Effect of composition and coating on the interparticle interactions and magnetic hardness of MFe2O4 (M = Fe, Co, Zn) nanoparticles.

Single domain superparamagnetic ferrite nanoparticles with the composition MFe2O4 (M = Fe, Co, Zn) have been prepared by thermal decomposition of metal acetylacetonates in diphenyl ether or dibenzyl ether, using oleic acid in the presence of oleylamine as a stabilizing agent. The Fe, Co and Zn ferrite nanoparticles are monodisperse with diameters of 4.9, 4.4 and 4.7 nm, respectively. The TG and IR results indicate that four or six carboxylate groups per nm2 are bonded at the surface of the particles acting as chelating and/or bridging bidentate ligands depending on the composition. The oleate groups minimize the interparticle interactions in Fe and Zn ferrite samples, while in the Co ferrite sample dipolar interactions produce broad maxima in the ZFC and energy barriers distribution curves. The inversion degree has been estimated from the Raman spectra and the obtained x values have been used to calculate the saturation magnetization and compare them with the experimental MS values. Compared to bulk materials, the magnetization value is higher for the Zn ferrite sample due to its mixed spinel cation distribution. For the Co ferrite sample, and probably for the Fe one, the low value of saturation magnetization seems to be due to the surface disordered layer of canted spins. Compared to non-coated nanoparticles with the same composition and similar size, the oleate groups, covalently bonded to the superficial cations, increase the anisotropy field and decrease the magnetization.

High pressure exploration in the Li-Ln-V-O system.

Using in situ high pressure Raman spectroscopy, two structural changes were observed in a sample of the composition LiLa5O5(VO4)2. Taking this into account and by combining different conditions, three new compounds were further obtained from high pressure-high temperature synthesis. Their crystal structure description was done using the antiphase approach, which implies the presence of oxygen-centered [OLn4] building units, where Ln is La for (1) ß-LiLa5O5(VO4)2 and (2) ß-LiLa2O2(VO4) or Nd for (3) LiNd5O5(VO4)2 compounds. (1) crystallizes in the triclinic space group P1[combining macron] with unit cell parameters of a = 5.8167(15) Å, b = 12.2954(28) Å, c = 18.7221(69) Å, α = 102.03(2)°, ß = 98.76(2)°, and γ = 103.54(2)°; a 3D structure was deduced from the ambient pressure polymorph. (2) also crystallizes in P1[combining macron] with a = 5.8144(7) Å, b = 5.8167(7) Å, c = 8.5272(1) Å, α = 98.184(7)°, ß = 100.662(7)° and γ = 92.579(7)°. It shows a 2D structure with [La2O2]2+ layers surrounded by [LiO4] and [VO4] tetrahedra sharing corners and edges. (3) exhibits a 3D architecture isotypic with AP-LiLa5O5(VO4)2. The crucial role of high pressure in such types of synthesis and materials is also discussed.

Ferromagnetic Cu-O-Cu coupling in CaCu3Sn4O12 probed by neutron diffraction.

The A-site ordered perovskite oxide with the formula CaCu(3)Sn(4)O(12) has been synthesized in polycrystalline form under moderate pressure conditions (3.5 GPa) in combination with high temperature (1000 °C). This oxide crystallizes in the cubic space group [Formula: see text] (no. 204) with the unit-cell parameter a = 7.64535(6) Å at 300 K. The SnO(6) network is extremely tilted, giving rise to a square planar coordination for Cu(2+) cations. The non-magnetic character of Sn(4+) offers an excellent opportunity to probe the magnetism of Cu(2+) at the A sublattice in CaCu(3)Sn(4)O(12). Magnetic susceptibility shows that this compound is ferromagnetic below T(C) = 10 K, which is an unusual magnetic behaviour in cuprates. This peculiar aspect has been examined by neutron powder diffraction. The refinement of the magnetic structure at 4 K indeed indicates a parallel coupling between Cu(2+) spins with a magnetic moment of 0.5 µ(B)/Cu atom.

High-pressure synthesis and neutron diffraction investigation of the crystallographic and magnetic structure of TeNiO3 perovskite.

TeNiO(3) has been prepared under moderate pressure conditions (3.5 GPa), starting from a reactive TeO(2) and Ni(OH)(2) mixture contained in a sealed platinum capsule under the reaction conditions (850 °C for 2 h). The sample has been studied by neutron powder diffraction (NPD) data and magnetization measurements. TeNiO(3) crystallizes in an orthorhombically-distorted perovskite structure (space group Pnma) with the unit cell parameters a = 5.9588(1) Å, b = 7.5028(1) Å and c = 5.2143(1) Å. The NiO(6) octahedral network is extremely tilted, shaping a trigonal-pyramidal environment for the Te, where it is effectively coordinated to three oxygen atoms at Te-O distances of 1.92 Å. Below T(N) ≈ 130 K, it experiences an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. Above the Néel temperature, a paramagnetic moment of 3.24(1) µ(B)/f.u. and θ(Weiss) = -199(1) K are obtained from the reciprocal susceptibility. Below T(N), the magnetic reflections observed in the neutron patterns can be indexed with a propagation vector k = 0. The magnetic structure corresponds to the magnetic mode G(y)F(z). The magnetic moments are oriented along the y-direction, with a canting along the z-axis. This ferromagnetic component explains the weak ferromagnetism observed in the magnetization isotherms; the infrequent shape of the magnetization cycles suggests a metamagnetic transition below 0.2 T. At T = 2.5 K, the ordered magnetic moment for the Ni(2+) ions is 1.88(5) µ(B) for the G(y) mode and 0.9(2) µ(B) for the F(x) mode.

Magnetic and structural features of the NdNi(1 - x)Mn(x)O3 perovskite series investigated by neutron diffraction.

Selected members of the perovskite series NdNi(1 - x)Mn(x)O(3) (0 ≤ x ≤ 1) have been prepared by a soft chemistry technique, followed by thermal treatments either under high oxygen pressure (x ≤ 0.5) or in air (x > 0.5). The crystal and magnetic structures have been studied by means of neutron diffraction, complemented with magnetic susceptibility measurements. For x = 0.25, 0.75, the crystal structure of the perovskites can be defined in the orthorhombic Pbnm space group, with Ni and Mn distributed at random over the octahedral sites of the structure. In contrast, the x = 0.5 compound crystallizes in a monoclinic P 2(1)/n structure containing two different octahedral positions, occupied by Ni and Mn, respectively. This is a result of the charge disproportionation of Ni(3+) + Mn(3+) to give Ni(2+) + Mn(4+) cations. The Ni(2+)O(6) octahedra are considerably larger than the Mn(4+)O(6) octahedra. This compound can be considered as a double perovskite of composition Nd(2)NiMnO(6). Unlike NdNiO(3) and NdMnO(3), which exhibit an antiferromagnetic ordering at low temperatures, the intermediate samples for x = 0.25, 0.50, 0.75 exhibit a ferromagnetic arrangement of (Ni, Mn) spins, with the moments aligned along the z axis, as probed using neutron diffraction. A maximum T(C) of 200 K is observed for x = 0.5, whereas T(C) = 150 K and 130 K are observed for x = 0.25 and 0.75, respectively. While NdNiO(3) is metallic above 200 K, a semiconducting behavior is determined between 120-300 K for the intermediate compositions.

Magnetic ground state of an experimental S=1/2 kagome antiferromagnet.

We present a detailed analysis of the heat capacity of a near-perfect S=1/2 kagome antiferromagnet, zinc paratacamite Zn(x)Cu(4-x)(OH)(6)Cl(2), as a function of stoichiometry x-->1 and for fields of up to 9 T. We obtain the heat capacity intrinsic to the kagome layers by accounting for the weak Cu2+/Zn2+ exchange between the Cu and the Zn sites, which was measured independently for x=1 using neutron diffraction. The evolution of the heat capacity for x=0.8...1 is then related to the hysteresis in the magnetic susceptibility. We conclude that for x>0.8 zinc paratacamite is a spin liquid without a spin gap, in which unpaired spins give rise to a macroscopically degenerate ground state manifold with increasingly glassy dynamics as x is lowered.

High-spin M2+ carboxylate triangles from the microwave.

The reaction of M(O2CMe)2.4H2O (M = Ni, Co) with NaN3 in pyridine/MeOH under microwave irradiation and controlled pressure/temperature leads to the formation of the trimetallic species [M3(N3)3(O2CMe)3(py)5] (M = Ni, 1; Co, 2) in 4 min and in high yields. Both complexes display dominant ferromagnetic interactions and high-spin ground states.