R. Vert

Magnetisation and neutron diffraction studies of HoFe12−xTaxXy (0.5≤x≤0.7, X=H, C)

Magnetisation measurements, in particular the angular dependences of the components of the magnetisation vector, parallel and perpendicular to the applied field were measured on oriented powder samples of the HoFe11.4Ta0.6 alloy and its hydride and carbide. These experiments allow a study of the magnetocrystalline anisotropy and an accurate determination of the spin reorientation process occurring in the hydride. Moreover powder neutron diffraction experiments led to a determination of the crystallographic and magnetic structures of these materials, in particular the values of the iron and holmium moments on the different sites. Analysis of the results allows an estimate of the magnetocrystalline anisotropy and of the exchange interactions. Accordingly their changes after hydrogen or carbon insertion in the starting alloy are determined.

New RFe12-xTax compounds and their related hydrides and carbides (R = Tb to Lu, 0.5≤x≤0.7)

Abstract New RFe 12− x M x alloys as well as parent hydrides and carbides have been synthesised with M=Ta and for rare earth metal R belonging to the series Tb to Lu. The range of stability of the compounds was found rather narrow, i.e. 0.5≤ x ≤0.7 making this system very similar to that recently reported for M=Nb. The crystal cell parameters and the Curie temperature of the compounds and their parent interstitial have been measured, the latter being particularly high.

Magnetic properties of RFe12−xMox compounds (R = Y, Er)

Abstract The magnetic properties of the RFe 12− x Mo x compounds have been studied for R = Y and Er, with x ranging from 1 to 3. Increasing the molybdenum content leads to a decrease of (i) the Curie temperature, (ii) the 3d moment and (iii) the 3d anisotropy. All compounds with Y are weak ferromagnets but as the Mo content increases the shift between weak and strong ferromagnetism passes through a minimum around x = 1.5. In the erbium-based ferrimagnets, the thermal variation of the spontaneous magnetisation depends on the balance between the f and d metal magnetisations. In particular, for x = 2.5 the two sublattice magnetisations are almost equal at low temperature and a well-marked compensation point is observed around 10 K. For smaller Mo contents spin reorientations are observed at low temperature, which can be understood taking into account fourth- and sixth-order terms in the Hamiltonian of the crystalline electric field acting on erbium.

On the new (R = rare earth metals) alloys and their related hydrides and carbides

New interstitial hydrides and carbides (R = rare earth metal) have been synthesized. Structural characteristics and preliminary magnetic properties ( and ), together with those of the starting alloys, are presented and compared to the previously studied nitrides. Large increases of the Curie temperature and of the iron moment, particularly large in the carbides, are obtained. The evolutions of the deduced Fe-Fe and R-Fe exchange interactions are discussed. The large Curie temperature obtained with the carbides makes this series particularly attractive for permanent magnet applications, owing to the easy route proposed for the synthesis.

Spin reorientation and crystal field in a single crystal

Magnetization measurements performed on a single crystal of the tetragonal ferrimagnetic compound are presented together with their quantitative analysis. Between K and a spin reorientation temperature K, the spontaneous magnetization is along the crystallographic c direction. Below the spontaneous magnetization progressively shifts away from c and is directed between this axis and the [100] direction. The results are rather well accounted for by considering the iron sublattice characteristics measured on a single crystal and diagonalising the rare earth Hamiltonian through a system of coupled equations. A set of crystalline electric field (CEF) parameters and Er-Fe exchange field coefficient is proposed. It is shown that the spin reorientation does not come from a competition between iron anisotropy and the effect of the second order CEF parameter acting on the rare earth, as is frequently observed in this type of compounds. It results rather from the interaction between (i) the effect of these two contributions both of which favour the c axis, and (ii) that of the fourth order CEF terms, in particular , which favour an intermediate direction of magnetization. The relative importance of this latter contribution is greatest at low temperature.

Magnetic properties of RFe12−xMox compounds with R=Y, Gd, Ho, Er versus the molybdenum content

Abstract Magnetic properties of the tetragonal RFe 12− x Mo x compounds with R=Y, Gd, Ho, Er and for x ranging from one to three have been analysed. For compounds with Curie temperatures larger than room temperature (RT), measurements were performed on oriented powders allowing an estimate both of the easy magnetisation direction and of the magnetocrystalline anisotropy. The Fe anisotropy always favours the c -axis whereas rare earth anisotropy depends on the sign of second order Stevens factor favouring the c -axis for Ho and the basal plane for Er. Then in the latter compound spin reorientations are observed at temperatures which increase with x . In these ferrimagnetic compounds the Fe sublattice magnetisation prevails over that of the rare earth sublattice except for R=Er and x =2.5 where a compensation temperature is observed. Analysis of the Curie temperatures allows an estimate of the R–Fe and Fe–Fe exchange interactions in the different compounds. They are comparable to those determined in other 1–12 compounds and tend to increase with the Fe amount as observed in a large number of rare earth-3d transition metal materials.

Study of exchange and anisotropy parameters versus insertion of H and C in the RFe11.35Nb0.65 compounds (R=Ho, Lu)

Abstract Magnetisation measurements were performed on oriented powder samples of the RFe 11.35 Nb 0.65 compounds (R=Ho, Lu) and their hydrides and carbides. The study of the angular dependence of the parallel and perpendicular components of the magnetisation vector, reference to the applied field, allows to determine accurately the magnetocrystalline anisotropy parameters. Powder neutron diffraction experiments led to a determination of the crystallographic and magnetic structures of these materials, in particular the values of the iron and holmium moments on the different sites. Analysis of the results allowed estimation of the exchange interactions and of the magnetocrystalline anisotropy originating from both sublattices and an explanation of the spin reorientation process occurring in the Ho-based hydride.

Magnetic properties of tetragonal YFe10.5Mo1.5 single crystal

Abstract Magnetic properties of the ferromagnetic YFe 10.5 Mo 1.5 tetragonal compound ( T c = 383 K) were measured on a single crystal. The spontaneous magnetisation along the easy fourfold axis follows rather well the Langevin function. The anisotropy within the basal plane remains negligible. The field dependence of the magnetisation perpendicular to c yields to the determination of the second- and fourth-order anisotropy constants K 1 and K 2 at any temperature below 300 K. At 10 K they reach 5.9 × 10 5 and 5.7 × 10 4 J/m 3 , respectively. Besides, we compare the K 1 and K 2 determinations to those obtained from magnetisation measurements on oriented powder.

Synthesis and magnetic properties of HoFe10.5Mo1.5Cx carbides

Abstract HoFe 10.5 Mo 1.5 C x carbides ( x = 0.22, 0.48, 0.67, 0.88 and 0.99) were prepared by decomposition of anthracene on the alloy. The carbon insertion causes a considerable increase of the Curie temperature. The magnetic moments were determined from neutron diffraction data. Contrary to the moments of the three Fe sites that are sensitive to the carbon content, the Ho moment remains almost constant with x .

Magnetic anisotropy of a DyFe10.5Mo1.5 single crystal

Abstract The angular dependence of the magnetisation parallel and perpendicular to the applied field were carried out on a single crystal of the tetragonal DyFe10.5Mo1.5 compound. The spin reorientation was studied with a much better accuracy than previously from other techniques.