Fast-field-cycling NMR study of magnetomechanically induced relaxation enhancement in ferrogelatine

We describe a field-cycling magnetic resonance experiment aimed to study the nuclear spinlattice relaxation in the simultaneous presence of an oscillating magnetic field and a magnetomechanically induced perturbation across the sample volume. The studied system is a gelatine doped with surfacted nanosize ferromagnetic particles. The spin system relaxes in coexistence with the magnetic interaction between the dopants and a weak external alternating magnetic field. Due to the interaction between the alternating field and the high magnetic moment of the dopants, a mechanical shaking of the particles becomes effective at low evolution fields. In this way, the particles act as local sonic sources, thus transmitting the vibrations to the gelic matrix where they are trapped. The impact produced on the molecular dynamics is observed through a magnetic resonance relaxation experiment. Saturation of the proton magnetization produced by the alternating magnetic field is also discussed. Results are compared with the direct effects produced by a pure sonic perturbation at the same frequency, introduced by coupling a piezoelectric transducer in direct contact with the sample.     

Fixed lock-time relaxation dispersion in the rotating frame

The spin-lattice relaxation dispersion may be probed in the laboratory frame through field-cycling NMR relaxometry. The experiment, as usually done, has the basic weakness that the low frequency end of the measured dispersion can be blurred by the presence of local fields. An understanding of the nature of such local fields was found to be essential to the interpretation of the dispersion profile. In this work, an attempt was made to determine the extent to which specific information can be obtained from a rotating frame experiment. The technique consists in the study of the NMR signal dispersion at a fixed spin-lock time, as a function of the radio frequency field intensity. Within this scheme, a strong dispersion can be attributed to the presence of a non-zero magnetic field component along the laboratory-frame Zeeman-axis in the rotating-frame. At on-resonance condition, this component is exclusively due to the presence of local fields as projected on that axis.     

A 13C field-cycling NMR relaxometry investigation of proton tunnelling in the hydrogen bond: dynamic isotope effects, the influence of heteronuclear interactions and coupled relaxation

Concerted double proton transfer in the hydrogen bonds of a carboxylic acid dimer has been studied using 13C field-cycling NMR relaxometry. Heteronuclear 13C-1H dipolar interactions dominate the 13C spin-lattice relaxation which is significantly influenced by the polarisation state of the 1H Zeeman reservoir. The methodology of field-cycling experiments for such heteronuclear spin-coupled systems is studied experimentally and theoretically, including an investigation of various saturation-recovery and polarisation-recovery pulse sequence schemes. A theoretical model of the spin-lattice relaxation of this coupled system is presented which is corroborated by experiment. Spectral density components with frequencies omega(C), omega(C) + omega(H), and omega(C) - omega(H) are mapped out experimentally from the magnetic field dependence of the 13C and 1H spin-lattice relaxation and the proton transfer rate at low temperature is determined from their widths. Any dynamic isotope effect on the proton tunnelling in the hydrogen bond arising from 13C enrichment in the skeletal framework of the dimer is found to be smaller than experimental uncertainties (approximately 5%).     

Nonlinear response to external magnetic fields

The purpose of this paper is to study conditions under which a system of itinerate spin-$\text{1}\text{2}$ fermions might exhibit a macroscopic linear response to external magnetic fields after long times. Exact expressions are obtained for the nonlinear response of the magnetization and the total energy. We find that for a constant field there is no response (our model contains no mechanism for the relaxation of spins). For an oscillatory field there is a response in which secular terms (in the time) appear which are associated both with nonlinear terms in the external field and with contributions from the background medium. The secular terms involving the magnetic field would not be seen if one used the usual approximations of microscopic linear response theory. They give rise to new conditions which must be satisfied if the system is to exhibit a macroscopic linear response in the long-time limit.     

Strain engineering of graphene nanoribbons: pseudomagnetic versus external magnetic fields

Bandgap opening due to strain engineering is a key architect for making graphene’s optoelectronic, straintronic, and spintronic devices. We study the bandgap opening due to strain induced ripple waves and investigate the interplay between pseudomagnetic fields and externally applied magnetic fields on the band structures and spin relaxation in graphene nanoribbons (GNRs). We show that electron-hole bands of GNRs are highly influenced (i.e. level crossing of the bands are possible) by coupling two combined effects: pseudomagnetic fields (PMF) originating from strain tensor and external magnetic fields. In particular, we show that the tuning of the spin-splitting band extends to large externally applied magnetic fields with increasing values of pseudomagnetic fields. Level crossings of the bands in strained GNRs can also be observed due to the interplay between pseudomagnetic fields and externally applied magnetic fields. We also investigate the influence of this interplay on the electromagnetic field mediated spin relaxation mechanism in GNRs. In particular, we show that the spin hot spot can be observed at approximately B = 65 T (the externally applied magnetic field) and B₀ = 53 T (the magnitude of induced pseudomagnetic field due to ripple waves) which may not be considered as an ideal location for the design of straintronic devices. Our analysis might be used for tuning the bandgaps in strained GNRs and utilized to design the optoelectronic devices for straintronic applications.     

Broadening of paramagnetic resonance lines by charged point defects in neodymium-doped scheelites

We study paramagnetic resonance linewidth in a series of CaWO₄ and CaMoO₄ crystals with different concentrations of neodymium ions (0.0031–0.81 at %). Experimental data are interpreted in the framework of the statistical theory of line broadening by charged point defects. In our calculations, three different contributions are singled out: arising from the local electric fields, electric field gradients and magnetic fields of the nearby point defects. The interaction parameters are determined from the spectroscopic data available for Nd:CaWO₄ crystal. Direct calculations of the linewidth are performed for different crystal orientations with respect to external magnetic field. We conclude that major contribution to the broadening comes from the interactions with random electric fields produced by neodymium and charge compensator ions.     

Magnetic Resonance of Impurity Spin in Slow Fluctuating Local Field

A basic model of magnetic resonance is considered. The model takes into account external static and orthogonal to it rotating magnetic fields together with fluctuating (local) field directed along the static field. The local field is considered as smooth normal stochastic process. New solutions for longitudinal relaxation are obtained in the region of adiabatic slow fluctuations and nonadiabatic losses are estimated.     

Effect of temperature,electric and magnetic field on spin relaxation in single layer graphene: A Monte Carlo simulation study

In this article, we employ the semiclassical Monte Carlo approach to study the spin polarized electron transport in single layer graphene channel. The Monte Carlo method can treat non-equilibrium carrier transport and effects of external electric and magnetic fields on carrier transport can be incorporated in the formalism. Graphene is the ideal material for spintronics application due to very low Spin Orbit Interaction. Spin relaxation in graphene is caused by D'yakonov-Perel (DP) relaxation and Elliott-Yafet (EY) relaxation. We study effect of electron electron scattering, temperature, magnetic field and driving electric field on spin relaxation length in single layer graphene. We have considered injection polarization along z-direction which is perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. This theoretical investigation is particularly important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene.     

外加磁场微波等离子推力器内流场数值模拟

为了提高微波等离子推力器性能，改善等离子体对电磁波能量的吸收状况，提高核心区温度，提出外加磁场的方案，并对热等离子体进行了数值模拟.假设局域热平衡条件，采用Navier-Stokes，Maxwell和Saha方程，利用压力修正的半隐格式和时域有限差分求解方法，建立了径向磁镜场下推力器内等离子体流场的数值计算模型.数值模拟结果表明：外加磁场后的磁感应强度小于0.5 T时，推力器内热等离子体核心区最高温度随磁感应强度的增加而迅速提高.外加磁场后的磁感应强度大于0.5 T时，核心区最高温度随磁感应强度的增加而缓慢提高.磁感应强度为0.5 T时，热等离子体核心区最高温度与不加磁场相比提高了24%.外加磁场对等离子体流场速度分布影响不大. 关键词： 等离子体模拟 等离子体相互作用 等离子体流动     

Study of magnetic properties of Pd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>0.95</Subscript>Mn<Subscript>0.05</Subscript> alloy using polarized muons and neutrons

The Pd_1?x Fe _x )_0.95Mn_0.05 alloy with random competing interaction was studied by measuring the muon spin relaxation in an external transverse magnetic field and in a zero magnetic field. Using the measured temperature dependence of the dynamic relaxation rate λ and the characteristics of the distribution of local static fields, the phase states of the sample under study are refined. In particular, it is shown that the ferromagnetic and spin-glass states coexist simultaneously in the sample below 25 K. Combined studies of the sample using the μSR and neutron depolarization methods made it possible to determine the size of magnetic inhomogeneities to be 2–6 μm in the temperature range 5–40 K.     

Features of the hyperfine interaction of tin impurities in metallic holmium

The Mössbauer effect has been used to study the hyperfine fields for the¹¹⁹Sn atom as an impurity in metallic Ho in the 4.5–136 K temperature range. Two values of the field relevant to the features of the Ho magnetic structure have been found in the ferro- and antiferromagnetic regions of ordering. The temperature dependence of the fields differs drastically from the magnetization function and is similar to the case of a Dy host studied earlier. The experiments with a 40 kOe external magnetic field at 25 K have revealed a strong (up to 19%) hysteresis in the hyperfine fields which can be explained by a rearrangement of the magnetic structure. The field sign has been found to be negative, from which fact a peculiar compensation of the local and hyperfine fields is deduced. The experiment is also indicative of a possible new phase transition at 95 K.     

Spin relaxation in the impurity band of a semiconductor in the external magnetic field

Spin relaxation in the impurity band of a 2D semiconductor with spin-split spectrum and hyperfine interaction in the external magnetic field is considered. Two contributions to the spin relaxation are shown to be relevant: the one given by optimal impurity configurations with the hop-waiting time inversely proportional to the external magnetic field and another one related to electron motion over large distances. The average spin relaxation rate is calculated. The article is published in the original.     

Mössbauer relaxation spectra of LiNbO3: Fe(III)

Mössbauer spectra of LiNbO₃: Fe(III)-monocrystals were measured in zero external magnetic field (5mK to 673K). They all show typical lineshapes due to relaxation effects. Below 77K temperature independent and above 77K a temperature-dependent relaxation is observable. In spectra measured in external magnetic fields relaxation effects occur only above 77K. We present least square fits of spectra measured at 300K and external magnetic fields using symmetry invariant reservoir spectral densities. The applied numerical formalism takes the complete electronuclear manifold of⁵⁷Fe(III) into account. We discuss the problems concerning the interpretation of zero field spectra.     

Mössbauer spectra of a paramagnetic ion in an axial crystal field in the presence of spin-lattice relaxation

The influence of spin-lattice relaxation on the Mössbauer hyperfine spectra of the Fe³⁺ ion is investigated in the case of an axial crystal field. All the various influences of the relaxation process on the spectra can be explicitly described using two relaxation parameters. A detailed analysis of the Mössbauer relaxation spectra for the various temperatures, relations between the relaxation parameters and external magnetic field directions is carried out. When the magnetic field direction is nearly collinear with the symmetry axis and one of the relaxation parameters is small, the dynamics of Mössbauer spectra is shown to have anomalous features. The influence of random magnetic fields is shown to give an unconventional development of patterns as a function of the relaxation parameters.     

Field Calculations for Permanent Magnets used for Glow Discharges

If a permanent magnet has both a homogeneous polarization inside the material and a linear demagnetization characteristic in external fields, its magnetic field can be expressed using a surface pole model. For magnets satisfying these conditions and, in addition, having a rectangular shape, the fields at any given point in space can be calculated analytically. An algorithm for this calculation is presented in a form that can easily be implemented into a computer program. In our experiments we used Nd₂Fe₁₄B magnets to support low pressure glow discharges by magnetic fields. The magnets can be seen as composed of elementary magnets with rectangular shape, for which the magnetic field distribution is calculable. We present results of field calculations for various configurations of permanent magnets that we used in hollow cathode and Penning discharges.     

优化磁共振纳米医学中磁性纳米粒子的热疗效率

磁共振热疗(magnetic resonance hyperthermia)是近年来新兴的一种纳米医学治疗方法,由磁共振的硬件架构产生特定交变磁场,有效地加热磁性纳米粒子,以直接或间接地杀死癌细胞,体现诊疗一体化。提高磁性纳米粒子的加热效率是当前磁共振热疗领域亟待解决的难题之一。磁性纳米粒子的加热效率不仅与粒子本身的大小、性质以及尺寸分布有关,还和聚集状态有关。该研究利用3D Metropolis蒙特卡罗模拟方法,模拟了不同温度下磁性纳米粒子的磁共振热动力学行为及其团聚与分离现象;并通过修正过的郎之万方程,建立了相变临界温度与外加磁场频率的函数关系。模拟结果显示,磁性纳米粒子悬浮液中多聚体的相对含量随着温度的升高而降低,达到临界温度后,多聚体完全分离成单体;而提高交变磁场频率可以显著降低临界温度,且存在临界频率,高于此临界频率后临界温度不再受外加磁场频率影响,达到稳定。因而在临界频率下预热磁性纳米粒子悬浮液,使得多聚体分离成单体,可优化磁性纳米粒子的热疗效率。     

Effect of electric field and magnetic field on spin transport in bilayer graphene armchair nanoribbons: A Monte Carlo simulation study

In this article we study the effect of external magnetic field and electric field on spin transport in bilayer armchair graphene nanoribbons (GNR) by employing semiclassical Monte Carlo approach. We include D'yakonov-Perel' (DP) relaxation due to structural inversion asymmetry (Rashba spin-orbit coupling) and Elliott-Yafet (EY) relaxation to model spin dephasing. In the model we neglect the effect of local magnetic moments due to adatoms and vacancies. We have considered injection polarization along z-direction perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. To the best of our knowledge there has been no theoretical investigation of the effects of external magnetic field on spin transport in graphene nanoribbons. This theoretical investigation is important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene GNRs.     

Quadrupole spin-echo envelopes for Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> single crystals doped with the atoms of transition and rare-earth elements

The influence of a weak (below 50 Oe) constant magnetic field on a quadrupole spin-echo envelope was studied for an undoped single crystal Bi₄Ge₃O₁₂, in which local magnetic fields on the order of 20–30 G were previously found, as well as for single Bi₄Ge₃O₁₂ crystals doped with the atoms of transition and rare-earth elements. In all of these cases, the spin-echo envelopes were strongly influenced. A considerable increase in the nuclear spin-spin relaxation time T ₂ was observed for the undoped sample upon the switching of weak external magnetic fields. For the doped samples, the spin-echo envelope decay became much slower already in the zero field. The external magnetic fields exhibited a markedly weaker influence on the spin-echo envelope for the doped samples. The text was submitted by the authors in English.     

Spin precession and alternating spin polarization in spin-3/2 hole systems

The spin density matrix for spin-3/2 hole systems can be decomposed into a sequence of multipoles which has important higher-order contributions beyond the ones known for electron systems [R. Winkler, Phys. Rev. B 70, 125301 (2004)]. We show here that the hole spin polarization and the higher-order multipoles can precess due to the spin-orbit coupling in the valence band, yet in the absence of external or effective magnetic fields. Hole spin precession is important in the context of spin relaxation and offers the possibility of new device applications. We discuss this precession in the context of recent experiments and suggest a related experimental setup in which hole spin precession gives rise to an alternating spin polarization.     

Spin-lattice relaxation in hyperfine-coupled systems: Applications to interstitial diffusion and molecular dynamics

The solid state diffusion of hydrogen, or of its pseudo-isotope muonium, provides an interesting example of spin-lattice relaxation in a 2-spin, 4-level system. The local field experienced by the interstitial atom fluctuates as it moves, inducing transitions between the coupled electron and nuclear spin states. Rate equations governing the populations of these states may be solved numerically to simulate the different relaxation functions which would be displayed by ESR, ENDOR and μSR spectroscopies and to assist in extracting motional correlation times from the experimental data. Spin relaxation in molecular radicals may be treated similarly, with different selection rules for different mechanisms: this paper treats the spin rotation mechanism and perturbation to anisotropic or isotropic components of the hyperfine interaction, caused by inter or intra-molecular motion. Conventional magnetic resonance monitors the population differences appropriate to particular transitions; only in sufficiently high fields do these distinguish the electronic and nuclear response. Muon spin relaxation is remarkable in separating out the nuclear spin projection whatever the degree of mixing of the spin states,via the asymmetry in the muon radioactive decay. Experimentally it has the advantage that measurements can be made over a wide range of field, from null external field up to thelevel crossing where the relaxation rate exhibits a striking peak.