J. Mestnik-Filho

The effect of hybridization on local magnetic interactions at highly diluted Ce ions in tetragonal intermetallic compounds RERh2Si2 (RE=Ce, Pr, Nd, Gd, Tb, Dy).

The contribution of the 4f electron to the local magnetic field at highly diluted Ce atoms in RERh(2)Si(2) (RE = Ce, Pr, Nd, Gd, Tb, Dy) has been investigated as a function of temperature through the measurement of the magnetic hyperfine field in (140)Ce nuclei by time differential perturbed gamma-gamma angular correlation spectroscopy. Samples of the studied compounds were characterized by x-ray diffraction and zero-field resistance to determine the crystal structure and transport properties. DC magnetic susceptibility was measured for NdRh(2)Si(2). It was observed that the variation of the magnetic hyperfine field with temperature follows the expected behaviour for the host magnetization, with the exception of GdRh(2)Si(2), which showed a strong deviation from such a behaviour. It is shown that the hybridization of the d band of the host with the f band of the Ce impurity, which is stronger in GdRh(2)Si(2) than in other compounds, is responsible for the observed deviation from the expected temperature dependence of the hyperfine field. The origin of this stronger hybridization is ascribed to the relatively small magnetic anisotropy observed in GdRh(2)Si(2) when compared with the other compounds of the series, as shown by resistance measurements.

Influence of Cd impurity on the electronic properties of CuAlO2 delafossite: first-principles calculations

We report on first-principles band-structure calculations of the semiconducting CuAlO2 delafossite compound in the pure form and also with Cd impurity occupying either a Cu or Al position. The computational tool was a full-potential linear augmented plane-wave method, with the generalized gradient approximation accounting for the exchange and correlation effects. The changes caused by the presence of Cd are studied by the analysis of the electronic structure and the electric field gradient (EFG) in both Cd-doped and pure CuAlO2 systems. Good agreement between the calculated and measured EFGs at Cd substituting for Cu or Al atoms in CuAlO2 indicates that the calculations were able to correctly describe the ground state of the system containing the impurity. It is shown that a specific hybridization scheme, involving Cu (and Cd) s and dz2 orbitals and neighbouring O pz orbitals, takes place at the Cu sites in CuAlO2 as proposed earlier. The results of the calculations indicate that the Cd-doped system changes its electrical properties when Cd replaces Cu atoms (producing an n-type semiconductor), but not when it substitutes for Al atoms.

Local investigation of magnetism at R and In sites in RNiIn "R=Gd, Tb, Dy, Ho… compounds

The local magnetism at R and In sites in RNiIn (R=Gd, Tb, Dy, Ho) compounds was investigated by measuring hyperfine interactions via perturbed gamma-gamma angular correlation (PAC) technique using La140–Ce140 and In111–Cd111 probe nuclei. PAC measurements carried out below respective TC for each compound showed a combined electric quadrupole plus magnetic dipole interaction for Cd111 probe at In sites and a pure magnetic interaction for Ce140 at R sites. The temperature dependence of the magnetic hyperfine field (Bhf) shows that, whereas the Bhf values at rare-earth sites measured with Ce140 drop to zero at temperatures around the expected TC for each compound, the respective fields at In sites measured with Cd111 drop to zero at lower temperatures. The difference between the temperatures at which Bhf is zero for Ce140 and Cd111 probes when compared with TC of each compound shows that this difference decreases with decreasing TC values. The results are discussed in terms of the Ruderman-Kittel-Kasuya-Yosi...

Temperature dependence of electric field gradient in LaCoO3 perovskite investigated by perturbed angular correlation spectroscopy

The time differential perturbed angular correlation (TDPAC) technique was used to study the temperature dependence of electric field gradient (EFG) in LaCoO3 perovskite using and nuclear probes. The radioactive parent nuclei 111In and 181Hf were introduced into the oxide lattice through a chemical process during sample preparation and were found to occupy only the Co sites in LaCoO3. The PAC measurements with 111Cd and 181Ta probes were made in the temperature range of 4.2–1146 K and 4.2–1004 K, respectively. No long-range magnetic order was observed up to 4.2 K. The EFGs at 111Cd and 181Ta show very similar temperature dependences. They increase slowly between 4.2 and about 77 K and then decrease almost linearly with increasing temperature until about 500–600 K, where a broad peak-like structure is observed, followed by linear decrease at still higher temperatures. These discontinuities at about 77 K and 500–600 K have been interpreted as thermally activated spin state transitions from the low-spin (t2g6eg0) ground state configuration to the intermediate-spin (t2g5eg1) state and from the intermediate-spin to the high-spin (t2g4eg2) state of the Co3+ ion, confirming previous observation in other recent studies. An indication of a Jahn–Teller distortion, which stabilizes the intermediate-spin state with orbital ordering, is also pointed out.

Structural, magnetic and hyperfine properties of Zr(Cr1−xFex)2 hydrides

Abstract Laves-phase compounds with the hexagonal C14-type structure Zr(Cr 1− x Fe x ) 2 ( x =0.3–0.7) were charged to maximum H capacity, which ranged from 3.6 to 2.5 H/f.u. for increasing x . The biggest cell volume expansion was 22%. Magnetic properties were investigated with magnetometry and Mossbauer spectroscopy. The hydrides exhibited spin-glass behavior, similar to their parent compounds, but with larger Fe moments and lower freezing temperatures. Electric hyperfine interactions were measured in the 78–350 K temperature range at the Fe sites and at the Zr site, using 57 Fe MS and 181 Ta TDPAC, respectively. Activation energies for H jump diffusion were determined by these two complementary techniques, yielding 72(4) and 246(30) meV respectively.

Investigation of Hyperfine Interactions in CeIn3 byTDPAC

Magnetic hyperfine fields (mhf) at 111Cd and 140Ce nuclei, dilutely substituting the In and Ce sites, respectively, have been measured in the intermetallic compound CeIn3 using perturbed angular correlation technique. A pure electric quadrupole interaction with an axially symmetric electric field gradient was observed at 111In(EC) 111Cd probe nuclei at room temperature while a combined magnetic dipole and electric quadrupole interaction is observed below 10K. Below the ordering temperature, only a magnetic interaction is observed at 140La(β−) 140Ce probe. The values of mhf measured experimentally as a function of temperature are discussed in terms of critical behavior.

Study of the effect of disorder on the local magnetism in Heusler alloys

A local study of the influence of annealing temperature that can provoke structural defects and chemical disorder on the magnetism of Heusler alloys has been carried out by measuring hyperfine interactions in Pd2MnZ (Z=Ge, In, Sn, and Sb). Perturbed angular correlation method has been used to measure the hyperfine fields at Cd111 probe nuclei substituting the Mn and Z sites as a function of temperature. The fraction of In111-nuclei-occupying Mn sites was found to increase with the annealing of sample at higher temperatures followed by quenching that can leave the alloy in a disordered state. As a consequence, it was possible to observe the effect of temperature in the ordering of the alloys. Moreover, electric quadrupole interaction measurements clearly indicated chemical disorder after quenching in Pd2MnSn and an improvement in the ordering of the Pd2MnGe structure at high temperatures.

Study of hyperfine interactions in GdIn3

In the present work, an experimental and theoretical study of the hyperfine interactions at Gd and In sites in GdIn3 were performed. The experimental measurements were carried out by perturbed angular correlation spectroscopy using 140Ce and 111Cd nuclear probes substituting Gd and In sites, respectively. Results for 111Cd probe at In sites in GdIn3 revealed only electric quadrupole interactions, differently from the results for CeIn3 where, in addition to quadrupole interactions, a magnetic hyperfine field (Bhf) was also observed at In sites. The temperature dependence of Bhf at 140Ce on Gd sites in GdIn3 could be fitted by a Brillouin curve, and the extrapolated Bhf value to 0 K was found to be much smaller than that at 140Ce in CeIn3. Ab-initio electronic structure calculations for GdIn3 matrix doped with Ce were confronted with experimental data in order to explain such differences. The calculations were carried out within density functional theory using Augmented Plane Waves plus local orbitals basis...

The low-temperature magnetism of cerium atoms in CeMn2Si2 and CeMn2Ge2 compounds

The low-temperature magnetic properties of the Ce atoms in the intermetallic compounds CeMn2Ge2 and CeMn2Si2 were studied. Previous neutron scattering measurements did not detect an ordered moment at Ce atoms in either compound despite the fact that they are surrounded by the Mn moments ordered ferromagnetically in the CeMn2Ge2 and antiferromagnetically in the CeMn2Si2 .C ontrasting with this result, a recen tm easurement performed with the tim ed ifferential perturbed angular correlation (TDPAC) technique showed th ep resence of a pronounced magnetic hyperfine field (MHF) at Ce sites in the CeMn2Ge2 compound and no MHF in CeMn2Si2 .T he absence of the Ce magnetic moment and MHF in the silicide can be understood in terms of too weak a Ce–Ce magnetic interaction while in the germanide the TDPAC result suggests that some magnetic ordering of Ce atoms may occur. Aiming to understand the effects which result in the quenching of the Ce 4f moment in both cases, we performed first-principle sb and-structure calculations for both systems, using the full potential linear augmented plane wave method. It is show nt hat the magnetism of the Ce sublattice has fundamentally different nature in CeMn2Si2 and CeMn2Ge2 .W hile the Ce atoms are intrinsically nonmagnetic in the silicide, having a zero magnetic moment with both spin and orbital contributions identically zero, they display magnetic properties in the CeMn2Ge2 since their very small total moment is composed of finite spin and orbital components which almost cancel each other accidentally.

Impurities in Magnetic Materials Studied by PAC Spectroscopy

Perturbed gamma-gamma angular correlation (PAC) spectroscopy is a precise and highly efficient tool to follow the temperature dependence of local magnetic fields in any material. Its resolution and efficiency does not depend on temperature and therefore can measure local fields at low as well as high temperature with the same accuracy. Due its versatility in using different probe nuclei it can sense the local magnetic fields at different sites in the crystalline structure of materials. In this review, important results obtained with PAC spectroscopy are shown in two classes of materials: transition metal and transition-metal based compounds and rare earth elements and rare-earth-element based compounds using mainly three different probe nuclei: 111Cd, 181Ta and 140Ce. PAC spectroscopy has contributed to the systematic study of the magnetic hyperfine field in impurities in matrices of Fe, Co and Ni as well as in transition-metal based Heusler alloys. It has also provided important contribution to the investigation of magnetism in rare-earth elements and intermetallic compounds. An still open issue concerning the local fields in metallic magnetic compounds and elements is the exchange interaction between the magnetic ions of the host and a dilute magnetic impurity, which acts as a defect in the magnetic lattice. PAC spectroscopy has been contributing to study this problem with success. Also shown in this review is the crucial role of ab-initio first principle calculations in the interpretation of PAC results.