N.A. de Oliveira

Investigations on magnetic refrigeration: Application to RNi2 (R=Nd, Gd, Tb, Dy, Ho, and Er)

In this article we report the thermodynamic investigations on the Ericsson cycle with application on RNi2 (R=Nd, Gd, Tb, Dy, Ho, and Er) series. Besides the Zeeman and exchange interactions, these compounds present an important contribution from crystalline electrical field interaction. The Ericsson coefficient of performance and refrigerant capacity was investigated under the crystal field influence. An optimum molar composite of Er–Dy–TbNi2 was proposed to work as refrigerant material in the temperature interval from 7 to 22 K.

Entropy change upon magnetic field and pressure variations

In this letter it is theoretically discussed the magnetocaloric effect under applied pressure and the barocaloric effect under applied magnetic field in compounds undergoing a first order magnetic phase transition. The theoretical findings of this letter point out that the magnetocaloric effect upon simultaneous variation of the magnetic field and pressure as well as the barocaloric effect in first order phase transition compounds can be very important for magnetic refrigeration technology.

Magnetocaloric effect in

Abstract In this paper we calculate the magnetocaloric effect in the compound Gd ( Pd 1 - x Rh x ) by using the Heisenberg Hamiltonian where the indirect exchange interaction parameter between localized spins depends on the Rh concentration and the spin–spin interaction is treated in the molecular field approximation. The calculated adiabatic temperature changes upon magnetic field variations are in good agreement with the available experimental data.

Absolute atomic oxygen and nitrogen densities in radio-frequency driven atmospheric pressure cold plasmas: Synchrotron vacuum ultra-violet high-resolution Fourier-transform absorption measurements

Reactive atomic species play a key role in emerging cold atmospheric pressure plasma applications, in particular, in plasma medicine. Absolute densities of atomic oxygen and atomic nitrogen were measured in a radio-frequency driven non-equilibrium plasma operated at atmospheric pressure using vacuum ultra-violet (VUV) absorption spectroscopy. The experiment was conducted on the DESIRS synchrotron beamline using a unique VUV Fourier-transform spectrometer. Measurements were carried out in plasmas operated in helium with air-like N2/O2 (4:1) admixtures. A maximum in the O-atom concentration of (9.1 ± 0.7)×1020 m−3 was found at admixtures of 0.35 vol. %, while the N-atom concentration exhibits a maximum of (5.7 ± 0.4)×1019 m−3 at 0.1 vol. %.

Barocaloric and magnetocaloric effects in La(Fe0.89Si0.11)13

In this paper we calculate the barocaloric and the magnetocaloric effects under applied pressure in the compound La(Fe0.89Si0.11)13. We use a microscopical model in the framework of the band theory of magnetism, where the Coulomb interaction between itinerant electrons is treated in the mean field approach. Our theoretical calculations point out that both the barocaloric potentials [ΔS]B and [ΔTad]B at fixed magnetic field, and the magnetocaloric potentials ΔS and ΔTad under simultaneous variation of magnetic field and pressure, reach large values in a wide range of temperatures.

The giant anisotropic magnetocaloric effect in DyAl2

We report on calculations of the anisotropic magnetocaloric effect in DyAl2 using a model Hamiltonian including crystalline electrical field effects. The anisotropic effect is produced by the rotation of a constant magnetic field from the easy to a hard magnetic direction in the crystal and is enhanced by the first order nature of the field induced spin reorientation transition. The calculated results indicate that for a field with modulus of 2 T rotating from a hard to the easy direction, the isothermal magnetic entropy (ΔSiso) and adiabatic temperature (ΔTad) changes present peak values higher than 60% the ones observed in the usual process, in which the field direction is kept constant and the modulus of the field is varied.

Theoretical calculations of the magnetocaloric effect in MnFeP0.45As0.55: a model of itinerant electrons

In this paper we calculate the magnetocaloric effect in the compound MnFeP0.45As0.55. We use a microscopical model in the picture of the band theory, including a magnetoelastic interaction. The theoretically calculated isothermal entropy changes upon magnetic field variations are in good agreement with the available experimental data.

Barocaloric effect and the pressure induced solid state refrigerator

The current refrigerators are based on the heating and cooling of fluids under external pressure variation. The great inconvenience of this refrigeration technology is the damage caused to the environment by the refrigerant fluids. In this paper, we discuss the magnetic barocaloric effect, i.e., the heating or cooling of magnetic materials under pressure variation and its application in the construction of refrigerators using solid magnetic compounds as refrigerant materials and pressure as the external agent. The discussion presented in this paper points out that such a pressure induced solid state refrigerator can be very interesting because it is not harmful to the environment and can exhibit a good performance.

Magnetocaloric effect under applied pressure and the barocaloric effect in the compounds RCo2 (R = Er, Ho and Dy)

In this work, I discuss the magnetocaloric effect under applied pressure, as well as the barocaloric effect, in the Laves phase compounds RCo2 (R = Er, Ho and Dy). To this end, I use a model Hamiltonian including both localized 4f spins and itinerant 3d electrons. The calculations point out that (i) for an applied pressure of 1.0 GPa the peaks of the magnetocaloric potentials [ΔS]M and [ΔTad]M are shifted to lower temperatures, but their magnitudes remain almost unchanged; (ii) the magnetocaloric potentials exhibit sizable values in a wider range of temperatures, when both magnetic field and pressure are changed; (iii) the peaks of the barocaloric potentials [ΔS]B and [ΔTad]B can be as large as the magnetocaloric ones.

Local magnetization and hyperfine field systematics of s−p and noble impurities in Gd and Ni hosts

Magnetic hyperfine data of s−p and noble impurities diluted in ferromagnetic Gd and Ni hosts are described within a simple model which is an extension on that one of Daniel and Friedel. We also include in the present model the effect of next-neighbor perturbation, due to the translational invariance break introduced by the impurity. Performing a self-consistent calculation of the local magnetic moments at the impurity site, one obtains the conduction electron polarization (CEP) hyperfine field. It is found that the model can explain the different hyperfine field trends observed in Gd and Ni hosts, e.g., a remaining negative value along the s−p series in the Gd case and a change of sign behavior in the Ni case. Period effects observed in noble impurities are also discussed and the theoretical calculations are in good agreement with available experimental data.