Spin Relaxation In Systems Research Articles

Spin Relaxation in Kondo Lattice Systems with Anisotropic Kondo Interaction

We study the influence of the Kondo effect on the spin relaxation in systems with anisotropic Kondo interaction at temperatures both high and low as compared with the static magnetic field. In the absence of the Kondo effect, the electron spin resonance linewidth is not narrowed in the whole temperature range due to the high anisotropy of the Kondo interaction. The Kondo effect leads to the universal energy scale, which regulates the temperature and magnetic field dependence of different kinetic coefficients and results in a mutual cancelation of their singular parts in a collective spin mode.

Current patterns and orbital magnetism in mesoscopic dc transport.

We present ab initio calculations of the local current density j(r) as it arises in dc-transport measurements. We discover pronounced patterns in the local current density, ring currents ("eddies"), that go along with orbital magnetism. Importantly, the magnitude of the ring currents can exceed the (average) transport current by orders of magnitude. We find associated magnetic fields that exhibit drastic fluctuations with field gradients reaching 1 T nm⁻¹ V⁻¹. The relevance of our observations for spin relaxation in systems with very weak spin-orbit interaction, such as organic semiconductors, is discussed. In such systems, spin relaxation induced by bias driven orbital magnetism competes with relaxation induced by the hyperfine interaction and appears to be of similar strength. We propose a NMR-type experiment in the presence of dc-current flow to observe the spatial fluctuations of the induced magnetic fields.

NMR and spin relaxation in systems with magnetic nanoparticles: effects of size andmolecular motion

To better understand the specifics of nuclear magnetic resonance and spin relaxationin systems with magnetic nanoparticles and test the limits of the outer spheremodel for the diffusion-related relaxation, iron oxide nanoparticle suspensionsare studied in dependence on the particle concentration and size (5–40 nm). Themodel is modified to account for aggregation of the particles into clusterswith an enlarged effective radius. For liquid suspensions containing smallparticles or clusters, both the longitudinal and transverse spin relaxation rates,T1−1 andT2−1, correspond well to the theory, which predicts passing ofT1−1 through a maximum and monotonic increase inT2−1 with increasing particle size. For the largest particle sizes, as well as in the case of strongaggregation, the relaxation rates are significantly lower than theoretical predictions. Anabrupt change in both the relaxation rates is observed in a narrow temperature rangearound the melting point of paraffin wax doped with magnetic nanoparticles. Theapplicability of fast-motion and fast-diffusion approximations is discussed for large effectivesizes and limiting molecular motion cases.

Effects of Magnetic Nanoparticles on Magnetic Resonance and Spin Relaxation in Systems of Different Viscosity

AbstractNuclear Magnetic Resonance (NMR) technique is a convenient method to monitor magnetic nanoparticles in different biomedical applications and observe changes induced by the particles. To better understand the specifics of the magnetic resonance and spin relaxation in the systems with magnetic nanoparticles, the NMR spectra and magnetization dynamics of the host protons are studied in the model systems of different viscosity and some biological systems in the presence of magnetic nanoparticles. The results confirmed that nanoparticles affect the proton relaxation kinetics of liquid solutions, changing the relaxation time (T1) significantly, whereas in systems of high viscosity the relaxation times are unchanged. The kinetics in intermediate systems is multi-exponential. A complicated picture is observed in biological systems, demonstrating contributions of liquid-like and solid-like behavior.

Observations outside the light cone: Algorithms for nonequilibrium and thermal states

We apply algorithms based on Lieb-Robinson bounds to simulate time-dependent and thermal quantities in quantum systems. For time-dependent systems, we modify a previous mapping to quantum circuits to significantly reduce the computer resources required. This modification is based on a principle of ``observing'' the system outside the light cone. We apply this method to study spin relaxation in systems started out of equilibrium with initial conditions that give rise to a very rapid entanglement growth. We also show that it is possible to approximate time evolution under a local Hamiltonian by a quantum circuit whose light cone naturally matches the Lieb-Robinson velocity. Asymptotically, these modified methods allow a doubling of the system size that one can obtain as compared to a direct simulation. We then consider a different problem of thermal properties of disordered spin chains and use quantum belief propagation to average over different configurations. We test this algorithm on one-dimensional systems with mixed ferromagnetic and antiferromagnetic bonds, where we can compare to quantum Monte Carlo, and then we apply it to the study of disordered, frustrated spin systems.

SPIN-GALVANIC EFFECT AND SPIN ORIENTATION BY CURRENT IN NON-MAGNETIC SEMICONDUCTORS

The spin-galvanic effect and the inverse effect, which yeilds current induced spin polarization, in low dimensional semiconductor structures are reviewed. Both effect are caused by asymmetric spin relaxation in systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian.

NMR and spin relaxation in systems with magnetic nanoparticles

The 1H NMR spectra and spin dynamics of the host systems have been studied in liquid and solid suspensionsof γ-Fe2O3 nanoparticles.Significant broadening of 1H NMR spectra and growing relaxation rates were observed with increasedconcentration of nanoparticles in the liquid systems, with the relationT1/T2 depending on the particular host. Solid systems demonstrateinhomogeneous broadening of the spectra and practically no dependence ofT1 upon the nanoparticle concentration. We explain the experimental results taking intoaccount the predomination of self-diffusion as a source of the relaxation in liquidsuspensions, and estimate effective parameters of relaxation in the systems understudy.

NMR and Spin Relaxation in Systems with Magnetic Nanoparticles

AbstractTo better understand the effects of magnetic nanoparticles to nuclear spectra and spin relaxation in different systems, we have studied 1H NMR spectra and spin dynamics of the host system in liquid and solid suspensions of γ-Fe2O3 nanoparticles. Significant line broadening of 1H NMR spectra and growing relaxation rates were observed with increased concentration of nanoparticles in the liquid systems, with the relation T1/T2 depending on the particular host. Solid systems demonstrate inhomogeneous broadening of the spectra and practically no dependence of T1 upon the nanoparticle concentration. We explain the experimental results taking into account predomination of diffusion as a source of the relaxation, and estimate effective parameters of relaxation in the systems in study.

Relaxation of a quadrupolar nucleus in a strong magnetic field

Longitudinal relaxation of spin systems (S = 1, 3/2, 3) containing isolated quadrupolar nuclei are studied. The characteristic features of the relaxation behavior are identified in strong magnetic fields in the presence of chemical shift anisotropy. Two mechanisms are established that favor involvement of high rank multipoles in the relaxation process: an autocorrelation mechanism and a cross correlation mechanism. Multipoles of odd rank are involved in the relaxation of nuclei with spin S > 1 as a result of the autocorrelation mechanism (in this case, due to quadrupole interactions). The cross correlation mechanism, due to correlations between the chemical shift anisotropy and the quadrupole interactions, favors the appearance of multipoles of even rank. Expressions are presented for the multipole cross relaxation and longitudinal relaxation rates for spin systems with S = 1, 3/2, 3.

Role of Orbital Dynamics in Spin Relaxation and Weak Antilocalization in Quantum Dots

We develop a semiclassical theory for spin-dependent quantum transport to describe weak (anti)localization in quantum dots with spin-orbit coupling. This allows us to distinguish different types of spin relaxation in systems with chaotic, regular, and diffusive orbital classical dynamics. We find, in particular, that for typical Rashba spin-orbit coupling strengths, integrable ballistic systems can exhibit weak localization, while corresponding chaotic systems show weak antilocalization. We further calculate the magnetoconductance and analyze how the weak antilocalization is suppressed with decreasing quantum dot size and increasing additional in-plane magnetic field.

Nuclear spin relaxation in systems of magnetic spheres

A new approach to the NMR relaxation theory for a system of magnetic spheres (sufficiently big spherical molecules) is presented. In this paper the NMR spin–lattice relaxation time T 1 and spin–spin relaxation time T 2 are calculated for nuclear spins I → j, taking into account intermolecular dipole–dipole interactions between the spins I → j, and spins S → k in the magnetic spheres. By an expansion of the dipole–dipole interaction in a series of spherical harmonics, it is possible to separate spatial variables of the interacting spins in a laboratory frame. A simultaneous effect of isotropic rotational and translation diffusion of the spins and relaxation rate of spins S → k is also taken into account.

Muon Spin Relaxation in Spin Systems

Muon Spin Relaxation in Spin Systems

Paramagnetic proton nuclear magnetic relaxation in the Ni2+hexa-aquo complex

The nuclear magnetic relaxation of the protons in the Ni2+(H2O)6 complex is analysed using a previously developed formalism (Benetis et al. 1983, Molec. Phys., 48, 329) for the description of paramagnetic nuclear spin relaxation in systems with a complex electron spin relaxation. The nuclear spin relaxation can be described within the Redfield theory and the transverse relaxation rate is expressed in terms of a spectral density K 1, -1(ωI), which is the Fourier-Laplace transform of a complex correlation function. In the Ni2+(H2O) complex the electron spin relaxation is caused by the zero field splitting (ZFS) and to evaluate the correlation function it is necessary to specify the dynamics of the ZFS. Three models are considered for this motion: (i) modulation of the ZFS by quantized vibrations, (ii) a classical pseudo-rotation of the ZFS at constant amplitude and (iii) a classical motion of the ZFS in an harmonic potential governed by the Smoluchowski equation.

On the critical dynamics of spin systems with random fields

The dynamical exponent z for the critical relaxation of spin systems in quenched random fields has been calculated in three-loop order. For both Heisenberg and Ising systems the result is z=2+2 eta , where the static exponent eta has its usual meaning. It is suggested that this result may be exact to all orders, leading to unusually slow relaxation, with z=4, at the lower critical dimension of the random-field Ising model.

Consequences of nonuniqueness in spin-relaxation theory

Various aspects of a general theorem, which asserts the lack of uniqueness of the density operator ρA for a composite system having subsystems B and C, are exploited in a systematic manner to obtain the maximum simplification in the equation of motion for the density operator ρB describing the subsystem. In particular, these ideas are applied to relaxation in spin systems in which only the second order terms in the interaction between subsystems B and C are retained in the master equation.

Kinetic theory of resonance and relaxation in spin systems: III. Spin bistability phenomena

The possibility of existence of nonequilibrium phase transitions, bistability phenomena and other critical properties of the two-level spin systems considered in the two previous papers is investigated. Under suitable assumptions, an evolution equation for the mean value of the Zeeman operator is obtained from kinetic equations derived in part I. Qualitative methods of analysis of autonomous differential equations are used to study properties of the evolution equation. Vector fields generated by this equation are analyzed. Stationary solutions and their stability are discussed. It is shown that under suitable conditions, the steady state solutions exhibit a hysteresis cycle. Time-dependent solutions of the evolution equation are presented and discussed.

Kinetic theory of resonance and relaxation in spin systems: II. Case of absence of resonance frequency shift

The kinetic equations which describe relaxation processes in EPR are analysed in the case of absence of the spin resonance frequency shift. The boundary conditions problem for integro- differential equations in dependence on the choice of initial time t 0 (the lower limit of integration) is discussed in detail. It is explicitly exhibited that the asymptotic states of a system described by the non-Markovian equations as well as the asymptotic states of a system described by the Markovian ones (which are approximate in relation to the integro-differential ones) are determined by equations of the same form. The question of the Markovian approximation is considered. Universality and uniformity of the applied method is demonstrated by showing that Brya and Wagner's, Bloch's, the BPP and the Provotorov-type equations can be obtained as special cases of generalized kinetic equations derived in part I.

Kinetic theory of resonance and relaxation in spin systems I: Kinetic equations for two-level open systems

The dynamics of the paramagnetic spin ( S= 1 2 ) system, treated as an open system interacting with an alternating magnetic field and a lattice (phonons), is considered. Starting with the generalized kinetic equations, the non-Markovian equations describing the Zeeman, the dipole interaction and the resonant phonon subsystems are obtained with accuracy to terms of the second order in a weak perturbation. The passage from a set of integro-differential equations (so called equations with memory) to equations in the Markovian limit is studied. The manner of transition to equations in the high-temperature approximation is explicitly exhibited. The connections with other equations are discussed.

Optical‐acoustic two‐phonon relaxation in spin systems

AbstractThe two phonon Raman process of spin–lattice relaxation is investigated in spin systems having electronic properties appropriate to both optical and acoustic branches of phonon spectrum. Apart from exponential temperature dependence of relaxation times for resonance processes, T−3‐, T−4‐, T−5, and T−6‐processes are found for both branches under several conditions. However, when the Debye energy is very high, the T−2‐dependence dominates with corrections of T−3 and T−4. The level structure of the spin system is found to be important for the temperature dependence of relaxation times.

Spin Relaxation on Yttrium Iron Garnet near Curie Temperature Studied by High Frequency Magnetic Susceptibility

The high frequency magnetic susceptibilities in yttrium iron garnet are observed in the temperature range from 546 K and 553 K in the high frequency range from 100 kHz to 100 MHz and the low frequency range lower than 100 Hz. The anomalous sharp peak appears in the temperature dependence curves of the imaginary part of the magnetic susceptibility in MHz range and is broadened with increasing frequency. From frequency dependence of the magnetic susceptibilities it is made clear that the relaxation of spin system occurs in the above two frequency ranges and relaxation process near the Curie temperature is not monodispersive but polydispersive. The temperature dependence of the relaxation time in spin system is obtained near the Curie temperature and is corresponding with the critical slowing down phenomena.