Spin Order Research Articles

Homogeneous Catalysts for Hydrogenative PHIP Used in Biomedical Applications

AbstractAt present, two competing hyperpolarization (HP) techniques, dissolution dynamic nuclear polarization (DNP) and parahydrogen (para‐H2) induced polarization (PHIP), can generate sufficiently high liquid state 13C signal enhancement for in vivo studies. PHIP utilizes the singlet spin state of para‐H2 to create non‐equilibrium spin populations. In hydrogenative PHIP, para‐H2 is irreversibly added to unsaturated precursors, typically in the presence of a homogeneous catalyst. The hydrogenation catalyst plays a crucial role in converting the singlet spin order of para‐H2 into detectable nuclear polarization. Currently, rhodium(I) bisphosphine complexes are the most widely employed catalysts for PHIP, capable of catalyzing the addition of para‐H2 to unsaturated precursors in organic solvents or aqueous media, depending on the ligand. Chiral catalysts enable the stereoselective production of hyperpolarized substrates. Ruthenium(II) piano stool complexes are capable of trans addition and are used to generate hyperpolarized fumarate. However, these catalysts systems are not optimal, and the greatest source of nuclear spin polarization loss is attributed to the mixing of singlet and triplet states of the protons derived from the para‐H2 during the hydrogenation process. Hence, future efforts should focus on enhancing the efficiency and kinetics of these catalysts.

Quasinormal modes of slowly-spinning horizonless compact objects

One of the main predictions of general relativity is the existence of black holes featuring a horizon beyond which nothing can escape. Gravitational waves from the remnants of compact binary coalescences have the potential to probe new physics close to the black hole horizons. This prospect is of particular interest given several quantum-gravity models that predict the presence of horizonless and singularity-free compact objects. The membrane paradigm is a generic framework that allows one to parametrize the interior of compact objects in terms of the properties of a fictitious fluid located at the object’s radius. It has been used to derive the quasinormal mode spectrum of static horizonless compact objects. Extending the membrane paradigm to rotating objects is crucial to constrain the properties of the spinning merger remnants. In this work, we extend the membrane paradigm to linear order in spin and use it to analyze the relationships between the quasinormal modes, the object’s reflectivity, and the membrane parameters. We find a breaking of isospectrality between axial and polar modes when the object is partially reflecting or the compactness differs from the black hole case. We also find that in reflective ultracompact objects some of the modes tend toward instability as the spin increases. Finally, we show that the spin enhances the deviations from the black-hole quasinormal mode spectrum as the compactness decreases. This implies that spinning horizonless compact objects may be more easily differentiated than nonspinning ones in the prompt ringdown. Published by the American Physical Society 2024

Spin dynamics and Phonons of the Chromites CoCr2O4 and MnCr2O4

Abstract Due to the geometric magnetic frustration inherent to their lattice and the resulting complex magnetic states, the spinel compounds are of great interest in both fundamental and application-oriented perspectives. Here, we applied x-ray diffraction, magnetization, heat capacity and powder inelastic neutron scattering measurements, along with theoretical calculations, to study the exotic properties of chromite-spinel oxides, CoCr2O4 and MnCr2O4. The temperature dependence of the phonon spectra provides an insight into the correlation between oxygen motion and the magnetic order, as well as the magnetoelectric effect in the ground state of MnCr2O4. Moreover, spinwave excitations in CoCr2O4 and MnCr2O4 are compared with Heisenberg model calculations. This approach enables the precise determination of exchange energies and offers a comprehensive understanding of the spin dynamics and relevant exchange interactions in complicated spiral spin ordering.

Suppressing energy migration via antiparallel spin alignment in one‐dimensional Mn2+ halide magnets with high luminescence efficiency

Photoexcited energy migration is prone to causing luminescence quenching in Mn2+ luminescent materials, presenting a formidable challenge for optoelectronic applications. Although various strategies and mechanisms have been proposed to mitigate this issue, the role of spin alignment between adjacent Mn2+ ions has remained largely unexamined. In this study, we have elucidated the influence of spin alignment on energy migration within the one‐dimensional Mn2+‐metal halide compound (CH3)4NMnCl3 (TMMC) through variable‐temperature photoluminescence (PL) and magnetic‐optical spectroscopy. This investigation was conducted with reference to (CH6N3)2MnCl4 (GUA) with isolated [Mn3Cl12]6‐ trimers and Cd2+‐doped TMMC. The spin order in TMMC below approximately 55 K is demonstrated by the disorder‐order transition observed in the temperature‐dependent magnetic susceptibility. This finding is further corroborated by the negligible shift in the temperature‐ and field‐dependent emission peaks, a consequence of magnetic saturation. Our results indicate that the antiparallel spin alignment along the Mn2+ chain in TMMC effectively suppresses energy migration and multiphonon relaxation, thereby reducing nonradiative transitions and enhancing the photoluminescence quantum yield (PLQY).This research casts new light on the potential for developing high‐performance Mn2+‐doped phosphors for optoelectronic and spin‐photonic applications, offering insights into the manipulation of spin and energy dynamics in these materials.

Suppressing energy migration via antiparallel spin alignment in one-dimensional Mn2+ halide magnets with high luminescence efficiency.

Photoexcited energy migration is prone to causing luminescence quenching in Mn2+ luminescent materials, presenting a formidable challenge for optoelectronic applications. Although various strategies and mechanisms have been proposed to mitigate this issue, the role of spin alignment between adjacent Mn2+ ions has remained largely unexamined. In this study, we have elucidated the influence of spin alignment on energy migration within the one-dimensional Mn2+-metal halide compound (CH3)4NMnCl3 (TMMC) through variable-temperature photoluminescence (PL) and magnetic-optical spectroscopy. This investigation was conducted with reference to (CH6N3)2MnCl4 (GUA) with isolated [Mn3Cl12]6- trimers and Cd2+-doped TMMC. The spin order in TMMC below approximately 55 K is demonstrated by the disorder-order transition observed in the temperature-dependent magnetic susceptibility. This finding is further corroborated by the negligible shift in the temperature- and field-dependent emission peaks, a consequence of magnetic saturation. Our results indicate that the antiparallel spin alignment along the Mn2+ chain in TMMC effectively suppresses energy migration and multiphonon relaxation, thereby reducing nonradiative transitions and enhancing the photoluminescence quantum yield (PLQY).This research casts new light on the potential for developing high-performance Mn2+-doped phosphors for optoelectronic and spin-photonic applications, offering insights into the manipulation of spin and energy dynamics in these materials.

Spin effect in the ferromagnetic organic photovoltaics cells

In organic photovoltaic devices, the separation and transport of photogenerated charges play crucial roles for power conversion efficiency. Magnetic doping in organic solar cells can effectively enhance the power conversion efficiency by introducing a static magnetic field. In this study, we observed that in pure organic magnetic solar cells, the spin-polarization-induced spin scattering effect can also efficiently modulate the photocurrent in solar cells. Compared to the demagnetized state, the short-circuit current of PTB7:nw-P3HT:PCBM solar cells increased by approximately 0.3% after magnetization. The dielectric constant only increased by about 0.05%. However, above the Curie temperature 310 K, the long-range spin order in PTB7:nw-P3HT:PCBM solar cells disappears, resulting in consistent circuit currents before and after magnetization. Therefore, magnetic doping can enhance the short-circuit current in organic solar cells by weakening the spin scattering effect and enhancing the charge carrier mobility.

A Spin‐Polarized Electron‐Induced MXene‐KBi0.9Co0.1Fe2O5 Photocatalyst for Enhanced Dye Degradation

In photocatalysts aimed to degrade environmental pollutants, photons are generally used as primary stimuli to generate electron–hole pairs to target the chemical bonds to disintegrate. Recently, electron spin has been reported as an essential degree of freedom to improve the performance of photocatalysts. In this work, spin‐polarized electrons and holes are introduced into a brownmillerite KBi0.9Co0.1Fe2O5 semiconductor and a two‐dimensional (2D) MXene composite system by application of an external magnetic field. The spin polarization properties are monitored by measuring the magnetoresistance effect of the MXene‐KBCFO device, which is related to the carrier transfer efficiency. Remarkably, the photocatalytic performance of MXene‐KBCFO is significantly enhanced by the simultaneous application of an external magnetic field and illumination due to the increased number of spin‐polarized photoexcited carriers caused by the synergy effect of the 2D MXene heterostructure together with the ferromagnetic spin ordering. This results in increased reaction hotspots, effective built‐in electric field, extended carrier lifetime, and reduced charge recombination owing to parallel alignment of electron spin orientation. The results show that the efficiency and stability of photocatalytic dye degradation can be effectively enhanced by manipulating the spin‐polarized electrons in the MXene‐based oxide‐perovskite heterostructure photocatalyst.

Stability of spin-spiral magnetic structures in Mn2PtSn

The stability of a long-periodic homogeneous spin-spiral configuration in an inverse tetragonal Heusler compound, Mn2PtSn, is studied with the help of density functional theory calculations. The energetically most stable collinear magnetic state in this system is the ferrimagnetic one. However, the existence of negative phonon frequency makes this configuration dynamically unstable. The energy dispersion plots reveal that an energy minimum exists at q=0.1 along [100] and [110] propagating directions, which correspond to a stable non-collinear configuration compared to the collinear spin states. The inclusion of spin–orbit coupling further reduces the ground-state energy without changing the q-vector of the energy minima. The cycloidal spiral configuration, where the spins rotate at an angle of 36° along the propagating direction, is found to be more stable than the screw spiral configuration. The calculated density of state plots further supports the stability of the non-collinear cycloidal spin order. This stable, non-collinear spin-spiral configuration of Mn2PtSn makes this compound a prospective material for spintronics device applications.

Spontaneous reversal of spin chirality and competing phases in the topological magnet EuAl4

AbstractMaterials exhibiting a spontaneous reversal of spin chirality have the potential to drive the widespread adoption of chiral magnets in spintronic devices. Unlike the majority of chiral magnets that require the application of an external field to reverse the spin chirality, we observe the spin chirality to spontaneously reverse in the topological magnet EuAl4. Using resonant elastic x-ray scattering we demonstrate that all four magnetic phases in EuAl4 are single-k, where the first two magnetic phases are characterized by spin density wave order and the last two by helical spin order. A single spin chirality was stabilised across the 1mm2 sample, and the reversal of spin chirality occurred whilst maintaining a helical magnetic structure. At the onset of the helical magnetism, the crystal symmetry lowers to a chiral monoclinic space group, explaining the asymmetry in the chiral spin order, and establishing a mechanism by which the spin chirality could reverse via magnetostructural coupling.

From higher-spin gauge interactions to Compton amplitudes for root-Kerr

We develop massive higher-spin theory as a framework for describing dynamics of rotating compact objects, such as Kerr black holes. In this paper, we explore gauge interactions up to quartic order and corresponding Compton amplitudes of higher-spin massive objects coupled to electromagnetism and Yang-Mills theory. Their classical counterparts are known as root-Kerr gauge-theory solutions, whose amplitudes are closely related to those of Kerr black holes. We use three distinct approaches: (i) massive higher-spin gauge symmetry to introduce cubic interactions for all spins and the quartic interactions up to spin 3, which is implemented both off shell and via Ward identities; (ii) a chiral higher-spin approach to construct quartic Lagrangians with correct degrees of freedom to all spins; (iii) on-shell functional patterns before and after taking the classical limit to constrain the Compton amplitudes. As final results, we arrive at simple local formulae for the candidate root-Kerr Compton amplitudes both in the quantum regime and classical limit, to all orders in spin. This is a precursor to the gravitational Kerr case, which is presented in a follow-up paper.

Avoided metallicity in a hole-doped Mott insulator on a triangular lattice

Doping of a Mott insulator gives rise to a wide variety of exotic emergent states, from high-temperature superconductivity to charge, spin, and orbital orders. The physics underpinning their evolution is, however, poorly understood. A major challenge is the chemical complexity associated with traditional routes to doping. Here, we study the Mott insulating CrO2 layer of the delafossite PdCrO2, where an intrinsic polar catastrophe provides a clean route to doping of the surface. From scanning tunnelling microscopy and angle-resolved photoemission, we find that the surface stays insulating accompanied by a short-range ordered state. From density functional theory, we demonstrate how the formation of charge disproportionation results in an insulating ground state of the surface that is disparate from the hidden Mott insulator in the bulk. We demonstrate that voltage pulses induce local modifications to this state which relax over tens of minutes, pointing to a glassy nature of the charge order.

Dielectric, magnetic, and magnetodielectric change mechanisms in Y-type BaSrZn2Fe12O22 ceramics regulated by doping Al3+ and Ga3+ ions

We systematically investigated the dielectric, magnetic, and magnetodielectric (MD) properties of Y-type hexaferrite doped with Al3+ and Ga3+ ions. Al3+ ions preferentially enter the spin-up octahedral lattice sites while Ga3+ ions occupy the spin-down octahedral lattice sites first and then, enter the spin- octahedral sites with the increase in doping amount. The initial magnetization reaches saturation from outside of the hysteresis loops, exhibiting a non-collinear spin order. The step-like increase in magnetization with magnetic field indicates the ferroelectric changes in longitudinal conical spin order. The positive MD effect at low temperatures and the negative MD effect after warming for the doped samples are controlled by non-collinear spin ordering and electron hopping, respectively. The Al-doped and Ga-doped samples exhibit different ferroelectric behaviors driven by magnetic fields. Their ferroelectric phase diagrams were proposed on the basis of magnetic and MD properties. The results associate the physical properties with the doped ions in Y-type hexaferrite.

Multiple magnetic orders discovered in the superconducting state of EuFe2(As1−xPx)2

Abstract The interplay between superconductivity and magnetism is an important subject in condensed matter physics. EuFe2As2-based iron pnictides could offer an interesting plateau to study their relationship that has attracted considerable attention. So far, two magnetic phase transitions were observed in EuFe2As2-based crystal, which were deemed to originate from the itinerant Fe moments ( 190 K) and the localized Eu2+ moments ( 19 K), respectively. Here, we systematically studied the heat capacity for the EuFe2(As1−xPx)2 crystals with x = 0.21 (optimally doped) and x = 0.29 (overdoped). We have found two new magnetic orders in the superconducting state (ranging from 0.4 to 1.2 K) in the optimally doped crystal. As more P was introduced into the As site, one of the magnetic orders becomes absent in the overdoped crystal. Additionally, we observed strong field and orientation dependence in heat capacity. The present findings in EuFe2(As1−xPx)2 have detected the new low-temperature magnetic orders, which may originate from the localized Eu2+ spins order or the spin reorientation.

Spin order and dynamics in the topological rare-earth germanide semimetals

The REAl(Si,Ge) (RE = rare earth) family, known to break both the inversion- and time-reversal symmetries, represents one of the most suitable platforms for investigating the interplay between correlated-electron phenomena and topologically nontrivial bands. Here, we report on systematic magnetic, transport, and muon-spin rotation and relaxation (uSR) measurements on (Nd,Sm)AlGe single crystals, which exhibit antiferromagnetic (AFM) transitions at TN = 6.1 and 5.9 K, respectively. In addition, NdAlGe undergoes also an incommensurate-to-commensurate ferrimagnetic transition at 4.5 K. Weak transverse-field µSR measurements confirm the AFM transitions, featuring a ∼90% magnetic volume fraction. Zero-field (ZF) µSR measurements reveal a more disordered internal field distribution in NdAlGe than in SmAlGe, reflected in a larger transverse muon-spin relaxation rate λT at T ≪ TN. This may be due to the complex magnetic structure of NdAlGe, which undergoes a series of metamagnetic transitions in an external magnetic field, while SmAlGe shows only a robust AFM order. In NdAlGe, the topological Hall effect (THE) appears between the first and the second metamagnetic transitions for H ∥ c, while it is absent in SmAlGe. Such THE in NdAlGe is most likely attributed to the field-induced topological spin textures. The longitudinal muon-spin relaxation rate λL, diverges near the AFM order, followed by a clear drop at T < TN. In the magnetically ordered state, spin fluctuations are significantly stronger in NdAlGe than in SmAlGe. In general, our longitudinal-field μSR data indicate vigorous spin fluctuations in NdAlGe, thus providing valuable insights into the origin of THE and of the possible topological spin textures in REAl(Si,Ge) Weyl semimetals.

Spin-Mechanical Coupling in 2D Antiferromagnet CrSBr.

Spin-mechanical coupling is vital in diverse fields including spintronics, sensing, and quantum transduction. Two-dimensional (2D) magnetic materials provide a unique platform for investigating spin-mechanical coupling, attributed to their mechanical flexibility and novel spin orderings. However, studying their spin-mechanical coupling presents challenges in probing mechanical deformation and thermodynamic property changes at the nanoscale. Here we use nano-optoelectromechanical interferometry to mechanically detect the phase transition and magnetostriction effect in multilayer CrSBr, an air-stable antiferromagnet with large magnon-exciton coupling. The transitions among antiferromagnetism, spin-canted ferromagnetism, and paramagnetism are visualized. Nontrivial magnetostriction coefficient 2.3 × 10-5 and magnetoelastic coupling strength on the order of 106 J/m3 have been found. Moreover, we demonstrate the substantial tunability of the magnetoelastic constant by nearly 50% via gate-induced strain. Our findings demonstrate the strong spin-mechanical coupling in CrSBr and pave the way for developing sensitive magnetic sensing and efficient quantum transduction at the atomically thin limit.

Signatures of polarized chiral spin disproportionation in rare earth nickelates

In rare earth nickelates (RENiO3), electron-lattice coupling drives a concurrent metal-to-insulator and bond disproportionation phase transition whose microscopic origin has long been the subject of active debate. Of several proposed mechanisms, here we test the hypothesis that pairs of self-doped ligand holes spatially condense to provide local spin moments that are antiferromagnetically coupled to Ni spins. These singlet-like states provide a basis for long-range bond and spiral spin order. Using magnetic resonant X-ray scattering on NdNiO3 thin films, we observe the chiral nature of the spin-disproportionated state, with spin spirals propagating along the crystallographic (101)ortho direction. These spin spirals are found to preferentially couple to X-ray helicity, establishing the presence of a hitherto-unobserved macroscopic chirality. The presence of this chiral magnetic configuration suggests a potential multiferroic coupling between the noncollinear magnetic arrangement and improper ferroelectric behavior as observed in prior studies on NdNiO3 (101)ortho films and RENiO3 single crystals. Experimentally-constrained theoretical double-cluster calculations confirm the presence of an energetically stable spin-disproportionated state with Zhang-Rice singlet-like combinations of Ni and ligand moments.

Investigation of thermodynamical stability and generation of large half-metallic gap along with optical properties in N-doped YHfO3 (YHO) (Y = Ca, Sr, and Ba) perovskite materials by first principle calculations

Considerable energy gap (Eg) in insulating channel, high spin-filtering, significant mean free path, and high magnetic transition temperature (TC) escalate half-metallic (HM) ferromagnetic (FM) materials very enticing for spintronic applications. Herein, ab-initio calculations have been executed comprehensively to inquest the structural, thermodynamical, electronic, magnetic, and optical properties of pure as well as nitrogen (N)-doped YHfO3 (YHO) (Y = Ca/Sr/Ba) materials with one N atom added at one of the oxygen (O) site. First, the thermodynamical stability of both bulk and doped YHO systems has been examined by estimating the formation enthalpies (ΔHf) by Convex Hull analysis, which confirms the practical realization of these materials on experimental grounds. Moreover, mechanical stability of these systems is affirmed by estimating the necessary elastic constants and respective parameters. Strikingly, all doped systems possess complete (100%) splitting in spin↑ and spin↓ channels at Fermi level and depict HM FM character with significant Eg of 3.31, 3.49, and 3.24 eV in spin↓ channels for CaHO (CHO), SrHO (SHO), and BaHO (BHO) systems, respectively. It is also found that N 2p orbitals are totally responsible for the emergence of metatallicity and magnetic character in all these doped structures. Moreover, the ground state long range FM stability between two added N atoms in doped systems is affirmed by computing the difference of total energies of FM and anti-ferromagnetic (AFM) spin ordering (ΔE) along with estimated TC at different distances. Lastly, it is found that N-doped systems have a lower optical energy loss than pristine YHO systems in the incident photon energy range of 0-50 eV. Hence, the present study proposes that an unconventional doping approach with intrinsically non-magnetic element in a variety of YHO perovskite systems could be a useful method to alter their various physical properties and forecast their potential applications in the development of novel spintronic and optoelectronic devices.

Real-Space Imaging of Intrinsic Symmetry-Breaking Spin Textures in a Kagome Lattice.

The electronic orders in kagome materials have emerged as a fertile platform for studying exotic quantum states, and their intertwining with the unique kagome lattice geometry remains elusive. While various unconventional charge orders with broken symmetry is observed, the influence of kagome symmetry on magnetic order has so far not been directly observed. Here, using a high-resolution magnetic force microscopy, it is, for the first time, observed a new lattice form of noncollinear spin textures in the kagome ferromagnet in zero magnetic field. Under the influence of the sixfold rotational symmetry of the kagome lattice, the spin textures are hexagonal in shape and can further form a honeycomb lattice structure. Subsequent thermal cycling measurements reveal that these spin textures transform into a non-uniform in-plane ferromagnetic ground state at low temperatures and can fully rebuild at elevated temperatures, showing a strong second-order phase transition feature. Moreover, some out-of-plane magnetic moments persist at low temperatures, supporting the Kane-Mele scenario in explaining the emergence of the Dirac gap. The observations establish that the electronic properties, including both charge and spin orders, are strongly coupled with the kagome lattices.

Compton Amplitude for Rotating Black Hole from QFT.

We construct a candidate tree-level gravitational Compton amplitude for a rotating Kerr black hole, for any quantum spin s=0,1/2,1,…,∞, from which we extract the corresponding classical amplitude to all orders in the spin vector S^{μ}. We use multiple insights from massive higher-spin quantum field theory, such as massive gauge invariance and improved behavior in the massless limit. A chiral-field approach is particularly helpful in ensuring correct degrees of freedom, and for writing down compact off-shell interactions for general spin. The simplicity of the interactions is echoed in the structure of the spin-s Compton amplitude, for which we use homogeneous symmetric polynomials of the spin variables. Where possible, we compare to the general-relativity results in the literature, available up to eighth order in spin.

Magnetic splitting induced ferromagnetism in chromium-doped HfSe2

Two-dimensional (2D) magnetic layered materials have gained significant interest in spintronic for memory storage devices. Herein, the electronic and magnetic properties of high-quality single crystals of chromium (Cr)-doped hafnium diselenide were studied. The electronic study indicated that the Cr-doped HfSe2 reveals the semiconducting nature with a proper bandgap as identified by angle-resolved photoemission spectroscopy and first-principle density functional theory (DFT) calculations. The magnetic results indicate that the HfSe₂ exhibited ferromagnetic characteristics with the substitution of Cr atoms in a perpendicular direction. The Cr-doped HfSe2 sample shows a ferromagnetic phase below ∼58 K, indicating the critical temperature (Tc) of magnetic order, while above this temperature, it shows a paramagnetic phase that can follow the Curie-Weiss Law. It can be seen that the magnetic moment is approximately 0.16 emu/g at 5 K, which further decreases with increasing temperature until 60 K and then transforms to a paramagnetic phase. The sample can clearly show a weak out-of-plane magnetic anisotropy with a coercivity of around 50.56 Oe and a saturation magnetization value of 0.029 emu/g. Convincingly, it was observed from the ARPES results that the valence band is split due to the spin-orbit interaction of Cr atoms, which can induce ferromagnetic order. At the same time, the temperature-dependent ARPES data shows that the splitting is higher below the Tc, while it is weaker above the Tc. The DFT results revealed that near the Fermi level, the spin-up and spin-down are spin-polarized with asymmetric trends. It can be observed that substituting Cr on the octahedral sites of Hf atoms influences the electron spin order created by Hf vacancies (as validated by the magnetic coupling measured by electron paramagnetic resonance spectroscopy experiments), which can change the magnetic alignment to induce the magnetism in HfSe2. This study highlights that doping transition metal in HfSe2 has promising potential for future applications, especially in memory devices and spintronic technologies.