Spin Dynamics Research Articles

Spin dynamics investigation and structural analysis of ambient scalable Sr2+ doped zirconium ferrite nanoparticles synthesized by low temperature auto combustion route

Microwave spin resonance behaviour of Sr2+ doped Zirconium ferrite Zr1-xSrxFe2O4; (0 ≤ x ≥ 0.20) nanoparticles synthesized by auto combustion route has been investigated by electron paramagnetic resonance (EPR) which show a broad ferromagnetic resonance signal with a superimposed hump signal which is attributed to ferrite Fe3+ ions and oxygen vacancies which is in agreement with XRD and XPS. EPR shows low spin concentration and smaller relaxation time for Zr0.80Sr0.20Fe2O4 which can be used for hyperthermia and for use as a MRI contrast agent. EDS and XPS confirms the presence of Zr, Sr, Fe, O, and C and HRTEM images confirm growth along (220) and (311) with an interatomic distance of 0.3327 nm and 0.2622 nm respectively, thus, authenticating the formation of spinel structure in agreement with FTIR studies. The magnetic behaviour (M − H loop) shows high saturation magnetization and higher value of effective magnetocrystalline anisotropy (Keff) for Zr0.80Sr0.20Fe2O4.

Direct Observation and Analysis of Low-Energy Magnons with Raman Spectroscopy in Atomically Thin NiPS3.

van der Waals (vdW) magnets have rapidly emerged as a fertile playground for fundamental physics and exciting applications. Despite the impressive developments over the past few years, technical limitations pose a severe challenge to many other potential breakthroughs. High on the list is the lack of suitable experimental tools for studying spin dynamics on atomically thin samples. Here, Raman scattering techniques are employed to directly observe the low-lying magnon (∼1 meV) even in bilayer NiPS3. The advantage is that it offers excellent energy resolutions far better on low-energy sides than most inelastic neutron spectrometers can offer. More importantly, with appropriate theoretical analysis, the polarization dependence of the Raman scattering by those low-lying magnons also provides otherwise hidden information on the dominant spin-exchange scattering paths for different magnons. By comparing with high-resolution inelastic neutron scattering data, these low-energy Raman modes are confirmed to be indeed of magnon origin. Because of the different scattering mechanisms involved in inelastic neutron and Raman scattering, this information is fundamental in pinning down the final spin Hamiltonian. This work demonstrates the capability of Raman spectroscopy to probe the genuine two-dimensional spin dynamics in atomically thin vdW magnets, which can provide insights that are obscured in bulk spin dynamics.

Fabricating Large-Area Thin Films of 2D Conductive Metal-Organic Frameworks.

ConspectusRecent years have witnessed significant interest in two-dimensional metal-organic frameworks (MOFs) due to their unique properties and promising applications across various fields. These materials offer distinct advantages, including high porosity and excellent charge transport properties. Their tunability allows precise control over various factors, including the electronic structure adjustments and local reactivity modulation, facilitating a wide range of properties and applications, such as material sensing and spin dynamics control. Moreover, the precise crystal structure of 2D MOFs supports rational design and mechanism studies, providing insights into their potential applications and enhancing their utility in various scientific and technological endeavors.To fully unveil the latent capabilities of 2D MOFs and advance their applications across diverse fields, thin film synthesis is crucial. Thin films provide a platform for investigating the intrinsic electrical properties of 2D MOFs with anisotropic structures, enabling the exploration of their unique characteristics comprehensively. Additionally, thin films offer the advantage of minimizing interference at contacts and junctions, thereby enhancing the performance of 2D MOFs for various applications. Furthermore, the properties of thin films can vary with thickness, presenting an opportunity to tailor their characteristics based on specific requirements.In this Account, we present an overview of our research focusing on the synthesis of 2D conductive MOF thin films encompassing two primary methods: chemical vapor deposition and solution processing. The chemical vapor deposition method allows for one-step, all-vapor-phase processes resulting in MOFs with edge-on orientation, controllable film thicknesses ranging from 55 to 662.7 nm, and smooth, homogeneous surfaces. On the other hand, solution-processing introduces a novel MOF, Cu3(HHTATP)2, by reducing interlayer interactions through the addition of pendent Brønsted bases on a ligand, enabling spin coating for thin film synthesis. This method yields a concentrated 2D MOF solution, enabling spin coating for thin film synthesis, where reversible electrical conductivity changes occur through doping with an acid and dedoping with a base. Additionally, we discuss various other synthesis methods, such as interfacial synthesis, layer-by-layer assembly, and microfluidic-assisted synthesis, offering versatile approaches for fabricating large-area thin films with tailored properties. Finally, we address ongoing challenges and potential strategies for further advancement in 2D conductive MOF thin film synthesis. We hope that this Account provides insights for selecting synthesis methods tailored to specific purposes, contributes to the development of varied synthesis techniques, and facilitates the exploration of diverse applications, unlocking the full potential of 2D conductive MOFs for next-generation technologies.

Near-infrared luminescent open-shell π-conjugated systems with a bright lowest-energy zwitterionic singlet excited state.

Open-shell systems with extensive π-conjugation have fascinating properties due to their narrow bandgaps and spin interactions. In this work, we report neutral open-shell di- and polyradical conjugated materials exhibiting intriguing optical and magnetic properties. Our key design advance is the planarized geometry allowing for greater interaction between adjacent spins. This results in absorption and emission in the near infrared at 803 and 1050 nanometers, respectively, and we demonstrate a unique electronic structure where a bright zwitterionic excited state is the lowest-accessible electronic transition. Electron paramagnetic resonance spectroscopy and superconducting quantum interference device measurements reveal that our materials are open-shell singlets with different degrees of spin interactions, dynamics, and antiferromagnetic properties, which likely contributed to the formation of their emissive zwitterionic singlet excited state and near-infrared emission. In addition, our materials show reversible and stable electrochromic switching with more than 500 cycles, indicating their potential for optoelectronic and electrochemical energy storage applications.

Microscopic Study on Superexchange Dynamics of Composite Spin-1 Bosons.

We report on an experimental simulation of the spin-1 Heisenberg model with composite bosons in a one-dimensional chain based on the two-component Bose-Hubbard model. Exploiting our site- and spin-resolved quantum gas microscope, we observed faster superexchange dynamics of the spin-1 system compared to its spin-1/2 counterpart, which is attributed to the enhancement effect of multi-bosons. We further probed the nonequilibrium spin dynamics driven by the superexchange and single-ion anisotropy terms, unveiling the linear expansion of the spin-spin correlations, which is limited by the Lieb-Robinson bound. Based on the superexchange process, we prepared and verified the entangled qutrits pairs with these composite spin-1 bosons, potentially being applied in qutrit-based quantum information processing.

Single-Shot MRI in parahydrogen hyperpolarized samples

The site-specific signal enhancement provided by parahydrogen induced polarization (PHIP) may be combined with magnetic resonance imaging (MRI) to study chemical and biomolecular processes. However, imaging of hydrogen nuclei (1H) is hampered by background signals arising from the presence of thermally polarized nuclei. Additionally, fast imaging sequences are commonly based on multiple radio-frequency pulses, where the signals resulting from PHIP oscillate due to the evolution with a J-coupling Hamiltonian. In this article, an innovative imaging scheme for single-scan MRI is presented that effectively detects hyperpolarized components while simultaneously canceling out thermal contributions. This method is based on the quenching of inherent oscillations of PHIP-originated signals due to J-couplings during the multipulse sequence and the suppression of thermal signals by spin dynamics and a tailored restructuring of the k-space. A series of numerical simulations on specific two- and three-spin systems serve to support the feasibility of the approach. Furthermore, this theoretical study demonstrates the potential of combining hyperpolarization and long-lived states (PHIP and LLS) in the selected molecules, which could be seen as a preliminary step towards the development of fast imaging techniques, for example in the field of biomolecular research.

The silicon vacancy centers in SiC: determination of intrinsic spin dynamics for integrated quantum photonics

The negatively charged silicon vacancy center (VSi−\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\rm{V}}}_{{\\rm{Si}}}^{-}$$\\end{document}) in silicon carbide (SiC) is an emerging color center for quantum technology covering quantum sensing, communication, and computing. Yet, limited information currently available on the internal spin-optical dynamics of these color centers prevents us from achieving the optimal operation conditions and reaching the maximum performance especially when integrated within quantum photonics. Here, we establish all the relevant intrinsic spin dynamics of the VSi−\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\rm{V}}}_{{\\rm{Si}}}^{-}$$\\end{document} center at cubic lattice site (V2) in 4H-SiC by an in-depth electronic fine structure modeling including the intersystem-crossing and deshelving mechanisms. With carefully designed spin-dependent measurements, we obtain all the previously unknown spin-selective radiative and non-radiative decay rates. To showcase the relevance of our work for integrated quantum photonics, we use the obtained rates to propose a realistic implementation of time-bin entangled multi-photon GHZ and cluster state generation. We find that up to three-photon GHZ or cluster states are readily within reach using the existing nanophotonic cavity technology.

Theoretical study of electronic structures, magnetic properties, and ultrafast spin manipulation in transition metal adsorbed polycyclic-aromatic-hydrocarbon molecules.

We present a first-principles study of the structural, electronic, and magnetic properties of TM(PAH)0/+ (TM = Fe, Co, Ni; PAH = C10H8, C16H10, C24H12, C32H14) complexes and explore the laser-induced spin dynamics as well as their stability with respect to various laser parameters. For each complex, the most stable configuration shows that the TM atom prefers to adsorb at the hollow site of the carbon ring with a slight deviation from the center. The electronic structure and spin localization of the complexes are found to be largely affected by the TM type. Driven by various laser pulses, spin-crossover scenarios are achieved in all structures, while spin-transfer between TM and PAH is achieved in Ni(C10H8), Ni(C16H10), and Ni(C24H12). The influence of the laser energy and chirp on the dynamics is also investigated, providing important information regarding the stability and sensitivity of the dynamical process. All results are believed to reveal the physics nature of the TM-PAH systems, to guide the experimental realization of their ultrafast spin dynamics and thus to promote their applications in future spintronics.

Colossal Zero-Field-Cooled Exchange Bias via Tuning Compensated Ferrimagnetic in Kagome Metals.

Exchange bias (EB) is a crucial property with widespread applications but particularly occurs by complex interfacial magnetic interactions after field cooling. To date, intrinsic zero-field-cooled EB (ZEB) has only emerged in a few bulk frustrated systems and their magnitudes remain small yet. Here, enabled by high temperature synthesis, we uncover a colossal ZEB field of 4.95 kOe via tuning compensated ferrimagnetism in a family of kagome metals, which is almost twice the magnitude of known materials. Atomic-scale structure, spin dynamics, and magnetic theory revealed that these compensated ferrimagnets originate from significant antiferromagnetic exchange interactions embedded in the holmium-iron ferrimagnetic matrix due to supersaturated preferential manganese doping. A random antiferromagnetic order of manganese sublattice sandwiched between ferromagnetic iron kagome bilayers accounts for such unconventional pinning. The outcome of the present study outlines disorder-induced giant bulk ZEB and coercivity in layered frustrated systems.

Phenolic syringyl end groups in 13C-enriched hardwoods detected and quantified by solid-state NMR

While syringyl units are the most abundant monolignols in hardwood lignin, their phenolic (i.e. hydroxyl) end group concentration has not been measured. In two uniformly 13C-enriched young hardwoods, from beech and oak, the syringyl units were quantitatively investigated by advanced solid-state 13C NMR. Small signals of OH-terminated syringyl units were resolved in spectrally edited two-dimensional 13C–13C NMR spectra of the two hardwoods. Their distinct peak positions predicted based on literature data were validated via the abundant OH-terminated syringyl units in hydrolyzed 13C-beechwood. In a two-dimensional 13C–13C exchange spectrum with diagonal-ridge suppression, a well-resolved peak of phenolic syringyl units was observed at the characteristic C–H peak position of syringyl rings, without significant overlap from guaiacyl peaks. Accurate 13C chemical shifts of regular and end-group syringyl units were obtained. Through spectrally edited 2D NMR after 1H inversion recovery, phenols of condensed tannin complexed with arginine were carefully analyzed and shown to overlap minimally with signals from phenolic syringyl units. The local structure and resulting spin dynamics of ether (chain) and hydroxyl (end-group) syringyl units are nearly the same, enabling quantification by peak integration or deconvolution, which shows that phenolic syringyl end groups account for 2 ± 1 % of syringyl units in beechwood and 5 ± 2 % in oakwood. The observed low end-group concentration needs to be taken into account in realistic molecular models of hardwood lignin structure.

Medium dynamic field range linear bipolar spin valve sensor through soft pinning the sensing layer

Magnetic sensor with spin valve-GMR technology with medium dynamic range is designed for a diversity of applications, including linear and rotary position measurements, proximity switches, and current sensors. For this, the sensing layer (SL) of the spin valve stack was modified by a soft pinning layer (SPL) through an exchange bias field created by an antiferromagnetic layer which has a lower blocking temperature than the one that is kept adjacent to the pinned layer. Numerical simulation was carried out to control the bias field by keeping a non-magnetic Ru spacer layer between the SPL and SL layers and the results were experimentally verified. The magnetic sensor was fabricated with linear operating field range of the order ±100 Oe having a sensitivity of the order of 0.1 m V V−1 Oe−1 near zero field. The thermal performance confirms that the device can be operated in the temperature range of −40 ∘C to 125 ∘C and it has a thermal coefficient of voltage around 15 µV V−1∘C−1.

Effect of monolayer ratio on single-shot all-optical switching in Gd/Fe multilayers

Ultrafast thermally induced magnetization switching (TIMS) with femtosecond lasers has attracted much attention due to its ability to trigger a single switching on the picosecond time scale. Currently, most of the studies on TIMS have focused on various ferrimagnetic alloys. In this paper, TIMS of Gd/Fe multilayers in different monolayer ratios is investigated by atomic spin dynamics. The results show that an increase in the monolayer Gd ratio narrows the energy density window of the switching. Further studies found that a lower damping ratio decreases the laser energy density threshold for magnetization reversal. Moreover, reducing the ratio of Gd in the monolayer at the appropriate energy density can shorten the duration of the transient ferromagnetic-like state, which can lead to faster realization of TIMS. Our simulation results provide new insights to explore the physical mechanism of TIMS in Gd/Fe multilayers.

The importance of long range Fe–Fe interactions in magnetically ordered 2D Fe0.2Ni0.8/Ni(001) and Fe0.3Ni0.7/Ni(001) alloy systems

We develop a theoretical approach to study the magnonic properties of the 2D ordered surface Fe–Ni alloys with 20% and 30% Fe on a face-centered cubic Ni(001) surface substrate. The surface alloy nanostructures in stable equilibrium are determined by computing the lowest energy configurations of constitutive supercells of the ordered alloys. The magnetic exchange constants of the ground state of the stable systems are calculated to define their Heisenberg Hamiltonians using the DFT Spin-Polarized Relativistic Korringa–Kohn–Rostoker (SPRKKR) method and considering Fe–Fe interactions up to third nearest neighbors. To determine the propagating exchange spin-wave modes, evanescent modes, and the consequent magnonic properties, the spin dynamics of the magnetic surface alloy nanostructures are solved using the phase field matching theory (PFMT). Our results indicate that the Fe–Fe exchange interactions up to third nearest neighbors are vital for accurately describing their magnonics. In particular, the exchange interactions influence the form of dispersion branches for the high-energy spin-wave modes of the Fe–Ni surface alloys considered. Appropriate Green’s functions are also derived from the PFMT method to compute the local densities of states (LDOS) for the atomic sites at the surface boundary. For the case of 20% Fe surface alloy, the Fe–Fe exchange interactions shift the Fe LDOS peaks to lower frequency intervals. We also distinguish the emergence of remarkable LDOS peaks for the he case of the 30% Fe surface alloy at specific Ni sites. These peaks change positions under second and third nearest neighbor Fe–Fe interactions, heralding new accessible spin-wave channels. Our results contribute to establishing a systematic understanding of the magnonics of the magnetic surface alloys of transition metal (TM) materials. This new knowledge will enable the study of the quantum transport of spin waves at interfacial alloy nanojunctions composed of such materials. The present DFT-PFMT theoretical approach is general and can be applied to other surface TM alloy magnetic nanostructures.

Free electrons spin-dependent Kapitza–Dirac effect in two-dimensional triangular optical lattice

Abstract The free electron spin dynamics in Kapitza–Dirac (KD) effect had been studied theoretically in one-dimensional standing wave of EUV to X-ray laser with extremely high intensity, which is far beyond experimental realization. Here, we propose to achieve the free electron spin-dependent KD effect in two-dimensional triangular optical lattice with spatial inversion symmetry breaking, and the theoretical results reveal that laser with wavelength in visible or near-IR and five orders of magnitude decreased intensity could lead to obvious spin-dependent KD effect. This work provides the way to realize the free electron spin-dependent KD effect experimentally.

Influence of local defects in the generation of memory effect in Dy2Ti2O7 compound: Fe-doped study

The study of defect-induced effects in functional materials is high-priority research for the miniaturization of smart devices. To elucidate the influence of local defects stemming from intrinsic or extrinsic factors on the enigmatic behavior of cooperative spin dynamics, we investigated the Dy2-xFexTi2O7 compound within a narrow Fe substitution range (x = 0–0.15). Our experimental findings unequivocally demonstrate that the dynamic response of cooperative spins and the emergence of anomalous memory effect are intricately linked to local defects. Depending on the nature and dynamics of these defects, the dynamic response of cooperative spins undergoes alteration. Notably, observed anomalous effect becomes more pronounced in scenarios characterized by slower defect dynamics and when cooperative spins are further from equilibrium. The emergence and sensitivity of thermomagnetic hysteresis (memory effect) to cooperative spin dynamics and external stimuli underscore the richness of Dy2Ti2O7 as a material for captivating physics investigations.

Laser induced ultrafast Gd 4f spin dynamics at the surface of amorphous CoxGd100-x ferrimagnetic alloys

We have investigated the laser induced ultrafast dynamics of Gd 4f spins at the surface of CoxGd100-x alloys by means of surface-sensitive and time-resolved dichroic resonant Auger spectroscopy. We have observed that the laser induced quenching of Gd 4f magnetic order at the surface of the CoxGd100-x alloys occur on a much longer time scale than that previously reported in “bulk sensitive” time-resolved experiments. In parallel, we have characterized the static structural and magnetic properties at the surface and in the bulk of these alloys by combining Physical Property Measurement System (PPMS) magnetometry with X-ray Magnetic Circular Dichroism in absorption spectroscopy (XMCD) and X-Ray Photoelectron spectroscopy (XPS). The PPMS and XMCD measurements give information regarding the composition in the bulk of the alloys. The XPS measurements show non-homogeneous composition at the surface of the alloys with a strongly increased Gd content within the first layers compared to the nominal bulk values. Such larger Gd concentration results in a reduced indirect Gd 4f spin-lattice coupling. It explains the “slower” Gd 4f demagnetization we have observed in our surface-sensitive and time-resolved measurements compared to that previously reported by “bulk-sensitive” measurements.

Magnetic Structure-Dependent Ultrafast Spin Relaxation in Magnet CrI3: A Time-Domain ab Initio Study.

Two-dimensional magnet CrI3 is a promising candidate for spintronic devices. Using nonadiabatic molecular dynamics and noncollinear spin time-dependent density functional theory, we investigated hole spin relaxation in two-dimensional CrI3 and its dependence on magnetic configurations, impacted by spin-orbit and electron-phonon interactions. Driven by in-plane and out-of-plane iodine motions, the relaxation rates vary, extending from over half a picosecond in ferromagnetic systems to tens of femtoseconds in certain antiferromagnetic states due to significant spin fluctuations, associated with the nonadiabatic spin-flip in tuning to the adiabatic flip. Antiferromagnetic CrI3 with staggered layer magnetic order notably accelerates adiabatic spin-flip due to enhanced state degeneracy and additional phonon modes. Ferrimagnetic CrI3 shows a transitional behavior between ferromagnetic and antiferromagnetic types as the magnetic moment changes. These insights into the spin dynamics of CrI3 underscore its potential for rapid-response spintronic applications and advance our understanding of two-dimensional materials for spintronics.

Impact of dimensionality on the magnetocaloric effect in two-dimensional magnets

Magnetocaloric materials, which exploit reversible temperature changes induced by magnetic field variations, are promising for advancing energy-efficient cooling technologies. The potential integration of two-dimensional materials into magnetocaloric systems represents an emerging opportunity to enhance the magnetocaloric cooling efficiency. In this study, we use atomistic spin dynamics simulations based on first-principles parameters to systematically evaluate how magnetocaloric properties transition from three-dimensional (3D) to two-dimensional (2D) ferromagnetic materials. We find that 2D features such as reduced Curie temperature, sharper magnetic transition, and higher magnetic susceptibility are beneficial for magnetocaloric applications, while the relatively higher lattice heat capacity in 2D can compromise achievable adiabatic temperature changes. We further propose GdSi2 as a promising 2D magnetocaloric material. Our calculation predicts that GdSi2 exhibits an isothermal entropy change ΔSM of 22.5 J kg−1 K−1 and an adiabatic temperature change ΔTad of 6.2 K, near the hydrogen liquefaction temperature (TC≈25 K). Our analysis offers valuable theoretical insights into the magnetocaloric effect in 2D ferromagnets and demonstrates that 2D ferromagnets hold promise for cooling and thermal management applications in compact and miniaturized nanodevices.

Observation of stacking engineered magnetic phase transitions within moiré supercells of twisted van der Waals magnets

Recent demonstrations of moiré magnetism, featuring exotic phases with noncollinear spin order in the twisted van der Waals (vdW) magnet chromium triiodide CrI3, have highlighted the potential of twist engineering of magnetic (vdW) materials. However, the local magnetic interactions, spin dynamics, and magnetic phase transitions within and across individual moiré supercells remain elusive. Taking advantage of a scanning single-spin magnetometry platform, here we report observation of two distinct magnetic phase transitions with separate critical temperatures within a moiré supercell of small-angle twisted double trilayer CrI3. By measuring temperature-dependent spin fluctuations at the coexisting ferromagnetic and antiferromagnetic regions in twisted CrI3, we explicitly show that the Curie temperature of the ferromagnetic state is higher than the Néel temperature of the antiferromagnetic one by ~10 K. Our mean-field calculations attribute such a spatial and thermodynamic phase separation to the stacking order modulated interlayer exchange coupling at the twisted interface of moiré superlattices.

Microscopic evaluation of spin and orbital moment in ferromagnetic resonance

Time-resolved X-ray magnetic circular dichroism under the effects of ferromagnetic resonance (FMR), known as X-ray ferromagnetic resonance (XFMR) measurements, enables direct detection of precession dynamics of magnetic moment. Here we demonstrated XFMR measurements and Bayesian analyses as a quantitative probe for the precession of spin and orbital magnetic moments under the FMR effect. Magnetization precessions in two different Pt/Ni-Fe thin film samples were directly detected. Furthermore, the ratio of dynamical spin and orbital magnetic moments was evaluated quantitatively by Bayesian analyses for XFMR energy spectra around the Ni L2,3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$L_{2,3}$$\\end{document} absorption edges. Our study paves the way for a microscopic investigation of the contribution of the orbital magnetic moment to magnetization dynamics.