Coherent control of spin precession motion with impulsive magnetic fields of half-cycle terahertz radiation

Coherent control of the precession motion of magnetizations in a single crystal YFeO3 with double half-cycle pulse terahertz waves was demonstrated. Quasiferromagnetic (0.299 THz) and quasiantiferromagnetic (0.527 THz) precession modes were selectively excited by choosing an appropriate interval of two pulses and were observed as free induction decay (FID) signals from the spin system. By observing the circularly polarized FID signals due to ferromagnetic resonance, we also succeeded in confirming directly the energy storage in the spin system and recovery from that to the electromagnetic radiation.     

铁磁异质结构中的超快自旋流调制实现相干太赫兹辐射

利用飞秒激光脉冲在生长于二氧化硅衬底上的W/CoFeB/Pt和Ta/CoFeB/Pt两类铁磁/非磁性金属异质结构中实现高效、宽带的相干THz脉冲辐射.实验中, THz脉冲的相位随外加磁场的反转而反转,表明THz辐射与样品的磁有序密切相关.为了考察三层膜结构THz辐射的物理机制,分别研究了构成三层膜结构的双层异质结构(包括CoFeB/W, CoFeB/Pt和CoFeB/Ta)的THz辐射.实验结果都与逆自旋霍尔效应相符合, W/CoFeB/Pt和Ta/CoFeB/Pt三层膜结构所辐射的THz强度优于同等激发功率下的ZnTe (厚度0.5 mm)晶体.此外,还研究了两款异质结构和ZnTe的THz辐射强度与激发光脉冲能量密度的关系,发现Ta/CoFeB/Pt的饱和能量密度略大于W/CoFeB/Pt的饱和能量密度,表明自旋电子在Ta/CoFeB/Pt中的界面积累效应相对较小.     

Long-lived ultrafast spin precession in manganese alloys films with a large perpendicular magnetic anisotropy

Spin precession with frequencies up to 280 GHz is observed in Mn(3-δ)Ga alloy films with a perpendicular magnetic anisotropy constant K(u)～15 M?erg/cm(3). The damping constant α, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the δ=1.46 (0.88) film. Those are about one-tenth of α values for known materials with large K(u). First-principles calculations well describe both low α and large K(u) for these alloys.     

Coherent structural dynamics and electronic correlations during an ultrafast insulator-to-metal phase transition in VO2

We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.     

Coherent spin precession via photoinduced antiferromagnetic interactions in La0.67Ca0.33MnO3

Pronounced spin precessions are observed in a geometry with negligible canting of the magnetization in ferromagnetic La(0.67)Ca(0.33)MnO(3) thin films using the time-resolved magneto-optical Kerr effect. The precession amplitude monotonically decreases with increasing field, indicating that the coherent spin rotation may be triggered by a transient exchange field and not by demagnetization and/or anisotropy field modulation. We attribute the transient exchange field to emergent antiferromagnetic interactions due to charge transfer and modification of the kinetic energy of e(g) electrons under optical excitation.     

Positive muon spin precession in magnetic oxides MnO and V2O3; local fields and phase transition

Muon spin precession is observed in the ordered state of antiferromagnetic oxides MnO and V₂O₃. An anomalous temperature dependence of muon precession frequency and the absence of critical behavior atT _N are found in MnO and a hysterisis of the transition temperature is observed in V₂O₃. These results are explained in terras of the crystal distortion of these materials belowT _N.We would like to thank Dr. T. Suzuki, Dr. S. Takagi, Dr. Y. Kuno and Dr. C.Y. Huang for their help in the measurements, and Dr. K. Kosuge and Dr. S. Kachi for providing the specimen of V₂O₃. We are grateful to the financial support from Japan Society for Promotion of Science.     

Configurational spin reorientation phase transition in magnetic nanowire arrays

Classical microscopic spin reorientation phase transitions (RPT) are the result of competing magnetocrystalline anisotropies. RPTs can also be observed in discrete macroscopic systems induced by competing shape anisotropies and magnetostatic coupling. Such a configurational RPT was recently observed in series of self-organized hexagonal arrays of 2.5 μm long, 25-60 nm diameter circular permalloy nanowires grown in anodic alumina matrix. This RPT is a crossover transition from a one-dimensional easy axis “wire” behavior of weakly interacting uniaxial nanowires to a two-dimensional behavior of strongly coupled “wire film” having an easy plane anisotropy. It is shown that RPT takes place due to the competition between the intrinsic dipolar forces in individual wires and the external dipolar field of interacting nanowires in the array. The crossover occurs at a volume ratio of 0.38 for 65 nm periodicity. The experimental results are in agreement with the semi-analytical calculations of the dipolar interaction fields for these arrays of circular ferromagnetic nanowires, and are interpreted in terms of the Landau phase transition theory. The conditions for the crossover and the order of the phase transition are established. Based on the contribution to the magnetic energy from the flower state at the ends of the wires, it is concluded that the observed transition is of the first order.     

Manipulating magnetic anisotropy and ultrafast spin dynamics of magnetic nanostructures

We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si(111) substrate by a novel method: varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a Co Fe B/Mg O/Co Fe B magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect(TRMOKE) for both the parallel state(P state) and the antiparallel state(AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two Co Fe B layers via the tunneling of hot electrons through the Mg O barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.     

Coherent control of the self-trapping transition

We discuss the dynamics of two weakly coupled Bose-Einstein condensates in a double-well potential, contrasting the mean-field picture to the exact N-particle evolution. On the mean-field level, a self-trapping transition occurs when the scaled interaction strength exceeds a critical value; this transition essentially persists in small condensates comprising about 1000 atoms. When the double-well is modulated periodically in time, Floquet-type solutions to the nonlinear Schr?dinger equation take over the role of the stationary mean-field states. These nonlinear Floquet states can be classified as “unbalanced” or “balanced”, depending on whether or not they entail long-time confinement of most particles to one well. Since the emergence of unbalanced Floquet states depends on the amplitude and frequency of the modulating force, we predict that the onset of self-trapping can efficiently be controlled by varying these parameters. This prediction is verified numerically by both mean-field and N-particle calculations. Received 5 November 2000 and Received in final form 16 February 2001     

The magnetic phase transition in amorphous ferromagnets and in spin glasses

The magnetic phase transition in materials with exchange disorder (amorphous ferromagnets, spin glasses) is discussed. In the critical temperature range the behavior of amorphous ferromagnetic transition metal-metalloid glasses is found to be similar to the one derived for a three-dimensional homogeneous Heisenberg ferromagnet. The most prominent difference between disordered and homogeneous materials is manifested in a large temperature range of deviations from the mean field behavior beyond the critical region, as observed experimentally for the temperature dependence of the linear susceptibility of amorphous ferromagnets and of the nonlinear susceptibility of spin glasses. A molecular field theory with correlations in space and time is developed, which relates the deviations from the mean field behavior to the interplay between the temperature dependent thermal correlations in the spin system and the spatial fluctuations of the material. Application to dynamical processes (kinetic critical slowing down) is discussed.     

Coherent spin precession in a separated solution of 3He in 4He

It is suggested that a coherently precessing spin structure in a normal Fermi liquid should be used to study the interface of two Fermi liquids. It is shown that the interface makes an additional contribution to the attenuation of the precessing structure. This contribution is determined by the kinetic coefficient which relates the magnetization flux across the interface to the magnetization jump at the interface. A relationship is established between this kinetic coefficient and the nature of the scattering of Fermi quasiparticles at the interface. Results of numerical calculations of the attenuation of the structure are presented for a specific object, a separated solution of ³He in ⁴He.     

Analogue of a spin flop phase transition for an array of magnetic moments with dipole interaction

In a two-dimensional array of magnetic moments with planar magnetization and relatively weak anisotropy in the basal plane, a stepwise phase transition is induced by an external magnetic field parallel to the easy axis of the system. This transition is similar to the spin flop phase transition in weakly anisotropic Heisenberg antiferromagnets with the significant difference that it is accompanied by the rearrangement of the sublattice structure of the magnet; i.e., it can be interpreted as a topological transition. The transition should manifest itself for arrays of submicron magnetic particles (magnetic dots) on nonmagnetic substrates, which have recently become the object of intensive research.     

Photoinduced phase transition in VO2 nanocrystals: ultrafast control of surface-plasmon resonance

We study the ultrafast insulator-to-metal transition in nanoparticles of VO2, obtained by ion implantation and self-assembly in silica. The nonmagnetic, strongly correlated compound VO2 undergoes a reversible phase transition, which can be photoinduced on an ultrafast time scale. In the nanoparticles, prompt formation of the metallic state results in the appearance of surface-plasmon resonance. We achieve large, ultrafast enhancement of optical absorption in the near-infrared spectral region that encompasses the wavelength range for optical-fiber communications. One can further tailor the response of the nanoparticles by controlling their shape.     

Coherent spin precession of electrons and excitons in charge tunable InP quantum dots

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.     

Optical studies of spin precession in magnetic two-dimensional electron gases

Magnetic two-dimensional electron gases are studied using time-resolved Kerr and Faraday rotation spectroscopy in the Voigt geometry. The data directly reveal both electron and Mn spin precession in modest transverse fields. Scattering by Mn ions dominates the electron spin relaxation processes in these materials, and prevents the electron gas from acquiring a long-lived spin polarization as observed in non-magnetic structures. Nonetheless, a persistent Mn spin polarization occurs which creates a oscillating magnetic field within the electron gas for hundreds of picoseconds.     

Electron spin precession in the field of an electromagnetic wave

The solution of the Dirac equation for an electron in the field of a plane circularly polarized electromagnetic wave having an arbitrary intensity and a phase velocity v _ph different from the speed of light c is obtained. This solution is shown to describe the previously unknown effect of electron spin precession that exceeds the known precession effects caused by radiative corrections by six and more orders of magnitude and opens new possibilities for control of the polarization of electrons and for realization of various resonance methods associated with the spin precession. The predicted effect is in agreement with symmetry principles; however, it disappears at v _ph=c and is not described by the known Volkov solution.     

A study of the stripe domain phase at the spin reorientation transition of two-dimensional magnetic system

Stripe domain phase in a two-dimensional magnetic system was studied using model calculation and Monte-Carlo simulation in the spin reorientation transition (SRT) region. We find that near the SRT point the stripe domains evolve into a static spin wave structure with a fractional in-plane magnetization along the stripe direction and a fractional out-of-plane magnetization in a sinusoidal form. With increase in the uniaxial perpendicular anisotropy, both the wavelength and the height of the static spin wave increase slowly until the saturation of the wave height, after which the stripe width increases exponentially with the magnetic anisotropy. Our theoretical result is in good agreement with experimental observations.     

Coherent control of the spin current through a quantum dot

Within the weak-coupling regime the spin current through a quantum dot system is calculated using a quantum master equation approach which includes a sum over Matsubara terms. To be able to efficiently calculate, also at low temperatures, the time evolution of the reduced density matrix a high-temperature approximation was derived which proves to be rather accurate in comparison to the exact results. In the present model it is assumed that the energy levels of the dot are split by a constant magnetic field. An additional external (laser) field is used to control the currents of the two spin polarizations. This is either done using the phenomenon of coherent destruction of tunneling or optimal control theory. Scenarios are studied in which the spin current is reversed while the charge current is kept constant.     

Coherent combining of a fiber laser array via cascaded internal phase control technique

We experimentally demonstrated a cascaded internal phase control technique. A laser array with 12 channels was divided into three sub-arrays and a stage array, and phases of the sub-arrays and the stage array were locked by four phase controllers based on the stochastic parallel gradient descent(SPGD) algorithm, respectively. In this way, the phases of the whole array were locked, and the visibility of the interference pattern of the whole emitted laser array in the far field was ～93%. In additi...