Showing papers on "Exchange interaction published in 2000"

Room Temperature Ferromagnetism in Novel Diluted Magnetic Semiconductor Cd1¡xMnxGeP2

Abstract: High concentration of Mn atoms has been incorporated in the surface region of II-IV-V2 type chalcopyrite semiconductor CdGeP2 Photoluminescence spectrum at 20 K shows a peak around 32 eV, suggesting that the incorporation of Mn introduces an energy gap much higher than that of the host semiconductor (Eg=183 eV) Prominent magnetic hysteresis loops with coercivity of 05 kOe has been observed at room temperature Magnetic force microscope (MFM) measurements reveal a stripe-shaped domain pattern on the top surface Magneto-optical Kerr ellipticity spectrum measured at room temperature show a prominent peak at 17 eV and a broad tail up to 35 eV We tentatively attribute the ferromagnetism to the double exchange interaction between Mn2+ and Mn3+ states due to the structural feature of II-IV-V2 type chalcopyrite compounds

Size-dependent electron-hole exchange interaction in Si nanocrystals

Abstract: Silicon nanocrystals with diameters ranging from [approximate]2 to 5.5 nm were formed by Si ion implantation into SiO2 followed by annealing. After passivation with deuterium, the photoluminescence (PL) spectrum at 12 K peaks at 1.60 eV and has a full width at half maximum of 0.28 eV. The emission is attributed to the recombination of quantum-confined excitons in the nanocrystals. The temperature dependence of the PL intensity and decay rate at several energies between 1.4 and 1.9 eV was determined between 12 and 300 K. The temperature dependence of the radiative decay rate was determined, and is in good agreement with a model that takes into account the energy splitting between the excitonic singlet and triplet levels due to the electron-hole exchange interaction. The exchange energy splitting increases from 8.4 meV for large nanocrystals ([approximate]5.5 nm) to 16.5 meV for small nanocrystals ([approximate]2 nm). For all nanocrystal sizes, the radiative rate from the singlet state is 300–800 times larger than the radiative rate from the triplet state.

First-principles calculations of magnetic interactions in correlated systems

Abstract: We present a method to calculate the effective exchange interaction parameters based on the realistic electronic structure of correlated magnetic crystals in local approach with the frequency dependent self-energy. The analog of ``local force theorem'' in the density-functional theory is proven for highly correlated systems. The expressions for effective exchange parameters, Dzialoshinskii-Moriya interaction, and magnetic anisotropy are derived. The first-principles calculations of magnetic excitation spectrum for ferromagnetic iron, with the local correlation effects from the numerically exact QMC scheme, are presented.

Direct coulomb and exchange interaction in artificial atoms

Abstract: We determine contributions from the direct Coulomb and exchange interactions to the total interaction in artificial semiconductor atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. The electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle-state degeneracy, and find that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.

Mesoscopic magnetization fluctuations for metallic grains close to the Stoner instability

Abstract: This paper is devoted to the magnetic properties of isolated mesoscopic grains. We demonstrate that under very general conditions the electron-electron interactions in such grains can be taken into account by a simple interaction Hamiltonian. This Hamiltonian involves only three coupling constants, which correspond to charging, exchange interaction, and superconducting correlations. The most important condition for such a description is that Thouless conductance of each grain is large. Under this condition sample-to-sample fluctuations of the coupling constants can be neglected. However, the thermodynamic properties can still remain sample specific due to the one-electron part of the Hamiltonian. If the grain is made from a material that is close to the threshold of ferromagnetic instability, the mesoscopic fluctuations of the magnetization are especially strong. Moreover, the situation becomes multistable: free energy of each grain as a function of the magnetization is characterized by a large number of local minima. We analyze the statistics of these minima and show that it possesses simple scaling properties. Numerical simulations confirm this scaling.

Theory of exciton doublet structures and polarization relaxation in single quantum dots

Abstract: The mechanism of exciton doublet structures in quantum dots is identified as the long-range part of the electron-hole exchange interaction which is emphasized by the anisotropic shape of quantum dots. The physical origin of the energetic order of the orthogonally polarized exciton states of each doublet is clarified by inspecting the spatial distribution of the exciton polarization. The key concepts to understand the energetic order are the node configuration of the distribution function of exciton polarization and the dipole-dipole interaction energy originating from the long-range electron-hole exchange interaction. The population relaxation and the polarization relaxation of excitons are studied and the extremely slow polarization relaxation within exciton doublet states is predicted. It is also found that the inter-doublet cross-relaxation between orthogonally polarized exciton states occurs as efficiently as the population relaxation.

Order induced by dipolar interactions in a geometrically frustrated antiferromagnet

Abstract: We study the classical Heisenberg model for spins on a pyrochlore lattice interacting via long-range dipole-dipole forces and nearest-neighbor exchange. Antiferromagnetic exchange alone is known not to induce ordering in this system. We analyze low-temperature order resulting from the combined interactions, both by using a mean-field approach and by examining the energy cost of fluctuations about an ordered state. We discuss behavior as a function of the ratio of the dipolar and exchange interaction strengths and find two types of ordered phase. Below a certain value of this ratio, we find that the system orders in a four-sublattice N\'eel state. For interaction strengths above this critical ratio, the system orders with an incommensurate wave vector. We relate our results to the recent experimental work and reproduce and extend the theoretical calculations on the pyrochlore compound, ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7},$ by N. P. Raju, M. Dion, M. J. P. Gingras, T. E. Mason, and J. E. Greedan, Phys. Rev. B 59, 14 489 (1999).

The effect of Coulomb correlation and magnetic ordering on the electronic structure of two hexagonal phases of ferroelectromagnetic YMnO3

Abstract: The electronic structure of YMnO3 in its high- and low-temperature hexagonal phases has been investigated within the local spin-density approximation (LSDA) and by the LSDA + U method which takes into account the local Coulomb interaction between d electrons of transition-metal ions. In contrast to the case for orthorhombic manganites, the d4-configuration degeneracy is already lifted in the high-temperature symmetric hexagonal phase, indicating that Mn3+ is not a Jahn-Teller ion; hence, we argue that the lowering of the symmetry is not connected with Jahn-Teller instability in hexagonal YMnO3. Each of these two hexagonal phases is found to be semiconducting, with a band gap of about 1.5 eV. It is shown that magnetism and correlation effects are important in band-gap formation for both crystal structures. Using the Green function method, we estimated the Neel temperature from the calculated effective exchange interaction parameters, and found it to be in good agreement with experiment.

Quantum dots with even number of electrons: kondo effect in a finite magnetic field

Abstract: We show that the Kondo effect can be induced by an external magnetic field in quantum dots with an even number of electrons. If the Zeeman energy B is close to the single-particle level spacing Delta in the dot, the scattering of the conduction electrons from the dot is dominated by an anisotropic exchange interaction. A Kondo resonance then occurs despite the fact that B exceeds by far the Kondo temperature T(K). As a result, at low temperatures T<

Suppression of exchange bias by ion irradiation

Abstract: The exchange bias effect in ferromagnetic/antiferromagnetic sandwich structures is generally believed to be sensitive on the interface exchange interaction, the magnetization, and the thickness of the ferromagnetic layer. Also the interface structure plays a crucial role. We show that, by irradiating samples with He ions, we can manipulate the exchange bias field in a controlled manner. Depending on the dose (1014–1017 ions/cm2) and the acceleration voltage (10–35 kV) of the ions, the shift of the hysteresis can be reduced or even fully suppressed. Potential applications of this effect for magnetic patterning on the nanoscale will be discussed.

Magnetic Exchange Across the Cyanide Bridge

Abstract: The superexchange interaction in cyano-bridged transition metal dimers is analysed by a valence bond configuration interaction model in combination with fourth order perturbation theory. Ferro- and antiferromagnetic contributions to the exchange splitting are expressed in terms of metal-ligand hybridization matrix elements, metal-to-ligand, lig-and-to-metal, metal-to-metal charge transfer energies, and intra-atomic exhange integrals. The formalism is simplified and we arrive at a two-parameter model, with which it is possible to estimate magnetic ordering temperatures of cubic stoichiometric Prussian Blue like phases containing first row transition metals. The relevance and applicability of our theoretical results is demonstrated on critical temperatures reported in the literature. The model accounts for both ferri- and ferromagnetic ordering temperatures. It is found that cyanide is an efficient mediator of exchange due to the participation of the π and π* orbitals to equal extent.

Electron-phonon effects in graphene and armchair (10,10) single-wall carbon nanotubes

Abstract: The electron-phonon interaction in low-dimensional tight-binding systems is discussed. A sheet of graphite, which is two-dimensional, and an armchair single-wall carbon nanotube (SWNT), which is quasi-one-dimensional, are taken as examples. For the modulated hopping the matrix elements for both systems are derived in the context of a two parameter model for the phonon vibrational spectrum. It is found that they (for both structures) display a deformation type of potential, and are reduced by a factor of $(1\ensuremath{-}R),$ where R depends on the phonon parameters. It is also shown that the ordinary electron-phonon coupling displays a deformation type of approximation for both systems. Next, a different type of interaction is considered---the phonon modulated electron-electron interaction. It gives two contributions---random phase approximation with one phonon line and exchange interaction with one phonon line. We find that for the two-dimensional (2D) graphene and for the quasi-1D (10,10) SWNT, the modulated hopping and exchange coupling govern the electron transport at room temperatures.

EPR linewidth variation, spin relaxation times, and exchange in amorphous hydrogenated carbon

Abstract: Electron paramagnetic resonance ~EPR! measurements have been made of amorphous hydrogenated carbon ~a-C:H! films grown by plasma enhanced chemical vapor deposition ~PECVD! with negative self-bias voltages Vb in the approximate range 10–540 V. For Vb,100 V, as the film changes from polymerlike to diamondlike, the changes in linewidth and shape are interpreted in terms of changes to two contributions—one due to dipolar interactions between the unpaired spins and one due to unresolved lines arising from hyperfine interactions with H1. The former yields a Lorentzian line, the latter a Gaussian, and the resultant spectrum has the Voigt shape. The empirical relation DBpp G ~in Gauss!5~0.1860.05!3~at.%H) between the peak-to-peak Gaussian contribution ~in Gauss! DBpp G and the hydrogen content in atomic percentage is obtained. For Vb.100 V the linewidth is shown to be dominated by the dipolar interactions and exchange and it decreases as Vb increases; the change is shown to arise primarily from a change in the exchange interaction. Evidence for this comes from measurements which show that the spin-lattice relaxation time appreciably shortens and the spin-spin relaxation time lengthens as the bias voltage is increased. The magnitude and variation with bias of the linewidth are consistent with the EPR signal originating from the p-type radicals.

Magnetization drive method, magnetic functional device, and magnetic apparatus

Abstract: A magnetic functional device allowing magnetization switching at a high speed even if the size of a magnetic substance is made finer on the sub-micron order. The magnetic functional device includes an information carrier layer formed by a magnetic substance; and a strain-imparting layer (such as piezoelectric layer) formed below the information layer and operably configured to impart a drive force to change the direction of a magnetization vector lying in a first plane of the information carrier thereby processing binary or more information by magnetization directions of the information carrier layer; wherein the drive force is applied in pulse to the information carrier layer in a direction nearly perpendicular to the first plane in which lies the magnetization vector of the information carrier layer when the magnetization vector is in an initial state before the application of the drive force. An effective field derived from a magnetic anisotropy or exchange interaction is used as the drive force.

Double resonance mechanism of ferromagnetism and magnetotransport in (Ga-Mn)As

Abstract: We calculate the electronic states of the Mn-doped semiconductors and show that resonant states are formed at the top of the down spin valence band due to magnetic impurities and that they give rise to a strong and long-ranged ferromagnetic coupling between Mn moments. We propose that the coupling of the resonant states, in addition to the intra-atomic exchange interaction between the resonant and nonbonding states, is the origin of the ferromagnetism of (Ga-Mn)As. The mechanism is thus called "double resonance." The resonant states bring about the spin-dependent resistivity to produce magnetoresistive properties in (Ga-Mn)As and their junctions.

Many-body Green's function theory for the magnetic reorientation of thin ferromagnetic films

Abstract: The field-induced reorientation of the magnetization of ferromagnetic films is treated within the framework of many-body Green's function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green's functions. This formulation allows a straightforward extension of the monolayer case to thin films with many layers and for arbitrary spin and moreover provides a practicable procedure for numerical computation. The model Hamiltonian includes a Heisenberg term, an external magnetic field, a second-order uniaxial single-ion anisotropy, and the magnetic dipole-dipole coupling. We utilize the Tyablikov (RPA) decoupling for the exchange interaction terms and the Anderson-Callen decoupling for the anisotropy terms. The dipole coupling is treated in the mean-field approximation, a procedure which we demonstrate to be a sufficiently good approximation for realistic coupling strengths. We apply the new method to monolayers with spin \(\) and to multilayer systems with S=1. We compare some of our results to those where mean-field theory (MFT) is applied to all interactions, pointing out some significant differences.

Microscopic imaging of Fe magnetic domains exchange coupled with those in a NiO(001) surface.

Abstract: We revealed the micromagnetic structure of an Fe thin film exchange interacting with the spins of a fully compensated (001) surface of antiferromagnetic NiO. The interface exchange interaction causes the Fe domains to follow the NiO domains. The Fe spin polarization is in plane and the spin polarization in each domain is roughly perpendicular to an easy-spin axis of the NiO. These results agree with numerically calculated spin directions. Our numerical results also show that the NiO spins at the interface cant from the easy-spin axis towards the Fe spin because of exchange coupling.

Effects of Localized Spins in Quasi-Two Dimensional Organic Conductors

Abstract: Theoretical study on the effect of high Fe spin S =5/2 on the organic two-dimensional electronic system of BETS compounds is made by the calculation with the mean-field approximation for the Coulomb interaction. It turned out that a relatively smaller value of on-site Coulomb interaction, U = U BETS , on BETS sites compared with that of ET makes the nature of ground states sensitive to the band structure which are characterized by two different types of molecular arrangement, λ- and κ-type. The λ-type system with a smaller band overlap shows a Mott-like insulating state, while the large band overlap of the κ-type favors a nobel spin density wave (SDW)-like state newly found in our study. In both states, the spin amplitude is increased by the exchange interaction, J , between Fe spins and BETS electrons. Our results are summarized in the unified phase diagram, where at a fixed value value of U BETS close to the metal-insulator boundary, even a small value of J as well as the difference in the band structur...

Field-induced magnetic reorientation and effective anisotropy of a ferromagnetic monolayer within spin wave theory

Abstract: The reorientation of the magnetization of a ferromagnetic monolayer is calculated with the help of many-body Green's function theory This allows, in contrast to other spin wave theories, a satisfactory calculation of magnetic properties over the entire temperature range of interest since interactions between spin waves are taken into account A Heisenberg Hamiltonian plus a second-order uniaxial single-ion anisotropy and an external magnetic field is treated by the Tyablikov (Random Phase Approximation: RPA) decoupling of the exchange interaction term and the Anderson-Callen decoupling of the anisotropy term The orientation of the magnetization is determined by the spin components $$\langle {S^\alpha }\rangle (\alpha = x,y,z)$$ , which are calculated with the help of the spectral theorem The knowledge of the orientation angle $${\theta _0}$$ allows a non-perturbative determination of the temperature dependence of the effective second-order anisotropy coefficient Results for the Green's function theory are compared with those obtained with mean-field theory (MFT) We find significant differences between these approaches

Critical points of condensation in Coulomb systems

Abstract: The critical points of condensation in Coulomb systems are described here by a modified van der Waals equation of state taking into account a many-particle exchange interaction between virtual atoms with overlapping classically accessible spheres of valence electrons. A characteristic feature of the Coulomb critical points is strong electron – ion coupling caused by the proximity to the metal – insulator transition. We consider a cell model of the exchange interaction of virtual atoms and examples of Coulomb critical points in a system of charged hard spheres, in alkali metals, in metal – ammonia solutions, and in excitonic systems. The Coulomb critical point parameters of transition metals are determined. We consider examples of insulator – metal transitions in semiconducting and dielectric fluids which form the Coulomb systems only in the liquid phase, and discuss a semiconducting critical state of mercury.

Ordered magnetic phase in Cd1-xMnxTe/Cd1-y-zMgyZnzTe : N heterostructures: magnetooptical studies

Abstract: Photoluminescence magnetospectroscopy is employed to examine the low-temperature magnetic phase that is induced by the carrier-mediated ferromagnetic exchange interaction in modulation-doped Cd1-xMnxTe quantum well. Unusual properties of the domain structure are linked to tendency towards spin-density wave formation in this low-dimensional magnetic system. (C) 2000 Elsevier Science B.V. All rights reserved.

Ising model for disordered ferromagnetic Fe-Al alloys

Abstract: A simple site-diluted Ising model is proposed to study the magnetic properties of ${\mathrm{Fe}}_{p}{\mathrm{Al}}_{q}$ alloys (with $p+q=1)$ in the structural disordered phase. It is assumed that Al atoms, which are nonmagnetic, can induce an extra ferromagnetic interaction between second-neighbor Fe atoms. It is further assumed that this second-neighbor interaction, as well as the nearest-neighbor one, decreases as q increases. The critical properties are obtained by the variational approach based on Bogoliubov inequality for the free energy in the pair approximation. Quite good fittings to the experimental results of the ordering temperature are obtained as a function of q. A negative value of the extra exchange interaction for some range of the Al concentration is obtained. It is argued that this negative exchange can drive a spin-glass-like phase in these compounds, a fact that should be sought experimentally.

Electromagnetic properties of decuplet hyperons in a chiral quark model with exchange currents

Abstract: We calculate static electromagnetic properties of the decuplet hyperons in the chiral constituent quark model including electromagnetic exchange currents between quarks. We study the effect of exchange currents on charge and magnetic radii, magnetic and quadrupole moments and radiative decay observables. SUF (3) flavour symmetry breaking due to the mass difference between strange and nonstrange quarks is included. Exchange currents dominate in observables (e.g. quadrupole moments), where one-body currents are suppressed, due to selection rules. The quadrupole moment of the ¯ is mainly sensitive to gluon degrees of freedom; its measurement could be used to determine the strength of the effective gluon exchange interaction between light quarks. We compare our results with the sparse experimental data and with predictions from other models.

Parameter-free exchange functional

Abstract: Conventional generalized-gradient approximations for exchange energy are derived to obey the fundamental conditions of the exact exchange functional. We present a simple analytic exchange functional that does not contain a semiempirical parameter or an adjusted fundamental constant. We show that this functional satisfies several significant and strict fundamental conditions, and gives accurate exchange energies for the atoms, hydrogen through argon, within a margin of error of a few percent. It can be updated for any kind of kinetic-energy density. Surprisingly, the present formalism exactly gives the gradient expansion coefficient for slowly varying density.

Interdependence of magnetism and superconductivity in the borocarbide TmNi2B2C

Abstract: We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron diffraction. The ordering vector is Q(A) = (0.48,0,0) and the phase appears above a critical in-plane magnetic field of 0.9 T. The field was applied in order to test the assumption that the zero-field magnetic structure at Q(F) = (0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity were destroyed. We present theoretical calculations which show that two effects are important: a suppression of the ferromagnetic component of the RKKY exchange interaction in the superconducting phase and a reduction of the superconducting condensation energy due to the periodic modulation of the moments at Q(A).

Double exchange and the cause of ferromagnetism in doped manganites

Abstract: The coexistence of ferromagnetism and metallic conduction in doped manganites has long been explained by a double exchange model (ferromagnetic Kondo-lattice model) in which the ferromagnetic exchange arises from the carrier hopping. We evaluate the zero-temperature spin stiffness $D(0)$ and the Curie temperature ${T}_{C}$ on the basis of the double exchange model using the measured values of the bare bandwidth W and the Hund's rule coupling ${J}_{H}.$ The calculated $D(0)$ and ${T}_{C}$ values are too small compared with the observed ones even without considering a strong electron-phonon coupling. We thus suggest that the ferromagnetism in doped manganites should not originate from the double exchange interaction. On the other hand, an alternative model based on the $d\ensuremath{-}p$ exchange and a strong electron-phonon coupling can quantitatively explain the magnetoresistance and ${T}_{C}$ values.

Magnetoluminescence in quantum dots and quantum wires of II–VI diluted magnetic semiconductors

Abstract: We report optical properties of quantum dots and quantum wires of diluted magnetic semiconductors. The quantum dots of Cd1−xMnxSe (x=0.03) show the exciton luminescence at around 2.4 eV, which indicates a strong confinement effect of the exciton energy corresponding to the dot size of 4–6 nm. The Zeeman shift of the exciton luminescence was observed with an effective g value of 91, showing a significant exchange interaction of the excitons with the Mn ions in the dots. The exciton luminescence from the quantum wires of Cd1−xMnxSe (x=0.08) shifts by 5.2 meV to the higher energy side with decreasing the wire width from 126 to 26 nm. The high energy shift in the narrow wires indicates the influence of the one-dimensional quantum confinement effect for the exciton states. The effective g value of the exciton in these quantum wires is 100–150. The exciton luminescence from the wires is linearly polarized (up to 80%) parallel to the wire direction at zero field, which indicates one-dimensional properties of the...

High-frequency electron spin resonance in magnetic systems

Abstract: We review the recent progress in the study of magnetic materials using a high-frequency electron spin-resonance (ESR) technique. First, we show how useful high-frequency ESR is for studying antiferromagnetic materials, where the ESR frequency and magnetic field depend greatly on the exchange interaction and anisotropy energy of the materials. Next, we review the recent high-frequency ESR experiments made on spin S = 1 quasi-one-dimensional Heisenberg antiferromagnets (Q1D HAFs) and the spin-Peierls system CuGeO3. Then, we review the ESR studies performed on more complex systems, such as an S = 1 Q1D HAF with bond alternation, spin-ladder compounds and quasi-two-dimensional magnets. Each of these systems has a singlet ground state of quantum origin and an energy gap to the lowest excited state. On applying an external magnetic field, these systems show a transition from the non-magnetic to a magnetized state, and in some cases, long-range magnetic ordering occurs. Efforts are made to explain the underlying physics intuitively at the expense of rigour.

Low Temperature Properties of Erbium In Gold

Abstract: The properties of dilute alloys of Er in Au have been studied, principally below 100 mK, in connection with the use of this system in thermometry and in microcalorimetry for particle detection. Measurements are reported of (1) the magnetization at high temperatures and high field, (2) the magnetization in low fields with temperatures extending down to 0.1 mK, and (3) the heat capacity as a function of temperature and field at low temperatures. These measurements are analyzed to provide information of several properties of the Er3+ ion in the Au lattice, in particular, the crystal field parameters, the exchange interaction of the 4f electrons with the conduction electrons, and the spin glass freezing temperature.