Magnetic Correlation Functions Research Articles

Quantitative MFM study of the global and local magnetic roughness of the DC-demagnetized state of thin film media

A magnetic height-height correlation function was used to quantitatively analyze the magnetic force microscope (MFM) images of the DC-demagnetized state of two thin film media. This function provided important information about the micromagnetic features of these media, including the global magnetic roughness, the local magnetic roughness, the strength of long-range magnetostatic interaction, and the degree of local intergranular exchange coupling. These features are accountable for the noise performance of these media detected by the recording head.

Magnetism and Jahn-Teller effect in LaMnO3

Combining charge transfer with the Jahn-Teller electron-phonon coupling, the magnetic properties of LaMnO3 and the influence of the Jahn-Teller effect are studied. The interaction parameters of super exchange coupling between manganese spins are obtained. In the ground state, manganese spins are antiferromagnetically ordered but canted by a small angle. The spectrum of the low-energy spin excitation is very close to that observed experimentally. Behaviors of magnetization and spin correlation functions of LaMnO3 exhibit certain two-dimensional ferromagnetic characters, even though it is an A-type antiferromagnet with anisotropic super exchange coupling. These results agree with several experiments. Furthermore, it is expected that the super exchange interactions depend on the electron-phonon coupling strength exponentially, and the Néel temperature will exhibit isotope shift.

Critical behavior and the Néel temperature of quantum quasi-two-dimensional Heisenberg antiferromagnets

The nonlinear sigma-model and its generalization on N-component spins, the O(N) model, are considered to describe thermodynamics of a quantum quasi-two-dimensional (quasi-2D) Heisenberg antiferromagnet. A comparison with standard spin-wave approaches is performed. The sublattice magnetization, Neel temperature and spin correlation function are calculated to first order of the 1/N-expansion. A description of crossover from a 2D-like to 3D regime of sublattice magnetization temperature dependence is obtained. The values of the critical exponents derived are \beta =0.36, \eta =0.09. An account of the corrections to the standard logarithmic term of the spin-wave theory modifies considerably the value of the Neel temperature. The thermodynamic quantities calculated are universal functions of the renormalized interlayer coupling parameter. The renormalization of interlayer coupling parameter turns out to be considerably temperature dependent. A good agreement with experimental data on La2CuO4 is obtained. The application of the approach used to the case of a ferromagnet is discussed.

The Behavior of the Critical Magnetic Field and Correlation Functions in Finite-Size Hubbard-Like Models

We present exact analytical results for a critical field and correlations functions for N site ring and arbitrary size two dimensional coupled chains in a two component Hubbard model with a strong magnetic field and exactly at half-filling. The critical field near the Nagaoka instability with one spin and two spin flips is driven by Coulomb interaction, lattice geometry, dimensionality and crucially depends on the number of atomic sites N. Unsaturated ferromagnetism, reminiscent of Nagaoka-like behavior, and spin liquid state are examined in the structure of off-diagonal spin correlation at large U.

Classical and wave-mechanical aspects of magneto-transport fluctuations in ballistic quantum dots

We examine the magneto-conductance fluctuations in quantum dot structures, for a weak field strength, using both the recursive Green's function technique and a classical simulation of ballistic motion employing semi-classical theory. We analyze the spectral content of the conductance fluctuations by both techniques, where the underlying classical dynamics are regular and chaotic, respectively, by constructing magnetic field conductance correlation functions and compare these with those obtained experimentally. We find that our numerical quantum mechanical correlation function results, for the chaotic dot, are in agreement with the predictions of semiclassical theory and consistent with experimental result.

Calculations of magneto-transport fluctuations in ballistic quantum dots

We examine the magneto-conductance fluctuations in ballistic quantum dot structures, for a weak field strength, using the recursive Green's function technique. We analyze the spectral content of the conductance fluctuations for two differently shaped dots, where the underlying classical dynamics are either regular or chaotic, by constructing magnetic field conductance correlation functions. We find our numerical quantum mechanical correlation function results for the chaotic dot are in agreement with semiclassical theory predictions.

Indications for RVB superconductivity in a generalized t- J model for the cuprates

We evaluate numerically several superconducting correlation functions in a generalized t- J model derived for hole-doped CuO 2 planes. The model includes a three-site term t″ similar to that obtained in the large- U limit of the Hubbard model but of opposite sign for realistic OO hopping. For parameters close to realistic values, we obtain strong evidence of superconductivity of predominantly d x 2− y 2 character. The ground state has a large overlap with a very simple resonating-valence-bond (RVB) wave function with off-diagonal long-range order. This function reproduces the main features of the magnetic and superconducting correlation functions.

Magnetic effects in transverse elastic constants of bcc Fe(Si)

We report measurements of the transverse elastic constants C′ = (C11 - C12)/2 and C44 of Fe(Si) single crystals from room temperature to T/TC ≌ 1.2 obtained from ultrasonic velocities determined by a pulse superposition method. Results are interpreted assuming that the elastic constants are the sum of an intrinsic magnetic contribution, proportional to the near-neighbour magnetic correlation function, and a ‘non-magnetic’ contribution (including indirect magnetostrictive contributions), approximated by a function of the lattice parameter only.

Superconducting, magnetic, and charge correlations in the doped two-chain Hubbard model.

We have studied the superconducting, magnetic, and charge correlation functions and the spin excitation spectrum in the doped two-chain Hubbard model by projector Monte Carlo and Lanczos diagonalization methods. The exponent of the interchain singlet superconducting correlation function, \ensuremath{\gamma}, is found to be close to 2.0 as long as two distinct noninteracting bands cross the Fermi level. Magnetic and charge correlation functions decay more rapidly than or as fast as the interchain singlet superconducting correlation function along the chains. The superconducting correlation in the doped two-chain Hubbard model is the most long-range correlation studied here. Implications of the results for the possible universality class of the doped two-chain Hubbard model are discussed.

Exact diagonalization study of the two-dimensional t-J model with adiabatic Holstein phonons: Single-hole case.

We present exact results for the ground-state properties of the two-dimensional single-hole Peierls t-J model on finite square lattices, up to 18 effective sites in size. Using a Lanczos technique for unrestricted energy minimization with respect to local lattice distortians, we study the competition between intrasite electron-phonon and intersite antiferromagnetic (AFM) exchange coupling on the hole state aver a wide range of interaction strengths. The structure of the doping state is examined by calculating local expectation values of spin and electron densities as well as magnetic and charge correlation functions. As the local electron-phonon interaction increases, we find a self-trapping transition from a delocalized hole (polaronlike) state to a mainly localized hole state

The half-filled Hubbard model in the pair approximation of the cluster variation method

The half filled Hubbard model is studied in the pair approximation of the Cluster Variation Method. The use of the $SO(4)$ symmetry of the model makes possible to give a complete analytical characterization of the ground state, by means of explicit expressions for the double occupancy and the nearest neighbor correlation functions. The finite temperature analysis is reduced to the numerical solution of only two coupled transcendental equations. The behavior of local magnetic moment, specific heat and correlation functions is given for some typical cases in one and two dimensions. We obtain good qualitative agreement with exact and numerical results in one dimension. The results for finite temperatures show a rapid evolution, with increasing temperature, from a strongly antiferromagnetic behavior to a disordered one; in the high temperature region a maximum (which has been related to a "gradual" metal--insulator transition) is found in the specific heat for very large values of the Coulomb repulsion.

A theoretical treatment of the magnetic resonance properties of Sc and Ru

The authors evaluate the low-temperature susceptibilities, the Knight shifts, and the spin-lattice relaxation times of the hexagonal d transition metals Sc and Ru and give a detailed account of the individual contributions expressed through magnetic correlation functions. The results are in reasonable agreement with tabulated experimental data for Sc and very recent measurements for Ru.

Ground-state disorder in the spin-1/2 kagomé Heisenberg antiferromagnet.

We search for broken symmetry in the ground state of the spin-1/2 Heisenberg antiferromagnet on a kagome$iaa--- lattice. Specifically, we calculate magnetic, spin-Peierls, spin-nematic, and chiral correlation functions in the ground states of clusters of 9, 12, 15, and 18 sites. All four correlation functions fall rapidly with distance. An analysis of their system-size dependence suggests that the ground state of this model is probably a spin liquid with the full symmetry of the Hamiltonian.

Investigations of magnetic excitations in a La2CuO4 single crystal

A detailed neutron scattering study is carried out of magnetic excitations in pure La2CuO4 in the low energy region up to 15 meV, at temperatures in the vicinity of the transition temperatureTN to the paramagnetic state. In particular the question of existence of propagating spin waves in this energy region is addressed. The experimental results show that the energy region studied (up to 15 meV) is characterized by aquasielastic peak of the width of order 4 meV (at 300 K). Thus the magnetic correlation function has relaxational behavior in the studied energy region, no indications for propagating spin waves are obtained. The measured quasielastic width agrees well with existing theories. Polarized neutrons and polarization analysis are also used to identify the magnetic origin of the observed scattering.

Magnetic short-range order in iron above Tc? Statistical mechanics with many-atom interactions

An effective spin Hamiltonian for iron has been derived previously from electronic structure calculations; there are strong many-atom interactions. The exchange interactions Jij depend on the surrounding magnetic order. The authors investigate the magnetization and spin correlation function in iron on this basis in two ways: by fitting the Jij to an analytically soluble Hamiltonian (an extended spherical model) and by Monte Carlo simulation. Short-range many-atom interactions do not lead to substantial short-range order in the paramagnetic state; the behaviour closely resembles that of the nearest-neighbour Heisenberg model. The main effect is a slight distortion of the magnetization curve below TC. Longer-range oscillatory pair interactions are needed for short-range order, but are not supported by the electronic structure data. They conclude that calculations of the electronic energy in static configurations do not lead to unusual short-range order.

Hole pairing on oxygen sites in high- Tc copper-oxide superconductors induced by an effective exchange interaction: A theoretical and numerical investigation on planar lattices

A previous theoretical-numerical investigation on the π structure of copper-oxygen high- T c superconductors, based on a model Hamiltonian which includes an effective exchange interaction between nearest oxygen sites, is extended here to planar lattices. Binding energy and magnetic correlation functions have been computed. The quasiparticle behaviour of the low-energy excitations has also been investigated through the spectral function S( k , ω). The obtained results confirm that, according to the present model, the holes arrange in singlet bonds corresponding to O 2 2−.

Magnetization Curve and Correlation Function of Haldane-Gap Antiferromagnet in Strong Magnetic Field

The shape of the magnetization curve of the S =1 Heisenberg antiferromagnet at zero temperature is discussed. It is found that the magnetization behaves as \(\sqrt{h-h_{c}}\) at the beginning of magnetization as a function of external field h . The transverse correlation function < S 0 x S n x >=< S 0 y S n y > of this system decays algebraically at zero temperature when it is partially magnetized by the field to the z -direction (< S 0 x S n x >≃(-1) n n -η ). The exponent η is 0.3∼0.5. This exponent can be calculated by the Luttinger liquid assumption and diagonalization results of finite chains. This is in contrast to the exponential decay of this chain in zero field.

Perturbation theory of the electronic properties in strongly correlated solids.

In an attempt to obtain a better description than the local-density approximation (LDA) for the electronic structure of realistic narrow-band systems, we have studied a perturbation-theory method for the self-energy and have tested it on the electronic properties of the periodic one-dimensional Anderson model. For a number of models we have calculated a wide range of properties such as quasiparticle energies, spectral densities, reduced hybridizations, and various magnetic correlation functions. For two different models we were able to compare with Monte Carlo results and found good agreement. In general, we have obtained a description of narrow-band systems that is more reasonable than the one given by the LDA.

Amplitude, wavevector and frequency dependence of magnetic fluctuations in YBa2Cu3O6+?

Polarized neutrons and polarization analysis were used to determine the total amplitude of magnetic fluctuations in tetragonal YBa2Cu3O6+δ for energies below 30 meV, both in the magnetically ordered state below and in the paramagnetic state above the Neel temperature. An energy analysis of the local magnetic correlation function (=S(r=0,ω)) was performed. The wavevector dependence of the energy integrated correlation function at high temperatures is dominated by local (i.e.q-independent) fluctuations. Total intensity, energy and wavevector dependence are substantially different from those of stable moments with quasi two-dimensional couplings. Magnetic amplitude fluctuation might be responsible for the observed behaviour.

On the short range magnetic order of iron aboveT c

A derivation of the ordering part of the free energy functional based on an expansion about the free energy of a random CPA tight-binding paramagnetic effective medium is presented. Considering the dominant term of this functional, which is of Heisenberg form, the static magnetic correlation function is calculated within a spherical model approximation. For some parameters a behaviour characteristics for systems with tendency to form helicoidal ordered structure belowTc is observed.