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Aczel, Adam A

Publications and source records attributed to Aczel, Adam A.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Magnetic and Structural Phase Transitions in the Spinel Compound Fe1+xCr2 xO4

Neutron diffraction, X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magnetic and structural phase transitions of the spinel system Fe1+xCr2 xO4 (0.0 x 1.0). The temperature versus Fe concentration (x) phase diagram contains two magnetically ordered phases and four structural phases below 420 K. The complicated transitions are closely related to the change in the spin and orbital degrees of freedom induced by substitution of Fe ions for Cr ions. The systematic change in the crystal structure is explained by the combined effects of Jahn-Teller distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among Fe2+, Fe3+, and Cr3+ ions.

Ma, Jie↗
DOE OSTI2014

Using Monte Carlo ray tracing simulations to model the quantum harmonic oscillator modes observed in uranium nitride

Recently an extended series of equally spaced vibrational modes was observed in uranium nitride (UN) by performing neutron spectroscopy measurements using the ARCS and SEQUOIA time-of- flight chopper spectrometers [A.A. Aczel et al, Nature Communications 3, 1124 (2012)]. These modes are well described by 3D isotropic quantum harmonic oscillator (QHO) behavior of the nitrogen atoms, but there are additional contributions to the scattering that complicate the measured response. In an effort to better characterize the observed neutron scattering spectrum of UN, we have performed Monte Carlo ray tracing simulations of the ARCS and SEQUOIA experiments with various sample kernels, accounting for the nitrogen QHO scattering, contributions that arise from the acoustic portion of the partial phonon density of states (PDOS), and multiple scattering. These simulations demonstrate that the U and N motions can be treated independently, and show that multiple scattering contributes an approximate Q-independent background to the spectrum at the oscillator mode positions. Temperature dependent studies of the lowest few oscillator modes have also been made with SEQUOIA, and our simulations indicate that the T-dependence of the scattering from these modes is strongly influenced by the uranium lattice.

Lin, J. Y. Y.↗
DOE OSTI2014

Magnons and a two-component spin gap in FeV2O4

The spinel vanadates have become a model family for exploring orbital order on the frustrated pyrochlore lattice, and recent debate has focused on the symmetry of local crystal fields at the cation sites. Here, we present neutron scattering measurements of the magnetic excitation spectrum in FeV2O4, a recent example of a ferrimagnetic spinel vanadate which is available in single-crystal form.We report the existence of two emergent magnon modes at low temperatures, which draw strong parallels with the closely related material, MnV2O4. We were able to reproduce the essential elements of both the magnetic ordering pattern and the dispersion of the inelastic modes with semiclassical spin-wave calculations, using a minimal model that implies a sizable single-ion anisotropy on the vanadium sublattice. Taking into account the direction of ordered spins, we associate this anisotropy with the large trigonal distortion of VO6 octahedra, previously observed via neutron powder diffractionmeasurements. We further report on the spin gap, which is an order of magnitude larger than that observed in MnV2O4. By looking at the overall temperature dependence, we were able to show that the gap magnitude is largely associated with the ferro-orbital order known to exist on the iron sublattice, but the contribution to the gap from the vanadium sublattice is in fact comparable to what is reported in the Mn compound. This reinforces the conclusion that the spin canting transition is associated with the ordering of vanadium orbitals in this system, and closer analysis indicates closely related physics underlying orbital transitions in FeV2O4 and MnV2O4.

MacDougall, Prof. Gregory J.↗
DOE OSTI2013

Coupled Nd and B' spin ordering in the double perovskites Nd2NaB'O6 (B' = Ru, Os)

We present a neutron powder diffraction study of the monoclinic double perovskite systems Nd$_2$NaB$'$O$_6$ (B$'$~$=$~Ru, Os), with magnetic atoms occupying both the A and B$'$ sites. Our measurements reveal coupled spin ordering between the Nd and B$'$ atoms with magnetic transition temperatures of 14~K for Nd$_2$NaRuO$_6$ and 16~K for Nd$_2$NaOsO$_6$. There is a Type I antiferromagnetic structure associated with the Ru and Os sublattices, with the ferromagnetic planes stacked along the c-axis and [110] direction respectively, while the Nd sublattices exhibit complex, canted antiferromagnetism with different spin arrangements in each system.

double perovskites↗