Spin-glass surface disorder on the magnetic behaviour of antiferromagnetic small particles

We studied the magnetic properties of an antiferromagnetic small particle of Ising spins. The particle is represented by a two-dimensional array of spins, where the coupling between spins in the core is antiferromagnetic, and its surface is modelled by a shell of spins with competing interactions, simulating a spin-glass type ordering at the surface. We investigated this model by mean-field approximation and Monte Carlo simulations. The magnetic behaviour was studied as a function of the temperature and external magnetic field. Some of the experimental findings observed in real antiferromagnetic nanoparticles, like hysteresis, shifted loops and coercive field, are obtained for this model. We showed that these properties strongly depend on the degree of surface disorder.     

Ferromagnetism versus superconductivity in ramdom alloys

Magnetic, superconducting narrow-band alloys are studied within a random lattice model. Coulomb repulsion and contact-type attraction of the itinerant electrons, and the indirect exchange between local magnetic moments are taken into account. Averaging the free energy expansion in coherent potential approximation, the Ginzburg-Landau functional is derived with two biquadratically coupled order parameters. Coherent orderings and concentration-dependent critical temperatures are determined. The coefficients of theq ²-terms, i.e. the stiffness and diffusion constants, involve vertex corrections due to impurity scattering. The coexistence criterion resulting from the 4th-order couplings cannot be fulfilled, referring to a first order transition between ferromagnetism and superconductivity. Tetracritical behaviour is found by including the direct exchanges between the localized spins.     

On the susceptibility of the one-dimensional Ising chain

The elements of the tensor of the susceptibility and the staggered susceptibility of a one-dimensional I Ising chain, with xx couplings between the spins are investigated in the presence of a magnetic field b parallel with or perpendicular to the x direction. Exact expressions are given for all susceptibilities, apart from the parallel susceptibilities in a transverse field which are evaluated by perturbation calculation. Special attention is paid to the conditions under which a susceptibility can have a minimum for b = 0. Furthermore, a system with weakly coupled Ising chains is considered on the basis of a model hamiltonian with separable interchain interactions.     

The local autocorrelation time in thin film and semi-infinite model

The nearest-neighbour Ising model of a film in which exchange couplings in surface layers can differ from exchange couplings in other layers is considered. The dependence of the local autocorrelation time on distances to surfaces of the film, temperature and surface exchange couplings is discussed. The behaviour of the local autocorrelation time in a three-dimensional semi-infinite model is obtained assuming that the thickness of the film tends to infinity.     

Spin, charge and lattice couplings in Cu-deficient oxysulphide BiOCu0.94S

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.94)S are investigated using neutron diffraction, ac magnetic susceptibility, magnetization and electrical resistivity measurements. The Cu spins order in a ferromagnetic arrangement below T(C) = 250 K. An antiferromagnetic component develops below 180 K when the crystalline unit cell experiences a sharp thermal contraction upon cooling, resulting in a canted ferromagnetic spin arrangement at low temperatures. In the magnetically ordered state the electrical transport can be described using three-dimensional variable range hopping conduction. An applied magnetic field can effectively reduce the hopping barrier. Spin-charge couplings are clearly revealed when the resistivity departs from the hopping conduction and begins to increase with increasing temperatures above 250 K where the Cu spins become disordered.     

Field-dependent nuclear relaxation of spins 1/2 induced by dipole-dipole couplings to quadrupole spins: LaF3 crystals as an example

A general theory of spin-lattice nuclear relaxation of spins I=1/2 caused by dipole-dipole couplings to quadrupole spins S1, characterized by a non-zero averaged (static) quadrupole coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through dipole-dipole couplings with certain transitions of quadrupole spins. The averaged quadrupole coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual dipole-dipole coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the 'classical' Solomon relaxation theory.     

The modulation of coupled relaxation in porous media.

This work investigates the effects of modulation of the transverse and longitudinal relaxation of the surface-fluid/pore-fluid spin system in porous media. Important new NMR well logging applications identify pore fluids by varying the CPMG T(2) pulse spacing to discriminate on the basis of fluid diffusivities in applied and local static magnetic field gradients. However, anomalous laboratory CPMG T(2) results have been reported repeatedly over 25 years for various porous media filled with a single fluid. In relatively large pores, at near bulk conditions, the transverse relaxation of diffusing molecular spins should be proportional to the square of the CPMG pulse spacing tau, the susceptibility contrast at the pore wall and the applied gradient. Observed is a markedly linear tau dependence that saturates at a plateau for large tau. The effect is not quadratic in applied gradient or susceptibility. For large pores, the tau dependence and the saturation value are proportional to the surface-to-volume ratio of the pores. This is in distinct contrast to the behavior observed by Borgia, Brown and Fantazzini for systems with much smaller pores at higher magnetic fields. The large-pore anomalous behaviors can be explained as a modulation of the exchange between surface-fluid and pore-fluid spins, such as observed by Luz and Meiboom in 1963 for water enriched with quadrupolar 17O. Scalar coupling of the solid-surface spins to the fluid-surface spins was postulated by Kleinberg, Kenyon and Mitra as a dominant relaxation mechanism for the surface fluid. The CPMG tau effect can be described as the modulation of the exchange coupling by the CPMG pi pulses, which mix the magnetizations between the exchanging, strongly coupled spin systems of the pore-fluid and the surface-fluid, which is, in turn, weakly coupled by scalar or pseudo-scalar interactions to the fast-relaxing solid surface.     

Functional magnetic resonance imaging with intermolecular multiple-quantum coherences

For the first time, we demonstrate here functional magnetic resonance imaging (fMRI) using intermolecular multiple-quantum coherences (iMQCs). iMQCs are normally not observed in liquid-state NMR because dipolar interactions between spins average to zero. If the magnetic isotropy of the sample is broken through the use of magnetic field gradients, dipolar couplings can reappear, and hence iMQCs can be observed. Conventional (BOLD) fMRI measures susceptibility variations averaged over each voxel. In the experiment performed here, the sensitivity of iMQCs to frequency variations over mesoscopic and well-defined distances is exploited. We show that iMQC contrast is qualitatively and quantitatively different from BOLD contrast in a visual stimulation task. While the number of activated pixels is smaller in iMQC contrast, the intensity change in some pixels exceeds that of BOLD contrast severalfold.     

Dynamics of Genuine Three-Qubit Entanglement in Ising Spin Systems

We investigate the dynamics of genuine three-qubit entanglement in the Ising model of three spins. A scheme is presented for generating the genuine three-qubit entanglement by the nearest-neighbour couplings. The effect of magnetic fields on the dynamics of genuine three-qubit entanglement is also discussed.     

Splitting of satellites in a time-dependent Kondo model

A time-dependent Kondo model, where both the voltage and the Kondo couplings oscillate on time, is considered. The bosonization technique at the Toulouse limit is applied to study the conductance and the magnetic susceptibility. It is shown that in addition to the satellites of the Kondo peak that appear in the conductance and the susceptibility as a function of the magnetic field or dc voltage, when the voltage oscillates, these satellites further split when the Kondo couplings also oscillate on time. The distance of the satellite splitting solely depends on the ratio between the oscillation frequencies of the voltage and of the Kondo couplings. When the Kondo couplings oscillate more rapidly than the voltage, the distance of the satellite peaks can be smaller than the voltage oscillation frequency.     

Spin glass model based on the Onsager reaction field

Changes in the Onsager reaction field are used to account quantitatively for aging (the decrease in the magnetic susceptibility when cooling in zero field is halted below the glass temperature) and rejuvenation (the disappearance of aging phenomena on further cooling only to reappear at Tw on heating) that characterize spin glasses. These effects must be caused by interactions between the spins since, absent the interactions, the magnetic properties of N spins are just N times the magnetic property of a single spin that cannot display aging. A spin introduced at an empty site with a nonzero field becomes polarized, and the polarized spin in turn polarizes its neighbors, thereby changing the local field. This additional field is the Onsager reaction field. Ma's theory for the reaction field in spin glasses [PRB 22, 4484 (1980)10.1103/PhysRevB.22.4484] has been extended to provide a spin-glass model that can account for the experimental data.     

Kondo screening in a magnetically frustrated nanostructure: exact results on a stable non-fermi-liquid phase

Triangular symmetry stabilizes a novel non-Fermi-liquid phase in the three-impurity Kondo model with frustrating antiferromagnetic interactions between half-integer impurity spins. The phase arises without fine-tuning of couplings, and is stable against magnetic fields and particle-hole symmetry breaking. We find a conformal field theory describing this phase, verify it using the numerical renormalization group, and extract various exact, universal low-energy properties. Signatures predicted in electrical transport may be testable in scanning tunneling microscopy or quantum-dot experiments.     

Theoretical study of magnetic spin correlations and the magnetocapacitance effect in BiMnO<Subscript>3</Subscript>

The magnetic correlations between Mn ion spins and their effects on the magnetocapacitance in BiMnO₃ are studied based on a microscopic model. Above the phase transition temperature of the magnetic subsystem, the magnetic correlations including the longitudinal and transverse parts are nonzero, and they lead to a nonzero specific heat of magnetic subsystem. We show that the magnetocapacitance effect is ascribed to the fluctuation of magnetic correlation. The linear relation between electric susceptibility and the magnetic correlation is obtained and the relevant experiments are explained.     

D.C. electrical resistivity as a functional of generalized dynamical susceptibilities—Comparison of different methods

The d.c. electrical resistivity is considered for a general solid state model characterized by local interactions of the charge carriers with a system of scatterers which may be e.g. quasiparticles, spins or impurities. A basic relationship - valid over the full temperature range - between the electrical resistivity of the charge carrier system (without magnetic field) and the generalized dynamical susceptibility of the scattering system is derived in the weak scattering limit. The equivalence of the following methods is shown: Boltzmann equation approach (variation procedure), Mori formalism, force-force correlation function method (for the total forces on the charge carriers), and the method of the transformed Liouville equation; herewith the equivalence with the Kubo or Green function method, the Zubarev formalism, the information-theoretical method and the Zwanzig formalism is also given. As a specific example, the s-d exchange model for the scattering of conduction electrons by localized spins is discussed including the corresponding electron-magnon system as a low-temperature approximation.     

Entanglement in a quantum mixed-spin chain

The pairwise entanglement between neighboring spins in a general mixed-spin chain with arbitrary spins S and 1/2 is investigated in the thermodynamical limit. The entanglement is witnessed by the magnetic susceptibility which determines a characteristic temperature for an entangled thermal state. The characteristic temperature is nearly proportional to the interaction J and the mixed-spin S. The bound of negativity is obtained on the basis of the magnetic susceptibility. It is found that the macroscopic magnetic properties are affected by the quantum entanglement in the real solids.     

Antiferromagnetic interactions between localized states in amorphous germanium

The temperature dependence of the magnetic susceptibility (300 → 1.6°K) and ESR (300 → 6°K) of amorphous germanium have been determined. There is a temperature dependent paramagnetic term to the magnetic susceptibility due to a density of localized unpaired spins (dangling bonds) of 10¹⁹ spins/cm³. There is an antiferromagnetic interaction between at least some of these localized unpaired spins with an exchange energy estimated by various models to be on the order of a degree Kelvin.     

Ordering in nematic liquid crystals from NMR cross-polarization studies

The measurement of dipolar couplings between nuclei is a convenient way of obtatining directly liquid crystalline ordering through NMR since the coupling is dependent on the average orientation of the dipolar vector in the magnetic field which also aligns the liquid crystal. However, measurement of the dipolar coupling between a pair of selected nuclei is beset with problems that require special solutions. In this article the use of cross polarization for measuring dipolar couplings in liquid crystals is illustrated. Transient oscillations observed during cross polarization provide the dipolar couplings between essentially isolated nearest neighbour spins which can be extracted for several sites simultaneously by employing two-dimensional NMR techniques. The use of the method for obtaining heteronuclear dipolar couplings and hence the order parameters of liquid crystals is presented. Several modifications to the basic experiment are considered and their utility illustrated. A method for obtaining proton-proton dipolar couplings, by utilizing cross polarization from the dipolar reservoir, is also presented.     

1-D EPR imaging of conducting and lossy-dielectric samples

Radiation power absorbed by a half-infinite sample irradiated by microwave field incident normally to the sample surface is calculated. Local properties of the sample, such as electric conductivity, magnetic susceptibility, concentration of localized spins, etc., are supposed to be nonhomogeneous in the direction perpendicular to the surface. In linear on magnetic susceptibility approximation, algorithms of electron paramagnetic resonance imaging are suggested for the case of conducting and lossy-dielectric materials. These algorithms are tested by numerical simulations. Obtained results can be applied to nuclear magnetic resonance imaging as well.     

A Monte Carlo study of the thermodynamic properties of a quasi-one-dimensional organic polymer ferromagnet

The thermodynamic properties of a quasi-one-dimensional organic ferromagnet at different temperatures and in different applied magnetic fields have been investigated by means of the Heisenberg model combined with the Monte Carlo method. The results indicate that a peak in the magnetic susceptibility is obtained at low temperatures. Furthermore, the effect of dimerization on the magnetic properties has also been studied. We find that the dimerization suppresses the magnetization in this model. The ferromagnetic couplings between the side-free radicals stabilize the ferromagnetism and increase the apparent Curie temperature.     

Double-exchange model for molecule-based magnets

We report a detailed study of a double-exchange model proposed for the molecule-based magnets. The model is applied to a two-dimensional periodic complex made of a transition metal and an organic molecule in which the electronic structure is described by effective d orbitals of the transition metal ion at infinite Hund's coupling limit and the lowest unoccupied molecular orbital of the organic molecule, π^∗. Depending on the average electron density of the organic molecules and various superexchange couplings between metal ions' core spins, magnetic states of the complex are investigated. Performing Monte Carlo calculations on a model Hamiltonian for various electron densities of the organic molecule, the average magnetization and critical magnetic ordering temperatures are determined.