Relaxation in Powdered Metallic Antimony in Contact with Liquid 3He

The nuclear spin-lattice relaxation in finely powdered antimony metal immersed in liquid ³ He has been studied at low temperatures. For T < 75 mK, a new surface relaxation mechanism becomes significant. This surface relaxation is due to the modulation by the quantum zero-point motion of the dipole-dipole interaction between the Sb spins at the metal surface and the ³ He spins in the solid-like layer on the surface. Replacement of the liquid ³ He by ⁴ He restores the relaxation rates to the bulk value. This work helps explain anomalies found in previous relaxation studies on powdered Pt immersed in liquid ³ He.     

Nuclear Spin Relaxation Characteristic of Submonolayer ^3He Films in Nanochannels

In order to obtain information on dynamics of helium films in the nondegenerate fluid region, we have performed a pulsed-NMR experiment at 3.29 MHz on $^3$ He films adsorbed in straight 2.4 nm channels of FSM silicates down to 0.54 K. In general, the spin-lattice and spin-spin relaxation times $T_1$ and $T_2$ were explained in terms of the two-dimensional Bloembergen–Purcell–Pound model for dipolar relaxation. Temperature dependences of $T_1$ in submonolayer $^3$ He films show a minimum, indicating that the dipolar-field correlation time $\tau _\mathrm {c}$ is about $\omega ^{-1}=4.8\times 10^{-8}$ s. The temperature $T_\mathrm {min}$ of the $T_1$ minimum monotonically lowers with increasing coverage, suggesting that $^3$ He adatoms become more mobile at higher coverages. The low-dimensional property of $^3$ He adatoms is observed as the separation of $T_1$ and $T_2$ above $T_\mathrm {min}$ where $\omega \tau _\mathrm {c}<1$ . On the other hand, several features specific to films in the nanochannel geometry were also found. Especially, the temperature dependence of $T_2$ becomes very small just below $T_\mathrm {min}$ and shows a shoulder at lower temperatures. This anomaly has not been observed in $^3$ He adsorbed in wider pores or on flat surfaces, so that it is considered to be characteristic of $^3$ He films confined in narrow channels with a diameter of a few nm.     

Study of Bulk Longitudinal Spin Relaxation in 3He-4He Liquid Mixtures

Longitudinal spin relaxation has been investigated in spin polarised liquid ³ He- ⁴ He mixtures in cesium-coated glass cells. The contribution of wall relaxation is found to be either negligible or easy to separate, and bulk relaxation times can be derived from systematic studies. We report on relaxation measurements at saturated vapour pressure, for temperatures ranging from 1.1 to 3K. ³ He densities vary from 0.07 to 11 mmol.cm^–3 ( ³ He concentrations between x ₃ 0.3 and 34%), so that both normal and superfluid ³ He- ⁴ He solutions are studied. At fixed temperature, the longitudinal relaxation time is inversely proportional to the ³ He number density over the whole range of investigated concentrations. In contrast, it is almost independent of temperature (changes do not exceed 10%). Comparison with published data and expected behaviours is discussed.     

Theory of the Anomalous Nuclear Spin Relaxation in U2D2 Solid 3He

In pulsed NMR experiments on U2D2 solid ³ He it has been observed that in some cases free induction signals decay very quickly. It was also found that in such cases a large negative frequency shift from the Larmor frequency appears. We investigated the mechanism of this anomalous nuclear spin relaxation theoretically, using the Holstein-Primakoff method for the Hamiltonian including dipole and exchange interaction. It is shown that the Suhl Instability which had been observed only in the electron spin systems occurs also in the nuclear spin systems and these observed behaviors are attributed to the instability of the uniform precession of magnetization due to the excitation of spin waves.     

Visualization of Spatially Inhomogeneous Spin Relaxation Phenomena In Liquid 3He-4He Mixtures

By using the Magnetic Resonance Imaging (MRI) technique, we studied the spatial recovery of the longitudinal magnetization of liquid ³ He- ⁴ He mixtures in a 2mm × 2mm × 20mm rectangular parallelepiped sample cell below 1 K. We applied saturation pulses to destroy the magnetization and took two-dimensional pictures of the recovery of the magnetization in three orthogonal planes by the MRI as a function of waiting times. We found from the MRI pictures that the bulk relaxation process was so slow that the surface relaxation and the subsequent diffusion process were dominant mechanisms for the 3He spin relaxation. When the mixture was phase-separated, the magnetization in the concentrated side recovered much faster than that in the diluted side. It was also shown that there was no extra magnetization recovery at the phase boundary. As a result, the magnetization diffused from the concentrated side, flowed through the phase boundary into the diluted side and caused a magnetization recovery in the diluted side.     

Spin Diffusion Anisotropy in Liquid 3He

We report new experiments on the transverse and longitudinal spin dynamics of liquid ³ He polarized by an 11.3T field. At the lowest temperatures probed, the transverse spin diffusion coefficient saturates with an apparent anisotropy temperature of T_a=12±2 mK. This temperature dependence is in accord with the general idea of spin-diffusion anisotropy but the value of T_a is half that expected from extrapolating earlier experiments at a lower field. Strong magnetostatic effects in the form of long-lived spin modes are also observed for small gradients and small tipping angles.     

Recent Experiments on Spin Polarized Liquid 3He

The importance of spin fluctuations on the physical properties of liquid ³ He can be probed by studying the polarization dependence of its magnetic susceptibility and viscosity. We discuss the present state of two such experiments. Using the rapid melting technique, to strongly polarize liquid ³ He, we have measured the viscosity enhancement induced by the polarization up to unprecedented large polarizations at a low temperature (80 mK). At a polarization of 60 %, the enhancement reaches a factor 2.5, comparable to that of the inverse susceptibility at the same polarization. Now at Oxford Instruments Research Instruments     

Intrinsic Spin Relaxation Processes in Metallic Magnetic Multilayers

Spin relaxation processes in metallic magnetic nanostructures are reviewed. First a brief review of the phenomenology of magnetic damping is presented using the Landau Lifshitz Gilbert (LLG) equations of motion. It is shown that the Gilbert damping in bulk metallic layers is caused by the spin orbit interaction and itinerant character of 3d and 4s-p electrons. Spin dynamics in magnetic nanostructures acquires an additional nonlocal damping. This means that a part of the magnetic damping is not given by the local Gilbert damping but arises from the proximity to other layers. Spin pumping and spin sink concepts will be introduced and used to describe the interface nonlocal Gilbert damping in magnetic multilayers. The modified LLG equation of motion in magnetic multilayers will be introduced and tested against the ferromagnetic resonance (FMR) data around the accidental crossover of FMR fields. The spin pumping theory will be compared to the early theories introduced in the 1970s for the interpretation of transmission electron spin resonance (TESR) measurements across ferromagnet/normal metal sandwiches.     

Spin Diffusion Measurements in Spin-Polarised Liquid 3He-4He Mixtures

We report on investigations of spin diffusion in isotopic helium liquid mixtures using a multi-echo technique in applied magnetic field gradients. Samples (up to 1 cm ³ in volume) are obtained by liquefaction of optically polarised gas. NMR measurements are performed at saturated vapour pressure in a low magnetic field (2.2 mT). Signal-to-noise ratios stay high down to ³ He concentrations of order a few parts per thousand at 1K for instance. We demonstrate that the experimental accuracy of spin diffusion measurements is substantially improved using optically polarised mixtures. We have observed that the dynamics of the precession of the transverse magnetisation can be altered due to large dipolar fields at high magnetisation densities. Effects on spin-echo signals and implications for the extension of spin diffusion measurements with our technique are discussed.     

Charging Effects on Nuclear Spin Relaxation in Quantum Dots

We study the mechanism of nuclear spin relaxation in quantum dots due to the electron exchange with 2D gas. We show that the nuclear spin relaxation rate T ₁ ^–1 is dramatically affected by the Coulomb blockade (CB) and can be controlled by gate voltage. In the case of strong spin–orbit (SO) coupling the relaxation rate is maximal in the CB valleys whereas for the weak SO coupling the maximum of 1/T ₁ is near the CB peaks. The physical mechanism of nuclear spin relaxation rate at strong SO coupling is identified as Debye–Mandelstam–Leontovich–Pollak–Geballe relaxational mechanism.     

Measurements of the Nuclear Spin Relaxation Times for Small Grains of Solid Hydrogen Suspended in Liquid Helium

Measurements of the nuclear spin-lattice relaxation times (T ₁) and the nuclear spin-spin relaxation times (T ₂) are reported for suspensions of small grains of solid hydrogen in liquid helium. The measurements have been carried out for temperatures 1.8>T>4.9 K and for a nuclear Larmor frequency of 4.0 MHz. The samples were prepared by rapid condensation of hydrogen and helium gas in a precooled cell. The hydrogen pressure was maintained below the triple point to prepare a “slush” of hydrogen particles in liquid helium. The characteristic relaxation times are much shorter, typically by an order of magnitude for T ₂, than those observed and reported previously for bulk samples of solid hydrogen for the same ortho-hydrogen concentrations and applied magnetic fields. Possible origins for this difference between the relaxation in bulk and micro-geometries are discussed.     

Magnetic Relaxation of Solid and Liquid 3He Confined Inside a Silver Sinter

We present T ₁ measurements performed in liquid and solid ³ He confined in sintered silver. In the liquid case, we find a close to linear relationship between T ₁ and the applied magnetic field, similar to other (non metallic) systems, and up to higher frequencies (corresponding to 11 T) than previously reported. In the solid, the relaxation rate is found to increase spectacularly at large polarizations, which may indicate a polarization dependence of the exchange motion. Now atNow at     

Damping of a Vibrating Wire in Spin Polarized Liquid 3He-4He Mixtures

In this paper we report measurements of the damping of a vibrating wire in spin polarized liquid helium mixtures. The dilute helium solutions have been cooled by nuclear demagnetisation in high magnetic fields such that the spin polarization for a 0.04% mixture is as high 75%. A PtW alloy vibrating wire has been used to probe the viscosity of the liquid. The increase in viscosity on changing the magnetic field from 3 tesla to 11.5 tesla is a factor 3.5 at the lowest temperatures. The results are compared to theory allowing for both s-wave and p-wave scattering.     

Gate Voltage Control of Nuclear Spin Relaxation in GaAs Quantum Well

Gate-voltage dependences of nuclear spin relaxation and decoherence times in a Schottky-gated n-GaAs/AlGaAs (110) quantum well (QW) are investigated by time-resolved Kerr-rotation measurements combined with pulsed-rf nuclear magnetic resonance (NMR). We show that the nuclear spin relaxation and decoherence times decrease with decreasing electron density, indicating that the hyperfine interaction is enhanced as the electronic states becomes localized in an impurity-doped QW.     

Spin Dynamics of 3He in Aerogel

We report continuous-wave NMR measurements on superfluid ³ He contained inside high porosity aerogel at 3.24 MPa, in a 28.4 mT magnetic field, and down to 0.9 mK. Three different purities of ³ He were used: pure ³ He, ³ He with enough ⁴ He to replace the first localized layer, and ³ He with enough ⁴ He to replace both localized layers. Below 2.26mK, the NMR spectrum does not consist of a single Lorentzian, but a distribution of NMR resonant frequencies both above and below the Larmor frequency, _L . Upon cooling, the component of the spectrum at frequencies below the Larmor frequency moves to higher frequencies over a narrow temperature range and does not return to < _L upon warming. The higher frequency component of the spectrum displays two resolvable peaks which become a complex structure upon removal of the localized ³ He spins. The spectrum changes as the magnetic field is rotated about the axis of the aerogel sample, indicating that the ³ He is able to detect a preferred direction in the aerogel. The shape of the spectrum shows no rotation or magnetic field hysteresis. We find that the average frequency shifts of our NMR spectra do not depend on the presence of the localized ³ He spins. These frequency shifts and the presence of a temperature-independent magnetization for the liquid ³ He suggest that the aerogel stabilizes a single equal-spin-pairing state in our experiments.     

Coupling of the Nuclear Spin Alignment of Quadrupolar Nuclei to 3He at Interfaces

Measurements of the relaxation of nuclear spin alignments have been carried out using NQR spin echo studies of powdered antimony in contact with liquid ³ He. This system differs from previous studies of the relaxation and magnetization coupling of ³ He at surfaces because the magnetizations < S _z > 0. The spin alignments < S _z ² > are the relevant dynamical variables. The analysis of the experimental results shows that the coupling of the nuclear spin alignment of the antimony nuclei to the magnetization of the ³ He nuclei is determined by the spectral density of the fluctuations of the spin-spin dipolar couplings at the interface. This power density is attributed to the zero-point motions (atom-atom and atom-vacancy exchange) of the ³ He atoms on the surface of the metal particles.     

Simulations of Non-linear Spin Dynamics in Spin Polarized Dilute 3He–4He Mixtures

Studies of spin dynamics of highly polarized dilute mixtures of ³He in superfluid ⁴He have been performed by various researchers over the past three decades. One series of experiments performed at Cornell University in the early 1990’s revealed a novel long timescale excitation. We present the numerical solution of the non-linear Leggett spin dynamics equation in one spatial dimension subject to boundary conditions consistent with the Cornell experiments. Experimentally observed phenomena are composed of trains of bursts in the transverse magnetization lasting several seconds. The simulations capture the time evolution of the individual bursts localized in time. Preliminary results of two dimensional simulations are also presented.     

Pulsed NMR in the Nuclear Spin Ordered Phases of Solid 3He in a Silver Sinter

To obtain the exact spin structure of the nuclear magnetically ordered phases of solid ³He, which in the bee lattice are the U2D2 and the high field phase, both occurring below about 1 mK, we started a project of neutron scattering at the Hahn-Meitner Institut, Berlin. This experiment faces three main difficugties: to grow a single crystal within the sinter needed for cooling, to cool the solid to temperatures below 1 mK (or even much lower in the case of the hcp lattice), and to keep it there long enough, even under neutron irradiation. As a first step we have performed pulsed NMR measurements in the ordered phases of solid ³He in a Ag sinter of 700 Å particle size down to temperatures of 400 µK at various molar volumes. The samples remained in the ordered state for as long as 140 h. In the low field phase a strong reduction of the Larmor line to nearly zero intensity was observed. With a sample grown over 6 h at the megting curve around 50 bars three lines at the Larrnor frequency and on its high frequency side were observed which seem to indicate a partly crystalline U2D2 phase in our sinter. The origin of the strong intensity drop, however, is not clear and needs further investigation.     

Spin lattice relaxation in BCC3He

Analysis is made of spin lattice relaxation measurements made at low temperature in various BCC samples of solid³He with concentrations of⁴He of 60 and 82 ppm, at Larmor frequencies of 50 and 200 MHz, and for molar volumes covering the range 23.05V24.82 cm³. From the characteristic features of the relaxation we deduce the mechanisms involved and their relative strengths. We stress the importance of a one-phonon process in relaxing the exchange energy at large molar volume and give an estimate of the bandwidth of the vacancy waves. The combination of the various mechanisms gives a coherent picture of the relaxation at low temperature.     

Spin waves in anisotropic superfluid3He

Spin waves in the A phase of superfluid³He are studied theoretically. It is assumed that the condensate in the A phase consists of theP-wave triplet pairs of the type suggested by Anderson and Brinkman. We show that the spin wave mode with a finite energy gap exists throughout the A phase. In particular atT=0 K the dispersion of the spin wave is given by, forvq<Δ, $$\omega ^2 = \omega _0^2 + 2(1 - \bar I)A\Delta ^2 + \tfrac{1}{3}(1 - \bar I)v^2 q^2 $$ where θ is the spin wave energy, ω _T =[ω ₀ ² +2(1?ī)AΔ²]^1/2 is the nuclear magnetic resonance energy,v is the Fermi velocity,q is the wave vector of the spin wave, andī=IN(0)=?1/4Z ₀ is the Landau-Fermi liquid parameter.