We performed inelastic neutron scattering experiments on PrMgNi4 with the cubic MgSnCu4-type structure. The magnetic excitations were observed at 1.1, 2.5, 5.9, 10.7, and 11.7 meV. Since the energy of the excitations is constant for | Q | < 5 Å−1, they are ascribed to the crystalline electric field (CEF) excitations of the Pr3+ ions. We adopted CEF parameters of W = −3.3 K and x = 0.8 for the cubic T d point group to reproduce the strong excitations at 1.1, 10.7, and 11.7 meV. The calculated CEF level scheme reveals the Γ3 doublet ground state with the quadrupolar degrees of freedom and the excited states of Γ4 triplet (1.4 meV), Γ1 singlet (3.4 meV), and Γ5 triplet (13.5 meV). This scheme, however, does not explain the observed two excitations at 2.5 and 5.9 meV. These additional excitations may arise from splitting of the doublet by symmetry lowering due to excess Mg atoms occupying the Pr sites. This splitting is probably responsible for the absence of the long-range quadrupole order in PrMgNi4.

We report the synthesis and characterization of polycrystalline Na2RuO3, a layered material in which the Ru4+ (4d(4) configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na2RuO3 (C2/c phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism [chi(0) similar to 1.42(2) x 10(-3) emu/mol Oe] with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility (chi(0)) together with the Sommerfeld coefficient of gamma = 11.7(2) mJ/Ru mol K-2 estimated from heat capacity measurements gives an enhanced Wilson ratio of R-w approximate to 8.9(1), suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behavior, photoemission spectroscopy indicates a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near-surface and interface effects, all other probes indicate that Na2RuO3 is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na2RuO3 is an antiferromagnetic insulator at low temperatures.

L. S. I. Veiga ,1,2 M. Etter,3 E. Cappelli,4 H. Jacobsen ,5,6 J. G. Vale,1 C. D. Dashwood,1 D. Le ,7 F. Baumberger,4,8 D. F. McMorrow,1 and R. S. Perry9 1London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom 2Diamond Light Source Ltd., Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom 3Deutsches Elektronen-Synchrotron (DESY), Hamburg 22607, Germany 4Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland 5Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom 6Paul Scherrer Institute, Laboratory for Neutron Scattering and Imaging, 5232 Villigen, Switzerland 7ISIS Facility, Rutherford Appleton Laboratory, STFC, Chilton, Didcot OX11 0QX, United Kingdom 8Swiss Light Source, Paul Scherrer Institute, CH-5232 Villigen, Switzerland 9London Centre for Nanotechnology and Institute for Materials Discovery, University College London, Gower Street, London WC1E 6BT, United Kingdom

The antiferromagnetic mixed valence ternary oxide $\alpha$-CoV$_{3}$O$_{8}$ displays disorder on the Co$^{2+}$ site that is inherent to the $Ibam$ space group. The zero field structural and dynamic properties of $\alpha$-CoV$_{3}$O$_{8}$~have been investigated using a combination of neutron and x-ray diffraction, DC susceptibility, and neutron spectroscopy. The low temperature magnetic and structural properties are consistent with a random macroscopic distribution of Co$^{2+}$ over the 16$k$ metal sites. However, by applying the sum rules of neutron scattering we observe the collective magnetic excitations are parameterized with an ordered Co$^{2+}$ arrangement and critical scattering consistent with a three dimensional Ising universality class. The low energy spectrum is well-described by Co$^{2+}$ cations coupled $via$ a three dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the $ab$ plane and along $c$, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long range antiferromagnetic order at $T\rm{_{N}}\sim$ 19 K. A crystal field analysis finds two bands of excitations separated in energy at $\hbar \omega$ $\sim$ 5 meV and 25 meV, consistent with a $j\rm{_{eff}}=\frac{1}{2}$ ground state with little mixing between spin-orbit split Kramers doublets. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated $j\rm{_{eff}}$ manifolds may play a key role in making the dynamics largely insensitive to disorder.

Neutron spectroscopy was applied to study the magnetic interactions of orbitally degenerate Co2+ on a host MgO rocksalt lattice where no long-range spin or orbital order exists. The paramagnetic nature of the substituted monoxide Co0.03Mg0.97O allows for the disentanglement of spin exchange and spin-orbit interactions. By considering the prevalent excitations from Co2+ spin pairs, we extract seven exchange constants out to the fourth coordination shell. An antiferromagnetic next-nearest-neighbor 180 degrees exchange interaction is dominant; however, dual ferromagnetic and antiferromagnetic interactions are observed for pairings with other pathways. These interactions can be understood in terms of a combination of orbital degeneracy in the t(2g) channel and the Goodenough-Kanamori-Anderson rules. Our work suggest that such a hierarchy of exchange interactions exists in transition-metal-based oxides with a t(2g) orbital degeneracy.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Ordered magnetism in the intrinsically decorated j eff = α-CoV 3 O 8 P. Sarte, A. Arévalo-López, M. Songvilay, D. Le, T. Guidi, V. García-Sakai, S. Mukhopadhyay, S. Capelli, W. Ratcliff, K. Hong, et al.

The antiferromagnetic mixed valence ternary oxide alpha-CoV3O8 displays disorder on the Co(2+ )site that is inherent to the Ibam space group resulting in a local selection rule requiring that one Co2+ and one V4+ reside next to each other, thus giving rise to an intrinsically disordered magnet without the need for external influences such as chemical dopants or porous media. The zero-field structural and dynamic properties of alpha-CoV3O8 have been investigated using a combination of neutron and x-ray diffraction, dc susceptibility, and neutron spectroscopy. The low-temperature magnetic and structural properties are consistent with a random macroscopic distribution of Co2+ over the 16k metal sites. However, by applying the sum rules of neutron scattering we observe that the collective magnetic excitations are parametrized with an ordered Co2+ arrangement and critical scattering consistent with a three-dimensional Ising universality class. The low-energy spectrum is well described by Co2+ cations coupled via a three-dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the ab plane and along c, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long-range antiferromagnetic order at T-N similar to 19 K. A crystal field analysis finds two bands of excitations separated in energy at (h) over bar omega similar to 5 meV and 25 meV, consistent with a j(eff) = 1/2 aground state with little mixing between spin-orbit split levels. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated j(eff) manifolds, may play a key role in making the dynamics largely insensitive to disorder.

The antiferromagnetic mixed valence ternary oxide $alpha$-CoV$_{3}$O$_{8}$ displays disorder on the Co$^{2+}$ site that is inherent to the $Ibam$ space group. The zero field structural and dynamic properties of $alpha$-CoV$_{3}$O$_{8}$~have been investigated using a combination of neutron and x-ray diffraction, DC susceptibility, and neutron spectroscopy. The low temperature magnetic and structural properties are consistent with a random macroscopic distribution of Co$^{2+}$ over the 16$k$ metal sites. However, by applying the sum rules of neutron scattering we observe the collective magnetic excitations are parameterized with an ordered Co$^{2+}$ arrangement and critical scattering consistent with a three dimensional Ising universality class. The low energy spectrum is well-described by Co$^{2+}$ cations coupled $via$ a three dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the $ab$ plane and along $c$, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long range antiferromagnetic order at $Trm{_{N}}sim$ 19 K. A crystal field analysis finds two bands of excitations separated in energy at $hbar omega$ $sim$ 5 meV and 25 meV, consistent with a $jrm{_{eff}}=frac{1}{2}$ ground state with little mixing between spin-orbit split Kramers doublets. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated $jrm{_{eff}}$ manifolds may play a key role in making the dynamics largely insensitive to disorder.