D. Givord

Magnetic properties and crystal structure of Nd2Fe14B

Abstract Outstanding permanent magnet properties have recently been reported to occur in ternary compound Nd-Fe-B 1 . The tetragonal crystallographic structure of this compound Nd 2 Fe 14 B is determined (space group P4 2 /mnm). This structure is related to the hexagonal CaCu 5 -type structure which is the fundamental basis for the crystal structure of many rare earth-transition metal compounds.

Evidence in rare-earth (R)-transition metal (M) intermetallics for a systematic dependence of R-M exchange interactions on the nature of the R atom

In rare earth (R)–transition metal (M) compounds, large R‐M magnetic interactions can occur, which give rise to higher values of the ordering temperature (TC,TN) for compounds with magnetic R elements than for compounds with R nonmagnetic, i.e., La, Lu, Y. Due to the localized character of the 4f shell, these R‐M interactions are indirect, mediated by the 5d, 6s conduction electrons. The highest value of the ordering temperature is obtained for Gd compounds, and as a first approximation it is reasonable to write the interaction energy as ER‐M=−nRMMSRMSM, where MSR and MSM are the rare‐earth and transition metal spin moments, respectively. The molecular field coefficient nRM is generally assumed to be a constant throughout a given series, owing to the similarities of band structure for all R elements. In this paper, the molecular field coefficient nRM has been obtained for a number of series of rare earth‐transition metal compounds. The analysis reveals that nRM is not a constant going across a given serie...

Polarized neutron study of the compounds Y2Fe14B and Nd2Fe14B

Outstanding permanent magnet properties were recently observed in a Nd‐Fe‐B compound which was shown to crystallize in a new phase, R2Fe14B. Polarized neutron measurements are reported on Y2Fe14B and Nd2Fe14B single crystals. The 3d Fe moment on different sites in Y2Fe14B at 4.2 K is closely related to local environment. Its value is maximum for atoms in σ‐like layers at the center of an Fe antiprism. On the contrary, it is reduced to 1.95 μB by 4d‐3d electron transfer and hybridization for Fe atoms which have the largest coordinance number of Y atoms, 4. The measurements at 250 K reveal a larger thermal decrease of the 3d moment for Fe atoms which exhibit shortest Fe‐Fe interatomic distances. This property reveals a reduction of 3d magnetic interactions for short distances as was previously observed in R2Fe17 compounds. In Nd2Fe14B, the low values obtained at 4.2 K for Nd magnetic moments suggest that the magnetic structure, determined by a competition between 3d‐4f exchange interactions and crystal‐fiel...

Coercivity mechanisms in ferrites and rare earth transition metal sintered magnets (SmCo/sub 5/, Nd-Fe-B)

Measurements of the temperature dependence of the coercive field, of the magnetic viscosity, and of the angular dependence of the coercive field are reported for ferrite and rare-earth-transition-metal sintered magnets. A satisfactory explanation for the observed properties is obtained in a simple model, considering that magnetization reversal is initiated in a volume equal to the activation volume and is determined by the formation of a domain wall. From magnetic viscosity measurements, the activation volume is found to be proportional to the cube of the domain wall width, delta . The observed angular dependence of the coercive field reveals that, in the activation volume, the anisotropy is much larger than the coercive field and is not strongly reduced with respect to the bulk. If the coercivity is determined by true nucleation in a fully saturated sample, this is unlike the usual assumption that the magnetocrystalline anisotropy is strongly reduced in the volume of the nucleus. >

High performance hard magnetic NdFeB thick films for integration into Micro-Electro-Mechanical-Systems

5μm thick NdFeB films have been sputtered onto 100mm Si substrates using high rate sputtering (18μm∕h). Films were deposited at ⩽500°C and then annealed at 750°C for 10min. While films deposited at temperatures up to 450°C have equiaxed grains, the size of which decreases with increasing deposition temperature, the films deposited at 500°C have columnar grains. The out-of-plane remanent magnetization increases with deposition temperature, reaching a maximum value of 1.4T, while the coercivity remains constant at about 1.6T. The maximum energy product achieved (400kJ∕m3) is comparable to that of high-quality NdFeB sintered magnets.

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 .

Co energy and magnetization anisotropies in RCo5 intermetallics between 4.2 K and 300 K

The magnetocrystalline anisotropy has been known for a long time to be very large in the RCo5 intermetallics. The recent availability of high D.C. magnetic fields up to 200 kOe has enabled us to carry out an extensive experimental study of the energy and magnetization anisotropies in YCo5 from 4.2 K to 300 K. The magnetization anisotropy reaches 4% at 4.2 K. From a polarized neutron study on NdCo5 we find that this anisotropy of magnetization originates mainly from the 2c site. Previous measurements on YCo5 had shown that the orbital contribution to the 2c moment is large. Hence, we conclude that the Co magnetization anisotropy in RCo5 comes from a large orbital contribution to the magnetic moment. Finally, we account for the observed discontinuity in the magnetization at its reorientation in RCo5 ferrimagnets, and for the absence of a similar discontinuity in NdCo5 which is a ferromagnet.

Evolutionary conservation of early mesoderm specification by mechanotransduction in Bilateria

The modulation of developmental biochemical pathways by mechanical cues is an emerging feature of animal development, but its evolutionary origins have not been explored. Here we show that a common mechanosensitive pathway involving β-catenin specifies early mesodermal identity at gastrulation in zebrafish and Drosophila. Mechanical strains developed by zebrafish epiboly and Drosophila mesoderm invagination trigger the phosphorylation of β-catenin–tyrosine-667. This leads to the release of β-catenin into the cytoplasm and nucleus, where it triggers and maintains, respectively, the expression of zebrafish brachyury orthologue notail and of Drosophila Twist, both crucial transcription factors for early mesoderm identity. The role of the β-catenin mechanosensitive pathway in mesoderm identity has been conserved over the large evolutionary distance separating zebrafish and Drosophila. This suggests mesoderm mechanical induction dating back to at least the last bilaterian common ancestor more than 570 million years ago, the period during which mesoderm is thought to have emerged.

Magnetic viscosity in Nd-Fe-B sintered magnets

Abstract Magnetic viscosity in Nd-Fe-B sintered magnets was measured at different fields between 4.2 and 300 K. At a given temperature, the viscosity S ( H ) and the irreversible susceptibility χ irr are found to be proportional. The temperature dependence of the coefficient S v = S / χ irr is deduced and an expression for the activation energy is derived. It is found that the intrinsic activation energy E 0 is proportional to the domain wall energy, whilst the activation volume is proportional to the cube of the domain wall width.

Magnetostatic interactions between magnetic arrays and superconducting thin films

Abstract Two-dimensional arrays of amorphous Gd-Co square particles with thickness around 200 nm were fabricated on 20 nm thick Nb films by means of high-resolution e-beam lithography and sputter deposition. Interesting oscillations due to the fluxoid quantization were observed in the magnetoresistance measured around the superconducting transition temperature of Nb films. The superconducting state of the Nb films seems to be modulated by stray fields produced by the square magnetic particles.