Crystal Field Interaction Research Articles

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Tuning Magnetic Relaxation in Square-Pyramidal Dysprosium Single-Molecule Magnets Using Apical Alkoxide Ligands

Mixed Spin-1/2 and Spin-3/2 Blume-Capel Ising Ferromagnetic System in a Longitudinal Magnetic Field

By means of the recursion relations method, the exact formulations of magnetizations for a mixed spin-1/2 and spin-3/2 Blume-Capel Ising ferromagnetic model on Bethe lattice in a longitudinal magnetic field are presented. The exact expressions for sublattice magnetizations and average magnetization per site are obtained, respectively, and the temperature dependence of the magnetizations for the Bethe lattice with coordination number 4 is numerically studied. The effects of the longitudinal field and crystal-field interaction on the magnetizations are also studied. It is found that the sublattice magnetizations and average one have two different saturation values in the case of low temperature, and decrease from three different saturation values to the same constant as D/J decreases in the case of high temperature.

Ultrafast fs coherent excitonic dynamics in CdSe quantum dots assemblies addressed and probed by 2D electronic spectroscopy.

We show in a joint experimental and theoretical study that ultrafast femto-second (fs) electronic coherences can be characterized in semi-conducting colloidal quantum dot (QD) assemblies at room temperature. The dynamics of the electronic response of ensembles of CdSe QDs in the solution and of QD dimers in the solid state is probed by a sequence of 3 fs laser pulses as in two-dimensional (2D) electronic spectroscopy. The quantum dynamics is computed using an excitonic model Hamiltonian based on the effective mass approximation. The Hamiltonian includes the Coulomb, spin-orbit, and crystal field interactions that give rise to the fine structure splittings. In the dimers studied, the interdot distance is sufficiently small to allow for an efficient interdot coupling and delocalization of the excitons over the two QDs of the dimer. To account for the inherent few percent size dispersion of colloidal QDs, the optical response is modeled by averaging over an ensemble of 2000 dimers. The size dispersion is responsible for an inhomogeneous broadening that limits the lifetimes of the excitonic coherences that can be probed to about 150 fs-200 fs. Simulations and experimental measurements in the solid state and in the solution demonstrate that during that time scale, a very rich electronic coherent dynamics takes place that involves several types of intradot and interdot (in the case of dimers) coherences. These electronic coherences exhibit a wide range of beating periods and provide a versatile basis for a quantum information processing device on a fs time scale at room temperature.

Determination of Crystal-Field Splitting Induced by Thermal Oxidation of Titanium.

The electronic structure of transition-metal oxides is a key component responsible for material's optical and chemical properties. Specifically for metal-oxide structures, the crystal-field interaction determines the shape, strength, and occupancy of electronic orbitals. Consequently, the crystal-field splitting and resulting unoccupied state populations can be foreseen as modeling factors of the photochemical activity. Herein, we study the formation of crystal-field effects during thermal oxidation of titanium in an ambient atmosphere and range of temperatures. The X-ray absorption spectroscopy is employed for quantitative analysis of average t2g-eg crystal-field splitting (Δoct) and relative t2g/eg bands occupancy. The obtained result shows that Δoct changes as a function of temperature from 1.97 eV for a passive oxide layer created on a Ti metal surface at room temperature to 2.41 eV at 600 °C when the material changes into the TiO2 rutile phase. On the basis of XAS data analysis, we show that the Δoct values determined from L2 and L3 absorption edges are equal, indicating that the 2p1/2 and 2p3/2 core holes screen the t2g and eg electronic states in a similar manner.

Dynamic phase of transitions of the mixed spin (1/2, 3/2) Ising model in the presence of a time-varying magnetic field by using the path probability method

Dynamic phase transitions (DPTs) of the mixed spin (1/2, 3/2) Ising ferrimagnetic model on the two interpenetrating square lattice with the bilinear and crystal-field interactions under a time-varying (sinusoidal) magnetic field were examined by using the path probability method (PPM). We studied stationary solutions of dynamic average magnetizations and obtained phases and then we investigated thermal behaviors of dynamic magnetizations to designate the nature of the DPTs and find the DPT temperatures. We also constructed the dynamic phase diagrams (DPDs) in two different planes. DPDs exhibit the paramagnetic (p), ferrimagnetic (i), and mixed phases (i+p), and one or two dynamic tricritical points and dynamic double critical end points. The reentrant behavior is also seen. We observed that the PPM is a more appropriate dynamic method to study the dynamic features of ferrimagnetism.

The thermal properties of the mixed spin-1/2, 1, 3/2 Ising model on the Bethe lattice

A triple mixed-spin Ising system defined on the Bethe lattice is numerically investigated by means of exact recursion relations (ERRs) calculations. The lattice is constituted by three types of magnetic atoms A, B, C with spins [Formula: see text], [Formula: see text], [Formula: see text] respectively arranged in the form ABCABC. The effects of bilinear exchange and crystal-field interactions as well as those of thermal fluctuations on the order parameters and phase diagrams are thoroughly studied and specified. First-order transitions and tricritical points are present for the coordination number [Formula: see text] whereas at [Formula: see text] they are absent. Global compensation phenomena are absent for the magnetic system. Instead, it is shown that it can only occur between the sublattice magnetizations B and C of the system. Several novel kinds of reentrance of the phase boundaries while varying the values of model parameters have been reported.

Specific Heat of Holmium in Gold and Silver at Low Temperatures

The specific heat of dilute alloys of holmium in gold and in silver plays a major role in the optimization of low temperature microcalorimeters with enclosed ^{163}{{text {Ho}}}, such as the ones developed for the neutrino mass experiment ECHo. We investigate alloys with atomic concentrations of x_{{{text {Ho}}}}=0.01{-}4% at temperatures between 10 and 800,{{{hbox {mK}}}}. Due to the large total angular momentum J=8 and nuclear spin I=7/2 of {{text {Ho}}}^{3+} ions, the specific heat of Au:Ho and Ag:Ho depends on the detailed interplay of various interactions, including contributions from the localized 4f electrons and nuclear contributions via hyperfine splitting. This makes it difficult to accurately determine the specific heat of these materials numerically. Instead, we measure their specific heat by using three experimental setups optimized for different concentration and temperature ranges. The results from measurements on six holmium alloys demonstrate that the specific heat of these materials is dominated by a large Schottky anomaly with its maximum at Tapprox 250,{{{hbox {mK}}}}, which we attribute to hyperfine splitting and crystal field interactions. RKKY and dipole–dipole interactions between the holmium atoms cause additional, concentration-dependent effects. With regard to ECHo, we conclude that for typical operating temperatures of Tle 20,{{{hbox {mK}}}}, silver holmium alloys with x_{{{text {Ho}}}}gtrsim 1% are suited best.

Compensation and Hysteresis Behaviors in a Heisenberg Ferrimagnetic Mixed Spin (½, 1) System

The hysteresis loops and compensation behaviors of a ferrimagnetic mixed system, consisting of two sublattices (A and B) with different spins (SA = 1 and SB = ½) and described by Heisenberg Hamiltonian on a cubic lattice, are investigated by using the pair approximation method. We have discussed the dependence of the compensation temperature on the D- crystal field interaction and J-exchange anisotropy. The influence of the longitudinal magnetic field and the magnetic hysteresis loops are also carried out. It is found that the model of mixed spin exhibits the possibility of two compensation points for several given sets of parameters. The model yields also second-order and first-order phase transitions, in addition to tricritical points. The reentrant phenomena can occur in some narrow ranges of D /|Jz| and Jxy /|Jz |. We have also observed the existence of triple hysteresis loop behaviors under the effects of special intrinsic parameters.

Wasp – Waisted loop and spin frustration in Dy2−xEuxTi2O7 pyrochlore

The Raman spectroscopy and AC and DC magnetization of Dy2−xEuxTi2O7 pyrochlore materials have been investigated. A systematic shift in all phonon modes with higher Eu content in Dy2−xEuxTi2O7 observed in Raman spectra confirms the successful substitution of Dy3+ ion by Eu3+ ions. High concentration of Eu induces the exchange interactions between atoms and alters the crystal-field interactions in Dy2−xEuxTi2O7. Rich Eu content samples (x = 1.5, 1.8 and 1.9) show the existence of wasp-waisted hysteresis loop which is explained on the basis of dipolar field, anisotropy exchange interaction and spin frustration present in the samples. Interestingly, AC susceptibility shows two single ion spin freezing transitions corresponding to Dy3+ and Eu3+ ions respectively in Eu rich samples only.

Dynamic magnetic hysteresis loop features of molecular-based magnetic materials and ferrimagnetic systems by the path probability method

Dynamic magnetic hysteresis loops (DMHL) features of molecular-based magnetic materials and ferrimagnetic systems are investigated by a mixed spin (1/2, 1) model by means of the path probability method (PPM). The PPM contains two rate constants; hence the DMHL features have been studied by using the method with two rate constants. In particular, the effects of the repulsive (J<0) and attractive (J>0) bilinear nearest-neighbor pair interactions, temperature (T), crystal-field interaction (D), angular frequency (ω) of the oscillating magnetic field and especially the rate constant (k2), which corresponds to the wheel speed in the melt spinning technique (MST), on the DMHL are investigated. The shapes and areas of hysteresis loops are affective sensibility by these system parameters. We also investigate the magnetic coercive fields and remanents magnetizations as a function ω. We found that the most of our results are in a good agreement with some theoretical and experimental reported works. We also observe the novel and interesting DMHL behaviors for lower values of T, and higher values of ω, k2 and D.

Ground States Phase Diagrams and Magnetizations Properties of Ferrimagnetic Model on Decorated Hexagonal Nanolattice: Monte Carlo Study

The ground states phase diagrams and magnetizations properties of ferrimagnetic model on decorated hexagonal nanolattice have been studied via Monte Carlo simulations. The shape of the phase diagrams depends of the values of the magnetic parameters given in the Hamiltonian. The phase diagrams show some key features: coexistence between regions, points where two, three, four and five and states can coexist. This richness of the ground state phase diagrams can serve as a guide for exploring interesting regions of the model’s finite temperature phase diagrams. The transition temperatures have been deduced for different values of exchange interactions between the atoms of spins moments(S). The effect of crystal field and exchange interactions on magnetization has been investigated. The magnetic coercive field and remanent magnetization are obtained for different parameters such as: exchange couplings and temperatures values. The superparamagnetic behavior has been found at the transition temperature.

Large enhancement of magnetocaloric effect driven by hydrostatic pressure in HoCuSi compound

Dual field driven caloric effect has aroused great interest in recent years. Hydrostatic pressure can shift the temperature zone of magnetocaloric effect(MCE), but seldom enhance its magnitude. Here, a large enhancement of MCE driven by pressure under magnetic field available by permanent magnet has been achieved in HoCuSi compound without introducing hysteresis loss, which is favorable for developing hybrid field driven refrigeration applications at low temperature regions. A distinct mechanism is revealed. Pressure largely regulates non-collinear spin structure and magnetizing process through affecting crystal field interactions and spin-spin coupling, hence resulting in the significant impact on magnetic properties and MCE.

Perovskite titanates at the nanoscale: Tunable luminescence by energy transfer and enhanced emission with Li+ co-doping

Three series of highly luminescent Ca1-xTiO3:xDy3+, Ca0.98-yDy0.02TiO3:yEu3+ and Ca0.91-zDy0.02Eu0.07TiO3:zLi+ nanophosphors have been synthesized by solution combustion method. X-ray diffraction analysis and Rietveld refinements of these phosphors confirms the pnma (62) structure and the occupancy of substituent ions in the matrix. Increase in intensity of the Raman modes viz., O–Ti–O bending mode (249 ​cm−1) and local vibrational mode (800 ​cm−1) signifies the occupancy of rare earth ions in the lattice. XPS analysis verifies the change in coordination environment around the RE3+ ions due to the variation in electron density by the observed shift in binding energy corresponding to the chemical states of Dy 3d and Eu 3d when Li+ ions are codoped. Ca1-xTiO3:xDy3+ exhibits a luminescence close to standard white and tuning of correlated color temperature is achieved by varying the excitation wavelength. In Ca0.98-yDy0.02TiO3:yEu3+ phosphors, co-doping of Eu3+ enables red component in white emission and at higher concentration of Eu3+, emission changes from orange red to purplish red, when wavelength of excitation is altered from 366 to 388 ​nm by virtue of energy transfer mechanism. An enhancement of luminescence intensity by 2.1 times has been observed due to Li+ substitution at optimum concentration in Ca0.91-zDy0.02Eu0.07TiO3:zLi+. This evidences the role of Li+ ions in modifying the crystal field interaction in perovskite luminescent materials.

Crystal-field approach to rare-earth higher borides: Dimerization, thermal, and magnetic properties ofRB50 (R=Tb,Dy,Ho,Er,Tm)

The microscopic justification for the dimerization in ladder-type rare-earth higher borides $R{\mathrm{B}}_{50}$ assumed by Mori [in Handbook on the Physics and Chemistry of Rare Earths, edited by K. A. Gschneidner, Jr., J.-C. G. B\unzli, and V. K. Pecharsky (North-Holland, Amsterdam, 2008), Vol. 38, p. 105] is presented. We consider a phenomenological model of crystal-field interactions and introduce Heisenberg antiferromagnetic exchange interactions between the rare-earth ions on the ladder rungs and the indirect exchange fields generated by magnetic interactions along the ladder legs. The crystal-field parameters and the exchange integrals for the rare-earth dimers are determined from modeling the temperature dependences of the heat capacity and static magnetic susceptibility. The field dependence of the low-temperature magnetization of polycrystalline samples of borides $R{\mathrm{B}}_{50}$ is simulated by making use of the proposed empirical expression for the renormalized magnetic moments of rare-earth ions. The characteristic anomalies of the thermal and magnetic properties of $\mathrm{Tb}{\mathrm{B}}_{50}, \mathrm{Dy}{\mathrm{B}}_{50}, \mathrm{Ho}{\mathrm{B}}_{50}$ and $\mathrm{Er}{\mathrm{B}}_{50}$ are successfully reproduced within the framework of this approach.

Dynamic magnetic hysteresis loop behaviors of a mixed spin (2, 5/2) Ising model on two interpenetrating square lattices

We investigate dynamic magnetic hysteresis loop (DMHL) behaviors of a mixed spin (2, 5/2) Ising model on two interpenetrating square lattices under an oscillating magnetic field within the effective-field theory based on Glauber–type stochastic dynamics. We study the DMHL properties for various values of reduced temperatures (T/zJ), crystal-field interaction (D/zJ) and frequency () of the magnetic field. We also examine T/zJ, D/zJ and dependences of the coercive fields (CFs) and remanent magnetizations (RMs). We found that for very low and high values of T/zJ and D/zJ, the hysteresis loop area is narrower and thinner that correspond soft magnets that desirable for transformers and motor cores and AC applications. We observed that for high values of the hysteresis loop areas are wide and big that corresponds hard magnets which can be useful for permanent magnets, magnetic recording, small motors and magnetic locks. We also found that the DMHLs, CFs and RMs behaviors are in quantitatively good agreement with some theoretical for Ising systems and experimental works for magnetic materials, such as cobalt films, Fe films and some magnetic nanomaterials.

Analysis of crystal field effects and interactions using X-ray restrained ELMOs

The purpose of this study is to assess the capability of X-ray restrained molecular wavefunction methods to reproduce the perturbation due to the crystal field.The effect of the crystal environment on the molecular charge densities has been thoroughly investigated experimentally and theoretically. However, the same analysis in terms of molecular orbitals is unprecedented. We specifically analysed the extremely localized molecular orbitals (ELMOs) because they allow for a valence bond like interpretation that is closer to the typical chemical reasoning.For a set of test molecules, we calculated first principles wavefunctions (called primary wavefunctions) and the corresponding scattered intensities (primary structure factors), which we afterwards used for X-ray restrained Hartree-Fock and ELMO calculations. This mimics the typical procedure of modelling wavefunctions from experimental X-ray diffraction, having the primary wavefunction as benchmark.This study follows a previous investigation concerning the effects of electron correlation, where it was shown that those subtle features could be retrieved only to a minor extent. The electric field generated by surrounding molecules is instead more substantial. Therefore, retrieving this information should be more successful and, in case, extremely useful for supramolecular chemistry and crystal engineering. The work also provides useful indications to experimentalists who want to exploit the potentialities offered by the X-ray restrained wavefunction methods.

Possible absence of trimeron correlations above the Verwey temperature in Fe3O4

The origin of the metal to insulator transition in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ remains a challenge due to the complexity of the system: it is a mixed valent, strongly correlated system where many interactions such as Jahn-Teller distortion, exchange, and phonons are very close in energetics. A recent interpretation of the Verwey transition as an ordering of a three-site magnetic polaron, the trimeron, has been put forward. Here we investigate the existence of the trimeron correlations in the high-temperature phase of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ using high-resolution iron $2p3d$ resonant inelastic scattering magnetic linear dichroism. Guided by theoretical simulations, we reveal that the polarization dependence of the low-energy spin-orbital excitations is incompatible with tetragonal Jahn-Teller trimeron-type distortion. We conclude that the lowest-energy state of the high-temperature phase of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ arises from an intricate interplay between trigonal crystal-field, exchange, and spin-orbit interactions.

Renormalization Group Study of the Mixed Spin-1 and Spin-3/2 Blume-Emery-Griffiths Model with Attractive Biquadratic Coupling

The mixed spin-1 and spin-3/2 Blume-Emery-Griffiths model with attractive biquadratic coupling is investigated in the framework of the Migdal-Kadanoff Renormalization Group method. By changing the ratio R > 0 of biquadratic and bilinear exchange interactions and according to the different values of crystal field interactions, we have determined six main types of phase diagrams. The full flow in the parameters space of the Hamiltonian was established and the fixed points obtained are drawn up in a table. In addition, we have determined the eigenvalues of the transformation of the group in the vicinity of the critical points. Finally, the introduction of a positive biquadratic interaction was discussed.

Multiband k·p Model for Tetragonal Crystals: Application to Hybrid Halide Perovskite Nanocrystals.

We investigate the theoretical band structure of organic-inorganic perovskites APbX3 with tetragonal crystal structure. Using D4h point group symmetry properties, we derive a general 16-band Hamiltonian describing the electronic band diagram in the vicinity of the wave-vector point corresponding to the direct band gap. For bulk crystals, a very good agreement between our predictions and experimental physical parameters, as band gap energies and effective carrier masses, is obtained. Extending this description to three-dimensional confined hybrid halide perovskite, we calculate the size dependence of the excitonic radiative lifetime and fine structure. We describe the exciton fine structure of cube-shaped nanocrystals by an interplay of crystal-field and electron-hole exchange interaction (short- and long-range parts) enhanced by confinement. Using very recent experimental results on FAPbBr3 nanocrystals, we extract the bulk short-range exchange interaction in this material and predict its value in other hybrid compounds. Finally, we also predict the bright-bright and bright-dark splittings as a function of nanocrystal size.

Simulation of an extended 3D mixed Ising model by Monte Carlo method

Owing to their useful properties, the ferromagnetic mixed spins Ising have been extensively investigated. In this context, the paper aims to study the phase diagram of 3d mixed spin-1 and spin-3/2 Ising system and its critical properties. The adapted Hamiltonian of the considered mixed spin-(1, 3/2) Ising model is parameterized by three exchange interaction between first nearest neighbors spins Ji (with i = 1, 2 and 3), two crystal field interactions and the external magnetic field. The competition between these parameters can generate rich critical behavior with the coexistence of several phases in the ground state phase diagram.In this work, Monte Carlo simulation based on Metropolis algorithm, was used to study the critical properties in different phases of ground state. The analysis of the behavior of total magnetization and susceptibility as a function of temperature allowed us to study the behavior of the critical temperature of the ferromagnetic – paramagnetic transition TC as a function of different parameters. The obtained results shows that the critical temperature TC has linear Ji dependency in strong spin–spin interaction regime.