Magnetic structures in the rich magnetic phase diagram of Ho2_2RhIn8_8

The magnetic phase diagram of the tetragonal Ho$_2$RhIn$_8$ compound has similar features to many related systems, revealing a zero magnetic field AF1 and a field-induced AF2 phases. Details of the magnetic order in the AF2 phase were not reported yet for any of the related compounds. In addition, only the Ho$_2$RhIn$_8$ phase diagram contains a small region of the incommensurate zero-field AF3 phase. We have performed a number of neutron diffraction experiments on single crystals of Ho$_2$RhIn$_8$ using several diffractometers including experiments in both horizontal and vertical magnetic fields up to 4 T. We present details of the magnetic structures in all magnetic phases of the rich phase diagram of Ho$_2$RhIn$_8$. The Ho magnetic moments point along the tetragonal $c$ axis in every phase. The ground-state AF1 phase is characterized by propagation vector $\textbf{k}$ = (1/2, 0, 0). The more complex ferrimagnetic AF2 phase is described by four propagation vectors $\textbf{k}_{0}$ = (0, 0, 0), $\textbf{k}_{1}$ = (1/2, 0, 0), $\textbf{k}_{2}$ = (0, 1/2, 1/2), $\textbf{k}_{3}$ = (1/2, 1/2, 1/2). The magnetic structure in the AF3 phase is incommensurate with $\textbf{k}_{AF3}$ = (0.5, $\delta$, 0). Our results are consistent with theoretical calculations based on crystal field theory.Comment: submitted to PR

Magnetic structures of non-cerium analogues of heavy-fermion Ce2RhIn8: case of Nd2RhIn8, Dy2RhIn8 and Er2RhIn8

R2RhIn8 compounds (space group P4/mmm, R is a rare-earth element) belong to a large group of structurally related tetragonal materials which involves several heavy-fermion superconductors based on Ce. We have succeeded to grow single crystals of compounds with Nd, Dy and Er and following our previous bulk measurements, we performed neutron-diffraction studies to determine their magnetic structures. The Laue diffraction experiment showed that the antiferromagnetic order below the N\'eel temperature is in all three compounds characterized by the propagation vector k = (1/2, 1/2, 1/2). The amplitude and direction of the magnetic moments, as well as the invariance symmetry of the magnetic structure, were determined by subsequent experiments using two- and four-circle diffractometers. The critical exponents were determined from the temperature dependence of the intensities below TN.Comment: 10 pages, 7 figures, submitted to PR

Direct observation of phase coherence in 3-<b>k</b> magnetic configurations

International audienceWe report the observation by neutron diffraction of phase coherent Bragg reflections in a multi-k magnetic configuration with a spatial periodicity outside the conventional scattering cross-section. The peaks, which exist in the 3-k state of UAs0.8Se0.2, display long-range order with a wavevector dependence characteristic of a magnetic interaction. The results confirm the long-range order and temperature dependence reported in an earlier study of similar peaks in this material using x-ray resonant scattering by (a) the non-trivial extension to the technique of neutron diffraction, and (b) the observation of similar 3-k phase-coherent reflections in other samples by x-ray resonant scattering. The importance of the neutron diffraction results lies primarily in the fact that magnetic neutron diffraction is well established as a weak probe operating on thermodynamic time scales. This alleviates concern that the rapid (10-15 - 10-14 s), strong interaction, characteristic of the resonant x-ray technique, is imaging a transient or non-equilibrium configuration. Likewise, the extension of the x-ray resonant scattering results to other samples establishes the generality of this effect. The enigma of how to understand the observed diffraction, which appears to lie strictly outside both the conventional neutron and x-ray scattering cross sections, remains

Ab initio calculations of the crystal field and phonon dispersions in CePd2Al2 and LaPd2Al2

CePd2Al2 crystallizes in the CaBe2Ge2-type tetragonal structure (P4/nmm, 129) and undergoes a phase transition to the orthorhombic Cmme structure at around 13 K. Its inelastic neutron spectra reveal an additional magnetic excitation that was ascribed to electron-phonon interaction leading to a formation of a new quantum quasi-bound vibron state. We present the first-principles calculations of the crystal field excitations and lattice dynamics calculations of the phonon dispersions to compare with the experimental data. The calculated crystal field energy splitting in CePd2Al2 agrees well with the model used to describe the experimental neutron scattering spectra. The first excited crystal field level moves to higher energies when undergoing the transformation from tetragonal to orthorhombic structure, in agreement with the experiment. The analysis based on calculated elastic constants and lattice dynamics calculations show that in both tetragonal and orthorhombic structures there are no imaginary modes for any q-wave vector within the Brillouin zone, and therefore the lattice structures are stable. The phonon dispersions and density of states are calculated for both crystal structures of CePd2Al2 and its nonmagnetic counterpart LaPd2Al2. The results generally agree well with the experimental data including the high phonon density of states around 12 meV. The phonon density of states is also used to calculate the mean squared displacement, Debye temperature, lattice heat capacity and compared with similar properties of the available experiment.Web of Science3223art. no. 23540

Lattice dynamics in CePd2Al2 and LaPd2Al2

The interaction between phonons and 4f electrons, which is forming a new quantum state (quasi-bound state) beyond Born-Oppenheimer approximation, is very prominent and lattice dynamics plays here a key role. There is only a small number of compounds in which the experimental observation suggest such a scenario. One of these compounds is CePd2Al2. Here the study of phonon dispersion curves of (Ce,La)Pd2Al2 at 1.5, 7.5, 80 and 300 K is presented. The inelastic X-ray scattering technique was used for mapping the phonon modes at X and Z points as well as in ? and Delta directions, where the symmetry analysis of phonon modes was performed. The measured spectra are compared with the theoretical calculation, showing very good agreement. The measurements were performed in several Brillouin zones allowing the reconstruction of phonon dispersion curves. The results are discussed with respect to the magneto-elastic interaction and are compared with other cerium compounds. The phonon mode symmetry A(1g) was found to be unaffected by the interaction, which is in contrast to previous assumptions.Web of Science111art. no. 2087