Magnetic properties of solid 3He down to 0.3 mK

Nuclear magnetization of solid ³He has been studied by static magnetization measurements from 10 mK down to 0.3 mK for molar volumes ranging from V = 24.14 to 21.02 cm³/mole in the bcc phase and from V= 19.83 to 19.26 cm³/mole in the hcp phase. In the bcc phase, both the antiferromagnetic transition temperature T _N and the reciprocal of the maximum magnetization V _max ^–1 at T _N vary in proportion to V ^16.5±1, and the magnetization below T _N is constant. The magnetization reduced by M _max is found to be represented by a universal function of the reduced temperature T/T _N. In the hcp phase, the magnetization can approximately be represented by Curie's law, and the estimated Weiss temperatures are below 50 K. We also observed that the boundary magnetism of liquid ³He depends considerably on pressure. The transition temperature of solid ³He to the antiferromagnetic phase coexisting with liquid in a restricted geometry is 15% higher than that of the bulk solid on the melting curve.     

Observation of3He crystallization down to 1 mK using a cooled CCD camera

We describe a new setup for optical observations (particularly of helium crystals) at ultralow temperatures. It is conceptually different from the usual optical setups, which require the use of optical windows in the cryostat. Our optical system, including the light source and camera, is placed inside the vacuum can of a dilution refrigerator cryostat. We have succeeded in filming the growth of³He crystals in contact with the superfluid B phase, down to 1 mK. The optical system can easily be combined with a nuclear demagnetization stage, and a simple extension of the image transport system will allow us to study³He crystals in high magnetic fields. The entire setup is described, and some typical images are shown.     

A Holmium-in-gold nuclear orientation thermometer for use from 1 K to 30 mK

A new -ray anisotropy thermometer which may ultimately be capable of primary thermometry from 1 K down to ~30 mK is reported. The thermometer is a dilute solid solution, ~0.1 at %, of ^166mHo in gold. Two different sample preparation schemes and other relevant details are discussed. The anisotropy of the 712-keV -ray as a function of B and T is presented. The interpretation of the anisotropy in terms of the crystal field, electronic Zeeman, and hyperfine interaction is discussed. It is concluded that incorrect crystal field parameters for HoAu are the reason for the discrepancy between theory and experiment. Relaxation effects are briefly considered.Supported by the Australian Research Grants Committee and the Australian Institute of Nuclear Science and Engineering.     

An investigation of Au/Mn contacts to p-type InP

The performance of a Au/Mn contact metallization to p-type InP has been reported. Electrical resistance measurements done on annealed contacts have been correlated to the accompanying microstructural changes, by means of electron microscopy and X-ray diffraction techniques. Manganese was found to react readily with the underlying InP, leading to the formation of Mn₂P followed by MnP. Subsequent outward diffusion of indium towards the gold layer led to the formation of Au₃In, which replaced the original gold layer. Inward diffusion of gold resulted in the formation of an Au-In-Mn ternary phase at the MnP-InP interface. This phase may have supplied the necessary manganese for InP doping required to lower the contact resistance. A minimum resistance of 6 x 10^-4Ωcm² was obtained.     

Thermal expansion coefficient of 3He near 1 mK

A measurement of the thermal expansion coefficient of ³He at 0 bar from 0.6 to 3 mK is presented. At the superfluid transition the expansion coefficient changes by 1.1 × 10^\s-6 mK^\s-1. This change is consistent with predictions based on thermodynamic arguments and existing heat capacity data.This work was supported by NSF grant DMR 79-25098.     

Vibrating Wire Measurements in Superfluid 3He at the Melting Curve Down to 0.53 mK

Measurements of the vibrating wire spectrum have been carried out in superfluid ³ He along the melting curve down to 0.53mK. We have observed that at temperatures below 0.3 T_c the width of the mechanical resonance of the wire decreases exponentially with 1/T, indicating the ballistic regime of collisions with quasiparticles. The value of the superfluid energy gap was found to be (1.99±0.05)T_c, in good agreement with the values obtained from heat capacity measurements. The vibrating wire was thereby calibrated for further experiments at temperatures below 0.5mK, where the sensitivity of the melting curve thermometry becomes rather poor.     

Morphology and Growth Kinetics of 3He Crystals Below 1 mK

The shapes as well as the growth and melting properties of bcc-³He single crystals have been investigated with a low temperature Fabry-Pérot interferometer. Eleven types of facets were clearly identified during slow crystal growth at the temperature of 0.55 mK, where the solid is in the antiferromagnetically ordered u2d2 phase. The growth rates of the individual facets have been measured and the results indicate significant growth anisotropy. The observed linear dependence of the growth velocity on the driving force shows that facets grow due to the presence of screw dislocations, while the step velocity is limited by the spin wave velocity due to the strong interaction of the moving step with magnons in the solid. The measured growth rates of the facets and the assumed growth mechanism gave us the unique opportunity to obtain the step free energies for ten different types of facets observed during a single growth sequence. The dependence of the free energy of the step on the step height is compared with predictions of the weak- and strong-coupling models. Our results suggest that ³He crystals have rather strong coupling of the liquid/solid interface to the crystal lattice and that the step-step interactions are of elastic origin.     

Viscosity of saturated3He-4He mixture below 200 mK

The shear viscosity of saturated³He-⁴He mixture has been measured at temperatures between 7 mK and 200 mK using a vibrating-wire viscometer and a calibrated pressure cell. The reliability of the vibrating-wire technique was tested by measuring the viscosity of pure⁴He. The results are internally consistent. A phenomenological expression is given for the viscosity of saturated³He-⁴He mixture between 7 mK and 80 mK.     

Measurements on the Surface Tension of 3He Crystals near 100 mK

No Heading ³He crystals start to show facets on their surface only at about 100 mK, well below the roughening transition temperature, and the reason for this change of the surface state is not clear yet. However, the most important characteristics of the crystal surface, the surface tension, was not measured in this temperature range before. We report our observations on the equilibrium shape of the ³He crystals in the temperature range of 77...100 mK. The surfaces tension was found to be isotropic and temperature—independent, and the corresponding value of the capillary length, = 0.93 ± 0.10 mm, is in a good agreement with the value measured at higher temperatures by Rolley et al.²PACS numbers: 68.35.Md 68.08.–p 67.80.–s     

Observations on Faceting of 3He Crystals at T=0.55 mK

We report on our recent observations on growing ³ He crystals in which altogether eleven different types of facets were found. The crystals were imaged with a novel low-temperature Fabry-Pérot interferometer. Intensity based analysis methods combined with a phase shift technique were used to identify the observed facets.     

Surface tension of liquid3He down to 0.3 K

The surface tension of liquid³He was measured by determining the surface wave velocity in the temperature range from 0.32 to 3.02 K. The decrease of the surface tension from the value at 0 K can be described as 22.3T ² mdyn/cm up to about 0.9 K and is quite different from that of⁴He. The extrapolated value to 0 K is 155.7±0.5 mdyn/cm. The data below 1.6 K were compared with Saam's model, where the surface tension is assumed to be given by the bulk free energy times the surface width and the quasi-two-dimensional free energy. Agreement between the data and the results calculated from the model is much improved if ripplons are taken into account. Data near the gas-liquid critical point, contrary to earlier results, show that the surface tension vanishes with a critical exponent of 1.24±0.05. This is in agreement with Widom's theory.     

Improved Optical Setup to Measure Growth and Megting of Solid 3He Below 1 mK in Magnetic Fields Up to 9 Tesla

Recent experiments on crystal growth and crystal shape in ³He at zero magnetic field at 0.55 mK by Tsepelin et al. ^{1, 2} in Helsinki have revealed many new facets and strong anisotropy in the growth rates. Since ³He presents the unique possibility of investigating the influence of a magnetic field on crystal growth properties, we intend to extend our experimental studies of the shape and growth rate of solid ³He to magnetic fields up to 9 T at temperatures as low as possible.For that purpose our optical access cryostat with low temperature CCD has been upgraded and modified. The low temperature LED is replaced by a room temperature source coupled to the cell with an optical fiber to obtain clear images of the growth process. Our Pomeranchuk cell has been fitted with a new optical section with pure quartz windows. It has a larger diameter, a larger distance between the optical windows, and an improved thermal link. Also the heat switches between the nuclear stage and the dilution refrigerator were replaced according to a more sturdy design. Initial resugts show clear crystals with sharp facet edges down to T=0.4 mK.     

Characterization of spin polarized dilute3He-4He solutions below 500 mK

We have prepared a large sample of spin polarized liquid ³ He- ⁴ He at 500 mK. Polarization is obtained by laser optical pumping, and studied by pulsed NMR. We have observed a nuclear magnetization lifetime T ₁ longer than 1 hour, and an anomalously long, non linear decay of tranverse magnetization.Unité de recherche de l'Ecole Normale Supérieure et de l'Université Pierre et Marie Curie, associé au CNRS (URA 18).     

Ni52 Mn23Ga24.5 Sm0.5合金的马氏体相变和磁致伸缩性能

研究了多晶Ni52 Mn23 Ga24．合金添加微量的稀土元素Sm后，对合金的马氏体相变和磁致伸缩性能的影响。结果发现，微量稀土元素Sm的掺入，降低了合金的马氏体相变温度和居里温度，但并未改变合金的晶体结构，同时由于晶粒细化作用，使合金室温时的磁致应变性能下降。