Spin States Research Articles

Quadruple-Q Skyrmion Crystal in Centrosymmetric Body-Centered Tetragonal Magnets

We conduct a numerical investigation into the stability of a quadruple-Q skyrmion crystal, a structure generated by the superposition of four spin density waves traveling in distinct directions within three-dimensional space, hosted on a centrosymmetric body-centered tetragonal lattice. Using simulated annealing applied to an effective spin model that includes momentum-resolved bilinear and biquadratic interactions, we construct a magnetic phase diagram spanning a broad range of model parameters. Our study finds that a quadruple-Q skyrmion crystal does not emerge within the phase diagram when varying the biquadratic interaction and external magnetic field. Instead, three distinct quadruple-Q states with topologically trivial spin textures are stabilized. However, we demonstrate that the quadruple-Q skyrmion crystal can become the ground state when an additional high-harmonic wave–vector interaction is considered. Depending on the magnitude of this interaction, we obtain two types of quadruple-Q skyrmion crystals exhibiting the skyrmion numbers of one and two. These findings highlight the emergence of diverse three-dimensional multiple-Q spin states in centrosymmetric body-centered tetragonal magnets.

Stable Luminescent Diradicals: The Emergence and Potential Applications.

Stable luminescent diradicals, characterized by the presence of two unpaired electrons, exhibit unique photophysical properties that are sensitive to external stimuli such as temperature, magnetic fields, and microwaves. This sensitivity allows the manipulation of their spin states and luminescence, setting them apart from traditional closed-shell luminescent molecules and luminescent monoradicals. As a result, luminescent diradicals are emerging as promising candidates for a variety of applications. This minireview discusses recent advances in the design and synthesis of luminescent diradicals, explores their photophysical properties, and potential applications. It also examines the challenges and prospects in the development of these materials, shedding light on their potential to drive technological innovation.

Stable Luminescent Diradicals: The Emergence and Potential Applications

Stable luminescent diradicals, characterized by the presence of two unpaired electrons, exhibit unique photophysical properties that are sensitive to external stimuli such as temperature, magnetic fields, and microwaves. This sensitivity allows the manipulation of their spin states and luminescence, setting them apart from traditional closed‐shell luminescent molecules and luminescent monoradicals. As a result, luminescent diradicals are emerging as promising candidates for a variety of applications. This minireview discusses recent advances in the design and synthesis of luminescent diradicals, explores their photophysical properties, and potential applications. It also examines the challenges and prospects in the development of these materials, shedding light on their potential to drive technological innovation.

Tuning the Spin State of Co Accelerates Hydrogen Evolution Reaction of Pt Nanoparticles

ABSTRACTThe quest for dynamic and cost‐effective electrocatalysts to substitute carbon‐supported platinum (Pt) in alkaline hydrogen evolution reaction (HER) remains a pressing challenge. The incorporation of transition metal atoms through electron donation and spin regulation dominates the HER performance of Pt nanoparticles. Herein, we demonstrate that Co‐N coordination was utilized to regulate and stabilize the chemical microenvironment of Pt nanoparticles to fabricate hybrid electrocatalysts (Pt/CoNC). The resultant Pt/CoNC delivers ultralow overpotentials of 15.2 and 171.2 mV at current densities of 10 and 100 mA cm−2, surpassing commercial Pt/C. The poisoning tests, where η10 values of Pt/CoNC depict negative shifts of 161 and 13 mV by potassium thiocyanide (KSCN) and ethylenediaminetetraacetic acid disodium (EDTA), suggest the combined impact of Pt nanoparticles and Co‐N coordination on HER, with Pt nanoparticles playing a decisive role. The magnetic characterization and spin density diagrams reveal that Pt induces a higher spin state of Co2+, creating a wider spin‐related channel for electron donation to Pt. Moreover, Co‐N effectively modifies the electronic structure of Pt, thereby reducing the energy barriers for H2O dissociation (from 0.41 to −0.22 eV) and H2 generation (from −0.35 to 0.03 eV). This finding provides insights to fabricate advanced electrocatalysts through regulating spin state and modulating interfacial electron transfer.

Improved Ammonia Synthesis and Energy Output from Zinc-Nitrate Batteries by Spin-State Regulation in Perovskite Oxides.

Electrocatalytic nitrate reduction to ammonia (eNRA) is a promising route toward environmental sustainability and clean energy. However, its efficiency is often limited by the slow conversion of intermediates due to spin-forbidden processes. Here, we introduce a novel A-site high-entropy strategy to develop a new perovskite oxide (La0.2Pr0.2Nd0.2Ba0.2Sr0.2)CoO3-δ (LPNBSC) for eNRA. The LPNBSC possesses a higher concentration of high-spin (HS) cobalt-active centers, resulting from an increased concentration of [CoO5] structural motifs compared to conventional LaCoO3. Consequently, this material exhibits a significantly improved electrocatalytic performance toward ammonia (NH3) production, resulting in a 3-fold increase in yield rate (129 μmol h-1 mgcat.-1) and a 2-fold increase in Faradaic efficiency (FE, 76%) compared to LaCoO3 at the optimal potential. Furthermore, the LPNBSC-based Zn-nitrate battery reaches a maximum FE of 82% and an NH3 yield rate of 57 μmol h-1 cm-2. Density functional theory calculations reveal that A-site high-entropy management in perovskites facilitates nitrate activation and potentially optimizes the thermodynamic rate-determining step of the eNRA process, namely, *HNO3 + H+ + e- → *NO2 + H2O. This work presents an efficient concept for modulating the spin state of the B-site metal in perovskites and offers valuable insights for the design of high-performance eNRA catalysts.

Proton-Induced Switching of Paramagnetism: Reversible Conversion between a Low and High Spin CoIII Center within a Heterobimetallic Core.

The development of molecular species with switchable magnetic properties has been a long-standing challenge in chemistry. One approach involves binding an analyte, such as protons, to a compound to trigger a change in magnetism. Transition metal complexes have been targeted for this type of magnetic modulation because they can undergo changes in their spin states. However, heterobimetallic complexes have had limited utility because of a lack of ligands that create differentiated structures around each metal center that are often necessary to regulate the electronic and magnetic properties. To circumvent this problem, we have used a tripodal ligand with phosphinic amido groups to prepare a complex with a discrete [CoIII(μ-OH)FeIII] core and an overall system spin of ST = 5/2. Deprotonation readily produces a species with a unique [CoIII(μ-O)FeIII] core and an ST = 1/2 system spin. X-ray diffraction studies, electron paramagnetic resonance spectroscopy, and Mössbauer spectroscopy pinpoint the hexacoordinate CoIII center as the cause of this spin change: the typical SCo = 0 spin state of the CoIII center in the [CoIII(μ-OH)FeIII] complex switches to a rare SCo = 2 spin state in the [CoIII(μ-O)FeIII] analogue; this change turns on antiferromagnetic coupling between the two metal centers. Computational studies link an increase in π bonding within the Co-oxido unit to the change in the CoIII spin state. The conversion is reversible and provides a blueprint for using oxido/hydroxido ligands within a heterobimetallic core to regulate the spin state of a metal site and thus modulate the paramagnetism of a system.

Proximity-induced flipped spin state in synthetic ferrimagnetic Pt/Co/Gd heterolayers

To develop new devices based on synthetic ferrimagnetic heterostructures, understanding the material’s physical properties is pivotal. Here, the induced magnetic moment (IMM), magnetic exchange coupling, and spin textures are investigated in Pt(1 nm)/Co(1.5 nm)/Gd(1 nm) multilayers using a multiscale approach. The magnitude and direction of the IMM are interpreted in the framework of both X-ray magnetic circular dichroism and density functional theory. The IMM transferred by Co across the Gd paramagnetic thickness leads to a nontrivial flipped spin state (FSS) within the Gd layers, in which their magnetic moments couple antiparallel/parallel with the ferromagnetic Co near/far from the Co/Gd interface, respectively. The FSS depends on the magnetic field, which, on average, reduces the Gd magnetic moment as the field increases. For the Pt, in both Pt/Co and Gd/Pt interfaces, the IMM follows the same direction as the Co magnetic moment, with negligible IMM in the Gd/Pt interface. Additionally, zero-field spin spirals were imaged using scanning transmission X-ray microscopy, whereas micromagnetic simulations were employed to unfold the interactions, stabilizing the ferrimagnetic configurations, where the existence of a sizable Dzyaloshinskii-Moriya interaction is demonstrated to be crucial.

Direct Electrical Access to the Spin Manifolds of Individual Lanthanide Atoms.

Lanthanide atoms show long magnetic lifetimes because of their strongly localized 4f electrons, but electrical control of their spins has been difficult because of their closed valence shell configurations. We achieved electron spin resonance of individual lanthanide atoms using a scanning tunneling microscope to probe the atoms bound to a protective insulating film. The atoms on this surface formed a singly charged cation state having an unpaired 6s electron, enabling tunnel current to access their 4f electrons. Europium spectra display a rich array of transitions among the 54 combined electron and nuclear spin states. In contrast, samarium's ground state is a Kramers doublet with a very large g-factor of 5. These results demonstrate that all-electronic sensing and control of individual lanthanide spins is possible for quantum devices and spin-based electronics by using their rarely observed monovalent cation state.

On-Surface Synthesis and Characterization of Radical Spins in Kagome Graphene.

Flat bands in Kagome graphene might host strong electron correlations and frustrated magnetism upon electronic doping. However, the porous nature of Kagome graphene opens a semiconducting gap due to quantum confinement, preventing its fine-tuning by electrostatic gates. Here we induce zero-energy states into a semiconducting Kagome graphene by inserting π-radicals at selected locations. We utilize the on-surface reaction of tribromotrioxoazatriangulene molecules to synthesize carbonyl-functionalized Kagome graphene on Au(111), thereafter modified in situ by exposure to atomic hydrogen. Atomic force microscopy and tunneling spectroscopy unveil the stepwise chemical transformation of the carbonyl groups into radicals, which creates local magnetic defects of spin state S = 1/2 and zero-energy states as confirmed by density functional theory. The ability to imprint local magnetic moments opens up prospects to study the interplay between topology, magnetism, and electron correlation in Kagome graphene.

Enhancing Stability and Activity of Fe-Based Catalysts for Propane Dehydrogenation via Anchoring Isolated Fe-Cl Sites.

The eco-friendly features and desirable catalytic activities of Fe-based catalysts make them highly promising for propane dehydrogenation (PDH). However, simultaneously improving their stability and activity remains a challenge. Here, we present a strategy to address these issues synergistically by anchoring single-atom Fe-Cl sites in Al3+ vacancies of Al2O3. The as-synthesized Fe-Cl/Al2O3 catalyst exhibited greater charge transfer between Cl and Fe than that between O and Fe in conventionally impregnated single-atom Fe/Al2O3 catalysts, resulting in higher effective magnetic moments for Fe-Cl/Al2O3 compared to Fe/Al2O3. When tested in PDH, the durability of Fe-Cl/Al2O3 exceptionally lasted for 250 h under continuous regeneration conditions comprising 60 % C3H8 (40 % N2), followed by pure C3H8 at 600 °C while maintaining a high propylene space-time yield of 1.2 molC3H6 gFe -1 h-1, surpassing the performance of previously developed Fe-based PDH catalysts. We demonstrate that anchoring Fe-Cl into Al3+ vacancies simultaneously enhances stability and suppresses coke formation, owing to unique atomically dispersed Fe-Cl active structures. Compared with Fe/Al2O3 catalysts, charge transfer between Cl and Fe active centers reduces the activation energy barrier for C-H activation during C3H8 dehydrogenation, thereby improving catalytic activity; this may be related to their spin state as observed in in-situ X-ray emission spectroscopy studies during PDH.

The Pauli-Poisson equation and its semiclassical limit

. The Pauli-Poisson equation is a semi-relativistic model for charged spin-1∕2-par-ticles in a strong external magnetic field and a self-consistent electric potential computed from the Poisson equation in three space dimensions. It is a system of two magnetic Schrödinger equations for the two components of the Pauli 2-spinor, representing the two spin states of a fermion, coupled by the additional Stern-Gerlach term representing the interaction of magnetic field and spin. We study the global well-posedness in the energy space and the semiclassical limit of the Pauli-Poisson to the magnetic Vlasov-Poisson equation with Lorentz force and the semiclassical limit of the linear Pauli equation to the magnetic Vlasov equation with Lorentz force. We use Wigner transforms and a density matrix formulation for mixed states, extending the work of P. L. Lions & T. Paul as well as P. Markowich & N.J. Mauser on the semiclassical limit of the non-relativistic Schrödinger-Poisson equation.

Spin effects in regulating the adsorption characteristics of metal ions.

Understanding the adsorption behavior of intermediates at interfaces is crucial for various heterogeneous systems, but less attention has been paid to metal species. This study investigates the manipulation of Co3+ spin states in ZnCo2O4 spinel oxides and establishes their impact on metal ion adsorption. Using electrochemical sensing as a metric, we reveal a quasi-linear relationship between the adsorption affinity of metal ions and the high-spin state fraction of Co3+ sites. Increasing the high-spin state of Co3+ shifts its d-band center downward relative to the Fermi level, thereby weakening metal ion adsorption and enhancing sensing performance. These findings demonstrate a spin-state-dependent mechanism for optimizing interactions with various metal species, including Cu2+, Cd2+, and Pb2+. This work provides new insights into the physicochemical determinants of metal ion adsorption, paving the way for advanced sensing technologies and beyond.

A mononuclear iron(II) complex constructed using a complementary ligand pair exhibits intrinsic luminescence-spin-crossover coupling.

Molecular materials that exhibit synergistic coupling between luminescence and spin-crossover (SCO) behaviors hold significant promise for applications in molecular sensors and memory devices. However, the rational design and underlying coupling mechanisms remain substantial challenges in this field. In this study, we utilized a luminescent complementary ligand pair as an intramolecular luminophore to construct a new Fe-based SCO complex, namely [FeL1L2](BF4)2·H2O (1-Fe, L1 is a 2,2':6',2''-terpyridine (TPY) derivative ligand and L2 is 2,6-di-1H-pyrazol-1-yl-4-pyridinecarboxylic acid), and two isomorphic analogs (2-Co, [CoL1L2](BF4)2·H2O and 3-Zn, [ZnL1L2](BF4)2·H2O). Magnetic studies reveal that 1-Fe exhibits thermally induced SCO within the temperature range of 150-350 K. Variable-temperature fluorescence emission spectral analysis of the three complexes confirmed the occurrence of SCO-luminescence coupling in 1-Fe. Furthermore, variable-temperature UV-vis absorption spectra and time-dependent density functional theory (TD-DFT) calculations elucidate the intramolecular luminescence emission behavior, highlighting the critical role of charge transfer processes between the L1 ligand and FeII ions with different spin states. Our research presents a novel construction strategy for synthesizing synergistic SCO-luminescent materials and contributes to the understanding of the mechanisms underlying SCO-luminescence coupling.

Modulating the electron spin states of Na2FePO4F cathode via high entropy strategy for enhanced sodium storage and ultra-high cycling stability

Modulating the electron spin states of Na2FePO4F cathode via high entropy strategy for enhanced sodium storage and ultra-high cycling stability

Helical magnetism in poly(aniline-co-ferrocene): structure and magnetism.

This study reports the synthesis and characterisation of a ferrocene-based conjugated polymer and a chiral composite. The precursor copolymer was synthesised from 1,3-phenylenediamine and 1,1'-dibromoferrocene via Buchwald-Hartwig polycondensation. This polymerisation process increased the effective conjugation length and led to magnetic spin interactions along the main chain, resulting in a ground triplet spin state at 25 °C. Furthermore, the polymer was oxidised with m-chloroperoxybenzoic acid and doped with sulfuric acid. A chiral polymer blend was also prepared by combining the copolymer with hydroxypropyl cellulose to form helical polymer liquid crystals. The physical properties of these polymers were analysed using superconducting quantum interference devices, optical spectroscopy, electron spin resonance, and X-ray fluorescence spectroscopy. The resulting polymer blend exhibits helical magnetism. This approach to fabricating a chiral magnetic material from an achiral polymer via supramolecular chirality introduces a new method for preparing chiral magnets.

Spin state modulation and kinetic control of thermal contraction in a [Fe2Co2] discrete Prussian blue analogue.

Stimuli-responsive switchable molecules represent an important category of magnetic materials with significant potential for functional devices. However, engineering complexes with controlled switchability remains challenging due to their sensitivity to lattice interactions. Herein, we report a [Fe2Co2] square complex [FeTp(CN)3]2[Co{(F5-Ph)Py}2]2·2ClO4·4CH3OH·2H2O {1·4CH3OH·2H2O; (F5-Ph)Py = (Z)-N'-(perfluorophenyl) picolinimidamide and Tp = hydrotris(1-pyrazolyl)borate}, tailored with hydrogen bonding (HB) donor and acceptor moieties for effective lattice interactions. The alteration in HB interactions in crystal phases obtained via single-crystal-to-single-crystal (SC-SC) transformation (i.e., 1·4CH3OH·2H2O ↔ 1·2H2O ↔ 1) led to a remarkable change in magnetic properties. Complexes 1·4CH3OH·2H2O and 1 possess [FeII LS(μ-CN)CoIII LS] and [FeIII LS(μ-CN)CoII HS] configurations, respectively. Meanwhile, 1·2H2O demonstrates multi-responsive (thermo-, photo- and pressure) reversible two-step electron transfer coupled spin transition (ETCST) accompanied by thermal contraction and expansion. The complete diamagnetic [FeII LS(μ-CN)CoIII LS] configuration in 1·2H2O was obtained at an intermediate temperature accompanied by thermal contraction, an unusual behaviour observed for the first time in cyanide-bridged systems. Additionally, 1·2H2O displayed temperature-induced excited spin state trapping (TIESST) of ∼70% of the paramagnetic [FeIII LS(μ-CN)CoII HS] configuration at low temperatures. The isothermal relaxation of the thermally trapped paramagnetic state shows a much faster and complete conversion to a diamagnetic state at ∼140 K, compared to the relaxation observed at other temperatures (100-190 K), corroborating the observed unique magnetic behaviour. Hence, this result provides valuable insight into the strategic design of complexes with enhanced and controlled switchability for potential applications such as actuators and sensors.

Magnetoresistance effect of pyridine-capped s-indacene-based conjugated radicals.

Owing to their unique and tunable optoelectronic and magnetic properties, organic conjugated radicals have great potential in information storage and communication through modulating the molecular spin states. However, few electronic/spintronic devices based on these materials have been reported to date due to various intrinsic constraints such as poor material stability and processability. In this work, we have synthesized a stable singlet ground state organic conjugated diradical 5,7-dimesityl-s-indaceno[1,2-b:7,6-b']dipyridine (mNIF) with narrow band gap (1.16 eV) and small singlet-triplet energy gap (ΔES-T = -1.05 kcal mol-1). mNIF showed good ambient stability and processability, and we have successfully fabricated a single ferromagnetic electrode device based on it with the structure of Ti/Au/mNIF/Co/Au. Distinct interface magnetoresistance effects were observed when the device was tested at different temperatures, which were attributed to the temperature anisotropy of the interface magnetic layer due to the small ΔES-T. Nevertheless, no interface magnetoresistance effect was observed in the device based on its syn analogous closed-shell molecule. Our work demonstrates the potential application of organic conjugated radicals in quantum memory.

Exploring acidity-dependent PCET pathways in imino-bipyridyl cobalt complexes.

The electrochemical proton reactivity of transition metal complexes has received intensive attention in catalyst research. The proton-coupled electron transfer (PCET) process, influenced by the coordination geometry, determines the catalytic reaction mechanisms. Additionally, the pKa value of a proton source, as an external factor, plays a crucial role in regulating the proton transfer step. Understanding the effects of variations in the pKa values of Brønsted acids on the PCET process is therefore essential. This study compares the PCET pathways of two high-spin cobalt (Co) complexes with contrasting exchange coupling interactions under acidic conditions with high and low pKa values. These findings reveal how proton reduction reactions in high-spin Co complexes are affected by the internal factor of the spin state, as well as an external factor related to the proton source. The corresponding reaction mechanisms are also proposed based on these observations.

Revisiting the Oxygen Reduction Reaction Activity of Two-Dimensional TM-C2N Electrocatalysts via Constant-Potential Density Functional Theory: Crucial Impact of Spin State and Coordination

Single atom catalysts (SACs) have shown great potentiality in catalyzing the oxygen reduction reaction (ORR) in fuel cell batteries. In the carbon-based SACs, besides the most representative TM-N4-C, other 2D...

Direct Observation of Fully Spin-Polarized Tunnel Current Between Quantum Spins Using a Single Molecule Sensor.

Controlling spin-polarized currents at the nanoscale is of immense importance for high-density magnetic data storage and spin-based logic devices. As electronic devices are miniaturized to the ultimate limit of individual atoms and molecules, electronic transport is strongly influenced by the properties of the individual spin centers and their magnetic interactions. In this work, we demonstrate the precise control and detection of spin-polarized currents through two coupled spin centers at a tunnel junction by controlling their spin-spin interactions. We attach a nickelocene (Nc) molecule to a scanning probe tip and place it over a spin center (either an Fe atom or another Nc molecule) located on a surface. By changing the adsorption orientation of Nc at the tip apex and adjusting the tip-sample distances, we control the wave function overlap between two spin systems, resulting in strong changes in their magnetic exchange coupling, quantum spin states, and spin excitation energies. Coupling the Nc molecule to the surface spin induces exchange-split spin states, enabling the quantitative determination of the spin polarization of tunnel currents. Strongly asymmetric tunneling spectra reveal almost 100% spin-polarized currents through the coupled Nc-Fe spin system. Our findings highlight the potential of these spin systems at the tunnel junction for high-performance spin-based devices engineered at the atomic scale.