Magnetization studied on spin alignments in organic molecule-based ferrimagnetics

A physics picture of spin alignments in molecule-based ferrimagnets is presented from studying the temperature dependence of the effective sublattice magnetic moments and the total reduced magnetization by means of Green’s function theory combined with the Jordan-Wigner transformation. The ferrimagnetic chain includes an S=1 biradical and an S=1/2 monoradical with antiferromagnetic alternating interactions, and the S=1 site in the chain is composed of two S=1/2 spins coupled by a finite ferromagnetic interaction. From the calculations of the sublattice magnetic moments, the magnetic moment of the S=1 biradical is negative, while that of the S=1/2 monoradical is positive, leading to a ferrimagnetic ground state. With the different kinds of the elementary excitations and the competition between the magnetic interactions and thermal fluctuations, the temperature dependence of the magnetization displays rich thermodynamic properties. Meanwhile, the external magnetic field dependence of the magnetization has a clear plateau at one third of the saturation magnetization, which can be compared with the possible experimental findings.     

Phase transitions in a two-dimensional dipole ferrimagnet

The magnetic configurations of the system of magnetic dipoles that have different values and are arranged in a staggered order on a square lattice are studied. A numerical simulation is used to study the phase transitions in the system when the mismatch between the dipoles changes. The restructuring of the magnetic configuration of the system induced by a change in the mismatch is shown to proceed via sequential second-order phase transitions between collinear and noncollinear phases. The numerical simulation results are supported by analytical calculations performed with trial functions.     

Magnetization transport and quantized spin conductance

We analyze transport of magnetization in insulating systems described by a spin Hamiltonian. The magnetization current through a quasi-one-dimensional magnetic wire of finite length suspended between two bulk magnets is determined by the spin conductance which remains finite in the ballistic limit due to contact resistance. For ferromagnetic systems, magnetization transport can be viewed as transmission of magnons, and the spin conductance depends on the temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin conductance is quantized in units of order (gmu(B))(2)/h at T=0. Magnetization currents produce an electric field and, hence, can be measured directly. For magnetization transport in electric fields, phenomena analogous to the Hall effect emerge.     

Magnetization,Mössbauer and neutron studies of α- and β-LiMnFeF6; Evidence of idle spin behaviour in a ferrimagnet

The trigonal phases α- and β-LiMnFeF₆ have been investigated by neutron diffraction, magnetization measurements, remanent magnetization and Mössbauer spectroscopy in an external field. Below 158 K, α-LiMnFeF₆ is antiferromagnetic, with the Mn and Fe spins aligned antiparallel along the c axis. Below 113 K, the β-phase is ferrimagnetic, with the Mn(3e) and Fe(2d) spins aligned antiparallel along the c axis. Fe(1a) is weakly coupled to the net magnetization and behaves as an “idle spin”. The data are consistently analysed and discussed with respect to the connecting mode of the MF₆ octahedra.     

Magnetization at corners in two-dimensional Ising models

We investigate the corner spin magnetization of two-dimensional ferromagnetic Ising models in various wedge geometries. Results are obtained for triangular and square lattices by numerical studies on finite wedges using the star-triangle transformation, as well as analytic calculations using corner transfer matrices and a fermionic representation of the row-to-row transfer matrix. The corner magnetizations vanish at the bulk critical temperature T_c with an exponent _c differing from the bulk exponent. For isotropic systems with free edges we find that _c is given simply by _c =/2, where is the angle at the corner. For apex magnetizations of conical lattices we obtain the strikingly similar result _a=/4. These formulas apply equally well to anisotropic lattices if the angle is interpreted as an effective angle, ^eff, determined by the relative strengths of the interactions.     

Electrical spin injection from an organic-based ferrimagnet in a hybrid organic-inorganic heterostructure

We report the successful extraction of spin-polarized current from the organic-based room temperature ferrimagnetic semiconductor V[TCNE](x) (x～2, TCNE: tetracyanoethylene; T(C)～400 K, E(G)～0.5 eV, σ(300?K)～10(-2) S/cm) and its subsequent injection into a GaAs/AlGaAs light-emitting diode. The spin current tracks the magnetization of V[TCNE](x～2), is weakly temperature dependent, and exhibits heavy-hole-light-hole asymmetry. This result has implications for room temperature spintronics and the use of inorganic materials to probe spin physics in organic and molecular systems.     

Magnetization of two-dimensional superconductors with defects

A new method is developed for numerical simulation of the magnetization of layered superconductors with defects that is based on the Monte Carlo algorithm. The minimization of the free energy functional of a two-dimensional vortex system enables one to obtain equilibrium configurations of vortex density and calculate the magnetization of a superconductor with arbitrary distribution of defects in a wide temperature range. Magnetization curves are obtained for the first time for a defective superconductor under conditions of cyclic variation of the external magnetic field for different temperatures. The magnetic induction profiles and the magnetic flux distribution inside a superconductor are calculated, which support the validity of Bean’s model. It is demonstrated that the process of magnetization reversal is accompanied by the emergence of an annihilation wave, i.e., the motion of a zone with zero magnetic induction at the leading front of the incoming magnetic flux.     

Magnetization reversal in spin patterns with complex geometry

We study field-driven dynamics of spins with antiferromagnetic interactions along the links of a complex substrate geometry, which is modeled by graphs of a controlled connectivity distribution. The magnetization reversal occurs in avalanches of spin flips, which are pinned by the topological constraints of the underlying graph. The hysteresis loop and avalanche sizes are analyzed and classified in terms of the graph's connectivity and clustering. The results are relevant for magnets with a hierarchical spatial inhomogeneity and for design of nanoscale magnetic devices.     

Magnetization curves of deposited finite spin chains

The European Physical Journal B - Two-dimensional arrays of interacting magnetic nanostructures offer a remarkable playground for simulating, experimentally, lattice spin models. Initially designed...     

ac Magnetization transport and power absorption in nonitinerant spin chains

We investigate the ac transport of magnetization in nonitinerant quantum systems such as spin chains described by the XXZ Hamiltonian. Using linear response theory, we calculate the ac magnetization current and the power absorption of such magnetic systems. Remarkably, the difference in the exchange interaction of the spin chain itself and the bulk magnets (i.e., the magnetization reservoirs), to which the spin chain is coupled, strongly influences the absorbed power of the system. This feature can be used in future spintronic devices to control power dissipation. Our analysis allows us to make quantitative predictions about the power absorption, and we show that magnetic systems are superior to their electronic counterparts.     

Magnetization measurements of magnetic two-dimensional electron gases

We directly measure the magnetization of both the conduction electrons and Mn2+ ions in (Zn,Cd,Mn)Se two-dimensional electron gases (2DEGs) by integrating them into ultrasensitive micromechanical magnetometers. The interplay between spin and orbital energy in these magnetic 2DEGs causes Landau level degeneracies at the Fermi energy. These Landau level crossings result in novel features in the de Haas-van Alphen oscillations, which are quantitatively reproduced by a simple model.     

Muon spin relaxation in spin gap state of BaVS3

Positive muon spin relaxation measurements were performed on a spin-1/2 system BaVS₃, which shows a metal-insulator transition at T_MI= 70 K. We found a marked muon-spin depolarization below T_X= 30 K without appreciable critical divergence. The possibility of muonium formation in the insulating state rather than electron spin freezing is discussed taking into account the quenching of V spins evidenced by ⁵¹V NMR and NQR measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Conduction-electron spin resonance in two-dimensional structures

The influence of the conduction-electron spin magnetization density, induced in a two-dimensional electron layer by a microwave electromagnetic field, on the reflection and transmission of the field is considered. Because of the induced magnetization and electric current, both the electric and magnetic components of the field should have jumps on the layer. A way to match the waves on two sides of the layer, valid when the quasi-two-dimensional electron gas is in the one-mode state, is proposed. By following this way, the amplitudes of transmitted and reflected waves as well as the absorption coefficient are evaluated.     

Energy gap of spin nanotube

Recently some quantum spin systems on tube lattices, so-called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As a first step of theoretical study on the spin nanotube, we investigate the three-leg spin tube, which is the simplest one, using numerical analyses of finite clusters and a finite-size scaling technique. The spin gap, which is one of the most interesting quantities reflecting the macroscopic quantum effect, was revealed to be open for any finite rung exchange couplings, in contrast to the three-leg spin ladder system which is gapless. We also found a quantum phase transition caused by an asymmetric rung interaction. When one of the three rung coupling constants is changed, the spin gap vanishes very rapidly.     

Magnetization in the two-dimensional Hubbard model: a numerical study

The Projector Quantum Monte Carlo method is used to study the two-dimensional Hubbard model with generalized boundary conditions at half-filling. The convergence of the algorithm depends strongly on the initial trial state. Spin-density waves provide an excellent trial state for the case of weak and of strong correlations. This choice of a trial state with broken symmetry allows us to calculate directly the staggered (or sublattice) magnetization m ₀ as a function of the on-site repulsion U. The use of general boundary conditions strongly reduces finite size effects in m ₀.     

Continuum of compensation points in the mixed spin Ising ferrimagnet with four-spin interaction and next-nearest neighbor coupling

We investigate the magnetic properties of the mixed spin-1/2 and spin-1 Ising ferrimagnetic system with four-spin interaction J₄ and next-nearest neighbor (NNN) coupling J′. We perform exact ground-state calculations and use the finite cluster approximation, based on a single cluster theory, to derive the state equations for the two-dimensional square lattice. The main attention has been paid to the study of the phase diagram for both the transition and compensation temperatures. We find a number of characteristic behaviors. The model with only NNNs induces one compensation point while the four-spin interaction does not. The investigation of the model with both interactions shows a number of characteristic behaviors. In particular, the presence of the four-spin interaction, according to J₄ and J′, may lead to one, two or possibly a continuum of compensation points. This phenomenon may have important applications in technology such as thermomagnetic writing and erasing at the compensation point.     

Energy band gap and electron spin polarization in photoemission

A contribution to the electron spin polarization in photoemission is investigated for the transition from occupied states in conduction bands to the states whose energies lie in a band gap. This contribution can be either positive or negative.