Compressible Sherrington-Kirkpatrick spin-glass model

We introduce a Sherrington-Kirkpatrick spin-glass model with the addition of elastic degrees of freedom. The problem is formulated in terms of an effective four-spin Hamiltonian in the pressure ensemble, which can be treated by the replica method. In the replica-symmetric approximation, we analyze the pressure-temperature phase diagram, and obtain expressions for the critical boundaries between the disordered and the ordered (spin-glass and ferromagnetic) phases. The second-order para-ferromagnetic border ends at a tricritical point, beyond which the transition becomes discontinuous. We use these results to make contact with the temperature-concentration phase diagrams of mixtures of hydrogen-bonded crystals.

Antiferromagnetic spherical spin-glass model

We study the thermodynamic properties and the phase diagrams of a multi-spin antiferromagnetic spherical spin-glass model using the replica method. It is a two-sublattice version of the ferromagnetic spherical p-spin glass model. We consider both the replica-symmetric and the one-step replica-symmetry-breaking solutions, the latter being the most general solution for this model. We find paramagnetic, spin-glass, antiferromagnetic and mixed or glassy antiferromagnetic phases. The phase transitions are always of second order in the thermodynamic sense, but the spin-glass order parameter may undergo a discontinuous change.

Surface spin disorder in nickel ferrite nanomagnets studied by in-field Mossbauer spectroscopy

The magnetic structure of NiFe(2)O(4) nanoparticles has been investigated by means of Mossbauer spectra at T=4.2 K in applied fields up to 12 T. Four samples were studied, with mean particle diameters ranging from 4.3 to 8.9 nm. All spectra could be decomposed into three sextets, two corresponding to the ferrimagnetic sublattices of Fe ions in the spinel structure (core) and the third one to randomly frozen spins near the particle surface (shell). The shell thickness, calculated from the fraction of disordered spins, was found to be about one-third of the particle radius at H (app)=e0 and to decrease with the applied field toward a common limit of similar to 0.4 nm. The mean canting angle relative to the field was also found to decrease for increasing fields, at a rate inversely correlated to the particle size.

Observation of two-magnon bound states in the spin-1 anisotropic Heisenberg antiferromagnetic chain system NiCl2-4SC(NH2)(2)

Results of systematic tunable-frequency ESR studies of the spin dynamics in NiCl2-4SC(NH2)(2) (known as DTN), a gapped S = 1 chain system with easy-plane anisotropy dominating over the exchange coupling (large-D chain), are presented. We have obtained direct evidence for two-magnon bound states, predicted for S = 1 large-D spin chains in the fully spin-polarized (FSP) phase. The frequency-field dependence of the corresponding excitations was calculated using the set of parameters obtained earlier [S.A. Zvyagin, et al., Phys. Rev. Lett. 98 (2007) 047205]. Very good agreement between the calculations and the experiment was obtained. It is argued that the observation of transitions from the ground to two-magnon bound states might indicate a more complex picture of magnetic interactions in DTN, involving a finite in-plane anisotropy. (C) 2007 Elsevier B.V. All rights reserved.

Spin current in the Mobius cyclacene belts

We report on density functional theory studies of the electronic structure and magnetic properties of Mobius-[n]cyclacenes. The geometry of Mobius bands presents a modulation of bond lengths that is needed to accommodate the twist. This modulation takes the form of bond alternation defects analogous to those of solitons in polyacetylene. The ground state of all Mobius bands is a triplet, with a spin density distribution that follows the bond length modulation. A molecular dynamics simulation of the Mobius cyclacene at 300 K shows that the twist travels around the belt inducing a magnetic current. (C) 2009 Elsevier B. V. All rights reserved.

Experimental evidence of the spin-glass transition in the diluted magnetic semiconductor Zn(1-x)Mn(x)In(2)Se(4)

This work reports on magnetic measurements of the quasi-two-dimensional (quasi-2D) system Zn(1-x)Mn(x)In(2)Se(4), with 0.01 <= x <= 1.00. For x > 0.67, the quasi-2D system seems to develop a spin-glass behaviour. Evidence of a true phase transition phenomenon is provided by the steep increase of the nonlinear susceptibility chi(nl) when approaching T(C) from above. The static scaling of chi(nl) data yields critical exponents delta = 4.0 +/- 0.2, phi = 4.37 +/- 0.17 and TC = 3.4 +/- 0.1 K for the sample with x = 1.00 and similar values for the sample with x = 0.87. These critical exponents are in good agreement with values reported for other spin-glass systems with short-range interactions.

sigma- and pi-defects at graphene nanoribbon edges: Building spin filters

The presence of certain kinds of defects at the edges of monohydrogenated zigzag graphene nanoribbons changes dramatically the charge transport properties inducing a spin-polarized conductance. Using an approach based on density functional theory and nonequilibrium Green`s function formalism to calculate the transmittance, we classify the defects in different classes depending on their distinct transport properties: (i) sigma-defects, which do not affect the transmittance close to the Fermi energy (E(F)); and (ii) pi-defects, which cause a spin polarization of the transmittance and that can be further divided into either electron or hole defects if the spin transport polarization results in larger transmittance for the up or down spin channel, respectively.

Symmetry controlled spin polarized conductance in Au nanowires

The fact that the resistance of propagating electrons in solids depends on their spin orientation has led to a new field called spintronics. With the parallel advances in nanoscience, it is now possible to talk about nanospintronics. Many works have focused on the study of charge transport along nanosystems, such as carbon nanotubes, graphene nanoribbons, or metallic nanowires, and spin dependent transport properties at this scale may lead to new behaviors due to the manipulation of a small number of spins. Metal nanowires have been studied as electric contacts where atomic and molecular insertions can be constructed. Here we describe what might be considered the ultimate spin device, namely, a Au thin nanowire with one Co atom bridging its two sides. We show that this system has strong spin dependent transport properties and that its local symmetry can dramatically change them, leading to a significant spin polarized conductance.

Characterization, photoluminescence, thermally stimulated luminescence and electron spin resonance studies of Eu(3+) doped LaAlO(3) phosphor

Europium-doped lanthanum aluminate (LaAlO(3)) powder was prepared by using a combustion method. The crystallization, surface morphology, specific surface area and luminescence properties of the samples have been investigated. Photoluminescence studies of Eu doped LaAlO(3) showed orange-reddish emission due to Eu(3+) ions. LaAlO(3):Eu(3+) exhibits one thermally stimulated luminescence (TSL) peak around 400 degrees C. Room temperature electron spin resonance spectrum of irradiated phosphor appears to be a superposition of two centres. One of them (centre I) with principal g-value 2.017 is identified as an O(-) centre while centre II with an isotropic g-value 2.011 is assigned to an F(+) centre (singly ionized oxygen vacancy). An additional defect centre observed during thermal annealing around 300 degrees C grows with the annealing temperature. This centre (assigned to F(+) centre) originates from an F-centre (oxygen vacancy with two electrons) and the F-centre along with the associated F(+) centre appear to correlate with the observed TSL peak in LaAlO(3):Eu(3+) phosphor. The activation energy for this peak has been determined to be 1.54 eV from TSL data. (C) 2010 Elsevier Masson SAS. All rights reserved.

Quantum spin tunneling of magnetization in small ferromagnetic particles

An approach is presented that can also account for the description of small ferromagnetic particle magnetization tunneling. An estimate of the saturation value of an external applied magnetic field along the easy axis is obtained. An analytic expression for the tunneling factor in the absence of an external magnetic field is deduced from the present approach that also allows one to obtain the crossover temperature characterizing the regime where tunneling is dominated by quantum effects. (C) 2009 Published by Elsevier B.V.

Spin Hall Effect in Symmetric Wells with Two Subbands

We investigate the spin Hall conductivity sigma (xy) (z) of a clean 2D electron gas formed in a two-subband well. We determine sigma (xy) (z) as arising from the inter-subband induced spin-orbit (SO) coupling eta (Calsaverini et al., Phys. Rev. B 78:155313, 2008) via a linear-response approach due to Rashba. By self-consistently calculating eta for realistic wells, we find that sigma (xy) (z) presents a non-monotonic (and non-universal) behavior and a sign change as the Fermi energy varies between the subband edges. Although our sigma (xy) (z) is very small (i.e., a parts per thousand(a)`` e/4 pi aEuro(3)), it is non-zero as opposed to linear-in-k SO models.

Phase diagram and spectral properties of a new exactly integrable spin-1 quantum chain

The spectral properties and phase diagram of the exactly integrable spin-1 quantum chain introduced by Alcaraz and Bariev are presented. The model has a U(1) symmetry and its integrability is associated with an unknown R-matrix whose dependence on the spectral parameters is not of a different form. The associated Bethe ansatz equations that fix the eigenspectra are distinct from those associated with other known integrable spin models. The model has a free parameter t(p). We show that at the special point t(p) = 1, the model acquires an extra U(1) symmetry and reduces to the deformed SU(3) Perk-Schultz model at a special value of its anisotropy q = exp(i2 pi/3) and in the presence of an external magnetic field. Our analysis is carried out either by solving the associated Bethe ansatz equations or by direct diagonalization of the quantum Hamiltonian for small lattice sizes. The phase diagram is calculated by exploring the consequences of conformal invariance on the finite-size corrections of the Hamiltonian eigenspectrum. The model exhibits a critical phase ruled by the c = 1 conformal field theory separated from a massive phase by first-order phase transitions.

Shot Noise in a Spin-Diode Geometry

We apply the master equation technique to calculate shot noise in a system composed of single level quantum dot attached to a normal metal lead and to a ferromagnetic lead (NM-QD-FM). It is known that this system operates as a spin-diode, giving unpolarized currents for forward bias and polarized current for reverse bias. This effect is observed when only one electron can tunnel at a time through the dot, due to the strong intradot Coulomb interaction. We find that the shot noise also presents a signature of this spin-diode effect, with a super-Poissonian shot noise for forward and a sub-Poissonian shot noise for reverse bias voltages. The shot noise thus can provide further experimental evidence of the spin-rectification in the NM-QD-FM geometry.

Spin-density functional for exchange anisotropic Heisenberg model

Ground-state energies for anti ferromagnetic Heisenberg models with exchange anisotropy are estimated by means of a local-spin approximation made in the context of the density functional theory. Correlation energy is obtained using the non-linear spin-wave theory for homogeneous systems from which the spin functional is built. Although applicable to chains of any size, the results are shown for small number of sites, to exhibit finite-size effects and allow comparison with exact-numerical data from direct diagonalization of small chains. (C) 2009 Elsevier B.V. All rights reserved.

WRITING ELECTRONIC FERROMAGNETIC STATES IN A HIGH-TEMPERATURE PARAMAGNETIC NUCLEAR SPIN SYSTEM

In this paper, we use Nuclear Magnetic Resonance (NMR) to write electronic states of a ferromagnetic system into high-temperature paramagnetic nuclear spins. Through the control of phase and duration of radio frequency pulses, we set the NMR density matrix populations, and apply the technique of quantum state tomography to experimentally obtain the matrix elements of the system, from which we calculate the temperature dependence of magnetization for different magnetic fields. The effects of the variation of temperature and magnetic field over the populations can be mapped in the angles of spin rotations, carried out by the RF pulses. The experimental results are compared to the Brillouin functions of ferromagnetic ordered systems in the mean field approximation for two cases: the mean field is given by (i) B = B(0) + lambda M and (ii) B = B(0) + lambda M + lambda`M(3), where B(0) is the external magnetic field, and lambda, lambda` are mean field parameters. The first case exhibits second order transition, whereas the second case has first order transition with temperature hysteresis. The NMR simulations are in good agreement with the magnetic predictions.