The electrochemical synthesis of nanoparticles of γ-Fe2O3 was performed in an organic medium. The size was directly controlled by the imposed current density, and the resulting particles were stabilized as a colloidal suspension by the use of cationic surfactants. The size distributions of the particles were narrow, with the average sizes varying from 3 to 8 nm. The amorphous character of the nanoparticles was clearly established by X-ray powder diffraction and TEM analysis. The microstructure of this phase could nevertheless be spectroscopically related to maghemite, γ-Fe2O3. 57Fe Mossbauer spectroscopy and magnetization measurements indicated that the dry powders exhibit superparamagnetic behavior at room temperature.

No abstract

No abstract

No abstract

The members of the CuMo1-xWxO4 series (0alpha transition can occur between 260 and 360 K, and the alpha-->gamma transition between 175 and 275 K as a function of x. The control of the alpha/gamma transition temperatures with x is related to the larger propensity of tungsten compared to molybdenum, to adopt a tetrahedral environment.

The effects of substituting nonmagnetic Mg 2+ and Zn 2+ ions for the Mn 2+ (S = 5/2) ions on the structural, magnetic and dielectric properties of the multiferroic frustrated antiferromagnet MnWO 4 were investigated. Polycrystalline samples of Mn 1-x Mg x WO 4 and Mn 1-x Zn x WO 4 (0 ≤ x ≤ 0.3) solid solutions were prepared by a solid-state route and characterized via X-ray and neutron diffraction, magnetization, and dielectric permittivity measurements. Mg and Zn substitutions give rise to very similar effects. The Néel temperature T N , the AF3-to-AF2 magnetic phase transition temperature T 2 , and the critical ferroelectric temperature T c = T 2 of MnWO 4 are reduced upon the nonmagnetic doping.At the lowest temperature (T = 1.5 K), the incommensurate magnetic structure for x(Mg) = 0.15 and x(Zn) = 0.15 corresponds to either a sinusoidal spin arrangement or an elliptical spin-spiral phase similar to the polar AF2 structure observed in MnWO 4 . These findings were discussed by considering the effects of the Mg and Zn substitutions on the crystal lattice and on the spin exchange network of MnWO 4 .

We present local probe results on the honeycomb lattice antiferromagnet Ba(3)CuSb(2)O(9). Muon spin relaxation measurements in a zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.

No abstract