J. Deportes

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.

Observations of ferromagnetic correlations at high temperatures in paramagnetic iron

Abstract Polarised neutron scattering with polarisation analysis has been used to obtain a unique measurement of the paramagnetic fluctuations in iron at temperatures between 1273 and 1573 K. The results clearly demonstrate almost complete ferromagnetic correlation over distances up to 15 A. The average moment per atom taking part in the correlation and giving rise to paramagnetic scattering is about 1.3μ B . These findings should lead to a better understanding of paramagnetism in metals.

Evidence of short range magnetic order at four times Tc in a metallic compound containing Fe: Susceptibility and paramagnetic scattering in CeFe2

Below 230 K CeFe2 orders with a saturation magnetic moment of 1.15 μB/Fe at 4.2 K. The paramagnetic effective moment deduced from the Curie‐Weiss law is 3.7 μB. Reported here are the results of neutron paramagnetic scattering measurements obtained between 300 K and 870 K on CeFe2. The technique of polarization analysis has been used to obtain a unique measurement of the magnetic cross‐section. The Fe moment determined in this way is about 1 μB, close to that observed in the ferromagnetic state. However, strong ferromagnetic correlations are found to persist up to the highest temperature used in this study, i.e., 870 K. The results are in complete agreement with recent theories which tend to reconcile 3d band magnetism with the persistence of a moment in the paramagnetic state. In conclusion, effective moments derived from susceptibility measurements do not allow an understanding of the 3d magnetism in the paramagnetic state.

Magnetization properties of RE2Fe17Hx compounds: RE = Nd, Ce, Ho

Abstract A structural and magnetic characterization of RE 2 Fe 17 alloys and their corresponding hydrides has been carried out. Hydrogenation induces an anisotropic cell expansion which is found to occur mainly in the basal plane of the structure. Magnetization measurements have been performed in a continuous magnetic field up to 6 T in the temperature range of 4.2 to 300 K for both alloys and hydrides. The behaviour of the Curie temperature and magnetization upon hydrogenation is discussed.

Spatial correlation of magnetisation in the paramagnetic phases of iron and nickel

Abstract The diffuse scattering of polarised neutrons with polarisation analysis has revealed the presence of ferromagnetic short range order in the paramagnetic phases of iron and nickel. Unlike the residual long range correlations which occur close to T C , the short range order arises from the itinerant nature of magnetic electrons.

Ferromagnetic correlations in both the α and γ-phases of paramagnetic iron

Abstract Paramagnetic scattering experiments using polarised neutrons and polarisation analysis in the paramagnetic phase of iron are reported. The measurements have been performed at 1120 K in the α-bcc phase and 1320 K in the γ-fcc phase. Iron moments to respectively 1.3 (0.1)μ B and 0.9 (0.1)μ B have been shown to persist at these temperatures. In both phases, ferromagnetic correlations are present and are especially strong in the α-phase. The ferromagnetic correlations also determine the magnetic character of iron in the γ-phase, contrary to previous suggestions of antiferromagnetic behaviour postulated in the literature on the basis of indirect experimental arguments.

Hyperfine field anisotropy in RE-Fe compounds

Abstract A large anisotropy of the magnetization associated with a large magnetocrystalline anisotropy is observed on a single crystal of the hexagonal Y2Fe17 compound. An anisotropy of the orbital contribution to the hyperfine field is deduced from a Mossbauer experiment on the nucleus whose surroundings are uniaxial.

Mn magnetism and magnetic structures in RMn2

Because of the tetrahedral arrangement of the Mn atoms in the Laves phases, the magnetic structures of the RMn2 are frustrated. To reduce this frustration, very unusual magnetic structures are stabilized with large crystallographic distortion.