Field-dependent nuclear relaxation of spins 1/2 induced by dipole-dipole couplings to quadrupole spins: LaF3 crystals as an example

A general theory of spin-lattice nuclear relaxation of spins I=1/2 caused by dipole-dipole couplings to quadrupole spins S1, characterized by a non-zero averaged (static) quadrupole coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through dipole-dipole couplings with certain transitions of quadrupole spins. The averaged quadrupole coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual dipole-dipole coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the 'classical' Solomon relaxation theory.     

Spin relaxation of Mn ions in (CdMn)Te/(CdMg)Te quantum wells under picosecond optical pumping

Spin relaxation of Mn ions in a Cd_0.97Mn_0.03Te/Cd_0.75Mg_0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (_B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively. Zh. éksp. Teor. Fiz. 112, 1440–1463 (October 1997)     

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.     

Anisotropic spin relaxation in graphene

Spin relaxation in graphene is investigated in electrical graphene spin valve devices in the nonlocal geometry. Ferromagnetic electrodes with in-plane magnetizations inject spins parallel to the graphene layer. They are subject to Hanle spin precession under a magnetic field B applied perpendicular to the graphene layer. Fields above 1.5 T force the magnetization direction of the ferromagnetic contacts to align to the field, allowing injection of spins perpendicular to the graphene plane. A comparison of the spin signals at B=0 and B=2 T shows a 20% decrease in spin relaxation time for spins perpendicular to the graphene layer compared to spins parallel to the layer. We analyze the results in terms of the different strengths of the spin-orbit effective fields in the in-plane and out-of-plane directions and discuss the role of the Elliott-Yafet and Dyakonov-Perel mechanisms for spin relaxation.     

Optical pumping of quantum-dot nuclear spins

Hyperfine interactions with randomly oriented nuclear spins present a fundamental decoherence mechanism for electron spin in a quantum dot, that can be suppressed by polarizing the nuclear spins. Here, we analyze an all-optical scheme that uses hyperfine interactions to implement laser cooling of quantum-dot nuclear spins. The limitation imposed on spin cooling by the dark states for collective spin relaxation can be overcome by modulating the electron wave function.     

Coherent spin dynamics of carriers in ferromagnetic semiconductor heterostructures with an Mn δ layer

The coherent spin dynamics of carriers in the heterostructures that contain an InGaAs/GaAs quantum well (QW) and an Mn δ layer, which are separated by a narrow GaAs spacer 2–10 nm thick, is comprehensively studied by the magnetooptical Kerr effect method at a picosecond time resolution. The exchange interaction of photoexcited electrons in QW with the ferromagnetic Mn δ layer manifests itself in magnetic-field and temperature dependences of the Larmor precession frequency of electron spins and is found to be very weak (several microelectron volts). Two nonoscillating components related to holes exist apart from an electron contribution to the Kerr signal of polarization plane rotation. At the initial stage, a fast relaxation process, which corresponds to the spin relaxation of free photoexcited holes, is detected in the structures with a wide spacer. The second component is caused by the further spin dephasing of energyrelaxed holes, which are localized at strong QW potential fluctuations in the structures under study. The decay of all contributions to the Kerr signal in time increases substantially when the spacer thickness decreases, which correlates with the enhancement of nonradiative recombination in QW.     

Canted ferromagnetism in PdMn and PdMnFe alloys

From nuclear orientation measurements on Pd 3% Mn and Pd 5% Mn 0.35% Fe we find that at low temperatures the alloys behave as canted ferromagnets. We can estimate the probability distribution for the individual spin orientations and can observe the reduction in canting in strong applied fields. In the PdMnFe sample the Fe spins are much more highly aligned than the Mn spins.     

Unusual behavior in the spin—lattice relaxation of oriented nuclei

In NMR studies of oriented ⁵⁴Mn spins in antiferromagnetic MnCl₂.4H₂O we have observed an unusual behaviour of the γ-ray anisotropy during the spin—lattice relaxation process. It can be understood in terms of the separate responses of the second and fourth rank tensor polarizations.     

Evolution of solid state systems containing mutually coupled dipolar and quadrupole spins: perturbation treatment

Perturbation approach to time evolution of multi-spin systems containing quadrupole and dipolar spins has been presented and discussed. The treatment comprises polarization transfer effects, field-dependent relaxation processes of dipolar as well as quadrupole spins and combined results of both of them. Complete theories dealing with various aspects of the spin dynamic processes have been proposed. Because of an educational character of this paper, relevant assumptions, limitations and even particular steps of the proposed treatments have been discussed in detail. Special emphasis is put on understanding of validity regimes of the perturbation treatment, depending on relative strengths of spin interactions and timescales of relevant motional processes affecting them. Motional regimes required for spins to be involved in essentially different evolution pathways like polarization transfers or relaxation have been illustrated by experimental examples.     

Low-frequency spin fluctuations in Skyrmions confined by wires: measurements of local nuclear spin relaxation

We investigate low-frequency electron spin dynamics in a quantum Hall system with wire confinement by nuclear spin relaxation measurements. We developed a technique to measure the local nuclear spin relaxation rate T(1)(-1). T(1)(-1) is enhanced on both sides of the local filling factor ν(wire)=1, reflecting low-frequency fluctuations of electron spins associated with Skyrmions inside the wire. As the wire width is decreased, the fast nuclear spin relaxation is suppressed in a certain range of Skyrmion density. This suggests that the multi-Skyrmion state is modified and the low-frequency spin fluctuations are suppressed by the wire confinement.     

Optical orientation of Mn2+ ions in GaAs in weak longitudinal magnetic fields

We report on optical orientation of Mn2+ ions in bulk GaAs subject to weak longitudinal magnetic fields (B≤100 mT). A manganese spin polarization of 25% is directly evaluated by using spin-flip Raman scattering. The dynamical Mn2+ polarization occurs due to the s-d exchange interaction with optically oriented conduction band electrons. Time-resolved photoluminescence reveals a nontrivial electron spin dynamics, where the oriented Mn2+ ions tend to stabilize the electron spins.     

Korringa relaxation time of magnetic ion system near a two-dimensional electron gas

Spin relaxation of Mn ions in a (Cd,Mn)Te quantum well with quasi-two-dimensional carriers (Q2DEG) is investigated. The mechanism of energy transfer is spin-flip scattering of Mn spin with electrons making transitions between spin subbands accompanied by a change in the Mn spin. A calculation of the spin-flip scattering rate shows that the Mn spin relaxation rate is proportional to the coupling constant squared, the density of states squared, and the electron temperature, the so called Korringa relaxation rate. It was found that for small Mn ion concentration, the relaxation time ≈10−7-10−6s is in a good agreement with experimental results. Moreover, the relaxation rate scales with L⁻², L being the well width, and it can be enhanced over its value in bulk.     

Change of direction of dipolar field in dilute Ising system: Paradoxical concentration independence

The local dipolar field produced by fluctuating Ising spins cannot change direction unless it is produced by two or more such spins. Nonetheless, I show that the change of orientation is independent of concentration in a dilute system. The implications fo for zero-field muon spin and NMR relaxation are discussed.     

Spin excitations in granular structures with ferromagnetic nanoparticles

Spin excitations and relaxation in a granular structure which contains metallic ferromagnetic nanoparticles in an insulating amorphous matrix are studied in the framework of the s-d exchange model. As the d system, we consider the granule spins, and the s system is represented by localized electrons in the amorphous matrix. In the one-loop approximation with respect to the s-d exchange interaction for a diagram expansion of the spin Green’s function, the spin excitation spectrum is found, which consists of spin-wave excitations in the granules and of polarized spin excitations. In polarized spin excitations, a change in the granule spin direction is accompanied by an electron transition with a spin flip between two sublevels of a split localized state in the matrix. We considered polarized spin relaxation (relaxation of the granule spins occurring by means of polarized spin excitations) determined by localized deep energy states in the matrix and the thermally activated electronic cloud of the granule. It is found that polarized spin relaxation is efficient over a wide frequency range. Estimates made for structures with cobalt granules showed that this relaxation could be observed in centimetric, millimetric, and submillimetric wavelength ranges.     

Electrically driven spin dynamics of paramagnetic impurities

The spin dynamics of dilute paramagnetic impurities embedded in a semiconductor GaAs channel of a conventional lateral spin valve has been investigated. It is observed that the electron spin of paramagnetic Mn atoms can be polarized electrically when driven by a spin valve in the antiparallel configuration. The transient current through the MnAs/GaAs/MnAs spin valve bears the signature of the underlying spin dynamics driven by the exchange interaction between the conduction band electrons in GaAs and the localized Mn electron spins. The time constant for this interaction is observed to be dependent on temperature and is estimated to be 80 ns at 15 K.     

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.     

Intrinsic spin fluctuations reveal the dynamical response function of holes coupled to nuclear spin baths in (In,Ga)As quantum dots

The problem of how single central spins interact with a nuclear spin bath is essential for understanding decoherence and relaxation in many quantum systems, yet is highly nontrivial owing to the many-body couplings involved. Different models yield widely varying time scales and dynamical responses (exponential, power-law, gaussian, etc.). Here we detect the small random fluctuations of central spins in thermal equilibrium [holes in singly charged (In,Ga)As quantum dots] to reveal the time scales and functional form of bath-induced spin relaxation. This spin noise indicates long (400 ns) spin correlation times at a zero magnetic field that increase to ～5 μs as dominant hole-nuclear relaxation channels are suppressed with small (100 G) applied fields. Concomitantly, the noise line shape evolves from Lorentzian to power law, indicating a crossover from exponential to slow [～1/log(t)] dynamics.     

室温下(Ga,Mn)As中载流子的自旋弛豫特性

运用飞秒时间分辨抽运-探测克尔光谱技术,研究了室温下退火及未退火(Ga,Mn)As的载流子自旋弛豫的激发能量密度依赖性,发现电子自旋弛豫时间随激发能量密度增加而增大,而在同一激发能量密度下,退火样品比未退火样品具有更短的载流子复合时间、电子自旋弛豫时间和更大的克尔转角,显示DP机理是室温下(Ga,Mn)As的电子自旋弛豫的主导机理.退火(Ga,Mn)As的超快克尔增强效应显示其在超高速全光自旋开关方面的潜在应用价值,也为(Ga,Mn)As铁磁性起源的p-d交换机理提供了证据. 关键词： (Ga Mn)As稀磁半导体 时间分辨克尔光谱 电子自旋弛豫 DP机理     

Exchange relaxation as a mechanism of the ultrafast reorientation of spins in a two-sublattice ferrimagnet

An exact solution for the evolution of the spins of sublattices in a two-sublattice ferrimagnet has been obtained in the exchange approximation. The nonlinear regimes of the spin dynamics including the longitudinal evolution of the spin magnitudes of the sublattices and the spin precession have been found taking into account exchange relaxation. In particular, these regimes describe the switching of the directions of spins observed in GdFeCo alloy under the action of a femtosecond laser pulse.     

Suppression of spin relaxation in an InAs nanowire double quantum dot

We investigate the triplet-singlet relaxation in a double quantum dot defined by top gates in an InAs nanowire. In the Pauli spin blockade regime, the leakage current can be mainly attributed to spin relaxation. While at weak and strong interdot coupling relaxation is dominated by two individual mechanisms, the relaxation is strongly reduced at intermediate coupling and finite magnetic field. In addition we observe a characteristic bistability of the spin-nonconserving current as a function of magnetic field. We propose a model where these features are explained by the polarization of nuclear spins enabled by the interplay between hyperfine and spin-orbit mediated relaxation.