F. Damay

Ordering of (Cr,V) Layers in Nanolamellar (Cr0.5V0.5)n+1AlCn Compounds

Nanolamellar MAX phase compounds (Cr0.5V0.5)n+1AlCn are formed with nu2009=u20091, 2 and 3, and their 300u2005K structure is studied in detail by high-resolution neutron diffraction. While the nu2009=u20091 compound is found to have complete disordering of vanadium and chromium in the metallic layers, the nu2009=u20092 and 3 compounds show strong tendency for these elements ordering, with the layer in the 2a(0,0,0) site of (Cr0.5V0.5)3AlC2 fully occupied by vanadium. The thermal expansion dependency of temperature is also studied by neutron diffraction for 2u2009<u2009Tu2009<u2009550u2005K, revealing a negligible thermal expansion below 100u2005K for all of the compounds.

Oxygen storage capacity and structural flexibility of LuFe2O4+x (0≤x≤0.5)

Combining functionalities in devices with high performances is a great challenge that rests on the discovery and optimization of materials. In this framework, layered oxides are attractive for numerous purposes, from energy conversion and storage to magnetic and electric properties. We demonstrate here the oxygen storage ability of ferroelectric LuFe2O4+x within a large x range (from 0 to 0.5) and its cycling possibility. The combination of thermogravimetric analyses, X-ray diffraction and transmission electron microscopy evidences a complex oxygen intercalation/de-intercalation process with several intermediate metastable states. This topotactic mechanism is mainly governed by nanoscale structures involving a shift of the cationic layers. The ferrite is highly promising because absorption begins at a low temperature (~=200 °C), occurs in a low oxygen pressure and the uptake of oxygen is reversible without altering the quality of the crystals. The storage/release of oxygen coupled to the transport and magnetic properties of LnFe2O4 opens the door to new tunable multifunctional applications.

Spin dynamics in the geometrically frustrated multiferroic CuCrO2

The spin dynamics of the geometrically frustrated triangular antiferromagnet multiferroic CuCrO2 have been mapped out using inelastic neutron scattering. The relevant spin Hamiltonian parameters modelling the incommensurate modulated helicoid have been determined, and correspond to antiferromagnetic nearest and next-nearest neighbour interactions in the ab plane, with a strong planar anisotropy. The weakly dispersive excitation along c reflects the essentially two-dimensional character of the magnetic interactions and according to classical energy calculations it is weakly ferromagnetic. Our results clearly point out the relevance of the balance between a ferromagnetic coupling between adjacent planes and a weakly antiferromagnetic next-nearest neighbour interaction in stabilising the three-dimensional ferroelectric magnetic order in CuCrO2. This novel insight on the interplay between magnetic interactions in CuCrO2 should provide a useful basis in the design of new delafossite-based multiferroic materials.

Zigzag ladders with staggered magnetic chirality in the S=3/2 compound beta-CaCr2O4

The crystal and magnetic structures of the S=3/2 antiferromagnet β-CaCr2O4 have been investigated by means of specific heat, magnetization, muon relaxation, and neutron powder diffraction between 300 and 1.5 K. In this compound, in which the unusual topology of the Cr3+ magnetic lattice can be described as a network of triangular “zigzag” ladders with legs parallel to c, a complex antiferromagnetic ordering with an incommensurate propagation vector k=(0,0,q) (q∼0.477 at 1.5 K) is evidenced below TN=21u2002K. This complex magnetic ordering can be described as a honeycomblike arrangement of cycloids, running along c, and presenting a unique pattern of staggered chirality. To understand the experimental observation of this staggered chirality, we propose to use antisymmetric Dzyaloshinskii-Moriya terms in the exchange Hamiltonian.

Thermal conductivity and magnetic transitions in Mn3+/Mn4+ manganites

We report on a comparative study of three manganites: Pr0.5Sr0.5MnO3, Pr0.5Ca0.5MnO3, and Pr0.5Sr0.3Ca0.2MnO3, which all exhibit the same Mn3+/Mn4+=1:1 concentration but show a very distinct magnetic behavior. The Pr0.5Sr0.5MnO3 sample is a ferromagnet (FM) below TC=265u2009K and becomes the A-type antiferromagnet at TN=140u2009K. In Pr0.5Ca0.5MnO3 the charge order develops at TCO=245u2009K and the CE-type antiferromagnetism occurs below TN=175u2009K. The Pr0.5Sr0.3Ca0.2MnO3 sample exhibits the FM transition at TC=225u2009K and the CE type antiferromagnetism and charge ordering occur simultaneously at TN=TCO=160u2009K (185 K) upon cooling (heating), respectively. The study is complemented by the data on two systems of 30% Mn4+, the ferromagnetic metal Pr0.7Sr0.1Ca0.2MnO3, and insulator Pr0.85K0.15MnO3, both with similar Curie temperature TC∼140u2009K. The electrical resistivity, thermoelectric power, thermal conductivity, and volume thermal expansion data are analyzed with respect to observed magnetic transitions and reveal the sign...

Helical magnetic state in the distorted triangular lattice ofα-CaCr2O4

The magnetic properties of the high temperature alpha form of the CaCr2O4 compound have been investigated for the first time by magnetic susceptibility, specific heat and powder neutron diffraction. The system undergoes a unique magnetic phase transition at 43K to a long range order incommensurate helical phase with magnetic propagation vector k=(0,0.3317(2),0). The magnetic model proposed from neutron diffraction data shows that the plane of rotation of the spins is perpendicular to the wave-vector, and that the magnetic modulation is consistent with two modes belonging to distinct irreducible representations of the group. The magnetic point group 2221 is not compatible with ferroelectricity unlike the CuCrO2 delafossite [Kimura et al., Phys. Rev. B, 78 140401 (2008)] but predicts the existence of quadratic magnetoelectric effects, discussed based on a Landau analysis.

Localised Ag(+) vibrations at the origin of ultralow thermal conductivity in layered thermoelectric AgCrSe2.

In materials science, the substructure approach consists in imagining complex materials in which a particular property is associated with a distinct structural feature, so as to combine different chosen physical characteristics, which otherwise have little chance to coexist. Applied to thermoelectric materials, it has been used to achieve simultaneously phonon-glass and electron-crystal properties. Mostly studied for its superionic conductivity, AgCrSe2 is a naturally layered compound, which achieves very low thermal conductivity, ~0.4u2009W.K−1.m−1 at RT (room temperature), and is considered a promising thermoelectric. The Cr atoms of the [CrSe2]∞ layer bear a spin Su2009=u20093/2, which orders below TNu2009=u200955u2009K. Here we report low temperature inelastic neutron scattering experiments on AgCrSe2, alongside the magnetic field evolution of its thermal and electrical transport. We observe a very low frequency mode at 3u2009meV, ascribed to large anharmonic displacements of the Ag+ ions in the [Ag]∞ layer, and 2D magnetic fluctuations up to 3 TN in the chromium layer. The low thermal conductivity of AgCrSe2 is attributed to acoustic phonon scattering by a regular lattice of Ag+ oscillating in quasi-2D potential wells. These findings highlight a new way to achieve localised phonon modes in a perfectly crystalline solid.

Phase transitions and magnetic structures in MnW1-x Mo x O4 compounds (x ⩽ 0.2).

Temperature-dependent specific heat, magnetization and neutron diffraction data have been collected in zero magnetic field for polycrystalline samples of MnW1-x Mo x O4 (xu2009u2009⩽u2009u20090.2) solid solution whose end-member MnWO4 exhibits a magnetoelectric multiferroic phase (AF2 phase) between T 1u2009u2009≈u2009u20098u2009K and T 2u2009u2009=u2009u200912.5u2009K. In MnW1-x Mo x O4, diamagnetic W(6+) are replaced with diamagnetic Mo(6+) cations and magnetic couplings among Mn(2+) (3du2009(5), Su2009u2009=u2009u20095/2) ions are modified due the doping-induced tuning of the orbital hybridization between Mn 3d and O 2p states. It was observed that magnetic phase transition temperatures which are associated with the second-order AF3-to-paramagnetic (T N) and AF2-to-AF3 (T 2) transitions in pure MnWO4 slightly increase with the Mo content x. Magnetic specific heat data also indicate that the first-order AF1-to-AF2 phase transition at T 1 survives a weak doping xu2009u2009⩽u2009u20090.05. This latter phase transition becomes invisible above the base temperature 2u2009K for higher level of doping xu2009u2009⩾u2009u20090.10. Neutron powder diffraction datasets collected above 1.5u2009K for a sample of MnW0.8Mo0.2O4 were analyzed using the Rietveld method. The magnetic structure belowu2009u2009≈u2009u200914u2009K is a helical incommensurate spin order with a temperature-independent propagation vector ku2009u2009=u2009u2009(-0.217(6), 0.5, 0.466(4)). This cycloidal magnetic structure is similar to the polar AF2 structure observed in MnWO4. The AF1 up-up-down-down collinear spin arrangement observed in MnWO4 is absent in our MnW0.8Mo0.2O4 sample.

Neutron diffraction study of magnetic order inNdFe2Al10

The orthorhombic compound NdFe

Effect of Tb3+ doping in mixed-valence manganites and cobaltites

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