M.B. Fontes

Effect of the Ni site substitution on superconducting properties of YNi2B2C

Abstract Structural and superconducting properties of Y(Ni1−xMx)2B2C compounds with MCo, Fe and Ru have been studied. A fast decrease of Tc with the concentration of the dopants was found. This effect can be attributed to the shift of the Fermi level induced by the dopant concentration with the additional contributions from the chemical-pressure effects and from the decrease of the electronic mean free path.

Magnetoresistance in CeTGe3 (T=Fe, Co)

Abstract We present the results of magnetoresistance ρ(H, T) measurements for CeFeGe3 and CeCoGe3 as a function of temperature in fixed applied fields and as a function of field at fixed temperatures. For CeCoGe3 the H–T phase diagram was constructed from ρ(H, T) results. Temperature dependent magnetoresistance of CeCoGe3 in applied field was fitted taking into account electron–magnon scattering in different regions of the H–T diagram.

Magnetic properties of Y1−xGdxCo2B2C series of borocarbides

Abstract Magnetic properties of Y 1− x Gd x Co 2 B 2 C series of compounds were studied. Two magnetically ordered phases were observed for 0.7 ≤ x ≤ 1. Change of the magnetic ordering temperatures T 1 and T 2 with the Gd concentration is argued to be related to the dilution of magnetic Gd by nonmagnetic Y and to the reduction of the distance between ( Y 1− x Gd x ) C planes. However, significant contribution to the changes in T 1 between pure GdNi 2 B 2 C and GdCo 2 B 2 C samples is suggested to be due to the different 3 d band filling in these compounds.

Effects of Ru doping on the transport and magnetic properties of a La1.32Sr1.68Mn2 ? yRuyO7 layered manganite system

The low temperature magnetization, specific heat, electrical resistance and magnetoresistance have been studied for the Ru-doped La(1.32)Sr(1.68)Mn(2 - y)Ru(y)O(7) (y = 0.0, 0.04, 0.08 and 0.15) layered manganite system. The undoped compound (y = 0.0) shows a sharp ferromagnetic transition (T(C)) accompanied by a metal-insulator transition (T(MI)) at 118 K. The Ru substitution decreases the T(C) and T(MI) temperatures significantly. The temperature dependence of specific heat measurement confirms the decrease in T(C) by observing the anomaly corresponding to T(C). The decreased effective moments from 3.48 μ(B) for the undoped compound to 1.82 μ(B) for the highly doped compound at 5 K indicates the Ru substitution weakens the ferromagnetic order in the low temperature regime and reduces the number of Mn pairs in the highly doped sample. The field dependence of magnetization measurements exhibits an enhancement of the coercive field with increased Ru concentration and gives evidence for the mixed magnetic phase for the highly doped compound. For the undoped sample, a large negative magnetoresistance of 300% at T(C) and 128% at 4.2 K in a 5 T field were observed. The magnetoresistance ratio decreases gradually with increasing Ru substitution. We find that the doped Ru in the Mn site drives the layered manganite system towards a magnetically mixed state. The effects of Ru doping in the transport and magnetic properties will be explained by the antiferromagnetically coupled Ru and Mn sublattices.

Magnetoresistance of the compound CeRu2Ge2

In this work we present a magnetoresistance study on the CeRu2Ge2 compound. We analyze the ρ(T) curves for several applied magnetic fields using the electron–magnon scattering model for a ferromagnetic spin arrangement. From this analysis, the field dependence of the energy gap of the magnon spectrum is obtained. The magnetoresistance ρ(H) at various temperatures arises from a normal metal contribution with an additional scattering mechanism due to electron–magnon interaction.

Magnetoresistance, susceptibility and magnetization measurements on RNiBC compounds (R=Er, Ho, Dy, Tb, AND Gd)

Abstract We studied magnetic behavior of the RNiBC compounds by magnetic and transport measurements. At low temperatures, each compound has different magnetic structure. Magnetoresistivity data are in good accordance with theory of magnetic elementary interactions, susceptibility reveals the magnetic transition and Curie Weiss behavior and magnetization shows low value of the saturation compared with the free R+3 ion that confirm that crystalline electric field is strong as in the RNi2B2C series.

Magnetic structure of as seen from the transferred magnetic hyperfine field at the Ni site

Mossbauer spectroscopy has been used to investigate the magnetic structure of doped with 1 at.% . Our experiments on polycrystalline reveal the known spin-rearrangement magnetic phase transition at , which until now has not been observed in polycrystalline samples. A spiral-like spin structure with a variable angle between the moment direction and the b- or c-axis seems to be the correct model for the spin structure of .

A comparison of the effect of pressure and Co substitution on the Néel temperature of the series

The effect of hydrostatic pressure and alloying on the magnetic ordering temperature in the series was studied. The observed behaviour was related to changes in the density of states at the Fermi level caused by pressure or Ni substitution. In addition, the elastic constants of , which is a representative member of the family, were measured.

Effect of pressure on the resistivity of the Ce(Ru1−xFex)2Ge2 series

Abstract The effect of hydrostatic pressure on the resistivity of Ce(Ru 1− x Fe x ) 2 Ge 2 was studied. Chemical pressure (alloying) and hydrostatic pressure produce similar changes in the physical properties of the series driving it from the ferromagnetic (CeRu 2 Ge 2 ) to the heavy fermion (CeFe 2 Ge 2 ) ground state with (decreasing) unit cell volume as a unique parameter; f-p(CeGe) hybridization is, probably, also giving some contribution to the changes.

Quantum Criticality of CePt and YbFe2Ge2 Heavy Fermions under Pressure

This work reports the study of the quantum criticality of the ferromagnetic (FM) CePt Kondo lattice and the new non-magnetic YbFe 2 Ge 2 heavy fermion compound using electrical resistivity measurements under high pressures ( P ≤20 GPa). Pressure drives these systems to different types of magnetic quantum critical point, at different critical pressures ( P c ): FM-QCP for CePt ( P c ∼12.1 GPa) and AF-QCP YbFe 2 Ge 2 ( P c ∼9.4 GPa). Despite of that, the quantum critical behavior of both compounds at the two sides of the zero temperature transition is dominated by two dimensional (2D) spin fluctuations.