Hall breakdown in a modulation-doped GaAs/AlAs heterostructure

The effect of dc current I _dc on the electron transport in a GaAs quantum well with AlAs/GaAs superlattice barriers is studied experimentally at a temperature of 4.2 K in magnetic fields up to 2 T. A sharp increase in resistance R _xx is observed in magnetic fields higher than the critical field B _c. The value of B _c is found to decrease with increasing the current I _dc. The phenomenon observed in the experiment is qualitatively explained by the electric breakdown of superlattice barriers under the action of the Hall field.     

Static critical behavior of 3D frustrated Heisenberg model on stacked triangular lattice with variable interlayer exchange coupling

Several Monte Carlo algorithms are used to examine the critical behavior of the 3D frustrated Heisenberg model on stacked triangular lattice with variable interlayer exchange coupling for values of the interlayer-to-intralayer exchange ratio R = |′/J| in the interval between 0.01 and 1.0. A finite-size scaling technique is used to calculate the static magnetic and chiral critical exponents α (specific heat), γ and γ_k (susceptibility), β and β_k(magnetization), ν and ν_k(correlation length), and the Fisher exponent η. It is shown that 3D frustrated Heisenberg models on stacked triangular lattice with R > 0.05 constitute a new universality class of critical behavior. At lower R, a crossover from 3D to 2D critical behavior is observed.     

The hyperfine properties of a hydrogenated Fe/V superlattice

We study the effect of hydrogen on the electronic, magnetic and hyperfine structures of an iron-vanadium superlattice consisting of three Fe monolayers and nine V monolayers. The contact charge density (ρ), the contact hyperfine field (B_hf) and the electronic field gradient (EFG) at the Fe sites for different H locations and H fillings are calculated using the first principle full-potential linear-augmented-plane-wave (FP-LAPW) method. It is found that sizeable changes in the hyperfine properties are obtained only when H is in the interface region.     

Critical phenomena in ferromagnetic superlattices

Within the framework of the high-temperature series expansions technique, we examine the phase transition and the critical phenomena of a two-component superlattice with simple cubic structure, through three models: Ising, XY and Heisenberg. The reduced critical temperature of the system is studied as a function of the thickness of the constituents and the exchange interactions in each material, and within the interface. We show the existence of a critical thickness of the unit cell at which the reduced critical temperature of the binary superlattice remains insensitive to the exchange coupling within the interfaces. The values of the effective critical exponent γ _eff associated with the magnetic susceptibility agreed with the universal classes in the limit cases where the superlattice is still comparable to an infinite simple cubic lattice. We attribute the breakdown in the universality hypothesis to the crossover effects.     

Nonequilibrium states and anomalies of Young’s modulus in rare-earth cobaltites RBaCo4O7 (R = Dy-Er,Y, Lu) caused by short-range magnetic order

The elastic properties of rare-earth cobaltites RBaCo₄O₇ (R = Dy, Ho, Er, Y, Lu) have been investigated experimentally. It has been found that the temperature dependences of the Young’s modulus exhibit significant hysteresis and irreversible effects over a wide range (80–280 K) between the structural and magnetic phase transition temperatures. These effects indicate that the short-range magnetic order in the Co sub-system of the studied rare-earth cobaltites gradually develops below the structural phase transition, when the distortion of the structure relieves the frustration of exchange interactions both in the Kagome lattice and in the triangular lattice of the cobalt subsystem. At the magnetic phase transition temperature, there are weak and smoothed anomalies of the Young’s modulus, which correlate with the low dimensionality and frustration of the exchange interactions in the Co subsystem of the studied rare-earth cobaltites.     

Investigating phase transitions in a model Fe/V magnetic superlattice

We propose a model of an iron-vanadium superlattice for investigating phase transitions in multilayers. Several basic types of temperature dependences of spontaneous (resultant) magnetization in a magnetic superlattice model are revealed for the first time for various values of an interlayer exchange interaction.     

A comparative study of the dynamic critical behavior of the four-state Potts like models

We investigate the short-time critical dynamics of the Baxter-Wu (BW) and n=3 Turban (3TU) models to estimate their global persistence exponent θ_g. We conclude that this new dynamical exponent can be useful in detecting differences between the critical behavior of these models which are very difficult to obtain in usual simulations. In addition, we estimate again the dynamical exponents of the four-state Potts (FSP) model in order to compare them with results previously obtained for the BW and 3TU models and to decide between two sets of estimates presented in the current literature. We also revisit the short-time dynamics of the 3TU model in order to check if, as already found for the FSP model, the anomalous dimension of the initial magnetization x₀ could be equal to zero.     

Magnetic Structures and Magnetic Phase Transitions in Rare-Earth <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>Si<Subscript>2</Subscript> Intermetallic Compounds (<Emphasis Type="Italic">R</Emphasis> = Sm,Tb)

The magnetic structures that form in La_1–xR_xMn₂Si₂ (R = Sm, Tb) layered compounds with various concentrations x have been determined by magnetic neutron diffraction and magnetic measurements, and the magnetic phase diagrams have been built. It is shown that the formation of the magnetic structures is dependent not only on exchange interactions, but also on the type of the magnetic anisotropy of a rare-earth atom. It is found that, in La_1–xTb_xMn₂Si₂ compounds with 0.2 < x < 0.5, the competition of the Tb–Mn and Mn–Mn interlayer exchange interactions and the existence of a strong uniaxial magnetic anisotropy in the Mn and Tb sublattices leads to the frustrated magnetic state and prevents the formation of the long-range magnetic order in the Tb sublattice.     

Tunable magnetic anisotropy of antiferromagnetic superlattice and resultant exchange bias of ferromagnetic layer on it

Exchange biasing of ferromagnetic layer deposited on the antiferromagnetic superlattice was investigated in (Co₇₀Fe₃₀/Ru)_29.5/Ru/Co₉₀Fe₁₀ multilayers. Uniaxial magnetic anisotropy (K_AF) was induced and tuned in the antiferromagentic superlattice by uniaxial substrate bending method through the inverse effect of magnetostriction. The exchange bias increased and tended to be saturated with increasing the K_AF, while it was not observed at K_AF=0.     

Hydrogen induced changes of the interlayer coupling in Fe(3)/V(x) superlattices (x=11–16)

The effect of hydrogen on the magnetic exchange coupling between iron layers through vanadium spacer layers has been studied with magneto-optical Kerr effect experiments in Fe₍₃₎/V_(x) superlattices. Here x refers to the number of V monolayers varying from 11 to 16 and the Fe layer thickness is fixed at three monolayers. Without hydrogen the superlattice is antiferromagnetic (AFM) for x between 12 and 14 and ferromagnetic (FM) in all other cases. With hydrogen loading the coupling can be switched from AFM to FM and vice versa. As previously observed with neutron reflectivity measurements (Hjörvarsson et al., Phys. Rev. Lett. 79 (1997) 901) the change of the interlayer coupling upon hydrogen uptake is not simply due to the expansion of the non-magnetic vanadium spacer layer but more likely to the distortion of the Fermi surface. Bilinear and biquadratic exchange couplings can be recognized by the magnetic hysteresis loops and their coupling energies have been extracted by fits to the curves. For all samples the easy axis of the magnetization is in the plane without any preferred in-plane direction. Hydrogen loading does not affect the magnetic anisotropy of these samples.     

Critical Relaxation of a Three-Dimensional Fully Frustrated Ising Model

Critical relaxation from the low-temperature ordered state of the three-dimensional fully frustrated Ising model on a simple cubic lattice is studied by the short-time dynamics method. Cubic systems with periodic boundary conditions and linear sizes of L = 32, 64, 96, and 128 in each crystallographic direction are studied. Calculations were carried out by the Monte Carlo method using the standard Metropolis algorithm. The static critical exponents for the magnetization and correlation radius and the dynamic critical exponents are calculated.     

The Si(111) 7 × 7 TO “1 × 1” transition

We have made a detailed study of the Si(111) 7 × 7 to “1 × 1” transition at 1140 K using low-energy electron diffraction. Excess diffuse scattering appears during the transition whose temperature dependence and magnitude are consistent with the loss of the 7th order superlattice intensities showing the transition is to a disordered surface phase rather than a true unreconstructed ×1. This strongly favors the class of models for the 7 × 7 selvedge involving an ordered arrangement of subunits like vacancies or adatoms rather than models with periodic distortion. The superlattice intensity shows the transition is continuous and reversible at least for temperatures below 0.995 T_c. The critical exponent for the long-range order is β = 0.11 ± 0.015 similar to those for 2D Ising or Potts models. However, the system does not show the critical scattering properties expected for simple models. The relaxation time seems to diverge near T_c but only for negative temperature increments.     

Optical spectroscopy of low-dimensional rare-earth iron borates

The family of RFe₃(BO₃)₄ borates (R=Pr, Nd, Eu-Er, Y) was studied by high-resolution optical absorption and Raman spectroscopies. Structural and magnetic phase transitions were detected and the types of magnetic structure were determined. Energy of crystal-field (CF) levels and exchange splitting of the ground state of the R³⁺ ion were obtained from the analysis of optical spectra. CF calculations were carried out. Effective magnetic field at the R³⁺ site was found, using the calculated value of the magnetic g-factor. Some peculiarities of modeling the paramagnetic susceptibility of NdFe₃(BO₃)₄ are discussed.     

Magnetic structure simulation of Fe/V superlattices with a variable thickness of iron layers

The model of the magnetic structure of Fe/V superlattices is discussed. The discrepancy in estimation of the critical temperature for the Fe₂/V _n /Fe₃/V _n superlattice obtained by neutron scattering and the magneto-optical Kerr effect is caused by inhomogeneity of the magnetization distribution in a finite superlattice.     

The magnetic properties of a finite superlattice with two disordered interfaces

Within the framework of the effective field theory, based on a probability distribution technique, we examine the critical and compensation behaviors of a ferrimagnetic alternating superlattice on a simple cubic structure. The superlattice consists of k unit cells each of which consists of L layers of spin-1/2 A atoms, L layers of spin-1 B atoms and a disordered interface with two layers in between that is characterized by a random arrangement of A and B atoms A_pB_1−pA_1−pB_p with a negative coupling A − B. Considering a finite and infinite superlattices, the effect of the thickness of the film and the surface exchange coupling on the magnetic properties are studied. The obtained results show a number of characteristic phenomena.     

Magnetic interaction and the critical dynamics above the curie point

It is shown that near T_c, where 4πχ ? 1 magnetic interaction completely change the long-wave critical dynamics of the ferromagnets and the new dynamic critical exponent is z_d = (5 - η)/2 - ν^-1 ≈ 1.     

Phase transitions and critical properties of the frustrated Heisenberg model on a layer triangular lattice with next-to-nearest-neighbor interactions

The critical behavior of the three-dimensional antiferromagnetic Heisenberg model with nearest-neighbor (J) and next-to-nearest-neighbor (J ₁) interactions is studied by the replica Monte Carlo method. The first-order phase transition and pseudouniversal critical behavior of this model are established for a small lattice in the interval R = |J ₁/J| = 0?C0.115. A complete set of the main static magnetic and chiral critical indices is calculated in this interval using the finite-dimensional scaling theory.     

Controllable spin-dependent transport in silicene superlattice

Based on the transfer-matrix method, we theoretically investigate the spin-dependent transport properties in magnetic silicene superlattice in the presence of extrinsic Rashba spin–orbit interaction (RSOI). It is found that the spin transmission probability and spin conductivities can be efficiently controlled by the number of magnetic barriers. As the number of magnetic barriers increases, spin conductivities strongly decrease, and reduce to zero in the large on-site potential difference between A and B sublattices (Δ_z) region. The results indicate that a magnetic silicene superlattice exhibits a remarkable wavevector-dependent spin filtering effect. Also, the magnetoresistance (MR) ratio exhibits an oscillatory behavior with the Fermi energy. The MR ratio can be tuned by the Fermi energy, number of magnetic barriers and extrinsic RSOI. Specifically, in the presence of magnetic field the spin polarization can be observed, and increases by increasing the magnetic field.     

Structural and magnetic properties of Cr1+x(Se1−yTey)2 compounds

The structural and magnetic properties of Cr_1+x(Se_1−yTe_y)₂ having a NiAs structure has been studied for (1+x)=1.27, 1.32 and 1.36 and y=0.75 by means of the Korringa-Kohn-Rostoker (KKR) band structure method. The sub-stoichiometry and the disorder on the chalcogenide sub-lattice has been treated by means of the coherent potential approximation (CPA) alloy theory. From total energy calculations a preferential site occupation on the Cr sub-lattice was found together with an antiparallel alignment of the magnetic moments on the two inequivalent Cr layers. The magnetic properties at finite temperature has been studied by means of Monte Carlo simulations on the basis of a classical Heisenberg Hamiltonian and the exchange coupling parameters calculated from first principles. This approach allowed to determine the critical temperature in good agreement with experiment.