenched ultrafast high-temperature sintering (qUHS) technique is developed to sinter metastable brownmillerite SrCoO2.5 (SCO) in less than a minute. Surprisingly, SCO fabricated by qUHS shows higher activity for the oxygen evolution reaction (OER) compared to solid-state-reaction-synthesized SCO. Comparing samples produced by these two techniques, the increased OER performance of SCO qUHS is likely due to the synergistic combination of surface Co chemical state, higher mesoporosity and enhanced hydroxyl ion (OH-) adsorption. This work demonstrates the potential of qUHS for producing high-performance electrocatalysts and provides detailed insights into the impact of ultrafast sintering on the materials' physical...

Inorganic Chemistry, 2021

A 4H-type BaMnO3 single crystal was prepared by combining the floating zone method with high-pres... more A 4H-type BaMnO3 single crystal was prepared by combining the floating zone method with high-pressure treatment at 5 GPa and 1023 K. The crystal crystallizes to a hexagonal structure with space group P63/mmc and lattice parameters a = 5.63723(5) Å and c = 9.22355(8) Å. In this structure, face-sharing MnO6 octahedral dimers connect with each other by corner O atoms along the c-axis direction, forming an -A-B-A-C-type 4H arrangement. A long-range antiferromagnetic (AFM) phase transition is found to occur at TN ≈ 263 K. When the synthesis pressure increases to 20 GPa, a new polymorphic phase is obtained. This higher-pressure phase still possesses the hexagonal P63/mmc symmetry, but the lattice parameters change to be a = 5.61349(2) Å and c = 13.66690(9) Å with a unit cell volume reduction of 2.05%. In this new phase, the c-axis MnO6 dimers are separated by MnO6 octahedral layers in the ab plane, forming an -A-B-C-A-C-B-type 6H structure. The 6H phase exhibits two long-range AFM orderings at TN1 ≈ 220 K and TN2 ≈ 25 K, respectively. The different magnetic properties are discussed on the basis of the detailed structural constitutions of 4H- and 6H-BaMnO3.

Applied Materials Today, 2021

Abstract Materials with room-temperature two-dimensional itinerant ferromagnetism enable ultracom... more Abstract Materials with room-temperature two-dimensional itinerant ferromagnetism enable ultracompact device density and quantum computing in next-generation nano-spintronics. Among numerous candidates, perovskite-based heterostructures have offered an excellent platform to manipulate spin-orbital-charge coupling, unveiling a series of emergent spin-dependent phenomena. In this work, we have fulfilled a robust room-temperature soft ferromagnetism together with a metallic characteristic based on a high quality SrRuO3-monolayer-based superlattice. Further examination on this short-range ferromagnetism has been carried out by synchrotron-related spectroscopy, where charge-transfer-induced antiferromagnetic coupling between partial Ru and Ti ions have been found. Although such local antiferromagnetic coupling would break long range ferromagnetic order, it simultaneously stabilize the localized intralayer ferromagnetic order in the SrRuO3 monolayers, which is confirmed by theoretical calculations. Our study not only represents a methodological advance of achieving fantastic magnetic properties in functional oxide monolayer, but is promising to unlock new opportunities for oxide-based spintronic devices toward room-temperature applications.

Physical Review B, 2020

We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie tem... more We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie temperature T c = 69 and T c = 36 K, respectively. These materials are useful for magnetic refrigerator applications at low temperature as they exhibit a large magnetocaloric effect near T c. We have used hard x-ray photoemission spectroscopy (HAXPES) to investigate the core-level and valence-band electronic states of GdNi and HoNi. HAXPES measurements of the Gd and Ho 3d, 4d, 4p, 5p, and 4s core-level spectra have been compared with atomic multiplet calculations of Gd 3+ and Ho 3+ ionic configurations. The good match between the experimental and calculated spectra clarify the important role of spin-orbit coupling, as well as Coulomb and exchange interactions in the intermediate-coupling scheme. The core-level spectra also show plasmons in addition to the atomic multiplets. The Gd and Ho 4s spectra show clear evidence of exchange splitting. The Ni 2p and 3s spectra of GdNi and HoNi show a correlation satellite at a binding energy of 7 eV above their main peaks. The Ni 2p and Ni 3s spectra could be reproduced using charge transfer multiplet calculations. Valence-band HAXPES of GdNi and HoNi shows that the Gd 4 f and Ho 4 f features are also consistent with atomic multiplets and occur at high binding energies away from the Fermi level. The Ni 3d density of states are spread from the Fermi level to about 3 eV binding energy. The results indicate a partially filled Ni 3d band and show that the charge transfer model is not valid for describing the electronic structure of GdNi and HoNi.

Physical Review B, 2020

GdNi is a ferrimagnetic material with a Curie temperature TC = 69 K which exhibits a large magnet... more GdNi is a ferrimagnetic material with a Curie temperature TC = 69 K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at T = 25 K in the ferrimagnetic phase. We analyze the Gd M4,5-edge (3d-4f) and Ni L2,3-edge (2p-3d) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd M4,5-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized 4f states. On the other hand, a model cluster calculation for Ni L2,3-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest neighbour Gd states, resulting in a d-electron count of 8.57. The Gd M4,5-edge XMCD spectrum is consistent with a ground state configuration of S = 7/2 and L=0. The Ni L2,3-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite total magnetic moment (mtot ∼ 0.12 µB) with the ratio mo/ms ∼ 0.11. Valence band hard x-ray photoemission spectroscopy shows Ni 3d features at the Fermi level, confirming a partially filled 3d band, while the Gd 4f states are at high binding energies away from the Fermi level. The results indicate that the Ni 3d band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni 3d and Gd 4f states.

Scientific Reports, 2019

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and ext... more Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential {\bf{3}}{{\boldsymbol{d}}}_{{{\bf{x}}}^{{\bf{2}}}{\boldsymbol{-}}{{\bf{y}}}^{{\bf{2}}}}$$3dx2−y2 orbital at high temperature (300–150 K) to the {\bf{3}}{{\boldsymbol{d}}}_{{{\bf{3}}{\bf{z}}}^{{\bf{2}}}{\boldsymbol{-}}{...

Physical Review B, 2018

We study the electronic structure of the skutterudite Ce 3 Co 4 Sn 13 , which is known to exhibit... more We study the electronic structure of the skutterudite Ce 3 Co 4 Sn 13 , which is known to exhibit a charge density wave (CDW) transition, at temperature T CDW ∼ 160 K, coupled to a metal-to-metal transition. We use temperature dependent hard x-ray photoemission spectroscopy (HAXPES) and x-ray absorption spectroscopy (XAS) to investigate the occupied and unoccupied electronic states of Ce 3 Co 4 Sn 13. The Co 2p and Sn 3p core level spectra show small but finite shifts in binding energy positions across T CDW while Ce 3d core level spectra do not show any change across the transition. The Ce M 4,5-edge XAS spectrum compared with calculations indicate a typical trivalent ionic Ce 3+ spectral shape, ruling out Kondo screening in Ce 3 Co 4 Sn 13. In contrast, the Co L 2,3-edge XAS spectrum compared with a calculated spectrum shows evidence for hybridization with neighboring Sn atoms in a trigonal prismatic coordination. Temperature dependent XAS across the Co L 2,3-edge shows a small shift across T CDW , consistent with HAXPES results. Detailed XAS measurements as a function of temperature show that the spectral shifts occur with a hysteresis across T CDW , indicative of a first-order transition. Valence band spectra show a normal Fermi edge above and below T CDW. The Co 3d states are observed at a binding energy of ∼2 eV while the Ce 4f states occur as a weak feature within 0.5 eV of the Fermi level. The results suggest an unusual CDW transition coupled to a metal-to-metal transition in Ce 3 Co 4 Sn 13 .

Physical Review B, 2021

Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and s... more Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and spin states of the transition metal ions in stoichiometric YCu 3 Co 4 O 12 and CaCu 3 Co 4 O 12. The trivalent and low-spin nature of both the Cu and Co ions in YCu 3 Co 4 O 12 makes this correlated system to be effectively a nonmagnetic band semiconductor. The substitution of Y by Ca produces formally tetravalent Co ions but the doped holes are primarily on the oxygen ligands. Concerning the spin degrees of freedom, the trivalent Co ions in YCu 3 Co 4 O 12 remain low spin upon the Y-Ca substitution, very much unlike the La 1-x Sr x CoO 3 system. The tetravalent Co ions in CaCu 3 Co 4 O 12 are interestingly also in the low-spin state, which then explains the good electrical conductivity of CaCu 3 Co 4 O 12 since charge exchange between neighboring Co 3+ and Co 4+ ions will not be hampered by the spin-blockade mechanism that otherwise would be in effect if the Co 4+ and Co 3+ spin quantum numbers were to differ by more than one-half. We infer that the stability of the Co low-spin state is related to the very short CoO bond lengths.

ACS Applied Materials & Interfaces

physica status solidi (RRL) - Rapid Research Letters

Physical Review Applied

In conventional spintronic devices, ferromagnetic materials are used, which have a magnetization ... more In conventional spintronic devices, ferromagnetic materials are used, which have a magnetization dynamics timescale of around nanoseconds, setting a limit for the switching speed. Increasing the magnetization switching speed has been one of the major challenges for spintronic research. In this work we take advantage of the ultrafast magnetic dynamics in ferrimagnetic materials instead of ferromagnets, and we use femtosecond laser pulses and a plasmonic photoconductive switch to create THz electrical pulses for ferrimagnetic switching by spin-orbit torque. By anomalous Hall and magneto-optic Kerr effect (MOKE) measurement, we demonstrate the robust THz-electrical-pulse-driven magnetization switching of ferrimagnetic Gd-Fe-Co. The time-resolved MOKE shows more than 50-GHz magnetic resonance frequency of Gd-Fe-Co, indicating faster than 20-ps magnetic dynamics. X-ray magnetic circular dichroism demonstrates the antiferromagnetically coupled Fe and Gd sublattices. Our work provides a promising route to realize ultrafast operation speed for nonvolatile magnetic memory and logic applications.

Applied Physics Letters, 2021

An A-site ordered but B-site disordered quadruple perovskite oxide PbMn3Cr3MnO12 was synthesized ... more An A-site ordered but B-site disordered quadruple perovskite oxide PbMn3Cr3MnO12 was synthesized by high-pressure and high-temperature methods. The compound crystallizes in space group Im-3 with the charge distribution of Pb2+Mn3+3Cr3+3Mn4+O12. Three antiferromagnetic phase transitions are found to occur at TN1 ≈ 155, TN2 ≈ 81, and TN3 ≈ 74 K, respectively, due to the complex B-site and A′-B intersite spin interactions. Compared with the isostructural LaMn3Cr4O12 with negligible A′-B intersite spin coupling, the substitution of Mn4+, which has identical electronic configuration to that of Cr3+ (t2g3), into the B site can introduce A′-B intersite spin interactions. As a result, the A′-site spin-related ordering temperature increases significantly, while the B-site one remains little changed in the current PbMn3Cr3MnO12. This work opens up a way to enhance the A′-site spin ordering temperature in quadruple perovskite oxides.

Transition metal oxides are a fascinating class of materials showing a wealth of fascinating prop... more Transition metal oxides are a fascinating class of materials showing a wealth of fascinating properties. These include metal-insulator transition in the early transition metal oxides, the colossal magnetoresistance in the manganates, the occurrence of stripe phases in the nickelates, and the high-T c superconductivity in the cuprates. The cobaltates form a special subclass in the sense that this single subclass covers a very wide range of different properties. For example, one can find not only giant magnetoresistance in RENi 0.3 Co 0.7 O 3 , and superconductivity in Na x CoO 2 •yH 2 O, but also exceptionally high thermopower in NaCo 2 O 4 , and even unique phenomena like the spin-blockade behavior in HoBaCo 2 O 5.5 and La 1.5 Sr 0.5 CoO 4. Much of these spectacular phenomena have their origin in the interplay among the charge, orbital, and spin degrees of freedom of the constituent transition metal ions. Special to the cobaltates with respect to the other subclasses like the cuprates, nickelates and manganates, is that the Co ions possess not only the"spin-up" and "spin-down" degrees of freedom, but also the possibility for a spin-state change: Co 3+ and Co 4+ ions can be tuned to have a high spin (HS) (S=2; S=5/2), a low spin (LS) (S=0; S=1/2), and an intermediate spin (IS) (S=1; S=3/2) state. These different spin states have also quite different orbital occupation. This enormous flexibility of the Co ions to have quite different local electronic states forms a challenge for experimentalists and theorists to explain and predict the properties of cobaltates. In this thesis we investigate the electronic structure of selected cobaltates using X-ray absorption and photoemission spectroscopy combined with configuration-interaction (CI) cluster calculations to further understand their electronic and magnetic properties. This thesis starts with the study on the spin state of the Co 3+ ions in Sr 0.7 Y 0.3 CoO 2.63 , which is claimed to have orbital ordering from the octahedral IS Co 3+ ions. The Co-L 2,3 and O-K X-ray absorption spectroscopy (XAS) measurements reveal that Co 3+ ions at octahedral, pyramidal, and tetrahedral sites are all in the HS state. Then we investigate valence and spin states of Co in SrCo 0.5 Ru 0.5 O 3 to understand its magnetic behavior. Ru-L 2,3 XAS for SrCo 0.5 Ru 0.5 O 3 clearly reveals a Co 3+ /Ru 5+ state, and not a Co 4+ /Ru 4+ state. The octahedral Co 3+ ions are in the unusual HS state, which is consistent with the large CoO distance obtained from Co-K EXAFS data. Under high pressure the Co Kβ X-ray emission spectra show a collapse of spin moment, reflecting a HS to a LS transition for Co 3+ ions.

Physical Review B, 2019

We investigate and compare the electronic structure of a bulk single crystal of Y 3 Fe 5 O 12 gar... more We investigate and compare the electronic structure of a bulk single crystal of Y 3 Fe 5 O 12 garnet [YIG, a high-T C (= 560 K) ferrimagnet] with that of an epitaxial ultrathin (3.3 nm) film of YIG with a reduced ferrimagnetic temperature T C = 380 K, using bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES), x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The Fe 2p HAXPES spectrum of the bulk single crystal exhibits a purely trivalent Fe 3+ state for octahedral and tetrahedral sites. The Fe 3s spectrum shows a clear splitting which allows us to estimate the on-site Fe 3s-3d exchange interaction energy. The valence band HAXPES spectrum shows Fe 3d, O 2p, and Fe 4s derived features and a band gap of ∼2.3 eV in the occupied density of states, consistent with the known optical band gap of ∼2.7 eV. Fe Ledge XAS identifies the octahedral Fe 3+ and tetrahedral Fe 3+ site features. XMCD spectra at the Fe L 2,3 edges show that bulk single-crystal YIG exhibits antiferromagnetic coupling between the octahedral-and tetrahedral-site spins. The calculated Fe 2p HAXPES, Fe Ledge XAS, and XMCD spectra using full multiplet cluster calculations match well with the experimental results and confirm the full local spin moments. In contrast, HAXPES, XAS, and XMCD of the Pt/YIG (3.3 nm) ultrathin epitaxial film grown by a pulsed laser deposition method show a finite Fe 2+ contribution and a reduced Fe 3+ local spin moment. The Fe 2+ state is attributed to a combination of oxygen deficiency and charge transfer effects from the Pt capping layer to the ultrathin film. However, the conserved XMCD spectral shape for the ultrathin film indicates that the 3.3-nm epitaxial film is genuinely ferrimagnetic, in contrast to recent studies on films grown by radio-frequency magnetron sputtering which have shown a magnetic dead layer of ∼6 nm. The presence of Fe 2+ and the reduced local spin moment in the epitaxial ultrathin film lead to a reduced Curie temperature, quantitatively consistent with well-known mean-field theory. The results establish a coupling of the local Fe spin moments, valency, and long-range magnetic ordering temperature in bulk single crystal and epitaxial ultrathin-film YIG.

Applied Physics Letters, 2017

The unconventional doping in YBa2Cu3O7x/La0.7Ca0.3MnO3 heterostructures by termination control.

Physical Review B, 2013

Here we present linear-polarization-dependent soft x-ray absorption spectroscopy data at the Ce M... more Here we present linear-polarization-dependent soft x-ray absorption spectroscopy data at the Ce M 4,5 edges of CeOs 2 Al 10 and CeFe 2 Al 10. Despite the strong hybridization between 4f and conduction electrons and the existence of spin gaps as seen in inelastic neutron scattering, we were able to determine the crystal field ground state wave functions by combining our spectroscopy data with magnetization measurements. The results quantitatively explain the small ordered moment along c and the measured magnetic moment along the easy a axis in CeOs 2 Al 10 .

Physical Review B, 2013

The physical properties of Sr 14−x Ca x Cu 24 O 41 are determined by the hole distribution betwee... more The physical properties of Sr 14−x Ca x Cu 24 O 41 are determined by the hole distribution between the edge-shared CuO 2 chain and the corner-shared Cu 2 O 3 two-leg ladder, but inconsistent results on the hole distribution were obtained in various experimental works in recent decades. In this work we reinvestigate the hole distribution by soft x-ray absorption spectroscopy at the Cu L 3 edge. By comparing with the pure ladder and the pure chain systems, we can unambiguously distinguish between the hole distributions in the chain and the ladder. We have found that there are 5.3 holes in the edge-shared chain and 0.7 holes in the corner-shared ladder on average for Sr 14 Cu 24 O 41. Upon Ca substitution, the holes gradually transfer from the edge-shared chain to the corner-shared ladder, and there are up to 1.21 holes in the corner-shared ladder for Sr 1.8 Ca 12.2 Cu 24 O 41 .

Chinese Journal of Physics, 2021

Bulletin of the American Physical Society, 2018

The family of perovskite related Sr and Co containing compounds has received considerable attenti... more The family of perovskite related Sr and Co containing compounds has received considerable attention as oxygen-permeating membranes, sensors, or cathodes for Solid oxide fuel cell (SOFC) because of their high oxygen permeability values [1]. SrCoO3-δ phases with 3C-like crystal structures are presented as mixed conductors with very high oxygen permeability [2]. On the other hand, the SrCoO3-δ samples with 2H-like hexagonal structures at room temperature (RT) have shown to be almost non-oxygen permeable [3].

The synthesis of transition metal oxides is typically time- and energy-consuming. Recently, fast ... more The synthesis of transition metal oxides is typically time- and energy-consuming. Recently, fast sintering methods have demonstrated great potential to reduce ceramic sintering time and energy use, improving the commercial prospects of these materials. In this article, a quenched ultrafast high-temperature sintering (qUHS) technique is developed to sinter metastable brownmillerite SrCoO2.5 (SCO) in less than a minute. Surprisingly, SCO fabricated by qUHS shows higher activity for the oxygen evolution reaction (OER) compared to solid-state-reaction-synthesized SCO. Comparing samples produced by these two techniques, the increased OER performance of SCO qUHS is likely due to the synergistic combination of surface Co chemical state, higher mesoporosity and enhanced hydroxyl ion (OH-) adsorption. This work demonstrates the potential of qUHS for producing high-performance electrocatalysts and provides detailed insights into the impact of ultrafast sintering on the materials' physical...

Inorganic Chemistry, 2021

A 4H-type BaMnO3 single crystal was prepared by combining the floating zone method with high-pres... more A 4H-type BaMnO3 single crystal was prepared by combining the floating zone method with high-pressure treatment at 5 GPa and 1023 K. The crystal crystallizes to a hexagonal structure with space group P63/mmc and lattice parameters a = 5.63723(5) Å and c = 9.22355(8) Å. In this structure, face-sharing MnO6 octahedral dimers connect with each other by corner O atoms along the c-axis direction, forming an -A-B-A-C-type 4H arrangement. A long-range antiferromagnetic (AFM) phase transition is found to occur at TN ≈ 263 K. When the synthesis pressure increases to 20 GPa, a new polymorphic phase is obtained. This higher-pressure phase still possesses the hexagonal P63/mmc symmetry, but the lattice parameters change to be a = 5.61349(2) Å and c = 13.66690(9) Å with a unit cell volume reduction of 2.05%. In this new phase, the c-axis MnO6 dimers are separated by MnO6 octahedral layers in the ab plane, forming an -A-B-C-A-C-B-type 6H structure. The 6H phase exhibits two long-range AFM orderings at TN1 ≈ 220 K and TN2 ≈ 25 K, respectively. The different magnetic properties are discussed on the basis of the detailed structural constitutions of 4H- and 6H-BaMnO3.

Applied Materials Today, 2021

Abstract Materials with room-temperature two-dimensional itinerant ferromagnetism enable ultracom... more Abstract Materials with room-temperature two-dimensional itinerant ferromagnetism enable ultracompact device density and quantum computing in next-generation nano-spintronics. Among numerous candidates, perovskite-based heterostructures have offered an excellent platform to manipulate spin-orbital-charge coupling, unveiling a series of emergent spin-dependent phenomena. In this work, we have fulfilled a robust room-temperature soft ferromagnetism together with a metallic characteristic based on a high quality SrRuO3-monolayer-based superlattice. Further examination on this short-range ferromagnetism has been carried out by synchrotron-related spectroscopy, where charge-transfer-induced antiferromagnetic coupling between partial Ru and Ti ions have been found. Although such local antiferromagnetic coupling would break long range ferromagnetic order, it simultaneously stabilize the localized intralayer ferromagnetic order in the SrRuO3 monolayers, which is confirmed by theoretical calculations. Our study not only represents a methodological advance of achieving fantastic magnetic properties in functional oxide monolayer, but is promising to unlock new opportunities for oxide-based spintronic devices toward room-temperature applications.

Physical Review B, 2020

We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie tem... more We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie temperature T c = 69 and T c = 36 K, respectively. These materials are useful for magnetic refrigerator applications at low temperature as they exhibit a large magnetocaloric effect near T c. We have used hard x-ray photoemission spectroscopy (HAXPES) to investigate the core-level and valence-band electronic states of GdNi and HoNi. HAXPES measurements of the Gd and Ho 3d, 4d, 4p, 5p, and 4s core-level spectra have been compared with atomic multiplet calculations of Gd 3+ and Ho 3+ ionic configurations. The good match between the experimental and calculated spectra clarify the important role of spin-orbit coupling, as well as Coulomb and exchange interactions in the intermediate-coupling scheme. The core-level spectra also show plasmons in addition to the atomic multiplets. The Gd and Ho 4s spectra show clear evidence of exchange splitting. The Ni 2p and 3s spectra of GdNi and HoNi show a correlation satellite at a binding energy of 7 eV above their main peaks. The Ni 2p and Ni 3s spectra could be reproduced using charge transfer multiplet calculations. Valence-band HAXPES of GdNi and HoNi shows that the Gd 4 f and Ho 4 f features are also consistent with atomic multiplets and occur at high binding energies away from the Fermi level. The Ni 3d density of states are spread from the Fermi level to about 3 eV binding energy. The results indicate a partially filled Ni 3d band and show that the charge transfer model is not valid for describing the electronic structure of GdNi and HoNi.

Physical Review B, 2020

GdNi is a ferrimagnetic material with a Curie temperature TC = 69 K which exhibits a large magnet... more GdNi is a ferrimagnetic material with a Curie temperature TC = 69 K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at T = 25 K in the ferrimagnetic phase. We analyze the Gd M4,5-edge (3d-4f) and Ni L2,3-edge (2p-3d) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd M4,5-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized 4f states. On the other hand, a model cluster calculation for Ni L2,3-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest neighbour Gd states, resulting in a d-electron count of 8.57. The Gd M4,5-edge XMCD spectrum is consistent with a ground state configuration of S = 7/2 and L=0. The Ni L2,3-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite total magnetic moment (mtot ∼ 0.12 µB) with the ratio mo/ms ∼ 0.11. Valence band hard x-ray photoemission spectroscopy shows Ni 3d features at the Fermi level, confirming a partially filled 3d band, while the Gd 4f states are at high binding energies away from the Fermi level. The results indicate that the Ni 3d band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni 3d and Gd 4f states.

Scientific Reports, 2019

Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and ext... more Temperature-dependent X-ray absorption near-edge structures, X-ray linear dichroism (XLD) and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques were used to investigate the valence state, preferred orbital and local atomic structure that significantly affect the electrical and magnetic properties of a single crystal of YBaCuFeO5 (YBCFO). An onset of increase of resistivity at ~180 K, followed by a rapid increase at/below 125 K, is observed. An antiferromagnetic (AFM)-like transition is close to the temperature at which the resistivity starts to increase in the ab-plane and is also observed with strong anisotropy between the ab-plane and the c-axis. The XLD spectra at the Fe L3,2-edge revealed a change in Fe 3d eg holes from the preferential {\bf{3}}{{\boldsymbol{d}}}_{{{\bf{x}}}^{{\bf{2}}}{\boldsymbol{-}}{{\bf{y}}}^{{\bf{2}}}}$$3dx2−y2 orbital at high temperature (300–150 K) to the {\bf{3}}{{\boldsymbol{d}}}_{{{\bf{3}}{\bf{z}}}^{{\bf{2}}}{\boldsymbol{-}}{...

Physical Review B, 2018

We study the electronic structure of the skutterudite Ce 3 Co 4 Sn 13 , which is known to exhibit... more We study the electronic structure of the skutterudite Ce 3 Co 4 Sn 13 , which is known to exhibit a charge density wave (CDW) transition, at temperature T CDW ∼ 160 K, coupled to a metal-to-metal transition. We use temperature dependent hard x-ray photoemission spectroscopy (HAXPES) and x-ray absorption spectroscopy (XAS) to investigate the occupied and unoccupied electronic states of Ce 3 Co 4 Sn 13. The Co 2p and Sn 3p core level spectra show small but finite shifts in binding energy positions across T CDW while Ce 3d core level spectra do not show any change across the transition. The Ce M 4,5-edge XAS spectrum compared with calculations indicate a typical trivalent ionic Ce 3+ spectral shape, ruling out Kondo screening in Ce 3 Co 4 Sn 13. In contrast, the Co L 2,3-edge XAS spectrum compared with a calculated spectrum shows evidence for hybridization with neighboring Sn atoms in a trigonal prismatic coordination. Temperature dependent XAS across the Co L 2,3-edge shows a small shift across T CDW , consistent with HAXPES results. Detailed XAS measurements as a function of temperature show that the spectral shifts occur with a hysteresis across T CDW , indicative of a first-order transition. Valence band spectra show a normal Fermi edge above and below T CDW. The Co 3d states are observed at a binding energy of ∼2 eV while the Ce 4f states occur as a weak feature within 0.5 eV of the Fermi level. The results suggest an unusual CDW transition coupled to a metal-to-metal transition in Ce 3 Co 4 Sn 13 .

Physical Review B, 2021

Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and s... more Using soft x-ray absorption spectroscopy we were able to determine unambiguously the charge and spin states of the transition metal ions in stoichiometric YCu 3 Co 4 O 12 and CaCu 3 Co 4 O 12. The trivalent and low-spin nature of both the Cu and Co ions in YCu 3 Co 4 O 12 makes this correlated system to be effectively a nonmagnetic band semiconductor. The substitution of Y by Ca produces formally tetravalent Co ions but the doped holes are primarily on the oxygen ligands. Concerning the spin degrees of freedom, the trivalent Co ions in YCu 3 Co 4 O 12 remain low spin upon the Y-Ca substitution, very much unlike the La 1-x Sr x CoO 3 system. The tetravalent Co ions in CaCu 3 Co 4 O 12 are interestingly also in the low-spin state, which then explains the good electrical conductivity of CaCu 3 Co 4 O 12 since charge exchange between neighboring Co 3+ and Co 4+ ions will not be hampered by the spin-blockade mechanism that otherwise would be in effect if the Co 4+ and Co 3+ spin quantum numbers were to differ by more than one-half. We infer that the stability of the Co low-spin state is related to the very short CoO bond lengths.

ACS Applied Materials & Interfaces

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