J. Chaussy

Nanocalorimeter for high resolution measurements of low temperature heat capacities of thin films and single crystals

An innovative nanocalorimeter has been developed for measuring specific heats of thin films, multilayers (typical thickness: 1000 A) and single crystals (mass: 1 μg) in the temperature range of 1.5–20 K. The addenda of the device are as small as 3 nJ/K at 4 K (0.5 nJ/K at 1.5 K), thus samples with a heat capacity of the order of nJ/K at 4 K can be measured. Heat capacity differences as a function of temperature or an external magnetic field (5 T) were determined with a resolution of ΔC/C≃10−4. This way we have seen heat capacity variations of less than a pJ/K. We present as an example measurements on very small Mn12O12 acetate single crystals and a measurement of a thin superconducting Pb layer. In the latter measurement we could evidence via specific heat a finite size effect.

Thermodynamic and transport properties of UPt3

In the normal phase of UPt3, the magnetoresistivity is large and positive with the striking feature of a quasi-independence of the temperature and magnetic field terms; the thermoelectric power has aT2 dependence and the susceptibility is almost constant up to 4 K. The superconducting transition is broadened and the specific heat jump is weak due to the strong anisotropy of its normal phase. FromTc to 146 mK (the lowest experimental temperature), a largeT2 contribution is observed in the specific heat, but the thermal conductivity has the same dependence only below 150 mK. These results are compared with the predictions given for polar odd-pairing superconductivity.

Attojoule Calorimetry of Mesoscopic Superconducting Loops

We report the first experimental evidence of nontrivial thermal behavior of the simplest mesoscopic system--a superconducting loop. By measuring the specific heat C of an array of 450,000 noninteracting aluminum loops with very high accuracy of approximately 20 fJ/K, we show that the loops go through a periodic sequence of phase transitions (with a period of an integer number of magnetic flux quanta) as the magnetic flux threading each loop is increased. The transitions are well described by the Ginzburg-Landau theory and are accompanied by discontinuities of C of only several thousands of Boltzmann constants kB.

Anisotropy and angular variation of the giant magnetoresistance in magnetic multilayers (invited)

The giant magnetoresistance (GMR) of magnetic multilayers is usually considered as isotropic, i.e., independent of the direction of the sensing current with respect to the applied field. In spin‐valve samples of the form NiFe/Cu/NiFe/FeMn it is possible to accurately determine the amplitude of the GMR (without any contribution from the usual anisotropic magnetoresistance) for various direction of the current with respect to the direction of the magnetization of the two ferromagnetic layers, both in the parallel and antiparallel magnetic configurations. In three series of spin‐valve samples of the composition F tF/Cu tCu/NiFe/FeMn, we have observed that the GMR amplitude is larger when the current is perpendicular to the magnetizations than when it is parallel to it. This intrinsic anisotropy in the GMR shows a pronounced maximum (relative amplitude of the anisotropy of the order of 10% at the maximum) for a thickness of the ferromagnetic layer of the order of 150 A. In contrast, this anisotropy depends ve...

Possible Existence of Intrinsic Geometrical Boundaries for the Critical Currents inside YBa2Cu3O7-δ

The irreversible magnetization of YBa2Cu3O7-δ crystals of different sizes have been measured and compared to other results published in the literature. We conclude that even the superconducting crystals contain normal barriers or weak links which strongly limit the spatial scale of supercurrents. These barriers are not the twin boundaries but could be related to a possible secondary decomposition of the off-stoichiometric crystals in high and low oxygen concentration regions. Supercurrents of about 4 106 A/cm2 at 4 K would be contained inside regions into the crystals having a perimeter of about 1 mm if the Bean model can be applied.

Physical kinetics and thermodynamics of phase transitions probed by dynamic nanocalorimetry

A specific ac nanocalorimeter is presented for the study of kinetics in phase transitions. This apparatus designed by means of microfabrication technologies requires a 50‐μm-thick solid or liquid sample. The use of the ac calorimetric method allows an operating frequency range around two decades combined with a very high sensitivity. The ability of this apparatus for thermal spectroscopy is illustrated by measuring the thermal behavior of a homopolymer, the polytetrafluoroethylene, which exhibits two phase transitions at room temperature. The variation of the thermal frequency allows the observation of kinetic effects directly seen on the thermodynamic properties (enthalpy and heat capacity). We demonstrate the possibility of extracting a quantitative value of the kinetic time constant occurring at a first-order phase transition and consequently the capabilities offered by highly sensitive ac calorimetry to investigate direct thermal dynamics in macromolecular samples.

Magnetization and energy relaxations in spin glass AuFe 4 at% below Ting

Abstract We present data on Au Fe 4% showing: a relaxation of the saturated remanent magnetization significative of a wide distribution of barriers; an associated energy relaxation implying that the barriers separate levels of different energies; a time-temperature correlation of these relaxations which implies thermally activated processes.

Influence of cooling rate on the heat capacity and thermal transitions of amorphous polyhexene-1

Abstract Heat capacity measurements have been made on an amorphous polyhexene-1 sample between 20 and 300K, with an adiabatic calorimeter giving a low dispersion on the C p curve (0·2–0·4%). The measured sample was prepared by a new method increasing its thermal diffusivity. The heat capacities were measured for different cooling rates (250 K/h and 0·15 K/h) through the glass transition. This large difference of the cooling rates did not change T g very much (215·5 to 213·5K), but a peak was added to the usual ΔC p at T g . An anomalous behaviour was observed at 80K, which is consistent with an experiment described elsewhere. Around 80K dielectric and mechanical relaxations were found in other experiments. In conclusion, the importance of the glass transition is pointed out, its effect being visible down to the lowest temperatures.

Measurement of the Derivatives of the Magnetization of a Superconducting Network

Measurements are reported of the derivatives dM/dH and dM/dT of the magnetization of a superconducting network near the superconducting transition temperature in a weak transverse magnetic field. The a.c. susceptibility dM/dH exhibits sharp peaks at integer and fractional number of flux quanta per cell as expected from the phase diagram (H, T) of the network. In contrast, the dM/dT measurements reveal distinct features, characteristic of the equilibrium state below the superconducting transition. These results are compared to recent theoretical predictions for the mixed state of a superconducting network.

Specific heat of Ce3Al11 and CeAl3 compounds

Abstract Specific heat of the Ce 3 Al 11 compound has been measured to find its properties and connect them to those of the other cerium-aluminum compounds. Two peaks are observed at 3.2 and 6.2 K. Kondo effect and ferromagnetic order coexist between 3.2 and 6.2 K. A modulated phase exists below 3.2 K.