Ph. L’Héritier

Crystal structure, magnetic properties and electronic structure of the MnBi intermetallic compound

The low-temperature phase of the MnBi alloy has a coercivity μ0Hc of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product (BH)max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6μB, and that the Bi atom has a magnetic moment of −0.15μB which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6μB at a volume expansion rate of ΔV/V ≈ 100%.

Magnetic and structural properties of Nd2Fe17−xMnx solid solutions

A series of Nd2Fe17−xMnx solid solutions with x values between 0 and 6 were prepared and analyzed using magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. All of the Nd2Fe17−xMnx samples crystallized in the Th2Zn17−x-type rhombohedral structure. The lattice parameters and unit cell volumes decrease with increasing manganese content up to ∼x equal to 2, and then increase for higher manganese content. The magnetizations of Nd2Fe17−xMnx decrease with increasing manganese content and Nd2Fe17−xMnx is paramagnetic at room temperature for x greater than 3. The Curie temperature in Nd2Fe17−xMnx solid solutions is maximum for x equal to 0.5 and decreases at a rate of ∼10° per substituted manganese up to x equal to 3, after which it drops sharply. These results are discussed in terms of the manganese site occupancies in Nd2Fe17−xMnx.

Neutron diffraction and Mössbauer spectral study of Nd2Fe16Ti and its nitride

The 295 K powder neutron diffraction patterns and the temperature dependence of the Mossbauer spectra of Nd2Fe17−xTix and Nd2Fe17−xTixNy have been measured. A Rietveld refinement of the neutron diffraction patterns yields the Nd2Fe16.32Ti0.68 and Nd2Fe16.32Ti0.68N2.7 stoichiometries for the two compounds and indicates that titanium occupies only the 6c crystallographic site and nitrogen only the 9e site in the rhombohedral Th2Zn17 structure. The insertion of interstitial nitrogen into Nd2Fe16.32Ti0.68 produces a 6.8% increase in the unit cell volume. The relative areas of the Mossbauer spectral components indicate that iron–iron, iron–titanium, and titanium–titanium 6c–6c dumbbell pairs exist in both of these materials. The magnetic hyperfine fields observed for the iron on the 6c site in the iron–titanium dumbbell pair in both compounds is substantially reduced from that found in Nd2Fe17 and Nd2Fe17N2.6 because of the titanium dumbbell near neighbor. For all the remaining hyperfine parameters there is a ...

Spin rotation in R2Fe14BHx

We have studied by 57Fe Mossbauer spectroscopy and low field magnetization on oriented powders the temperature and hydrogen dependence of the magnetic structure of R2Fe14B compounds with R=Dy, Ho, and Y. We show that the iron sublattice may be canted by an angle ≂15° at high temperature. Hydrogen increases this canting at low temperature. It increases the spin reorientation temperature measured by bulk magnetization for Ho2Fe14B and induces a spin reorientation in the Dy alloys. These effects show that the strength of the crystal field terms of fourth (and maybe sixth) order compared to the terms of second order is increased by hydrogen.

The effect of nitrogen insertion in the Nd2-xGdxFe17Ny gadolinium substituted compounds

Abstract The structural and magnetic properties of Nd 2− x Gd x Fe 17 N y ( x =0–2 and y ≈3) gadolinium substituted compounds were studied. X-Ray diffraction characterization showed that all our samples are single phase and crystallize in the rhombohedral Th 2 Zn 17 -type structure. Nitrogen insertion in Nd 2− x Gd x Fe 17 samples induced an increase in the unit-cell volume. This increase was found to be about 5.6% for all gadolinium contents. Magnetization measurements showed that nitrogen insertion induced a strong increase in the Curie temperature, Δ T c = T cy − T c0 , which depends on the gadolinium content. The values of Δ T c decrease with increasing gadolinium concentration. It was found to decrease from 125% for x =0 to 70% for x =2. Nitrogen insertion leads to an increase in the saturated magnetization, μ s ( y )− μ s (0)/ μ s (0), which increases with increasing gadolinium content. It was found to increase from 26% for x =0 to 53% for x =2. The volume effects on the Curie temperature of our samples have been studied. The evolution of Gruneisen parameter defined by Γ versus gadolinium contents was linear.

Magnetic and crystallographic properties of NdMn6−xFexSn6 intermetallics

Intermetallic compounds of NdMn6?xFexSn6 () were studied by means of x-ray and neutron diffraction techniques and SQUID magnetic measurements in the temperature range of 30?400?K. The substitution of iron for manganese leads to a phase transition whereby NdMn6Sn6 with the HoFe6Sn6 structure (space group Immm) changes to TbFe6Sn6 (space group Cmcm) for NdMn6?xFexSn6 with . The iron atoms prefer to occupy the 8g sites at iron content x<2.0 due to the longest Mn/Fe?Sn bond distance. The Curie temperature (TC) increases from x = 0 to?1.5 and then decreases for the larger iron content. The magnetic moment of the iron sublattice couples ferromagnetically with the manganese and neodymium moments for the x<2.0 samples. Spin reorientation is observed in samples with iron content up to 1.5, and the spin reorientation temperature (Ts) increases with increasing iron content. Except for NdMn4Fe2Sn6, the easy direction of magnetization for all samples is parallel and perpendicular to the (bc) plane of the unit cell at 300 and 30?K, respectively. The easy direction of magnetization for NdMn4Fe2Sn6 is parallel to the a-axis in the entire temperature range mentioned above, as a result of the anisotropic contraction of the unit cell along the (bc) plane.

New method of insertion of hydrogen in R2Fe16Ti alloys with RY and Nd

Abstract A new method of inserting hydrogen in R 2 Fe 16 Ti compounds with RY and Nd was employed with success. The insertion of hydrogen in the structure increases the lattice parameters and, in turn, the unit cell volume. The changes in lattice constants are in good agreement with those obtained by the classic hydrogenation method. The Curie temperature as well as the saturation magnetization increase after insertion of hydrogen.

PRESSURE INDUCED REVERSAL OF THE VOLUME EXPANSION CAUSED BY INTERSTITIAL NITROGEN IN ND2FE17N3

Crystallographic properties of Nd2Fe17N3 have been investigated using powder neutron diffraction techniques at pressures up to 7.3 GPa. With increasing pressure, the unit cell volume of Nd2Fe17N3 decreases almost linearly at a rate of 6.1 A3/GPa without undergoing a phase transformation or expelling the interstitial nitrogen atoms. The observed rate of contraction corresponds to a bulk modulus of 1.4×1011 N/m2 for Nd2Fe17N3. The unit cell volume of Nd2Fe17N3 at 7.3 GPa is approximately equal to that of Nd2Fe17 at atmospheric pressure. Even though the unit cell of Nd2Fe17 expands anisotropically due to nitrogenation, contraction of the Nd2Fe17N3 unit cell under pressure appears to be isotropic with lattice parameters a and c decreasing by approximately 1.8% as the sample pressure increases from ambient to 7.3 GPa.

Crystal Chemistry, Physical Properties, Hydridation Properties of Nd2Fe14B-Type of Compounds

Structure and magnetic characteristics of RE2Fe14B have been determined on powder and single crystal samples, both by X-ray and neutron diffraction. All the magnetic and structural parameters have been measured versus the absorbed hydrogen. Hydrogen acts indirectly on the magnetic structure via the magnetoelastic couplings, but also directly via the “screenning” effect of hydrogen filling interstices between RE and Fe. Results from various techniques as Mossbauer spectrocopy, μ+SR, magnetization measurements… obtained on pure and hydrided samples are reported.