Sophie Tencé

Huge influence of hydrogenation on the magnetic properties and structures of the ternary silicide NdMnSi

The hydride NdMnSiH obtained by exposure of the ternary silicide NdMnSi under a pressure of 4 MPa of hydrogen at 523 K crystallizes in the tetragonal ZrCuSiAs-type structure where H atom occupies the tetrahedral [Nd4] sites. The hydrogenation of NdMnSi induces an increase in the unit cell volume close to 3.3%. The investigation of NdMnSiH by magnetization and specific heat measurements reveals the existence of two antiferromagnetic ordering, respectively, at TN1=565(5) K and TN2=103(4) K. Neutron powder diffraction shows that these Neel temperatures are associated with an antiferromagnetic arrangement of the (i) Mn substructure (TN1) and (ii) Nd substructure linked to a reorientation of the Mn one (TN2). Comparison of the TN1 and TN2 temperatures of NdMnSiH to those reported for the initial compound NdMnSi indicates a strong increase in TN1 (280 K→565 K) and a significant decrease in TN2 (185 K→103 K). The magnetic properties, magnetic structures, and values of Nd and Mn ordered magnetic moments are discu...

Modulated magnetic structures of the antiferromagnetic hydride CeRuSiH

The hydride CeRuSiH is derived from the non-magnetic heavy-fermion ternary silicide CeRuSi that crystallizes in the tetragonal CeFeSi-type structure (P4/nmm space group). This hydride exhibits two antiferromagnetic transitions at TN1 = 7.5(2) K and TN2 = 3.1(2) K, evidenced by magnetization and specific heat measurements. Crystal and magnetic structures of CeRuSiH have thus been investigated by means of neutron powder diffraction, which has shown firstly that H-atoms are fully inserted in the [Ce4]-tetrahedra. Moreover, between TN1 and TN2, the Ce magnetic moments are ordered in a collinear antiferromagnetic sinusoidal structure associated with the propagation vector k = (kx, kx,1/2) (kx<1/3 and tends to 1/3 with decreasing temperature). At TN2, the magnetic structure becomes square wave modulated and commensurate with k = (1/3,1/3,1/2) as propagation vector. At 1.5 K, the structure is antiphase with a Ce magnetic moment of 1.68(2) µB. These magnetic structures are compared with those previously reported on the substituted ternary silicides (Ce1−xLax)Ru2Si2 and CeRu2(Si1−xGex)2, which also present an antiferromagnetic behaviour.

Intermediate-valent CeCoAl - a commensurate modulated structure with short Ce-Co distances

Abstract CeCoAl was synthesized by melting of the elements in a sealed niobium tube in an induction furnace. Annealing of the sample gave access to a single phase sample. Its structure was refined on the basis of single-crystal X-ray diffractometer data at different temperatures. Above 271 K CeCoAl crystallizes in its own structure type in the space group C2/m [a = 1107.4(2), b = 440.6(1) and c = 479.6(1) pm, β = 104.6(1)°]. Data obtained at 300 K lead to 511 F2 values with 20 variables and a residual of [I ≥ 3σ(I)] wR = 0.0539. Below 271 K satellites give rise to the superspace group C2/m(α0γ)00; α = 2/3, γ = 2/5 with a temperature independent q-vector. For the 90 K data (also for 180 and 220 K) the commensurate modulated structure could be refined with 4817 F2 values, 129 variables and residuals of wR = 0.0347 (main), wR = 0.1927 (satellites 1st order), wR = 0.1541 (satellites 2nd order) and wR = 0.1768 (satellites 3rd order) [a = 1107.5(1), b = 440.3(1) and c = 479.0(1) pm, β = 104.7(1)°]. For the three temperatures only minor variations of the modulation amplitudes are observed. The relation of the low temperature (3+1)D 3a × 5c approximant and the room temperature 3D structure is discussed on the basis of a group–subgroup relation. By investigation of the heat capacity, the phase transition could be identified as a second order one with a transition temperature of 271 K. Magnetic measurements clearly prove the intermediate cerium valence which is in line with the short Ce–Co distances.

From antiferromagnetic to ferromagnetic ordering induced by hydrogenation of the compounds NdCoSi and NdCoGe

NdCoSi and NdCoGe order antiferromagnetically at TN = 7.5 and 8.3 K respectively. These ternary compounds absorb easily hydrogen at 523 K under a hydrogen pressure of 4 MPa; the resulting hydrides are stable in ambient conditions and crystallize in the tetragonal ZrCuSiAs-type structure. Investigation of the hydrides by magnetization and electrical resistivity measurements and also by neutron powder diffraction reveals their ferromagnetic behaviours appearing below TC = 20.5 and 15.9 K respectively for NdCoSiH and NdCoGeH. Their magnetic structures are collinear with Nd-moments (2.28 and 2.35 μB at 1.5 K for NdCoSiH and NdCoGeH respectively) aligned along the c-axis as observed previously for NdFeSi. The antiferromagnetic-ferromagnetic transition evidenced during the hydrogenation can be explained by a change of the number of conduction electrons linked to the H-insertion as observed by electronic structure determination.

Multi-magnetic phases in the ferromagnetic ternary silicides Nd6Co1.67Si3 and Tb6Co1.67Si3

Neutron powder diffraction experiments have been carried out on the hexagonal ternary silicides Nd6Co1.67Si3 and Tb6Co1.67Si3 in order to determine their magnetic structures. These compounds exhibit two successive ferromagnetic transitions at TC = 84 K and T1 = 40 K for Nd6Co1.67Si3 and at TC = 186 K and T1 = 171 K for Tb6Co1.67Si3. Single-crystal x-ray diffraction and specific heat measurements have been realized on Tb6Co1.67Si3 in order to complete its characterization. Neutron diffraction investigation at 1.5 K reveals for both compounds a canted ferromagnetic structure with a propagation vector k = (0 0 0). The Nd moment values are, respectively, equal to μ1 = 2.06(10)μB and μ2 = 3.14(9)μB for Nd1 and Nd2 atoms whereas Tb1 and Tb2 atoms carry a magnetic moment of μ1 = 7.9(2)μB and μ2 = 8.9(2)μB in Tb6Co1.67Si3. Moreover, a transition from a canted ferromagnet to a pure ferromagnet occurs at T1 = 40 K in Nd6Co1.67Si3 while a spin reorientation of the non-collinear ferromagnetic structure occurs at T1 = 171 K for Tb6Co1.67Si3. These magnetic structures are discussed on the basis of the atomic disorder existing for Co atoms in these ternary silicides around the Nd1 and Tb1 atoms.

Neutron Diffraction Study of the Kondo Systems Ce6Ni1.67Si3 and Ce5Ni1.85Si3

Neutron powder diffraction experiments were performed on the moderate Kondo systems Ce6Ni1.67Si3 and Ce5Ni1.85Si3. These measurements have shown that Ce6Ni1.67Si3 does not present a long range magnetic order down to 1.6 K, but only short range ferromagnetic correlations. Ce5Ni1.85Si3 undergoes an antiferromagnetic transition at TN ≈ 6 K. Its magnetic structure at 1.5 K and zero field is collinear with the magnetic moments along the hexagonal axis and can be described with the propagation vector k = (1/2 0 0). This antiferromagnetic order coexists with a short range ferromagnetic component.