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

Influence of Mg, Ag and Al substitutions on the magnetic excitations in the triangular-lattice antiferromagnet CuCrO2

Magnetic excitations in CuCrO$_{2}$, CuCr$_{0.97}$Mg$_{0.03}$O$_{2}$, Cu$_{0.85}$Ag$_{0.15}$CrO$_{2}$, and CuCr$_{0.85}$Al$_{0.15}$O$_{2}$ have been studied by powder inelastic neutron scattering to elucidate the element substitution effects on the spin dynamics in the Heisenberg triangular-lattice antiferromagnet CuCrO$_{2}$. The magnetic excitations in CuCr$_{0.97}$Mg$_{0.03}$O$_{2}$ consist of a dispersive component and a flat component. Though this feature is apparently similar to CuCrO$_{2}$, the energy structure of the excitation spectrum shows some difference from that in CuCrO$_{2}$. On the other hand, in Cu$_{0.85}$Ag$_{0.15}$CrO$_{2}$ and CuCr$_{0.85}$Al$_{0.15}$O$_{2}$ the flat components are much reduced, the low-energy parts of the excitation spectra become intense, and additional low-energy diffusive spin fluctuations are induced. We argued the origins of these changes in the magnetic excitations are ascribed to effects of the doped holes or change of the dimensionality in the magnetic correlations.Comment: 7 pages, 5 figure

Structural and magnetic characterization of the complete delafossite solid solution (CuAlO2){1-x}(CuCrO2){x}

We have prepared the complete delafossite solid solution series between diamagnetic CuAlO2 and the t2g^3 frustrated antiferromagnet CuCrO2. The evolution with composition x in CuAl(1-x)Cr(x)O2 of the crystal structure and magnetic properties has been studied and is reported here. The room-temperature unit cell parameters follow the Vegard law and increase with x as expected. The effective moment is equal to the Cr^3+ spin-only S = 3/2 value throughout the entire solid solution. Theta is negative, indicating that the dominant interactions are antiferromagnetic, and its magnitude increases with Cr substitution. For dilute Cr compositions, J_BB was estimated by mean-field theory to be 2.0 meV. Despite the sizable Theta, long-range antiferromagnetic order does not develop until very large x, and is preceeded by glassy behavior. Data presented here, and that on dilute Al-substitution from Okuda et al., suggest that the reduction in magnetic frustration due to the presence of non-magnetic Al does not have as dominant an effect on magnetism as chemical disorder and dilution of the magnetic exchange. For all samples, the 5 K isothermal magnetization does not saturate in fields up to 5 T and minimal hysteresis is observed. The presence of antiferromagnetic interactions is clearly evident in the sub-Brillouin behavior with a reduced magnetization per Cr atom. An inspection of the scaled Curie plot reveals that significant short-range antiferromagnetic interactions occur in CuCrO2 above its Neel temperature, consistent with its magnetic frustration. Uncompensated short-range interactions are present in the Al-substituted samples and are likely a result of chemical disorder

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic (T_N = 24 K) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t_2g orbitals of Cr^3+ and that spin polarization is predicted with 3 mu_B per Cr^3+. The structural characterizations of CuCr1-xMgxO2 reveal a very limited range of Mg^2+ substitution for Cr^3+ in this series. As soon as x = 0.02, a maximum of 1% Cr ions substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01. This explains the saturation of the Mg^2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01. Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr^4+ "holes", created by the Mg^2+ substitution, in the matrix of high spin Cr^3+ (S = 3/2) are responsible for the transport properties of these compounds.Comment: 9 pages, 11 figures, more information at http://www.physik.uni-augsburg.de/~eyert

Hydrothermal synthesis of crednerite CuMn1-x M (x) O-2 (M = Mg, Al; x=0-0.08) structural characterisation and magnetic properties

A series of CuMn1-x M (x) O-2 (M = Mg, Al; x = 0-0.08) samples was prepared using the low-temperature hydrothermal method. Crednerite-type materials are obtained for a low level of substitution, i.e. up to x = 0.08, and transmission electron microscopy observations indicate that the average crystallite size decreases with an increase in x. The evolution of unit cell parameters in function of x, from Rietveld refinements using X-ray powder diffraction data, presents a distinct behaviour for both series, but corresponds in both cases to a regularisation of the triangular network in the (a, b) plane. The investigation of the structural, thermal and magnetic properties reveals that the substitution has a significant role on the magnetism in CuMn0.94M0.06O2 (M = Mg, Al). It was found that the Mg and Al substitution on the Mn site leads to a small increase in the magnetisation values at low temperature, although the particle size decreases, which can be related to a release of magnetic frustration

Hydrothermal synthesis of crednerite CuMn1-x M (x) O-2 (M = Mg, Al; x=0-0.08) structural characterisation and magnetic properties

International audienceA series of CuMn1-x M (x) O-2 (M = Mg, Al; x = 0-0.08) samples was prepared using the low-temperature hydrothermal method. Crednerite-type materials are obtained for a low level of substitution, i.e. up to x = 0.08, and transmission electron microscopy observations indicate that the average crystallite size decreases with an increase in x. The evolution of unit cell parameters in function of x, from Rietveld refinements using X-ray powder diffraction data, presents a distinct behaviour for both series, but corresponds in both cases to a regularisation of the triangular network in the (a, b) plane. The investigation of the structural, thermal and magnetic properties reveals that the substitution has a significant role on the magnetism in CuMn0.94M0.06O2 (M = Mg, Al). It was found that the Mg and Al substitution on the Mn site leads to a small increase in the magnetisation values at low temperature, although the particle size decreases, which can be related to a release of magnetic frustration

Oxygen storage capacity and structural flexibility of LuFe2O4Cx (0 < x< 0.5)

International audienceCombining 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 LuFe2O4Cx 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 ( D 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