High-spin Stability Research Articles

Two-dimensional transition metal triborides: Monolayers with robust intrinsic magnetism and high spin stability

Two-dimensional materials with intrinsic magnetism have provoked comprehensive attentions due to their potential application in spintronics. In this work, a class of stable transition metal triborides, $\mathrm{TM}{\mathrm{B}}_{3}$ ($\mathrm{TM}=\mathrm{Ti}$, V, Cr, Mn, Fe) with various electronic and magnetic features were predicted by density functional theory. Based on static energy, phonon spectrum, and $ab\phantom{\rule{0.16em}{0ex}}initio$ molecular dynamics simulations, all the $\mathrm{TM}{\mathrm{B}}_{3}$ monolayers have thermal and kinetic stability. Meanwhile, $\mathrm{TM}{\mathrm{B}}_{3}\mathrm{s}$ exhibit mechanical anisotropy reflected by Young's modulus and Poisson's ratio. Moreover, $\mathrm{Ti}{\mathrm{B}}_{3}$ and $\mathrm{Fe}{\mathrm{B}}_{3}$ are robust ferromagnetic metals with the respective Curie temperatures of 248 and 367 K. $\mathrm{Cr}{\mathrm{B}}_{3}$ is antiferromagnetic semiconductor with direct bandgap of 513.6 meV and magnetic transform temperatures of as high as 378 K. Finally, it is predicted that the magnetic properties can be further modulated by exerting external strains. Our results may provide new clues for searching and designing two-dimensional magnetic materials.

Development of analytical prediction method for designing organic ferromagnets with heteroatoms

It is still very tough to predict the high-spin stability of conjugated organic radicals to design organic ferromagnets as there is difficulty in calculating the accurate energy of the open-shell low spin state. We have developed an analytical prediction method for designing organic ferromagnets containing heteroatoms without quantum chemical calculations. We have proved that the high-spin stability reflected by the energy difference between the lowest and highest spins is proportional to the high-spin stability index LAP. Therefore, the LAP method is useful in designing organic ferromagnets containing heteroatoms.

Stabilizing spin systems via symmetrically tailored RKKY interactions

Spins of single atoms adsorbed on substrates are promising building blocks for spintronics and quantum computation schemes. To process spin information and for increased magnetic stability, these spins have to be coupled to arrays. For a single atom, a high symmetry of the environment increases its spin stability. However, little is known about the role of the symmetry of the magnetic couplings in the arrays. Here, we study arrays of atomic spins coupled via Ruderman−Kittel−Kasuya−Yosida interaction, focusing on Dzyaloshinskii−Moriya and symmetric anisotropic exchange. We show that the high spin stability of a trimer can be remotely detected by a nearby atom, and how the Dzyaloshinskii−Moriya interaction leads to its destabilization. Adding more nearby atoms further destabilizes the trimer, due to a non-local effective transverse anisotropy originating in the symmetric anisotropic exchange. This transverse anisotropy can be quenched for highly symmetric structures, where the spin lifetime of the array increases drastically.

Computational Study of Cu–Containing Artificial DNA: Twist Angle Dependence of Magnetism

AbstractThe magnetic properties of a copper‐containing artificial DNA (a 2,6‐dipicolinate−Cu(II)−pyridine complex‐based artificial DNA) were theoretically investigated at the ab initio molecular orbital (MO) theory level from the standpoint of the relationship between the twist angle about the DNA chain axis and its high‐spin (HS) stability. It was found that HS stability gradually increased with increasing twist angle at the Hartree‐Fock level. Consideration of the electron correlation effects using a perturbation theory emphasized on the twist angle dependence of HS stability (leading to an increase in the magnetic contrast for the twist angle). Detailed analyses based on MOs revealed that the trend in HS stability is related to the twist angle dependence of the delocalization of singly occupied MOs for the highest spin state and that of the highest doubly occupied MO for the lowest spin state.

Development of molecular fragment interaction method for designing organic ferromagnets

The design and preparation of organic ferromagnets are important and challenging from both experimental and theoretical viewpoints. We propose a molecular fragment interaction method for predicting the high-spin stability of conjugated hydrocarbon radicals without quantum chemistry calculations, which comprise three steps: (1) division of the system into fragments and numbering of the carbon atoms, (2) construction of a secular equation, and (3) solving of the secular equation. If there are two or more nonbonding molecular orbitals (NBMOs) and the smallest number of bonds between two carbon atoms with unpaired electrons is an even number, then the conjugated hydrocarbon radical has a high-spin ground state. This method can also be used to compare the high-spin stability between different conjugated hydrocarbon radicals. If a conjugated hydrocarbon radical has a high-spin ground state, then its derivative has a high-spin ground state as long as the degeneracy of the NBMOs is kept. This method is useful for the rapid design of organic ferromagnets.

Development of minimized mixing molecular orbital method for designing organic ferromagnets.

Predicting the high spin stability of organic radicals correctly for designing organic ferromagnets remains a significant challenge. We have developed a method with an index (L(min)) for predicting the high spin stability of conjugated organic radicals at the restricted open-shell Hartree-Fock level. Unitary transformations were performed for localizing the coefficients of nonbonding molecular orbitals, and subsequently the localized coefficients were used to calculate L(min) that indicates the high spin stability of conjugated organic radicals. This method can be combined with the elongation method to treat huge high spin open-shell systems. Thus, this method is useful for designing organic ferromagnets.

An Analytical Approach to Predict High-spin Stability of Conjugated Hydrocarbon Radical Polymers using Minimized Mixing Nonbonding Molecular Orbitals

A method of analytical prediction (AP) and the corresponding index LAP used for predicting high-spin stability of alternant conjugated hydrocarbon radical polymers were generalized for both non-disjoint closed-type and open-type systems. The general formulas of LAP were derived analytically as the function of the number of unpaired electrons, denoted as N, without performing any localization procedure to attain the minimum mixing between nonbonding molecular orbitals (NBMOs). To examine the reliability of this analytical treatment, the relationship between the LAP and the energy stability in high-spin state was investigated by ab initio method for open-shell systems. It was shown that this approach can be a useful tool for qualitative prediction of ferromagnetism in alternant conjugated hydrocarbon radical polymers. Keywords: Alternant conjugated hydrocarbon, Analytical prediction, Disjoint system, Ferromagnetism, High-spin, Hückel method, LMO, Localization, Molecular orbital method, NBMO method, Non-disjoint system, Polyradical, Quantum chemistry, Radical polymer, ROHF, Unpaired electrons

High-spin stability of urea dication

High-spin stability of urea dication is analyzed as a trimethylenemethane analogue. MCSCF calculations revealed that the ground-state 3B 2 lies below any other singlet state. The lowest singlet state 1B 1 with a π– n configuration lies 9.74 kcal/mol above the 3B 2 state, and a closed-shell singlet 1A 1 lies far above the 3B 2 state. The high-spin stability is due to pseudo non-bonding molecular orbitals and one-centered exchange integrals on cationic nitrogens. Band structure of polyuret is also presented as a design for novel ferromagnetic polymers.

Ab Initio MO Analysis of Interaction Paths between Radicals in Ferromagnetic Organic Systems

Interaction path analyses for pi-conjugated organic systems were performed at the ab initio molecular orbital level to examine the relationship between inter-radical interactions and the high-spin stability of the system. It was found that the high-spin stability results from through-bond interactions between radicals, not from through-space interactions, in relation to the stabilization of a low-spin state due to the effects of electron correlation. L(ij)(min) value for estimating the mixing of nonbonding molecular orbitals well predicted the relationship between the through-bond interactions and the high-spin stability. Furthermore, molecular orbital calculations revealed that the all-trans type interaction path between radicals produces long-range exchange interactions, and the additivity of high-spin stability is observed by keeping short-range through-bond interaction paths.

Analytical Method for Predicting Ferromagnetic Properties of Benzyl-Radical Polymers Based on NBMO Theory.

It has been demonstrated that the high-spin stability of benzyl radical oligomers can be predicted without any quantum chemical calculations. This method is composed of three steps: (1) predicting the shapes of nonbonding molecular orbitals (NBMOs), (2) counting up the mixings of NBMOs, and (3) formulating the mixings for the number of radical centers (N). This treatment enables us to predict the high-spin stability evaluated by ab initio MO calculations involving the post-Hartree-Fock method or the density functional theory (DFT) method.

High-Spin Stability of Triiminiummethane Ion -A Comparative Study with Trimethylenemethane-

High-spin stability of non-Kekule-type molecule triiminiummethane ion (TIM3+) is discussed using the semiempirical molecular orbital method. The triplet ground state A2’ with a D3h geometry is predicted to lie below the Jahn-Teller distorted singlet states similar to trimethylenemethane (TMM). The lowest singlet states B1, B2 and A1 with C2v geometries are nearly degenerate. The spin-density distribution of the A2’ state is calculated and the origin of the spin alignment is discussed relating to the spin-polarization rule characteristic of high-spin organic molecules. The valence bond analysis on the spin states of TIM3+ is also carried out.

A computational study of the Dougherty model for the prediction of high-spin states in organic chemistry

The model proposed by Dougherty for the design of high-spin organic systems has been studied from a quantitative point of view using a Heisenberg Hamiltonian formalism. This analysis leads to a decomposition of the phenomenological coupling parameter, J, into contributions from individual active orbital sites and a decomposition of the spin multiplicity into terms from the ferromagnetic coupling unit and the spin-containing units. An analysis of the origin of quintet stability has been carried out for four molecular systems with quintet ground states that have previously been synthesized by Dougherty and by Adam. The results indicate that the ferromagnetic coupling unit plays the dominant role in determining high-spin stability as suggested by Dougherty and gives some insights that may be useful in the rational design of high-spin systems.

On the Validity of the McConnell-I Model of Ferromagnetic Interactions: The [2.2]Paracyclophane Example

A formal comparison between a rigorous implementation of a Heisenberg Hamiltonian model in a VB space with the McConnell-I model shows that the validity of the McConnell model rests upon a heuristic one-to-one correspondence between spin density products in the latter with the difference of the spin exchange density matrix elements in the former. Using a rigorous Heisenberg Hamiltonian, a numerical model computation of the singlet/quintet stability for pseudoortho, pseudometa, and pseudopara bis(phenylmethylenyl)[2.2]paracyclophanes (modeled with the correponding singlet/triplet bis(methyl)[2.2]paracyclophanes) shows that the McConnell model makes the correct prediction of low and high spin stability only because the contributions from “closest contact” carbon atoms that are not directly aligned is rather small. In systems where the alignment is not perfect, this cancelation may not hold. The association between spin density products in the McConnell model with the difference of the spin exchange density ...