Spin Coupling Mechanism Research Articles

Study of magnetic and magnetocaloric properties of calcium doped La0.97−xCaxHo0.03MnO3 compound

This study reports the magnetic and magnetocaloric properties of different concentrations of alkaline earth metal, such as Ca-doped La0.97−xCaxHo0.03MnO3 (x = 0.3, 0.33, and 0.37) composites, which were synthesized via autocombustion technique. The second-order paramagnetic-to-ferromagnetic phase transition temperature appeared at the temperature-dependent field-cooled magnetization curve. The result shows an increase in the Curie temperature of the compound with Ca2+ doping. In addition, increasing the Ca2+ doping concentration further increased the change in magnetic entropy, − ∆SM, up to 0.206 J kg−1 K−1, resulting in a higher RCP value up to 30 J kg−1 at x = 0.37. This work’s key aspect is demonstrating the potentiality of enhancing the magnetocaloric effect in the framework via the spin coupling mechanism of Ca2+-doped rare-earth perovskite compounds.Graphical abstract

Atomic spin and phonon coupling mechanism of nitrogen-vacancy center

The nitrogen-vacancy center structure of diamond has attracted widespread attention due to its high sensitivity in quantum precision measurement. In this paper, a coupled phonon field is used to resonantly regulate the atomic spins of the nitrogen-vacancy center for improving the spin transition efficiency. Firstly, the interaction between phonons and lattice energy is analyzed based on the relationship between the wave function and the lattice displacement vector. The spin transition mechanism is investigated based on phonon resonance regulation, and the strain-induced energy transferable phonon-spin interaction coupling excitation model is established. Secondly, the coefficient matrix satisfying Bloch’s theorem is adopted to develop the phonon spectrum model of the first Brillouin zone characteristic region for different axial nitrogen-vacancy centers. Considering the thermal expansion, the thermal balance properties of phonon resonance system are analyzed and its specific heat model is studied based on the Debye model. Finally, the structure optimization model of different axial nitrogen-vacancy centers under the phonon model is built up based on the molecular dynamics simulation software CASTEP and density functional theory for first-principles research. The structural characteristics, phonon characteristics, and thermodynamic properties of nitrogen-vacancy centers are analyzed. The research results show that the evolution of phonon mode depends on the occupation of the nitrogen-vacancy center. A decrease in thermodynamic entropy accompanies the strengthening of the phonon mode. The covalent bond of diamond with nitrogen-vacancy center is weaker than that of a defect-free diamond. The thermodynamic properties of a defect-free diamond are more unstable. The primary phonon resonance frequency of diamond with nitrogen-vacancy centers are on the order of THz, and the secondary phonon resonance frequency is about in a range of 800 and 1200 MHz. A surface acoustic wave resonance mechanism with an interdigital width of 1.5 μm is designed according to the secondary resonance frequency, and its center frequency is about 930 MHz. The phonon resonance control method can effectively increase the spin transition probability of nitrogen-vacancy center under suitable phonon resonance control parameters, and thus realizing the increase of atomic spin manipulation efficiency.

Multiferroic behavior in EuTiO3 films constrained by symmetry

We have elucidated the spin, lattice, charge and orbital coupling mechanism underlying the multiferroic character in tensile strained EuTiO3 films. Symmetry determined by oxygen octahedral tilting shapes the hybridization between the Eu 4f and Ti 3d orbitals and this inhibits predicted Ti displacement proper ferroelectricity. Instead, phonon softening emerges at low temperatures within the pseudo-cube (110) plane, orthogonal to the anticipated ferroelectric polarization symmetry. Additionally, the magnetic anisotropy is determined by orbital distortion through hybridization between the Ti 3d and typically isotropic Eu2+ 4f. This unique scenario demonstrates the critical role symmetry plays in the coupling of order parameters defining multiferroic behaviour.

Magneto-optical control and spin coupling with non-reciprocal surface waves for nanoscale thermotronics -INVITED

Magneto-optical materials have been proposed as promising candidates for an active control of the directionality of nanoscale heat radiation. Here, we discuss the possibility to rectify nanoscale radiative heat fluxes by means of non-reciprocal surface waves and propose a nanoscale heat flux rectifier or diode which can be controlled actively by means of externally applied fields. We furthermore, reveal the spin coupling mechanism behind the observed heat flux rectification

Dynamics of quantum discord of two coupled spin-1/2’s subjected to time-dependent magnetic fields

Abstract We describe the dynamics of quantum discord of two interacting spin-1/2’s subjected to controllable time-dependent magnetic fields. The exact time evolution of discord is given for various input mixed states consisting of classical mixtures of two Bell states. The quantum discord manifests a complex oscillatory behaviour in time and is compared with that of quantum entanglement, measured by concurrence. The interplay of the action of the time-dependent magnetic fields and the spin-coupling mechanism in the occurrence and evolution of quantum correlations is examined in detail.

On the Border between Low-Nuclearity and One-Dimensional Solids: A Unique Interplay of 1,2,4-Triazolyl-Based {CuII5(OH)2} Clusters and MoVI-Oxide Matrix.

A pentanuclear CuII5-hydroxo cluster possessing an unusual linear-shaped configuration was formed and crystallized under hydrothermal conditions as a result of the unique cooperation of bridging 1,2,4-triazole ligand ( trans-1,4-cyclohexanediyl-4,4'-bi(1,2,4-triazole) ( tr2 cy)), MoVI-oxide, and CuSO4. This structural motif can be rationalized by assuming in situ generation of {Cu2Mo6O22}4- anions, which represent heteroleptic derivatives of γ-type [Mo8O26]4- further interlinked by [Cu3(OH)2]4+ cations through [ N- N] bridges. The framework structure of the resulting compound [Cu5(OH)2( tr2 cy)2Mo6O22]·6H2O (1) is thus built up from neutral heterometallic {Cu5(OH)2Mo6O22} n layers pillared with tetradentate tr2 cy. Quantum-chemical calculations demonstrate that the exclusive site of the parent γ-[Mo8O26]4- cluster into which CuII inserts corresponds with the site that has the lowest defect ("MoO2 vacancy") formation energy, demonstrating how the local metal-polyoxomolybdate chemistry can express itself in the final crystal structure. Magnetic susceptibility measurements of 1 show strong antiferromagnetic coupling within the Cu5 chain with exchange parameters J1 = -500(40) K (-348(28) cm-1), J2 = -350(10) K (-243(7) cm-1) and g = 2.32(2), χ2 = 6.5 × 10-4. Periodic quantum-chemical calculations reproduce the antiferromagnetic character of 1 and connect it with an effective ligand-mediated spin coupling mechanism that comes about from the favorable structural arrangement between the Cu centers and the OH-, O2-, and tr2 cy bridging ligands.

Tuning the Spin Coupling Interactions in the Nitroxide-Based Bisphenol-Like Diradicals.

The intramolecular spin coupling interactions of bisphenol-like trinary-bridged diradicals [nitroxide-(para/meta)phenylene-X-phenylene(para/meta)-nitroxide, X=C=CH2 , O, BH, NH and SO2 ] were explored with an emphasis on the tuning role of the X coupler at the (U)B3LYP/6-311++G(d,p) level. Our results indicate that all designed trinary-bridged diradicals featuring a V-type structure with a bending angle of 104-130° and torsional angles of two phenylene rings being 20-90° exhibit different diradical character and magnetism, depending on the structures and properties of the X bridges. More interestingly, although meta/para-phenylene supports a ferromagnetic (FM)/antiferromagnetic (AFM) coupling, their combinations by using X as a trinary bridge can mediate spin coupling, but the coupling magnitude strongly depends on X. In general, a para/para or meta/meta combination with X leads to an open-shell singlet ground state and thus AFM, but the meta/meta combination considerably decreases the spin coupling interaction. In contrast, a para/meta combination with X produces a triplet ground state and FM. Their combination with a single-electron conjugation end coupler (C=CH2 ) leads to an inverse coupling regularity. All results can be reasonably explained by the spin alternation rule, molecular structures, and properties. In particular, detailed spin coupling mechanisms are suggested to involve cooperative through-space and through-bond pathways with different levels of cooperativity. This is rationalized with the X-induced bending of the diradicals not only modifying the through-bond (extended π conjugation) pathway but also provididng a through-space (face-to-face vs. side-to-side π-π interaction) possibility for spin coupling, in conjunction with twisting of the phenylene rings. Different contributions of the through-space and through-bond couplings are quantitatively distinguished and depend on the structure and property of the X coupler. Clearly, this work reports interesting aspects of the trinary bridged diradicals and also provides important information for the design of molecules for functional magnetic materials and tuning their magnetic properties.

A Landau‐Lifshitz‐Gilbert‐Type equation and torsion effects on the dynamics of magnetization

Based on the non‐relativistic regime of electrons described by a Dirac equation coupled to a torsion pseudo‐vector, we study the dynamics of magnetization and how it may be affected by the presence of this (background) torsion field. The latter describes some anisotropy of the bulk and it may be accounted for in two ways: by means of a minimal coupling in a covariant derivative endowed with spin connection and a gauge magnetic field; in addition, through a non‐minimal interaction term with the spin density. We show, within this framework, that it is possible to attain a version of the Landau‐Lifshitz equation in presence of the torsion background where its effects are included, re‐enforcing that torsion corresponds to a geometric anisotropy with relevant unfolds on the spin coupling mechanism. We also conclude that the torsion terms can open up two important landscapes in the magnetization dynamics: one of them, related with a damping effect; the other, related with the screw dislocation that gives us a global effect like a sharped‐helix damping. The whole effort here is then to pursue an investigation of torsion in connection with the precession dynamics of the magnetization. image

Ferromagnetic Exchange Coupling between Fe Phthalocyanine and Ni(111) Surface Mediated by the Extended States of Graphene

The interface spin coupling mechanism is studied in a hybrid structure made of Fe phthalocyanine molecules sublimed in ultrahigh vacuum on graphene grown on the magnetic substrate Ni(111). By using synchrotron X-ray magnetic circular dichroism, the field-dependent magnetization of the isolated FePc molecules and of the Ni substrate has been measured at low temperature (8 K). Along with density functional theory calculations, the role of the graphene interlayer in transmitting the magnetic coupling is addressed. Both experiments and theory show a ferromagnetic coupling between the molecules and the substrate which is weakened by the insertion of graphene. DFT calculations indicate that the key role is played by the π orbitals of graphene, which hybridize with the underlying magnetic Ni, giving rise to a sizable spin polarized continuum at the molecular interface. The resulting overlap with the Fe orbitals favors a direct coupling of ferromagnetic nature, as evidenced by our spin density distribution plots.

Cation Order–Disorder Transition in Fe-Doped 6H-BaTiO3 for Dilute Room-Temperature Ferromagnetism

Understanding the fundamental spin coupling mechanisms in dilute magnetic oxides is not possible without exact atomistic understanding of the solids. This is even more important for few systems that show dilute ferromagnetism at room temperature and could potentially be used in spintronic and multiferroic technologies. Here, we show the first known example of a cation ordering in hexagonal 6H-perovskites. Fe3+ ions in Fe-doped 6H-BaTiO3 are initially randomly distributed over two Ti4+ crystallographic sites but can be, by a prolonged heat treatment, ordered onto one of the sites. The cation ordering is driven by reduction in the electrostatic repulsion, which occurs when Fe3+ substitutes the highly charged Ti4+ ions that are located at unusual proximity to each other. We show that such a continuous order–disorder transition is associated with induction of the room-temperature dilute ferromagnetism.

Influence of exchange correlation on the symmetry and properties of siderite according to density-functional theory

Density-functional theory (DFT) without spin-orbit coupling (SOC), as implemented in CASTEP, yields structural, energetic, and elastic properties of siderite $({\text{FeCO}}_{3})$ in good agreement with available data from the literature. The electronic ground state is however found ferromagnetic (FM) with a nonzero density of states at the Fermi level, whereas real siderite is antiferromagnetic (AFM) and insulating. We show in both cases how the paradox can be removed. Concerning the metallic/insulating character, a symmetry analysis of the electronic structure performed with the help of ISOTROPY shows that the absence of gap is due to an essential degeneracy of monoelectronic wave functions in the experimental $R\overline{3}c$ symmetry. The small dispersion of individual bands establishes a clear difference with the case of a real metal. Complemented by $\text{DFT}+\text{SOC}$ computations performed using ABINIT this shows the consistency of the DFT description with crystal-field theory data taken from the literature. Thus for DFT the electronic structure of siderite is characterized by correlation, but of exchange origin, as opposed to electrostatic origin. The AFM nature of the ground state is also restored by SOC. Without SOC the total energies of the FM and AFM states are very close. After introducing some modifications in a classical model of superexchange we obtain a satisfactory interpretation of our DFT results in terms of spin polarization in the iron-carbonate bonds, spin-coupling mechanisms between neighboring iron ions and finally of the observed FM over AFM stability. Hence, siderite emerges as an original model system among transition-metal compounds for the application of DFT to open-shell electronic structures.

A solid-state 31P NMR study of 1:1 silver–triphenylphosphine complexes — Interpretation of 1J(107,109Ag,31P) values,

High-resolution solid-state 31P NMR spectroscopy was used to investigate a series of 1:1 silver–triphenylphosphine complexes, [Ph3PAgX]n, where X is a monovalent anion and n = 1, 2, 3, 4, or ∞. The 31P CP MAS NMR spectra reveal the number of distinct phosphorus sites in these complexes as well as the |1J(109Ag,31P)| values, which range from 401 ± 10 Hz (X = N3–) to 869 ± 10 Hz (X = SO3CF3–). The data obtained here and in earlier investigations indicate that |1J(109Ag,31P)| values for silver–tertiary phosphine complexes decrease as Ag–P bond lengths increase. This experimental conclusion is supported by DFT calculations, which also indicate that the Fermi-contact mechanism is the only important spin–spin coupling mechanism for 1J(109Ag,31P) in these complexes. In addition, the crystal structure of a silver–triphenylphosphine trifluoroacetate tetramer was determined using X-ray crystallography, and the structure of a silver–triphenylphosphine chloride tetramer was reinvestigated.

Electron−Nuclear Spin Dynamics in a Bacterial Photosynthetic Reaction Center

The solid-state photo-CIDNP effect is known to occur in natural photosynthetic reaction centers (RCs) where it can be observed by magic-angle spinning (MAS) NMR as strong modification of signal intensities under illumination compared to experiments performed in the dark. The origin of the effect has been debated. In this paper, we report time-resolved photo-CIDNP MAS NMR data of reaction centers of quinone depleted Rhodobacter sphaeroides. It is demonstrated that the build-up of nuclear polarization on the primary donor and the bacteriopheophytin acceptor depends on the presence and lifetimes of the molecular triplet states of the donor and carotenoid. Analysis of the data proves that up to three electron−nuclear spin-coupling mechanisms and two transient effects are working concomitantly in the spin-chemical machinery of the reaction center.

Spin‐Polarized Structures and Solid‐State NMR Spectroscopy of Paramagnetic Compounds

On an atomic scale and with high sensitivity, solid-state NMR spectroscopy can provide information about the electronic spin density and coupling mechanisms in paramagnetic compounds. The picture shows how the hyperfine splitting collapses through relaxation. Insights into which compounds are suitable and which approximations have to be made are given.

Cyano‐bridged Ln3+‐Cr3+ Binuclear Complexes [Ln(L)x(H2O)yCr(CN)6]·mL·nH2O (Ln=La–Nd, x=5, y=2, m=1 or 2, n=2 or 2.5; Ln=Sm–Dy, Er, x=4, y=3, m=0, n=1.5 or 2.0; L=2‐pyrrolidinone): Structure, Magnetism and Spin Density Map

AbstractIn order to shed light upon the nature and mechanism of 4f‐3d magnetic exchange interactions, a series of binuclear complexes of lanthanide(3+) and chromium(3+) with the general formula [Ln(L)5(H2O)2Cr(CN)6]·mL·nH2O (Ln=La (1), Ce (2), Pr (3), Nd (4); x=5, y=2, m=1 or 2, n=2 or 2.5; L=2‐pyrrolidinone) and [Ln(L)4(H2O)3Cr(CN)6] ·nH2O (Ln=Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Er (10); x=4, y=3, m=0, n=1.5 or 2.0; L2‐pyrrolidinone) were prepared and the X‐ray crystal structures of complexes 2, 6 and 7 were determined. All the compounds consist of a Ln‐CN‐Cr unit, in which Ln3+ in a square antiprism environment is bridged to an octahedral coordinated Cr3+ ion through a cyano group. The magnetic properties of the complexes 3 and 6–10 show an overall antiferromagnetic behavior. The fitting to the experimental magnetic susceptibilities of 7 give g=1.98, J=0.40 cm−1, zJ′=−0.21 cm−1 on the basis of a binuclear spin system (SGd=7/2, SCr=3/2), revealing an intra‐molecular Gd3+‐Cr3+ ferromagnetic interaction and an inter‐molecular antiferromagnetic interaction. For 7 the calculation of quantum chemical density functional theory (DFT), combined with the broken symmetry approach, showed that the calculated spin coupling constant was 20.3 cm−1, supporting the observation of weak ferromagnetic intra‐molecular interaction in 7. The spin density distributions of 7 in both the high spin ground state and the broken symmetry state were obtained, and the spin coupling mechanism between Gd3+ and Cr3+ was discussed.

Calculation and analysis of NMR spin–spin coupling constants

The analysis of NMR spin-spin coupling leads to a unique insight into the electronic structure of closed-shell molecules, provided one is able to decode the different features of the spin-spin coupling mechanism. For this purpose, the physics of spin-spin coupling is described and the way how spin-spin coupling constants (SSCCs) can be quantum mechanically determined. Based on this insight, a set of requirements is derived that guide the development of a quantum mechanical analysis of spin-spin coupling. It is demonstrated that the J-OC-PSP (=J-OC-OC-PSP: Decomposition of J into orbital contributions using orbital currents and partial spin polarization) analysis method fulfills all requirements. J-OC-PSP makes it possible to partition the isotropic indirect SSCC J or its reduced analogue K as well as the four Ramsey terms (Fermi contact (FC), spin dipole (SD), diamagnetic spin orbit (DSO), paramagnetic spin orbit (PSO)) leading to J (or K) into Cartesian components (for the anisotropic Ramsey terms SD, DSO, PSO), orbital contributions or electron interaction terms. For the purpose of decoding the spin-spin coupling mechanism, FC, SD, DSO, and PSO coupling is discussed in detail and related to electronic and bonding features of the molecules in question. The myth of empirical and semiempirical relationships between SSCCs and bonding features is unveiled. It is found that most relationships are only of limited, partly dubious value, often arising from a fortuitous cancellation of terms that cannot be expected in general. These relationships are replaced by quantum chemical relations and descriptions that directly reflect the complex electronic processes leading to spin-spin coupling.

Analysis of the magneto-structural correlations in the meso-tetraphenylporphyrinatomanganese(iii) tetracyanoethenide family of molecule-based magnets

The dependence of the nearest neighbor magnetic interaction, JAB, as a function of geometry for [Mn(porphyrin)][TCNE] materials has been evaluated by the results of CASSCF, CASPT2 and B3LYP calculations. The B3LYP results were validated for seven representative [Mn(porphyrin)][TCNE] molecule-based magnets and were found to be in good agreement with the experimental data for observed values of JAB/kB less than −150 K, and the CASPT2 method for JAB/kB greater than −150 K. Then, the JAB was systematically evaluated as a function of the intermolecular coordinates that define the position and orientation of [TCNE]˙− relative to [Mn(porphyrin)]+. The calculated JAB/kB (Ĥ = −2ΣJABŜAŜB) are antiferromagnetic in all regions, with values in the −112 to −457 K range. The most negative JAB values occur for the smallest plane angles between the [Mn(porphyrin)]+ and [TCNE]˙− planes. The B3LYP results were then compared to those obtained using the three spin exchange mechanisms (i.e. orthogonality, spin coupling, and CI mechanisms), and none of these mechanisms give trends in accord with the B3LYP results. Therefore, first principles methods, such as DFT or CASPT2, are needed for more detailed calculations.

Variable-Temperature 19F NMR and Theoretical Study of 1,9- and 1,7-C60F(CF3) and Cs- and C1-C60F17(CF3): Hindered CF3 Rotation and Through-Space JFF Coupling

Milligram amounts of the new compounds 1,9- and 1,7-C60F(CF3) (ca. 85:15 mixture of isomers) and C60F3(CF3) were isolated from a high-temperature C60/K2PtF6 reaction mixture and purified to 98 mol % compositional purity by two-dimensional high-performance liquid chromatography using Buckyprep and Buckyclutcher columns. The previously observed compounds C60F5(CF3) and C60F7(CF3) were also purified to 90+ mol % for the first time. Variable-temperature 19F NMR spectra of the mixture of C60F(CF3) isomers and the previously reported mixture of C(s)- and C1-C60F17(CF3) isomers demonstrate for the first time that fullerene(F)n(CF3)m derivatives with adjacent F and CF3 substituents exhibit slow-exchange limit hindered CF3 rotation spectra at -40 +/- 10 degrees C. The experimental and density functional theory (DFT) predicted deltaH++ values for CF3 rotation in 1,9-C60F(CF3) are 46.8(7) and 46 kJ mol(-1), respectively. The DFT-predicted deltaH++ values for 1,7-C60F(CF3), C(s)-C60F17(CF3), and C1-C60F17(CF3) are 20, 44, and 54 kJ mol(-1), respectively. The (> or = 4)J(FF) values from the slow-exchange-limit 19F spectra, which vary from ca. 0 to 48(1) Hz, show that the dominant nuclear spin-spin coupling mechanism is through-space coupling (i.e., direct overlap of fluorine atom lone-pair orbitals) rather than coupling through the sigma-bond framework. The 2J(FF) values within the CF3 groups vary from 107(1) to 126(1) Hz. Collectively, the NMR data provide an unambiguous set of (> or = 4)J(FF) values for three different compounds that can be correlated with DFT-predicted or X-ray diffraction derived distances and angles and an unambiguous set of 2J(FF) values that can serve as an internal standard for all future J(FF) calculations.

One-electron versus electron–electron interaction contributions to the spin–spin coupling mechanism in nuclear magnetic resonance spectroscopy: Analysis of basic electronic effects

For the first time, the nuclear magnetic resonance (NMR) spin-spin coupling mechanism is decomposed into one-electron and electron-electron interaction contributions to demonstrate that spin-information transport between different orbitals is not exclusively an electron-exchange phenomenon. This is done using coupled perturbed density-functional theory in conjunction with the recently developed J-OC-PSP [=J-OC-OC-PSP: Decomposition of J into orbital contributions using orbital currents and partial spin polarization)] method. One-orbital contributions comprise Ramsey response and self-exchange effects and the two-orbital contributions describe first-order delocalization and steric exchange. The two-orbital effects can be characterized as external orbital, echo, and spin transport contributions. A relationship of these electronic effects to zeroth-order orbital theory is demonstrated and their sign and magnitude predicted using simple models and graphical representations of first order orbitals. In the case of methane the two NMR spin-spin coupling constants result from totally different Fermi contact coupling mechanisms. (1)J(C,H) is the result of the Ramsey response and the self-exchange of the bond orbital diminished by external first-order delocalization external one-orbital effects whereas (2)J(H,H) spin-spin coupling is almost exclusively mitigated by a two-orbital steric exchange effect. From this analysis, a series of prediction can be made how geometrical deformations, electron lone pairs, and substituent effects lead to a change in the values of (1)J(C,H) and (2)J(H,H), respectively, for hydrocarbons.

Analysis of the NMR through-space coupling mechanism between 19F atoms

The through-space NMR spin–spin coupling mechanism between 19F nuclei was investigated at the coupled perturbed density functional level using a BLYP(60:40) functional and a large basis set. Through-space coupling decays exponentially with the distance R(F,F) and is strongly angular dependent. The lp(σ) and lp(σ)–lp(σ) contributions to the FC term are larger than the corresponding lp(π ip) contributions. The passive lp(π) contributions are responsible for 10% of the FC mechanism. The PSO term is not negligible. In 1,8-difluoronaphthalene, the FC term of 56 Hz results from through-space coupling (75 Hz) and σ through-bond coupling (−19 Hz). The magnitude of through-space coupling can be predicted with a suitable model.