Electron spin resonance of proton-irradiated graphite

In the case of colossal magnetoresistance in the perovskite manganites, "double exchange" mediated by the itinerant spins is believed to play a key role in the ferromagnetism. In contrast, the conventional "Heisenberg" interaction, i.e., direct (unmediated) interaction between the localized spins produced by the proton irradiation, is identified as the origin of proton irradiation-induced ferromagnetism in graphite.     

Optical pumping of the electronic and nuclear spin of single charge-tunable quantum dots

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.     

Effect of optical spin injection on ferromagnetically coupled Mn spins in the III-V magnetic alloy semiconductor (Ga,Mn)As

We report on the new type of photoinduced magnetization in ferromagnetic (Ga,Mn)As thin films. Optically generated spin-polarized holes change the orientation of ferromagnetically coupled Mn spins and cause a large change in magnetization, being 15% of the saturation magnetization, without the application of a magnetic field. The memorization effect has also been found as a trace after the photoinduced magnetization. The observed results suggest that a small amount of nonequilibrium carrier spins can cause collective rotation of Mn spins presumably through the p-d exchange interaction.     

Effect of magnetic doping on the electronic states of Ni

Angle-resolved photoemission is used to determine the change in the electronic states of Ni induced by doping with Fe and Cr. Well-defined spin and k states are selected using high energy and k resolution combined with single crystal alloys. Iron suppresses the mean free path of minority spins only, while chromium suppresses both spins and decreases the magnetic splitting. The strong variation of these effects from one impurity to the other supports the concept of magnetic doping.     

Nuclear spin-lattice relaxation mechanisms in kaolinite confirmed by magic-angle spinning

Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in ¹H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between ¹H spins is strong. ²⁹Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the ²⁹Si relaxation of relatively pure samples under static and slow-spinning conditions. ²⁷Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between ²⁷Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated.     

Reduction of dipolar interaction by off-resonance phonon avalanche

An off-resonance phonon avalanche is shown to reduce effectively the magnetic dipolar interaction between paramagnetic spins

$\text{1}\text{2}$

in a crystal and to change the Larmor frequency of the spins. Both effects are obtained by a unitary transformation which eliminates approximately the phonon field from the Hamiltonian of the system.     

On Relativistic Quantum Information Properties of Entangled Wave Vectors of Massive Fermions

We study special relativistic effects on the entanglement between either spins or momenta of composite quantum systems of two spin- \frac12\frac{1}{2} massive particles, either indistinguishable or distinguishable, in inertial reference frames in relative motion. For the case of indistinguishable particles, we consider a balanced scenario where the momenta of the pair are well-defined but not maximally entangled in the rest frame while the spins of the pair are described by a one-parameter (η) family of entangled bipartite states. For the case of distinguishable particles, we consider an unbalanced scenario where the momenta of the pair are well-defined and maximally entangled in the rest frame while the spins of the pair are described by a one-parameter (ξ) family of non-maximally entangled bipartite states. In both cases, we show that neither the spin-spin (ss) nor the momentum-momentum (mm) entanglements quantified by means of Wootters’ concurrence are Lorentz invariant quantities: the total amount of entanglement regarded as the sum of these entanglements is not the same in different inertial moving frames. In particular, for any value of the entangling parameters, both ss and mm-entanglements are attenuated by Lorentz transformations and their parametric rates of change with respect to the entanglements observed in a rest frame have the same monotonic behavior. However, for indistinguishable (distinguishable) particles, the change in entanglement for the momenta is (is not) the same as the change in entanglement for spins. As a consequence, in both cases, no entanglement compensation between spin and momentum degrees of freedom occurs.     

Effect of rf field on spin diffusion

A study was made of the effect of an rf field on spin diffusion. The interaction of spins with the rf field is described quantum mechanically. It is shown that the effect of the rf field on the system of spins can, in some approximation, be interpreted as the effect of a change in the Larmor frequency of the spin and a decrease in the magnitude of the dipole-dipole interaction between spins. These conclusions were obtained on the basis of a unitary transformation which eliminates the explicit form of the spin-photon interaction operator in the Hamiltonian of the system considered. An expression is derived for the spin-diffusion coefficient under saturation. The presence of the rf field results in a decrease in the diffusion coefficient.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 7–11, September, 1979.     

The edge state of nanographene and the magnetism of the edge-state spins

Nanographene has unique edge-shape dependence of the electronic structure with non-bonding edge states being created in its zigzag edges. The presence of the edge state is experimentally confirmed in well-defined hydrogen-terminated zigzag edges by scanning tunneling microscopy/spectroscopy (STM/STS) observations. In the three-dimensional (3D) disordered network of nanographite domains in nanoporous carbon (activated carbon fibers), the localized edge-state spins are in a spin-glass-like ordered state at low temperatures with the aid of exchange interactions whose strengths varies randomly in space, when the strengths of inter-nanographene and nanographite interactions are tuned. Chemical and structural modifications of nanographene edges change the magnetism of edge-state spins through covalent bond formation and charge transfer.     

Collective relaxation of nuclear spins and “superradiance” of magnons in ferromagnets

The Heisenberg model of ferromagnet, which contains nuclear spins interacting with electronic spins of own atoms by means of hyperfine interaction, is considered. A nonlinear evolution equation for collective nuclear spin z-component is obtained for the initial condition with the inverted nuclear spins (of the value 1/2). The exact solution of this equation is obtained. The collective relaxation of nuclear spins reveals the cooperative behaviour of superradiant type accompanied by magnons generation.     

二维无规混合磁性系统磁特性的微磁学及Monte Carlo研究

采用能量极小原理的微磁学及Monte Carlo方法对异类自旋组成混合Heisenberg自旋体系进行模拟计算,研究了二维铁磁反铁磁无规混合系统的磁特性.发现了二维无规混合磁性系统存在M-H磁化曲线的阶梯效应.通过一维Ising模型及系统能量、自旋组态的研究,发现小自旋数目的反铁磁耦合系统是产生M-H阶梯效应的根本原因.     

Angular momentum transfer and partition in the deep-inelastic reaction 664 MeV 84Kr + natAg

In- and out-of-plane angular distributions have been measured for sequential α-decay from target-like fragments produced in fully relaxed heavy-ion collisions. At angles equal to or larger than the target-recoil direction, the α-particle energy spectra are evaporation-like and the in-plane angular distributions are consistent with isotropy in the rest frame of the target recoil. The out-of-plane distributions exhibit an anisotropy of approximately two. Fragment spins were extracted from these distributions as a function of mass asymmetry. These spins are in agreement with those obtained from a simultaneous γ-ray multiplicity measurement. Both the fragment kinetic energies and intrinsic spins are consistent with rigid rotation of an intermediate complex consisting of two substantially deformed spheroids in near proximity.     

Monte Carlo study of low/high spin transitions

We study the finite temperature property of a model on two dimensional square lattices with two Ising spins at each lattice site by Monte Carlo simulations. When those Ising spins at a lattice site are parallel the site is said to be in the high-spin state (HS), while when they are antiparallel the site is said to be in the low-spin state (LS). Throughout the study, the energy of HS is presumed to be higher than that of LS. Two Ising spins at each site are added to form a total spin, which interacts with its nearest neighbour total spins via spin-spin couplings. The spin-phonon coupling also is introduced via harmonic springs between nearest neighbour sites with spring constants and equilibrium distances depending on the spin states of the sites involved. In this system, we investigate the feature of transitions between LS and HS (to be called low/high spin transition (LHST)) by varying the temperature. As for the ferromagnetic interaction between total spins, the second order phase transition: pure HSmixed state of HS and LS is possible to occur in a pure spin system, as is expected from mean field calculations. The role of lattice distortions by the change of lattice spacings is shown to be essential for LHST: pure LS(pure)HS. In the model investigated, there appears an indication of the strong first order phase transition which reveals a conspicuous hysteresis.     

Ultrafast manipulation of antiferromagnetism of NiO

Photoexcitation of antiferromagnetic NiO leads to ultrafast reorientation of Ni2+ spins due to change of the magnetic anisotropy. Recovery of the magnetic ground state occurs as coherent oscillation of the antiferromagnetic order parameter between hard- and easy-axis states manifesting itself as quantum beating. The coherence time is approximately 1 ns with the beating frequency being determined by the anisotropy energy.     

Dynamics of multiple quantum coherences in dipole-coupling spins in solid

A perturbation method deals with dipolar coupling spins in solids is presented. As example of application the method, the multiple-quantum coherence dynamics in clusters of a linear chain of four nuclear spins and a ring of six spins coupled by dipole-dipole interaction are considered. The calculated 0Q and 2Q intensities in a linear chain of four nuclear spins and 6Q intensity in a ring of six spins vs. the duration of the preparation period agree well with the exact solutions (for linear chain of four nuclear spins) and simulation data (for linear chain of four nuclear spins and a ring of six spin).     

储锂硅材料的ESR谱研究

采用了电化学方法制备储锂硅材料,并用电子自旋共振(ESR)方法进行研究. 实验结果表明, 储锂前的硅ESR行为符合居里自旋的ESR特征, ESR信号主要来源于硅材料中的晶格缺陷、 表面悬空键等局域化自旋中心. 储锂后硅材料中产生了泡利自旋,居里自旋的强度比储锂前增大2～3倍. 此外,对硅和储锂硅ESR谱线的g因子和ΔH_pp随温度的变化情况也进行了分析. 硅材料电化学储锂时,与锂离子中和的电子主要参与形成Li-Si共价键,对ESR信号贡献很小.     

Ion-beam-induced ferromagnetism in Ca-doped LaMnO3 thin films grown on Si (100)

The ion-bean-induced room temperature ferromagnetic ordering in pulsed laser deposited Ca-doped LaMnO₃ thin films grown on Si (100) are presented in the present study. In addition to this, changes bought by the ion beam in structural, morphological and electrical properties are presented. Dense electronic excitation produced by high energy 120?MeV Ag⁹⁺ ion irradiation causes change in surface roughness, crystallinity and strain. It is also evident that these excitations induce the magnetic ordering in this system. The observed modifications are due to the large electronic energy deposited by swift heavy ion irradiation. The appearance of ferromagnetism at 300?K in these samples after irradiation may be attributed to the canting of the antiferromagnetically ordered spins due to the structural distortion. It is observed that the irradiated films show higher resistance than unirradiated films for all the compositions.     

Spin twists, domain walls, and the cluster spin-glass phase of weakly doped cuprates

We examine the role of spin twists in the formation of domain walls, often called stripes, by focusing on the spin textures found in the cluster spin glass phases of and . To this end, we derive improved analytic expressions for the spin distortions produced by a frustrating bond, both near the core region of the bond and in the far field, and then derive an improved expression for interaction energies between such bonds. We critique our analytical theory by comparison to numerical solutions of this problem and find excellent agreement. By looking at collections of small numbers of such bonds localized in some region of a lattice, we demonstrate the stability of small “clusters” of spins, each cluster having its own orientation of its antiferromagnetic order parameter. Then, we display a domain wall corresponding to spin twists between clusters of locally ordered spins showing how spin twists can serve as a mechanism for stripe formation. Since the charges are localized in this model, we emphasize that these domain walls are produced in a situation for which no kinetic energy is present in the problem. Received 10 January 2000     

MathNMR: spin and spatial tensor manipulations in Mathematica

Spin and spatial tensor manipulations are frequently required to describe the theory of NMR experiments. A Mathematica package is presented here, which provides some of the most common functions for these calculations. Examples are the calculation of matrix representations of operators, commutators, projections, rotations, Redfield matrix elements, matrix decomposition into basis operators, change of basis, coherence filtering, and the manipulation of Hamiltonians. The calculations can be performed for any spin system, containing spins 1/2 and quadrupolar spins alike, subject to computational limitations. The package will be available from upon acceptance of the article.