RESUMEN
Neutron powder diffraction patterns measured above T C have been used to determine the location of the excess Mn in MnxGa (1.15 ≤ x ≤ 1.8). This information has then been used to constrain the fits to neutron powder diffraction patterns measured at ambient temperature and so determine unambiguously the Mn moments in this system. We find that Mn randomly occupies the two Ga sites (2a and 2b) in the I4/mmm structure and propose that it is more appropriate to use a simpler structure based on the P4/mmm space group with a reduced unit cell. In this structure the two Ga sites are formally equivalent (they occupy the 1a site while Mn occupies the 1d site). Our experimental observations are supported by DFT calculations. Below T C we find that the Mn(1d) moment is constant at 2.45(3) µ B , while Mn on the 1a site carries a slightly larger moment (~3 µ B ) that is coupled antiparallel to the Mn(1d) moments, leading to the observed drop in magnetisation with increasing Mn content in MnxGa.
RESUMEN
Magnetization measurements have been carried out on a series of carefully prepared single-phase Mn(1+x)Ga (0
RESUMEN
The electronic structure and magnetic properties for Gd(3)NiSi(2) have been studied theoretically from a first-principles density functional calculation. The energy band structure is calculated in a local spin density approximation (LSDA), and in a LSDA+Hubbard U approach (LSDA+U), respectively. For Gd atoms, in the LSDA+U approximation, seven spin-up 4f bands are fully occupied and situated at the bottom of Si s states, while the spin-down 4f hole levels are completely unoccupied and well above the Fermi level. The calculated magnetic moments for the three Gd sites vary from 7.13 to 7.16 µ(B), leading to a total magnetization of 21.5 µ(B) per formula unit including the small induced moments at Ni and Si atoms. The exchange coupling parameters for the nearest Gd-Gd pairs (J(Gd-Gd)) are 0.16 mRyd, 0.14 mRyd and 0.19 mRyd in the three Gd sub-lattices, respectively. The inter-site distance dependence of J(Gd-Gd) shows a RKKY-like oscillation. The estimated Curie temperature is about 251 K from the calculated exchange coupling parameters based on the mean-field approximation, in good agreement with the experimental value (T(C)(exp.) = 215 K).
RESUMEN
Since it was first observed about 40 years ago [A. B. Pippard, Proc. R. Soc. A 216, 547 (1953)10.1098/rspa.1953.0040], the peak effect has been the subject of extensive research mainly impelled by the desire to determine its exact mechanisms. Despite these efforts, a consensus on this question has yet to be reached. Experimentally, the peak effect indicates a transition from a depinned vortex phase to a reentrant pinning phase at a high magnetic field. To study the effects of intrinsic pinning on the peak effect, we consider FexNi1-xZr2 superconducting metallic glasses in which the vortex pinning force varies depending on the Fe content and in which a huge peak effect is seen. The results show that the peak effect broadens with decreasing pinning force. Typically, pinning is increased by pinning centers, but here we show that reentrant pinning is due to the strengthening of interactions and collective effects (while decreasing pinning strength).
RESUMEN
In the mixed state of type II superconductors, vortices penetrate the sample and form a correlated system due to the screening of supercurrents around them. Interestingly, we can study this correlated system as a function of density and driving force. The density, for instance, is controlled by the magnetic field B, whereas a current density j acts as a driving force F=j x B on all vortices. To minimize the pinning strength, we study a superconducting glass in which the depinning current is 10 to 1000 times smaller than in previous studies, which enables us to map out the complete phase diagram in this new regime. The diagram is obtained as a function of B, driving current, and temperature, and leads to a remarkable set of new results, which includes a huge peak effect, an additional reentrant depinning phase, and a driving force induced pinning phase.