B. Malaman

Neutron diffraction study of CeMn2Ge2, PrMn2Ge2 and NdMn2Ge2: evidence of dominant antiferromagnetic components within the (001) Mn planes in ferromagnetic ThCr2Si2-type manganese ternary compounds

The magnetic properties of ThCr2Si2-type structure RMn2Ge2 (R = Ce, Pr, Nd) compounds have been reinvestigated by the use of neutron diffraction experiments. The ferromagnetic ordering previously proposed to take place on the manganese sublattice is revised. The three compounds are characterized by mixed (001) Mn planes where ferro- and antiferromagnetic components coexist. In the temperature range 2–300 K, CeMn2Ge2 (Tc = 320 K) exhibits an easy axis conical magnetic structure (cone semiangle α ≈ 60°) with a total Mn moment value of about 2.7μB at 2 K. Ce moments do not order above 1.6 K. At room temperature, PrMn2Ge2 (Tc = 330 K) and NdMn2Ge2 (Tc = 330 K) compounds exhibit an easy axis canted ferromagnetic structure (φ ≈ 60°) while, at lower temperature (T < 250 K), a conical magnetic structure takes place on the Mn sublattice (α ≈ 60°). The rare earth sublattices orders ferromagnetically below at about 100 K. In PrMn2Ge2, an easy axis prevails in the whole temperature range. At 2 K, the total Mn moment is about 2.8μB and the Pr moment value is about 2.9μB. On the contrary, in NdMn2Ge2 the easy axis rotates to the (001) plane below 250 K. At 2 K, the total Mn moment is about 2.7μB and the Nd moment value is about 2.3μB. In all cases, the thermal dependence of the AF component suggests that an ordering temperature TN not detected by magnetic moments, occurs at a temperature much higher than the Curie point. The results are compared with those of the related RMnSi, RMnGe and RMnSi2 compounds. The evolution of the magnetic behaviour within the ThCr2Si2-type structure RMn2X2 (X = Si, Ge) compounds is discussed.

Crystallographic data and magnetic properties of RT6Ge6 compounds (R Sc, Y, Nd, Sm, GdLu; TMn, Fe)

Abstract The iron and manganese germanides RT 6 Ge 6 (RSc, Y, Nd, Sm, GdLu; TMn, Fe) have been investigated by X-ray diffraction and susceptibility measurements in the temperature range 90–900 K. They crystallize in the known HfFe 6 Ge 6 , YCo 6 Ge 6 , TbFe 6 Sn 6 or HoFe 6 Sn 6 type of structure except GdFe 6 Ge 6 which is of a new type. All the compounds are anti-ferromagnetic except the RMn 6 Ge 6 (RNd, Sm, Gd) compounds which behave ferrimagnetically. Weak coercive fields (down to 1 kOe) are measured in RMn 6 Ge 6 (RNd, Sm).

Neutron diffraction study of Nd0.35La0.65Mn2Si2: A SmMn2Ge2-like magnetic behaviour compound

Abstract Magnetization and neutron diffraction measurements on the ThCr 2 Si 2 -type structure Nd 0.35 La 0.65 Mn 2 Si 2 compound are reported. This compound is characterized by re-entrant ferromagnetic behaviour, as observed in the SmMn 2 Ge 2 compound, with three characteristic magnetic transitions at T C = 295 K, T tl = 220 K (F → AF) and T t2 = 50 K (AF → F). Neutron diffraction study reveals the occurrence of an additional antiferromagnetic region (not detected by magnetometric measurements) characterized by antiferromagnetic (001) Mn planes, between T C = 295 K and T N = 395 K. On the other hand, the occurrence of an in-plane antiferromagnetic Mn component (obviously correlated with a critical MnMn intralayer spacing value of ∼ 2.87 A ) over the whole ordered range leads us to strongly revise the ferromagnetic ordering previously proposed to take place on the Mn sublattice in such compounds. A new description of the thermal dependence of the Mn sublattice magnetic orderings within the RMn 2 X 2 (X = Si, Ge) compounds is proposed.

Magnetic properties of RMn6Sn6 (R = Sc, Y, Gd−Tm, Lu) compounds with HfFe6Ge6 type structure

Ternary stannides RMn6Sn6 (R = Sc, Y, Gd−Tm, Lu) have been synthesized from the elements. All the compounds are isotypic and crystallize in the HfFe6Ge6 type structure (filled derivative of CoSn-B35). Their magnetic behaviours were studied in the temperature range 4.2 K < T < 900 K. All these compounds order ferrimagnetically (Gd−Ho) or antiferromagnetically (Sc, Y, Er, Tm, Lu), up to rather high temperatures (333 K < T < 435 K). With R = Gd−Ho, both R and Mn sublattices order simultaneously above the room temperature while Er and Tm sublattices order only at 75 and 58 K respectively. RMn6Sn6 compounds with R = Tb−Ho exhibit a second magnetic transition below the Curie point, related to a reorientation of the easy axis magnetization, from the basal plane to the c axis. Some of these compounds (Tb−Er) exhibit large coercive fields (up to 11 kOe for TbMn6Sn6 at 4.2 K) related to uniaxial anisotropy.

Neutron diffraction studies of LaMn2Ge2 and LaMn2Si2 compounds : evidence of dominant antiferromagnetic components within the Mn planes

Abstract The magnetic properties of ThCr 2 Si 2 -type structure LaMn 2 Ge 2 and LaMn 2 Si 2 compounds have been reinvestigated by neutron diffraction experiments. The ferromagnetic ordering previously proposed to take place on the manganese sublattice is revised. At high temperature, both compounds are purely collinear antiferromagnets (not detected by magnetic measurements), characterized by a stacking of antiferromagnetic (001) Mn planes. Below T c =310 and 325 K for LaMn 2 Ge 2 and LaMn 2 Si 2 , respectively, both compounds exhibit an easy-axis ferromagnetic behaviour. However, the occurrence of a dominant antiferromagnetic component within the (001) Mn planes yields a conical magnetic structure for the germanide (cone semi-angle α =58° at 2 K) and a canted magnetic structure for the silicide (φ=49°). At 2 K, the total Mn moments are about 3.0 and 2.4 μ B for LaMn 2 Ge 2 and LaMn 2 Si 2 , respectively. The results are compared with those of closely related RMnSi and RMnGe compounds and the magnetic properties of the ThCr 2 Si 2 -type structure RMn 2 X 2 (XSi, Ge) compounds are discussed.

Incommensurate magnetic structures of RMn6Sn6(R = Sc, Y, Lu) compounds from neutron diffraction study

Abstract Neutron diffraction measurements have been performed on the ternary compounds YMn 6 Sn 6 , LuMn 6 Sn 6 and ScMn 6 Sn 6 of HfFe 6 Ge 6 -type structure (space group, P6/mmm ). All three compounds order antiferromagnetically at 333 K, 353 K and 384 K for R = Y, Lu and Sc respectively. In the whole temperature range for YMn 6 Sn 6 and ScMn 6 Sn 6 and below room temperature for LuMn 6 Sn 6 , the magnetic structure is a flat spiral consisting of ferromagnetic (001) “MnSnSnMn” sheets (with the moments μ Mn ≈ 2.15 μ B at 2 K in the layers) stacked along the c axis in the sequence MnRMnSnSnMnRMn. In each case, according to the occurrence of several thermally dependent modulation vectors, the magnetic structure has been refined considering an inhomogeneous mixture of magnetic phases, each of them being characterized by a different wavevector. Under these conditions, the magnetic moments change their orientation by about 40°–60° within the basal plane on going from one R sheet to another. In the case of YMn 6 Sn 6 , a complex unique arrangement with a non-constant rotation of the moments has also been considered, i.e. a turn-angle sequence [1 × 160°−7 × 114.7°−2 × 27°−7 × 114.7°−1 × 160°] along one-half the c ′ parameter of a supercell with c ′ = 36 c . A discussion and some comparisons with the parent CoSn B35-type structure binary compounds, on the one hand, and the other RMn 6 Sn 6 compounds, on the other hand, are given in conclusion.

Magnetic properties of RFeSi (RLa-Sm, Gd-Dy) from susceptibility measurements and neutron diffraction studies

Abstract Investigations made by susceptibility measurements and neutron diffraction experiments are reported on the ternary silicides RFeSi (RLa-Sm, Gd-Dy) with the tetragonal CeFeSi-type structure (space group, P4/nmm ). This structure, which is closely related to the ThCr 2 Si 2 - and TbFeSi 2 -type structures, can be described as isolated ThCr 2 Si 2 blocks connected via R-R contacts. LaFeSi and CeFeSi are Pauli paramagnets, whereas PrFeSi is a Curie-Weiss paramagnet down to 2 K. The other compounds are ferromagnetic below T C = 25, 40, 135, 125 and 110 K for neodymium, samarium, gadolinium, terbium and dysprosium respectively. With the exception of GdFeSi, these compounds exhibit rather large coercive fields at 4.2 K (up to 9 kOe for terbium). Neutron diffraction experiments on RFeSi (R  Nd, Tb, Dy) led to the determination of collinear ferromagnetic structures with magnetic moments (with values close to the theoretical free ion values) localized on R ions only and aligned along the c axis. The results are discussed and compared with those of closely related RFe 2 Si 2 and RFeSi 2 compounds.

Magnetic structure of YMn6Ge6 and room temperature magnetic structure of LuMn6Sn6 obtained from neutron diffraction study

Neutron diffraction measurements have been performed on the ternary compounds YMn6Ge6 and LuMn6Sn6 of HfFe6Ge6-type structure (space group, P6/mmm). This structure can be described as a filled derivative of the CoSn-B35-type structure. Each of the rare earth (R) and Mn atoms are successively distributed in alternate layers, stacked along the c axis with the sequence MnRMnMnRMn. At 300 K, both compounds exhibit collinear antiferromagnetic arrangements and the magnetic structures consist of a stacking of ferromagnetic (001) layers of Mn with the coupling sequence Mn(+)RMn(−)Mn(−)RMn(+) (μMn ≈ 1.33(1)μB and 1.82(3)μB for LuMn6Sn6 and YMn6Ge6 respectively). For LuMn6Sn6, the magnetic moments lie in the (001) plane, while they are along the c axis in YMn6Ge6. At low temperature, a spin reorientation process occurs in both compounds, yielding incommensurate antiferromagnetic arrangements. For YMn6Ge6 (Tt ≈ 80 K), the Mn moments form a double-cone structure with a periodicity of about 105 A (μMn = 1.95(4)μB at 2 K), while only preliminary results are available for LuMn6Sn6 below about 200 K. The results are compared with those obtained on the CoSnB35-type structure binary compounds FeSn and FeGe, on one hand, and the RMn6Sn6 compounds, on the other hand.

Influence of the nature of milling media on phase transformations induced by grinding in some oxides

Abstract Polymorphic transformations induced by dry ball milling in an argon atmosphere have been investigated in various oxides (TiO2, SnO2, Y2O3, WO3) by X-ray diffraction, Mossbauer spectroscopy and transmission electron microscopy (TEM). The transformations have been found to depend on the nature of the milling media, particularly when reduction reactions take place between grinding tools and oxide particles. When ground with steel tools, cubic yttria is transformed into a monoclinic modification as reported in the literature, while tin oxide is reduced. When ground with zirconia tools, cubic yttria with a bixbyite type structure is transformed into cubic yttria with a fluorite type structure, while monoclinic tungsten oxide is transformed into a cubic oxide with an ReO3 type structure. Plausible structural explanations are proposed. In all cases, nanometre-sized domains have been observed by TEM even after grinding times as short as some minutes.

X-ray single crystal refinements on some RT2Ge2 compounds (R = Ca, Y, La, Nd, U; T = MnCu, RuPd): evolution of the chemical bonds

Abstract We report on single crystal refinement of 22 ThCr 2 Si 2 -type structure germanides of composition RT 2 Ge 2 (R = Ca, Y, La, Nd, U; T = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd). The interatomic distances and their variations with R size, R valency and the position of the transition metal in the Periodic Table are analysed from 35 refined structures. The TGe bond is always strong except in the Mn compounds. The GeGe bond is stronger in the right-hand side transition metal compounds. This effect is discussed in term of a previous theoretical work. Among the other contacts TT, RT and RR, the TT contacts seem to play an important role in the Ru and Rh compounds, whereas it is believed that the RT contacts control the cell dimensions of Fe and Co compounds. This paper examines the relationships between the interatomic distances and the magnetic behaviour of manganese ThCr 2 Si 2 -type structure compounds.