Spin Relaxation Mechanism Research Articles

Magnetic Properties, Spin-Vibration Couplings, and Temperature Effects in Phenalenyl and Triangulene.

We introduce a computational methodology for evaluating and analyzing spin-vibration couplings in molecular systems, enabling insights into the interplay between electronic spins and molecular vibrations. By mapping ab initio electronic structure calculations onto molecular spin Hamiltonians, our approach captures those vibrational interactions potentially driving spin relaxation process. Spin-vibration couplings, derived from Holstein and Peierls terms, highlight the pivotal role of vibrational mode symmetry in spin decoherence and efficient energy dissipation. Additionally, second-order couplings provide a framework to explore the temperature dependence of spin properties via the thermal population of higher vibrational levels. Applied to phenalenyl and [3]triangulene, the results indicate that direct spin transitions dominate over Orbach relaxation in both systems. Hyperfine interactions primarily dictate spin-vibration couplings and thermal effects in phenalenyl, whereas zero-field splitting contributions are dominant in [3]triangulene. These findings advance the understanding of spin relaxation mechanisms in organic molecular systems.

A Spectrochemical Series for Electron Spin Relaxation.

Controlling the rate of electron spin relaxation in paramagnetic molecules is essential for contemporary applications in molecular magnetism and quantum information science. However, the physical mechanisms of spin relaxation remain incompletely understood, and new spectroscopic observables play an important role in evaluating spin dynamics mechanisms and structure-property relationships. Here, we use cryogenic magnetic circular dichroism (MCD) spectroscopy and pulse electron paramagnetic resonance (EPR) in tandem to examine the impact of ligand field (d-d) excited states on spin relaxation rates. We employ a broad scope of square-planar Cu(II) compounds with varying ligand field strength, including CuS4, CuN4, CuN2O2, and CuO4 first coordination spheres. An unexpectedly strong correlation exists between spin relaxation rates and the average d-d excitation energy (R2 = 0.97). The relaxation rate trends as the inverse 11th power of the excited-state energies, whereas simplified theoretical models predict only an inverse second power dependence. These experimental results directly implicate ligand field excited states as playing a critical role in the ground-state spin relaxation mechanism. Furthermore, ligand field strength is revealed to be a particularly powerful design principle for spin dynamics, enabling formation of a spectrochemical series for spin relaxation.

Spin Pumping in Epitaxial Ge1‐xSnx Alloys

AbstractThe use of Ge1‐xSnx semiconductor alloys is generating significant interest in the scientific community due to their precisely tunable Sn content. This tunability makes them particularly attractive for applications in photonics, electronics, and, more recently, spintronics. Room‐temperature emission and detection of spin currents are observed in Ge1‐xSnx/Co hybrids through spin‐pumping ferromagnetic resonance. Experiments conducted over a wide range of compositions and strains show that spin current injection is enhanced in Ge1‐xSnx solid solutions compared to elemental Ge. The magnetization dynamics reveal an intriguing scenario where the Gilbert damping constant and the spin mixing conductance display a non‐monotonic behavior. The maximum spin‐pumping efficiency occurs at a Sn molar fraction of ≈10 at.% and remains unaffected by the elastic strain built up in Ge1‐xSnx films through epitaxial growth on Ge‐buffered Si substrates. These findings highlight the non‐trivial dependence of alloy scattering in defining spin accumulation and relaxation mechanisms, providing insightful information on phenomena at the forefront of spintronics and quantum technology research.

Spin relaxation in persistent spin textures

Abstract Spin relaxation due to the combined diffuse scattering and spin-orbit coupling (SOC) plays a crucial role for the efficient spin transport, which is a prerequisite for spintronic devices. Here, we investigate the spin relaxation in two-dimensional systems with different types of SOC, with a particular focus on the SOC with persistent spin texture (PSO). Based on the Boltzmann transport theory, we calculate spin diffusion matrices within the framework of Dyakonov-Perel mechanism. In the uniform case, it is found that the in-plane and out-of-plane spin relaxations for all considered SOCs are independent. Interestingly, the in-plane spin relaxation for certain PSOs reveals significant anisotropy characterized by the infinite spin relaxation time for the spin oriented along the direction of spin-orbit field. In the non-uniform case, we show that there always exists the static solution for the PSO with SU(2) symmetry, which corresponds the persistent spin helix in real space. Our work is expected to enrich the fundamental understanding of spin relaxation mechanism and provide new guidelines to design spin-orbitronic devices.

Spintronics in GaN‐Based Semiconductors: Research Progress, Challenges and Perspectives

AbstractSpintronics, exploiting the spin degree of electrons as the information vector, is an attractive field for implementing the beyond‐complementary metal‐oxide‐semiconductor (CMOS) devices. GaN‐based semiconductors, characterized by weak spin‐orbit coupling, long spin relaxation time, and Curie temperature higher than room temperature, are considered ideal materials for advancing spintronics. In addition, GaN‐based semiconductors possess a variety of heterostructures, and different properties can be combined through energy band engineering, this enables addressing the limitation of GaN‐based semiconductors. Nevertheless, there are still challenges in practical applications; for instance, the mechanism of spin relaxation in GaN‐based semiconductors is still unclear, and efficient spin gating has not yet been realized. This review examines the progress of spintronics in GaN‐based semiconductors, and systematically summarizes the advancements in spin injection, transport, manipulation, and device application. The current challenges and future perspectives on the studies of spintronic devices based on GaN‐based semiconductors are also highlighted.

Impact of γ-Irradiation on Separation of Nuclear Spin-Relaxation Mechanisms Under Magnetic Saturation in a NaF Crystal

Impact of γ-Irradiation on Separation of Nuclear Spin-Relaxation Mechanisms Under Magnetic Saturation in a NaF Crystal

A novel non-adiabatic spin relaxation mechanism in molecular qubits.

The interaction of electronic spin and molecular vibrations mediated by spin-orbit coupling governs spin relaxation in molecular qubits. We derive an extended molecular spin Hamiltonian that includes both adiabatic and non-adiabatic spin-dependent interactions, and we implement the computation of its matrix elements using state-of-the-art density functional theory. The new molecular spin Hamiltonian contains a novel spin-vibrational orbit interaction with a non-adiabatic origin, together with the traditional molecular Zeeman and zero-field splitting interactions with an adiabatic origin. The spin-vibrational orbit interaction represents a non-Abelian Berry curvature on the ground-state electronic manifold and corresponds to an effective magnetic field in the electronic spin dynamics. We further develop a spin relaxation rate model that estimates the spin relaxation time via the two-phonon Raman process. An application of the extended molecular spin Hamiltonian together with the spin relaxation rate model to Cu(II) porphyrin, a prototypical S = 1/2 molecular qubit, demonstrates that the spin relaxation time at elevated temperatures is dominated by the non-adiabatic spin-vibrational orbit interaction. The computed spin relaxation rate and its magnetic field orientation dependence are in excellent agreement with experimental measurements.

Tuning Spin-Polarized Lifetime at High Carrier Density through Deformation Potential in Dion-Jacobson-Phase Perovskites.

The control of spin relaxation mechanisms is of great importance for spintronics applications as well as for fundamental studies. Layered metal-halide perovskites represent an emerging class of semiconductors with rich optical spin physics, showing potential for spintronic applications. However, a major hurdle arises in layered metal-halide perovskites with strong spin-orbit coupling, where the spin lifetime becomes extremely short due to D'yakonov-Perel' scattering and Bir-Aronov-Pikus at high carrier density. Using the circularly polarized pump-probe transient reflection technique, we experimentally reveal the important scattering for spin relaxation beyond the electron-hole exchange strength in the Dion-Jacobson (DJ)-type 2D perovskites (3AMP)(MA)n-1PbnI3n+1 [3AMP = 3-(aminomethyl)piperidinium, n = 1-4]. Despite a more than 10-fold increase in carrier concentration, the spin lifetimes for n = 3 and 4 are effectively maintained. We reveal neutral impurity and polar optical phonon scatterings as significant contributors to the momentum relaxation rate. Furthermore, we show that more octahedral distortions induce a larger deformation potential which is reflected on the acoustic phonon properties. Coherent acoustic phonon analysis indicates that the polaronic effect is crucial in achieving control over the scattering mechanism and ensuring spin lifetime protection, highlighting the potential of DJ-phase perovskites for spintronic applications.

Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl2 Monolayer.

The development of two-dimensional (2D) magnetism is driven not only by the interest of low-dimensional physics but also by potential applications in high-density miniaturized spintronic devices. However, 2D materials possessing a ferromagnetic order with a relatively high Curie temperature (Tc) are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl2 on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl2, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature (Tc) of ∼147 K is revealed for monolayer FeCl2, based on our static magneto-optical Kerr effect measurements. Furthermore, temperature-dependent magnetization dynamics is investigated by the time-resolved magneto-optical Kerr effect. A transition from one- to two-step demagnetization occurs as the lattice temperature approaches Tc, which supports the Elliott-Yafet spin relaxation mechanism. The findings contribute to a deeper understanding of the underlying mechanisms governing ultrafast magnetization in 2D ferromagnetic materials.

Direct Observation of Circularly Polarized Nonlinear Optical Activities in Chiral Hybrid Lead Halides.

Circularly polarized light emission is a crucial application in imaging, sensing, and photonics. However, utilizing low-energy photons to excite materials, as opposed to high-energy light excitation, can facilitate deep-tissue imaging and sensing applications. The challenge lies in finding materials capable of directly generating circularly polarized nonlinear optical effects. In this study, we introduce a chiral hybrid lead halide (CHLH) material system, R/S-DPEDPb3Br8·H2O (DPED = 1,2-diphenylethylenediammonium), which can directly produce circularly polarized second harmonic generation (CP-SHG) through linearly polarized infrared light excitation, exhibiting a polarization efficiency as high as 37% at room temperature. To understand the spin relaxation mechanisms behind the high polarization efficiency, we utilized two models, so-called D'yakonov-Perel' (DP) and Bir-Aronov-Pikus (BAP) mechanisms. The unique zigzag inorganic frameworks within the hybrid structure are believed to reduce the dielectric confinement and exciton binding energy, thus enhancing spin polarization, especially in regions with a high excitation pump fluence based on the DP mechanism. In the case of low excitation pump fluence, the BAP mechanism dominates, as evidenced by the observed decrease in the polarization ratio from CP-SHG measurement. Using density functional theory analysis, we elucidate how the distinctive 8-coordination environment of lead bromide building blocks effectively suppresses spin-orbit coupling at the conduction band minimum. This suppression significantly diminishes spin-splitting, thereby slowing the spin relaxation rate.

New series of mononuclear β-diketonate cerium(III) field induced single-molecule magnets.

Five new β-diketonate Ce3+ mononuclear complexes, [Ce(Btfa)3(H2O)2] (1), [Ce(Btfa)3(phen)] (2), [Ce(Btfa)3(bipy)] (3), [Ce(Btfa)3(terpy)] (4) and [Ce(Btfa)3(bathophen)(DMF)] (5), where Btfa- = 4,4,4-trifluoro-1-phenyl-1,3-butanedionate, phen = 1,10-phenanthroline, bipy = 2,2'-bipyridyl, terpy = 2,2':6',2''-terpyridine and bathophen = 4,7-diphenyl-1,10-phenanthroline, have been synthesized and structurally characterized through X-ray diffraction of single crystals. The central Ce3+ atom displays a coordination number of 8 for 1, 2 and 3 and of 9 for 4 and 5. Under a 0 T external magnetic field, none of the given compounds exhibits single molecule magnet (SMM) behaviour. However, a small magnetic field, between 0.02 and 0.1 T, is enough for all the compounds to exhibit slow relaxation of the magnetization. A comprehensive magnetic analysis, with experimental magnetic data and ab initio calculations, was undertaken for all the complexes, and the study highlights the significance of the different spin relaxation mechanisms that must be considered for a Ce3+ lanthanide ion.

Optical Electron-Nuclear Resonance in Semiconductors

A considerable number of investigations have by now been made on optical pumping of spin-oriented electrons in semiconductors. The optical-pumping method makes it possible to produce easily a high polarization of the electron spins in the conduction band, and to detect it optically [1–3]. The use of this method for the investigation of semiconductors turned out to be very fruitful; the spin orientation of the minority [1–3] and majority [4] carriers was determined, the lifetimes of the non-equilibrium electrons were measured [2, 3, 5], the mechanisms of spin relaxation of “cold” [2, 5] and “hot” [2, 6] electrons were investigated, spin orientation of excitons was effected [7], and electron paramagnetic resonance with non-equilibrium electrons was registered [8].

T1 Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit.

Paramagnetic molecules offer unique advantages for quantum information science owing to their spatial compactness, synthetic tunability, room-temperature quantum coherence, and potential for optical state initialization and readout. However, current optically addressable molecular qubits are hampered by rapid spin-lattice relaxation (T1) even at sub-liquid nitrogen temperatures. Here, we use temperature- and orientation-dependent pulsed electron paramagnetic resonance (EPR) to elucidate the negative sign of the ground state zero-field splitting (ZFS) and assign T1 anisotropy to specific types of motion in an optically addressable S = 1 Cr(o-tolyl)4 molecular qubit. The anisotropy displays a distinct sin2(2θ) functional form that is not observed in S = 1/2 Cu(acac)2 or other Cu(II)/V(IV) microwave addressable molecular qubits. The Cr(o-tolyl)4 T1 anisotropy is ascribed to couplings between electron spins and rotational motion in low-energy acoustic or pseudoacoustic phonons. Our findings suggest that rotational degrees of freedom should be suppressed to maximize the coherence temperature of optically addressable qubits.

Isolated spin-pair analysis of 13C NMR relaxation rates in liquids

In small organic molecules, dipolar relaxation rate R1DD is the most important mechanism for 13C NMR spin relaxation. Herein, the R1DD was used to analyze force-field molecular dynamics trajectories, corresponding to some neat liquids and solutions, in order to simulate 13C relaxation. The results show a satisfactory correlation between simulated and experimental values, structure-dependent behavior and self-diffusion effects due to differences in inertia axes that could be observed by the isolated spin-pair approach that avoids the solution of relaxation matrices, which helps in interpreting 13C relaxation experimental data from a molecular dynamics approach.

Hybrid organic/inorganic interface: in situ and ex situ characterization in terms of structure, morphology, electronic and magnetic properties

Organic conjugated molecules are promising media to host spin-polarized carriers, due to their weak spin relaxation mechanisms. Here, we fabricated a metal–hybrid organic–inorganic complex oxide heterostructure made of Fe–Phthalocyanine (FePc): Co(0.5 nm)/FePc(0; 0.5; 1 nm)/La0.67Sr0.33MnO3/SrTiO3 and investigated the interface structure and chemical bonding, and overall magnetic properties. Details about the Co/FePc and FePc/LSMO interfaces were obtained through X-ray photoelectron and absorption characterizations. The combined in-situ and ex-situ magnetic measurements of the heterostructure indicates the contribution of FePc and Co moments to the overall magnetic response. Moreover, these measurements indicate that the LSMO bulk magnetic properties were not canceled out by the surface treatment, but favored the formation of a magnetic dead layer. The hybrid heterostructure presents a modification of the saturation magnetization attributed to the contribution of the molecular layer. The present results are a step forward in the engineering of hybrid organic/inorganic interfaces.

Comprehensive analysis on the magnetic field error of a K–Rb–21Ne comagnetometer with low-frequency bias magnetic field sensitivity

The spin-exchange relaxation-free comagnetometer (SERFC) is of important research value compared to existing high-precision gyroscopes because of its extremely high theoretical limit sensitivity and long-term stability, in which one significant limiting factor is the magnetic field error. First, the relationship between the magnetic field gradient and the nuclear spin relaxation mechanism is introduced into the frequency response and steady-state response models of SERFC. Then, a novel method for suppression of the low-frequency magnetic field error based on the modified bias magnetic field sensitivity model is proposed. Finally, the effectiveness of the proposed suppression methods is demonstrated by optimizing the cell temperature, pump light power, and compensation magnetic field gradient to increase the suppression factor by 72.19%, 20.24%, and 69.86%, and the corresponding bias instability increased by 55.41%, 20.84%, and 27.63%, respectively. This study contributes to improving the long-term zero bias stability of the SERFC.

Model for 1/f flux noise in superconducting aluminum devices: Impact of external magnetic fields

Superconducting quantum interference devices and related circuits made of aluminum are known to display 1/ω flux noise, where ω is frequency. A recent experiment showed that the application of an external magnetic field in the 10–100 G range changed the noise to a single Lorentzian peaked at ω = 0. Here, it is shown that a model based on independent impurity spin flips with coexisting cross and direct mechanisms of spin relaxation may explain these experiments. The model shows that application of an external magnetic field can be used to reduce the impact of flux noise in qubits.

Unlocking hidden spins in centrosymmetric 1T transition metal dichalcogenides by vacancy-controlled spin-orbit scattering

Spin current generation and manipulation remain the key challenge of spintronics, in which relativistic spin-orbit coupling (SOC) plays a ubiquitous role. In this paper, we demonstrate that hidden Rashba spins in the nonmagnetic, centrosymmetric lattice of multilayer ${\mathrm{SnSe}}_{2}$ can be efficiently activated by spin-orbit scattering introduced by Se vacancies. Via vacancy scattering, conduction electrons with hidden spin-momentum locked polarizations acquire out-of-plane magnetization components, which effectively break the chiral symmetry between the two Se sublattices of an ${\mathrm{SnSe}}_{2}$ monolayer when electron spins start precession in the strong built-in Rashba SOC field. The resulting spin separations are manifested in quantum transport as vacancy concentration- and temperature-dependent crossovers from weak antilocalization to weak localization, with the distinctive spin relaxation mechanism of the D'yakonov-Perel' type. In a nonlocal geometry, the generated spins exhibit a long diffusion length exceeding 5 $\textmu{}\mathrm{m}$, when fast momentum scattering protects the effective spin polarizations by driving the random-walk evolution of spin precessions subject to rapidly changing Rashba SOC field. Our study shows the great potential of two-dimensional systems with hidden-spin textures for spintronics.

Microscopic theory of spin relaxation of a single Fe adatom coupled to substrate vibrations

Understanding the spin-relaxation mechanism of single adatoms is an essential step towards creating atomic magnetic memory bits or even qubits. Here we present an essentially parameter-free theory by combining \textit{ab-initio} electronic and vibrational properties with the many-body nature of atomic states. Our calculations account for the millisecond spin lifetime measured recently on Fe adatoms on MgO/Ag(100) and reproduce the dependence on the number of decoupling layers and the external magnetic field. We show how the atomic interaction with the environment should be tuned in order to enhance the magnetic stability, and propose a clear fingerprint for experimentally detecting a localized spin-phonon excitation.

Scanning inverse spin Hall effect spectrometer by shorted coaxial probes.

In this work, a scanning inverse spin Hall effect measurement system based on a shorted coaxial resonator has been built, which provides a high throughput method to characterize spin transport properties. The system is capable of performing spin pumping measurements on patterned samples within an area of 100 × 100mm2. Its capability was demonstrated with Py/Ta bilayer stripes deposited on the same substrate with different thicknesses of Ta. The results show that the spin diffusion length is about 4.2nm with a conductivity of about 7.5 × 105Ω-1m-1, which leads to the conclusion that the intrinsic mechanism of spin relaxation of Ta is the Elliott-Yafet interactions. The spin Hall angle of Ta is estimated to be about -0.014 at room temperature. The setup developed in this work provides a convenient, efficient, and nondestructive way to obtain the spin and electron transportation characteristics of the spintronic materials, which will fertilize this community by developing new materials and figuring out their mechanism.