A. V. J. Edge

The magnetic structures of YbMn2Si2

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 Neel 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.

169Tm Mossbauer investigation of thulium pyrogermanate

A 169Tm Mossbauer investigation of thulium pyrogermanate is reported. All spectra recorded over the temperature range 4.2 K-740 K are pure quadrupole doublets. This observation and the detailed temperature dependence of the quadrupole interaction are inconsistent with recent claims of a fivefold local symmetry at the Tm3+ site. Initial attempts at analysing the data in terms of the full crystal-field Hamiltonian appropriate for C1 symmetry suggest that the electronic ground state consists of a pair of non-magnetic singlets well isolated from the next state.

57Fe Mössbauer and magnetic studies of ErFe12−xNbx

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 57Fe M?ssbauer 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 dFe?Fe(8i)> dFe?Fe(8j)> dFe?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 Tsr in ErFe12?xNbx compounds remain essentially unchanged (Tsr~42?44?K) with increasing Nb concentration, whereas a significant decrease in Tsr?(Tsr 1~236?204?K; Tsr 2~154?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 ?J. 57Fe hyperfine interaction parameters and magnetic moments values have been determined for the 8i, 8j and 8f sites from the M?ssbauer spectra. The weighted average 57Fe hyperfine field values were found to follow a T2 dependence; this suggests that a single-particle excitation mechanism is responsible for reduction of the 3d-sublattice magnetization with increasing temperature.

Valence and magnetic ordering in Y bMn2Si2−xGex

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.

Evidence for atomic scale disorder in indium nitride from perturbed angular correlation spectroscopy

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.

NMRON studies of ferromagnetic hcp and fcc Co

The high resolution hyperfine spectroscopy, modulated adiabatic passage of oriented nuclei (MAPON), has been applied for the first time to high purity, elemental systems. Detailed comparisons between the electric quadrupole hyperfine interactions (EQI’s) and, in particular, their distributions, are obtained for60CoCo where the hosts are a single crystal of hcp cobalt and a polycrystalline cobalt foil of predominantly fcc character. For hcp Co, with the electronic magnetization, M, parallel to the c-axis, the mode value P/h=3e2qQ/4I(2I−1)h=−48.5(5) kHz. This fractional distribution implies the sharpest electric field gradient (efg) measured in a metal to date, using MAPON spectroscopy, in excess of two times sharper than that of the most dilute impurity efg in a crystallographically cubic ferromagnetic host. The mode efg is Vzz=−27.3(32)×1019 Vm−2. For the polycrystalline, predominantly fcc foil, prepared by quenching, the EQI mode value is P/h=−6.2(4) kHz with a FWHM of 12.0(7) kHz yielding a mode efg of Vzz=−3.5(5)×1019 Vm−2.

Low temperature annealing of ball-milled Fe66B34

High energy ball-milling of Fe and B powders of starting composition Fe66B34 resulted in an alloy which revealed both a nanostructural state and a disordered amorphous-like state and a majority of unreacted components. The disordered phase is located in the interfacial regions between the Fe and B atoms. The ball-milled sample was annealed at low temperatures (250–350°C) to examine further the proposition that solid-state reaction is the mechanism of amorphization by mechanical alloying. A slight reaction between the Fe and B grains which were previously unreacted, probably limited to their contacting boundaries, has been detected.

MAPON EFG Result for 54Mn in Cubic Cobalt

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 Vzz=+1.3(2)×1019 V m−2. This result is consistent with established trends for Mn and Co probes in the 3d ferromagnetic hosts.