Viscoelasticity of dynamically self-assembled paramagnetic colloidal clusters

Paramagnetic particles in a liquid above a solid dynamically self-assemble into two-dimensional (2D) viscoelastic clusters in a processing magnetic field if the precession angle exceeds the magic angle. Hexagonal clusters rotate with a frequency proportional to the precession frequency of the magnetic field. The rotation is explained by viscoelastic shear waves excited in the clusters that can be visualized slightly above the magic angle. The cluster rotation and the visualization of viscoelastic modes are independent techniques to probe the rheological properties of the cluster. We find agreement between both techniques when determining the 2D cluster viscosity eta(c) approximately 10(-11) N s/m.     

Spin precession waves in superfluid 3He-B

Spin precession waves of homogeneously precessing domains (HPD) in superfluid 3He-B have been studied at 11 bars and temperatures down to 0.45T(c). The waves were excited by an alternating longitudinal magnetic field with an axial symmetry, applied as a small perturbation ranging from 1 nT up to a few micro T. When the spin precession wave is excited, two nuclear magnetic resonances simultaneously coexist: first, the high frequency resonance used for excitation of the HPD, and, second, the low-frequency resonance of the HPD wave mode. We report the first experimental evidence of the nonlinear behavior of low-frequency precession spin wave modes of the continuously maintained HPD.     

Two-magnon relaxation reversal in ferrite spheres

The reversal of two-magnon relaxation associated with linear scattering of oscillations of uniform magnetization precession from sample nonuniformities is studied theoretically and experimentally in ferrite spheres of yttrium iron garnet (YIG). Relaxation reversal is performed by parametric phase conjugation of dipole-exchange spin waves formed as a result of scattering of uniform precession from inhomogeneities. As a result of two-magnon backward scattering of dipole-exchange spin waves with a certain time delay, magnetization oscillations are renewed with an amplitude that could exceed the initial amplitude of uniform precession. The relaxation reversal is due to crystallographic anisotropy of the sample and is manifested most strongly when a YIG sphere is magnetized along the intermediate axis [110]. Experiments were carried out on YIG spheres of diameter 0.65–1.05 mm for a parallel pumping frequency ω _p /2π ≈ 9.4 GHz, which is about twice the uniform precession frequency. The maximal delay time for the restored signal of uniform precession was about 2 μs, while the maximal amplitude exceeded the initial uniform precession amplitude by a factor of about 5. The “latent” relaxation parameters of ferrites, e.g., the natural ferromagnetic resonance linewidth associated with many-particle processes and the linewidth associated with two-magnon scattering at bulk nonuniformities, are determined experimentally.     

Excitation of spin waves during ferromagnetic resonance

A non-linear equation is solved which includes the excitation of spin waves by Suhl non-linear coupling and by the scattering of homogeneous precession on inhomogeneities. The solution of this equation takes into account the dependence of the spin-wave frequencies on the degree of excitation of the homogeneous precession and on the degree of excitation of the spin waves. The result of the dependence just quoted is the finality of the amplitudes of spin waves for arbitrary values of the angle of homogeneous precession and the foldover effect, connected with hysteresis and discontinuities in the excitation of the spin waves. From the solution it is also apparent that for values of the homogeneous precession amplitude higher than the Suhl critical value the resonance curve splits into two curves, i.e. two groups of spin waves are excited. These two groups differ in frequency by a value of the same order as their relaxation frequency.     

Parametric instability in uniform spin precession in superfluid 3He-B

In order to explain the “catastrophic spin relaxation” observed in superfluid ³He-B, the stability of spatially uniform spin precession in this liquid relative to the parametric excitation of spin waves has been analyzed. It is shown that uniform spin precession becomes unstable at low temperatures (Suhl instability). At zero temperature, the growth increments are determined for all spin wave branches. The temperature at which the transition from stable spin precession to instability takes place is estimated.     

On the possibility of recording the sagnac effect using π-mesons

We propose to measure the Sagnac effect for counterpropagating de Broglie waves of π⁺ or⁻ mesons whose spin is equal to zero. The latter circumstance allows us to avoid the nonreciprocal phase shift of counterpropagating waves, which is related to both the Thomas precession (the effect of the special relativity theory) and the Larmor precession of the spin of elementary particles due to the presence of magnetic fields. Among the advantages of using π mesons, we should mention their electric charge, which allows us to use the well-known devices for focusing, separation, and joining the particle beam, and their comparatively long lifetime. A modulation method for processing the interference signal is proposed. The maximum sensitivity of the above method, which is limited by the shot noise, is estimated. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 6, pp. 767–774, June, 1998.     

Low-temperature stability limit of the coherent spin precession in <Superscript>3</Superscript>He-B

It has been shown that the instability of uniform spin precession in the bulk of the ³He-B superfluid phase is due to the joint action of the anisotropy of the velocities of spin waves and dipole interaction. In the leading approximation in the ratio of the Leggett frequency to the Larmor frequency, the growth increments of the amplitudes of spin waves for all of the possible decay channels have been found. The maximum increment has been determined for all of the angles of spin deviation from the equilibrium orientation. The minimum temperature to which precession is stable has been estimated.     

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.     

Measurement of the angular momentum of a rotating bose-einstein condensate

We study the quadrupole oscillation of a Bose-Einstein condensate of 87Rb atoms confined in an axisymmetric magnetic trap, after it has been stirred by an auxiliary laser beam. The stirring may lead to the nucleation of one or more vortices, whose presence is revealed unambiguously by the precession of the axes of the quadrupolar mode. For a stirring frequency Omega below the single vortex nucleation threshold Omega(c), no measurable precession occurs. Just above Omega(c), the angular momentum deduced from the precession is approximately Planck's over 2pi. For stirring frequencies above Omega(c) the angular momentum is a smooth and increasing function of Omega, until an angular frequency is reached at which the vortex lattice disappears.     

Electrical measurement of spin-wave interactions of proximate spin transfer nanooscillators

We have investigated the interaction mechanism between two nanocontact spin transfer oscillators made on the same magnetic spin valve multilayer. The oscillators phase lock when their precession frequencies are made similar, and a giant magnetoresistance signal is detectable at one contact due to precession at the other. Cutting the magnetic mesa between the contacts with a focused-ion beam modifies the contact outputs, eliminates the phase locking, and strongly attenuates the magnetoresistance coupling, which indicates that spin waves rather than magnetic fields are the primary interaction mechanism.     

Symmetry-breaking instabilities on a fluid surface

A sequence of symmetry-breaking instabilities leading to a chaotic state has been discovered in the surface deformations of a fluid layer subjected to a vertical oscillation. For driving amplitudes above a critical value, a primary instability leads to circularly symmetric standing waves at half the driving frequency. A second instability at a higher threshold breaks the circular symmetry and leads to a slow precession of the pattern, so that the overall motion is quasiperiodic. Beyond a third threshold, azimuthal modulations produce chaotic time dependence A fourth instability leads discontinuously to a spatially disordered flow. The spatial structure associated with each instability has been determined qualitatively, and the frequency spectrum of the local surface deformation has been measured using a sensitive laser deflection technique.     

Some properties of isospin waves in a neutron lattice

A dilute system of isospin waves in equilibrium with electrons in a neutron lattice should show Bose-Einstein condensation. The weak interaction of the condensate may cause damping of the free precession of neutron stars.     

X-ray detected magnetic resonance of YIG thin films in the nonlinear regime of spin waves

We discuss the information content of element/edge resolved X-ray detected magnetic resonance (XDMR) experiments carried out on yttrium iron garnet (YIG) thin films. Starting with a phenomenological approach, it is shown that the photoionisation of deep atomic core levels by circularly polarized X-rays can be used to probe the precession dynamics of spin or orbital magnetization components in empty final states of proper symmetry. Crude estimates of the opening angle of the uniform precession mode were tentatively deduced from the ratio of the XDMR and XMCD absorption cross-sections either at the iron or yttrium absorbing sites. The implications of the most recent experimental results collected at the ESRF are analyzed, keeping in mind that: (i) the Fe K-edge XDMR signal is largely dominated by the precession of orbital magnetization components at the tetrahedral iron sites; (ii) the Y L-edges XDMR signal essentially describes the precession of induced spin magnetization involving the 4d states of yttrium. In the magnetostatic regime, we produce clear experimental evidence of collective excitations of orbital magnetization waves, especially under high pumping power. Several coupling mechanisms could explain our observations, starting with pseudo-dipolar interactions in ferromagnetic systems. In ferrimagnetic systems in which orbital degeneracy and orbital ordering make the excitation of orbitons possible, one may envisage additional modes of excitation or relaxation of orbital magnetization waves. This interpretation looks fully consistent with the results of band structure calculations carried out recently on YIG with fully relativistic LMTO-LSDA methods.     

Experimental observation of the tilting mode of an array of vortices in a dilute Bose-Einstein condensate

We have measured the precession frequency of a vortex lattice in a Bose-Einstein condensate of Rb87 atoms. The observed mode corresponds to a collective motion in which all the vortices in the array are tilted by a small angle with respect to the z axis (the symmetry axis of the trapping potential) and synchronously rotate about this axis. This motion corresponds to excitation of a Kelvin wave along the core of each vortex and we have verified that it has the handedness expected for such helical waves, i.e., precession in the opposite sense to the rotational flow around the vortices.     

Jet precession in neutrino-cooled disks for gamma-ray bursts: The effects of the mass and spin of a black hole

We present a model of jet precession driven by a neutrino-cooled disk around a spinning black hole to explain the quasi-periodic features observed in some gamma-ray burst light curves.The different orientations of the rotational axes between the outer part of a neutrino-cooled disk and a black hole result in precessions of the central black hole and the inner part of the disk.Hence,the jet arising from the neutrino annihilation above the inner disk is driven to precession.We find that the period of precession is positively correlated with the mass as well as the spin of a black hole.     

肿瘤和正常组织的FT-Raman与FT-IR研究

本文运用ＦＴ－Ｒａｍａｎ光谱法研究肠、胃、十二指肠、口腔腮腺等部位正常组织与肿瘤组织，发现正常组织与肿瘤组织在Ｒａｍａｎ光谱上表现出较为明显的差别，这一结果与我们先前所得到的红外光谱结果相一致。此结果说明Ｒａｍａｎ光谱亦有潜力发展成为一种活体、原位、无损诊断肿瘤的新方法。     

A non-covariant theory of gravitation,II

This is a continuation of a previous paper, in which a theory of gravitation was developed based on the existence of a preferred frame of reference and a preferred time coordinate in the universe. The gravitational field equations are derived with the help of a variational principle containing three constants. Two relations among the constants are introduced, leaving one of them arbitrary. This constant does not affect the precession of the perihelion of Mercury but does affect the behaviour of gravitational waves. By changing one of the relations among the constants, one can account for the discrepancy in the precession of the perihelion associated with the oblateness of the sun, as found by Dicke and Goldenberg.     

折射率、色散变化量与宽谱段傅氏镜二级光谱变化量的分析

介绍了宽谱段傅里叶变换镜头中光学玻璃的折射率、色散变化对系统的成像质量的影响.推导了折射率、色散变化量所引起的光学系统二级光谱的变化量公式.重点讨论了在宽谱段光学系统中，光学玻璃在折射率、色散上的变化量，所造成的胶合薄透镜的二级光谱的变化量.其系数在本文例中达0.28，相当于变化量占理论二级光谱余量的28％，因此在宽谱段系统中的二级光谱余量的变化量不应该被忽略.实例表明光学玻璃的折射率、色散变化量对宽谱段傅里叶变换镜头的成像质量有显著的影响.此外，还考虑了傅里叶变换透镜的波像差问题，其设计值小于1/10波长，采用最优玻璃对组合，可以保证波像差小于1/10波长，完全满足使用要求.     

Zero-field muon spin rotation in monocrystalline chromium

Spin precession of positive muons in chromium in zero applied magnetic field is reported for the first time. The observations cover the temperature range from about 2.5 K to 10 K and thus pertain to the so-called longitudinal spin-density wave (LSDW) state of antiferromagnetic Cr. The conclusions that may be drawn from the existence of one rather sharp spin precession line are discussed, among them the estimateD _μ=2.4·10⁻¹⁴ m² s⁻¹ for the muon diffusivity at 4 K. Considerable evidence exists for a strong interactions of μ⁺ with the charge-density waves that are likely to accompany the LSDWs in Cr.     

All-optical probe of coherent spin waves

A novel, all-optical method to excite and detect spin waves in magnetic materials is presented. By exploiting the temperature dependence of the magnetic anisotropy, an ultrashort laser pulse is efficiently converted in a picosecond "anisotropy field" pulse that triggers a coherent precession of the magnetization. Recording the temporal evolution of the precessing spins by a time-delayed probe-pulse provides a quantitative method to study locally the magnetic anisotropy, as well as switching and damping phenomena in micromagnetic structures. Applications to nickel and permalloy ( Ni80Fe20) films are discussed, particularly showing the possibility to explore standing spin waves in thin films.