P. de la Presa

Investigation of the magnetic hyperfine field of 111Cd in the rare-earth Laves phases RCo2 and RNi2

The magnetic hyperfine field Bh f of the closed-shell probe nucleus 111 Cd on the cubic R site of the C15 Laves phase RCo2 (R = Gd, Tb, Dy, Ho, Er, Sm, Nd and Pr) and RNi2 (R = Gd, Sm) has been investigated at 9 K by perturbed angular correlation (PAC) spectroscopy. In RCo2 the hyperfine field increases with increasing R spin, both for the heavy and the light R. The experimental trend suggests that the total hyperfine field has two contributions of opposite sign: Bh f = B 3d - B 4 f , where B 3d is induced by the 3d Co moment and B 4 f is due to indirect 4f - 4f exchange. The 4f contribution can be estimated from the measurement of Bh f in RNi2 where the 3d moment vanishes. The 111 Cd hyperfine fields in GdNi2 and SmNi2 at 9 K are Bh f = 7.5(5) and 2.0(2) T, respectively. Assuming a linear dependence of B 4 f on the projection ( g - 1)J of the R spin on the total angular momentum J , as expected from the Ruderman-Kittel-Kasuya-Yosida (RKKY) theory of indirect 4f - 4f exchange, the variation of B 3d across the R series can be determined from the experimental values of Bhf . In heavy RCo2 (R = Gd, ... ,Er) B 3d follows very closely the variation of the Co moment ?Co across the R series with B 3d /?Co = 28.7(2) T ?-1 B . With the same ratio for the light RCo2 (R = Pr, Nd, Sm) the experimental Bh f values indicate an increase of the 3d moment from ?Co = 0.45 to 0.7 ?B between PrCo2 and SmCo2 .

Structure and grain growth of TiO2 nanoparticles investigated by electron and x-ray diffractions and Ta181 perturbed angular correlations

Bare and coated TiO₂ nanoparticles with particle sizes d 100 nm after 1300 K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO₂. The data suggest a critical grain size of d ~10 nm for the onset of the ordering process. The spectra of coarse-grained TiO₂ are reached at a particle size d >= 30 nm. In n-TiO₂ coated with Al₂O₃ and ZrO₂ both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO₂ cores and in the ZrO₂ coating of n-TiO₂/ZrO₂ at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO₂ kernels and the coatings.

Perturbed angular correlation study of magnetic and electric hyperfine interactions at 111Cd in GdAl2

The magnetic hyperfine field Bhf of the closed-shell probe nucleus 111Cd in the C15 Laves phase RNi2 (R= Gd, Sm) has been investigated as a function of temperature by perturbed angular correlation (PAC) spectroscopy. The saturation magnetic hyperfine fields at 10 K are Bhf=7.7(2) and 3.9(1) T for GdNi2 and SmNi2, respectively. Although the probe nucleus resides on the cubic rare-earth site, a strong axially symmetric electric quadrupole interaction (QI) is observed in the paramagnetic phase at T≤300 K. The possible relation of this unexpected QI to the structural instability of RNi2 is discussed.

The 3d magnetization at first-order transitions of the rare earth Laves phases R1−xYxCo2 studied by measurements of magnetic hyperfine fields

The magnetic phase transitions of pseudo-binary rare earth (R) Laves phases R1−xYxCo2 have been investigated by perturbed angular correlation (PAC) measurements of the magnetic hyperfine fields at the probe nucleus 111Cd for R = Tb, Sm and Ho at various Y concentrations x and for R = Gd, Dy, Er, Nd and Pr at the concentration x = 0.3. First-order transitions were observed in Tb1−xYxCo2 and Sm1−xYxCo2 for x≥0.3, in Ho1−xYxCo2 for x≤0.4 and in R0.7Y0.3Co2 for R = Dy, Ho, Er, Nd and Pr. For Gd0.7Y0.3Co2, the temperature dependence of the average magnetic hyperfine field is compatible with a second-order transition. The discontinuity of the magnetic hyperfine interaction at the first-order transitions of heavy R1−xYxCo2, which mainly reflects the jump of 3d magnetization of the Co subsystem at TC, was found to increase monotonically with decreasing order temperature. The TC dependence of the normalized magnetic frequency with T0 = 203(5) K for the boundary temperature between first- and second-order transitions can be explained by the temperature dependence of the coefficient of the M4 term of the free energy in the Wohlfarth–Rhodes–Shimizu theory of itinerant electron metamagnetism.