Magnetic anisotropy terms in [110] MBE grown REFe2 films involving the strain term ???

The magnetic anisotropy parameters in [110] MBE grown films of REFe2 compounds are not the same as those in the bulk. This is due to the presence of a shear strain εxy, frozen in during crystal growth. In this paper, calculated magnetic anisotropy parameters for [110] MBE grown REFe2 films, that directly involve the shear strain εxy, are presented and discussed. In addition to the usual first order Callen and Callen term K˜'2, there are nine second order terms six of which involve cross terms between εxy and the cubic crystal field terms B4 and B6. Two of the second order cross terms are identified as being important: K˜"242(T) and K˜"262(T). Of these, the rank-two term K˜"242(T) dominates over a large temperature range. It has the same angular dependence as the first order term K˜'2, but with a more rapid temperature dependence. The correction at T = 0K for TbFe2, DyFe2, HoFe2, ErFe2, and TmFe2, amounts to ~+9.2%, -13.9%, -11.6%, +22.7%, and 27.1%, respectively. Similar comments are made concerning the rank-four K˜"264(T) term

Random Field and Random Anisotropy Effects in Defect-Free Three-Dimensional XY Models

Monte Carlo simulations have been used to study a vortex-free XY ferromagnet with a random field or a random anisotropy on simple cubic lattices. In the random field case, which can be related to a charge-density wave pinned by random point defects, it is found that long-range order is destroyed even for weak randomness. In the random anisotropy case, which can be related to a randomly pinned spin-density wave, the long-range order is not destroyed and the correlation length is finite. In both cases there are many local minima of the free energy separated by high entropy barriers. Our results for the random field case are consistent with the existence of a Bragg glass phase of the type discussed by Emig, Bogner and Nattermann.Comment: 10 pages, including 2 figures, extensively revise

On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

Relationship of edge localized mode burst times with divertor flux loop signal phase in JET

A phase relationship is identified between sequential edge localized modes (ELMs) occurrence times in a set of H-mode tokamak plasmas to the voltage measured in full flux azimuthal loops in the divertor region. We focus on plasmas in the Joint European Torus where a steady H-mode is sustained over several seconds, during which ELMs are observed in the Be II emission at the divertor. The ELMs analysed arise from intrinsic ELMing, in that there is no deliberate intent to control the ELMing process by external means. We use ELM timings derived from the Be II signal to perform direct time domain analysis of the full flux loop VLD2 and VLD3 signals, which provide a high cadence global measurement proportional to the voltage induced by changes in poloidal magnetic flux. Specifically, we examine how the time interval between pairs of successive ELMs is linked to the time-evolving phase of the full flux loop signals. Each ELM produces a clear early pulse in the full flux loop signals, whose peak time is used to condition our analysis. The arrival time of the following ELM, relative to this pulse, is found to fall into one of two categories: (i) prompt ELMs, which are directly paced by the initial response seen in the flux loop signals; and (ii) all other ELMs, which occur after the initial response of the full flux loop signals has decayed in amplitude. The times at which ELMs in category (ii) occur, relative to the first ELM of the pair, are clustered at times when the instantaneous phase of the full flux loop signal is close to its value at the time of the first ELM

MAGNETIC DIFFUSE SCATTERING MEASUREMENTS USING POLARISED NEUTRONS

Deux techniques différentes de neutrons polarisés peuvent être utilisées pour l'étude de la diffusion magnétique diffuse. La première, pour les alliages ferromagnétiques, implique un faisceau incident polarisé sans analyse de la polarisation diffusée. Cette méthode a été largement utilisée, surtout par le groupe d'Oak Ridge, afin d'étudier les défauts de moment magnétique dans les alliages binaires ferromagnétique de métaux de transition. La deuxième technique se base sur l'analyse de polarisation et a été utilisée pour l'étude des corrélations de spin dans les verres de spin concentrés, ainsi que des défauts de moment magnétique dans les alliages antiferromagnetiques. Dans le cas dernier les mesures montrent que l'hypothèse habituelle d'un défaut de moment collinéaire n'est pas valable. On a trouvé que les corrélations de spin dans les verres de spin concentrés sont modulée d'une manière incommensurable.Two distinct polarised neutron techniques are applicable to the study of magnetic diffuse scattering. The first, used for ferromagnetic alloys, employs a polarised incident beam without analysis of the outgoing polarisation. This method has been widely used, chiefly by the Oak Ridge group, to study moment disturbances in ferromagnetic binary transition metal alloys. The second technique involves polarisation analysis and has been used to study spin correlations in concentrated spin glass materials, and moment defects in antiferromagnetic alloys. Measurements on antiferromagnetic γ-Mn alloys show that the usual assumption of a collinear moment defect is not valid. The spin correlations in certain concentrated spin glass alloys are found to have an incommensurate modulation. This may result from the modification of the antiferromagnetic couplings by the atomic correlations in the alloys

Inelastic neutron scattering from liquid cerium

Par des mesures de diffusion inélastique de neutrons, nous avons déterminé la dépendance en température de la susceptibilité des électrons 4f du métal cérium jusqu'au-delà de sa température de fusion. Les résultats confirment une délocalisation partielle des électrons 4f dans l'état liquide. L'importance de cette délocalisation semble augmenter avec la température. Nos résultats sont en désaccord avec d'autres mesures antérieures sur du cérium liquide.Inelastic neutron scattering measurements have allowed the temperature dependence of the 4f electron susceptibility to be determined through the melting temperature of cerium metal. The results confirm that there is a partial delocalisation of the 4f electron in the liquid state. The extent of the delocalisation appears to increase with temperature. Previous neutron diffraction data on liquid cerium are not compatible with our results