Magnetic properties of the iron sublattice in the YFe12-xMx compounds (M = Ti, Mo or V; x = 1-3.5)

International audienceThe magnetic properties of the YFe12-xMx compounds (M = Ti, Mo or V; x = 1-3.5) have been determined in the ordered ferromagnetic state as well as in the paramagnetic state. The iron magnetic moment has been determined from 4 K up to the Curie temperature whereas the analysis of the paramagnetic region has led to the determination of the effective iron magnetic moment. The number of spins has been calculated below and above the Curie temperature in order to discuss the degree of itinerancy of the Fe magnetic behavior in the YFe12-xMx compounds. All the YFe12-xMx compounds (M = Ti, Mo or V; x = 1-3.5) have very similar crystalline properties: they crystallize in the same crystal structure and all the M elements used here are known to substitute for iron on the same crystal site. In contrast, they exhibit a wide range of magnetic behavior; the Curie temperature varies from 63 to 539 K and the mean magnetic moment per iron atom is also very dependent upon the M element used and its concentration. Furthermore the degree of itinerancy of the iron is not preserved along YFe12-xMx compounds but is found to depend significantly upon the nature of the substituting element M and its concentration. The results are discussed and compared to earlier published results obtained on binary R-Fe and ternary R-Fe-B compounds

On the origin of the giant isotopic effect of hydrogen on the magnetic properties of YFe2A4.2 (A5H, D): A high pressure study

International audienceIn order to identify the origin of the giant isotope effect observed on the magnetic properties of YFe2A4.2 compounds (A¼H, D) and probe the role of the cell volume difference, we have compared the effect of hydrostatic pressure up to 1.0 GPa on the ferro-antiferromagnetic transition temperatures TF-AF and spontaneous magnetization of YFe2D4.2 and YFe2H4.2. Using compressibility value of 0.013 GPa 1 and the remarkably different negative pressure slopes of TF-AF, the existence of critical volume where the ferromagnetism is suppressed at 0K, VC¼501.760.3 ˚A3, was demonstrated. This consequently established the crucial role of volume on the huge isotope effect observed on the magnetic properties

On the pressure and temperature dependence of ordering temperature and magnetization of Y1-xThxCo4B compounds

International audienceWe present here a detailed investigation of the pressure dependence of the structural and magnetic properties in a wide temperature range from 5 to 340 K for the Y1-xThxCo4B series. The unit cell lattice compressibility has been determined from powder neutron diffraction technique in the 0 to 0.5 GPa pressure range. Substitution of Th for Y is found to reduce significantly the compressibility. The Curie temperature decreases dramatically upon the Th for Y substitution. Studies were performed on polycrystalline samples under hydrostatic pressure up to 1.1 GPa and in magnetic field up to 5 T. The evolution of the saturation magnetization under pressure is presented as well as the pressure dependence of the Curie temperature. Remarkable pressure effects on the Curie temperature and on the critical field at which the first order magnetization process occurs for Y0.8Th0.2Co4B are analyzed and discussed. The pressure derivatives values of both the Curie temperature and the magnetization are negative for all the studied compounds. This is an opposite tendency than that expected from chemical pressure effects. This supports the dominant role of valence electrons in the magnetic properties of the substituted Y1-xThxCo4B compound in the case of the chemical pressure effect. The magnetization and the Curie temperature exhibit different pressure dependence along the Y1-xThxCo4B series of compounds

Structural and magnetic properties of nanocrystalline ZnFe2O4 powder synthesized by reactive ball milling

International audienceThe zinc ferrite (ZnFe2O4) has been obtained in nanocrystalline state by reactive milling in a high energy planetary mill from a stoichiometric mixture of oxides (ZnO and α-Fe2O3). A post milling annealing promotes the solid state reaction, improves the ferrite crystalline state and removes internal stresses. The formation of zinc ferrite was studied by X-ray diffraction and magnetic measurements. The chemical homogeneity and morphology of the powders were studied by X-ray microanalysis and scanning electron microscopy. The mean crystallite size after 16 h of milling was found to be 18 ± 2 nm. The lattice parameter of the obtained ferrite depends on the milling time and subsequent annealing treatment. It is lower than that of zinc ferrite obtained by the ceramic method. The evolution of the magnetization versus milling time is discussed in terms of milling induced cations reorganisation into the spinel structure

Specific heat measurements and structural investigation of CeCu6 - xSnx compounds

International audienceThe evolution of the crystal structure and some magnetic properties of the heavy-fermion material CeCu6 - xSnx (x = 0, 0.25, 0.65, 0.75, 0.85 and 1.0) has been studied by powder neutron diffraction and by specific heat measurements. The substitution of Cu by Sn suppresses the temperature induced orthorhombic to monoclinic transition, known to occur in the pure CeCu6 phase. No structural phase transition has been observed in these samples as a function of x but the cell volume increases considerably in an anisotropic way. Sn occupies preferentially the special Cu crystallographic site which is next to each of the four Ce atoms in the unit cell. The transition to antiferromagnetic order, characterizing the samples with higher x, is sensitive to both x and magnetic field. The results are discussed in the context of the competition between Kondo and RKKY interactions in disordered or not heavy-fermion systems and reveal an interesting interplay between composition, structure and magnetism in CeCu6 - xSnx

Synthesis, structural and magnetic characterization of nanocrystalline nickel ferrite-NiFe2O4 obtained by reactive milling

International audienceNanocrystalline nickel ferrite (NiFe2O4) has been synthesized from a stoichiometric mixture of oxides NiO and alpha-Fe2O3 in a high energy planetary mill. An annealing at 350 degrees C, after milling, was used to improve the solid state reaction. The obtained powders were investigated by X-ray diffraction, magnetic measurements, scanning electron microscopy, X-ray microanalysis and differential scanning calorimetry. The particles size distribution was analyzed using a laser particle size analyser. The nickel ferrite begins to form after 4 h of milling and continuously form up to 16 h of milling. The obtained nickel ferrite has many inhomogeneities and a distorted spinel structure. The mean crystallites size at the final time of milling is 9 +/- 2 nm and the lattice parameter increases with increase the milling time. DSC measurements revealed a large exothermic peak associated with cations reordering in the crystalline structure. The magnetization of the obtained powder depends on the milling time and annealing. After the complete reaction between the starting oxides the milling reduces the magnetization of the samples. The magnetization increases after annealing, due to the reorganization of the cations into the spinel structure

Quasi-one-dimensional magnetic properties of NiNb2−xVxO6 compounds synthesized at high pressure in a nonstandard columbite-type structure

We report on the low-dimensional magnetic behavior of the series of compounds NiNb2−xVxO6 (0 x 2), with a columbite-type crystal structure stabilized at high pressure and temperature. Based on susceptibility, magnetization, and specific-heat measurements, the system is characterized as presenting quasi-one-dimensional magnetism, with Ni2+ magnetic moments that can be modeled as Ising spins, placed along zigzag chains in the crystal structure. The low-temperature phase is found to consist of an antiferromagnetic arrangement of ferromagnetic chains, and a metamagnetic transition to uniform ferromagnetic order is observed under magnetic fields slightly above μ0H = 1 T. We discuss the effects of substituting vanadium for niobium, maintaining the same crystal structure along the whole series of samples. In particular, the long-range magnetic order, most clearly seen for x = 0, tends to be suppressed as the vanadium content is increased. The exchange interactions are quantified, revealing that the ferromagnetic intrachain interactions vary from about 7 K for NiNb2O6 to 2 K for NiV2O6, remaining one order of magnitude larger than the mean antiferromagnetic interchain coupling

A high-energy inelastic neutron scattering investigation of the Gd-Co exchange interactions in GdCo4B: Comparison with density-functional calculations

International audienceInelastic neutron scattering is used to quantify the Gd-Co exchange interaction in GdCo4B. A significant reduction is observed in comparison with the GdCo5 compound. A mean value of 130 T is obtained for the exchange field on the two Gd sites in GdCo4B. The experimental results are compared with density-functional calculations. The local atomic magnetic moments calculated using the LSDA+U approximation are reported for each atomic site of the GdCo4B crystal structure. These calculations demonstrate that the two nonequivalent Gd crystal sites experience a significantly different exchange interaction, a difference that is discussed in the light of the local atomic environment. The observed reduction of the exchange field occurring upon substituting B for Co in GdCo5 is mainly caused by the decrease of the Co magnetic moment, whereas the Gd-Co coupling constant is found to be almost the same in both GdCo5 and GdCo4B

Magnetic and crystal structures of the one-dimensional ferromagnetic chain pyroxene NaCrGe2O6

International audienceWe investigated the magnetic and structural properties of the ferromagnetic pyroxene NaCrGe2O6 by superconducting quantum interference device magnetometry and powder neutron diffraction. This material is the only ferromagnetic member of the recently reported multiferroic pyroxene family AMX2O6 A=Li,Na; M =Fe, Cr; X=Si,Ge . Below TC=6 K, the magnetic structure is characterized by one-dimensional magnetic chains with spins aligned along the c axis of the monoclinic cell. The magnetic moment of Cr3+ is significantly reduced by about 25%. We show that this is likely the result of the low dimensionality of the system. The associated magnetic space group is C2 /c . This symmetry does not allow a linear magnetoelectric effect. No structural phase transition was observed down to 1.8 K