Showing papers on "Exchange interaction published in 2007"
TL;DR: In this article, the structural, magnetic and transport properties of double perovskites with ferromagnetism above room temperature are discussed, and the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity.
Abstract: We review the structural, magnetic and transport properties of double perovskites (A2BB'O6) with ferromagnetism above room temperature. Ferromagnetism in these compounds is explained by an indirect B?O?B'?O?B exchange interaction mediated by itinerant electrons. We first focus on the BB' =?FeMo-based double perovskites, with Sr2FeMoO6 (TC = 420?K) being the most studied compound. These compounds show metallic behaviour and low magnetic coercivity. Afterwards, we will focus on B' = Re compounds, where the significant orbital moment of Re plays a crucial role in the magnetic properties, for example in the large magnetic coercivity and magnetostructural coupling. More specifically, we first discuss the A2FeReO6 series, with maximum TC = 520?K for Ca2FeReO6, which shows a tendency to semiconducting behaviour. Finally, we describe the Sr2(Fe1?xCrx)ReO6 series, with maximum TC = 625?K for Sr2CrReO6, which is the highest TC in an oxide compound without Fe. This compound is metallic. We discuss the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity. In particular, we focus on the large intergrain magnetoresistance effect observed in polycrystalline samples and the possible implementation of these materials as electrodes in magnetic tunnel junctions.
435 citations
TL;DR: The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy and adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.
Abstract: The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 \AA{} we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.
157 citations
TL;DR: In this paper, an ab initio framework for calculating parameters of a high-temperature magnetic Hamiltonian was developed, which includes transverse and longitudinal magnetic excitation spectra on equal footing.
Abstract: We have developed an ab initio framework for calculating parameters of a high-temperature magnetic Hamiltonian. In the adiabatic approximation, this includes transverse and longitudinal magnetic excitation spectra on equal footing. The exchange interaction parameters of the Hamiltonian for bcc Fe and fcc Ni are determined from constrained local spin-density approximation calculations. Finite temperature magnetic properties of the resulting model Hamiltonian are then investigated by a Monte Carlo simulation technique. The calculated Curie temperatures and paramagnetic susceptibilities are in good agreement with experimental data for both metals. We demonstrate that the temperature-induced longitudinal spin fluctuations are important for high temperature properties such as susceptibility and magnetic specific heat.
141 citations
TL;DR: In this paper, the authors analyzed the optical emission of single II-VI quantum dots containing 0 or 1 magnetic atom (manganese) and a controlled number of carriers (0, $\ifmmode\pm\else\textpm\fi{}1$ electron).
Abstract: We analyze the optical emission of single II-VI quantum dots containing 0 or 1 magnetic atom (manganese) and a controlled number of carriers (0, $\ifmmode\pm\else\textpm\fi{}1$ electron). The emission of these quantum dots presents a large degree of linear polarization. This linear polarization is attributed to a valence-band mixing and we show that in nonmagnetic quantum dots combining both a shape anisotropy and an anisotropic in-plane strain distribution, the linear polarization direction of the exciton are controlled by an interplay between valence-band mixing and electron-hole (e-h) exchange interaction. Similarly, under strong transverse magnetic field, the direction of the linearly polarized emission of the charged excitons is simultaneously controlled by the valence-band mixing and the direction of the magnetic field. In quantum dots containing a Mn atom, the valence-band mixing allows simultaneous hole-Mn spin flips coupling bright and dark excitons. These spin flips are responsible for linearly polarized transitions in the emission of the charged excitons at zero magnetic field.
140 citations
TL;DR: In this article, the exchange interaction of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) was employed to explain the ferromagnetism of Fe-doped PbTiO3 nanocrystals.
Abstract: Room-temperature ferromagnetism has been observed in Fe-doped PbTiO3 nanocrystals. The magnetism of the nanocrystals develops from diamagnetism to ferromagnetism and the paramagnetism on increasing nominal Fe doping concentration from 0to4mol%. Transmission electron microscope (TEM) and high-resolution TEM data indicate that Fe-doped PbTiO3 nanocrystals with the size of ∼100nm are organized to form a planarlike self-assembly via oriented aggregation. These assembled nanostructures effectively improve room-temperature ferromagnetism of the sample. The exchange interaction of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is employed to explain the ferromagnetism of Fe-doped PbTiO3 nanocrystals.
127 citations
TL;DR: In this paper, the authors report on the observation of ultrafast photo-enhanced ferromagnetism in GaMnAs, which is manifested as a transient magnetization increase on a 100-ps time scale, after an initial sub-ps demagnetization.
Abstract: We report on the observation of ultrafast photo-enhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100-ps time scale, after an initial sub-ps demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature {Tc} and dominates the demagnetization component when approaching {Tc}. We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20K and a transient increase in {Tc}.
99 citations
TL;DR: In this paper, the influence of Coulomb interactions on the switching process of a single molecule magnet was investigated theoretically and the perturbation approach (Fermi golden rule) was applied to calculate currentvoltage characteristics.
Abstract: Magnetic switching of a single molecule magnet (SMM) due to spin polarized current is investigated theoretically. The charge transfer between the electrodes takes place via the lowest unoccupied molecular orbital (LUMO) of the SMM. Generally, the double occupancy of the LUMO level, and hence a finite on-site Coulomb repulsion, is allowed. Owing to the exchange interaction between electrons in the LUMO level and the SMM's spin, the latter can be reversed. The perturbation approach (Fermi golden rule) is applied to calculate current-voltage characteristics. The influence of Coulomb interactions on the switching process is also analyzed.
83 citations
TL;DR: In this paper, a theoretical model of ultrafast light-induced magnetization dynamics in systems of localized spins coupled to carriers' spins by sp-d exchange interaction is proposed and applied to ferromagnetic semiconductors.
Abstract: We propose and analyze a theoretical model of ultrafast light-induced magnetization dynamics in systems of localized spins that are coupled to carriers' spins by sp-d exchange interaction. A prominent example of a class of materials falling into this category are ferromagnetic (III, Mn)V semiconductors, in which ultrafast demagnetization has been recently observed. In the proposed model, light excitation heats up the population of carriers, taking it out of equilibrium with the localized spins. This triggers the process of energy and angular momentum exchange between the two spin systems, which lasts for the duration of the energy relaxation of the carriers. We derive the master equation for the density matrix of a localized spin interacting with the hot carriers and couple it with a phenomenological treatment of the carrier dynamics. We develop a general theory within the sp-d model and we apply it to the ferromagnetic semiconductors, taking into account the valence band structure of these materials. We show that the fast spin relaxation of the carriers can sustain the flow of polarization between the localized and itinerant spins leading to significant demagnetization of the localized spin system, observed in (III, Mn)V materials.
73 citations
TL;DR: In this article, the authors proposed materials design of high-T C wide band-gap dilute magnetic semiconductors (DMSs) based on first-principles calculations by using the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) method.
69 citations
TL;DR: In this article, the role of exchange interaction, magnetic anisotropy, and dipolar coupling for the ordering in chiral nanomagnets is investigated, and it is demonstrated that the periodicity of the modulated structure, which is determined by all interactions involved, is lattice dependent.
Abstract: Chiral magnetic ordering due to Dzyaloshinsky-Moriya interaction on two-dimensional lattices is studied theoretically. Several competing Dzyaloshinsky-Moriya vectors are introduced on the basis of symmetry arguments. The role of the exchange interaction, magnetic anisotropy, and dipolar coupling for the ordering in chiral nanomagnets is investigated. It is demonstrated that the periodicity of the modulated structure, which is determined by all interactions involved, is lattice dependent; the direction of spiral propagation and orientation of magnetization is determined by the competition between different Dzyaloshinsky-Moriya vectors and anisotropy; the anisotropy can induce a domain formation or destroy the chiral ordering depending on its orientation. We show that the Dzyaloshinsky-Moriya coupling is responsible for the chiral magnetic ordering in $\mathrm{Fe}∕\mathrm{W}(110)$.
TL;DR: In this article, the structural, magnetic and transport properties of double perovskites with ferromagnetism above room temperature are discussed, and the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity.
Abstract: We review the structural, magnetic and transport properties of double perovskites (A2BB'O6) with ferromagnetism above room temperature. Ferromagnetism in these compounds is explained by an indirect B?O?B'?O?B exchange interaction mediated by itinerant electrons. We first focus on the BB' =?FeMo-based double perovskites, with Sr2FeMoO6 (TC = 420?K) being the most studied compound. These compounds show metallic behaviour and low magnetic coercivity. Afterwards, we will focus on B' = Re compounds, where the significant orbital moment of Re plays a crucial role in the magnetic properties, for example in the large magnetic coercivity and magnetostructural coupling. More specifically, we first discuss the A2FeReO6 series, with maximum TC = 520?K for Ca2FeReO6, which shows a tendency to semiconducting behaviour. Finally, we describe the Sr2(Fe1?xCrx)ReO6 series, with maximum TC = 625?K for Sr2CrReO6, which is the highest TC in an oxide compound without Fe. This compound is metallic. We discuss the impact of these materials for spin electronics in the light of their high spin polarization at the Fermi level and metallicity. In particular, we focus on the large intergrain magnetoresistance effect observed in polycrystalline samples and the possible implementation of these materials as electrodes in magnetic tunnel junctions.
TL;DR: In this paper, the magnetic order and the exchange interactions in monoatomic transition-metal chains of V, Cr, Mn, Fe, and Co were analyzed based on first-principles calculations.
Abstract: Based on first-principles calculations we analyze the magnetic order and the exchange interactions in monoatomic $3d$ transition-metal chains of V, Cr, Mn, Fe, and Co. While freestanding Fe and Co chains remain ferromagnetic in the entire range of interatomic distances, V, Cr, and Mn chains change their magnetic state from antiferromagnetic (AFM) to ferromagnetic (FM) upon stretching. The corresponding distance-dependent exchange interaction is in striking resemblance to the Bethe-Slater curve. We demonstrate that in combination with the symmetry reduction on the $(110)$ surfaces of Cu, Pd, Ag, and $\mathrm{NiAl}$ even a weak chain-surface hybridization is sufficient to dramatically change the magnetic coupling in the chains. In particular, we find a tendency towards antiferromagnetic coupling. The obtained magnetic state of a specific chain depends sensitively on the chemical composition and the lattice constant of the surface. Surprisingly, Cr and Mn chains show a transition from ferromagnetic coupling in freestanding chains to antiferromagnetic coupling on the (110) surfaces of Pd, Ag, and $\mathrm{NiAl}$. For Fe and Co chains on $\mathrm{NiAl}(110)$ the FM and AFM states differ by only $2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, suggesting the possibility of a more complex, noncollinear magnetic ground state.
TL;DR: In this paper, the authors investigated the spin state of the [Pd(dmit)2] ion by means of 13C NMR measurements down to 1.37 K for polycrystalline samples.
Abstract: The series of [Pd(dmit)2] salts (dmit = 1,3-dithiole-2-thione-4,5-dithiolate) is a Mott insulator on an anisotropic triangular lattice and has an antiferromagnetic exchange constant J~ 200 K. While it has been revealed that most of them undergo antiferromagnetic transitions, the nature of the spin state of EtMe3Sb[Pd(dmit)2]2 which has a nearly isotropic triangular lattice is still unknown. We investigated the spin state by means of 13C NMR measurements down to 1.37 K for polycrystalline samples in which the carbon atoms in both ends of the Pd(dmit)2 molecule were selectively enriched with the 13C isotope. Both behaviours of the spin–lattice relaxation rate and the spectra indicate no spin ordering/freezing at least down to 1.37 K, which is lower than one-hundredth of the exchange interaction. Furthermore, the spin–lattice relaxation rate does not show an exponential temperature dependence and seems to retain a finite value at low temperatures. Thus, the ground state of this system is possibly the almost-gapless spin-liquid state.
TL;DR: Using electron energy-loss spectroscopy, many-electron atomic spectral calculations, and density functional theory, it is shown that angular-momentum coupling in the 5f states plays a decisive role in the formation of the magnetic moment in Cm metal.
Abstract: Using electron energy-loss spectroscopy in a transmission electron microscope, many-electron atomic spectral calculations and density functional theory, we examine the electronic and magnetic structure of Cm metal. We show that angular momentum coupling in the 5f states plays a decisive role in the formation of the magnetic moment. The 5f states of Cm in intermediate coupling are strongly shifted towards the LS coupling limit due to exchange interaction, unlike most actinide elements where the effective spin-orbit interaction prevails. It is this LS-inclined intermediate coupling that is the key to producing the large spin polarization which in turn dictates the newly found crystal structure of Cm under pressure.
TL;DR: Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using EPR spectroscopy.
Abstract: Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu 2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in exchange-coupled spin triads of nitroxide-copper(II)-nitroxide. The values of exchange interactions in spin triads of studied systems are typically on the order of tens to hundreds of inverse centimeters. The large magnitude of exchange interaction determines the specific and very informative peculiarities in EPR spectra due to the predominant population of the ground state of a spin triad and spin exchange processes. The variety of these manifestations depending on structure and magnetic properties of spin triads are described. EPR is demonstrated as an efficient tool for the characterization of spin transitions and for obtaining information on the temperature-dependent sign and value of the exchange interaction in strongly coupled spin triads.
TL;DR: In this paper, the magnetic susceptibility and specific heat of the kagome lattice with high-temperature expansions and exact diagonalizations were computed and compared with the experimental data on ZnCu3(OH)6Cl2 obtained by Helton et al.
Abstract: We compute the magnetic susceptibility and specific heat of the
spin-
$\frac{1}{2}$
Heisenberg model on the kagome lattice with
high-temperature expansions and exact diagonalizations. We compare
the results with the experimental data on ZnCu3(OH)6Cl2 obtained by Helton
et al. [Phys. Rev. Lett. 98, 107204 (2007)]. Down to
kBT/J≃0.2, our calculations reproduce accurately the
experimental susceptibility, with an exchange interaction
J≃190 K and a contribution of 3.7% of weakly interacting
impurity spins. The comparison between our calculations of the
specific heat and the experiments indicate that the low-temperature
entropy (below ~20 K) is smaller in ZnCu3(OH)6Cl2 than in the kagome
Heisenberg model, a likely signature of other interactions in the
system.
TL;DR: The spectra of 1 provide a rare example of a polymetallic system where detailed information on the spin-Hamiltonian parameters of excited spin-states is provided.
Abstract: A prerequisite to understand and subsequently to allow applications of the magnetic properties of polymetallic exchange coupled systems is a detailed insight into the magnetic properties of their elementary building blocks and the interaction between them. Such an understanding passes through the quantification of the relative contributions of single-ion and exchange terms to the overall magnetic properties of the system. In the general case, the quantification of these two kinds of contributions is possible when numerous spin-states are experimentally observed since single-ion and exchange terms project differently on the ground and various excited spin-states of an exchange coupled system. We present here highly resolved multifrequency (X-, K-, and Q-band) CW-EPR spectra obtained on a heterooctametallic ring [Me2NH2][Cr(III)7Cd(II)F8((CH3)3CCOO)16] (1). The spectra of 1 provide a rare example of a polymetallic system where detailed information on the spin-Hamiltonian parameters of excited spin-states is...
TL;DR: X-ray and magnetochemical studies of a series of mixed-ligand compounds revealed that an extra ligand (Py, NIT-mPy, or DABCO) coordinated to the metal atom produces a dramatic effect on the magnetic properties of the complex, changing the multiplicity of the ground state.
Abstract: Heterospin complexes [Cu(SQ)2Py]·C7H8, Cu(SQ)2DABCO, and [Cu(SQ)2NIT-mPy]·C6H6, where Cu(SQ)2 is bis(3,6-di-tert-butyl-o-benzosemiquinonato)copper(II), DABCO is 1,4-diazabicyclo(2,2,2)octane, and NIT-mPy is the nitronyl nitroxide 2-(pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, have been synthesized. The molecules of these complexes have a specific combination of the intramolecular ferro- and antiferromagnetic exchange interactions between the odd electrons of Cu(II) and SQ ligands, characterized by large exchange coupling parameters |J| ≈ 100−300 cm-1. X-ray and magnetochemical studies of a series of mixed-ligand compounds revealed that an extra ligand (Py, NIT-mPy, or DABCO) coordinated to the metal atom produces a dramatic effect on the magnetic properties of the complex, changing the multiplicity of the ground state. Quantum chemical analysis of magnetostructural correlations showed that the energy of the antiferromagnetic exchange interaction between the odd electrons of ...
TL;DR: The role of the different components of the AS in the low-temperature magnetic behavior of V15 was analyzed, and a perfect fit was reached to the experimental data on the staircaselike dependence of magnetization versus field in the whole temperature range including extremely low temperature.
Abstract: In this article, the low-temperature static (adiabatic) magnetization data of the nanoscopic V-15 cluster present in K-6[(V15As6O42)-As-IV(H2O)]center dot 8H(2)O is analyzed. The cluster anion, which attracted much attention in the past, contains a triangular V-3(IV) array causing frustration as a function of applied field and temperature. In the analysis, a three-spin (S = 1/2) model of V-15 was employed that includes isotropic antiferromagnetic exchange interaction and antisymmetric (AS) exchange in the most general form compatible with the trigonal symmetry of the system. It was shown that, along with the absolute value of AS exchange, the orientation of the AS vector plays a significant physical role in spin-frustrated systems. In this context, the role of the different components of the AS in the low-temperature magnetic behavior of V-15 was analyzed, and we were able to reach a perfect fit to the experimental data on the staircaselike dependence of magnetization versus field in the whole temperature range including extremely low temperature. Furthermore, it was possible for the first time to precisely estimate the two components of the AS vector coupling constant in a triangular unit, namely, the effective in-plane component, D, and the perpendicular part, D-n.
TL;DR: In this paper, the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime was systematically studied, and it was shown that the full recovery of the kondo resonance can be achieved not only by an appropriately tuned external magnetic field, but also via a appropriately tuned gate voltage.
Abstract: We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime. We employ Wilson’s numerical renormalization group method, extended to handle leads with a spin asymmetric density of states, to identify the effects of i a finite spin polarization in the leads at the Fermi surface, ii a Stoner splitting in the bands governed by the band edges, and iii an arbitrary shape of the lead density of states. For a generic lead density of states, the quantum dot favors being occupied by a particular spin species due to exchange interaction with ferromagnetic leads, leading to suppression and splitting of the Kondo resonance. The application of a magnetic field can compensate this asymmetry, restoring the Kondo effect. We study both the gate voltage dependence for a fixed band structure in the leads and the spin polarization dependence for fixed gate voltage of this compensation field for various types of bands. Interestingly, we find that the full recovery of the Kondo resonance of a quantum dot in the presence of leads with an energy-dependent density of states is possible not only by an appropriately tuned external magnetic field but also via an appropriately tuned gate voltage. For flat bands, simple formulas for the splitting of the local level as a function of the spin polarization and gate voltage are given.
TL;DR: In this paper, the magnetic susceptibility and specific heat of the spin-1/2 Heisenberg model on the kagome lattice with high-temperature expansions and exact diagonalizations were computed.
Abstract: We compute the magnetic susceptibility and specific heat of the spin-1/2 Heisenberg model on the kagome lattice with high-temperature expansions and exact diagonalizations. We compare the results with the experimental data on ZnCu3(OH)6Cl2 obtained by Helton et al. [Phys. Rev. Lett. 98, 107204 (2007)]. Down to k_BT/J~0.2, our calculations reproduce accurately the experimental susceptibility, with an exchange interaction J~190K and a contribution of 3.7% of weakly interacting impurity spins. The comparison between our calculations of the specific heat and the experiments indicate that the low-temperature entropy (below ~20K) is smaller in ZnCu3(OH)6Cl2 than in the kagome Heisenberg model, a likely signature of other interactions in the system.
TL;DR: In this paper, the magnetic structures of small clusters of Fe, Mn, and Cr supported on a Cu(111) surface have been studied with non-collinear first-principles theory.
Abstract: The magnetic structures of small clusters of Fe, Mn, and Cr supported on a Cu(111) surface have been studied with noncollinear first-principles theory. Different geometries such as triangles, pyramids, and wires are considered and the cluster sizes have been varied between two and ten atoms. The calculations have been performed using a real-space linear muffin-tin orbital method. The Fe clusters are found to order ferromagnetically regardless of the cluster geometry. For Mn and Cr clusters, antiferromagnetic exchange interactions between nearest neighbors are found to cause collinear antiferromagnetic ordering when the geometry allows it. If the antiferromagnetism is frustrated by the cluster geometry, noncollinear ordering is found. A comparison between the calculated structures and ground states obtained from simplified Heisenberg Hamiltonians show that the exchange interaction varies for different atoms in the clusters as a result of the different local structure.
TL;DR: In this paper, the electronic structure and magnetic properties of Fe-doped In2O3 were theoretically investigated based on the first-principles calculations, and it was shown that the ferromagnetic exchange is mediated by the donor impurity state, which mainly consists of Fe:3d and Fe:4s electrons trapped in oxygen vacancies.
Abstract: Based on the first-principles calculations, the electronic structure and magnetic properties of Fe-doped In2O3 were theoretically investigated. The presence of the predominating defects in oxide, i.e., oxygen vacancies, can lead to strong ferromagnetic coupling between the nearest neighboring Fe cations. Spin density and band-projected charge distribution in the vicinity of the oxygen vacancies reveal that the ferromagnetic exchange is mediated by the donor impurity state, which mainly consists of Fe:3d and Fe:4s electrons trapped in oxygen vacancies. Such results provide direct evidence for the F-center mediated exchange interaction in oxide-based magnetic semiconductors.
TL;DR: MagSaki(A) as mentioned in this paper was developed for the magnetic analysis of dinuclear high-spin cobalt(II) complexes and performed magnetic analysis to determine magnetic parameters using five types of theoretical susceptibility equations.
Abstract: MagSaki(A) software was developed for the magnetic analysis of dinuclear high-spin cobalt(II) complexes. This software performs magnetic analysis to determine magnetic parameters using five types of theoretical susceptibility equations. A characteristic feature of the software is that the exchange interaction can be treated anisotropically whereas the previous MagSaki software treats the exchange interaction isotropically.
TL;DR: In this article, the authors investigate how Kondo phenomenon occurs in the Anderson model dynamically coupled with local Jahn-Teller phonons, and they find that the total angular moment composed of electron pseudo-spin and phonon angular moments is screened by conduction electrons.
Abstract: We investigate how Kondo phenomenon occurs in the Anderson model dynamically coupled with local Jahn–Teller phonons. It is found that the total angular moment composed of electron pseudo-spin and phonon angular moments is screened by conduction electrons. Namely, phonon degrees of freedom essentially contribute to the formation of singlet ground state. A characteristic temperature of the Kondo effect due to dynamical Jahn–Teller phonons is explained by an effective s – d Hamiltonian with anisotropic exchange interaction obtained from the Jahn–Teller–Anderson model in a non-adiabatic region.
TL;DR: In this article, the spectral properties and quantum phase transitions in parallel triple quantum dot systems were investigated using the numerical renormalization-group method, and the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between the dots was identified in the spectra and is consistent with that obtained by the perturbation theory.
Abstract: The spectral properties and the quantum phase transitions in parallel triple quantum dot systems are investigated using the numerical renormalization-group method. For small interdot hopping $t$, the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between the dots is identified in the spectra and is consistent with that obtained by the perturbation theory. With increasing $t$, due to the competition between the RKKY exchange and the direct exchange between dots, there exist two first-order quantum phase transitions between the phases with the local spin ${S}_{\mathit{\text{dot}}}=3∕2$, $1∕2$, and 0 on the dots. At low temperature, the local spin ${S}_{\mathit{\text{dot}}}=3∕2$ is partially screened to a residual spin $S=1$, while the local spin ${S}_{\mathit{\text{dot}}}=1∕2$ is totally screened. In both local quadruplet and doublet states, the Kondo resonance accompanied with a conductance of the unitary limit $2{e}^{2}∕h$ is observed, while no Kondo effect is found in the local spin-singlet state.
TL;DR: It is obtained for the first time that the coercive field H(c), the permanent magnetization M(r) and the Curie temperature T(C) can increase or decrease for different kinds of doping ions.
Abstract: The influence of size, anisotropy and doping effects on the hysteresis loop of ferromagnetic nanoparticles is studied, based on the modified Heisenberg model. A Green's function technique in real space allows the calculation of the dependence of the magnetization on the temperature, magnetic field, anisotropy, defects and particle size. It is demonstrated that the coercive field Hc is very sensitive to the surface single-ion anisotropy, and to the exchange interaction constant on the surface Js and in the defect shells Jd. With respect to the strong surface single-site anisotropy Ds, we observe at small particle size, N = 4 shells, a maximum in the size dependence of the coercive field, whereas for the small surface anisotropy there is no maximum. Taking into account that J can be different in the defect shells compared to the case without defects, we have obtained for the first time that the coercive field Hc, the permanent magnetization Mr and the Curie temperature TC can increase or decrease for different kinds of doping ions. The dependence on the particle size is discussed, too. The results are in accordance with the experimental data.
TL;DR: It is proposed that the spin exchange process is caused by the modulation of exchange interaction between copper and nitroxides by lattice vibrations, and the estimations of the rate of exchange process and model calculations essentially support the observed phenomena.
Abstract: The complexes of Cu2+ hexafluoroacetylacetonate with two pyrazol-substituted nitronyl nitroxides are the choice systems to study the spin dynamics of strongly exchange-coupled spin triads. The large values of exchange coupling (ca. 100 cm-1) and high-resolution electron paramagnetic resonance (EPR) at Q- and W-bands (35 and 94 GHz) allowed us to observe and interpret specific characteristics of these systems. An electron spin exchange process has been found between different multiplets of the spin triad, which manifests itself as a significant shift of the EPR line position with temperature. We propose that the spin exchange process is caused by the modulation of exchange interaction between copper and nitroxides by lattice vibrations. The estimations of the rate of exchange process and model calculations essentially support the observed phenomena. The studied characteristics of strongly coupled spin triads explain previously obtained results, agree with literature, and should be accounted for in future investigations of similar spin systems.
TL;DR: The Ising model was introduced in 1920 to describe a uniaxial system of magnetic moments, localized on a lattice, interacting via nearest-neighbour exchange interaction.
Abstract: The Ising model was introduced in 1920 to describe a uniaxial system of magnetic moments, localized on a lattice, interacting via nearest-neighbour exchange interaction. It is the generic model for ...