R. Lemaire

Magnetic transition and anomalous thermal expansion in R 2 Fe 17 compounds

In R-Fe compounds, the magnetic properties are mainly determined by the Fe-Fe interatomic distances and the number of Fe nearest neighbors. Below the ordering temperatures, the R 2 Fe 17 compounds are generally ferromagnetic or ferrimagnetic colinear. The compounds with rare earth ions of small atomic radii, Ce 2 Fe 17 , Tm 2 Fe 17 , and Lu 2 Fe 17 , are exceptions; they are helimagnetic. At low temperatures they exhibit a transition either to a fan spin or to a ferrimagnetic colinear configuration. For all compounds, a negative thermal expansion is observed below their ordering temperatures. Negative interactions occuring mainly in the substitution zones are thus deduced. Varying strongly with distances, they lead to the observed anomalous thermal expansion, and being largest in the compounds with the shortest Fe-Fe distances, their helimagnetic structure is explained.

X-ray and neutron determination of a so-called Th2Ni17-type structure in the lutetium-iron system

Abstract The iron-rich lutetium compound crystallises in a hexagonal structure, space group P 6 3 / mmc . By X-ray and neutron diffraction single crystal studies we have shown that its structure does not belong to the ideal Th 2 Ni 17 -type as previously proposed. The structure under investigation is established by ordered substitutions occurring in the CaCu 5 subcells on two of the three chains of lutetium atoms, parallel to the c -axis. These substitutions lead to an increase of the c parameter which allows for some disordered substitutions on the third chain and the stoichiometry shifts from LuFe 8.5 to LuFe 9.5 .

Helimagnetism in the cubic Laves phase YMn2

Abstract Neutron diffraction experiments on YMn 2 using a wave length of λ N = 2.483 A show a splitting of the magnetic peaks. The magnetic structure is helimagnetic consistent with an angle modulation of the previously reported antiferromagnetic structure. The NMR spectrum can be explained as arising from a perturbation of the helix by the magnetocrystalline anisotropy. Below T N the observed frustration of the negative Mn interactions is inherent from the topology of the crystallographic structure. Above T N , it creates short range ordering whose thermal decrease may explain the increase in the paramagnetic neutron scattering as the temperature is increased.

Anisotropy of the magnetization and of the iron hyperfine field in R2Fe17 compounds

Abstract A single crystal of the hexagonal Y 2 Fe 17 compound has been prepared. The exact composition, Y 2 Fe 18.9 has been refined through X-rays measurements. A large anisotropy of the magnetization is associated with the large magnetocrystalline anisotropy. Mossbauer experiments have been performed at 4.2 K under high magnetic fields. A large anisotropy of the orbital contribution to the hyperfine field is reduced. This can explain the anomalies of the hyperfine field observed in Tm 2 Fe 17 and ErFe 3 when magnetization reorientations occur with temperature.

Influence of substitutional pairs of cobalt atoms on the magnetocrystalline anisotropy of cobalt-rich rare-earth compounds

Abstract The crystallographic structures of cobalt-rich rare-earth (R) compounds are determined by the ordering of substitutional pairs of Co atoms in the same hexagonal RCo5 basic structure. However, RCo5 compounds are metastable at room temperature and contain some disordered substitutions. These substitutions induce a large decrease of the anisotropy in Sm1−sCosCo5 and Y1−s2CosCo5 alloys. Consequently, with each substitution there is associated a large anisotropy with a negative value of K1. A comparison with the strong uniaxial anisotropy of the RCo5 basic structure allows one to explain the changes of the direction of easy magnetization in Y2(Co1−xMx) alloys, and the differences in the coercivity of RCo5 compounds.

Magnetic interactions in R6Mn23 rare earth intermetallics

Polarized neutron experiments on a single crystal of Y6Mn23 give evidence for a ferrimagnetic arrangement of Mn atoms. The value of the Mn moment depends on the crystallographic site. Magnetization measurements at low temperature suggest non‐collinear magnetic structures for single crystals of Gd6Mn23, Dy6Mn23, and Er6Mn23. The non‐collinearity results from the effects of negative R‐R interactions and the crystal field.

Evidence of disordered substitutions in the “Th2Ni17-type” structure. Exact structure determination of the ThNi, YNi and ErCo compounds

Abstract Hexagonal intermetallic compounds close to R2M17 stoichiometry in the ThNi, YNi and ErCo systems have been studied by single crystal X-ray techniques. All of these compounds have been previously reported as belonging to the Th2Ni17-type structure. In fact, this structure type does not exist but is an ideal case. The nature of the observed extinctions and refinement of the crystal structures show that substitutions must occur on all R rows parallel to the c -axis with the subsequent displacements of the M atoms. This leads to a non-stoichiometric composition, e.g., ThNi9.5.

Magnetic behavior of rare-earth iron-rich intermetallic compounds

The thermal variation of lattice parameters of rare earth-transition metal intermetallic compounds rich in Fe, Co or Ni in the temperature range of 25-900°K is studied. For the Fe compounds, negative thermal expansion is observed below their magnetic ordering temperatures regardless of the nature of substitutional ordering. For Co and Ni compounds, the thermal expansion behavior is normal. It is then concluded that for the Fe compounds, the magnetic properties are mainly determined by the Fe-Fe interatomic distances and the number of Fe nearest neighbors, whereas for Co and Ni compounds the magnetic properties are determined by the conduction electron transfer from the rare earth to the 3d band of Co or Ni, The anomalous thermal expansion of Fe compounds and the metamagnetic transition of the Lu compound is explained in terms of the distance dependence of the interaction energy as proposed by Neel.

Magnetic interactions in R/sub 6/Mn/sub 23/ rare earth intermetallics

Polarized neutron experiments on a single crystal of Y6Mn23 give evidence for a ferrimagnetic arrangement of Mn atoms. The value of the Mn moment depends on the crystallographic site. Magnetization measurements at low temperature suggest non‐collinear magnetic structures for single crystals of Gd6Mn23, Dy6Mn23, and Er6Mn23. The non‐collinearity results from the effects of negative R‐R interactions and the crystal field.

Magnetic properties of HoFe3

Abstract The magnetic structure of HoFe3 has been determined by powder neutron diffraction techniques. The moments of the holmium and the iron atoms are colinear, ferrimagnetically coupled, and parallel to the basal plane of the hexagonal structure. X-ray diffraction of the powder, oriented in a magnetic field, has shown that the easy direction of magnetization is the b-axis. Molecular field coefficients and average exchange fields for holmium and iron atoms have been calculated from the thermal variation of the magnetization and the paramagnetic susceptibility. The interactions between iron atoms are strong whereas the holmium-holmium and holmium-iron interactions are weak.