arXiv subjects

Kyungwha Park

Publications and source records attributed to Kyungwha Park.

At least 55 records · Page 3Linked to original sources

arXiv2010

Effects of bonding type and interface geometry on coherent transport through the single-molecule magnet Mn12

We examine theoretically coherent electron transport through the single-molecule magnet Mn$_{12}$, bridged between Au(111) electrodes, using the non-equilibrium Green's function method and the density-functional theory. We analyze the effects of bonding type, molecular orientation, and geometry relaxation on the electronic properties and charge and spin transport across the single-molecule junction. We consider nine interface geometries leading to five bonding mechanisms and two molecular orientations: (i) Au-C bonding, (ii) Au-Au bonding, (iii) Au-S bonding, (iv) Au-H bonding, and (v) physisorption via van der Waals forces. The two molecular orientations of Mn$_{12}$ correspond to the magnetic easy axis of the molecule aligned perpendicular [hereafter denoted as orientation (1)] or parallel [orientation (2)] to the direction of electron transport. We find that the electron transport is carried by the lowest unoccupied molecular orbital (LUMO) level in all the cases that we have simulated. Relaxation of the junction geometries mainly shifts the relevant occupied molecular levels toward the Fermi energy as well as slightly reduces the broadening of the LUMO level. As a result, the current slightly decreases at low bias voltage. Our calculations also show that placing the molecule in the orientation (1) broadens the LUMO level much more than in the orientation (2), due to the internal structure of the Mn$_{12}$. Consequently, junctions with the former orientation yield a higher current than those with the latter. Among all of the bonding types considered, the Au-C bonding gives rise to the highest current (about one order of magnitude higher than the Au-S bonding), for a given distance between the electrodes. The current through the junction with other bonding types decreases in the order of Au-Au, Au-S, and Au-H. Importantly, the spin-filtering effect in all the nine geometries stays robust and their ratios of the majority-spin to the minority-spin transmission coefficients are in the range of 10$^3$ to 10$^8$. The general trend in transport among the different bonding types and molecular orientations obtained from this study may be applied to other single-molecular magnets.

cond-mat.mes-hall↗

arXiv2009

First-principles study of electron transport through the single-molecule magnet Mn12

We examine electron transport through a single-molecule magnet Mn12 bridged between Au electrodes using the first-principles method. We find crucial features which were inaccessible in model Hamiltonian studies: spin filtering and a strong dependence of charge distribution on local environments. The spin filtering remains robust with different molecular geometries and interfaces, and strong electron correlations, while the charge distribution over the Mn12 strongly depends on them. We point out a qualitative difference between locally charged and free-electron charged Mn12.

cond-mat.mtrl-sci↗

arXiv2009

Spin-filtering effect in the transport through a single-molecule magnet Mn$_{12}$ bridged between metallic electrodes

Electronic transport through a single-molecule magnet Mn$_{12}$ in a two-terminal set up is calculated using the non-equilibrium Green's function method in conjunction with density-functional theory. A single-molecule magnet Mn$_{12}$ is bridged between Au(111) electrodes via thiol group and alkane chains such that its magnetic easy axis is normal to the transport direction. A computed spin-polarized transmission coefficient in zero-bias reveals that resonant tunneling near the Fermi level occurs through some molecular orbitals of majority spin only. Thus, for low bias voltages, a spin-filtering effect such as only one spin component contributing to the conductance, is expected. This effect would persist even with inclusion of additional electron correlations.

cond-mat.mtrl-sci↗

arXiv2007

Transition rates for a S \ge 1 model coupled to a phonon bath

We investigate transition rates between different spin configurations for $S \ge 1$ spins weakly coupled to a $d$-dimensional phonon bath. This study is motivated by understanding observed magnetization relaxation as a function of temperature in diverse magnetic systems such as arrays of magnetic nanoparticles and magnetic molecules. We assume that the magnetization of the spin system relaxes through consecutive emission or absorption of a single phonon. From a weak, linear spin-phonon coupling Hamiltonian, we derive transition rates that would be used to examine dynamic properties of the system in kinetic Monte Carlo simulations. Although the derived phonon-assisted transition rates satisfy detailed balance, in the case of two and three dimensional phonon baths, transitions between degenerate states are not allowed. Thus, if there are no alternative paths along which the spin system can relax, the relaxation time diverges. Otherwise, the system finds other paths, which leads to an increase in the relaxation time and energy barrier. However, when higher-order phonon processes are included in the transition rates, it is found that the system can reach the states which were inaccessible due to the forbidden transitions. As a result, the system recovers some of the dynamic properties obtained using the Glauber transition rate.

cond-mat.mtrl-sci↗

arXiv2007

First-principles study of a single-molecule magnet Mn_{12} monolayer on the Au(111) surface

The electronic structure of a monolayer of single-molecule magnets Mn$_{12}$ on a Au(111) surface is studied using spin-polarized density-functional theory. The Mn$_{12}$ molecules are oriented such that the magnetic easy axis is normal to the surface, and the terminating ligands in the Mn$_{12}$ are replaced by thiol groups (-SH) where the H atoms are lost upon adsorption onto the surface. This sulfur-terminated Mn$_{12}$ molecule has a total magnetic moment of 18 $μ_B$ in the ground state, in contrast to 20$μ_B$ for the standard Mn$_{12}$. The Mn$_{12}$ molecular orbitals broaden due to the interaction of the molecule with the gold surface and the broadening is of the order of 0.1 eV. It is an order of magnitude less than the single-electron charging energy of the molecule so the molecule is weakly bonded to the surface. Only electrons with majority spin can be transferred from the surface to the sulfur-terminated Mn$_{12}$ since the gold Fermi level is well above the majority lowest unoccupied molecular orbital (LUMO) but below the minority LUMO. The amount of the charge transfer is calculated to be 1.23 electrons, dominated by the tail in the electronic distribution of the gold surface. A calculation of level shift upon charging provides 0.28 electrons being transferred. The majority of the charge transfer occurs at the S, C, and O atoms close to the surface. The total magnetic moment also changes from 18 $μ_B$ to 20 $μ_B$, due to rearrangements of the magnetic moments on the S and Mn atoms upon adsorption onto the surface. The magnetic anisotropy barrier is computed including spin-orbit interaction self-consistently in density-functional theory. The barrier for the Mn$_{12}$ on the gold surface decreases by 6 K in comparison to that for an isolated Mn$_{12}$ molecule.

cond-mat.mtrl-sci↗

arXiv2004

Incommensurate Transverse Anisotropy Induced by Disorder and Spin-Orbit-Vibron Coupling in Mn12-acetate

It has been shown within density-functional theory that in Mn$_{12}$-acetate there are effects due to disorder by solvent molecules and a coupling between vibrational and electronic degrees of freedom. We calculate the in-plane principal axes of the second-order anisotropy caused by the second effect and compare them with those of the fourth-order anisotropy due to the first effect. We find that the two types of the principal axes are not commensurate with each other, which results in a complete quenching of the tunnel-splitting oscillation as a function of an applied transverse field.

cond-mat.mtrl-sci↗

arXiv2004

Electronic structure and magnetic anisotropy for nickel-based molecular magnets

Recent magnetic measurements on tetra-nickel molecular magnets [Ni(hmp)(ROH)Cl]$_4$, where R=CH$_3$, CH$_2$CH$_3$, or (CH$_2$)$_2$C(CH$_3$)$_3$ and hmp$^-$ is the monoanion of 2-hydroxymethylpyridine, revealed a strong exchange bias prior to the external magnetic field reversal as well as anomalies in electron paramagnetic resonance peaks at low temperatures. To understand the exchange bias and observed anomalies, we calculate the electronic structure and magnetic properties for the Ni$_4$ molecules with the three different ligands, employing density-functional theory. Considering the optimized structure with possible collinear spin configurations, we determine a total spin of the lowest-energy state to be S=0, which does not agree with experiment. We also calculate magnetic anisotropy barriers for all three types of Ni$_4$ molecules to be in the range of 4-6 K.

cond-mat.mtrl-sci↗

arXiv2004

Low-temperature nucleation in a kinetic Ising model under different stochastic dynamics with local energy barriers

Using both analytical and simulational methods, we study low-temperature nucleation rates in kinetic Ising lattice-gas models that evolve under two different Arrhenius dynamics that interpose between the Ising states a transition state representing a local energy barrier. The two dynamics are the transition-state approximation [T. Ala-Nissila, J. Kjoll, and S. C. Ying, Phys. Rev. B 46, 846 (1992)] and the one-step dynamic [H. C. Kang and W. H. Weinberg, J. Chem. Phys. 90, 2824 (1989)]. Even though they both obey detailed balance and are here applied to a situation that does not conserve the order parameter, we find significant differences between the nucleation rates observed with the two dynamics, and between them and the standard Glauber dynamic [R. J. Glauber, J. Math. Phys. 4, 294 (1963)], which does not contain transition states. Our results show that great care must be exercised when devising kinetic Monte Carlo transition rates for specific physical or chemical systems.

cond-mat.stat-mech↗

arXiv2004

Effect of extra electrons on the exchange and magnetic anisotropy in the anionic single-molecule magnet Mn12

To understand the effect of molecular environment on the electronic and magnetic properties of the single-molecule magnet (SMM) Mn$_{12}$, we explore two possible means for adding extra electrons to molecule. We explore both substitution of Mn ions by Fe ions and the inclusion of neighboring electronic donors. For both possibilities we calculate, within density-functional theory, the electronic structure, the total ground-state spin and ordering, the magnetic anisotropy barrier, the transverse magnetic anisotropy parameter $E$ which is responsible for some measured tunneling, and the tilting angle of the easy axis from the z axis. Our calculations show that the total spin increases with increasing number of extra electrons except for the case of Mn$_{8}$Fe$_4$ where the resulting ground state has a low spin. The calculated energy gaps between the unoccupied and the occupied orbitals exhibit no clear trend as a function of number of extra electrons. The calculated magnetic anisotropy barrier decreases with increasing number of extra electrons and can be directly chased to a quenching of Jahn-Teller distortions at the sites where the additional electrons are localized. The values of $E$ and the easy-axis tilting angles for the geometries with one- and two-extra electrons are significantly larger than those induced by solvent disorder.

cond-mat.mtrl-sci↗

arXiv2003

Second-order transverse magnetic anisotropy induced by disorders in the single-molecule magnet Mn12

For the single-molecule magnet Mn12, Cornia et al. recently proposed that solvent molecules may cause the quantum tunneling that requires a lower symmetry than S4. However, magnetic quantum tunneling and electron paramagnetic resonance experiments suggested that the proposed theory may not correspond to the measurements. In this regard, we consider positional disorder induced by the solvent molecules and orientational disorder by the methyl groups of a Mn12 molecule. We calculate, within density-functional theory, the second-order transverse magnetic anisotropy parameter E and an easy-axis tilting angle induced by the positional disorder and the E value by the orientational disorder. We also calculate the local magnetic anisotropy and the local easy axis for each inequivalent Mn site in different environments to investigate their effects on the global E value. We find that the hydrogen bonding between a Mn12 molecule and the solvent molecule is crucial to obtain a substantial E value and that the E value increases upon geometry relaxations. Our calculations on relaxed geometries show that the largest calculated E value is 0.016 K and that the largest tilting angle is 0.5 degrees. Our largest E value is comparable to experimental results but larger than Cornia et al.'s.

cond-mat.mtrl-sci↗

arXiv2003

Low-temperature nucleation in a kinetic Ising model with soft stochastic dynamics

We study low-temperature nucleation in kinetic Ising models by analytical and simulational methods, confirming the general result for the average metastable lifetime, = A*exp(beta*Gamma) (beta = 1/kT) [E. Jordao Neves and R.H. Schonmann, Commun. Math. Phys. 137, 209 (1991)]. Contrary to common belief, we find that both A and Gamma depend significantly on the stochastic dynamic. In particular, for a ``soft'' dynamic, in which the effects of the interactions and the applied field factorize in the transition rates, Gamma does NOT simply equal the energy barrier against nucleation, as it does for the standard Glauber dynamic, which does not have this factorization property.

cond-mat.stat-mech↗

arXiv2003

Properties of low-lying excited manifolds in the Mn12 acetate

Most experimental data on the single-molecule magnet Mn$_{12}$ acetate have been successfully explained by the assumption that the Mn$_{12}$ acetate has an effective ground-state spin of S=10. However, the effect of the low-lying excited manifolds caused by interactions between Mn spins has not been well understood. To investigate the features of the low-lying excited manifolds, the intramolecular exchange interactions are calculated using density-functional theory (DFT). With the calculated exchange parameters, the energy gap between the S=10 ground-state and the first excited-state manifold is calculated by diagonalization of the Heisenberg Hamiltonian. The upper limit on the energy gap is about 40.5 K which is likely to be overestimated due to incomplete treatment of the Coulomb potential within DFT. It is found that there are several S=9 low-energy excited-state manifolds above the S=10 ground-state manifold. The magnetic anisotropy barriers for the low-lying spin excitations are calculated using DFT. Based on the calculations, it is found that the anisotropy barriers for the low-lying excited manifolds are approximately the same as that for the ground-state manifold, which is applicable to other single-molecule magnets such as Mn$_4$.

cond-mat.mtrl-sci↗

arXiv2003

Electronic, magnetic, and vibrational properties of the molecular magnet Mn4 monomer and dimer

A new type of the single-molecule magnet [Mn_4 O_3 Cl_4 (O_2 CEt)_3(py)_3] forms dimers. Recent magnetic hysteresis measurements on this single-molecular magnet revealed interesting phenomena: an absence of quantum tunneling at zero magnetic field and tunneling before magnetic field reversal. This is attributed to a significant antiferromagnetic exchange interaction between different monomers. To investigate this system, we calculate the electronic structure, magnetic properties, intramolecular and intermolecular exchange interactions using density-functional theory within the generalized-gradient approximation. Our calculations agree with experiment. We also calculate vibrational infrared absorption and Raman scattering intensities for the monomer which can be tested experimentally.

cond-mat.mtrl-sci↗

arXiv2003

DFT calculation of the intermolecular exchange interaction in the magnetic Mn$_4$ dimer

The dimeric form of the single-molecule magnet [Mn$_4$O$_3$Cl$_4$(O$_2$CEt)$_3$(py)$_3$]$_2$ recently revealed interesting phenomena: no quantum tunneling at zero field and tunneling before magnetic field reversal. This is attributed to substantial antiferromagnetic exchange interaction between different monomers. The intermolecular exchange interaction, electronic structure and magnetic properties of this molecular magnet are calculated using density-functional theory within generalized-gradient approximation. Calculations are in good agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2003

Electron Paramagnetic Resonance Linewidths and Lineshapes for the Molecular Magnets Fe8 and Mn12

We study theoretically Electron Paramagentic Resonance (EPR) linewidths for single crystals of the molecular magnets Fe$_8$ and Mn$_{12}$ as functions of energy eigenstates $M_s$, frequency, and temperature when a magnetic field along the easy axis is swept at fixed excitation frequency. This work was motivated by recent EPR experiments. To calculate the linewidths, we use density-matrix equations, including dipolar interactions and distributions of the uniaxial anisotropy parameter $D$ and the Landé $g$ factor. Our calculated linewidths agree well with the experimental data. We also examine the lineshapes of the EPR spectra due to local rotations of the magnetic anisotropy axes caused by defects in samples. Our preliminary results predict that this effect leads to asymmetry in the EPR spectra.

cond-mat↗

arXiv2003

Effect of Defects on the Line shape of Electron Paramagnetic Resonance Signals from the Single-Molecule Magnet Mn12: A Theoretical Study

We herein estimate the effect of lattice defects on the line shape of electron paramagnetic resonance (EPR) signals from a single crystal of the S=10 single-molecule magnet Mn$_{12}$ with the external magnetic field along the crystal c axis. A second-order perturbation treatment of an effective single-spin Hamiltonian indicates that a small, random, static misorientation of the magnetic symmetry axes in a crystalline lattice can lead to asymmetric EPR peaks. Full spectra are simulated by calculating probability-distribution functions for the resonant fields, employing distributions in the tilt angle of the easy axis from the c axis, in the uniaxial anisotropy parameter, and in the $g$-factor. We discuss conditions under which the asymmetry in the EPR spectra becomes prominent. The direction and magnitude of the asymmetry provide information on the specific energy levels involved with the EPR transition, the EPR frequency, and the distribution in the tilt angle.

cond-mat↗

arXiv2003

Effects of D-strain, g-strain, and dipolar interactions on EPR linewidths of the molecular magnets Fe8 and Mn12

Electron paramagnetic resonance (EPR) measurements on single crystals of the molecular magnets Fe$_8$ and Mn$_{12}$ reveal complex nonlinearities in the linewidths as functions of energy eigenstate, frequency, and temperature. Using a density-matrix equation with distributions of the uniaxial anisotropy parameter $D$, the Landé $g$ factor, and dipolar interactions, we obtain linewidths in good agreement with experiments. Our study shows that the distribution in $D$ is common to the examined molecular magnets Fe$_8$ and Mn$_{12}$ regardless of the qualities of the samples. This could provide the basis for a proposed tunneling mechanism due to lattice defects. The distribution in $g$ is also quite significant for Mn$_{12}$.

cond-mat↗

arXiv2003

Role of dipolar and exchange interactions in the positions and widths of EPR transitions for the single-molecule magnets Fe8 and Mn12

We examine quantitatively the temperature dependence of the linewidths and line shifts in electron paramagnetic resonance experiments on single crystals of the single-molecule magnets Fe$_8$ and Mn$_{12}$, at fixed frequency, with an applied magnetic field along the easy axis. We include inter-molecular spin-spin interactions (dipolar and exchange) and distributions in both the uniaxial anisotropy parameter $D$ and the Landé $g$-factor. The temperature dependence of the linewidths and the line shifts are mainly caused by the spin-spin interactions. For Fe$_8$ and Mn$_{12}$, the temperature dependence of the calculated line shifts and linewidths agrees well with the trends of the experimental data. The linewidths for Fe$_8$ reveal a stronger temperature dependence than those for Mn$_{12}$, because for Mn$_{12}$ a much wider distribution in $D$ overshadows the temperature dependence of the spin-spin interactions. For Fe$_8$, the line-shift analysis suggests two competing interactions: a weak ferromagnetic exchange coupling between neighboring molecules and a longer-ranged dipolar interaction. This result could have implications for ordering in Fe$_8$ at low temperatures.

cond-mat↗