The magnetic hyperfine fields of Cd guest atoms in nickel have been measured by perturbed angular correlation spectroscopy with an unprecedented precision. The experiments exhibit the well-known oscillations which have been attributed to the substitutional and the cubic C-site of Cd in nickel, but also show the existence of previously unknown sites through the observation of combined interactions between magnetic hyperfine fields and electric field gradients. The contributions of a large number of possible sites in connection with the properties of the combined interactions are proposed to be responsible for the invisibility of sites as well as for the large amplitudes of some measured frequencies. Following the systematic of magnetic fields of vacancy-associated 5sp-elements in nickel, the measured fields are consistent with theoretical expectations which use the number of vacancies as the primary parameter.

(170)Yb and (57)Fe Mossbauer spectra are reported for hexagonal phase YbMnO(3) and Yb((57)Fe(x)Mn(1-x))O(3) (x = 0.005, 0.01), respectively. The dilute concentrations of (57)Fe are demonstrated to provide a reliable, non-perturbing Mossbauer spectroscopy probe of the Mn sub-lattice magnetization. Substitution of up to 1 at.% (57)Fe exerts negligible influence on the Neel temperature (T(N1) approximate to 88-89 K) and point charge model estimates of the electric field gradient agree well with experimental (57)Fe Mossbauer results in terms of both sign and magnitude. The (170)Yb Mossbauer spectrum recorded at 4.5 K provides support for strong crystal field quenching with isolated Kramers doublet ground states at both Yb sites. A 'static', five-line sub-spectrum is tentatively assigned to the 4b site, for which the slowed fluctuation of the Kramers doublet is attributed to non-zero magnetic exchange with the antiferromagnetic Mn sub-lattice. This is not the case for the 2a site whose sub-spectrum is a motionally narrowed single line. On the basis of this work, the saturation magnetization for the ordered Yb sub-lattice (T(N2) <= 3 K) is estimated at approximate to 1.8 mu(B) per formula unit, in close agreement with single-crystal magnetization data reported elsewhere.

Refinements of the X-ray diffraction patterns show that DyFe12−x Ta x compounds with x=0.5−0.7 crystallise in the ThMn12-type structure and that the Ta atoms occupy the 8i sites. Spin reorientations have been detected by ac magnetic susceptibility for all compounds below room temperature. First the moments shift direction from easy axis to easy cone at T srl, then to easy plane at T sr2. Both T srl and T sr2 increase with increasing Ta content up to x=0.65 before decreasing with further increase in Ta content. Analyses of the Mössbauer spectra indicate that the individual Fe site hyperfine fields derived at 4.5 K for DyFell.35TaO.65 are B hf=37.4 T, 32.2 T and 27.6 T for the 8i, 8j and 8f sites, respectively.

Refinements of the X-ray diffraction patterns show that DyFe12-x Ta (x) compounds with x = 0.5-0.7 crystallise in the ThMn12-type structure and that the Ta atoms occupy the 8i sites. Spin reorientations have been detected by ac magnetic susceptibility for all compounds below room temperature. First the moments shift direction from easy axis to easy cone at T (sr1), then to easy plane at T (sr2). Both T (sr1) and T (sr2) increase with increasing Ta content up to x = 0.65 before decreasing with further increase in Ta content. Analyses of the Mossbauer spectra indicate that the individual Fe site hyperfine fields derived at 4.5 K for DyFe11.35Ta0.65 are B (hf) = 37.4 T, 32.2 T and 27.6 T for the 8i, 8j and 8f sites, respectively.

DC magnetisation, electrical resistivity, specific beat and Gd-155 Mossbauer spectroscopy measurements demonstrate that the Gd sub-lattice of GdNiAl4 undergoes two magnetic transitions (T-N = 24.7 K, T-N = 20.8 K) as the temperature is decreased. Given that Gd3+ is an S-state ion, a crystal field mechanism can be ruled out as the origin of similar behaviour observed elsewhere for RNiAl4 with R = Pr1-xNdx (0 <= x <= 0.7) and Tb. Based on a comparison of point charge model calculations of the electric field gradient tensor with those measured at the Gd-155 nucleus, it is deduced that the magnetisation is aligned with either the b- or c-crystallographic axis of the orthorhombic structure. (c) 2004 Elsevier B.V. All rights reserved.

The lattice structure of both InN grains with submicron dimensions and a MBE-grown InN film have been studied with the radioisotope probe 111In/Cd and perturbed angular correlation spectroscopy. The quadrupole interaction frequency of the probe in this material has been measured to be of the order of 28 MHz, consistent with results for AlN and GaN. Strong damping of the signal indicates that InN has a highly defective lattice with diverse defect structures, which are not cured by annealing in the accessible temperature range

The crystal lattice of bulk grains and state-of-the-art films of indium nitride was investigated at the atomic scale with perturbed angular correlation spectroscopy using the 111In/Cd radioisotope probe. The probe was introduced during sample synthesis, by diffusion and by ion implantation. The mean quadrupole interaction frequency νQ = 28 MHz was observed at the indium probe site in all types of indium nitride samples with broad frequency distributions. The observed small, but non-zero, asymmetry parameter indicates broken symmetry around the probe atoms. Results have been compared with theoretical calculations based on the point charge model. The consistency of the experimental results and their independence of the preparation technique suggest that the origin of the broad frequency distribution is inherent to indium nitride, indicating a high degree of disorder at the atomic scale. Due to the low dissociation temperature of indium nitride, furnace and rapid thermal annealing at atmospheric pressure reduce the lattice disorder only marginally.

The structural and magnetic properties of ErFe12-xNbx compounds (x = 0.6, 0.7 and 0.8) have been investigated by x-ray diffraction, ac susceptibility and dc magnetization measurements and Fe-57 Mossbauer spectroscopy. Refinements of the x-ray diffraction patterns show that the Nb atoms preferentially occupy the 8i sites; this can be understood in the terms of enthalpy effects and differences in the metallic radii. The average Fe-Fe distance at the different sites is found to behave as d(Fe-Fe)(8i) > d(Fe-Fe)(8j) > d(Fe-Fe)(8f). The unit cell volume increases slightly with increasing Nb content, consistent with the larger radius of Nb compared with Fe. A spin reorientation from easy-axis at room temperature to easy-cone at low temperatures has been detected for all compounds. The spin reorientation temperatures T-sr in ErFe12-xNbx compounds remain essentially unchanged (T-sr similar to 42-44 K) with increasing Nb concentration, whereas a significant decrease in T-sr (T-sr1 similar to 236-204 K; T-sr2 similar to 154-94 K) is obtained in DyFe12-xNbx from x = 0.6 to 0.8. This can be understood by taking the different crystal-field terms responsible for the spin reorientation in the two systems into account. We find that the spin-reorientation process is particularly sensitive to the sixth-order term B60O60 of the crystal field acting on the Er3+ ion, due to its large and positive value of gamma(J). Fe-57 hyperfine interaction parameters and magnetic moments values have been determined for the 8i, 8j and 8f sites from the Mossbauer spectra. The weighted average Fe-57 hyperfine field values were found to follow a T-2 dependence; this suggests that a single-particle excitation mechanism is responsible for reduction of the 3d-sublattice magnetization with increasing temperature.

170Yb Mössbauer spectroscopy has been used to study YbMn2Si2−x Gex for 0 x 2. YbMn2Si2 contains only trivalent Yb. A Yb2+ component appears at x = 1.15(5) and the Yb in YbMn2Ge2 is fully divalent. Transferred hyperfine fields at the Yb site for x < 1.2 show that a canting of the Mn moments occurs between 35 and 65 K, while a second event is seen at ∼6 K for 0.4 x 1.0 which probably reflects a further reorganization of the Mn order rather than ordering of the Yb3+ moments.

The magnetic structures of (EuMn2Ge2)-Eu-153 and (EuMn2Si2)-Eu-153 have been determined by neutron diffraction measurements (similar to1.8-723 K). The Mn sublattices of both EuMn2Ge2 and EuMn2Si2 order antiferromagnetically [T-N=667(9) K; space group I4'/m'm'm and T-N=391(5) K; space group I(p)4/m'm'm', respectively] with no evidence for ordering of the Eu+2 ions in EuMn2Ge2 down to 1.5 K. EuMn2Si2 exhibits a thermally driven valence transition around T(v)similar to527 K with a pronounced increase in the unit cell volume of similar to7% from the Eu3+ state at low temperatures (Tsimilar to600 K). The valence-induced volume changes have been analyzed for the first time in terms of the interconfigurational fluctuation model with occupation probabilities for the Eu2+ state in EuMn2Si2 and the average Eu-val values found to agree well with our reanalysis of earlier (EuMn2Si2)-Eu-151 isomer shift data. The strength of the magnetic interaction for antiferromagnetic ordering of the Mn sublattice is decreased by similar to10%-15% as a result of the change in the electronic configuration of EuMn2Si2.

Spin reorientation effects in DyFe12-xNbx compounds with x=0.6, 0.7 and 0.8 have been investigated by ac magnetic susceptibility and Fe-57 Mossbauer effect measurements over the temperature range 4.2-300 K. Refinements of the X-ray diffraction patterns show that DyFe12-xNbx compounds crystallize in the ThMn12- type structure and that the Nb atoms occupy the 8i sites. A spin reorientation from easy axis to easy cone at T-sr1 then to easy plane at T-sr2 with decreasing temperature has been detected by ac susceptibility. Both T-sr1 and T-sr2 decrease monotonically with increasing Nb content (from T-sr1 similar to 236 K for x=0.6 to T-sr1 similar to 204 K for x=0.8 and from T-sr2 similar to 154 K for x=0.6 to T-sr2 similar to 94 K for x=0.8). Analyses of the Mossbauer spectra (4.2-300 K) reveal a discontinuity in the temperature dependences of the hyperfine parameters at Tsr(2) for the 8i Fe. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Yb M¨ ossbauer spectroscopy has been used to study YbMn2Si2−x Gex for 0 x 2. YbMn2Si2 contains only trivalent Yb. A Yb 2+ component appears atx = 1.15(5) and the Yb in YbMn2Ge2 is fully divalent. Transferred hyperfine fields at the Yb site for x < 1. 2s how that a canting of the Mn moments occurs between 35 and 65 K, while a second event is seen at ∼ 6K for 0.4 x 1.0 which probably reflects a further reorganization of the Mn order rather than ordering of the Yb 3+ moments.

Neutron diffraction and differential scanning calorimetry measurements (DSC) on YbMn2Si2−xGex provide clear evidence for a critical region around xc∼1.6. Below xc∼1.6 the compounds exhibit an axial antiferromagnetic structure while at higher Ge concentration the structure transforms to planar antiferromagnetism for T>∼200 K with canted antiferromagnetism prevailing at lower temperatures. The structural changes are consistent with a valence change between the trivalent behaviour of YbMn2Si2 and the divalent-like behaviour of YbMn2Ge2.

Modulated adiabatic passage on oriented nuclei (MAPON) spectroscopy has been used to measure the electric quadrupole interaction at dilute 54Mn impurity probes in crystallographically cubic (fcc) cobalt. The measured value is P/h=+5.2(5) kHz and using Q=+0.33(3) b, leads to an electric field gradient of V zz =+1.3(2)×1019 V m−2. This result is consistent with established trends for Mn and Co probes in the 3d ferromagnetic hosts.

The magnetic structures of YbMn2Si2 with the tetragonal ThCr2Si2-type structure have been determined by neutron diffraction measurements over the temperature range ∼1.5–538 K. Rietveld refinements demonstrate that YbMn2Si2 has a collinear antiferromagnetic structure below the Néel temperature TN1 = 526(4) K with the Mn moments parallel to the c-axis. Below TN2 ∼ 30(5) K, the Mn sublattice rearranges to a +−−+ antiferromagnetic structure with propagation vector k = 00½. The moment direction is along the c-axis with a total moment of 1.92(8) μB at 10 K. There is no indication of ordering of the Yb ions at 10 K, although a diffraction pattern at 1.5 K shows that the Yb ions are ordered at the latter temperature. Analysis reveals that the Yb sublattice orders antiferromagnetically. The antiferromagnetically ordered Yb sublattice exhibits the same propagation vector, k = 00½, as the Mn sublattice, although the Yb spin directions are found to be perpendicular to the c-axis compared with the parallel alignment of the Mn moments. The refined value of the Yb3+ magnetic moment at 1.5 K is μYb = 0.57(9) μB compared with the free ion value of about 4.5 μB, while at 1.5 K the Mn magnetic moment is μMn = 1.98(7) μB.

A Fe-57 Mossbauer study has been conducted on the Fe-57 doped novel rare-earth intermetallic compounds R3T29Si4B10 (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu, T = Ni, Co). The Fe-57 Site assignment has been investigated by Fe-57 Mossbauer spectroscopy combined with thermodynamic analysis and crystal field estimation. The investigation demonstrated that Fe-57 atoms preferentially occupy the 2c crystallographic site with (4) over bar m2 local symmetry in the R3T29Si4B10 compounds. Sequentially, the 8j1, 8j2 and 8i2 crystallographic sites are the second preferentially occupied and 8i3 and 16 K sites are the third preferential occupancy group of iron atoms. The magnetic hyperfine interaction at 4.2 K demonstrates the effect of a spin-induced Co magnetic moment.