Maria Poienar

On the strong impact of doping in the triangular antiferromagnet CuCrO2

Abstract 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 t2g orbitals of Cr3+ and that spin polarization is predicted with 3 μ B per Cr3+. The structural characterizations of CuCr1−xMgxO2 reveal a very limited range of Mg2+ substitution for Cr3+ in this series. As soon as x = 0.02 , a maximum of 1% Cr ions are 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 Mg2+ 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 Cr4+ “holes”, created by the Mg2+substitution, in the matrix of high spin Cr3+ ( S = 3 / 2 ) are responsible for the transport properties of these compounds.

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.

Mg substitution in CuCrO2 delafossite compounds

Abstract A detailed investigation of the series CuCr1−xMgxO2 ( x = 0.0 – 0.05 ) has been performed by making high-temperature resistivity and thermopower measurements, and by performing a theoretical analysis of the latter. Microstructure characterization has been carried out as well. Upon Mg2+ for Cr3+ substitution, a concomitant decrease in the electrical resistivity and thermopower values is found, up to x ∼ 0.02 – 0.03 , indicating a low solubility limit of Mg in the structure. This result is corroborated by scanning electron microscopy observations, showing the presence of MgCr2O4 spinels as soon as x = 0.005 . The thermopower is discussed in the temperature-independent correlation function ratio approximation as based on the Kubo formalism, and the dependence of the effective charge carrier density on the nominal Mg substitution rate is addressed. This leads to a solubility limit of 1.1% Mg in the delafossite, confirmed by energy dispersive X-ray spectroscopy analysis.

Effect of coupled ferroelectric and antiferromagnetic fluctuations on dielectric anomalies in spin induced multiferroics

Dielectric and magnetodielectric peaks have been evidenced by ε(T, H) measurements in spin induced ferroelectrics: CuCrO(2) and AgCrO(2) delafossites. Such behaviour, also found in several other improper ferroelectrics, can be explained in the frame of Landau analysis of phase transitions with two coupled order parameters: antiferromagnetic ordered moment, L, and polarization, P. The existence of such anomalies in the dielectric constant observed at T(N) is very general. The existence of this peak is not due to any linear coupling term between P and L in this system, but rather due to the L(2)P(2) term which always exists in every compound, whatever the symmetry/space group is.