mK Temperatures Research Articles

Nuclear orientation study of low temperature magnetization of54Mn impurity in ferromagnetic PtFe alloys

Low-temperature nuclear orientation of54Mn in ferromagnetic Pt95Fe5 and Pt90Fe10 hosts has been investigated in 12–50 mK temperature range at external magnetic fields up to 1.2 T. Temperature dependence of the effective magnetic field Beff on Mn has been observed and interpreted as an effect of misalignment of Mn magnetic moments with respect to the macroscopic magnetization direction at host magnetic saturation.

Dynamic measurement of the surface tension of liquid helium with a two-dimensional electron probe

Electrons deposited above the free surface of liquid helium exhibit coupled electron-ripplon modes whose frequencies are simply related to the surface tension α of helium. The hydrodynamic expression for the ripplon dispersion relation for pure4He is shown to be valid up to the wavevector range 105 cm−1<q<2×106 cm−1 of the coupled modes. For a small amount of3He (X3⋍10−6−10−8) added to pure4He, we measure the variation of the surface tension in the 60–600 mK temperature range. These experiments, which cover the frequency range v⋍10–300 MHz, agree fairly well with previous static (capillary) or low-frequency (surface sound velocity) experiments.

Local ordering in dilute ortho-para-hydrogen mixtures at low temperatures

We report experimental observations of the local orientational ordering of ortho-H2 molecules in dilute solid ortho-para mixtures for low temperatures. The results indicate a smooth, continuous freezing of the rotational degrees of freedom without any evidence for a distinct discontinuity in the temperature dependence of the local order parameters for the 37–600 mK temperature range.

Induced magnetic form factor of CeCu6

Polarized neutron scattering techniques have been used to study the spatial distribution and temperature dependence of the magnetization induced by an externally applied magnetic field in a single crystal of CeCu6, a heavy-fermion compound that remains paramagnetic and nonsuperconducting down to ∼10 mK. The measurements were performed at 4.2 K and 92 mK in an applied magnetic field of 50 kOe. We find that the induced magnetization is predominately of 4f electronic character and exhibits Pauli-like behavior in the 4.2 K–92 mK temperature region.

Neutron Scattering Experiments on Nuclear Magnets–Set-up and First Results

A two stage nuclear demagnetization cryostat has been constructed for neutron scattering studies of nuclear magnetism at mK, µK and nK temperatures. Special precautions needed in order to perform these ultra-low temperature experiments in a reactor environment are briefly discussed. The use of polarized neutrons and the two-magnet design enables a variety of unique experiments. We present data on the spin-lattice relaxation in copper at very low temperatures and in high magnetic fields.

Magnetic versus Nonmagnetic Mechanism for Thermal Boundary Conductances at Millikelvin Temperatures

It is pointed out that, for the anomalous thermal boundary conductances between metal particles and liquid 3 He observed at mK temperatures, randomly distributed spins near the surface and the soft mode of metal particles play a comparable role. This is concluded from the analysis of the recent measurements of boundary conductances by Osheroff and Richardson, where a novel magnetic field dependence has been found for the boundary between silver particles and liquid 3 He in the mK region.

Investigation of new lanthanum-, cerium- and uranium-based ternary intermetallics

While searching for new Ce- and U-based “heavy fermion” systems, we have investigated the following intermetallic compounds: CeCu 2Ge 2, CeCu 2Sn 2, CeAg 2Ge 2, CeZn 2Al 2 (tetragonal ThCr 2Si 2-structure and modifications thereof); CeRu 3Si 2, URu 3Si 2 (hexagonal LaRu 3Si 2-structure); Ce 2Cu 7Al 10, U 2Cu 7Al 10 (trigonal Th 2Mn 17-structure); CeCu 4Al 8, CeAg 4Al 8, UCu 4Al 8 (tetragonal CeMn 4Al 8-structure); LaCu 13− x Al x with x = 3, 5.5, and 6.5, as well as CeCu 6.5Al 6.5 (cubic NaZn 13-structure). Resistive and inductive measurements were undertaken between T = 30 mK and room temperature, and two new superconductors were found: LaCu 6.5Al 6.5 ( T c = 0.65 K) and CeRu 3Si 2 ( T c = 1.2 K). The specific heat and thermopower of two of these systems have also been measured. CeCu 4Al 8 shows interesting features that are presumably related to fine structure in the “heavy-fermion” density of states near the Fermi level. CeCu 6.5A1 6.5 shows a magnetic transition at T = 0.5 K which resembles a “spin-glass freezing” rather than a formation of long-range magnetic order.

The sign of the magnetic moment of191Pt(g.s) determined by Mössbauer effect on oriented nuclei

Radioactive191Pt(3/2−) alloyed to iron has been oriented at low temperature in a dilution refrigerator. The resonance absorption spectrum of the 82.4 keV transition of the polarized split source has been registered at high (≈ 1 K) and low (≤ 15 mK) temperatures. From the high temperature spectrum δ (E2/M1; 82.4) 0. This result is in agreement with deformed Hartree-Fock calculation predictions.

Spectral Types, Color Indices, and Abundances for Cepheids in the Magellanic Clouds and the Galaxy

Accurate MK temperature and luminosity classes have been obtained for 58 Cepheids in the Galaxy and in the Magellanic Clouds from 121 spectrograms. Classification criteria for the spectral region 3700X–6000X in the spectra of F-K supergiants are described. The spectra of galactic and LMC Cepheids are found to be similar, and Cepheids in both systems appear to obey the same relation between (B-V)o and spectral type. The low E(B-V) values of Parsons (1970) and certain other recent authors are confirmed. The spectra of SMC Cepheids show slightly weaker metal lines relative to galactic Cepheid spectra, and SMC Cepheids average about 1.3 subclasses earlier in spectral type than LMC and galactic Cepheids at the same value of (B-V)o. Reddenings for both Clouds are found to be less than 0.05. Among supergiants (Mv = −4 to −8), stars of the same spectral type have the same (B-V)o regardless of luminosity class. Spectral types at minimum light are found to be later than those reported by Feast (1974) when luminosity effects are allowed for. Spectroscopic surface gravities near minimum light appear to be lower than those near maximum and slightly fainter Mv values are found at each luminosity class than those usually cited. SMC Cepheids appear to have lower surface gravities than LMC Cepheids at the same absolute magnitude, as one would expect from the lower metal abundances.

Copper powder nuclear demagnetization?A feasibility study

We describe a straightforward computational model for the simulation of an experiment designed to cool liquid3He or dilute3He-4He solutions by means of the nuclear demagnetization of copper powder. The calculations show that3He temperatures of around 0.1 mK and dilute solution temperatures of around 0.2 mK should be attainable, assuming the sort of starting conditions, cell construction, and heat leaks currently achieved by our experimental group.

Nuclear orientation studies of the 51·5 min167Lu decay

Anisotropies of γ rays emitted in the decay of167Lu,T1/2=51·5 min, were measured. The167Lu was oriented by the hyperfine interaction when the lutetium activity was included in a gadolinium host which was cooled to 15 mK temperature. A top-loading3He∶4He dilution refrigerator capable of quickly cooling a short-lived sample was used. Multipole mixing ratios have been evaluated for many transitions. The spin assignments have been made for a number of levels in167Yb and their parities have been deduced. In particular, an unambiguous 11/2− assignment was made for the 571·5 keV level which was identified as the band head of the 11/2− [505] band observed by Lindblad. The rotational bands in167Yb are discussed.

Tunneling within the mK range for the investigation of crystal field effects in superconductors

Experimental techniques for tunneling into bulk superconductors have been tested for the investigation of crystal field effects in LaAl2, LaSn3 and LaPb3 doped with Tb, Nd, or Pr. The evaporation of counterelectrodes onto cleaved and oxidized crystals failed in most cases, but Schottky contacts with GaAs electrodes could be prepared reproducibly by mechanical adjustment of the junction resistance at mK temperature. Problems remain resulting from the specific mechanical and structural properties of the metallic compounds under investigation.

Magnetic phase diagram of gadolinium gallium garnet

To determine a possible phase transition in gadolinium gallium garnet (GGG), we have measured the ac susceptibility down to mK temperatures. In zero field, no long range order is found down to our lowest attainable temperature of 25 mK. However, the results show evidence for field induced antiferromagnetic phases.

Heat capacity and thermal conductivity measurements down to 25 mk on samples of poor thermal diffusivity and small specific heat

A heat pulse technique is described with a permanent heat link to the cold sink used for the measurement of small heat capacities, such as vitreous dielectrics, down to 25 mK. Total heat capacities of the order of 5μJ K −1 are measured below 50 mK with an accuracy of 5%. Two carbon resistor thermometers are used to avoid any systematic error. Thermal conductivity measurements are also performed in the same temperature range by the steady flow method. The accuracy of the carbon resistor thermometer calibration enables the measurement of 2 or 3 mK temperature gradients at 25 mK.

151Eu Low Temperature Mössbauer Thermometer

A low temperature Mossbauer thermometer suitable for the 25 to 250 mK temperature range with151Eu in EuS as the absorber has been discussed, and the problems of absorber thickness have been pointed out. The large correction for the apparent temperature deduced from the sum of Lorentzian analysis has been given for absorbers with a different number of resonance nuclei per cm2. With the correction procedure, without a least-squares fit using the transmission integral, one can obtain the temperature with an accuracy of better than ±2%.

Combined lattice-location-hyperfine-interaction experiment on Se implanted in Fe and Co

The channeling technique has been used to study the lattice location of Se implanted into Fe and Co (hcp) single crystals. For both systems the impurity is found to be fully substitutional at doses of 5 \ifmmode\times\else\texttimes\fi{} ${10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}2}$. Nuclear-orientation measurements at mK temperatures have been made on implanted $^{75}\mathrm{Se}$ in Fe and Co polycrystalline hosts. Nuclear magnetic resonance of oriented nuclei (NMR/ON) was observed for Fe $^{75}\mathrm{Se}$ at 141.6 MHz (${H}_{\mathrm{ext}}=0$) and a resonance-shift experiment yields $|\ensuremath{\mu}(^{75}\mathrm{Se})|=(0.67\ifmmode\pm\else\textpm\fi{}0.04){\ensuremath{\mu}}_{N}$ and ${H}_{\mathrm{hf}}(Fe\mathrm{Se})=690\ifmmode\pm\else\textpm\fi{}50$ kG. For CoSe no NMR/ON signal was observed, but the temperature-dependent $\ensuremath{\gamma}$ anisotropy is fitted to yield ${H}_{\mathrm{hf}}(Co\mathrm{Se})=+420\ifmmode\pm\else\textpm\fi{}40 \mathrm{kG} or\ensuremath{-}440\ifmmode\pm\else\textpm\fi{}40 \mathrm{kG}$ although these values make no allowance for possible quadrupole interactions at the hcp lattice site. In the analysis of the Fe $^{75}\mathrm{Se}$ nuclear-orientation experiment, the $\ensuremath{\delta}(\frac{E2}{M1})$ mixing ratios of the 280- and 265-keV transitions determined. These results are compared with recent intermediate-coupling collective-model calculations.

Nuclear orientation of 160Tb in gold and palladium

The hyperfine interaction parameters have been measured for 160Tb in gold and palladium in the 100-10 mK temperature range using nuclear orientation. Good agreement between theory and experiment is found assuming the crystal field splitting gives 〈 J z〉 ≃ 6.