An effective quantum parameter for strongly correlated metallic ferromagnets

The correlated motion of electrons in metallic ferromagnets is investigated in terms of a realistic interacting-electron model with N-fold orbital degeneracy and intra-orbital (U) and inter-orbital (J) Coulomb interactions. Correlation-induced self-energy and vertex corrections are incorporated systematically to provide a non-perturbative Goldstone-mode-preserving scheme. An effective quantum parameter [U2+(N-1)J2]/[U+(N-1)J]2 is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large N, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. Correlation effects are investigated for spin stiffness, magnon dispersion, electronic spectral function, density of states, and finite-temperature spin dynamics using realistic bandwidth, interaction, and lattice parameters for iron.     

Metal-insulator transitions: influence of lattice structure, jahn-teller effect, and Hund's rule coupling

We study the influence of the lattice structure, the Jahn-Teller effect, and the Hund's rule coupling on a metal-insulator transition in A(n)C60 (A = K,Rb). The difference in the lattice structure favors A3C60 (fcc) being a metal and A4C60 (bct) being an insulator, and the coupling to H(g) Jahn-Teller phonons favors A4C60 being nonmagnetic. The coupling to H(g) ( A(g)) phonons decreases (increases) the value U(c) of the Coulomb integral at which the metal-insulator transition occurs. There is an important partial cancellation between the Jahn-Teller effect and the Hund's rule coupling.     

Two-dimensional intrinsic ferromagnetism at nitride-boride interfaces

We predict theoretically novel two-dimensional interface ferromagnetism at AlN/MgB(2)(0001) using first-principles calculations, where the interface is employed as an ordered structure of spin sites instead of point defects. Although N dangling bonds are apparently saturated, interfacial states exhibit spin polarization. Hund's coupling of the two N p(∥) orbitals as well as low density of states at the Fermi energy contribute to strong band ferromagnetism. Furthermore, first-principles electron transport calculations demonstrate that this interfacial spin polarization is responsible for quantum spin transport. The magnetization can be controlled by applied gate bias voltages.     

Segmented band mechanism for itinerant ferromagnetism

We introduce a novel mechanism for itinerant ferromagnetism, which is based on a simple two-band model, and, by using numerical and analytical methods, we show that the periodic Anderson model contains this mechanism. We propose that the mechanism, which does not assume an intra-atomic Hund's coupling, is present in both the iron group and some f electron compounds.     

Microscopic conditions favoring itinerant ferromagnetism: Hund's rule coupling and orbital degeneracy

The importance of Hund's rule coupling for the stabilization of itinerant ferromagnetism is investigated within a two-band Hubbard model. The magnetic phase diagram is calculated by finite-temperature quantum Monte-Carlo simulations within the dynamical mean-field theory. Ferromagnetism is found in a broad range of electron fillings whereas antiferromagnetism exists only near half filling. The possibility of orbital ordering at quarter filling is also analyzed. Received: 26 February 1998 / Accepted: 17 April 1998     

Non-perturbative approaches to magnetism in strongly correlated electron systems

The microscopic basis for the stability of itinerant ferromagnetism in correlated electron systems is examined. To this end several routes to ferromagnetism are explored, using both rigorous methods valid in arbitrary spatial dimensions, as well as Quantum Monte Carlo investigations in the limit of infinite dimensions (dynamical mean-field theory). In particular we discuss the qualitative and quantitative importance of (i) the direct Heisenberg exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a high spectral density near the band edges caused by an appropriate lattice structure and/or kinetic energy of the electrons. We furnish evidence of the stability of itinerant ferromagnetism in the pure Hubbard model for appropriate lattices at electronic densities not too close to half-filling and large enough U. Already a weak direct exchange interaction, as well as band degeneracy, is found to reduce the critical value of U above which ferromagnetism becomes stable considerably. Using similar numerical techniques the Hubbard model with an easy axis is studied to explain metamagnetism in strongly anisotropic antiferromagnets from a unifying microscopic point of view.     

Magnetic properties of nitrogen doped ZnO

Using the first principle method based on density functional theory,this paper studies the electronic structure and the ferromagnetic stability in N-doped ZnO.The calculated results based on local density approximation(LDA) and LDA+U method show that ferromagnetism coupling between N atoms is more energetically favourable for eight geometrically distinct configurations.The dominant ferromagnetic interaction is due to the hybridization between O 2p and N 2p.The origin of the ferromagnetic state in N doped ZnO is discussed by analysing coupling between N 2p levels.We also analyse N dopant concentration and lattice strain effect on ferromagnetism.     

Ferromagnetic ground state in the Kondo lattice model

We present a series of rigorous examples of the Kondo lattice model that exhibit full ferromagnetism in the ground state. The models are defined in one-, two- and three-dimensional lattices, and are characterized by a range of hopping terms, specific electron filling, and large ferromagnetic coupling. Our examples show that a sufficient strong but finite exchange coupling between conduction electrons and localized spins could overcome the competition from mobility of a finite density of electrons and drive the system from a paramagnetic phase to a ferromagnetic phase. We also establish a relation of ferromagnetism between the Hubbard model and Kondo lattice model. Meanwhile some rigorous results on ferromagnetism in the corresponding Hubbard model are presented. Received: 10 September 1997 / Revised: 15 October 1997 / Accepted: 17 October 1997     

Coexistence of antiferromagnetic and ferromagnetic phase for ferromagnetic Kondo lattice model

To study the proposed phase separations in doped manganites, we performed Monte-Carlo calculations for the ferromagnetic Kondo lattice model with strong Hund's coupling between conduction electrons and localized spins. For the practical calculations, we adopted a one dimensional lattice and treated the spins of the localized t_2g electrons semi-classically. A direct evidence of the phase separation is observed from a snapshot of the spatial dependence of localized spins. No indication of the canted or spiral phases is found in the results of simulations. Further, the calculated results of the spin structure factor in the phase separation region are well compared with recent experiments. Received: 1st September 1998 / Revised: 30 October 1998 / Accepted: 27 November 1998     

Dimerization versus orbital-moment ordering in a Mott insulator YVO3

We use exact diagonalization combined with mean-field theory to investigate the phase diagram of the spin-orbital model for cubic vanadates. The spin-orbit coupling competes with Hund's exchange and triggers a novel phase, with the ordering of t(2g) orbital magnetic moments stabilized by the tilting of VO6 octahedra. It explains qualitatively spin canting and reduction of magnetization observed in YVO3. At finite temperature, an orbital instability in the C-type antiferromagnetic phase induces modulation of magnetic exchange constants even in the absence of lattice distortions. The calculated spin structure factor shows a magnon splitting at q-->=(0,0,pi / 2) due to the orbital dimerization.     

Half-metallic ferromagnetism of zinc-blende CrS and CrP: a first-principles pseudopotential study

The electronic structure and the ferromagnetism of CrS and CrP in the zinc-blende (ZB) phase are investigated by spin-polarized calculations with first-principles plane-wave pseudopotential method within the generalized gradient approximation for the exchange-correlation potential. From the analysis of the spin-dependent density of states, band structure and magnetic moment, we predict that ZB CrS and CrP at their respective equilibrium lattice constant are half-metallic ferromagnets with a magnetic moment of 4.00 and 3.00μ_B per formula unit, respectively. We also find that the ZB CrS maintains half-metallic ferromagnetism up to 3% compression of lattice constant while the half-metallic ferromagnetism for ZB CrP exists only near its equilibrium lattice constant.     

Odd-parity triplet pair induced by Hund's rule coupling

We discuss microscopic aspects of the odd-parity triplet pair in orbital degenerate systems. From the concept of off-diagonal long-range order, a pair state is unambiguously defined as the eigenstate with the maximum eigenvalue of a pair correlation function. Performing this scheme by a numerical technique, we clarify that the odd-parity triplet pair occurs as an out-of-phase combination of local triplets induced by Hund's rule coupling for the lattice including two sites in the unit cell.     

On the Ground State of Spin Systems with Orbital Degeneracy

In order to understand the properties of the spin system with orbital degeneracy,we first study the ground state of the SU(4) spin-orbital model on a square lattice.The mean-field results suggest that for a small Hund‘s interaction,the flavor liquid state is stable against the solid state,but with sufficient deviation from the SU(4) limit the long-range order may be attained in 2D system.Furthermore,we employ a variational approach to calculate the phase diagram of the ground state and the temperature-dependent susceptibility by taking into account the Hund‘s interaction and the anisotropy in orbital wavefunctions.Finally,the implications for the experimental observations on the material,LiNiO2,are discussed.     

Saturated ferromagnetism from statistical transmutation in two dimensions

The total spin of the ground state is calculated in the U-->infinity Hubbard model with uniform magnetic flux perpendicular to a square lattice, in the absence of Zeeman coupling. It is found that the saturated ferromagnetism emerges in a rather wide region in the space of the flux density phi and the electron density ne. In particular, the saturated ferromagnetism at phi=ne is induced by the formation of a spin-1/2 boson, which is a composite of an electron and the unit flux quantum.     

Structural and magnetic phase transitions in rare-earth-cadmium equiatomic compounds

The tendency to structural instability and the nature of the magnetic ordering are investigated in all the cubic rare-earth-cadmium equiatomic compounds from measurements of resistivity and magnetic susceptibility. The CsCl-type structure is stable at room temperature in all the compounds. However, LaCd exhibits a lattice change at 61 K, while CeCd and PrCd undergo two transitions at 107 and 216 K, and 125 and 190 K, respectively. The low-temperature phases are unknown, but seem to have a symmetry lower than tetragonal. Other compounds are cubic at least in their paramagnetic phase. In connection with the change in the lattice symmetry, a change of the magnetic ordering is observed from ferromagnetism towards antiferromagnetism. Among the heavy rare-earth compounds, cubic thus ferromagnetic, DyCd plays a peculiar role since it undergoes a structural transition in its ordered range, the magnetoelastically stressed lattice becoming unstable again. The strength of bilinear interactions and the occurrence of quadrupolar pair coupling are then discussed.     

Ferromagnetism in the Hubbard Model with a Gapless Nearly-Flat Band

We present a version of the Hubbard model with a gapless nearly-flat lowest band which exhibits ferromagnetism in two or more dimensions. The model is defined on a lattice obtained by placing a site on each edge of the hypercubic lattice, and electron hopping is assumed to be only between nearest and next nearest neighbor sites. The lattice, where all the sites are identical, is simple, and the corresponding single-electron band structure, where two cosine-type bands touch without an energy gap, is also simple. We prove that the ground state of the model is unique and ferromagnetic at half-filling of the lower band, if the lower band is nearly flat and the strength of on-site repulsion is larger than a certain value which is independent of the lattice size. This is the first example of ferromagnetism in three dimensional non-singular models with a gapless band structure.     

Effects of the 3d transition metal doping on the structural,electronic, and magnetic properties of BeO nanotubes

First-principles calculations have been performed on the structural, electronic, and magnetic properties of seven 3d transition-metal(TM) impurities(V, Cr, Mn, Fe, Co, Ni, and Cu) doped armchair(5,5) and zigzag(8,0) beryllium oxide nanotubes(BeONTs). The results show that there exists a structural distortion around the 3d TM impurities with respect to the pristine BeONTs. The magnetic moment increases for V- and Cr-doped BeONTs and reaches a maximum for Mn-doped BeONT, and then decreases for Fe-, Co-, Ni-, and Cu-doped BeONTs successively, consistent with the predicted trend of Hund’s rule to maximize the magnetic moments of the doped TM ions. However, the values of the magnetic moments are smaller than the predicted values of Hund’s rule due to the strong hybridization between the 2p orbitals of the near O and Be ions of BeONTs and the 3d orbitals of the TM ions. Furthermore, the V-, Co-, and Ni-doped(5,5) and(8,0) BeONTs with half-metal ferromagnetism and thus 100% spin polarization character are good candidates for spintronic applications.     

Orbital ordering, new phases, and stripe formation in doped layered nickelates

Ground-state properties of layered nickelates are investigated based on the orbital-degenerate Hubbard model coupled with lattice distortions, by using numerical techniques. The Néel state composed of spin S=1 ions is confirmed in the undoped limit x = 0. At x = 1/2, novel antiferromagnetic states, called CE- and E-type phases, are found by increasing the Hund's coupling. (3x2-r2/3y2-r2)-type orbital ordering is predicted to occur in a checkerboard-type charge-ordered state. At x = 1/3, both Coulombic and phononic interactions are found to be important, since the former stabilizes the spin stripe, while the latter leads to the striped charge order.     

Emergence of strong exchange interaction in the actinide series: the driving force for magnetic stabilization of curium

Using electron energy-loss spectroscopy, many-electron atomic spectral calculations, and density functional theory, we show that angular-momentum coupling in the 5f states plays a decisive role in the formation of the magnetic moment in Cm metal. 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. Hund's rule coupling is the key to producing the large spin polarization that dictates the newly found crystal structure of Cm under pressure.     

Coupling of magnetic and ferroelectric hysteresis by a multicomponent magnetic structure in Mn2GeO4

The olivine compound Mn(2)GeO(4) is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism. Symmetry suggests that the direct coupling between the ferromagnetism and ferroelectricity is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multicomponent magnetic structures found in magnetically frustrated materials like Mn(2)GeO(4) provide a new route towards functional materials that exhibit coupled ferromagnetism and ferroelectricity.